Variational preparation of finite-temperature states on a quantum computer

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.

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• Demonstration of Two-Qubit Algorithms with a Superconducting Quantum Processor
• Simple pulses for elimination of leakage in weakly nonlinear qubits
• Suppressing Charge Noise Decoherence in Superconducting Charge Qubits
• Leakage detection for a transmon-based surface code
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