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Experimental Investigation of Quantum Discord in Multipartite Systems Vanessa Chille1,2, Christian Peuntinger1,2, Christian Gabriel1,2, Ladislav Mišta3,4, Natalia Korolkova3, Christoph Marquardt1,2, Gerd Leuchs1,2 [1] Max Planck Institute for the Science of Light, Günther-Scharowsky-Straße 1, 91058 Erlangen, Germany [2] Institute of Optics, Information and Photonics, University Erlangen-Nuremberg, Germany [3] School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews KY16 9SS, United Kingdom [4] Department of Optics, Palacký University, 17. listopadu 50, 772 07 Olomouc, Czech Republic Content 1) Motivation 2) Mutual Information 3) Distribution of Entanglement without Entanglement 4) Polarization Squeezing 5) Experimental Studies on Quantum Discord Motivation Is entanglement a necessary property for quantum computation? Yes! - But only for pure quantum states, where quantum correlations coincide with entanglement. For separable mixed quantum states: quantum correlations apart from entanglement Quantum Discord quantifies these correlations. 2) Mutual Information Mutual Information H ( A) H ( B) selection of the signal receipt of the signal information transferred by the communication = mutual information Quantum Information, Stephen M. Barnett, Oxford University Press, 2009 Mutual information - classical equivalent H ( B) H ( A) H ( A | B) H ( A, B) H ( B | A) G. Adesso and A. Datta, PRL 105, 030501 (2010) Mutual information - quantum Mutual information One-way classical correlation Quantum Discord: G. Adesso and A. Datta, PRL 105, 030501 (2010) 3) Distribution of Entanglement without Entanglement Distribution of continuous-variable entanglement by separable Gaussian states Initially, three modes A, B and C, which form a fully mixed separable Gaussian state, are prepared. The modes A and B are finally entangled while the mode C remains always separable from the pair of modes (AB). L. Mista and N. Korolkova, Phys. Rev. A 77, 050302 (2008) 4) Polarization Squeezing Description of polarization - in the classical domain Stokes parameters Poincaré sphere Description of polarization - in the quantum domain Classical Stokes Parameters Quantum Stokes Operators Stokes operators do not commute, obey the SU(2) Lie algebra N. Korolkova et al., Phys. Rev. A 65, 052306 Polarization Squeezing commutation relations for the Stokes operators Heisenberg inequalities For our experiment, we chose for example: Just as position and momentum in phase space, the Stokes parameters S1 and S2 can be squeezed in the dark plane S1/S2 . R. Dong et al., New J. Physics, 9 (410), (2007) 5) Experimental Studies on Quantum Discord Experimental Studies Bright circularly polarized light beam, squeezed in S1, antisqueezed in S2. Experimental Studies EOM, sinusoidal voltage Modulation of S1 electric local oscillator Displacement of the squeezed state in its dark plane Experimental Studies displacements with Gaussian distribution Gaussian mixed state Experimental Studies the prepared state enters a quantum network Measurement of the Covariance Matrix at different points of this network Quantum Discord take-home messages Quantum Discord… … is a measure of quantum correlations, also beyond entanglement. … may be studied using polarization squeezing, which allows a quite simple measurement scheme. … is the cause of astonishing phenonomena, such as the distribution of entanglement with separable states.