Quantum magnetic gradiometer with entangled twin light beams

Abstract

In the past few decades, optical magnetometry has experienced remarkable development and reached to an outstanding sensitivity. For magnetometry based on optical readout of atomic ensemble, the fundamental limitation of sensitivity is restricted by spin projection noise and photon shot noise. Meanwhile, in practical applications, ambient magnetic noise also greatly limits the sensitivity. To achieve the best sensitivity, it is essential to find an efficacious way to eliminate the noises from different sources, simultaneously. Here, we demonstrate a quantum magnetic gradiometer with sub-shot-noise sensitivity using entangled twin beams with differential detection. The quantum enhancement spans a frequency range from 7 Hz to 6 MHz with maximum squeezing of 5.5 dB below the quantum noise limit. The sensitivity of gradiometer reaches 18 fT/cm Hz at 20 Hz. Our study inspires future possibilities to use quantum-enhanced technology in developing sensitive magnetometry for practical applications in noisy and physically demanding environments.

Recent Developments in Fabrication Methods and Measurement Schemes for Optically Pumped Magnetic Gradiometers: A Comprehensive Review

“…That gradiometer would have eliminated ambient magnetic noise and photon shot noise at the same time. Two beams of light were quantum-mechanically entangled through four-wave mixing (FWM) to squeeze intensity-difference noise [ 57 , 58 ]. The possibility of realizing subshot-noise magnetometry in practical applications was demonstrated.…”

An optically pumped magnetic gradiometer for the detection of human biomagnetism

“…Such a configuration suppresses the common mode magnetic field noise, but enables only reduced sensitivity [32]. Semi-intrinsic OPMGs use a single laser beam split into either two [33][34][35] or multiple measurement channels [36]. The signals produced are electronically subtracted enabling superior sensitivity.…”