Constant-Depth Quantum Imaginary Time Evolution Using Dynamic Fan-out Circuits

TL;DR

This paper introduces a constant-depth quantum imaginary time evolution method using dynamic fan-out circuits, improving ground-state preparation efficiency on noisy quantum hardware by reducing entanglement and circuit depth.

Contribution

It presents a novel reduced-parameter QITE ansatz with constant two-qubit gate depth using dynamic fan-out circuits, enabling more practical ground-state simulations.

Findings

Higher success probability in noiseless simulations with the reduced ansatz

Semi-classical variant performs better on hardware

Dynamic fan-out circuits can outperform unitary implementations with reduced errors

Abstract

Dynamic quantum circuits combine mid-circuit measurement with classical feed-forward, enabling circuit constructions with reduced entangling-gate depth. Here, we investigate their use in Quantum Imaginary Time Evolution (QITE), where circuit depth and parameter growth limit practical implementations of ground-state preparation. For dense classical optimization Hamiltonians, we introduce a reduced-parameter QITE ansatz that restricts entanglement generation via a small set of control qubits, enabling each QITE layer to be implemented with constant two-qubit gate depth using fan-out-based dynamic circuits. In noiseless simulations of exact cover and set partitioning instances, the reduced ansatz yields a higher success probability than standard QITE approaches. We implement unitary, dynamic fan-out, and semi-classical adaptive variants on IBM superconducting hardware. The semi-classical variant performs favorably to the unitary implementation, while the fully dynamic construction exposes the trade-offs between entangling-depth reduction and measurement and feed-forward overhead associated to dynamic circuit implementations. Using a fidelity threshold of 0.5 relative to the noiseless QITE ansatz, we show that dynamic fan-out based QITE would outperform unitary implementations on current devices when the measurement and two-qubit gate errors are reduced by 65% and the feedback latency is halved.