Magnetic Sensitivity Beyond the Projection Noise Limit by Spin Squeezing

Abstract

We report the generation of spin squeezing and entanglement in a magnetically sensitive atomic ensemble, and entanglement-enhanced field measurements with this system. A maximal m(f) = ± 1 Raman coherence is prepared in an ensemble of 8.5 × 10(5) laser-cooled (87)Rb atoms in the f = 1 hyperfine ground state, and the collective spin is squeezed by synthesized optical quantum nondemolition measurement. This prepares a state with large spin alignment and noise below the projection-noise level in a mixed alignment-orientation variable. 3.2 dB of noise reduction is observed and 2.0 dB of squeezing by the Wineland criterion, implying both entanglement and metrological advantage. Enhanced sensitivity is demonstrated in field measurements using alignment-to-orientation conversion.

Entanglement with negative Wigner function of almost 3,000 atoms heralded by one photon

“…Metrologically useful entangled states of large atomic ensembles have been experimentally realized [1][2][3][4][5][6][7][8][9][10], but these states display Gaussian spin distribution functions with a non-negative Wigner function. Non-Gaussian entangled states have been produced in small ensembles of ions [11,12], and very recently in large atomic ensembles [13][14][15].…”

“…Entanglement can be verified in a variety of ways, with one of the strictest criteria being a negative-valued Wigner function [16,17], that necessarily implies that the entangled state has a non-Gaussian wavefunction. To date, the metrologically useful spin-squeezed states [1][2][3][4][5][6][7][8][9][10] have been produced in large ensembles. These states have Gaussian spin distributions and therefore can largely be modeled as systems with a classical source of spin noise, where quantum mechanics enters only to set the amount of Gaussian noise.…”

Three-dimensional light-matter interface for collective spin squeezing in atomic ensembles

“…In a real-world metrological application such as an optically probed atomic magnetometer [47,48], spin rotations are measured by passing the probe through the atom sample and measuring the resulting Faraday rotation in a polarimeter. In addition to spin projection noise, the measurement resolution is then subject also to "technical noise," including probe shot noise, detector electronic noise, and atom number fluctuations.…”

Robust spin squeezing preservation in photonic crystal cavities

“…For spin or angular momentum systems, it is often named as spin squeezing if the variance of one angular momentum component, e.g., (∆J x ) 2 or (∆J y ) 2 is smaller than J z /2 [1,2]. In recent years, spin squeezing has attracted considerable attention and has been studied both theoretically and experimentally [3][4][5][6], since it has potential applications in entanglement detection [7][8][9], quantum information processing [10] and high-precision measurement, such as Ramsey spectroscopy [11], atom clocks [12], gravitational-wave interferometers [13] and quantum metrology [14][15][16].…”