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Reduced spin measurement back-action for a phase sensitivity ten times beyond the standard quantum limit

Nature Photonics volume 8, pages 731–736 (2014)Cite this article

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Abstract

Fundamental quantum noise limits the precision of quantum-based detectors, for example limiting the ultimate precision of atomic clocks, which have applications in communication, navigation and tests of fundamental physics. Collective measurements of many quantum spins can project the ensemble into an entangled, spin-squeezed state with improved quantum-limited measurement resolution. However, measurement back-action has limited previous implementations of collective measurements to only modest observed enhancements in precision. Here, we experimentally demonstrate a collective measurement with reduced measurement back-action to produce and directly observe, with no background subtraction, a spin-squeezed state with phase resolution improved by a factor of 10.5(1.5) in variance, or 10.2(6) dB, compared to the initially unentangled ensemble of N = 4.8 × 105 87Rb atoms. The measurement uses a cavity-enhanced probe of an optical cycling transition, mitigating back-action associated with state-changing transitions induced by the probe. This work establishes collective measurements as a powerful technique for generating useful entanglement for precision measurements.

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Figure 1: Spin-squeezing and measurement back-action.
Figure 2: Detection of a quantum phase with entanglement-enhanced sensitivity.
Figure 3: Spin-squeezing and probe-induced back-action.
Figure 4: Absolute phase sensitivity versus N.

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Acknowledgements

The authors acknowledge K. McAlpine's early contributions to building detectors and helpful discussions with A. M. Rey and K. W. Lehnert. The authors acknowledge financial support from the Defense Advanced Research Projects Agency Quantum Assisted Sensing and Readout project (DARPA QuASAR), the Army Research Office (ARO), the National Science Foundation Physics Frontier Center (NSF PFC) and the National Institute of Standards and Technology (NIST). J.G.B. acknowledges support from the National Science Foundation Graduate Research Fellowship (NSF GRF) and K.C.C. acknowledges support from the National Defense Science and Engineering Fellowship (NDSEG). This material is based upon work supported by the National Science Foundation (grant no. 1125844).

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Author notes

  1. J. G. Bohnet & Z. Chen

    Present address: Present addresses: National Institute of Standards and Technology, Boulder, Colorado 80305, USA (J.G.B.), Data Storage Institute, Agency for Science, Technology and Research, Singapore 117608, Singapore (Z.C.),

Authors and Affiliations

  1. JILA, University of Colorado and NIST, Boulder, 80309, Colorado, USA

    J. G. Bohnet, K. C. Cox, M. A. Norcia, J. M. Weiner, Z. Chen & J. K. Thompson

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  1. J. G. Bohnet
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Contributions

J.G.B., K.C.C., M.A.N. and J.M.W. designed and performed experiments and analysed data. Z.C. provided analytic support. J.K.T. designed the experiment and analysed data. J.G.B. wrote the manuscript and all authors provided feedback for the manuscript.

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Correspondence to J. K. Thompson.

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Bohnet, J., Cox, K., Norcia, M. et al. Reduced spin measurement back-action for a phase sensitivity ten times beyond the standard quantum limit. Nature Photon 8, 731–736 (2014). https://doi.org/10.1038/nphoton.2014.151

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