Variational preparation of finite-temperature states on a quantum computer
Ramiro Sagastizabal, Shavindra Premaratne, Berend Klaver, M. A. Rol, Victor Negîrneac, M. S. Moreira, Xiang Zou, Sonika Johri, Nandini Muthusubramanian, Marc Beekman, Christos Zachariadis, Viacheslav Ostroukh, Nadia Haider, Alessandro Bruno, A. Y. Matsuura, L. DiCarlo · npj Quantum Information · 2021
Abstract The preparation of thermal equilibrium states is important for the simulation of condensed matter and cosmology systems using a quantum computer. We present a method to prepare such mixed states with unitary operators and demonstrate this technique experimentally using a gate-based quantum processor. Our method targets the generation of thermofield double states using a hybrid quantum-classical variational approach motivated by quantum-approximate optimization algorithms, without prior calculation of optimal variational parameters by numerical simulation. The fidelity of generated states to the thermal-equilibrium state smoothly varies from 99 to 75% between infinite and near-zero simulated temperature, in quantitative agreement with numerical simulations of the noisy quantum processor with error parameters drawn from experiment.