Mapping coherence in measurement via full quantum tomography of a hybrid optical detector

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Quantum states and measurements exhibit wave-like (continuous) or particle-like (discrete) character. Hybrid discrete–continuous photonic systems are key to investigating fundamental quantum phenomena1,2,3, generating superpositions of macroscopic states4, and form essential resources for quantum-enhanced applications5 such as entanglement distillation6,7 and quantum computation8, as well as highly efficient optical telecommunications9,10. Realizing the full potential of these hybrid systems requires quantum-optical measurements sensitive to non-commuting observables such as field quadrature amplitude and photon number11,12,13. However, a thorough understanding of the practical performance of an optical detector interpolating between these two regions is absent. Here, we report the implementation of full quantum detector tomography, enabling the characterization of the simultaneous wave and photon-number sensitivities of quantum-optical detectors. This yields the largest parameterization to date in quantum tomography experiments, requiring the development of novel theoretical tools. Our results reveal the role of coherence in quantum measurements and demonstrate the tunability of hybrid quantum-optical detectors.

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Figure 1: Experimental set-up.
Figure 2: Experimentally reconstructed one-click POVM elements of a weak-homodyne APD.
Figure 3: Reconstructed POVM elements.


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The authors thank G. Donati, T. J. Bartley, J. Eisert, X. Yang and A. Feito for assistance and fruitful discussions. This work was funded in part by the Engineering and Physical Sciences Research Council of the UK (EPSRC, project EP/H03031X/1), the US European Office of Aerospace Research & Development (EOARD, project 093020), the European Commission (under Integrated Project Quantum Interfaces, Sensors, and Communication based on Entanglement (QESSENCE) and Specific Targeted Research Project (STREP) Hybrid Information Processing (HIP)) and the Alexander von Humboldt Foundation. I.A.W. acknowledges support from the Royal Society.

Author information

L.Z., H.B.C.-R. and A.D. contributed equally to this work. L.Z., H.B.C.-R., X.-M.J. and I.A.W. conceived the project and contributed to the design of the experiment and to laboratory measurements and data analysis. L.Z., A.D. and M.B.P contributed modelling and data analysis. G.P., J.S.L. and B.J.S. contributed to the initial conception of the project. All authors contributed to writing the manuscript.

Correspondence to Lijian Zhang.

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Zhang, L., Coldenstrodt-Ronge, H., Datta, A. et al. Mapping coherence in measurement via full quantum tomography of a hybrid optical detector. Nature Photon 6, 364–368 (2012) doi:10.1038/nphoton.2012.107

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