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Experimental estimation of the dimension of classical and quantum systems

Abstract

Experimental observations are usually described using theoretical models that make assumptions about the dimensionality of the system under consideration. However, would it be possible to assess the dimension of a completely unknown system only from the results of measurements performed on it, without any extra assumption? The concept of a dimension witness1,2,3,4,5,6 answers this question, as it allows bounding the dimension of an unknown system only from measurement statistics. Here, we report on the experimental demonstration of dimension witnesses in a prepare and measure scenario6. We use photon pairs entangled in polarization and orbital angular momentum 7,8,9 to generate ensembles of classical and quantum states of dimensions up to 4. We then use a dimension witness to certify their dimensionality as well as their quantum nature. Our work opens new avenues in quantum information science, where dimension represents a powerful resource10,11,12, especially for device-independent estimation of quantum systems13,14,15,16 and quantum communications17,18.

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Figure 1: Sketch of a device-independent test of the dimension of an ensemble of states.
Figure 2: Experimental set-up.
Figure 3: Dimension witness I4 for qubit (blue triangles), qutrit (red circles) and quart (green diamonds) states as a function of temporal delay τ.

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Acknowledgements

We thank C. Hadley and E. Nagali for contributions in early stages of this work. We acknowledge support from the ERC Starting Grant PERCENT, the EU Projects Q-Essence, QCS and PHORBITECH (FET OPEN grant number 255914), the UK EPSRC, the Project MSM6198959213 of the Czech Ministry of Education, the Spanish projects FIS2010-14830, FIS2010-14831 and Chist-Era DIQIP, an FI Grant of the Generalitat de Catalunya, CatalunyaCaixa, and Fundació Privada Cellex, Barcelona.

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R.G., N.B. and A.A. developed the theory, M.H., R.G., N.B., A.A. and J.P.T. designed the experiment, and M.H., M.M. and J.P.T. performed the experiment. All of the authors contributed to the writing of the manuscript.

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Correspondence to Antonio Acín or Juan P. Torres.

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The authors declare no competing financial interests.

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Hendrych, M., Gallego, R., Mičuda, M. et al. Experimental estimation of the dimension of classical and quantum systems. Nature Phys 8, 588–591 (2012). https://doi.org/10.1038/nphys2334

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