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 digitally tuned. We optimize the parameters of this system so that it reproduces the spectral function of a given system-bath problem. The optimization is performed 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 utilization of the noisy qubits to improve the simulation results.

Validity

System-bath simulations are best implemented when:

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

However, these restrictions can be lifted with the cost of reduced simulation accuracy or by 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 the spectral functions, leading to an increased effective temperature.