Single pixel structured imaging through fog.

We describe the application of structured imaging with a single-pixel camera to imaging through fog. We demonstrate the use of a high-pass filter on the detected bucket signals to suppress the effects of temporal variations of fog density and enable an effective reconstruction of the image. A quantitative analysis and comparison of several high-pass filters are demonstrated for the application. Both computational ghost imaging and compressive sensing techniques were used for image reconstruction and compressive sensing was observed to give a higher reconstructed image quality.

Free space quantum key distribution using modulating retro-reflectors.

Quantum key distribution (QKD) can be used to produce a cryptographic key whose security is guaranteed by quantum mechanics. The range of fiber-based QKD links is limited, by loss, to a few hundred kilometers, and cannot be used between mobile platforms. Free space QKD can, in principle, overcome these limitations. In practice, very narrow beam divergences must be used, requiring highly accurate pointing of the transmitting terminal to the receiver. This makes deployment very difficult. Here we describe the experimental implementation of a new type of free space QKD link, using modulating retro-reflectors (MRR). The MRR-QKD link eases the pointing requirements by more than three orders of magnitude, from microradians to degrees, while maintaining the narrow beam divergence necessary for long-range communication links. The system uses new, high extinction surface-normal multiple quantum well modulators with a modulation rate of 100 MHz. A laboratory-based BB84 QKD link using multiple quantum well MRRs is demonstrated, link budgets for possible applications are discussed, and security issues are considered.

Quantum memory in warm rubidium vapor with buffer gas.

The realization of quantum memory using warm atomic vapor cells is appealing because of their commercial availability and the perceived reduction in experimental complexity. In spite of the ambiguous results reported in the literature, we demonstrate that quantum memory can be implemented in a single cell with buffer gas using the geometry where the write and read beams are nearly copropagating. The emitted Stokes and anti-Stokes photons display cross-correlation values greater than 2, characteristic of quantum states, for delay times up to 4 µs.

Spatially resolved magnetometry using cold atoms in dark optical tweezers.

We use Faraday spectroscopy of atoms confined to crossed hollow beam tweezers to map magnetic fields over 3 millimeters with 200 micron resolution in a single trap loading cycle. The hollow beams are formed using spatial light modulation, and the trap location is scanned using acousto-optic deflectors. We demonstrate the technique by mapping a linear quadrupole magnetic field with 10 nT sensitivity.

Azimuthally and radially polarized light with a nematic SLM.

We demonstrate a technique for generating azimuthally and radially polarized beams using a nematic liquid crystal spatial light modulator and a pi phase step. The technique is similar in concept to prior techniques that interfere TEM(01) and TEM(10) laser modes, but the presented technique removes the requirement of interferometric stability. We calculate an overlap integral of >0.96 with >70% efficiency from an input Gaussian mode. The technique can easily switch between beams with azimuthal and radial polarization.

Efficient excitation of the TE(01) hollow metal waveguide mode for atom guiding.

We demonstrate excitation of the azimuthally-polarized TE(01) cylindrical waveguide mode in hollow glass and metal waveguides with 780 nm light. Experimentally, we demonstrate formation of the vectorial vortex beams, and measure attenuation lengths of the TE(01) mode in hollow optical fibers with diameters of 50-100 microns. By silver-coating the inner walls of the dielectric fibers, we demonstrate a approximately 200% increase in the attenuation length.

Cold atom Raman spectrography using velocity-selective resonances.

We have studied velocity-selective resonances in the presence of a uniform magnetic field and shown how they can be used for rapid, single-shot assessment of the ground state magnetic sublevel spectrum in a cold atomic vapor. Cold atoms are released from a magneto-optical trap in the presence of a small bias magnetic field ( approximately 300 mG) and exposed to a laser field comprised of two phase-locked counterpropagating beams connecting the two ground state hyperfine manifolds. An image of the expanded cloud shows the velocity-selected resonances as distinct features, each corresponding to specific magnetic sublevel, in a direct, intuitive manner. We demonstrate the technique with both 87Rb and 85Rb, and show the utility of the technique by optically pumping into particular magnetic sublevels. The results are shown to agree with a theoretical model, and are compared to traditional Raman spectroscopy.

Optical spin initialization and nondestructive measurement in a quantum dot molecule.

The spin of an electron in a self-assembled InAs/GaAs quantum dot molecule is optically prepared and measured through the trion triplet states. A longitudinal magnetic field is used to tune two of the trion states into resonance, forming a superposition state through asymmetric spin exchange. As a result, spin-flip Raman transitions can be used for optical spin initialization, while separate trion states enable cycling transitions for nondestructive measurement. With two-laser transmission spectroscopy we demonstrate both operations simultaneously, something not previously accomplished in a single quantum dot.

A single-shot imaging magnetometer using cold atoms.

We demonstrate a technique for imaging magnetic fields using velocity-selective two-photon resonances in a cold atom cloud. Freely expanding (85)Rb atoms released from a magneto-optical trap are exposed to a brief (approximately 1 ms), off-resonant, retro-reflected laser pulse in a lin-perp-lin configuration. Two-photon resonance between magnetic sublevels occurs only for atoms in narrow velocity classes dependent on the magnetic field strength. The momentum of resonant atoms is altered by the pulse, and this two-photon momentum change is easily visible after further ballistic expansion. When the momentum pulse is applied to an atom cloud with finite size, magnetic field variations across the sample result in position-dependent features in images of the expanded cloud. We demonstrate the technique by imaging magnetic field variations over approximately 5 mm with approximately 250 microm spatial resolution.

Focusing properties of high charge number vortex laser beams.

We investigate experimentally and numerically the propagation characteristics of laser beams formed by imparting an azimuthal phase lphi to a Gaussian beam, where l is an integer. We find that when high-l beams of a finite extent are focused through a lens, the beams achieve peak intensity and are most sharply defined before and after the focal plane. Additionally, in these regions of highest intensity the effect of aberrations on the beam quality is greatly reduced, which we also demonstrate experimentally and numerically. We present a simple geometrical picture that provides excellent estimates of the beam radius and propagation distance to the plane of peak intensity.

Generation of hollow beams by using a binary spatial light modulator.

We demonstrate the generation of hollow laser beams by using a binary spatial light modulator and compare the results with those for a continuous modulator. The binary phase modulator produces beams that have continuous, azimuthally varying phase profiles and can be dynamically changed with kilohertz refresh rates. The intensity and phase profiles are recorded through the focus of an imaging lens and are compared with scalar diffraction theory. We highlight properties of the beams relevant to optical dipole traps.

Analysis and optimization of channelization architecture for wideband slow light in atomic vapors.

We carry out an analysis of an earlier proposed "channelization" architecture for wideband slow light propagation and pulse delays in atomic vapors using electromagnetically induced transparency (EIT). In the channelization architecture, a wideband input signal pulse is spatially dispersed in the transverse dimension, sent through an EIT medium consisting of an initially spin-polarized atomic vapor illuminated by a monochromatic, co-propagating pump laser, then spatially recombined. An inhomogenous magnetic field is used to Zeeman shift the atomic vapor into two-photon (Raman) resonance with the signal-pump transitions at all locations. Extending on previous analyses, we show in detail how the reconstructed pulse will be delayed only if a slight mis-match from the two-photon resonance is introduced. If the desired delay is taken as a constrained parameter, we find the bandwidth can be increased by large factor. We present an analytic treatment which optimizes the bandwidth given a desired delay and constraints on the pump power and focusing. We find bandwidth increases on the order of 5 times (100 MHz versus 20 MHz) should be possible for delays of interest (10 ns) to applications in telecommunications and radar. Interestingly, due to the mis-match requirement, we find the channelization can not increase the optimal delay-bandwidth product over conventional slow light.

Stimulated Brillouin scattering in single-mode As(2)S(3) and As(2)Se(3) chalcogenide fibers.

Stimulated Brillouin scattering was investigated for the first time in As(2)S(3) single-mode fibers, and also in As(2)Se(3). The propagation loss and numerical aperture of the fibers at 1.56 mum, along with the threshold intensity for the stimulated Brillouin scattering process were measured. From the threshold values we estimate the Brillouin gain coefficient and demonstrate record figure of merit for slow-light based applications in chalcogenide fibers.