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Direct comparison of two spin-squeezed optical clock ensembles at the 10−17 level

Nature Physics volume 20, pages 208–213 (2024)Cite this article

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Abstract

Building scalable quantum systems that demonstrate performance enhancement based on entanglement is a major goal in quantum computing and metrology. The main challenge arises from the fragility of entanglement in large quantum systems. Optical atomic clocks utilizing a large number of atoms have pushed the frontier of measurement science, building on precise engineering of quantum states and control of atomic interactions. However, state-of-the-art optical atomic clocks are limited by a fundamental source of noise stemming from fluctuations of the population of many atoms—the quantum projection noise. Here, we present an optical clock platform integrated with collective strong-coupling cavity quantum electrodynamics for quantum non-demolition measurements. Optimizing the competition between spin measurement precision and loss of coherence, we measure a metrological enhancement for a large ensemble of atoms beyond the initial coherent spin state. Furthermore, a movable lattice allows the cavity to individually address two independent subensembles, enabling us to spin squeeze two clock ensembles successively and compare their performance without the influence of clock laser noise. Although the clock comparison remains above the effective standard quantum limit, the performance directly verifies 1.9(2) dB clock stability enhancement at the 10−17 level without subtracting any technical noise contributions.

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Fig. 1: Optical clock with cQED architecture.
Fig. 2: Non-demolition measurements of the collective spin state.
Fig. 3: Directly observed metrological enhancement versus QND probe strength.
Fig. 4: Differential clock comparison.

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Data availability

The experimental data presented in this article are available from the corresponding author upon reasonable request. Source data are provided with this paper.

Code availability

The code used for analysis and simulation in this work is available from the corresponding author upon reasonable request.

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Acknowledgements

We acknowledge funding support from National Science Foundation QLCI OMA-2016244, Department of Energy National Quantum Information Science Research Center – Quantum Systems Accelerator, Air Force Office for Scientific Research, DARPA, Vannevar Bush Faculty Fellowship, National Institute of Standards and Technology, and National Science Foundation Phys-1734006. M.M. acknowledges the NSF Graduate Research Fellowship. We gratefully acknowledge early technical contributions and discussions from J. Meyer, J. Uhrich and E. Oelker. We acknowledge A. Aeppli, K. Kim, C. Sanner, R. Hutson, W. Milner and L. Yan for many stimulating discussions. We thank A. M. Rey, C. Luo, E. Polzik, V. Vuletic`, J. Hall, M. Schleier-Smith and M. Kasevich for careful reading of the manuscript.

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Authors and Affiliations

  1. JILA, National Institute of Standards and Technology and University of Colorado, Department of Physics, University of Colorado, Boulder, CO, USA

    John M. Robinson, Maya Miklos, Yee Ming Tso, Colin J. Kennedy, Tobias Bothwell, Dhruv Kedar, James K. Thompson & Jun Ye

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All authors contributed to the design and operation of the experiment and data analysis and writing of the manuscript.

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Extended data

Extended Data Fig. 1 Independence of atomic ensembles.

a, Measured correlation coefficient between Jz,A and Jz,B versus the separation between the ensembles. (black circles). The blue line is a Monte Carlo simulation. b, Corresponding change of the QPN due to the finite overlap of the ensembles, with numerical Monte Carlo simulation (blue) and analytical calculation (orange). At our operating ensemble separation, the change to QPN is 0.04 dB.

Extended Data Fig. 2 Pulse sequence for SSS - SSS comparison.

a, Clock pulses are the black pulses, measurements of ensemble A are the red pulses, and the transports are shown as the green and purple pulses. The Bloch spheres depict the spin state distribution at various points during the sequence. We note that the phase evolution is exaggerated for clarity. b, Ramsey fringe measured by varying the phase of the final π/2 pulse. c, Pre and final measurements of ensemble A. d, Pre and final measurements of ensemble B. e, The final measurements of ensemble A and B show strong correlations, allowing for the subtraction of the common-mode laser phase noise. The data in panels (b-e) correspond to the raw data corresponding to the SSS-SSS comparison shown in Fig. 4b.

Source data

Source Data Fig. 1

Source data underlying Fig. 1.

Source Data Fig. 2

Source data underlying Fig. 2.

Source Data Fig. 3

Source data underlying Fig. 3.

Source Data Fig. 4

Source data underlying Fig. 4.

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Robinson, J.M., Miklos, M., Tso, Y.M. et al. Direct comparison of two spin-squeezed optical clock ensembles at the 10−17 level. Nat. Phys. 20, 208–213 (2024). https://doi.org/10.1038/s41567-023-02310-1

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