Molecular excited state calculations with adaptive wavefunctions on a quantum eigensolver emulation: reducing circuit...

Area of Science:

• Quantum computing
• Quantum chemistry
• Computational physics

Background:

• Accurate computation of electronic excited states is crucial for studying photochemical reactions but faces significant resource scaling challenges on classical computers.
• Variational Quantum Deflation (VQD) offers a potential quantum computing solution for excited state energy calculations, but near-term quantum hardware limitations necessitate reduced quantum circuit complexity.

Purpose of the Study:

• To investigate the efficacy of adaptive quantum circuit growth (ADAPT-VQE) and spin restrictions in reducing the quantum computational cost of VQD for excited state calculations.
• To develop and test a quantum eigensolver emulation package (QEBAB) for evaluating proposed adaptive VQD methods.

Main Methods:

• Implemented an adaptive quantum circuit growth strategy (ADAPT-VQE) within the VQD framework.
• Utilized spin restrictions to efficiently isolate and compute excited states with different spin symmetries.
• Emulated and compared the performance of the adaptive spin-restricted VQD against fixed-circuit VQD methods (UCCGSD-VQD, k-UpCCGSD-VQD) using the QEBAB package.

Main Results:

• The spin-restricted adaptive VQD approach yielded significantly more compact quantum circuits compared to existing fixed-circuit VQD methods.
• The adaptive method consistently recovered sufficient electron correlation energy for various nuclear geometries and excited states.
• Spin restrictions effectively separated singlet and triplet electronic states, reducing computational overhead and error accumulation.

Conclusions:

• Adaptive quantum circuit growth and spin restrictions represent a promising strategy for enhancing the efficiency of VQD algorithms for excited state quantum chemistry.
• These advancements pave the way for earlier utilization of real quantum computers for complex electronic excited state calculations.
• The developed QEBAB package serves as a valuable tool for testing and validating hybrid quantum algorithms in computational chemistry.