Synergistic integration of variational quantum eigensolver with a quantum subspace expansion

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 The accuracy of ground state energy estimation can be systematically improved by synergistically integrating two control variables: circuit depth that controls the expressivity of the variational wavefunction ansatz, and the dimension of the Krylov subspace generated by multi-qubit Pauli gates. 

Scientific Achievement

This work demonstrates an improved ground state energy estimation of spin-lattice models in the presence of hardware noise.

Significance and Impact

This presents a viable candidate approach to perform ground state calculations on large-size systems beyond 100 qubits to achieve quantum utility before fault tolerance.

Research Details

  • Demonstration of the approach by coordinating the number of layers of the Hamiltonian variational ansatz and the expansion order of the Krylov subspace.
  • Benchmark calculations of ground state energies of 1D and 2D mixed-field Ising spin models on noisy simulators and on IBM QPUs including ibmq_guadalupe (16 qubits).
  • Showcase of systematic energy improvement based on probabilistic error reduction.

J. C. Getelina, P. Sharma, T. Iadecola, P. P. Orth, and Y. -X. Yao, Quantum Subspace Expansion in the Presence of Hardware Noise, APL Quantum 1, 036127 (2024), https://doi.org/10.1063/5.0217294