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Quantum versus Classical Correlations in Gaussian States Gerardo Adesso joint work with Animesh Datta (Imperial College / Oxford) School of Mathematical Sciences Imperial College London 10/08/2010

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Outline Quantum versus classical correlations Quantum discord Gaussian quantum discord Structure of Gaussian correlations Open problems 2 Quantum versus Classical Correlations in Gaussian States Imperial College London 10/08/2010

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Correlations 3 Quantum versus Classical Correlations in Gaussian States Imperial College London 10/08/2010 A A B B Classical correlations Quantum correlations

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Correlations 4 Quantum versus Classical Correlations in Gaussian States Imperial College London 10/08/2010 Pure global composite states: ▫ entanglement = nonlocality = nonclassicality (quantum correlations) Mixed global composite states: ▫ Werner 1989: separable = classically correlated

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Quantumness in separable states Nonorthogonal separable states cannot be discriminated exactly Measuring a local observable on a separable bipartite state will perturb the state The eigenvectors of a separable state can be entangled superpositions … In general separable states have not a purely classical nature 5 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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A new paradigm 6 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States M. Piani, P. Horodecki, R. Horodecki, PRL 2008

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Quantum discord A measure that strives at capturing all quantum correlations, beyond entanglement, which can be nonzero also in separable states Introduced a decade ago in two independent works (Ollivier/Zurek and Henderson/Vedral) Recently became very popular: stats from arXiv:quant-ph… 7 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Quantum discord Almost all bipartite states have nonzero quantum discord (purely classically correlated states are of zero measure) A. Ferraro et al. PRA 2010 Reduces to the entropy of entanglement on pure bipartite states Quantum discord without entanglement may allow for a computational speed-up in the DQC1 model of quantum computation A. Datta et al. 2008-2010; experiment: M. Barbieri et al. PRL 2008 8 discord entanglement Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Mutual information: classical 9 measuring total correlations… all equal (Bayes’ rule) Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Mutual information: quantum 10 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Conditional entropy 11 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States Introduce POVM on B: Posterior state of A after B has been measured: looking for the “least disturbing measurement”:

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Bipartite correlations Total correlation One-way classical correlation Henderson, Vedral, JPA 2001 Quantum discord Ollivier, Zurek, PRL 2001 12 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Quantum discord For classical states (classical probability distribution embedded into density matrices) I=J hence the quantum discord vanishes Zurek introduced it in the context of environment-induced selection, identifying classical states with the pointer states The optimization involved in the conditional entropy is hard. Closed analytical formulas are available only for special families of two- qubit staes (X-shaped), not even for arbitrary states of two qubits Two recent independent works, including this one, defined a Gaussian version of the quantum discord for bipartite Gaussian states, where the optimization is restricted to Gaussian measurements P. Giorda & M.G.A. Paris PRL 2010; GA & A. Datta PRL 2010 We have solved the optimization problem and obtained a simple formula for the Gaussian quantum discord of arbitrary two-mode Gaussian states 13 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Gaussian states 14 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States Very natural: ground and thermal states of all physical systems in the harmonic approximation regime (M.S.Kim: like orange juice and sunshine) Relevant theoretical testbeds for the study of structural properties of entanglement and correlations, thanks to the symplectic formalism Preferred resources for experimental unconditional implementations of continuous variable protocols Crucial role and remarkable control in quantum optics -coherent states -squeezed states -thermal states

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Gaussian operations Gaussian states can be efficiently: displaced (classical currents) squeezed (nonlinear crystals) rotated (phase plates, beam splitters) measured (homodyne detection) 15 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Gaussian operations Gaussian states can be efficiently: displaced (classical currents) squeezed (nonlinear crystals) rotated (phase plates, beam splitters) measured (homodyne detection) 16 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Gaussian operations Gaussian states can be efficiently: displaced (classical currents) squeezed (nonlinear crystals) rotated (phase plates, beam splitters) measured (homodyne detection) 17 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Gaussian operations Gaussian states can be efficiently: displaced (classical currents) squeezed (nonlinear crystals) rotated (phase plates, beam splitters) measured (homodyne detection) 18 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Gaussian operations Gaussian states can be efficiently: displaced (classical currents) squeezed (nonlinear crystals) rotated (phase plates, beam splitters) measured (homodyne detection) 19 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Gaussian states: formalism Up to local unitaries, Gaussian states are completely specified by the covariance matrix… … or equivalently by the four symplectic invariants 20 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Gaussian POVMs 21 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States All the measurements that can be done by linear optics (appending Gaussian ancillas, manipulating with symplectic transformations, plus homodyne detection): The posterior state of A after measuring B has a covariance matrix (independent of the measurement outcome)

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Gaussian quantum discord The Gaussian quantum discord is the quantum discord of a bipartite Gaussian state where the optimization in the conditional entropy is restricted to Gaussian POVMs and can be rewritten as ▫where the symplectic eigenvalues are 22 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Gaussian quantum discord Optimal POVM: heterodyne for squeezed thermal states, homodyne for another class of states, something in- between for the other two-mode Gaussian states 23 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Discord/separability/entanglement By relating the nullity of discord with saturation of strong subadditivity of entropy, we demonstrated that (for finite mean energies) the only two-mode Gaussian states with zero Gaussian discord are product states All correlated Gaussian states (including all entangled states and all non-product separable mixed states) are quantumly correlated! This proves the truly quantum nature of Gaussian states despite their positive Wigner function… 24 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Consider this class of states (box=two-mode squeezing) ▫s: initial entanglement; r: entanglement degradation 25 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States Discord/separability/entanglement s ABCABC r

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26 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States Discord/separability/entanglement

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27 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States Discord/separability/entanglement pure 1

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Other results & comments Via the Koashi-Winter duality between entanglement and one-way classical correlations we can derive a closed formula for the Gaussian EoF of a family of three-mode Gaussian states Only in very special cases we can prove that the Gaussian quantum discord realizes the absolute minimum in the conditional entropy optimization not constrained to Gaussian POVMs (this is related to the problem of additivity of bosonic channel capacity etc…) It would be interesting to prove, or show counterexamples to it, that Gaussian POVMs are always optimal among all continuous variable measurements (including photodetection etc.) 28 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Summary The concept of quantum correlations goes beyond entanglement Quantum discord is a bona fide measure of such general quantum correlations Quantum discord can be computed for Gaussian states under Gaussian measurements All correlated Gaussian states have quantum correlations They are limited for separable states They admit upper and lower bounds as a function of the entanglement, for entangled states 29 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Open problems Maximum discord for separable states in any dimension. ▫known for qubits, numerically, to be 1/3 Al-Qasimi & James, arXiv:1007.1814 30 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Open problems Operational interpretation of discord Usefulness of quantum correlations in separable states for quantum information processing Understanding connection with other nonclassicality indicators in continuous variable systems (e.g. in terms of P function) Producing a theory of quantum correlations, with axioms to be satisfied by any valid measure of quantum correlations (e.g. nonincreasing under local operations and classical communication…) … 31 Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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Thank you Imperial College London 10/08/2010 Quantum versus Classical Correlations in Gaussian States

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