Abstract

Single-photon qubits have become increasingly important for many applications in quantum-information technology. In our work, we report a scheme for generation of entangled single-photon states coherently delocalized in two well-separated temporal modes. The proposed scheme employs strong nonclassical correlations between the Stokes and anti-Stokes photons generated via the protocol of Duan et al. [1] and represents a remotely source for Stokes time-bin qubit. The latter is generated by two consecutive phase-locked and well separated write pulses with overall duration T_w less than the atomic memory lifetime (Fig.l). After a controllable time delay, a read laser is applied which induces emission of anti-Stokes photon. The detection of the latter through single photon detectors (D_AS) guarantees the conditional projection of the Stokes photon into a single-photon state and simultaneously generates indistinguishability between different temporal modes propagating in the Stokes channel owing to the fact that the Stokes photon may have been generated by any write pulse. The Stokes beam propagates in free space before being mixed at a beam splitter (BS) with a local oscillator (LO) for time-domain balanced homodyne detection (HD). We analyze the entangled nature and nonlocality in Stokes qubit by employing the Bell's inequality formulated for two-mode Wigner function by Banaszek and Wodkiewicz [2]. We note the advantage of our mechanism consisting in entanglement of narrow-bandwidth, frequency tunable single photon with properties allowing to excite the narrow atomic resonances in contrast to the previous proposals employing spontaneous parametric down conversion (SPDC) sources which have too broad linewidth and low spectral brightness to allow excitation of atomic species. We show also that in our scheme a single Stokes photon can be coherently generated in more than two output temporal modes when a train of multi-write-pulses is used.

© 2007 IEEE