Cavity-enhanced optical clocks and spin squeezing via continuous measurements: status and perspectives at INRIM

Optical atomic clocks are rapidly advancing towards their quantum limit, which is set by the quantum noise of projective measurement of the atomic collective state. We report on the experimental status and the perspectives of the strontium optical lattice clock at INRIM and its development towards a cavity-enhanced optical frequency standard exploiting quantum techniques.

We first describe our experimental apparatus, which employs a cold atomic source for optical loading of the ultracold Sr frequency discriminator [1], and a multiwavelengh frequency stabilization system, which dramatically simplifies clock apparatus [2]. Optical lattice clock operation has been successfully demonstrated on ultracold 88Sr atoms by magnetic-field induced spectroscopy. We have recently developed a novel method to perform high-fidelity spectral purity transfer based on serrodyne optical frequency shifting [3]. We estimated the phase noise induced by the modulation setup by developing a novel composite selfheterodyne interferometer. The consequent short-term stability of our clock is presented, showing collision-limited interaction time in the bosonic 88Sr clock. We also outline the progress towards a cavity-enhanced optical lattice clock [4].

Regarding the theoretical work on this topic, we present the study of the generation of spin-squeezed states by coupling three-level atoms to an optical cavity and continuously measuring the cavity transmission in order to monitor the evolution of the atomic collective state [5]. We employ the QuTip libraries to perform cavity quantum electrodynamics simulations of the full conditional dynamics, and show that one can achieve significant metrologically relevant spin squeezing even without the continuous feedback that is proposed in previous approaches [6]. We characterize the different regimes for spinsqueezing generation and describe its scaling dependence on the atomic ensemble size, even when the adiabatic removal of the cavity field is not feasible [7].

References
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3. M. Barbiero, D. Calonico, F. Levi, and M. G. Tarallo, IEEE Trans. Instrum. Meas. 71, 1–9 (2022)
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