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Quantum Information and Measurement (QIM)

Creating Spin Squeezing in a Compact Atomic Clock

Théo Laudat, Konstantin Ott, Meng-Zi Huang, Vincent Dugrain, Alice Sinatra, P. Rosenbusch, Carlos L. Garrido Alzar, Jakob Reichel

Published Jan 1, 2017• 0 citations
Publisher

Abstract

We demonstrate spontaneous spin squeezing in a trapped-atom clock on a chip (TACC) using condensed atoms and present progress towards a squeezed clock with thermal atoms, which uses a fiber microcavity on the chip.
DOI
10.1364/qim.2017.qf2a.2

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Using the platform of a trapped-atom clock on a chip, we have generated spin-squeezed states and observed their time evolution on unprecedented timescales, showing that metrological spin squeezing is preserved for one second. We observe up to 8.1(9) dB of metrological squeezing in a cloud of 2times1042times 10^42times104 ultracold alkali atoms by quantum nondemolition (QND) measurement in a fiber Fabry-Perot microcavity. The experiment also reveals a surprising amplification effect in the final cavity measurement of the spin state. It results from a subtle interplay between the cavity coupling and the spin dynamics originating from cold collisions. Spin dynamics such as these are an important factor in real atomic clocks that previous proof-of-principle squeezing experiments had not been able to address. Our results open up encouraging perspectives for squeezing-enhanced atomic clocks in a metrologically relevant stability regime.
In general, the process of spin squeezing is employed for atomic sensors that are essentially free from the effect of spontaneous emission. In the case of an atomic clock based on coherent population trapping, spontaneous emission plays a key role. Specifically, when the clock is realized in the Ramsey configuration employing a pair of pulses separated in time, it is the spontaneous emission process during the first pulse that creates the coherent population trapping, by optically pumping the atoms into the so-called dark state. We show in this talk that it is still possible to apply the process of spin squeezing to enhance the sensitivity of such a clock, by designing carefully the sequence of steps in the protocol. Specifically, we consider the approach of using the maximally entangled Schroedinger Cat states, produced via one axis twist squeezing, to achieve Heisenberg Limited sensitivity, representing an improvement in sensitivity by as much as four orders of magnitude.
The Dick effect and quantum projection noise (QPN) are among the most stringent limiting factors for state-of-the-art atomic frequency standards, including both primary and secondary standards, as well as compact clocks. A possible solution to the Dick effect relies on Quantum Non-Destructive (QND) detection, which allows multiple interrogations with weak measurements on the same atomic ensemble. Furthermore, spin-squeezed states, which can also be prepared by such QND detection, have the potential to enable clock measurements beyond the standard quantum limit, by employing quantum correlations. Our experimental platform is a Trapped-Atom Clock on a Chip (TACC) that uses ultracold Rb atoms, having demonstrated metrological stability. In the second generation of this experiment, we target quantum enhancement by using QND detection and spin-squeezed ultra-cold atoms generated by cavity QED interactions. To reach this goal, we have integrated two fiber Fabry-Perot cavities on the clock chip, realizing a platform for cavity QED in the strong coupling and weak coupling regimes. With this new device we aim to explore spin squeezing at a metrologically relevant level of stability. Here, we report on the current status and perspectives of this project.
Using the platform of a trapped-atom clock on a chip, we have generated spin-squeezed states with up to 8.1(9) dB of metrological squeezing in a cloud of 2×1042\times 10^42×104 ultracold alkali atoms by quantum nondemolition (QND) measurement in a fiber Fabry-Perot microcavity. Observing the time evolution of the squeezed state on unprecedented timescales of more than one second reveals a surprising measurement amplification effect in the final measurement of the spin state. It results from a subtle interplay between the spin dynamics of interacting indistinguishable particles and energy-dependent cavity coupling and leads to an increased cavity shift per spin, and thus to a higher signal per photon read out. Metrological spin squeezing is preserved for 1 s. Both results open up encouraging perspectives for squeezing-enhanced atomic clocks in a metrologically relevant stability regime.