Ensemble Engineering to Overcome Destructive Cancellation in Quantum Measurements

TL;DR

This paper introduces quantum ensemble engineering to mitigate destructive cancellation in NISQ device measurements, improving the extraction of meaningful signals from expectation values.

Contribution

It presents a general framework and practical circuit implementations for aligning ensemble weights with operator structures to enhance measurement accuracy.

Findings

Engineered ensembles reveal operator-resolved contributions suppressed in uniform averaging.

Demonstrated on IBM quantum processors with up to 20 qubits, showing improved measurement signals.

Identified a tradeoff between amplification strength and noise robustness.

Abstract

On noisy intermediate-scale quantum (NISQ) devices, expectation values of many observables are obtained through sampling-based approximations to trace-like quantities. A central limitation of this approach is destructive cancellation under near-uniform ensembles, which can render physically relevant signals effectively unresolvable. Here we show that this limitation is not simply statistical, but reflects a structural mismatch between ensemble weights and the operator-dependent sign structure of the measured correlator. We introduce a general framework for mitigating this effect through quantum ensemble engineering, in which the sampling distribution is encoded directly in the prepared quantum state. By reformulating correlators in a basis-resolved representation, we make the origin of cancellation explicit and derive strategies for aligning ensemble weights with operator structure. We realize this approach using two complementary circuit constructions: a Grover-type amplitude amplification protocol that provides a structure-aligned benchmark, and an oracle-free shallow circuit designed for near-term hardware constraints. Using the infinite-temperature correlation function as a representative setting, we demonstrate on IBM quantum processors with up to 20 qubits that engineered ensembles expose operator-resolved contributions that are strongly suppressed under uniform averaging. We identify a practical tradeoff between amplification strength and noise robustness, extend the framework to multi-qubit diagonal observables, and outline a path toward non-diagonal generalizations. These results position ensemble engineering as a new tool for improving measurement efficiency in near-term quantum algorithms.