Dynamical Quantum Memories

We propose a dynamical approach to quantum memories using an oscillator-cavity model. This overcomes the known difficulties of achieving high quantum input-output fidelity with storage times long compared to the input signal duration. We use a generic model of the memory response, which is applicable to any linear storage medium ranging from a superconducting device to an atomic medium. The temporal switching or gating of the device may either be through a control field changing the coupling, or through a variable detuning approach, as in more recent quantum memory experiments. An exact calculation of the temporal memory response to an external input is carried out. This shows that there is a mode-matching criterion which determines the optimum input and output mode shape. This optimum pulse shape can be modified by changing the gate characteristics. In addition, there is a critical coupling between the atoms and the cavity that allows high fidelity in the presence of long storage times. The quantum fidelity is calculated both for the coherent state protocol, and for a completely arbitrary input state with a bounded total photon number. We show how a dynamical quantum memory can surpass the relevant classical memory bound, while retaining a relatively long storage time.Comment: 16 pages, 9 figure

Continuous variable entanglement using cold atoms

We present experimental demonstration of quadrature and polarization entanglement generated via the interaction between a coherent linearly polarized field and cold atoms in a high finesse optical cavity. The non linear atom-field interaction produces two squeezed modes with orthogonal polarizations which are used to generate a pair of non separable beams, the entanglement of which is demonstrated by checking the inseparability criterion for continuous variables recently derived by Duan et al. [Phys. Rev. Lett. 84, 2722 (2000)] and calculating the entanglement of formation [Giedke et al., Phys. Rev. Lett. 91, 107901 (2003)]

Spin rings in bi-stable planar semiconductor microcavities

A unique feature of exciton-polaritons, inherited from their mixed light-matter origin, is the strongly spin-dependent polariton-polariton interaction, which has been predicted to result in the formation of spin rings in real space [Shelykh et al., Phys. Rev. Lett. 100, 116401 (2008)]. Here we experimentally demonstrate the spin bi-stability of exciton-polaritons in an InGaAs-based semiconductor microcavity under resonant optical pumping. We observe the formation of spin rings whose size can be finely controlled in a spatial scale down to the micrometer range, much smaller than the spot size. We additionally evaluate the sign and magnitude of the antiparallel polariton spin interaction constant.Comment: 5 pages, 4 figure

Twin polaritons in semiconductor microcavities

The quantum correlations between the beams generated by polariton pair scattering in a semiconductor microcavity above the parametric oscillation threshold are computed analytically. The influence of various parameters like the cavity-exciton detuning, the intensity mismatch between the signal and idler beams and the amount of spurious noise is analyzed. We show that very strong quantum correlations between the signal and idler polaritons can be achieved. The quantum effects on the outgoing light fields are strongly reduced due to the large mismatch in the coupling of the signal and idler polaritons to the external photons

Electromagnetically induced transparency in inhomogeneously broadened Lambda-transition with multiple excited levels

Electromagnetically induced transparency (EIT) has mainly been modelled for three-level systems. In particular, a considerable interest has been dedicated to the Lambda-configuration, with two ground states and one excited state. However, in the alkali-metal atoms, which are commonly used, hyperfine interaction in the excited state introduces several levels which simultaneously participate in the scattering process. When the Doppler broadening is comparable with the hyperfine splitting in the upper state, the three-level Lambda model does not reproduce the experimental results. Here we theoretically investigate the EIT in a hot vapor of alkali-metal atoms and demonstrate that it can be strongly reduced due to the presence of multiple excited levels. Given this model, we also show that a well-designed optical pumping enables to significantly recover the transparency