Building Operators

Quantum Solver allows constructing quantum mechanical operators in a flexible and convenient way. As we have seen in the "Getting Started" section, quantum mechanical operators are constructed by providing a mathematical term and a vector space. The constructor of the Operator class, however, does not only accept single terms, but arbitrary linear combinations of terms, which makes this interface flexible.

To demonstrate this feature, let us consider the following example. Assume we want to construct the Hamiltonian $H$, $$H=-t\sum _{j=0}^{M-2}(c_j^{†}{c}_{j+1}+c_{j+1}^{†}{c}_j)+U\sum _{j=0}^{M-2}(n_j-\frac{1}{2})(n_{j+1}-\frac{1}{2}),$$ where $n_j=c_j^{†}{c}_j$. We start by creating a VectorSpace object, like in the "Getting Started" section, i.e.:

v = VectorSpace(sites=M, particle_numbers=N)

We can now consider the individual parts of the formula for the Hamiltonian. The term ${\sum }_{j=0}^{M-2}(c_j^{†}{c}_{j+1}+c_{j+1}^{†}{c}_j)$ can be obtained by passing the matrix $$\left[\begin{array}{ccccc}0& 1& & & \\ 1& 0& 1& & \\ & 1& \ddots & \ddots & \\ & & \ddots & \ddots & 1\\ & & & 1& 0\end{array}\right]$$ to the hopping function, and this matrix can be constructed by:

neighbors = toeplitz(unit_vector(v.sites, 1))

The term ${\sum }_{j=0}^{M-2}(n_j-\frac{1}{2})(n_{j+1}-\frac{1}{2})$ can be obtained by passing the matrix $$\left[\begin{array}{ccccc}0& 1& & & \\ & 0& 1& & \\ & & \ddots & \ddots & \\ & & & 0& 1\\ & & & & 0\end{array}\right]$$ to the interaction function, and this matrix can be constructed by:

right_neighbors = toeplitz(np.zeros(v.sites), unit_vector(v.sites, 1))

We can now combine the individual terms, to construct the desired Hamiltonian.

H = Operator(-t * hopping(neighbors) + U * interaction(right_neighbors), domain=v)

Using the same code as discussed in the "Getting Started" section, we can compute the expectation value of the occupation for different values of $U$.

Complete Code

# Title    : HQS Quantum Solver Hubbard Interaction
# Filename : hubbard_interaction.py

# NumPy, SciPy, and Matplotlib imports
import numpy as np
from scipy.linalg import toeplitz
from scipy.sparse.linalg import eigsh
import matplotlib.pyplot as plt

# HQS Quantum Solver imports
from hqs_quantum_solver.spinless_fermions import (
    VectorSpace,
    Operator,
    hopping,
    interaction,
    annihilation,
)
from hqs_quantum_solver.util import unit_vector

# ===== Parameters =====

M = 16  # site count
N = 2  # particle count
t = 1  # hopping strength
U = 10  # interaction strength

# ===== Creation of the Hamiltonian =====

v = VectorSpace(sites=M, particle_numbers=N)

neighbors = toeplitz(unit_vector(v.sites, 1))
right_neighbors = toeplitz(np.zeros(v.sites), unit_vector(v.sites, 1))

H = Operator(-t * hopping(neighbors) + U * interaction(right_neighbors), domain=v)

# ===== Computing the Eigenvalues and Eigenvectors =====

k = min(12, H.shape[0] - 1)  # Number of eigenvalues to compute
evals, evecs = eigsh(H, k=k, which="SA")
groundstate = evecs[:, 0]

# ===== Plotting the Site Occupations =====

v_minus = v.copy(particle_number_change=-1)


def site_occupation(psi, j):
    annihilation_operator = Operator(annihilation(site=j), domain=v, codomain=v_minus)
    c_psi = annihilation_operator.dot(psi)
    return c_psi.conj() @ c_psi


occupations = np.array([site_occupation(groundstate, j) for j in range(M)])
plt.figure("site_occupation", clear=True)
plt.title(f"Site Occupation (M = {M}, N = {N}, t={t}, U={U})")
plt.plot(occupations, "x-")
plt.axis((0, M - 1, 0, 1))
plt.xlabel("j")
plt.ylabel("Expectation Value")
plt.show()