Chengjie Zhang

Experimental investigation of classical and quantum correlations under decoherence

It is well known that many operations in quantum information processing depend largely on a special kind of quantum correlation, that is, entanglement. However, there are also quantum tasks that display the quantum advantage without entanglement. Distinguishing classical and quantum correlations in quantum systems is therefore of both fundamental and practical importance. In consideration of the unavoidable interaction between correlated systems and the environment, understanding the dynamics of correlations would stimulate great interest. In this study, we investigate the dynamics of different kinds of bipartite correlations in an all-optical experimental setup. The sudden change in behaviour in the decay rates of correlations and their immunity against certain decoherences are shown. Moreover, quantum correlation is observed to be larger than classical correlation, which disproves the early conjecture that classical correlation is always greater than quantum correlation. Our observations may be important for quantum information processing.

Quantum discord of two-qubitXstates

Quantum discord provides a measure for quantifying quantum correlations beyond entanglement and is very hard to compute even for two-qubit states because of the minimization over all possible measurements. Recently a simple algorithm to evaluate the quantum discord for two-qubit X states was proposed by Ali, Rau, and Alber [Phys. Rev. A 81, 042105 (2010)] with minimization taken over only a few cases. Here we shall at first identify a class of X states, whose quantum discord can be evaluated analytically without any minimization, for which their algorithm is valid, and also identify a family of X states for which their algorithm fails. And then we demonstrate that this special family of X states provides furthermore an explicit example for the inequivalence between the minimization over positive operator-valued measures and that over von Neumann measurements.

Experimental investigation of the non-Markovian dynamics of classical and quantum correlations

We experimentally investigate the dynamics of classical and quantum correlations of a Bell diagonal state in a non-Markovian dephasing environment. The sudden transition from a classical to a quantum decoherence regime is observed during the dynamics of a Bell diagonal state. Due to the refocusing effect of the overall relative phase, the quantum correlation revives from near zero and then decays again in the subsequent evolution. However, the non-Markovian effect is too weak to revive the classical correlation, which remains constant in the same evolution range. With the implementation of an optical {sigma}{sub x} operation, the sudden transition from a quantum to a classical revival regime is obtained, and correlation echoes are formed. Our method can be used to control the revival time of correlations, which would be important in quantum memory.

All Entangled Pure States Violate a Single Bell’s Inequality

We show that a single Bells inequality with two dichotomic observables for each observer, which originates from Hardys nonlocality proof without inequalities, is violated by all entangled pure states of a given number of particles, each of which may have a different number of energy levels. Thus Gisins theorem is proved in its most general form from which it follows that for pure states Bells nonlocality and quantum entanglement are equivalent.

Preservation of quantum correlation between separated nitrogen-vacancy centers embedded in photonic-crystal cavities

We investigate the non-Markovian dynamics of quantum correlation between two initially entangled nitrogen-vacancy centers (NVCs) embedded in photonic-crystal cavities (PCCs). We find that a finite quantum correlation is preserved even asymptotically when the transition frequency of the NVC is within the band gap of the PCC, which is quantitatively different from the result of approaching zero under the Born-Markovian approximation. In addition, once the transition frequency of NVC is far beyond the band gap of the PCC, the quantum correlation initially prepared in NVC will be fully transferred to the reservoirs in the long-time limit. Our result reveals that the interplay between the non-Markovian effect of the structured reservoirs and the existence of emitter-field bound state plays an essential role in such quantum correlation preservation. This feature may open new perspectives for devising active decoherence-immune solid-state optical devices. DOI: 10.1103/PhysRevA.87.022312

Entanglement detection beyond the computable cross-norm or realignment criterion

Separability problem, to decide whether a given state is entangled or not, is a fundamental problem in quantum information theory. We propose a powerful and computationally simple separability criterion, which allows us to detect the entanglement of many bound entangled states. The criterion is strictly stronger than the criterion based on Bloch representations, the computable cross-norm or realignment criterion and its optimal nonlinear entanglement witnesses. Furthermore, this criterion can be generalized to an analog of permutation separability criteria in the even-partite systems.

Observable estimation of entanglement for arbitrary finite-dimensional mixed states

We present observable upper bounds of squared concurrence, which are the dual inequalities of the observable lower bounds introduced in [F. Mintert and A. Buchleitner, Phys. Rev. Lett. 98, 140505 (2007)] and [L. Aolita, A. Buchleitner and F. Mintert, Phys. Rev. A 78, 022308 (2008)]. These bounds can be used to estimate entanglement for arbitrary experimental unknown finite-dimensional states by few experimental measurements on a twofold copy

Detecting the quantum discord of an unknown state by a single observable

\rho\otimes\rho

Entanglement detection via tighter local uncertainty relations

of the mixed states. Furthermore, the degree of mixing for a mixed state and some properties of the linear entropy also have certain relations with its upper and lower bounds of squared concurrence.

Dynamics of quantum correlation between separated nitrogen-vacancy centers embedded in plasmonic waveguide.

We propose a single observable to witness the nonzero quantum discord of an unknown quantum state provided that we have four copies of the state. The expectation value of this observable provides a necessary and sufficient condition for nonzero quantum discord in