The Parity-Gate Based On Nitrogen-Vacancy Centers Of Realization And Application In Decoherence-Free Subspace

It is well known that quantum entanglement plays a more and more impor-tant role because of its coherence, uncertainty and space nonlocality properties. It provides an important resources for quantum communication and various compu-tational tasks. Quantum entanglement is used to realize various quantum informa-tion processing tasks, such as quantum teleportation, quantum key distribution and quantum secret sharing and so on. The superiority of quantum information and com-puting comes from the quantum coherence, but the decoherence resulting from the unavoidable couple between the system and its environment will destroy the quan-tum coherence and reduce the fidelity of entanglement. Generally, there are several methods to deal with the decoherence, namely, error correction, geometric phase, dissipative dynamics, entanglement purification and decoherence-free subspace. For the case of decoherence-free subspace, two physical qubits are used to encode one logic qubit to avoid the influence originating from the collective-dephasing noise when the system and environment have some certain symmetry.Because of the optical controllability and fine electron spin coherence even at room temperature, the nitrogen-vacancy centers coupled to the low-Q microres-onator cavity with a quantized whispering-gallery modes is a promising solid-state system in the large-scale quantum information processing. We propose a parity-check gate for logical qubits encoded in decoherence-free subspace by the coherent optical pulse input-output process with nitrogen-vacancy centers fixed on the sur-face of microtoroidal resonators with quantized whispering-gallery modes. Using the proposed parity-check gate, we achieve controlled phase flip gate, generation of quantum entangled state, and multipartite entanglement purification, we em-ploy a beam of coherent light pulse instead of single-photon pulse to accomplish the input-output process. So, the low efficiencies of single-photon resource have been successfully avoided. And, the action of the photon detector in our protocol is to distinguish vacuum state|0) from nonvacuum state|(?)α), which is more feasible than a single-photon detector or homodyne measurement in other protocols. More-over, the scheme of multipartite entanglement purification can be iter to obtain a higher success probability.