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We theoretically discuss one-photon and two-photon double-slit interferences for spontaneous and stimulated parametric down-conversions. We show that the two-photon sub-wavelength interference can exist in a general spontaneous parametric down-conversion (SPDC) for both type I and type II crystals. We propose an alternative way to observe sub-wavelength interference by a joint-intensity measurement which occurs for only type I crystal in a higher gain of SPDC. When a signal beam injects into the crystal, it may create two interference patterns by two stimulated down-converted beams, showing no sub-wavelength interference effect.

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We analyze a double Λ atomic configuration interacting with two signal beams and two control beams. Because of the quantum interference between the two Λ channels, the four fields are phase-matched in electromagnetically induced transparency. Our numerical simulation shows that this system is able to manipulate synchronous optical signals, such as generation of optical twin signals, data correction, signal transfer and amplification in the atomic storage.PACS numbers: 42.50. Gy, 42.50.Hz, In modern communication, the transmission of information is usually carried out among multi-users which are organized in a network. The network consists of spatially separated nodes in which information can be stored and locally manipulated. Recently, experiments have demonstrated that optical signal can be stored in and then retrieved from an atomic ensemble[1] [2]. Light storage is implemented in a scheme of electromagnetically induced transparency (EIT) in which the atoms with a Λ configuration interact resonantly with both a signal beam and a control beam [3]. Theoretically, it has been proved that the scheme can also be used as a quantum memory[4]- [9]. Then, EIT is extended to a double Λ configuration in which two couples of probe and control beams interact resonantly with a four-level atom[10]- [16]. In this model the two Λ subsystems share a common dark state and the quantum interference exists not only between the two lower states, but also between the two Λ channels. Therefore, the EIT effect occurs only when the ratio of Rabi frequencies in each Λ channel is equal. Ref.[10] pointed out the lossless propagation of shape matched probe pulses interacting with one Λ channel when the strong identical pulses drive another Λ channel. However, the storage mechanism of optical pulse in the standard EIT interaction can be applied to double Λ configuration where two probe pulses can

We use two-mode squeezed vacuum state as a quantum channel to teleportate the quantum squeezed vacuum state and analyze the fidelity of the teleportation by the Wigner function. The result shows that the squeeze factor of the quantum squeezed vacuum state is smaller, the quantum features of the teleportation state is more easily preserved.