In Ga As/In P single photon avalanche diodes(SPADs) are more and more available in many research fields. They are affected by afterpulsing which leads to a poor single photon detection probability. We present an In Ga As/In P avalanche photodiode with an active quenching circuit on an application specific integrated circuit(ASIC). It can quench the avalanche rapidly and then reduce the afterpulse rate. Also this quenching circuit can operate in both free-running and gated modes.Furthermore, a new technique is introduced to characterize the influence of the higher order of afterpulses, which uses a program running on a field programmable gate array(FPGA) integrated circuit.
We propose a practical entanglement concentration protocol(ECP) for a hybrid entangled state using quantum dots and a microcavity coupled system.A hybrid less-entangled state can be concentrated to a most-entangled state with a certain probability using only one ancillary single photon.Moreover,using this protocol,we can also concentrate an arbitrary three-particle less-entangled W state using two ancillary photons and classical communication.The proposed protocols provide us with a useful method to concentrate less-entangled states,which can be implemented with current technology.
We present a full quantum network scheme using a modified BB84 protocol. Unlike other quantum network schemes,it allows quantum keys to be distributed between two arbitrary users with the help of an intermediary detecting user.Moreover, it has good expansibility and prevents all potential attacks using loopholes in a detector, so it is more practical to apply. Because the fiber birefringence effects are automatically compensated, the scheme is distinctly stable in principle and in experiment. The simple components for every user make our scheme easier for many applications. The experimental results demonstrate the stability and feasibility of this scheme.
The quantum discord(QD) of a three-qubit Heisenberg XXZ system with the Dzyaloshinskii-Moriya(DM) interaction parameter is investigated,and the three-qubit QD under Ornstein-Uhlenbeck noise is calculated.We find that both DM interaction parameter D z and anisotropic parameter J z can increase QD and entanglement but D z is a more efficient parameter in the antiferromagnetic case.We observe a complex range of QD with the change of D z and a sudden change of QD with the variation of J z in the ferromagnetic case.We show that the decrease durations of both QD and entanglement can be prolonged by increasing DM interaction and reducing noise bandwidth in the evolutionary process.
