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Subspace-Search Quantum Imaginary Time Evolution for Excited State Computations
Cameron Cianci1,2, Lea F Santos1, Victor S Batista3,4
1Physics Department, University of Connecticut, Storrs, Connecticut 06269, United States.
Journal of Chemical Theory and Computation
|October 1, 2024
Summary
We introduce a new quantum algorithm, subspace search quantum imaginary time evolution (SSQITE), for calculating excited states. This method shows promise for advancing quantum computations of excited states in various applications.
Area of Science:
- Quantum Computing
- Quantum Chemistry
Background:
- Noisy Intermediate-Scale Quantum (NISQ) devices are driving interest in quantum systems.
- Calculating ground states is common with algorithms like VQE, but excited states are less explored due to a lack of efficient methods.
Purpose of the Study:
- To introduce a novel quantum algorithm for calculating excited states of quantum systems.
- To address the limitations of current methods for studying excited states on quantum devices.
Main Methods:
- The subspace search quantum imaginary time evolution (SSQITE) method is proposed.
- SSQITE integrates elements of subspace search variational quantum eigensolver (SSVQE) and variational quantum imaginary time evolution (VarQITE).
Main Results:
- SSQITE successfully calculated low-lying excited states for H2 and LiH molecules.
- A toy Hamiltonian demonstrated VarQITE's robustness in avoiding local minima, which extends to SSQITE for excited states.
Conclusions:
- SSQITE is an effective method for calculating excited states on quantum devices.
- The algorithm's robustness in avoiding local minima suggests broad applicability for quantum excited state computations.
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