Full characterization of polarization states of light via direct measurement


Ascertaining the physical state of a system is vital in order to understand and predict its behaviour. However, due to their fragile nature, the direct observation of quantum states has, until recently, been elusive. Historically, determination of the quantum state has been performed indirectly through the use of tomography. We report on two experiments showing that an alternative approach can be used to determine the polarization quantum state in a simple, fast and general manner. The first experiment entails the direct measurement of the probability amplitudes describing pure polarization states of light, the first such measurement on a two-level system. The second experiment entails the direct measurement of the Dirac distribution (a phase-space quasi-probability distribution informationally equivalent to the density matrix), demonstrating that the direct measurement procedure is applicable to general (that is, potentially mixed) quantum states. Our work has applications to measurements in foundational quantum mechanics, quantum information and quantum metrology.

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Figure 1: Schematic representation of the experiment.
Figure 2: Results of experiment 1 with linearly polarized probe states.
Figure 3: Measured states on the Poincaré sphere.
Figure 4: Results of experiment 2.


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The authors thank K. Piché and F. Miatto for helpful discussions, and P.B. Corkum and C. Zhang for lending us the quartz crystal. This work was supported by the Canada Excellence Research Chairs (CERC) Program. In addition, R.W.B. acknowledges support from the DARPA InPho program.

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J.Z.S. initiated the study. The experiment was designed by J.Z.S., A.S.J., E.B. and J.L. The experiment was performed by J.Z.S., M.A. and A.S.J., and data analysis was performed by J.Z.S. R.W.B. supervised all aspects of the project. All authors contributed to the text of the manuscript.

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Correspondence to Jeff Z. Salvail.

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The authors declare no competing financial interests.

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Salvail, J., Agnew, M., Johnson, A. et al. Full characterization of polarization states of light via direct measurement. Nature Photon 7, 316–321 (2013). https://doi.org/10.1038/nphoton.2013.24

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