Unambiguous measurement in an unshielded microscale magnetometer with sensitivity below 1 pT/rHz

TL;DR

This paper presents a cold atom magnetometer using Hilbert-demodulated optical readout that achieves unambiguous, high-precision magnetic field measurements below 1 pT/Hz in an unshielded environment, overcoming fringe ambiguity issues.

Contribution

The authors introduce a novel Hilbert-demodulated optical magnetometry technique enabling unambiguous, high-sensitivity magnetic field measurements with cold atom sensors in unshielded environments.

Findings

Achieved dc sensitivity of 380 fT in 1000 ms

Demonstrated unambiguous magnetic field measurement in unshielded environment

Measured a test field with high precision in a single shot

Abstract

Cold atom magnetometers exploit a dense ensemble of quanta with long coherence times to realise leading sensitivity on the micrometer scale. Configured as a Ramsey interferometer, a cold atom sensor can approach atom shot-noise limited precision but suffers from fringe ambiguity, producing gross errors when the field falls outside a narrow predefined range. We describe how Hilbert-demodulated optical magnetometry can be realised on cold atom sensors to provide field measurements both precise and unambiguous. Continuous reconstruction of the Larmor phase allows us to determine the dc magnetic field unambiguously in an unshielded environment, as well as measure ac variation of the field, in a single shot. The ac measurement allows us to characterize, and then neutralise, line-synchronous magnetic interference, extending reconstruction times. Using $1.6 \times 10^6$ $^{87}$Rb atoms in a volume of $(68 \,\mathrm{\mu m})^3$, we measure a test field to be $ 86.0121261(4) \; \mathrm{\mu T}$ in a single shot, achieving dc sensitivity of 380 fT in a duration of 1000 ms. Our results demonstrate that Hilbert-demodulated optical readout yields metrologically-significant sensitivity without the fringe ambiguity inherent to Ramsey interferometry.