Fidelity Overhead for Nonlocal Measurements in Variational Quantum Algorithms

Area of Science:

• Quantum computing
• Quantum information science
• Computational chemistry

Background:

• Measuring quantum observables efficiently is crucial for near-term quantum computing.
• Transforming observables to Ising form (Pauli Z products) reduces measurement counts.
• Previous methods overlooked the impact of imperfect quantum gate fidelity.

Purpose of the Study:

• To analyze the impact of non-unitary gate fidelity on measurement schemes for quantum observables.
• To compare the efficiency of nonlocal vs. local qubit rotations in Ising form measurements under realistic error conditions.
• To determine if grouping strategies remain advantageous despite gate errors.

Main Methods:

• Modeling errors introduced by additional gates in Pauli product grouping schemes.
• Applying circuit fidelity reduction to account for gate imperfections.
• Calculating the number of measurements required for molecular electronic Hamiltonians using both nonlocal and local qubit rotations.

Main Results:

• Additional transformations introduce uncertainty, increasing the required number of measurements.
• Nonlocal qubit rotation schemes still require fewer measurements than local qubit rotation schemes, even with gate errors.
• The efficiency of grouping strategies persists despite accounting for gate fidelity reduction.

Conclusions:

• Grouping terms into Ising form is a robust strategy for efficient quantum observable measurement in near-term devices.
• Nonlocal qubit rotations offer an advantage over local rotations, even when gate errors are considered.
• This work provides a more realistic estimation of measurement resources needed for quantum algorithms.