# Near-Unitary Spin Squeezing in $^{171}$Yb

## TL;DR

This paper demonstrates nearly unitary spin squeezing in $^{171}$Yb atoms using cavity feedback, significantly enhancing measurement precision and improving atomic clock performance beyond the standard quantum limit.

## Contribution

It introduces a method for achieving substantial, nearly unitary spin squeezing in $^{171}$Yb, advancing quantum metrology for atomic clocks.

## Key findings

- Achieved 12.9 dB of spin squeezing limited by Bloch sphere curvature.
- Demonstrated a 3.7-fold improvement in averaging time over the SQL.
- Squeezing can be transferred to optical clock transitions in future work.

## Abstract

Spin squeezing can improve atomic precision measurements beyond the standard quantum limit (SQL), and unitary spin squeezing is essential for improving atomic clocks. We report substantial and nearly unitary spin squeezing in $^{171}$Yb, an optical lattice clock atom. The collective nuclear spin of $\sim 10^3$ atoms is squeezed by cavity feedback, using light detuned from the system's resonances to attain unitarity. The observed precision gain over the SQL is limited by state readout to 6.5(4) dB, while the generated states offer a gain of 12.9(6) dB, limited by the curvature of the Bloch sphere. Using a squeezed state within 30% of unitarity, we demonstrate an interferometer that improves the averaging time over the SQL by a factor of 3.7(2). In the future, the squeezing can be simply transferred onto the optical clock transition of $^{171}$Yb.