HQS Noise App User Guide

The HQS Noise App offers a novel approach to mapping problems from diverse fields such as chemistry, materials science, and other quantum mechanical problems onto a quantum computer. What sets the HQS Noise App apart is its scalable approach to quantum simulation based on simulating time evolutions. This approach is designed for simulation of mixed systems, e.g. spins coupled to fermions or bosons, which can be relevant for effects like light-matter interactions or microscopic vibrations in materials. In addition, it provides novel capabilities for applications like quantum machine learning. While the HQS Noise App is primarily tailored to NISQ devices, its relevance and significance extends to the era of fully error-corrected quantum computing. The Phase Estimation Algorithm, a key component for conducting quantum simulations on error-corrected quantum hardware, is grounded on the principle of time evolution. This principle is the cornerstone of the HQS Noise App, highlighting its continuing importance in quantum computing.

The HQS NoiseApp ca be used from two different perspectives:

1. To derive effective noisy algorithm models to gain insights into the effects of noise during circuit execution.
2. To use the noisy algorithm model to create optimized system-bath quantum circuits to simulate a open quantum system of interest on a noisy quantum computer.

The theoretical foundations to this software package have been published in two arxiv papers: noise mapping and bath fitting.

For a detailed scientific discussion we refer the readers to the arxiv papers. This User Guide will provide a concise summary of the fundamental physical concepts and an introduction to the Python interface of this software package.

Fundamental physical concepts

Noise models

We consider physical noise, meaning noise on the hardware-level, that is caused by qubits coupling to some fluctuating environment either during control operations or at all times. This is assumed to cause damping, dephasing, and/or depolarization of their quantum states. Our model of a noisy quantum computer is based on adding corresponding non-unitary (noise) operations after or before (ideal) unitary gate operations. This model is described in detail on page modeling.

Noise mapping

The first use of the HQS Noise App is to investigate how noise affects gate-based quantum simulation of noiseless systems. Particularly, it solves the Lindblad open-system model that the noisy quantum hardware is effectively running. Foundations of this mapping (between physical and simulated noise) are discussed on page mapping or in more detail in this arxiv paper. The shown mathematical derivation may be somewhat cumbersome in some places, but luckily the mathematics of connecting physical and effective noise for arbitrary quantum algorithms is done automatically by the HQS Noise App.

The system-bath approach

The second functionality of the HQS Noise App is to design system-bath quantum circuits to simulate an open quantum system of interest with the help of the physical noise on a quantum computer, thus turning parts of the noise on the device into a resource for computation. The basis of the system-bath approach is discussed in the system bath section of this user guide or in the arxiv paper.

Using the program

Python interface

On the interface page, we go through the Python interface of the HQS Noise App and discuss how to extract the noisy algorithm model, how to create system-bath simulation circuits, and how to fit a system-bath on a quantum computer to given bosonic baths, fermionic baths or a given spectral function.

Python examples

On the examples page, we show example analyses of several small-scale systems using the HQS Noise App. For more examples please also see our IPython-Notebook examples distributed along with the HQS Noise App.

API Documentation

Python API

Changelog

For a changelog starting from version 0.5 please see here

Modeling physical noise

Background

We consider incoherent errors as the predominant noise mechanism in quantum simulation. These errors can originate in interactions between qubits and a fluctuating environment. This mechanism influences the time evolution of the qubits in a way that the errors do not add up coherently. Simple examples of this are superconducting qubits coupling to lossy two-level systems, ion-trap devices with heated vibrational modes or unwanted control-pulse scattering, and spin-qubits in the presence of fluctuating background magnetic-field. During the last few decades, such noise mechanisms have been described successfully using the Lindblad master equation. The HQS Noise App applies this method to model the effect of noise in a digital quantum simulation.

Master equation

Lindbladian

The noise mapping performed by the HQS Noise App is valid in the regime of Markovian master equations, meaning for noise mechanisms with short memory times. In particular, we apply master equations in the Lindbladian form:

\[ \dot{\rho} = \sum_{ij} M_{ij} \left( A_i \rho A^\dagger_j - \frac{1}{2} A^\dagger_j A_i \rho -\frac{1}{2} \rho A^\dagger_j A_i \right) \equiv L[\rho] , \]

where \(\rho\) is the density matrix of the qubits (spins). Operators \(A_i\) offer a basis for representing the noise operators, while matrix \(M\) gives (generalized) noise rates. There is a freedom for definition of operators \(A_i\). Thus, only in combination with matrix \(M\) (and particularly its non-diagonal entries) can we make judgements about the dominant noise processes and rates. In the following section we detail our choice of operators \(A_i\) so that the user may understand how their physical noise model can be represented by \(M\).

Noise operators \(A\)

In our software, operators \(A_i\) are chosen to be Pauli-matrices, \(\sigma^x_n, \textrm{i}\sigma^y_n, \sigma^z_n\) (single-qubit noise on qubit \(n\)), and more generally Pauli products, \(\sigma^x_n\sigma^x_m,\sigma^x_n\textrm{i}\sigma^y_m,\sigma^x_n\sigma^z_m,\ldots\) (multi-qubit noise). An imaginary factor is added to Y-operators ( \(\sigma^y_n\rightarrow \textrm{i}\sigma^y_n\) ) so that we avoid having imaginary numbers in matrix \(M\) of the physical noise. (The effective rate matrix can however end up having imaginary entries.)

Rate matrix \(M\)

For clarity in the following, we provide our definition of the spin-lowering and spin-raising operators:

\[ \sigma^-=\frac{1}{2}\left( \sigma^x + \textrm{i}\sigma^y \right) = \begin{pmatrix} 0 & 1 \ 0 & 0 \end{pmatrix}, \\ \sigma^+=\frac{1}{2}\left( \sigma^x - \textrm{i}\sigma^y \right) = \begin{pmatrix} 0 & 0 \ 1 & 0 \end{pmatrix} \]

The Lindblad equation for damping of qubit \(1\) has the form:

\[ \dot{\rho} = \gamma_{\textrm{damping}} \left( \sigma^-_1 \rho \sigma^+_1 - \frac{1}{2} \sigma^+_1 \sigma^-_1 \rho - \frac{1}{2} \rho \sigma^+_1 \sigma^-_1 \right) \]

\[ \dot{\rho} = \gamma_{\textrm{damping}} \left[ \frac{1}{4} \left( \sigma^x_1 + \textrm{i} \sigma^y_1 \right) \rho \left( \sigma^x_1 - \textrm{i} \sigma^y_1 \right) -\frac{1}{8} \left( \sigma^x_1 - \textrm{i} \sigma^y_1 \right) \left( \sigma^x_1 + \textrm{i} \sigma^y_1 \right) \rho -\frac{1}{8} \rho \left( \sigma^x_1 - \textrm{i} \sigma^y_1 \right) \left( \sigma^x_1 + \textrm{i} \sigma^y_1 \right) \right] . \]

Our noise matrix then has four non-zero values:

\[ M_{1X, 1X} = \frac{\gamma_\textrm{damping}}{4}, \\ M_{1X, 1 \textrm{i} Y} = \frac{\gamma_\textrm{damping}}{4}, \\ M_{1 \textrm{i} Y, 1X} = \frac{\gamma_\textrm{damping}}{4}, \\ M_{1 \textrm{i} Y, 1 \textrm{i} Y} = \frac{\gamma_\textrm{damping}}{4}. \]

Similarly, dephasing of qubit 1 corresponds to noise matrix with non-zero entry:

\[ M_{1Z, 1Z} = \gamma_\textrm{dephasing}. \]

Depolarization corresponds to identical noise in all directions with rates:

\[ M_{1X, 1X} = \frac{\gamma_\textrm{depolarising}}{4}, \\ M_{1Y, 1Y} = \frac{\gamma_\textrm{depolarising}}{4}, \\ M_{1Z, 1Z} = \frac{\gamma_\textrm{depolarising}}{4}. \]

This definition is based on the density matrix (here) approaching a diagonal matrix with rate \(\gamma_\textrm{depolarising}\).

Form of noise matrix \(M\) for physical model

Since our physical noise acts individually on each qubit \(n\), our physical noise matrix is a sum over individual contributions \(m^n_{ij}\), where \(i,j\) can be \(nX,n\textrm{i}Y\) or \(nZ\). The matrix is real valued. In the case where noise acts on all qubits at all times, the total noise matrix has the form:

\[ M = m^0 \oplus m^1 \oplus m^2 \oplus \ldots , \]

whereas on the other hand, if noise affects only qubits that are being operated on, we have:

\[ M = \bigoplus_{n\in \textrm{acted qubits}} m^n . \] It is important to note that this is the model for physical gates: the form of the effective noise can also include multi-qubit operators and imaginary (non-diagonal) rates, see sections mapping and examples.

Noisy gates

Let us now introduce our model of gate-based quantum simulation with incoherent errors.

Super-operator matrix notation

For simplicity, we now use a notation where each noiseless gate operation (with a given unitary transformation \(U\)) is represented as matrix-multiplication:

\[ U \rho U^{\dagger} \rightarrow G \rho \\ U_2 U_1 \rho U_1^{\dagger} U_{2}^{\dagger} \rightarrow G_2 G_1 \rho . \]

On the right-hand side, the density "matrix" \(\rho\) has become a vector, and the unitary transformations \(G_i\) are matrices acting on it.

Model

In our modeling, we split each gate operation into the ideal unitary gate \(G\) and non-unitary noise \(N\). We then replace each gate in some gate decomposition with:

\[ G \rightarrow N G , \\ G_2 G_1 \rightarrow N_2 G_2 N_1 G_1 , \]

where the noise transformation is given by the Lindblad time-evolution over some physical gate time \(\tau_i\),

\[ N_i \equiv e^{L\tau_i} , \]

where \(L\) is the Lindblad operator. Such a description can always be established if physical gate times \(\tau_i\) are much shorter than the decoherence rates related to this gate (\(\tau_i \gamma_i \ll 1\)). The question of how correct this description is of some specific hardware realization is then redirected to the question of the correct choice of noise matrix \(M\).

In the context of correctly describing the interplay of unitary gates and noise (see also the mapping section) the notion of so called small-angle gates is also important. In short a small-angle gate is a unitary gate that is close to the identity gate compared to some other quantity of interest. The term is inspired by rotation gates (e.g. RotateX) that are close to the identity operation for small rotation angles. Small-anlges are not an absolute quantity but if we are talking about unitary gates G together with noise contributions N with a small prefactor p, we would consider a unitary a small-angle gate when the deviation D of G from the identity is small enough compared to p that

\[ p GN = p NG + p [D,N] \approx p NG \] The question of how correct this description is for some specific hardware realization is then redirected to the question of the correct choice of noise matrix \(M\).

Foundations of noise mapping

On this page, we go through the mathematical foundations of the noise mapping performed by the HQS Noise App. Physical noise on qubits transforms into effective noise on spins in the quantum simulation. For a detailed discussion see the full paper on noise mapping.

Trotterization of time evolution operator

Decomposition blocks

Digital quantum simulation is based on trotterization of the time-evolution operator, such that:

\[ e^{-\textrm{i} H t} = \left[ e^{-\textrm{i} H t/m} \right]^m \equiv \left[ e^{-\textrm{i} H \bar t} \right]^m \approx \left[ \Pi_{j} e^{-\textrm{i} H_j \bar t} \right]^m . \]

We consider a time-independent Hamiltonian \(H\) and timesteps \(\bar t = t/m\). The total Hamiltonian \(H\) is divided into elements \(H_j\),

\[ H=\sum_j H_j , \]

which correspond to "small-angle" unitary transformations in the time-evolution, \(\exp\left(-\textrm{i} H_j\bar t \right)\).

The goal of the division is to have a sequence of unitary transformations which can be implemented efficiently on hardware. In the HQS Noise App, such divisions are marked as circuit Decomposition_Blocks. An approximation (error) occurs when individual unitaries do not commute.

Small- and large-angle decompositions

Ideally, these unitaries can be implemented directly on hardware by the onset of single-qubit transitions or multi-qubit interactions, e.g., by control-field pulses. These operations then have one-to-one correspondence with equivalent small-angle gates,

\[ e^{-\textrm{i} H_j \bar t} \rightarrow G_j . \]

As described in section modeling, in the presence of noise, each gate \(G_j\) is assigned a noise operator \(N_j\).

On the other hand, some unitaries may need to be decomposed into a series of elementary gates,

\[ e^{-\textrm{i} H_j \bar t} \rightarrow \Pi_l G^l_j . \]

Here, each gate \(G^l_j\) comes with noise \(N^l_j\). In practice, such decompositions often include "large-angle" gates, such as CNOT gates or \(\pi\) rotations.

Trotter error

The error in the simplest, first order, Trotter expansion is of size:

\[ \epsilon_\textrm{error} \propto \bar g^2 , \] where \(\bar g= g \bar t\), and \(g\) is some typical energy of non-commuting terms in the Hamiltonian. Note that the error goes to zero in the limit \(\bar t\rightarrow 0\).

Effective noise in the simulated system

Here, we detail how we map the physical noise of a quantum computer to effective noise in the simulated system. For this, it is enough to analyze unitary gates and non-unitary noise operations within one Trotter step. It also turns out that analysis of noise rotations can be done on each decomposition block separately.

Noise transformations

Let us first consider a decomposition of some unitary operation \(\exp\left(-\textrm{i} H_j\bar t \right)\) by two large-angle gates. The generalization to an arbitrary number of gates is straightforward.

By using the fact that gates \(G_i\) are unitary, and thereby invertible, we can write:

\[ N_2 G_2 N_1 G_1 \rho = N_2 G_2 N_1 G_2^{-1} G_2 G_1 \rho \equiv N_2 N_1' G_2 G_1 \rho \equiv N G \rho , \]

where \(G\) corresponds to a small-angle unitary,

\[ G = G_2 G_1 , \]

and the effective noise operator is:

\[ N = N_2 N_1' , \\ N'_1 = G_2 N_1 G_2^{-1} . \] We see that the first noise operator got transformed by the (large-angle) unitary gate \(G_2\). Importantly, since both operators, \(G\) and \(N\), describe small-angle (or probability) processes, these operators will later define the effective Lindbladian of the simulated system.

More generally, for an arbitrary decomposition we can write:

\[ e^{-\textrm{i} H_j \bar t} \rightarrow \Pi_{l=\textrm{last}}^\textrm{first} N^l_j G^l_j = N_j G_j , \]

with:

\[ G_j \equiv \Pi_{l=\textrm{last}}^\textrm{first} G^l_j, \\ N_j \equiv \prod P_j \\ P_0 = N_j^\textrm{last} \\ P_1 = G_j^\textrm{last} N_j^\textrm{last-1} \left( G_j^\textrm{last}\right)^{-1} \\ P_2 = G_j^\textrm{last} G_j^\textrm{last-1} N_j^\textrm{last-2} \left( G_j^\textrm{last-1}\right)^{-1} \left( G_j^\textrm{last}\right)^{-1} \\ \ldots \]

It follows that the full time-evolution operator can be written as:

\[ \Pi_j \exp\left( -\textrm{i} H_j\bar t \right) = \Pi_{j} N_j G_j . \]

Here, each operator \(G_j\) as well as \(N_j\) is a "small-angle" (or probability) transformation. Finally, within the assumption that rotations over small-angle transformations can be neglected, we get the expression for the full Trotter step:

\[ \Pi_{j} N_j G_j \approx \left(\Pi_j N_j \right) \left(\Pi_j G_j \right) = \left( \Pi_j N_j\right) G. \]

(It should be noted that the SWAP algorithm of quantum simulation, available in later releases, includes large-angle gates between decomposition blocks. The effect on the noise mapping, described above, is however analogous to state swaps and can be accounted for by simple "reordering dictionaries" in decomposition-block definitions.)

Lindbladian in the simulated system

Noise mapping is most intuitively formulated in terms of unitary transformations of Lindbladian operators. When constructing the effective model Lindbladian, we include all consecutive noise operations \(N_j\), each of them being some multiplication of transformed Lindbladians \(N_j=\Pi_k\exp\left(L_j^k\tau_j^k\right)\) (see below), under one (exponentiated) Lindbladian,

\[ \Pi_j N_j = \Pi_{j} \Pi_{k} \exp\left(L_j^k\tau_j^k\right) \approx \exp\left(\sum_{jk} L_j^k\tau_j^k\right) \equiv \exp\left(L_\textrm{effective}\bar t\right). \]

Here, each summed Lindbladian \(L_j^k\) has noise operators \(A_i\) that are conjugated by the unitary gate defined by its corresponding decomposition, \(O_j^k\equiv U_j^\textrm{last}U_j^\textrm{last-1}\ldots U_j^\textrm{k+1}\),

\[ A_{i} \rightarrow A_i^{jk} \equiv O_j^k A_i \left(O_j^k\right)^\dagger. \]

(For native gates there will be no transformation, so that for these \(A_i^{jk} = A_{i}\).) The final effective Lindbladian then takes the form:

\[ L_\textrm{effective}[\rho] \equiv \sum_{ii'jk} \frac{\tau_j^k}{\bar t} M^{jk}_{ii'}\left( S_1- \frac{1}{2}S_2\right). \]

with

\[ S_1 = A_i^{jk} \rho (A^{jk}_{i'})^\dagger \]

and the anti-commutator

\[ S_2 = \lbrace(A^{jk}_{i'})^\dagger A_i^{jk} , \rho \rbrace \]

Here, an important noise scaling factor \(\tau / \bar t\) appears. We notice that the effective noise decreases with increasing Trotter timestep \(\bar t\). An example of this is given on page examples.

Error in the noise mapping

The above approximations are analogous to neglecting the Trotter error. First, we made the approximation that noise transformations can be restricted to individual decomposition blocks. The error of this approximation is of size:

\[ \epsilon_\textrm{error} \propto \bar g \gamma\tau , , \]

where \(\bar g= g \bar t\) and \(\gamma \tau\) are some typical Hamiltonian energy and noise probability of non-commuting elements between the two. Importantly, the error goes to zero in the limit \(\bar t\rightarrow 0\).

Second, in the derivation of the Lindbladian, an error occurs when we combine consecutive noise terms (Lindbladians) under one exponent. This step has an error of size:

\[ \epsilon_\textrm{error} \propto (\gamma\tau)^2 \]

The validity of the noise mapping can be investigated numerically by comparing the solution for the original noisy circuit and for the effective Lindbladian, as shown on page examples.

System-bath approach

The hqs-noise-app generates algorithms for quantum simulations of system-bath type problems. It is ideal for hardware with one or several good qubits, identified as the system qubits, which have weak noise in comparison to the other qubits, used as bath qubits. This version supports the simulation of spin-boson and spin-fermion systems. The boson or fermion baths are represented by bath spins.

Quick overview

Main idea: utilizing bath-qubit noise

In the HQS approach, noise originating in the bath qubits is effectively filtered and coupled to the system in a way that it appears to come from a bath with a continuous spectral function. This spectral function can be tuned digitally. We optimize the parameters of this system so that it reproduces the spectral function of a given system-bath problem. The optimization is made using a finite number of bath-mode frequencies, system-bath couplings, and bath-mode broadenings. In the quantum simulation, the frequencies and couplings can be implemented digitally. Their magnitude is scaled so that the energy-level broadenings due to the bath noise match to the fitting values. We also provide a SWAP algorithm specifically tailored for system-bath type problems, which takes into account that the bath is non-interacting.

In this version we provide functions to derive constraints for the broadening of the bath qubits from an available device. Providing these constraints to the optimization, the hqs-noise-app will automatically optimize the utilisation of the noisy qubits to improve the simulation results.

Validity

The system-bath simulations are best implemented when:

• system qubits have vanishing or weak noise
• the effective noise of bath qubits is predominantly damping and dephasing
• the quantum computer has a two-dimensional architecture or all-system to all-bath connectivity

These restrictions can, however, be lifted with the cost of reduced simulation accuracy or adding modifications to the system-bath model we simulate. For example, various types of noise mechanisms can be included to be a part of the simulation via changes in the form of the spin-boson Hamiltonian. Also, moderate system noise can be included as a constant background in spectral functions, leading to increased effective temperature.

Using the noise-app

The objective is to take in a system-bath (spin-boson and spin-fermion) model that one wishes to simulate. It is given as a mixed-system object (see also the struqture documentation) and can have a dense spectrum of bath modes or only several broad ones. Important is that it can be turned into a spectral function, which is later coarse-grained by the provided fitting tools. The output will be a coarse-grained spin-boson system with optimized parameters. This system will be then turned into a mixed spin-spin system. The final total system is ideally small enough so that it can be simulated on a quantum computer.

In this process, the fitting tool will also provide the best choice of a Trotter timestep when converting the spin-spin system into a quantum circuit that implements the time-propagation under the spin-spin Hamiltonian. Choosing the right Trotter timestep gives the best possible fit between the open system time evolution and the time propagation on the quantum computer under the influence of physical noise.

Coarse graining mixed Hamiltonians

More detailed, the goal of using the tool is to convert either a spin-boson system system (MixedLindbladOpenSystem) to a spin-spin system (MixedHamiltonianSystem). The spin-boson system corresponds to a Hamiltonian of type:

\[ H_\textrm{input} = H_\textrm{system} + \frac{1}{2}\sum_{i} \sigma^i_x X_i + \sum_{m} \omega_{m} a^\dagger(\omega_m) a(\omega_m) \nobreakspace , \]

where the coupling between the system and the bath is described by spin-boson operators, where the spin parts are single-spin operators, and bosonic part is a linear operator of the form:

\[ X_i = \sum_{m} g_{im} \left[a(\omega_m) + a^\dagger(\omega_m)\right] \nobreakspace . \]

The bosonic bath is non-interacting and has discretized frequencies \( \omega_m \) and widths \(d\omega_m\equiv \gamma_m\). The widths are identified as boson-mode damping rates, corresponding to lorentzian spectral peaks at their frequencies. All of the bosonic parts can also be given by a spectral function:

\[ S_{ij}(\omega) = 2\pi \sum_{m} g_{im}g_{jm} \delta(\omega-\omega_m + \textrm{i}\gamma_m) \nobreakspace , \]

added with the information of the interaction type, which is in this example \(\sigma_x \).

The final objective is to create a Hamiltonian of class MixedHamiltonianSystem, with system spins and bath spins of the form:

\[ H_\textrm{output} = H_\textrm{system} + \sum_{i\in\textrm{system}}\sum_{j\in\textrm{bath}} \frac{g_{ij}}{2} \sigma_x^i\sigma_x^j + \sum_{j\in\textrm{bath}} \frac{\epsilon_j}{2} \sigma_z^j \nobreakspace . \]

The first part of the mixed system is the original spin-system. The second part is the effective spin-bath, approximating the behaviour (spectral function) of the original bosonic bath. The broadening of the spin-bath modes is implemented by intrinsic noise of the qubits during the quantum simulation. This can be due to qubit damping and dephasing.

For a detailed technical description of the two main functions used in the coarse-graining see the sections on Bath fitter and the simulation examples

Creating mixed Hamiltonians and open systems

The system-bath model is defined in the interface as a mixed system. The mixed system can either have two spin subsystems, with the first the system and the second the bath, or one spin system and several boson subsystems. The number of boson subsystems needs to be equal to or smaller than the number of spins in the spin-subsystem. The boson subsystems are used to define the spin-boson problem. The mixed system used in quantum simulations here needs to have exactly two spin subsystems: the first subsystem is the system, the second the bath. Only non-interacting baths are allowed. The corresponding system-bath couplings are limited to products of two Pauli operators.

In the following we create a spin-boson Hamiltonian with struqture_py. Struqture is a library that supports building spin objects, fermionic objects, bosonic objects and mixed system objects that contain arbitrary many spin, fermionic and bosonic subsystems. The struqture documentation contains several examples how to define different kinds of Hamiltonians.

The spin-boson Hamiltonian we create has \(2\) system spins and \(3\) bosons. Each of the system spins is coupled to all of the bosons with the coupling matrix system_bath_couplings. The first part of the script will insert the system energies and the inner-system hopping. The next lines define the boson energies and the noise of the bosons. The last step is the insert the coupling between spins and bosons which are coupled through Pauli-z matrix.

from struqture_py import mixed_systems, spins, bosons
from hqs_noise_app import coupling_to_spectral_function
import numpy as np
import matplotlib.pyplot as plt
plt.rcParams.update({'font.size': 12})

# Set number of system spins.
system_spins = 2
# Set number of boson.
number_bosons = 3

system_energies = [0.1, 0.2]
# Hopping within the system spins.
system_hopping = 0.1

# Use three boson baths.
bath_energies = [0, 1, 2]
bath_broadenings = [0.1, 0.2, 0.3]

# System bath coupling matrix.
system_bath_couplings = [[0.3, 0.1, 0.3], [0.2, 0.4, 0.2]]

# Create a new mixed system with one spin and one boson subsystem.
spin_boson_hamiltonian = mixed_systems.MixedLindbladOpenSystem(
    [system_spins],
    [number_bosons],
    [],
)

# Insert system energies. 
for (system, system_energy) in enumerate(system_energies):
    index = mixed_systems.HermitianMixedProduct(
        [spins.PauliProduct().z(system)], [bosons.BosonProduct([], [])], []
    )
    spin_boson_hamiltonian.system_add_operator_product(index, 0.5 * system_energy)

# inserting inner-system hopping (0.5*xx + 0.5*yy)
for system in range(system_spins - 1):
    index_xx = mixed_systems.HermitianMixedProduct(
        [spins.PauliProduct().x(system).x(system + 1)],
        [bosons.BosonProduct([], [])],
        [],
    )
    index_yy = mixed_systems.HermitianMixedProduct(
        [spins.PauliProduct().y(system).y(system + 1)],
        [bosons.BosonProduct([], [])],
        [],
    )
    spin_boson_hamiltonian.system_add_operator_product(index_xx, 0.5 * system_hopping)
    spin_boson_hamiltonian.system_add_operator_product(index_yy, 0.5 * system_hopping)


# Set bath energies
for (bath_index, bath_energy) in enumerate(bath_energies):
    index = mixed_systems.HermitianMixedProduct(
        # Identity spin operator
        [spins.PauliProduct()],
        # Create a Boson occupation operator
        [bosons.BosonProduct([bath_index], [bath_index])],
        [],
    )
    spin_boson_hamiltonian.system_add_operator_product(index, bath_energy)

# Set bath noise
for (bath_index, bath_broadening) in enumerate(bath_broadenings):
    # create the index for the Lindblad terms.
    # We have pure damping
    index = mixed_systems.MixedDecoherenceProduct(
        # Identity spin operator
        [spins.DecoherenceProduct()],
        # Create a Boson occupation operator
        [bosons.BosonProduct([], [bath_index])],
        [],
    )
    spin_boson_hamiltonian.noise_add_operator_product((index, index), bath_broadening)

# Set couplings, use longitudinal (Z) coupling
for spin_index in range(system_spins):
    for (bath_index, system_bath_coupling) in enumerate(
        system_bath_couplings[spin_index]
    ):
        index = mixed_systems.HermitianMixedProduct(
            # Identity spin operator
            [spins.PauliProduct().z(spin_index)],
            # Create a Boson coupling  operator (always a + a^dagger)
            [bosons.BosonProduct([], [bath_index])],
            [],
        )
        spin_boson_hamiltonian.system_add_operator_product(index, system_bath_coupling)


print("Newly created system:")
print(spin_boson_hamiltonian.__repr__())

# create spectrum from Spin-Bath-System
min_frequency = -2
max_frequency = 4
number_frequencies = 1000
frequencies = np.linspace(min_frequency, max_frequency, number_frequencies)

spectrum = coupling_to_spectral_function(spin_boson_hamiltonian, frequencies)

print("Spectrum frequencies")
print(spectrum.frequencies())

print("Spectrum for 0, 0 spin indices with Z coupling")
print(spectrum.get(("0Z", "0Z")))

print("Spectrum for 0, 1 spin indices with Z coupling")
print(spectrum.get(("0Z", "1Z")))

System-bath circuits

We also provide algorithms specifically tailored for system-bath type problems, which take into account that the bath is non-interacting and that large-angle rotations of the bath qubits should be avoided. System-bath interactions can be generated optimally (at least) for two types of hardware connectivity/architechture.

Top: System-bath interactions generated using system to all-bath connectivity. Bottom: System-bath interactions generated in 2d-architecture using nearest-neighbour connectivity and a special SWAP algorithm.

Ideally, all system qubits are connected to all bath qubits. For system-bath problems connectivity between bath qubits is not needed, since the bath is usually assumed to be non-interacting. This approach is used when using the algorithm ParityBased (see also the description on page interface).

On the other hand, if only nearest-neighbour interactions are possible, we offer the user an automatic circuit generation based on a SWAP algorithm specifically optimized for system-bath type problems and 2d hardware-architecture. Here, the quantum state(s) of the system spin(s) are moved (or swapped) in a system-qubit network, located between the bath qubits. Within this modification, one avoids performing SWAPs between bath qubits, which can cause major modifications to the effective noise model. In the example shown above, the state of the spin is stored by two system qubits, one per time. This architechture most optimally stores a system consisting of \(n\) spins coupled to \(2n\) bath modes. This approach is used when using the algorithm QSWAP.

The noise-app

The noise-app allows to set properties of the noise model and several other features. A simple example that initialises the noise-app with the noise-mode and the algorithm is

from hqs_noise_app import HqsNoiseApp

# Initialize noise app.
noise_mode = "all_qubits"
algorithm = "VariableMolmerSorensen"

noise_app = HqsNoiseApp(noise_mode)
noise_app.time_evolution_algorithm = algorithm

# Print all available methods of the HqsNoiseApp
print(dir(HqsNoiseApp))

# Print docstring of algorithm
help(HqsNoiseApp.algorithm)

The possible input argument of the noise_mode are

• all_qubits: The noise model adds noise on all qubits that are involved in the whole quantum circuit after each gate operation.
• active_qubits_only: The noise model only adds noise on the qubits that are actively involved in the gate operation after each operation.
• parallelization_blocks: Parallelizes operations into blocks assuming that device supports it.

The available algorithms are

• ParityBased: An algorithm that implements the time evolution under a product of PauliOperators via the total Parity of the set of involved qubits.
• QSWAP: Terms in the Hamiltonian involving two qubits are implemented using a swap network that reorders the terms.
• QSWAPMolmerSorensen: The QSWAP algorithm using variable angle Mølmer Sørensen XX interaction gates.
• VariableMolmerSorensen: An algorithm that implements terms in the Hamiltonian involving two qubits with a basis change and variable angle Mølmer Sørensen gates

Besides noise_mode and algorithm, there are several other parameters that can be set with the HqsNoiseApp. A complete list of methods can be obtained with print(dir(HqsNoiseApp)). Information about the methods can then be obtained by help(HqsNoiseApp.algorithm) for example.

Device and Noise Model

Some of the functions of the noise-app require that we define a device and/or a noise model to run the algorithm. These are defined in qoqo. The documentation contains several examples about how to use qoqo. Important for the noise-app is mainly how to set up a device and create a noise model.

from qoqo import devices, noise_models
# Number of qubits. 
number_qubits = 3

# Setting up the device.
single_qubit_gates = ["RotateX", "RotateZ"]
two_qubit_gates = ["VariableMSXX"]

# Same time for all qubits.
gate_time = 1.0

# Setup device with all-to-all connectivity.
device = devices.AllToAllDevice(
    number_qubits, single_qubit_gates, two_qubit_gates, gate_time
)
# Set up a noise model
noise_model = noise_models.ContinuousDecoherenceModel()

# Damping and dephasing of qubits. 
qubit_damping_rates = [0.01, 0.02, 0.15]
qubit_dephasing_rates = [0.004, 0.006, 0.12]

# Add damping and dephasing to qubits.
for ii in range(number_qubits):
    noise_model = noise_model.add_damping_rate([ii], qubit_damping_rates[ii])
    noise_model = noise_model.add_dephasing_rate([ii], qubit_dephasing_rates[ii])

# Set single qubit gate times. 
for qubit in range(number_qubits):
    device.set_single_qubit_gate_time('RotateX', qubit, 0.03)
    device.set_single_qubit_gate_time('RotateZ', qubit, 0.01)

The above noise model has different rates for all qubits. We also set the gate times for single qubit gates to be faster than the gate times of the two-qubit gate.

System-bath functions

We provide several functions specifically for the system-bath analysis.

Spin spectrum

• coupling_to_spectral_function: A tool that takes a spin-boson system-bath and creates the effective spectral function seen by the system. The couplings in the system-bath correspond to \(g_{ism}\). If the spectral function is unphysical, the closest solution according to the Frobenius norm is returned.

  • Import:

    from hqs_noise_app import coupling_to_spectral_function
    

  • Input:

    • spin_boson_system (MixedLindbladOpenSystem): The struqture system-bath model that contains the couplings and baths from which the spectral function is constructed. The pure system part is ignored for the construction of the spectral fucntion.
    • frequencies (np.ndarray): The spectral function can be constructed for arbitrary n frequencies. For standard use-cases we recommend using a fine-grained sequence of evenly spaced values e.g. by using :code:np.linspace(start, stop, 1000) as the parameter
    • background (float): A constant background shift that is applied to the on-diagonal ((0,0), (1,1) etc.) parts of the spectral function. Can be used to represent direct decoherence on the system-parts of the system-bath model

  • Output:

    • spectrum (SpinBRNoiseOperator or FermionBRNoiseOperator): Spectral function with cross-correlations. Also contains the frequencies extracted from the coupling in the system-bath hamiltonian.

• spectral_function_to_coupling: The opposite of the previous function. It takes the spectral function seen by a spin system and constructs a collection of bosonic baths that would produce that spectral function. The constructed bosonic baths can include cross-correlations in the coupling. If the spectral function is unphysical, the closest solution according to the Frobenius norm is returned.

  • Import:

    from hqs_noise_app import spectral_function_to_coupling
    

  • Input:

    • spectrum (SpinBRNoiseOperator or FermionBRNoiseOperator): Spectral function with cross-correlations. The values of the spectral function are defined for given frequencies.
    • number_spins (int): Number of spins in the system part of the mixed system.

  • Output:

    • MixedLindbladOpenSystem: The bath and coupling parts of a spin-boson system-bath in the form of a struqture MixedLindbladOpenSystem. The pure system parts of the model will be empty.

• SpinBRNoiseOperator: A helper class that can represent the spectral function for a spin-system for defined frequencies. Includes cross-correlations.

  • Import SpinBRNoiseOperator

    from hqs_noise_app import SpinBRNoiseOperator
    

  • Input: - frequencies (np.ndarray): The frequencies for which the spectral functions are defined.

  • Output:

    • SpinBRNoiseOperator: New empty spin spectrum.

  • Functions:

    • set(key: (PauliProduct, PauliProduct), spectrum: np.ndarray): Set the component of the spectral function corresponding to the PauliProduct given by key.
    • get(key: (PauliProduct, PauliProduct)): Get the component of the spectral function corresponding to the PauliProduct given by key.
    • frequencies(): Return the frequencies the spectral function is defined for.
    • get_spectral_function_matrix(spectrum_index: int, number_spins: int): Gets the matrix of the spectral function of a spin-system at a specific energy index. A real spectral function needs to be symmetric in the coupling indices. The method will fail if the SpinBRNoiseOperator is not symmetric. The output of this function will be of size 3n, as for each index there are three possible coupling types: X, Y, and Z.
    • resample(frequencies: np.ndarray): Resamples the SpinBRNoiseOperator on a new set of frequencies. The SpinBRNoiseOperator defined on the old frequencies is interpolated for the new frequencies. The interpolated values are taken as the new values of the spectral function for these frequencies.

• FermionBRNoiseOperator: A helper class that can represent the spectral function for a spin-system for defined frequencies. Includes cross-correlations.

  • Import FermionBRNoiseOperator

    from hqs_noise_app import FermionBRNoiseOperator
    

  • Input: - frequencies (np.ndarray): The frequencies for which the spectral functions are defined.

  • Output:

    • FermionBRNoiseOperator: New empty spin spectrum.

  • Functions:

    • set(key: (FermionProduct, FermionProduct), spectrum: np.ndarray): Set the component of the spectral function corresponding to the FermionProduct given by key.
    • get(key: (FermionProduct, FermionProduct)): Get the component of the spectral function corresponding to the FermionProduct given by key.
    • frequencies(): Return the frequencies the spectral function is defined for.
    • get_spectral_function_matrix(spectrum_index: int, number_spins: int): Gets the matrix of the spectral function of a spin-system at a specific energy index. A real spectral function needs to be symmetric in the coupling indices. The method will fail if the FermionBRNoiseOperator is not symmetric. The output of this function will be of size n*n, as for each index there are n possible coupling types: c^\dagger_0 c_0 ... c^\dagger_0 c_n.
    • resample(frequencies: np.ndarray): Resamples the FermionBRNoiseOperator on a new set of frequencies. The FermionBRNoiseOperator defined on the old frequencies is interpolated for the new frequencies. The interpolated values are taken as the new values of the spectral function for these frequencies.

Bath-fitter

The BathFitteris a class that combines several methods to fit effective open quantum systems to original open quantum systems. ABathFitter instance is created by

from hqs_noise_app import BathFitter

bath_fitter = BathFitter(
    number_boson_modes=2,
    spins_per_bosonic_mode=1,
    broadening_constraint=[0.1, 0.1],
    background_broadening_ratio=0.1,
    minimum_eigenfrequencies=-2,
    maximum_eigenfrequencies=2,
    fitting_window=(-0.5, 1, 10),
    coupling_types=["X"],  # specified if we are fitting a system-bath with a spin system
    max_fitting_iterations=5,
    max_fitting_error=0.05,
)

# Print docstring of a function in BathFitter
help(bath_fitter.fit_spin_bath_to_boson_bath)

The parameters of BathFitter are:

• number_boson_modes: Number of bosonic modes used for the fit of bosonic baths
• spins_per_bosonic_mode: Number of spin modes used to represent one bosonic mode.
• broadening_constraint (Optional[List[float]]): The optional broadening constraints. When None broadenings of bosonic modes are fitted freely. When given as a list, the relative broadening of all modes is fixed and only a prefactor is fitted. The prefactor corresponds to the Trotter timestep in a quantum cirucit.
• background_broadening_ratio (float): Adds a constant background offset to the diagonal spectral functions when fitting. Given in terms of (as a ratio of) the average broadening.
• minimum_eigenfrequencies (Optional[float]): Minimal value allowed for bath eigenfrequencies.
• maximum_eigenfrequencies (Optional[float]): Maximum value allowed for bath eigenfrequencies.
• fitting_window (Optional[Tuple[float,float, int]]): The frequency window used for the fitting (start, end, steps). If no values are provided, the functions uses the whole frequency range to determine the fit.
• coupling_types (Optional[Union[Dict[Tuple[int, int], List[str]], List[str]]]): A list of the couplings to include. If None, all the couplings are used: X, Y, Z. The user can also specify this input as a dictionary, where the keys are tuples of couplings (system spin index, bath boson index), and the values are the list of couplings (X, Y, Z) which apply to this coupling.
• coupling_indices (Optional[List[Tuple[int, int]]]): A list of allowed fermionic hopping operators of the form c^dagger_j c_k that are allowed to couple to bosonic modes. For example [(0,0), (0,1)] only allows coupling operators c^dagger_0 c_0 and c^dagger_0 c_1.
• max_fitting_iterations (int): The number of retries allowed when fitting the spectrum.
• max_fitting_error (float): The maximum allowed fitting error in fitting the spectrum.

The bath fitter uses a simple metric for the quality of the fit: Let A be the sum of squares of the difference between the fitter and target and B be the sum of squares of the fitted spectral function. The quality of the fit is defined as the ratio A/B where a small ratio corresponds to a good fit. By default, a deviation of 5% is allowed. If the criterion is not met, the fitting is retried and if the number of retries exceeds the maximum the fit fails. By default, the number of retries is 5.

The BathFitter provides several functions specifically for the system-bath analysis. These functions are

1. fit_boson_bath_to_boson_bath
2. fit_spin_bath_to_boson_bath
3. fit_boson_bath_to_fermion_bath
4. fit_spin_bath_to_fermion_bath
5. fit_boson_bath_to_spectral_function
6. fit_spin_bath_to_spectral_function
7. spin_bath_trotterstep_from_boson_bath

More information about the input arguments and the returned systems can be obtained from help(bath_fitter.fit_spin_bath_to_boson_bath) for example.

More examples on how to use the BathFitter and different functions are in the examples section.

Simple example fit_boson_bath_to_boson_bath

In the following, we give an example of the basic usage of the function fit_boson_bath_to_boson_bath. We would like to fit a spin-boson model to a boson bath. First we create a mixed Hamiltonian consisting of a spin- and a boson system.

from struqture_py import mixed_systems, spins, bosons

# Number of bosons and spins. 
number_system_spins=1
number_bosons=2

# Boson energies and broadening. 
bath_energies = [0.5, 1.5]
bath_broadenings = [0.1, 0.2]

# Coupling between system and boson. 
system_boson_couplings = [0.3, 0.1]

# Create a new mixed system with one spin and one boson subsystem
spin_boson_hamiltonian = mixed_systems.MixedLindbladOpenSystem(
    [number_system_spins],
    [number_bosons],
    [],
)

# Set bath energies
for (bath_index, bath_energy) in enumerate(bath_energies):
    index = mixed_systems.HermitianMixedProduct(
        # Identity spin operator
        [spins.PauliProduct()],
        # Create a Boson occupation operator
        [bosons.BosonProduct([bath_index], [bath_index])],
        [],
    )
    spin_boson_hamiltonian.system_add_operator_product(index, bath_energy)

# Set bath noise
for (bath_index, bath_broadening) in enumerate(bath_broadenings):
    # create the index for the Lindblad terms.
    # We have pure damping
    index = mixed_systems.MixedDecoherenceProduct(
        # Identity spin operator
        [spins.DecoherenceProduct()],
        # Create a Boson occupation operator
        [bosons.BosonProduct([], [bath_index])],
        [],
    )
    spin_boson_hamiltonian.noise_add_operator_product((index, index), bath_broadening)

# Set couplings, use longitudinal (Z) coupling
for (bath_index, system_bath_coupling) in enumerate(system_boson_couplings):
    index = mixed_systems.HermitianMixedProduct(
        # Identity spin operator
        [spins.PauliProduct().z(0)],
        # Create a Boson coupling  operator (always a + a^dagger)
        [bosons.BosonProduct([], [bath_index])],
        [],
    )
    spin_boson_hamiltonian.system_add_operator_product(index, system_bath_coupling)

Now we can fit the spin_boson_hamiltonian to a boson bath with fit_boson_bath_to_boson_bath by

import numpy as np
from hqs_noise_app import BathFitter


# create spectrum from Spin-Bath-System
min_frequency = -2
max_frequency = 4
number_frequencies = 1000
frequencies = np.linspace(min_frequency, max_frequency, number_frequencies)

bath_fitter = BathFitter(
    number_boson_modes=2,
    spins_per_bosonic_mode=1,
    broadening_constraint=[0.1, 0.1],
    background_broadening_ratio=0.1,
    minimum_eigenfrequencies=-2,
    maximum_eigenfrequencies=2,
    fitting_window=(-0.5, 1, 10),
    coupling_types=["Z"]
)

fitted_spin_boson_system, _ = bath_fitter.fit_boson_bath_to_boson_bath(
    original_system=spin_boson_hamiltonian, frequencies=frequencies
)

Spin-boson model

In a nutshell

The spin-boson model studies the dynamics of a two-state system interacting with a continuous bosonic bath. Such systems are ubiquitous in nature. Examples include state decay of atoms, certain types of chemical reactions, and exciton transport in photosynthesis.

The great applicability of the spin-boson theory is based on the idea that the bath does not need to be microscopically bosonic, but needs only to behave like one. If an interaction with a bath is mediated by a operator \(Y\), this coupling operator can be replaced by linear bosonic operator \(X\) of a fictitious (non-interacting) bosonic bath, if:

• the coupling is weaker than the bath memory decay, or
• the coupling is strong but statistics of \(Y\) are approximately Gaussian

The corresponding spectral functions then need to match:

\[ \int_{-\infty}^\infty e^{i\omega t} \langle X(t) X(0)\rangle_0 dt = \int_{-\infty}^\infty e^{i\omega t} \langle Y(t) Y(0)\rangle_0 dt \nobreakspace . \]

This is imposed over some relevant frequency range \(\omega_\textrm{min}<\omega<\omega_\textrm{max}\) and "smoothening" \(d\omega\), corresponding to shortest and longest relevant timescales in the problem. Here the subscript-0 refers to an average during free-evolution.

Generally, solving the spin-boson model with numerical methods becomes more difficult when increasing the system-bath couplings as well as the spectral structure.

Hamiltonian, spectral function, and spectral density

The Hamiltonian of the "traditional" spin-boson problem is of the form:

\[ H = \frac{\Delta}{2}\sigma_z + \frac{1}{2}\sigma_x\left[ \epsilon + \sum_i (g_i a_i + g_i^* a^\dagger_i)\right] + \sum_i \omega_i a^\dagger_i a_i \nobreakspace , \]

with possible (trivial) exchange between \(\sigma_z\) and \(\sigma_x\). Here, the two-level system (spin) has a level-splitting \(\Delta\) and a transverse coupling to a bath position-operator:

\[ X=\sum_i g_i a_i + g_i^* a_i^\dagger \nobreakspace . \]

A central role is played by the (free-evolution) bath spectral function:

\[ S(\omega) = \int e^{i\omega t}\left< X(t)X(0)\right> _0 dt \nobreakspace , \] as it fully defines the effect of the bath on the system. In thermal equilibrium, we have:

\[ S(\omega) = \frac{2\pi J(\omega)}{1-\exp\left( -\frac{\omega}{k_{\rm B}T } \right)} \nobreakspace . \] where the spectral density is defined as:

\[ J(\omega) = \sum_i |g_i|^2 \delta(\omega-\omega_i) \quad \omega >0 \nobreakspace , \]

and thereby connects the spectral density to the couplings and frequencies in the Hamiltonian.

Representing a finite temperature bath by a zero-temperature bath

At zero temperature, the spectral function and density differ only by a front factor. It follows that the couplings in the Hamiltonian define the spectral function.

It is possible to represent the spectral function at \(T>0\) by a spectral function of some another system at \(T=0\). For this one needs to introduce negative boson frequencies. We also define \(T=0\) as the situation where all boson modes are empty. The corresponding new Hamiltonian couplings are then defined as:

\[ \sum_i |g_i|^2 \delta(\omega-\omega_i) \equiv \frac{S(\omega)}{2\pi} . \] We use this description of spin-boson systems due to the simplicity it provides, and since it is the description that the final coarse-grained system also corresponds to.

Other forms of the (single) spin-boson Hamiltonians

We can perform a generalization of the "standard" spin-boson model and include several baths coupling via different directions, each of them have their own coupling polarization (X, Y, or Z),

\[ \begin{align} H &= \frac{\Delta}{2}\sigma_z + \frac{1}{2}\sigma_x X + \frac{1}{2}\sigma_y Y + \frac{1}{2}\sigma_z Z \\ &+ \sum_i \omega_{Xi} a^\dagger_{Xi} a_{Xi}+ \sum_i \omega_{Yi} a^\dagger_{Yi} a_{Yi}+ \sum_i \omega_{Zi} a^\dagger_{Zi} a_{Zi} \end{align} \] Here, the spin is coupled to three independent baths.

A model that emerges when mapping electron-transport models to the spin-boson theory is:

\[ \begin{align} H &= \frac{\Delta}{2}\sigma_z + \frac{1}{2}\sigma^+\left( \sum_i g_i (a_{1i} + a^\dagger_{2i})\right) + \frac{1}{2}\sigma^-\left( \sum_i g_i (a^\dagger_{1i} + a_{2i})\right) \\ &+ \sum_i \omega_i \left( a^\dagger_{1i} a_{1i} + a^\dagger_{2i} a_{2i} \right) \nobreakspace . \end{align} \]

Here we have two sets of bosonic environments which have identical parameters. This is a special model since it allows for mapping the system-qubit damping to the bath spectral density.

Generalization to the multi-spin boson model

The above models can be straightforwardly generalized to multi-spin systems. An example is the following Hamiltonian:

\[ \begin{align} H &= \sum_{j \in \textrm{system}}\frac{\Delta}{2}\sigma^j_z + \sum_{jk \in \textrm{system}}\frac{1}{2}\left( g_{jk} \sigma^+_j \sigma^-k + g^*{jk} \sigma^-_j \sigma^+_k \right) \\ &+ \sum {j \in \textrm{system}} \sum {sm \in \textrm{bath}} \frac{1}{2} \sigma^z_j \left[ g{jsm} a_s(\omega_m) + g{jsm}^* a^\dagger_s(\omega_m)\right] + \sum _{sm} \omega_m a^\dagger_s(\omega_m) a_s(\omega_m) \end{align} \]

This is used to describe exciton transport in photosynthesis. The system spins describe the molecular vibrations of excitons and bosonic modes. All excitons couple, in principle, to the same bath. However, such cross-couplings are often neglected. In that case, each exciton has its own bath and we can set \(g_{ism}=0\), if \(i\neq s\).

Spectral function, spectral density, and cross correlations are generalized analogously. In the case of multi-spin-boson model, the spectral function is defined as:

\[ S_{ij}(\omega) = \int e^{i\omega t}\langle X_i(t)X_j(0)\rangle dt \nobreakspace , \] where:

\[ X_i = \sum_{sm} \left[g_{ism} a_s(\omega_m) + g_{ism}^* a^\dagger_s(\omega_m)\right] \nobreakspace , \]

and \(i, j\) refer to system spins. These functions fully define the effect of the bosonic bath on the system. In thermal equilibrium,

\[ S_{ij}(\omega) = \frac{2\pi J_{ij}(\omega)}{1-\exp\left( -\frac{\omega}{k_{\rm B}T } \right)} , , \] where the spectral density is generalized to:

\[ J_{ij}(\omega) = \sum_{sm} g_{ism}^* g_{jsm} \delta(\omega-\omega_m) \quad \omega >0 \] We see that cross-correlations between two spins, elements \(J_{ij}(\omega)\) with \(i\neq j\), can be finite only if different spins couple to the same bath mode.

As in the case of single-spin theory, we model the finite-temperature bath by the corresponding \(T=0\) bath, see section Representing a finite temperature bath by a zero-temperature bath.

Coarse graining details

By "coarse graining", we mean fitting the spectral function using a finite number of broad bosonic modes. It means we limit the number of bath modes to some finite number, but introduce broadenings \(\omega_i\rightarrow\omega_i+\textrm{i}\gamma/2\). A similar approach has been taken in many classical numerical methods, such as in the hierachy of equations of motion (HEOM), or various methods of introducing auxiliary modes with Lindblad decay.

Broad boson modes

Our goal is to construct a bath that reproduces the desired spectral function as well as possible, but includes only a finite number of boson modes. The approach is to make the boson modes broad, so that they effectively represent a continuous bath. Such a broadening can be established theoretically by introducing a damping Lindbladian:

\[ \dot \rho = \textrm{i}[\rho, H] + \gamma\left( a\rho a^\dagger -\frac{1}{2}a^\dagger a\rho -\frac{1}{2}\rho a^\dagger a \right) \nobreakspace . \]

On a Hamiltonian level, the equivalent is to introduce a bilinear coupling between a boson mode and a "flat" environment (bath of the bath). This coupling leads to broadening of spectral peaks,

\[ \begin{align} \langle a(t)a^\dagger\rangle &= e^{-\textrm{i}\omega_0 t - \gamma \vert t \vert/2} \\ \int_{-\infty}^{\infty} e^{\textrm{i}\omega t}\langle a(t)a^\dagger\rangle dt &= 2\pi \frac{1}{\pi}\frac{\gamma/2}{(\gamma/2)^2+(\omega-\omega_0)^2} \end{align} \nobreakspace . \]

Coupling such modes to a system operator such as \(\sigma_z/2\) via a coupling coefficient \(g\) gives the corresponding spectral function peak:

\[ S(\omega) = 2\pi g^2 \frac{1}{\pi}\frac{\gamma/2}{(\gamma/2)^2+(\omega-\omega_0)^2} \nobreakspace . \]

Broad boson modes represented by noisy spins

The coarse-grained bath is bosonic. We aim to represent this using a spin bath. Various efficient "digital" (such as binary or unary) mappings between qubit states and boson states have been proposed in recent literature. However, such encodings do not map the decay of (arbitrary) bath qubits to single-boson annihilation,which is the key correspondence in our approach.

The spin-boson simulator instead directly replaces the coarse-grained bath operators by spin-bath operators, in the simplest case by doing the following:

\[ \begin{align} g(a + a^\dagger) &\rightarrow g \sigma_x \\ \omega a^\dagger a &\rightarrow \frac{\omega}{2}\sigma_z \\ \sqrt{\gamma} a &\rightarrow \sqrt{\gamma}\sigma^- \end{align} \]

This can work, since spins can (collectively) behave like a bosonic field. The condition for this being true boils down to the requirement of coupling-operator statistics to be Gaussian, up to some relevant order. This is clearly valid for a spin-bath with a fast memory decay compared to the system-bath coupling, \(\gamma\gg g\), since here only two-operator correlation functions matter. A less trivial example is a group of identical spins, which can collectively behave like one bosonic mode with long memory time. Such a system is studied explicitly in example XXX.

More generally, we do not have to assume that the spins are identical. A central condition for the bath being bosonic is that individual bath spins are only weakly excited,

\[ \left< \sigma^+_i \sigma^-_i \right> _{\textrm{full solution}} \ll 1 \quad i\in \textrm{bath} \nobreakspace . \]

Note that the collective excitation number, \( \sum_{i\in \textrm{bath}}\left< \sigma^+_i \sigma^-_i \right> _\textrm{full solution}\), can still be fairly large. This relation should be valid for all times during the simulation. It can be probed by bath-qubit measurements.

If the demand is not respected, the "bosonicity" of the bath can be improved by representing a problematic mode by \(N \) spins, all of them with a down-scaled coupling:

\[ g \rightarrow \frac{g}{\sqrt{N}} \nobreakspace . \]

The down-scaling guarantees that the the total spectral function stays unchanged. We then use:

\[ \begin{align} g(a + a^\dagger) &\rightarrow \sum_{i=1}^N \frac{g}{\sqrt{N}} \sigma^x_i \\ \omega a^\dagger a &\rightarrow \sum_{i=1}^N \frac{\omega}{2}\sigma^z_i \\ \sqrt{\gamma}a & \rightarrow \sum_{i=1}^N \sqrt{\gamma}\sigma^-_i \end{align} \] It should be noted that it can also be enough to just increase the total number of broad modes in the spectral-function fitting range (keeping \(N=1\)).

In the coarse-graining theory, the broadening of the bosonic modes is always due to damping. However, the corresponding broadening of the spin modes can also be partly due to dephasing. The correspondence between the broadenings is straightforward:

\[ \gamma = \gamma_\textrm{damping} + \gamma_\textrm{dephasing} \nobreakspace . \]

Noiseless system

In the fitting, our target function is the frequency-discretized multi-dimensional spectral function:

\[ S_{ij}(\omega_m) \equiv S[i, j, m] , . \] It is spin-exchange symmetric, so \(S_{ij}(\omega_m)= S_{ji}(\omega_m)\). More generally, it is Hermitian, as we are dealing with complex couplings.

Our fitting finds an approximation of this function by replacing the original bath by \(n_\textrm{lorentzians}\) broad modes. Each broad mode has a central frequency \(\omega_k\), coupling to the \(i^\text{th}\) spin \(g_{ik}\), and width \(\gamma_k\). The (multi-dimensional) spectral function that follows is:

\[ S_{ij}^\textrm{cg}(\omega) = 2\pi\sum_{k=1}^{n_\textrm{lorentzians}} g_{ik}g_{jk}^*\frac{1}{\pi}\frac{\gamma_k/2}{(\gamma_k/2)^2+(\omega-\omega_k)^2} \nobreakspace . \] The current fitting routine optimizes the following cost function:

\[ C= \sum_{i\leq j} \left \lbrace\sum_k \left [S_{ij}^\textrm{cg}(\omega_k) - S_{ij}(\omega_k) \right]^2 \right \rbrace \nobreakspace , \]

by seeking the optimal values of \(\omega_k\), \(g_{ik}\), and \(\gamma_k\), starting from some initial guess. The general fitting problem then has \(n_\textrm{lorentzians}\) frequencies, \(n_\textrm{spins} \times n_\textrm{lorentzians}\) couplings, and \(n_\textrm{lorentzians}\) widths.

The fitting can also be done using the same broadening for all modes, \(\gamma_m=\gamma\). This can be a more realistic choice when mapping to hardware properties.

Note that if \(n_\textrm{spins}=1\), we are dealing with fitting only one function, \(S_{ij}(\omega_m) \equiv S(\omega_m)\). Furthermore, if \(n_\textrm{spins}>1\), but cross-correlations are very small and diagonal entries (\(i=j\)) of the spectrum are almost identical, it is recommended to do a comparison to the result of fitting the corresponding single-spin systems only.

Open system

We also allow the user to add system-qubit noise as a background to the spectral function and do the fitting with this taken into account. This is motivated by the possibility to add system damping exactly to fermion-transport models. We then introduce a system rate that is an average of bath rates, times some given factor \(f\),

\[ \gamma_\textrm{system} \equiv f \langle \gamma_i\rangle , , \]

and change the spectral function as:

\[ S_{ij}^\textrm{cg}(\omega) = 2\pi\sum_{k=1}^{n_\textrm{lorentzians}} g_{ik}g_{jk}^*\frac{1}{\pi}\frac{\gamma_k}{(\gamma_k/2)^2+(\omega-\omega_k)^2} + \delta_{ij} \gamma_\textrm{system} \nobreakspace . \]

The noise of different system qubits is assumed to be uncorrelated, hence the multiplication by \(\delta_{ij}\). The correct factor \(f\) to be used depends on the model one considers (as well as on hardware properties). Note that the fitting problem stays essentially the same.

Spectral function to coupling

Here, we assume that a spectral function is given as an input and we want to write down a Hamiltonian that reproduces it: we need to solve the system-bath couplings from the spectral function. The spectral function was discretized to frequencies \(\omega_m\), and the corresponding data values are \(S_{ij}(\omega_m)\equiv S[i, j, m]\). In the corresponding Hamiltonian description, the frequencies of the bath modes are going to be \(\omega_m\). It should be noted that if the given spectral function is unphysical, the software will find the closest physical solution (according to the Frobenius norm).

In the case of single-spin system, the couplings will be defined by the areas:

\[ S(\omega_m) d\omega_m = |g_m|^2 \nobreakspace , \]

where \(d\omega_m\) is the width of the frequency interval corresponding to mode \(\omega_m\). The couplings are then solved to be \(g_m=\pm\sqrt{S(\omega_m) d\omega_m}\). Here, the chosen sign is not important.

In the case of \(n\) system spins, the couplings are constructed similarly with the help of linear algebra. We introduce \(n\) bath-submodes per frequency and solve:

\[ S[i, j, m] d\omega_m = \sum_{s=1}^{n} g[i, s, m] g*[j, s, m] \nobreakspace . \]

Here \(g[i, s, m]\) is the coupling from spin \(i\) to bath submode \(s\) at frequency \(\omega_m\). The spectral function is Hermitian with respect to changing \(i\) and \(j\). There are multiple solutions to this equation, some of them having unreasonable and complicated cross-couplings. To find a reasonable solution, we can require:

\[ g[i, s, m] = 0 \qquad \textrm{for } i < s \ . \] In this form, the obtained cross-couplings are zero if there are no spectral-function cross-correlations. The solution can then be found by Cholesky decomposition:

\[ S[:, :, m] = L L^\dagger \ , \] where \(L\) is the solution for the coupling matrix at frequency \(\omega_m\).

If the given spectral function is unphysical (or is rank-deficient), this has no solution. In this case, the software will use an eigendecomposition to find the closest physical solution to \(L\) according to the Frobenius norm.

Using the HQS Noise App

In this section, we outline how to employ the HQS Noise App to simulate quantum systems with noisy quantum computers. The HQS Noise App works together with the HQS struqture library. In this regard, we first briefly show how to create Hamiltonians and open quantum systems using struqture. Then, we illustrate how to study noise models using the HQS Noise App. Furthermore, we discuss the different backends that the HQS Noise App offers for doing numerical simulations.

Struqture

struqture is a library enabling compact representation of quantum mechanical operators, Hamiltonians, and open quantum systems. The library supports building spin, fermionic and bosonic objects, as well as combinations thereof (mixed systems). Here we demonstrate its usage with simple spin systems. For more complicated examples, please check the user documentation of struqture.

Creating spin Hamiltonians

Let us now create a spin Hamiltonian using the struqture library. Here, arbitrary spin operators are built from PauliProduct operators. As the name implies, these are products of single-spin Pauli operators. Each Pauli operator in the PauliProduct operates on a different qubit. Not all qubits need to be represented in a PauliProduct (such qubits contribute via identity operators).

For instance, let us create a transverse-field Ising model,

\[ H= \sum_{i=0}^9 3\sigma_{i}^z + \sum_{i=0}^8 2\sigma_{i}^x\sigma_{i+1}^x , , \]

which is created by:

from struqture_py import spins
from struqture_py.spins import (SpinHamiltonianSystem, PauliProduct)

number_spins = 10
transverse_field = 3.0
spin_coupling = 2.0

hamiltonian = SpinHamiltonianSystem()
for site in range(number_spins):
    hamiltonian.add_operator_product(PauliProduct().z(site), transverse_field)
for site in range(number_spins-1):
    hamiltonian.add_operator_product(PauliProduct().x(site).x(site+1), spin_coupling)

Information can be accessed using the following functions:

hamiltonian.keys()             # operator keys (Pauli products / strings)
hamiltonian.get("0X1X")        # front factor of Pauli product "0X1X"
hamiltonian.number_spins()     # number of spins in the system

Creating spin Lindblad noise operators

Let us now create the Lindblad noise operators for a system of spins with the aid of struqture. Such noise operators are important to describe the noise part of the the Lindblad master equation, see the modeling chapter, for more information.

To describe the pure noise part of the Lindblad equation \( \sum_{i,j}M_{i,j} \left( A_{i}\rho A_{j}^{\dagger} - \frac{1}{2} \lbrace A_j^{\dagger} A_i, \rho \rbrace \right)\), we use DecoherenceProducts as the operator base. The object SpinLindbladNoiseOperator is given by a HashMap or dictionary with the tuple (DecoherenceProduct, DecoherenceProduct) as keys and the entries in the rate matrix \(M_{j,k} \) as values. Similarly to SpinOperators, SpinLindbladNoiseOperators have a system equivalent: SpinLindbladNoiseSystem, with a number of involved spins defined by the user. For more information on these, see the user documentation of struqture.

For instance, take \( A_0 = A_1 = \sigma_0^{x} \sigma_2^{z} \) with coefficient 1.0: \( 1.0 \left( A_0 \rho A_1^{\dagger} - \frac{1}{2} \lbrace A_1^{\dagger} A_0, \rho \rbrace \right) \)

from struqture_py import spins
from struqture_py.spins import SpinLindbladNoiseSystem, DecoherenceProduct
import scipy.sparse as sp

system = spins.SpinLindbladNoiseSystem(3)

dp = spins.DecoherenceProduct().x(0).z(2)

system.add_operator_product((dp, dp), 1.0)

# Accessing information:
system.current_number_spins()     # Result: 2
system.get((dp, dp))              # Result: CalculatorFloat(2)
system.keys()                     # Result: [("0Z1X", "0Z1X")]
dimension = 4**system.number_spins()
matrix = sp.coo_matrix(system.sparse_matrix_superoperator_coo(), shape=(dimension, dimension))

Creating physical noise models

Supported noise mechanisms in the present release

The current version of the HQS Noise App supports single-qubit physical noise in the form of damping, dephasing, and depolarization, with user-given decoherence rates. We assume that the noise is the same for all gate types.

Setting up the HQS Noise App

The HQS Noise App has to be initialized with :

• noise_mode : the way noise is added to a quantum circuit

The available options for noise_mode are:

• active_qubits_only : noise added only for qubits involved in operations
• all_qubits : noise added for all qubits at each step
• parallelization_blocks : noise added after each parallelization block

Additionally, there are settings that have default values and can be set with setter functions with the same name as the setting.

• number_measurements : the number of projective measurements used when measuring observables (defaults to 100000)
• use_bath_as_control: True/False to use/not use bath qubits as control when solving system-bath problems (defaults to False)
• noise_placement : before, after or symmetric- The noise is applied before the gate, after the gate based on gate duration or symmetrically (defaults to after)
• algorithm : options are: ParityBased, QSWAP, QSWAPMolmerSorensen, VariableMolmerSorensen, (defaults to ParityBased).
• noise_symmetrization : whether of not to apply noise symmetrization, which controls whether the Trotterstep is symmetrized with respect to damping noise. When the overall noise has the tendency to favor one state |0> or |1>, the Trotterstep can be symmetrized by doubling the Trotter circuit and flipping the definition of |0> and |1> for the second circuit. This process will bring the effective noise closer to a balanced noise at the cost of incuring larger decoherence overall. This option defaults to false.
• after_trotter_wait_time : the optional additional physical wait time after the Trotter circuit has been applied. This can be used to introduce additional decoherence when the decoherence in the Trotter step is too low or is dominated by the wrong type of decoherence compared to the background decoherence processes that are active during wait time. This defaults to None, for which no wait time is applied. no optimizations are applied, or 1 where SingleQubitGates are combined (defaults to 1)
• trotterization_order : trotterization order of time-evolution operator, 1 or 2 (defaults to 1)
• optimization_level : the level of optimization when performing the algorithm: either 0 where no optimizations are applied, or 1 where SingleQubitGates are combined (defaults to 1)

Creating device

We can define an AllToAllDevice device with the following settings:

• number_of_qubits : The number of qubits for the device
• single_qubit_gates : The list of single-qubit gates available on the quantum computer
• two_qubit_gates : The list of two-qubit gates available on the quantum computer
• default_gate_time : The default starting gate time.

The option single_qubit_gates can be any list of single qubit gates available in the HQS qoqo library as long as it contains one of the following combinations:

• RotateX and RotateZ
• RotateY and RotateZ
• RotateX and RotateY
• RotateZ and SqrtPauliX and InvSqrtPauliX

The supported choices for two_qubit_gates are:

• CNOT
• ControlledPauliZ
• ControlledPhaseShift
• MolmerSorensenXX
• VariableMSXX

An example code of setting device information reads as follows.

from qoqo import devices

# Setting up the device.
number_of_qubits=5
single_qubit_gates = ["RotateX", "RotateZ", "RotateY"]
two_qubit_gates = ["CNOT"]
default_gate_time = 1.0
device = devices.AllToAllDevice(
    number_of_qubits, single_qubit_gates, two_qubit_gates, default_gate_time
)

Creating noise model

We can define a ContinuousDecoherenceModel noise model with the following types of noise:

• dephasing: using the add_dephasing_rate function
• depolarisation: using the add_depolarising_rate function
• damping: using the add_damping_rate function
• excitations: using the add_excitation_rate function

Each of these functions takes a list of qubits to apply the noise to and a noise rate (float), and returns the modified ContinuousDecoherenceModel. An example code of setting noise model information, including the damping of qubits, reads as follows.

from qoqo import noise_models

# Setting up the noise model.
damping = 1e-3
dephasing = 5e-4
noise_model = noise_models.ContinuousDecoherenceModel().add_damping_rate([0, 1, 2], damping).add_dephasing_rate([3, 4], dephasing)

Creating QuantumPrograms

The HQS Noise App can create quantum programs to time propagate a spin state. The QuantumProgram will initialize a spin state on a quantum computer, time propagate the spin state with a quantum algorithm and measure the values of spin observables. The initialization and measured operators are defined by the user. The creation of quantum programs specifically for mixed systems is discussed more detailed on System Bath.

The QuantumPrograms can then be simulated using the Backend class of the qoqo-QuEST package.

CNOT algorithm

The QuantumPrograms can be constructed by different algorithms. One choice the HQS Noise App offers is a parity-based algorithm, which we call the CNOT algorithm. It trotterizes the time evolution by separating a Hamiltonian into a sum over Pauli-products. The time evolution under each Pauli-product is simulated by

• Rotating all involved qubits into the \(Z\) basis (by applying the Hadamard gate for \(X\) and a RotateX for \(Y\)
• Encoding the total parity of the involved qubits in the last involved qubits with the help of a chain of CNOT operations
• Applying a RotateZ operation on the last qubit, rotating by twice the prefactor of the PauliProduct
• Undoing the CNOT chain and basis rotations

For the example of simulating \(\exp(-\textrm{i} g \sigma^x_0\sigma^x_2)\), the circuit looks like:

 0 -H---o-------------------o----H--------
 1 -----|-------------------|-------------
 2 -H---x----RotateZ(2g)----x----H--------

An example of creating a circuit of a previously defined Hamiltonian and device is:

from hqs_noise_app import HqsNoiseApp
from struqture_py import spins
from qoqo import devices, noise_models

# define hamiltonian (transverse Ising Hamiltonian, with spin_coupling=0)
number_spins = 3
hamiltonian = spins.SpinHamiltonianSystem(number_spins)
for site in range(number_spins):
    hamiltonian.set("{}Z".format(site), 1.0)

# Setting up the device.
single_qubit_gates = ["RotateX", "RotateZ", "RotateY"]
two_qubit_gates = ["CNOT"]
gate_times = 1.0
damping = 1e-3
device = devices.AllToAllDevice(
    number_spins, single_qubit_gates, two_qubit_gates, gate_times
)
# While the noise model is not needed to generate the quantum program, it will be required
# when simulating the quantum program.
noise_model = noise_models.ContinuousDecoherenceModel().add_damping_rate([0, 1, 2], damping)

# Create the quantum program. The function HqsNoiseApp() uses the CNOT algorithm by default
trotter_timestep=0.01
operators = []
operator_names = []
for i in range(number_spins):
    operator = spins.SpinSystem(number_spins)
    operator.set(spins.PauliProduct().z(i), -0.5)
    operator.set(spins.PauliProduct(), 0.5)
    operators.append(operator)
    operator_names.append("population_site_{}".format(i))
initialisation = [0.0 for _ in range(number_spins)]

noise_app_sc = HqsNoiseApp("all_qubits")

quantum_program = noise_app_sc.quantum_program(hamiltonian, trotter_timestep, initialisation, operators, operator_names, device)
# Printing the circuits of the QuantumProgram:
print(quantum_program.measurement().circuits()) 

QSWAP algorithm

The QSWAP algorithm trotterizes the time evolution into sequences of nearest-neighbour interactions and quantum-state swaps. Using this approach, a quantum simulation of arbitrary two-body interactions can always be generated with linear depth. For the example of simulating arbitrary operation \(O_{02} = \exp(-\textrm{i} I_0I_2)\), the circuit looks like:

0 -----x-------------------x-----
      SWAP                SWAP
1 -----x--------x----------x-----
           Operation 02
2 --------------x----------------

An example of creating a circuit of a previously defined Hamiltonian and device is:

from hqs_noise_app import HqsNoiseApp
from struqture_py import spins
from qoqo import devices, noise_models

# define hamiltonian (transverse Ising Hamiltonian, with spin_coupling=0)
number_spins = 3
hamiltonian = spins.SpinHamiltonianSystem(number_spins)
for site in range(number_spins):
    hamiltonian.set("{}Z".format(site), 1.0)

#noise_app_sc = HqsNoiseApp("all_qubits", ["RotateX", "RotateZ"], "CNOT")

# Setting up the device.
single_qubit_gates = ["RotateX", "RotateZ", "RotateY"]
two_qubit_gates = ["CNOT"]
gate_times = 1.0
damping = 1e-3
device = devices.AllToAllDevice(
    number_spins, single_qubit_gates, two_qubit_gates, gate_times
)
# While the noise model is not needed to generate the quantum program, it will be required
# when simulating the quantum program.
noise_model = noise_models.ContinuousDecoherenceModel().add_damping_rate([0, 1, 2], damping)


# Create circuit. The function HqsNoiseApp() uses the CNOT algorithm per default
trotter_timestep=0.01
operators = []
operator_names = []
for i in range(number_spins):
    operator = spins.SpinSystem(number_spins)
    operator.set(spins.PauliProduct().z(i), -0.5)
    operator.set(spins.PauliProduct(), 0.5)
    operators.append(operator)
    operator_names.append("population_site_{}".format(i))
initialisation = [0.0 for _ in range(number_spins)]

noise_app_sc = HqsNoiseApp("all_qubits")
noise_app_sc.time_evolution_algorithm = "QSWAP"

quantum_program = noise_app_sc.quantum_program(hamiltonian, trotter_timestep, initialisation, operators, operator_names, device)
# Printing the circuits of the QuantumProgram:
print(quantum_program.measurement().circuits()) 

System-bath CNOT algorithm

This is used for system-bath Hamiltonians. It assumes all-to-all connectivity. The creation of Hamiltonians and circuits specific for mixed systems is discussed more in the System Bath chapter. The system-bath version CNOT algorithm tries to miminize bath qubit rotations when performing XX or ZX type interactions between system and bath qubits. The reasoning is to minimize large-angle rotations on bath qubits which we assume have stronger noise then system qubits. This is possible for the relevant cases of XX and ZX type of interaction between the system and the bath. Here, for simulating a XX interaction between spin qubit s and bath qubit b1, \(\exp(-\textrm{i} g \sigma^x_{s}\sigma^x_{b1})\), the circuit is chosen to be

s  -----o----RotateX(2g)----o-----
b0 -----|-------------------|-----
b1 -----x-------------------x-----

An example of creating a quantum program of a previously defined Hamiltonian (hamiltonian) and a given device is:

logical_to_physical_system = {0: 0, 1: 1, 2: 3}
logical_to_physical_bath = {0: 4, 1: 5, 2: 7}
bath_qubit_connectivity = {0: [4, 5], 1: [5, 6], 3: [7, 8]}

noise_app_sc = HqsNoiseApp("all_qubits")
noise_app_sc.bath_qubits_as_control_qubits(False)

quantum_program = noise_app_sc.system_bath_quantum_program(
    hamiltonian,
    trotter_timestep,
    initialisation,
    operators,
    operator_names,
    device,
    logical_to_physical_system,
    logical_to_physical_bath,
    bath_qubit_connectivity,
)

The use_bath_as_control option is to use bath qubits as control when solving system-bath problems and interacting between system and bath qubits with CNOT operations as shown in the above circuit example (affects the noise model, see examples).

System-bath QSWAP algorithm

The system-bath QSWAP algorithm trotterizes the time evolution into sequence of nearest-neighbour interactions in the system and following quantum-state SWAPs. Assumption is made that each system spin can have a nearest-neighbour bath qubit below and above (assuming 2d architecture). The bath spins are not swapped. When creating the circuit, new system qubits are added to the circuit, if needed. So if we have an input of mixed system with 1 system and 4 bath spins, the final circuit has 6 qubits ordered like [bath0, bath1, system0, system1, bath3, bath4]. Here, simulating the original model cross-coupling between spin-qubit s and bath qubit b1, \(\exp(-\textrm{i} g \sigma^x_{s}\sigma^x_{b1})\), looks like

     s0 -x---------------------------x-
        SWAP                        SWAP
     s1 -x---o----RotateX(2g)----o---x-
             |                   |
     b1 -----x-------------------x-----

An example of creating a quantum program of a Hamiltonian (H_mixed) and a given device is:

trotter_timestep=0.01
quantum_program_mixed = noise_app.system_bath_quantum_program(
    H_mixed, trotter_timestep, initialisation, operators, operator_names, device, None, None, None
)
quantum_program_mixed # printing the circuit

Noisy algorithm model

Extracting the noise model

The noisy algorithm model of a hamiltonian is generated by the function HqsNoiseApp.noisy_algorithm_model, with input arguments:

• hamiltonian: The Hamiltonian for which the noise algorithm model is created
• trotter_timestep: The simulation time the circuit propagates the simulated system
• device: The device determining the topology.
• noise_models: The noise models determining noise properties.

The noisy algorithm model represents the effective Lindblad noise that the system simulated with a given hamiltonian is exposed to due to qubit noise during a single Trotter step, see Mapping chapter for more details about effective noise mapping.

An example is:

from hqs_noise_app import HqsNoiseApp
from struqture_py import spins
from struqture_py.spins import (SpinHamiltonianSystem, PauliProduct)
from qoqo import devices, noise_models

# define hamiltonian 
number_spins = 4
hamiltonian = spins.SpinHamiltonianSystem(number_spins)
hamiltonian.add_operator_product(PauliProduct().z(0).z(2).z(3), 4.0)

# Setting up the device
single_qubit_gates = ["RotateX", "RotateZ"]
two_qubit_gates = ["CNOT"]
gate_times = 1.0
damping = 1e-3
device = devices.AllToAllDevice(
    number_spins, single_qubit_gates, two_qubit_gates, gate_times
)
# While the noise model is not needed to generate the quantum program, it will be required
# when simulating the quantum program.
noise_model = noise_models.ContinuousDecoherenceModel().add_damping_rate([0, 1, 2, 3], damping)

# create inputs
trotter_timestep=0.01
noise_app = HqsNoiseApp("all_qubits")

# obtain noisy algorithm model
noisy_model = noise_app.noisy_algorithm_model(hamiltonian, trotter_timestep, device, [noise_model])
noisy_model

noisy_model is an object of class 'struqture.spins.SpinLindbladNoiseSystem'.

Printing and modifying a noisy algorithm model

Information on the created noisy algorithm model can be obtained by

noisy_model                            # noise terms and rates
noisy_model.keys()                     # strings of the Lindbladian noise matrix M
noisy_model.get(("0Z", "0Z"))          # accessing the rate of a specific noise term

In creating the effective model, we can encounter terms that are effectively zero but appear as non-zero due to the finite numerical accuracy. To eliminate these terms one can use the truncate function:

# absolute values below the given threshold will be set to zero
truncated_model = noisy_model.truncate(1e-4)

Furthermore, when studying the effect of individual contributions, it may be useful to turn "off" or "on" individual noise channels. For this one can use add_operator_product function:

noisy_model.add_operator_product(("2Z", "2Z"), 0.5)
noisy_model.add_operator_product(("0Z2Z3Z", "0Z2Z3Z"), 0)

System-bath noisy algorithm model

The noisy algorithm model of a hamiltonian is generated by the function HqsNoiseApp.system_bath_noisy_algorithm_model, with input arguments:

• hamiltonian: The Hamiltonian for which the noise algorithm model is created
• trotter_timestep: The simulation time the circuit propagates the simulated system
• device: The device determining the topology.
• noise_models: The noise models determining noise properties.
• logical_to_physical_system: an optional mapping from logical to physical qubits for the system.
• logical_to_physical_bath: an optional mapping from logical to physical qubits for the bath.
• bath_qubit_connectivity - optional connectivity between physical system and bath qubits.

An example is detailed below. For this example, we use the following physical device:

0 - 1 - 2 - 3
|   |   |   |
4 - 5 - 6 - 7

where we choose the system qubits to be qubits [4, 5, 6] and the bath qubits to be qubits [0, 1, 2].

from hqs_noise_app import HqsNoiseApp
from struqture_py import spins
from struqture_py.spins import (SpinHamiltonianSystem, PauliProduct)
from qoqo import devices, noise_models

# define hamiltonian on the logical indices
hamiltonian = MixedHamiltonianSystem([3, 3], [], [])
# A XX interaction between spins 0 and 1 and spins 1 and 0
hamiltonian.set(HermitianMixedProduct(["0X1X", ""], [], []), 1.0)
# A XX interaction between spins 1 and 2 and spins 1 and 0
hamiltonian.set(HermitianMixedProduct(["1X2X", ""], [], []), 1.0)
hamiltonian.set(HermitianMixedProduct(["0Z", "0Z"], [], []), 0.1)
hamiltonian.set(HermitianMixedProduct(["1Z", "1Z"], [], []), 0.1)
hamiltonian.set(HermitianMixedProduct(["2Z", "2Z"], [], []), 0.1)

# Setting up the device
single_qubit_gates = ["RotateX", "RotateZ"]
two_qubit_gates = ["CNOT"]
gate_times = 1.0
damping = 1e-3
device = devices.AllToAllDevice(
    8, single_qubit_gates, two_qubit_gates, gate_times
)
# While the noise model is not needed to generate the quantum program, it will be required
# when simulating the quantum program.
noise_model = noise_models.ContinuousDecoherenceModel().add_damping_rate([0, 1, 2, 3, 4, 5, 6, 7], damping)
# The logical-to-physical mappings would be as follows:
logical_to_physical_system = {0: 4, 1: 5, 2: 6}
logical_to_physical_bath = {0: 0, 1: 1, 2: 2}
# The corresponding bath connectivity is:
connectivity = {4: [0], 5: [1], 6: [2]}

# create inputs
trotter_timestep=0.01
noise_app = HqsNoiseApp("all_qubits")

# obtain noisy algorithm model
noisy_model = noise_app.system_bath_noisy_algorithm_model(
    hamiltonian,
    trotter_timestep,
    device,
    [noise_model],
    logical_to_physical_system,
    logical_to_physical_bath,
    connectivity    
)
noisy_model

noisy_model is an object of class 'struqture.spins.SpinLindbladNoiseSystem'.

Backends

Simulation of the (original) noisy hamiltonian

The QuantumProgram objects created from the input hamiltonians can be simulated by the user using one of the qoqo interface packages, such as qoqo-QuEST, qoqo-for-braket or qoqo-qiskit.

NOTE: For Linux users, the function HqsNoiseApp.simulate_quantum_program numerically solves the time-evolution starting in the state defined by initial_density_matrix. The noise is added automatically to the QuantumProgram. The solver returns the density matrix at the end of the simulation.

One can use the HQS Noise App to add the noise to a quantum program and simulate the corresponding circuit using the qoqo-QuEST simulator (for instance), as follows

import numpy as np
from struqture_py import spins
from struqture_py.spins import (SpinHamiltonianSystem, PauliProduct)
from hqs_noise_app import HqsNoiseApp
from qoqo import devices, noise_models
from qoqo_quest import Backend


# define hamiltonian (transverse Ising Hamiltonian)
number_spins = 5
transverse_field = 3.0
spin_coupling = 2.0
hamiltonian = SpinHamiltonianSystem(number_spins)
for site in range(number_spins):
    hamiltonian.add_operator_product(PauliProduct().z(site), transverse_field)
for site in range(number_spins-1):
    hamiltonian.add_operator_product(PauliProduct().x(site).x(site+1), spin_coupling)    


# Setting up the device.
single_qubit_gates = ["RotateX", "RotateZ"]
two_qubit_gates = ["CNOT"]
gate_times = 1.0
damping = 0.0001
device = devices.AllToAllDevice(
    number_spins, single_qubit_gates, two_qubit_gates, gate_times
)
# While the noise model is not needed to generate the quantum program, it will be requried
# when simulating the quantum program.
noise_model = noise_models.ContinuousDecoherenceModel().add_damping_rate([0, 1, 2, 3, 4], damping)

noise_app = HqsNoiseApp("all_qubits")
noise_app.time_evolution_algorithm = "QSWAP"

trotter_timestep=0.01
operators = []
operator_names = []
for i in range(number_spins):
    operator = spins.SpinSystem(number_spins)
    operator.set(spins.PauliProduct().z(i), -0.5)
    operator.set(spins.PauliProduct(), 0.5)
    operators.append(operator)
    operator_names.append("population_site_{}".format(i))
initialisation = [0.0 for _ in range(number_spins)]

number_trottersteps = 20
quantum_program = noise_app.quantum_program(hamiltonian, trotter_timestep, initialisation, operators, operator_names, device)
quantum_program_with_noise = noise_app.add_noise(quantum_program, device, [noise_model])

backend = Backend(number_spins)
result = quantum_program_with_noise.run(backend, [number_trottersteps])

Exporting noisy algorithm model as Scipy sparse matrix

To run small-scale numerical simulations of the effective model, or to process the model further, one can export the superoperator as a Scipy sparse-matrix (acting on a density-matrix vector). The density matrix is flattened to a vector in row-major fashion [[1,2],[3,4]] -> [1,2,3,4]). A compact example:

import scipy.sparse as sparse

noisy_model = noisy_model.truncate(1e-4)
coo = noisy_model.sparse_matrix_superoperator_coo()
dimension = 4**noisy_model.number_spins()
matrix = sparse.coo_matrix(coo, shape=(dimension, dimension))
#dense_matrix = matrix.toarray()

Alternatively, one can export both the Hamiltonian and the noise model together as one Lindblad superoperator:

from struqture_py.spins import SpinLindbladOpenSystem
import scipy.sparse as sparse

# for exporting, combining the effective model and Hamiltonian into struqture.SpinLindbladOpenSystem
noisy_system = SpinLindbladOpenSystem(number_spins)
noisy_system  = SpinLindbladOpenSystem().group(hamiltonian, noisy_model)

# exporting hamiltonian and noise as super-operator matrix
dimension = 4**number_spins
coo = noisy_system.sparse_matrix_superoperator_coo()
sparse_matrix = sparse.coo_matrix(coo, shape=(dimension, dimension))
#lindblad_matrix = sparse_matrix.toarray()  # to dense matrix

Several examples of combining this with Scipy numerical methods are given in the Jupyter notebooks distributed with the HQS Noise App.

Exporting the noisy algorithm model to QuTiP

The effective model can also be exported to QuTip. For this, one can use the HQS package struqture_qutip_interface. Central functions here are SpinQutipInterface.pauli_product_to_qutip and SpinOpenSystemQutipInterface.open_system_to_qutip. With the QuTiP package, one can efficiently solve the time evolution of small-scale systems, for example, using the qutip.mesolve solver. An example of doing this is:

import numpy as np
from hqs_noise_app import  HqsNoiseApp
from struqture_py.spins import SpinHamiltonianSystem, SpinLindbladOpenSystem, PauliProduct
from struqture_qutip_interface import SpinQutipInterface, SpinOpenSystemQutipInterface
import qutip as qt
from qoqo import devices, noise_models

# constructing Hamiltonian
number_spins = 4
transverse_field = 1.0
spin_coupling = 1.0
hamiltonian = SpinHamiltonianSystem(number_spins)
for site in range(number_spins):
    hamiltonian.add_operator_product(PauliProduct().z(site), transverse_field)
for site in range(number_spins-1):
    hamiltonian.add_operator_product(PauliProduct().x(site).x(site+1), spin_coupling)


# Setting up the device.
single_qubit_gates = ["RotateX", "RotateZ"]
two_qubit_gates = ["CNOT"]
gate_times = 1.0
damping = 0.0001
device = devices.AllToAllDevice(
    number_spins, single_qubit_gates, two_qubit_gates, gate_times
)
# While the noise model is not needed to generate the quantum program, it will be requried
# when simulating the quantum program.
noise_model = noise_models.ContinuousDecoherenceModel().add_damping_rate([0, 1, 2, 3], damping)

# setting up the HQS Noise App
noise_app = HqsNoiseApp("all_qubits")

# automatic generation of the circuit corresponding to one Trotter step
trotter_timestep = 0.01
number_timesteps = 500

# extracting the effective noise model, corresponding to circuit of one trotterstep
noisy_model = noise_app.noisy_algorithm_model(hamiltonian, trotter_timestep, device, [noise_model])

# transforming Struqture open system (hamiltonian+noise) into QuTiP superoperator
noisy_system = SpinLindbladOpenSystem(number_spins)
noisy_system  = SpinLindbladOpenSystem().group(hamiltonian, noisy_model)

sqi = SpinOpenSystemQutipInterface()
(coherent_part, noisy_part) = sqi.open_system_to_qutip(noisy_system)
liouFull = coherent_part + noisy_part

# desired struqture operators to QuTiP
qi = SpinQutipInterface()
op_Z0X1 = PauliProduct().set_pauli(0, "Z").set_pauli(1, "X")
qt_Z0X1 = qi.pauli_product_to_qutip(op_Z0X1, number_spins, endianess="little")

# setting up an initial state
init_spin = [qt.basis(2, 1)] # first spin initially excited
for i in range(number_spins - 1):
    init_spin.append(qt.basis(2, 0)) # other spins at ground
init_spin_tensor = qt.tensor(list(reversed(init_spin)))
psi0 = init_spin_tensor * init_spin_tensor.dag()  # ro_init = |init> <init|

# QuTiP master-equation solver
time_axis = np.linspace(0, trotter_timestep * number_timesteps, number_timesteps + 1)
result = qt.mesolve(liouFull,
                    psi0,
                    time_axis,
                    [], # c_op_list is left empty, since noise is already in liouFull
                    [qt_Z0X1] # operator(s) to be measured
                    ).expect
time_evolution_Z0X1 = np.real(result[0])

Examples

In this page, we use the system-bath tool to solve dynamics of some simple system-bath problems and study the quality of the corresponding quantum simulation. Notebooks with the examples are available in the examples repository of the HQS Noise App.

Spin coupled to broad bosonic mode

We first look at a system coupled to one bosonic mode, described by the Hamiltonian

\[ H = \frac{\Delta}{2}\sigma_z + \frac{1}{2}\sigma_x(g a + g^* a^\dagger) + \omega_0 a^\dagger a \nobreakspace . \]

The coupling to the bosonic mode is ultrastrong, \(g\sim\omega_0\). This makes the representation of the bath using spins challenging, but not impossible. Below, we show how the bosonic bath can be represented by a spin bath. We assume that the system qubit is noiseless, the bath qubit noise is due to decay and dephasing, and that the noise is "on" constantly.

For each Trotter step, we need to correctly scale the phases of the digitally implemented small-angle gates, so that the physical broadening matches with the model broadening. For example, we need to fix the phase of the system-bath coupling so that:

\[ \begin{align} &\varphi_g \equiv g_\textrm{simulation}\tau_\textrm{simulation} = \frac{g_\textrm{fit}}{\gamma_\textrm{fit}} {\cal N} \\ & {\cal N} = D \gamma_\textrm{physical}\tau_\textrm{physical} \nobreakspace , \end{align} \]

where \(D\) is the circuit depth. It accounts for the accumulation of the noise. The parameter \({\cal N}\) is then a measure of total infidelity of one Trotter step.

To demonstrate the possibility to also include bath-qubit dephasing to effective broadening, we consider the case where the boson-mode-broadening \(\kappa\) is reproduced by an equal contribution from qubit decay \(\gamma\) and dephasing \(\Gamma\),

\[ \begin{align} \kappa &= \gamma + 2\Gamma \\ \gamma &= 2\Gamma = \frac{\kappa}{2} , . \end{align} \]

Furthermore, we consider the parameters:

\[ \begin{align} g &= 1 \\ \omega &= 1 \\ \Delta &= 0.9 \\ \kappa &= 0.5 \nobreakspace . \end{align} \]

We also use \({\cal N} = 0.02\), which is a relatively high value, but still does not lead to a noticeable Trotter error. The larger this parameter is, the more Trotter error the quantum simulation will have.

Below, we show a comparison between the exact spin-boson model and the quantum simulation for the (native) variable MS-decomposition, where the bath is described either by 1 spin or 6 identical spins. The simulation starts from the system-spin being in the +1 eigenstate of operator \(\sigma^x_\textrm{system}\) and the bath being in its groundstate. We track the time evolution of \(\langle \sigma^x_\textrm{system}(t) \rangle\).

We see that the quantum simulation with 1 spin gives incorrect dynamics, whereas for 6 spins the dynamics are almost identical to the correct result. The remaining difference is due to the finite number of bath spins. The trotter error is here negligible.

In the next figure, we study the same system, but run the simulation with a non-native gate decomposition (CNOT). Here, we use the bath qubits as control qubits when performing CNOT gates. The simulation is performed for 8 bath qubits.

We again find a very small difference between the ideal spin-spin model and the quantum simulation, though now with a marginally larger difference. This is due to a small difference between the ideal and effective noise models. The smallness of the error is still a rather surprising result.

In the last case, we study the CNOT decomposition where the bath qubits are target qubits. The simulation is performed for 8 bath qubits.

Now we can see larger differences in the results. This is due to significant changes in the form of the effective noise. It can be shown that here the effective noise model includes strong system-qubit dephasing. We then conclude that in the considered situations noise-utilising system-bath simulations are feasible and that small changes in gate decompositions can lead to large changes in the effective noise model.

Spin coupled to an ohmic bath

This model is a cornerstone of the spin-boson theory. It shows how system-bath dynamics can drastically change with system-bath interaction strength. Here the strength is characterized by only one parameter \(\alpha\) (the so-called Kondo parameter). As a rough overall picture, for \(\alpha\ll 1\), the system dynamics show damped oscillations, whereas for \(\alpha\lesssim 1/2\), the dynamics transform into slow incoherent relaxation. For \(\alpha > 1\) and \(T=0\) the system does not relax.

Using the system-bath tool, we can demonstrate this transition with a coarse grained bath (at a finite temperature). Below we consider a spectral function

\[ \begin{align} \Delta &= 1 \\ S(\omega) &= \frac{2\alpha\omega}{1 - \exp(-\omega / T) } \exp(-\vert \omega\vert / \omega_c) \\ \omega_c &= 10 \\ T &= 0.5 \nobreakspace. \end{align} \]

As earlier, we solve the time evolution when initially prepared in the eigenstate +1 of \(\sigma^x_\textrm{system}\). As a "correct" reference result, we use the so-called NIBA (non-interacting blip approximation) calculation, performing it with the original correct bath and with the coarse-grained baths. (The NIBA calculation is made by a piece of code not provided in the HQS Noise App.) The coarse-graining is performed by optimizing parameters of 8 bath modes having the same widths.

Below, we show the results for quantum simulation with variable-MS decomposition for \( \alpha=0.05 \)

for \(\alpha=0.3\)

and for \(\alpha=0.6\)

We see the transition from damped oscillations to incoherent decay. The results also agree well with the NIBA solutions. A noticeable difference to the original-bath NIBA result emerges for large \(\alpha\), which is due to increased sensitivity of dynamics to temperature (the effective coarse-grained bath temperature is elevated), rather than an increase in bath-spin populations. As this setup is analogous to the previous example, with the difference of distributed bath parameters, the conclusions made for the CNOT decompositions remain the same.

Exciton transport

Exciton transport has been studied extensively using multi-spin boson models. The spins correspond to bacteriochlorophyll pigment molecules. Their electronic excited states interact and can effectively "jump" between different sites. They couple longitudinally to a boson bath, which corresponds to vibrations in the central molecular structure and the surrounding protein. The total Hamiltonian describing this system is of the form:

\[ \begin{align} H &= \sum_{i \in \textrm{system}}\frac{\Delta}{2}\sigma^i_z + \sum_ {i<j \in \textrm{system}} \frac{g_{ij}}{2}\left( \sigma^+_i \sigma^-_j + \sigma^-_j \sigma^+_i \right) \\ &+\sum _{i \in \textrm{system}} \sum _{sm \in \textrm{bath}} \frac{g _{ism}}{2} \sigma^z_i \left[a_s(\omega_m) + a^\dagger_s(\omega_m)\right] + \sum _{sm} \omega_m a^\dagger_s(\omega_m) a_s(\omega_m) \end{align} \] Energy is transferred across a network of 8 spins (or in more general models, 3 sets of 8 spins = 24 spins). This energy transfer occurs with partial energy dissipation in the bath. The environment is often taken to be Ohmic or sub-Ohmic, but in a more detailed simulation it can be allowed to have more structure. Here we choose the Ohmic form of a spectral function:

\[ \begin{align} J(\omega) &= \frac{2}{\pi}\frac{\lambda \gamma \omega}{\gamma^2 + \omega^2} \\ \lambda &= 35~\textrm{cm}^{-1} \\ \gamma &= 110~\textrm{cm}^{-1} \\ T &= 100~\textrm{cm}^{-1} (144~K) \nobreakspace . \end{align} \]

and coarse grain it by 2 broad modes. In the simulation, the scaling of small-angle terms is made exactly in the same way as in the the previous examples. We assume the noise is purely qubit decay.

Furthermore, we greatly simplify the model and consider here a simplified version of the exciton transport model in photosynthesis: a system of 3 spins with system Hamiltonian:

\[ g_{ij\in\textrm{system}} = \begin{pmatrix} 420 & 95 & 0 \\ & 250 & 30 \\ & & 0 \end{pmatrix} \] These are given in units of \(\textrm{cm}^{-1}\).

The system is initially left to relax to a steady state where there are no excitations in the system. Then an exciton is inserted on site 1 (by \(\sigma^x\) operation). We then monitor the site populations and study how fast the exciton reaches its destination, site 3:

Even though we include here only 3 sites in the model, the result is very similar to those shown in the literature. This agreement despite such a simple model can be rationalized by noting that the excitation number of spins in the bath stays low, which is also consistent with the results in the literature.

In the second version of the calculation, we add cross correlations to the system-bath interaction. Specifically, we assume here that half of the spectrum seen by sites 1 and 3 is due to coupling to the bath of site 2. The system-bath coupling of spin 2 is kept unchanged as well as the diagonal spectral components of spins 0 and 2.

We find relatively strong changes in the solution: the transport to site 3 is essentially weaker. This implies that cross correlations can be harmful for efficient energy transport. The absence of cross-correlations is indeed the usual assumption taken in numerical simulations.

Changelog