Efficient Step-Merged Quantum Imaginary Time Evolution Algorithm for Quantum Chemistry.

Area of Science:

• Quantum computing
• Computational chemistry
• Quantum algorithms

Background:

• Solving for the ground state of molecular Hamiltonians is crucial in chemistry.
• Current quantum algorithms often require significant resources.
• Variational Quantum Eigensolver (VQE) is a common approach but involves complex optimization.

Purpose of the Study:

• To develop a resource-efficient quantum algorithm for determining molecular ground states.
• To assess the performance of the new algorithm against existing methods.
• To demonstrate the applicability of the algorithm on current quantum hardware.

Main Methods:

• Development of the step-merged quantum imaginary time evolution (smQITE) approach.
• Utilizing a fixed shallow quantum circuit depth for state evolution.
• Applying smQITE to calculate binding energy curves for molecules like H2, LiH, and H2O.
• Comparing smQITE with the Variational Quantum Eigensolver (VQE) using unitary coupled cluster ansatz.

Main Results:

• smQITE accurately determines the binding energy curves for a range of molecules.
• The algorithm achieves high accuracy comparable to VQE with the same ansatz.
• smQITE requires less complex optimization than VQE.
• Successful execution of smQITE on Rigetti quantum processing units.

Conclusions:

• smQITE is a resource-efficient and accurate method for ground state calculations on quantum computers.
• The algorithm offers a viable alternative to VQE, especially regarding optimization complexity.
• smQITE is practical for current noisy intermediate-scale quantum (NISQ) devices.