The aim of this work package is to explore theoretically and experimentally different methods capable of creating a strong spin squeezing in the cold atomic samples at the core of optical clocks.

Spin-squeezing is a technique to overcome the QPN limit in the clock frequency stability by reducing the quantum noise in the measured quadrature of the atomic population at the expense of the unmeasured quadrature. The maximum reduction in the QPN is the Heisenberg limit, which scales like the inverse of the number of atoms. There are well-established methods in dedicated cold atom experiments mimicking microwave clocks, achieving degrees of squeezing up to 20 dB [hos16]. They rely on a differential dispersive measurement of the population difference between the two hyperfine states. State-of-the-art atomic clocks are devices operating at, or close to, the QPN limit, therefore these quantum-engineering techniques could find their first practical application. Furthermore, optical lattice clocks (OLCs) are well adapted for this because a large set of atoms is confined in a small volume, making it easier to address them collectively.

Several methods can be used to produce the non-linearity required to generate spin-squeezing in an atomic ensemble, including dispersive QND measurements [lou10], cavity back-action [nie08], squeezing via atom-atom interactions, or via the lattice light field [mei08]. This WP aims to explore two different techniques to reach spin-squeezing in the clock transition of strontium atoms:

• the first is based on a weak QND measurement assisted by a high finesse cavity surrounding the atoms close to the strong ¹S₀ → ¹P₁ transition of strontium (led by OBSPARIS),
• the second uses Rydberg state dressing (led by UDUR and NPL).

These two techniques are chosen because their implementation is facilitated by the experience and the hardware available to the partners of this project, making a practical demonstration realistic within the duration of the project. Furthermore, being the two techniques based on different physical principles, the parallel development mitigates the risk of the ambitious goal of generating spin-squeezing in a state-of-the-art optical clock.

To successfully implement spin-squeezing techniques on optical transitions, it is essential to build on the experience acquired in the microwave domain, where seminal experiments have been performed. For this reason ICFO will participate in this WP. Furthermore, new promising spin-squeezing techniques envisioned in Rb atoms will be developed (led by ICFO), and their transfer to the optical domain will be evaluated.

 

Lead participant

OBSPARIS (Dr Jerome Lodewyck)

Co-participants

NPL, UDUR, ICFO, INRIM