Spin Squeezing in Atomic Ensembles and Optical Clocks

Short answer: Spin squeezing redistributes quantum noise in an ensemble of atoms so that one measured quantity becomes more precise than the standard quantum limit allows. Experiments in 2012 and 2023 report squeezing in cold-atom ensembles and in optical clocks, with gains of about 2 to 4 decibels in the authors' own measures. These are laboratory demonstrations in atoms, not a general sensing technology.

What is spin squeezing for?

The 2023 Nature article describes a main challenge in building scalable quantum systems that show performance enhancement from entanglement: entanglement is fragile in large systems. The authors note that state-of-the-art optical atomic clocks are limited by quantum projection noise, which comes from fluctuations in the population of many atoms. Squeezing is a way of producing metrologically useful entanglement to get below that limit.

What did the 2012 experiment show?

A 2012 Physical Review Letters paper reports spin squeezing and entanglement in a magnetically sensitive atomic ensemble of 8.5 x 10^5 laser-cooled rubidium-87 atoms. The collective spin is squeezed by a synthesized optical quantum nondemolition 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 metrological advantage. They also demonstrate enhanced sensitivity in field measurements using alignment-to-orientation conversion.

What did the 2023 clock experiments report?

A Nature Physics paper presents an optical clock platform integrated with collective strong-coupling cavity quantum electrodynamics for quantum non-demolition measurements. The authors measure a metrological enhancement for a large ensemble of atoms beyond the initial coherent spin state, and use a movable lattice so the cavity can address two independent subensembles and compare them without the influence of clock laser noise. They report that the comparison remains above the effective standard quantum limit, while directly verifying a 1.9(2) dB clock stability enhancement at the 10^-17 level, according to the visible abstract text, which is cut off after that point.

A separate 2023 Nature paper combines neutral-atom arrays with frequency metrology. The authors realize Rydberg-mediated spin squeezing in an optical atomic clock based on a programmable array of interacting optical qubits. They generate states with almost four decibels of metrological gain. In a synchronous comparison between independent squeezed states, they report a fractional-frequency stability of 1.087(1) x 10^-15 at one second averaging time, which is 1.94(1) decibels below the standard quantum limit, reaching a fractional precision at the 10^-17 level during a half-hour measurement. They also use local phase shifts from tweezer control to explore squeezing beyond the relative coherence time with the optical local oscillator.

How to read these results

Decibel figures describe how much noise was reduced relative to a reference, and the references differ between papers, so the 2012 and 2023 numbers are not a single progression. All three are laboratory experiments with their own conditions. This post is based on abstracts and visible summary text, and the full texts were not checked. It makes no claim about products, or about nuclear magnetic resonance applications, which the abstracts do not cover.

FAQ

What does squeezing actually do?

It lowers noise in the measured spin variable below the projection-noise level, at the cost of more noise in another variable, as the 2012 abstract describes for its mixed alignment-orientation variable.

How much squeezing was reported in 2012?

3.2 dB of noise reduction and 2.0 dB by the Wineland criterion, with 8.5 x 10^5 rubidium-87 atoms.

Did the 2023 clocks beat the standard quantum limit?

The neutral-atom array paper reports stability 1.94(1) dB below it. The cavity clock paper says its comparison remained above the effective standard quantum limit while verifying a 1.9(2) dB stability enhancement.

Is this about NMR sensing?

The abstracts concern atomic ensembles and clocks. They do not report nuclear magnetic resonance results.