Optimizing pointer states for dynamical evolution of quantum correlations under decoherence Bo You,Li-xiang Cen Department of Physics, SiChuan University

Outline 1.Dissipative channels and pointer bases 2. Two different optimizations in relation to quantum discord 2.1.Bell-diagonal state 2.2.two-qubit state of two ranks 3. Optimizing pointer bases to achieve maximal condition entropy

Dissipative channels and pointer bases Dynamical evolution under dissipative channel In this case of dephasing channel Pointer bases: selected by the system-environment coupling

Quantum discord: Where Geometry measure of Quantum Discord: Where is the set of zero-discord. Entanglement of formation: where H. Ollivier and W. H. Zurek, PhysRevLett B.Dakic,V.Vedral, and C. Brukner,Phys. Rev. Lett. 105,

Schematic illustration: dependence of decoherence dynamics on pointer bases

We investigate the dynamics of decorrelation under different choices of the pointer states of system- reservoir couplings. In detail, we consider a two- qubit system,initially prepared in certain states with non-zero quantum correlations, subjected to local dissipative channels responsible for various pointer states. Dynamical evolution of entanglement, quantum discord,and the mutual information sharing between the two qubits, is depicted. We elucidate various optimizations of the pointer states, e.g., minimization and maximization of the conditional entropy, as well as the geometric optimization via minimizing discord,and analyze the properties of the corresponding behavior of decorrelation. Motivation and main concerns

Case 1: Two different optimizations in relation to quantum discord It is proved that quantum discord and its geometry measure can be rewritten as: Where is the projector state achieving the minimize. S.L. Luo and S.S. Fu,PhysRevA Therefore we could choose the two projector states as our optimal bases,so that we could get some physics about dissipate process with different optimizations.

Bell-diagonal states We can express the state as: For Bell-diagonal state, the measurements for minimizing the quantum discord and geometry measure of QD can be easily calculated, which are the same, that is: S.L.Luo,Phys. Rev. A 77, (2008) Thus, the dynamic with the two optimizations is the same for Bell-diagonal states.

The state express as Where The minimizing discord can be obtained by purifying the state to a three-qubit pure state: In the state, the minimal conditional entropy of AC- system is equal to the entanglement of BC-system: L.X.Cen, et.al., Phys. Rev. A 83, (2011) Two qubits state of two ranks

Therefore, the minimizing conditional entropy of AC- system could be obtained by calculating the EoF of BC- system. Thus through the method conducted by Wootters(phys.rev.let ),the decomposition achieving the EoF can be deduced. So the measurement corresponding to minimizing conditional entropy can be determined by: Thus the measurement is: where is the normalized vector, and

The measurement for minimizing the geometry measure of can be obtained through the method conducted by B.Dakic(Phys.Rev.Lett ),write as: Where is the normalized eigenvector with the largest eigenvalue of,and the state is expressed as: It is clear that,generally,the measurement for minimizing condition entropy is not the one minimizing the geometry distance of discord,unless it satisfying: but the state is not necessarily the Bell-diagonal state.

the figure shows the correlation evolution in dephasing process, which set: where dashed line is the evolution with minimal geometry measure of discord,green line quantum discord.

Case 2: Optimizing pointer bases to achieve maximal condition entropy Quantum discord: If we select the pointer bases without min, otherwise, with max to conditional entropy,i.e.,how situation will be? Generally, the evolution of mutual information is slower than above choices, however the evolution of quantum discord has not easy relation, that is, max choice doesnt meaning the slower quantum discord evolution.

The right figure show the evolution of the state with: we can see QD line of max- selection is the bottle one,while the mutual information line of max selection is also the bottle one. The left figure show the evolution of quantum correlation of two- qubit of two ranks above,the red line is corresponding to max selection, we can see QD line of max-selection is the top one,while the mutual information line of max selection is the bottle one.

We could consider the amplitude damping channel, which in Kraus operator is: Select the pointer bases above, generally, we can find the evolution of the quantum discord has increasing region in max selection in above setting, showing below: