Adiabatic Spectroscopy and a Variational Quantum Adiabatic Algorithm
Preparation of the ground state of a Hamiltonian is a problem of great significance in physics, with deep implications in the field of combinatorial optimization. The adiabatic algorithm is known to return the ground state for sufficiently long preparation times that depend on the a priori unknown spectral gap. Our work relates in a twofold way. First, we propose a method to obtain information about the spectral profile of the adiabatic evolution. Second, we present the concept of a variational quantum adiabatic algorithm (VQAA) for optimized adiabatic paths. We aim at combining the strengths of the adiabatic and the variational approaches for fast and high-fidelity ground-state preparation while keeping the number of measurements as low as possible. Our algorithms build upon ancilla protocols that we present, which allow us to directly evaluate the ground-state overlap. We benchmark for a nonintegrable spin-1/2 transverse and longitudinal Ising chain with N=53 sites using tensor-network techniques. Using a black-box gradient-based approach, we report a reduction in the total evolution time for a given desired ground-state fidelity by a factor of 10, which makes our method suitable for the limited decoherence time of noisy-intermediate scale quantum devices.
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