Analysis On Robustness And Discrimination In Quantum System

For a two-qubit system under depolarizing channel, the most robustness state and themost fragile state can be obtained for a given concurrence or negativity. Under the aidedof the evolution function of entanglement given by Kronard et al [Nat. Phys.4,99(2008)],the most robustness states and the most fragile states of the one-sided noise channel, theuniform channel and the nonuniform channel are derived. By the method of numericalsimulation, the bound states under diferent channels are derived for a given concurrenceor negativity, these bound states are the most robustness states and the most fragile state.If the entanglement approaches zero, the general nonuniform channel reduce to the uni-form channel. Under the condition that the entanglement is zero, we study the procedurefor sequential unambiguous states discrimination. A qubit is prepared in one of two al-ready known nonorthogonal state, and measured by two observers sequentially. With thehelp of auxiliary system, a necessary condition for the unambiguous sates discriminatedby the observers is the absence of entanglement. In general, the procedure to recognizethe two nonorthogonal states when the entanglement is absence is relies on the availabilityof quantum discord, while the quantum discord is precisely the quantum dissonance if theentanglement is zero. In the measurement of the first observer, the left discord is positivelycorrelated with the information extracted by the observer, and the right discord enhancethe information left for the next observer. When the product of the discords reaches itsmaximum, the probability that both observers to discriminate the states achieves its optimalvalue.