Single Ion Quantum Lock-In Amplifier

TL;DR

This paper introduces a quantum lock-in amplifier using a single trapped ion, significantly improving sensitivity and coherence time for magnetic field and light shift measurements, with broad potential applications in quantum sensing.

Contribution

It demonstrates a quantum analog of the classical lock-in amplifier using a single ion, achieving unprecedented sensitivity and coherence, and showcases its application in precision measurements.

Findings

Sensitivity of 25 pT/√Hz for magnetic fields

Phase coherence extended to over one second

Measurement of light shifts with uncertainty below 140 mHz

Abstract

We report on the implementation of a quantum analog to the classical lock-in amplifier. All the lock-in operations: modulation, detection and mixing, are performed via the application of non-commuting quantum operators on the electronic spin state of a single trapped Sr+ ion. We significantly increase its sensitivity to external fields while extending phase coherence by three orders of magnitude, to more than one second. With this technique we measure magnetic fields with sensitivity of 25 pT/sqrt(Hz) and light shifts with an uncertainty below 140 mHz after 1320 seconds of averaging. These sensitivities are limited by quantum projection noise and, to our knowledge, are more than two orders of magnitude better than with other single-spin probe technologies. In fact, our reported sensitivity is sufficient for the measurement of parity non-conservation, as well as the detection of the magnetic field of a single electronic-spin one micrometer from an ion-detector with nanometer resolution. As a first application we perform light shift spectroscopy of a narrow optical quadruple transition. Finally, we emphasize that the quantum lock-in technique is generic and can potentially enhance the sensitivity of any quantum sensor.