The side effects of quantum error correction and how to cope with them

The performance of quantum sensors can be improved by quantum error correction. New theory work cautions that the approach can give rise to inaccurate and misleading results and shows how to correct them.

Classical systems can interact with one another in ways that are fundamentally different from quantum systems. In a quantum sensor, the particularities of these interactions are exploited to obtain characteristic information about the environment of the quantum system. When a device harnesses the laws of quantum mechanics, it can surpass what is possible with classical technologies.

The physical quantities of interest and noise are both exquisitely sensitive to quantum sensors. One way to suppress unwanted contributions is to apply quantum error correction. This approach might enable practical high-precision quantum sensors in a wider range of applications than is possible today. The benefits of error-corrected quantum sensing come with major potential side effects, as a team led by Florentin Reiter, an Ambizione fellow of the Swiss National Science Foundation working in the group of Jonathan Home at the Institute for Quantum Electronics has found. They show in their theoretical work that QEC can distort the output of quantum sensors and even lead to unphysical results in realistic settings. The researchers describe procedures on how to restore the correct results.

A person is drifting off a track.

Errors are corrected when the sensor acquires information about the target quantity. Imagine a car that keeps leaving from the center of the lane it is in. The drift is corrected by counter-steering. It has been shown that by constant error correction, the noise can be suppressed completely. The story is different when the driver can only correct the steering wheel at certain points in time. Experience tells us that the sequence of driving ahead and making corrective movements has to be adjusted. If the sequence didn't matter, the driver could simply perform all their steering maneuvers at home and then put their foot down on the pedal. The order in which the actions of one type or the other are executed changes the outcome.

The Hamiltonian operator of the sensor describes a similar situation with non-commuting actions. The sensor output experiences a systematic bias when there is a delay, according to Ivan Rojkov, a PhD researcher at the Massachusetts Institute of Technology. The dynamics of the quantum system can become contaminated by interference by error operators if the delay time is long. During the delay, the sensor acquires less information about the quantity of interest, such as a magnetic or electric field, compared to a situation in which no error has occurred. The different speeds result in a distortion of the output.

Senses of movement.

This bias matters. Estimates for the minimum signal that the quantum sensor can detect might end up being overly optimistic if they are not accounted for. Wrong estimates are deceptive for experiments that push the limits of precision. The team provides an escape route to overcome the bias. The amount of bias introduced by the finite-rate QEC can be calculated, and through appropriate measures, so that the sensor output makes sense again. The QEC can give rise to systematic shifts that can help to develop the ideal sensor protocol.

The results of this work will provide an import contribution to tweaking out the highest precision from a broad range or quantum sensors, and keep them on track to deliver on their promise of leading us to regimes.

More information: Ivan Rojkov et al, Bias in Error-Corrected Quantum Sensing, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.128.140503 Journal information: Physical Review Letters Citation: The side effects of quantum error correction and how to cope with them (2022, April 6) retrieved 6 April 2022 from https://phys.org/news/2022-04-side-effects-quantum-error-cope.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.