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Entanglement-enhanced measurement of a completely unknown optical phase

Nature Photonics volume 5, pages 43–47 (2011)Cite this article

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Abstract

Precise interferometric measurement is vital to many scientific and technological applications. Using quantum entanglement allows interferometric sensitivity that surpasses the shot-noise limit (SNL)1,2. To date, experiments demonstrating entanglement-enhanced sub-SNL interferometry3,4,5,6, and most theoretical treatments7,8,9,10,11,12,13, have addressed the goal of increasing signal-to-noise ratios. This is suitable for phase-sensing—detecting small variations about an already known phase. However, it is not sufficient for ab initio phase-estimation—making a self-contained determination of a phase that is initially completely unknown within the interval [0, 2π). Both tasks are important2, but not equivalent. To move from the sensing regime to the ab initio estimation regime requires a non-trivial phase-estimation algorithm14,15,16,17. Here, we implement a ‘bottom-up’ approach, optimally utilizing the available entangled photon states, obtained by post-selection5,6. This enables us to demonstrate sub-SNL ab initio estimation of an unknown phase by entanglement-enhanced optical interferometry.

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Figure 1: A Mach–Zehnder interferometer with the phase shift to be measured, φ, in one arm, and a controllable phase shift, θ, in the other arm.
Figure 2: Experimental layout.
Figure 3: Fits (lines) of measured (points) photon counts corresponding to fringes given by equations (1) to (4).
Figure 4: Standard deviations δ φ of phase estimates for varying total photon number N.

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Acknowledgements

The authors thank J. O'Brien for helpful discussions. This work was supported by the Australian Research Council.

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Authors and Affiliations

  1. Centre for Quantum Computer Technology, Centre for Quantum Dynamics, Griffith University, Brisbane, 4111, Australia

    G. Y. Xiang, B. L. Higgins, H. M. Wiseman & G. J. Pryde

  2. Key Laboratory of Quantum Information, University of Science and Technology of China, CAS, Hefei, 230026, China

    G. Y. Xiang

  3. Institute for Quantum Computing, University of Waterloo, Waterloo, N2L 3G1, Ontario, Canada

    D. W. Berry

Authors
  1. G. Y. Xiang
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  2. B. L. Higgins
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  3. D. W. Berry
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  4. H. M. Wiseman
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  5. G. J. Pryde
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Contributions

G.J.P. devised and designed the experiment. H.M.W. devised and analysed the theoretical approach. G.Y.X. constructed and operated the experiment, and collected the data. B.L.H. automated and experimentally implemented the feedback algorithm, and contributed to the operation of the apparatus. D.W.B. constructed and performed numerical simulations of the algorithm, and performed numerical processing of results. All authors contributed to the manuscript.

Corresponding authors

Correspondence to H. M. Wiseman or G. J. Pryde.

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The authors declare no competing financial interests.

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Xiang, G., Higgins, B., Berry, D. et al. Entanglement-enhanced measurement of a completely unknown optical phase. Nature Photon 5, 43–47 (2011). https://doi.org/10.1038/nphoton.2010.268

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