Signatures of incoherence in a quantum information processor

TL;DR

This paper investigates incoherent noise in quantum processors, distinguishing it from decoherence, and demonstrates methods to identify and control incoherence during entangling operations through simulations and NMR experiments.

Contribution

It introduces a way to detect and analyze incoherence during entangling operations, which is more challenging than in small Hilbert spaces, using fidelity decay signatures.

Findings

Incoherence can be distinguished from decoherence via fidelity decay signatures.

Experimental validation was performed using a three-qubit NMR system.

Numerical simulations support the identification of incoherence signatures.

Abstract

Incoherent noise is manifest in measurements of expectation values when the underlying ensemble evolves under a classical distribution of unitary processes. While many incoherent processes appear decoherent, there are important differences. The distribution functions underlying incoherent processes are either static or slowly varying with respect to control operations and so the errors introduced by these distributions are refocusable. The observation and control of incoherence in small Hilbert spaces is well known. Here we explore incoherence during an entangling operation, such as is relevant in quantum information processing. As expected, it is more difficult to separate incoherence and decoherence over such processes. However, by studying the fidelity decay under a cyclic entangling map we are able to identify distinctive experimental signatures of incoherence. This result is demonstrated both through numerical simulations and experimentally in a three qubit nuclear magnetic resonance implementation.