Quantum computation of chemical and nuclear energy levels

Two- and three-qubit single-step QITE quantum circuit.

Various methods have been developed for the quantum computation of the ground and excited states of physical and chemical systems, but many of them require either large numbers of ancilla qubits or high-dimensional optimization in the presence of noise.

The quantum imaginary-time evolution (QITE) and quantum Lanczos (QLanczos) methods proposed in Motta et al. (2020) eschew the aforementioned issues.

In this study, the researchers demonstrated the practical application of these algorithms to challenging quantum computations of relevance for chemistry and nuclear physics, using the deuteron-binding energy and molecular hydrogen binding and excited state energies as examples.

With the correct choice of initial and final states, they showed that the number of timesteps in QITE and QLanczos can be reduced significantly, which commensurately simplifies the required quantum circuit and improves compatibility with NISQ devices.

They have performed these calculations on cloud-accessible IBM Q quantum computers.

With the application of readout-error mitigation and Richardson error extrapolation, they have obtained ground and excited state energies that agree well with exact results obtained from diagonalization.

The paper has been published in npj Nature Quantum Information.