We demonstrate a method to preserve entanglement and improve fidelity of three-qubit quantum states undergoing amplitude-damping decoherence using weak measurement and quantum measurement reversal. It is shown that we are able to enhance entanglement to the greatest extent, and to circumvent entanglement sudden death by increasing the weak measurement strength both for the GHZ state and the W state. The weak measurement technique can also enhance the fidelity to the quantum region and even close to 1 for the whole range of the decoherence parameter in both of the two cases. In addition, the W state can maintain more fidelity than the GHZ state in the protection protocol. However, the GHZ state has a higher success probability than the W state.
The most severe problem of a two-way ＂plug-and-play＂ （p ＆： p） quantum key distribution system is that the source can be controlled by the eavesdropper. This kind of source is defined as an ＂untrusted source＂. This paper discusses the effects of the fluctuation of internal transmittance on the final key generation rate and the transmission distance. The security of the standard BB84 protocol, one-decoy state protocol, and weak＋vacuum decoy state protocol, with untrusted sources and the fluctuation of internal transmittance are studied. It is shown that the one-decoy state is sensitive to the statistical fluctuation but weak＋vacuum decoy state is only slightly affected by the fluctuation. It is also shown that both the maximum secure transmission distance and final key generation rate are reduced when Alice＇s laboratory transmittance fluctuation is considered.
We propose two schemes for the generation of the Wn state with three atoms separately trapped in two distant cavities coupled by an optical fibre. One is implemented by controlling the interaction time, the other is implemented via the adiabatic passage. The influence of various decoherence processes, such as spontaneous emission of the utoma and photon leakages of the cavities and the optical fibre, on the fidelity is also investigated. It is found that the Wn state can be generated with high fidelity even when these decoherence processes are present.
This paper analyses a system of two independent qubits off-resonantly coupled to a common non-Maxkovian reservoir at zero temperature. Compared with the results in Markovian reservoirs, we find that much higher values of entanglement can be obtained for an initially factorized state of the two-qubit system. The maximal value of the entanglement increases as the detuning grows. Moreover, the entanglement induced by non-Maxkovian environments is more robust against the asymmetrical couplings between the two qubits and the reservoir. Based on this system, we also show that quantum state transfer can be implemented for arbitrary input states with high fidelity in the non-Markovian regime rather than the Markovian case in which only some particular input states can be successfully transferred.
A scheme is proposed where two superconducting qubits driven by a classical field interacting separately with two distant LC circuits connected by another LO circuit through mutual inductance, are used for implementing quantum gates. By using dressed states, quantum state transfer and quantum entangling gate can be implemented. With the help of the time-dependent electromagnetic field, any two dressed qubits can be selectively coupled to the data bus （the last LC circuit）, then quantum state can be transferred from one dressed qubit to another and multi-mode entangled state can also be formed. As a result, the promising perspectives for quantum information processing of mesoscopic superconducting qubits are obtained and the distributed and scalable quantum computation can be implemented in this scheme.
