We design several single-photon-sources based on the emission of a quantum dot embedded in a semiconductor (GaAs) nanowire. Through various taper designs, we engineer the nanowire ends to realize efficient metallic-dielectric mirrors and to reduce the divergence of the far-field radiation diagram. Using fully-vectorial calculations and a comprehensive Fabry-Perot model, we show that various realistic nanowire geometries may act as nanoantennas (volume of approximately 0.05 lambda(3)) that assist funnelling the emitted photons into a single monomode channel. Typically, very high extraction efficiencies above 90% are predicted for a collection optics with a numerical aperture NA=0.85. In addition, since no frequency-selective effect is used in our design, this large efficiency is achieved over a remarkably broad spectral range, Deltalambda=70 nm at lambda=950 nm.

We show that very large nonlocal nonlinear interactions between pairs of colliding slow-light pulses can be realized in atomic vapors in the regime of electromagnetically induced transparency. These nonlinearities are mediated by strong, long-range dipole-dipole interactions between Rydberg states of the multi-level atoms in a ladder configuration. In contrast to previously studied schemes, this mechanism can yield a homogeneous conditional phase shift of π even for weakly focused singlephoton pulses, thereby allowing a deterministic realization of the photonic phase gate.PACS numbers: 42.50.Gy, 03.67.LxWhether or not quantum information processing and quantum computing [1] become practical technologies crucially depends on the ability to implement highfidelity quantum logic gates in a scalable way [2]. Among alternative routes to this challenging goal, are of particular interest the schemes operating with photons as qubits [3,4], since photons are ideal carriers of quantum information in terms of transfer rates, distances and scalability. A current trend makes use of linear optical elements and photodetectors for the implementation of key components of quantum communications and information processing in a probabilistic way [4]. The desirable objective though is a deterministic realization of entangling operations between individual photons, which require sufficiently strong nonlinearities or long interaction times. These are achievable, at the single-photon level, by tight spatial confinement of the photons, in the very demanding regime of strong atom-field coupling in high-Q cavities [5].A promising alternative is to enhance both the nonlinear susceptibility and interaction time, by employing the ultra-slow light propagation in resonant media subject to electromagnetically induced transparency (EIT) [6,7,8]. In a pioneering work, Schmidt and Imamoglu have suggested the possibility of enhanced, non-absorptive, crossphase modulation of two weak fields in the EIT regime [9], provided their interaction time is long enough. However, upon entering the EIT medium light pulses become spatially compressed by the ratio of group velocity v to the vacuum speed of light c [10], so that the interaction time of two colliding pulses is a constant independent of v. In order to maximize this time, copropagating pulses with nearly matched group velocities have been proposed [11,12]. The essential drawback of such an approach is the spatial inhomogeneity of the conditional phase shift, causing spectral broadening of the interacting pulses, thereby preventing the realization of a highfidelity quantum phase gate. Alternative approaches free of spectral broadening have been suggested [13,14,15]. In all of them, however, a rather tight transverse confinement through waveguiding or focusing of the pulses, close to the diffraction limit of λ 2 , is needed in order to attain a phase shift of π, which is technically challenging.When the light pulses enter EIT media, photonic excitations are temporarily transferred to atomic excitations ...

It has been proposed that the adiabatic loading of a Bose-Einstein Condensate (BEC) into an optical lattice via the Mott-insulator transition can be used to initialize a quantum computer [D. Jaksch, et al., Phys. Rev. Lett. 81, 3108 (1998)]. The loading of a BEC into the lattice without causing band excitation is readily achievable; however, unless one switches on an optical lattice very slowly, the optical lattice causes a phase to accumulate across the condensate. We show analytically and numerically that a cancellation of this effect is possible by adjusting the harmonic trap force-constant of the magnetic trap appropriately, thereby facilitating quick loading of an optical lattice for quantum computing purposes. A simple analytical theory is developed for a non-stationary BEC in a harmonic trap.

Using a fully vectorial frequency-domain aperiodic Fourier modal method, we study nanowire metallic mirrors and their photonic performance. We show that the performance of standard quarter-wave Bragg mirrors at subwavelength diameters is surprisingly poor, while engineered metallic mirrors that incorporate a thin dielectric adlayer may offer reflectance larger than 90% even for diameters as small as lambda/5.

We study the giant Kerr nonlinear interaction between two ultraweak optical fields in which the cross-phasemodulation is not accompanied by spectral broadening of the interacting pulses. This regime is realizable in atomic vapors, when a weak probe pulse, upon propagating through the electromagnetically induced transparency ͑EIT͒ medium, interacts with a signal pulse that is dynamically trapped in a photonic band gap created by spatially periodic modulation of its EIT resonance. We find that large conditional phase shifts and entanglement between the signal and probe fields can be obtained with this scheme. The attainable phase shift, accompanied by negligible absorption and quantum noise, is shown to allow a high-fidelity realization of the controlledphase universal logic gate between two single-photon pulses.