Scalable Quantum Simulation of Molecular Energies

Abstract

We report the first electronic structure calculation performed on a quantum computer without exponentially costly precompilation. We use a programmable array of superconducting qubits to compute the energy surface of molecular hydrogen using two distinct quantum algorithms. First, we experimentally execute the unitary coupled cluster method using the variational quantum eigensolver. Our efficient implementation predicts the correct dissociation energy to within chemical accuracy of the numerically exact result. Second, we experimentally demonstrate the canonical quantum algorithm for chemistry, which consists of Trotterization and quantum phase estimation. We compare the experimental performance of these approaches to show clear evidence that the variational quantum eigensolver is robust to certain errors. This error tolerance inspires hope that variational quantum simulations of classically intractable molecules may be viable in the near future.

Alternate abstract:

Plain Language Summary

Universal simulation of physical systems is among the most compelling applications of quantum computing. In particular, quantum simulation of molecular energies promises significant advances in our understanding of chemistry, including precise predictions of chemical reaction rates. These advances, which may require relatively few qubits to achieve, may have broad industrial impact given the wide importance of chemistry. Here, we report the first electronic structure calculation performed on a quantum computer without exponentially costly precompilation.

We use a programmable array of superconducting qubits held at a base temperature of 20 mK in a dilution refrigerator; each qubit is composed of a superconducting quantum interference device (SQUID) and a capacitor, and is coupled to its nearest neighbors. We compute the energy surface of molecular hydrogen (i.e., the potential energy curve) using two distinct quantum algorithms: the variational quantum eigensolver and the phase estimation algorithm with Trotterization. We compare the experimental performance of these approaches and show that the variational quantum eigensolver yields chemically accurate results. Our experiments also reveal that the variational quantum eigensolver is robust to certain systematic errors.

Our findings suggest that the quantum simulation of classically intractable molecules may be possible without the overhead of quantum error correction.