Input states for quantum gates

We examine three possible implementations of non-deterministic linear optical cnot gates with a view to an in-principle demonstration in the near future. To this end we consider demonstrating the gates using currently available sources such as spontaneous parametric down conversion and coherent states, and current detectors only able to distinguish between zero or many photons. The demonstration is possible in the co-incidence basis and the errors introduced by the non-optimal input states and detectors are analysed

Self-induced transparency and giant nonlinearity in doped photonic crystals

Photonic crystals doped with resonant atoms allow for uniquely advantageous nonlinear modes of optical propagation: (a) Self-induced transparency (SIT) solitons and multi-dimensional localized "bullets" propagating at photonic band gap frequencies. These modes can exist even at ultraweak intensities (few photons) and therefore differ substantially either from solitons in Kerr-nonlinear photonic crystals or from SIT solitons in uniform media. (b) Cross-coupling between pulses exhibiting electromagnetically induced transparency (EIT) and SIT gap solitons. We show that extremely strong correlations (giant cross-phase modulation) can be formed between the two pulses. These features may find applications in high-fidelity classical and quantum optical communications.Comment: 11 pages, 7 figures, to appear in JOSA-

Measurements of Relative Phase in Binary Mixtures of Bose-Einstein Condensates

We have measured the relative phase of two Bose-Einstein condensates (BEC) using a time-domain separated-oscillatory-field condensate interferometer. A single two-photon coupling pulse prepares the double condensate system with a well-defined relative phase; at a later time, a second pulse reads out the phase difference accumulated between the two condensates. We find that the accumulated phase difference reproduces from realization to realization of the experiment, even after the individual components have spatially separated and their relative center-of-mass motion has damped.Comment: 12 pages, 3 figure

Angle-resonant stimulated polariton amplifier

We experimentally demonstrate resonant coupling between photons and excitons in microcavities which can efficiently generate enormous single-pass optical gains approaching 100. This new parametric phenomenon appears as a sharp angular resonance of the incoming pump beam, at which the moving excitonic polaritons undergo very large changes in momentum. Ultrafast stimulated scattering is clearly identified from the exponential dependence on pump intensity. This device utilizes boson amplification induced by stimulated energy relaxation

Decoherence of Bose-Einstein condensates in traps at finite temperature

The phase diffusion of the order parameter of trapped Bose-Einstein condensates at temperatures large compared to the mean trap frequency is determined, which gives the fundamental limit of the line-width of an atom laser. In addition a prediction of the correlation time of the number fluctuations in the condensate is made and related to the phase diffusion via the fluctuation-dissipation relation.Comment: 4 pages Revtex, revised version, to appear in Phys. Rev. Letter

Combined encoding and decoupling solution to problems of decoherence and design in solid-state quantum computing

Proposals for scalable quantum computing devices suffer not only from decoherence due to the interaction with their environment, but also from severe engineering constraints. Here we introduce a practical solution to these major concerns, addressing solid state proposals in particular. Decoherence is first reduced by encoding a logical qubit into two qubits, then completely eliminated by an efficient set of decoupling pulse sequences. The same encoding removes the need for single-qubit operations, that pose a difficult design constraint. We further show how the dominant decoherence processes can be identified empirically, in order to optimize the decoupling pulses.Comment: 5 pages, Revtex4, updated, shortened version to appear in Phys. Rev. Let

Transparency induced via decay interference

Published versio

Phase control of spontaneous emission

Published versio