paper

Variational Quantum State Eigensolver

arXiv:2004.01372 · doi:10.1038/s41534-022-00611-6

Abstract

Extracting eigenvalues and eigenvectors of exponentially large matrices will be an important application of near-term quantum computers. The Variational Quantum Eigensolver (VQE) treats the case when the matrix is a Hamiltonian. Here, we address the case when the matrix is a density matrix . We introduce the Variational Quantum State Eigensolver (VQSE), which is analogous to VQE in that it variationally learns the largest eigenvalues of as well as a gate sequence that prepares the corresponding eigenvectors. VQSE exploits the connection between diagonalization and majorization to define a cost function $C=\Tr(\tildeρ H)$ where is a non-degenerate Hamiltonian. Due to Schur-concavity, is minimized when is diagonal in the eigenbasis of . VQSE only requires a single copy of (only qubits) per iteration of the VQSE algorithm, making it amenable for near-term implementation. We heuristically demonstrate two applications of VQSE: (1) Principal component analysis, and (2) Error mitigation.

13 pages, 7 figures, 1 algorithm. Updated to published version

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