Narrow Results

A scheme of quantum concentration for unknown atomic entangled states via cavity QED is proposed. During the preparation and the joint measurement of quantum states, the cavity is only virtually excited; thus, the scheme is insensitive to the cavity field states and the cavity decay. In the meanwhile, our setup also provides a demonstration of a quantum repeater in cavity QED in principle.

We review recent experiments [M. D. Eisaman et al., Nature438 (2005) 837] demonstrating the generation of narrow-bandwidth single photons using a room-temperature ensemble of ⁸⁷Rb atoms. Our method involves creation of an atomic coherence via Raman scattering and projective measurement, followed by the coherent transfer of this atomic coherence onto a single photon using electromagnetically induced transparency (EIT). The single photons generated using this method are shown to have many properties necessary for quantum information protocols, such as narrow bandwidths, directional emission, and controllable pulse shapes. The narrow bandwidths of these single photons (~MHz), resulting from their matching to the EIT resonance (~MHz), allow them to be stored in narrow-bandwidth quantum memories. We demonstrate this by using dynamic EIT to store and retrieve the single photons in a second ensemble for storage times up to a few microseconds. We also describe recent improvements to the single-photon fidelity compared to the work by M. D. Eisaman in Nature438 (2005) 837. These techniques may prove useful in quantum information applications such as quantum repeaters, linear-optics quantum computation, and daytime free-space quantum communication.

Quantum repeaters, which are indispensable for long-distance quantum communication, are necessary for extending the entanglement from short distance to long distance; however, high-rate entanglement distribution, even between adjacent repeater nodes, has not been realized. In a recent work by [C. Jones et al., New J. Phys. 18 (2016) 083015], the entanglement distribution rate between adjacent repeater nodes was calculated for a plurality of quantum dots, nitrogen-vacancy centers in diamond, and trapped ions adopted as quantum memories inside the repeater nodes. Considering practical use, arranging a plurality of quantum memories becomes so difficult with the state-of-the art technology. It is desirable that high-rate entanglement distribution is realized with as few memory crystals as possible. Here, we propose new entanglement distribution scheme with one quantum memory based on the atomic frequency comb which enables temporal multimode operation with one crystal. The adopted absorptive-type quantum memory degrades the difficulty of multimode operation compared with the previously investigated quantum memories directly generating spin-photon entanglement. It is shown that this scheme improves the distribution rate by nearly two orders of magnitude compared with the result in [C. Jones et al., New J. Phys. 18 (2016) 083015] and the experimental implementation is close by utilizing state-of-the-art technology.

A new technique of an entangled photon pair generation and regeneration characterization using an all fiber optic scheme is investigated. The proposed system is consisted of a fiber optic ring resonator. The Kerr nonlinearity effect in the fiber ring resonator is exploited for the generation of two independent beams. The advantage of such a system is that it requires a simple arrangement without any optical pumping part and bulky optical components. Polarized light pulse trains are launched randomly into a nonlinear fiber optic ring resonator. Where the superpositions of light pulses in a nonlinear fiber optics ring resonator are randomly occurred which is formed the entangled photon pairs. A polarization controller controls polarization states of light pulses while circulating in the ring resonator. The entangled photons are seen on the avalanche photo-detector. Then the output of the entangled photon states recovery by using a fiber ring resonator incorporating an erbium-doped fiber (EDF) has been investigated. We have shown that the weak entangled photon states can be recovered after circulating in the amplified fiber optic medium. The results obtained have shown that this system can be used to achieve the recovered polarization entangled states with the obtained high gain. The amplifying noise has also been detected and seen on the spectrum output. This is affected to the entangled photon visibility, which is discussed.

A key ingredient for a practical quantum repeater is a long memory coherence time. We describe a quantum memory using the magnetically-insensitive clock transition in atomic rubidium confined in a 1D optical lattice. We observe quantum lifetimes exceeding 6 milliseconds. We also demonstrate a dozen independent quantum memory elements within a single cold sample, and describe matter-light entanglement generation involving arbitrary pairs of these elements.