We propose a scheme to generate the entangled state of two Lambda-type three-level atoms trapped in distant cavities by using interference of polarized photons. Two possible spontaneous emission channels of each excited atom result in a coherent superposition of the states of two atoms. The subsequent detection of the different polarized photons reveals that both atoms are in different ground states, but an interference effect prevents us from distinguishing which atom is in which ground state; the atoms are thus entangled. In comparison with the original proposal of interference-induced entanglement [C. Cabrillo, J. Cirac, P. Garcia-Fernandez, and P. Zoller, Phys. Rev. A 59, 1025 (1999)]], in our scheme the weakly driven condition is not required, and the influence of atomic excitement and atomic recoil on the entanglement fidelity can be eliminated.

A scheme of atom localization based on the interference of resonance of double-dark states is proposed, in which the atom interacts with a classical standing-wave field. It is found that the localization property is significantly improved due to the interaction of double-dark resonances. It is realized that the atom is localized just at the nodes of the standing-wave field with higher precision. Moreover, an improvement by a factor of 2 in the detecting probability of a single atom within the subwavelength domain can be achieved by adjusting the probe-field detuning. This scheme shows more advantages than other schemes of atom localization.

The diamond nitrogen-vacancy (NV) center is an excellent candidate for quantum information processing, whereas entangling separate NV centers is still of great experimental challenge. We propose an one-step conditional phase flip with three NV centers coupled to a whispering-gallery mode cavity by virtue of the Raman transition and smart qubit encoding. As decoherence is much suppressed, our scheme could work for more qubits. The experimental feasibility is justified.As a promising building block for room-temperature quantum computing, 1 the nitrogen-vacancy (NV) center consisting of a substitutional nitrogen atom and an adjacent vacancy in diamond can feature near-unity quantum efficiency, a homogeneous line width, and long electronic spin decoherence time at room-temperature, 2 . Readout of spin state and single qubit gating have been achieved in optical fashion in individual NV centers, 3 and quantum information swapping and entanglement are available between electronic and the nuclear spins. 4 However, scalability is the main obstacle in such a system because entanglement of NV centers in distant diamonds has never been accomplished experimentally. Recently, Benjamin et al 5 suggested to entangle different NV electron spins by detecting the emitted photons, but met some difficulties due to the particular characteristic of the NV centers, such as the fact that 96% of the emitted photons reside in broad photon sidebands to the resonant zero phonon line (ZPL) at 637 nm even in cryogenic situation. 5 It implies that the most photons emitted from the NV centers could not effectively interfere in the beam splitter.We study a potential idea to entangle separate NV centers using the quantized whispering-gallery mode (WGM) of a fused-silica high-Q microsphere cavity. So far there has been much development in WGM cavities with, such as the microtoroidal, 6 microcylinders, 7 microdisks, 8 and microspheres. 9 Especially, microsphere cavity had gained widespread attention because of their ultrahigh Q factor (≥ 10 8 even up to 10 10 ), 10 very small volume (V m ≤ 100 µm 3 ) 10 and simple fabrication technique. In the fused-silica microsphere cavity, WGMs form via total internal reflection along the curved boundary, and the small radius of 10 µm could lead to a vacuum electric field of 150 V /cm at the sphere surface (with wavelength 600 nm) and to the Q factor exceeding 10 9 . On the other hand, the lowest-order WGM correa) Electronic sponding to the light traveling around the equator of the microsphere 11 offers predominant conditions for reaching strong coupling regime. Recent experimental progresses about the nanocrystal-microsphere system also provide experimental evidence for strong coupling between NV centers and the WGM of silica microsphere 12 or polystyrene microsphere, 13 respectively.The key point of our proposal is a conditional phase flip (CPF) on the NV center electron-spins, based on recent experimental and theoretical progresses, e.g., the possible Λ-type configuration of the optical transitions in NV cente...

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