Spin squeezing in atomic ensembles

Short answer: Spin squeezing is an operation that produces entanglement useful for measurement by reducing quantum projection noise in an ensemble of atoms. A 2023 Nature Physics paper reports a 1.9 dB clock stability enhancement from squeezing in a cavity-based optical clock, though the comparison stayed above the effective standard quantum limit. A 2023 Nature paper reports a Rydberg-array clock with a stability 1.94 dB below the standard quantum limit, and a 2012 Physical Review Letters paper reports squeezing in a magnetically sensitive rubidium ensemble. These are laboratory experiments.

The noise being reduced

The Nature Physics authors say that state-of-the-art optical atomic clocks are limited by a fundamental source of noise from fluctuations of the population of many atoms, the quantum projection noise. They describe building scalable quantum systems that show a performance enhancement based on entanglement as a major goal in quantum computing and metrology, and the fragility of entanglement in large quantum systems as the main challenge.

A cavity-based optical clock

In the Nature Physics experiment, the authors present an optical clock platform integrated with collective strong-coupling cavity quantum electrodynamics for quantum non-demolition measurements. By optimizing the competition between spin measurement precision and loss of coherence, they measure a metrological enhancement for a large ensemble of atoms beyond the initial coherent spin state. A movable lattice lets the cavity address two independent subensembles one at a time, so they can squeeze two clock ensembles in succession and compare them without the influence of clock laser noise. They say the clock comparison remains above the effective standard quantum limit, but directly verifies a 1.9(2) dB clock stability enhancement at the 10 to the minus 17 level without subtracting technical noise.

A Rydberg-array clock

The Nature authors realize spin squeezing in an optical clock based on a programmable array of interacting optical qubits, using Rydberg-mediated interactions. They report states with almost four decibels of metrological gain. In a synchronous frequency comparison between independent squeezed states they report a fractional-frequency stability of 1.087(1) times 10 to the minus 15 at one second of averaging, which they say is 1.94(1) decibels below the standard quantum limit and reaches a fractional precision at the 10 to the minus 17 level during a half-hour measurement. They also use local phase shifts to explore squeezing in measurements that operate beyond the relative coherence time with the optical local oscillator.

A magnetic sensor

The 2012 Physical Review Letters paper reports spin squeezing and entanglement in a magnetically sensitive atomic ensemble, and entanglement-enhanced field measurements. A maximal Raman coherence is prepared in an ensemble of 8.5 times 10 to the 5 laser-cooled rubidium-87 atoms, and the collective spin is squeezed by a synthesized optical quantum non-demolition measurement. The authors observe 3.2 dB of noise reduction and 2.0 dB of squeezing by the Wineland criterion, which they say implies both entanglement and a metrological advantage, and they demonstrate enhanced sensitivity in field measurements using alignment-to-orientation conversion.

Limits

The numbers come from specific setups and averaging times. The cavity clock comparison did not beat the effective standard quantum limit, as its authors state. None of the papers describes a deployed clock or sensor.

FAQ

What does spin squeezing do?

It reduces noise in one measured quantity of an atomic ensemble below the projection-noise level, which the papers use for more precise measurement.

Did the experiments beat the standard quantum limit?

The Rydberg-array clock reports a stability 1.94 dB below it. The cavity clock comparison reports a 1.9 dB enhancement but remained above the effective limit.

Can it be used for magnetic fields?

The 2012 paper reports enhanced sensitivity in field measurements with a squeezed rubidium ensemble.

How big were the ensembles?

The 2012 experiment used 8.5 times 10 to the 5 rubidium atoms. The clock papers describe large ensembles and a programmable atom array.

Is it deployed?

No. These are laboratory demonstrations.