Electromagnetically induced transparency in inhomogeneously broadened solid media.

We study, theoretically and experimentally, electromagnetically induced transparency (EIT) in two different solid-state systems. Unlike many implementations in homogeneously broadened media, these systems exhibit inhomogeneous broadening of their optical and spin transitions typical of solid-state materials. We observe EIT lineshapes typical of atomic gases, including a crossover into the regime of Autler-Townes splitting, but with the substitution of the inhomogeneous widths for the homogeneous values. We obtain quantitative agreement between experiment and theory for the width of the transparency feature over a range of optical powers and inhomogeneous linewidths. We discuss regimes over which analytical and numerical treatments capture the behavior. As solid-state systems become increasingly important for scalable and integratable quantum optical and photonic devices, it is vital to understand the effects of the inhomogeneous broadening that is ubiquitous in these systems. The treatment presented here can be applied to a variety of systems, as exemplified by the common scaling of experimental results from two different systems.

Photon number squeezing in repeated parametric downconversion with ancillary photon-number measurements.

We present a realistic numerical simulation of a source of number-squeezed photon states employing a cavity-based parametric downconversion (PDC) process. A cavity containing the PDC medium is pumped repeatedly. The cavity recycles only one of the PDC output modes, allowing it to be amplified with each subsequent pump pulse. A photon number resolved (PNR) measurement is made on the other PDC output mode following each pump pulse. Once the PNR measurements indicate that the target number of photons has accumulated in the cavity, the pumping is stopped and the resulting photon state is released. The photon number uncertainty in the resulting state is ~3 dB below that of a mean-equivalent coherent state and furthermore the probability of generating the target photon number is similarly increased.

Topologically robust transport of photons in a synthetic gauge field.

Electronic transport is localized in low-dimensional disordered media. The addition of gauge fields to disordered media leads to fundamental changes in the transport properties. We implement a synthetic gauge field for photons using silicon-on-insulator technology. By determining the distribution of transport properties, we confirm that waves are localized in the bulk and localization is suppressed in edge states. Our system provides a new platform for investigating the transport properties of photons in the presence of synthetic gauge fields.

Implementation of generalized quantum measurements for unambiguous discrimination of multiple non-orthogonal coherent states.

Generalized quantum measurements implemented to allow for measurement outcomes termed inconclusive can perform perfect discrimination of non-orthogonal states, a task which is impossible using only measurements with definitive outcomes. Here we demonstrate such generalized quantum measurements for unambiguous discrimination of four non-orthogonal coherent states and obtain their quantum mechanical description, the positive-operator valued measure. For practical realizations of this positive-operator valued measure, where noise and realistic imperfections prevent perfect unambiguous discrimination, we show that our experimental implementation outperforms any ideal standard-quantum-limited measurement performing the same non-ideal unambiguous state discrimination task for coherent states with low mean photon numbers.

Storage and retrieval of collective excitations on a long-lived spin transition in a rare-earth ion-doped crystal.

Robust, long-lived optical quantum memories are important components of many quantum information and communication protocols. We demonstrate coherent generation, storage, and retrieval of excitations on a long-lived spin transition via spontaneous Raman scattering in a rare-earth ion-doped crystal. We further study the time dynamics of the optical correlations in this system. This is the first demonstration of its kind in a solid and an enabling step toward realizing a solid-state quantum repeater.

Ancilla-assisted calibration of a measuring apparatus.

A quantum measurement can be described by a set of matrices, one for each possible outcome, which represents the positive operator-valued measure (POVM) of the sensor. Efficient protocols of POVM extraction for arbitrary sensors are required. We present the first experimental POVM reconstruction that takes explicit advantage of a quantum resource, i.e., nonclassical correlations with an ancillary state. A POVM of a photon-number-resolving detector is reconstructed by using strong quantum correlations of twin beams generated by parametric down-conversion. Our reconstruction method is more statistically robust than POVM reconstruction methods that use classical input states.

Invited review article: Single-photon sources and detectors.

We review the current status of single-photon-source and single-photon-detector technologies operating at wavelengths from the ultraviolet to the infrared. We discuss applications of these technologies to quantum communication, a field currently driving much of the development of single-photon sources and detectors.

Experimental realization of a low-noise heralded single-photon source.

We present a heralded single-photon source with a much lower level of unwanted background photons in the output channel by using the herald photon to control a shutter in the heralded channel. The shutter is implemented using a simple field programable gate array controlled optical switch.

Heralded, pure-state single-photon source based on a Potassium Titanyl Phosphate waveguide.

We analyze the generation of single spatial mode, spectrally uncorrelated photon pairs via type II spontaneous parametric down-conversion in a Potassium Titanyl Phosphate (KTP) waveguide using real experimental parameters. We show that this source can be used as an efficient, heralded, pure-state single-photon source.

Enhancing image contrast using coherent states and photon number resolving detectors.

We experimentally map the transverse profile of diffractionlimited beams using photon-number-resolving detectors.We observe strong compression of diffracted beam profiles for high detected photon number. This effect leads to higher contrast than a conventional irradiance profile between two Airy disk-beams separated by the Rayleigh criterion.

Scalable multiplexed detector system for high-rate telecom-band single-photon detection.

We present an actively multiplexed photon-counting detection system at telecom wavelengths that overcomes the difficulties of photon-counting at high rates. We find that for gated detectors, the heretofore unconsidered deadtime associated with the detector gate is a critical parameter, that limits the overall scalability of the scheme to just a few detectors. We propose and implement a new scheme that overcomes this problem and restores full scalability that allows an order of magnitude improvement with systems with as few as 4 detectors. When using just two multiplexed detectors, our experimental results show a 5x improvement over a single detector and a greater than 2x improvement over multiplexed schemes that do not consider gate deadtime.

Measuring optical tunneling times using a Hong-Ou-Mandel interferometer.

We report a prediction for the delay measured in an optical tunneling experiment using Hong-Ou-Mandel (HOM) interference, taking into account the Goos-Hänchen shift generalized to frustrated total internal reflection situations. We precisely state assumptions under which the tunneling delay measured by an HOM interferometer can be calculated. We show that, under these assumptions, the measured delay is the group delay, and that it is apparently 'superluminal' for sufficiently thick air gaps. We also show how an HOM signal with multiple minima can be obtained, and that the shape of such a signal is not appreciably affected by the presence of the optical tunneling zone, thus ruling out the explanation of the anomalously short tunneling delays in terms of a reshaping of the wavepacket as it goes through the tunneling zone. Finally, we compare the predicted tunneling delay to a relevant classical delay and conclude that our predictions involve no non-causal effect.

Increased cross-correlation in cascaded four-wave mixing processes.

We report the measurement of increased noise cross-correlation between stokes and anti-stokes beams created in cascaded four-wave mixing processes with dual pump wavelengths. This method may be useful in creating highly correlated twin beams for various applications including quantum information processing.

Quantum state tomography of a fiber-based source of polarization-entangled photon pairs.

We report an experimental demonstration of a bright high-fidelity single-mode-optical-fiber source of polarization-entangled photon pairs. The source takes advantage of single-mode fiber optics, highly nonlinear microstructure fiber, judicious phase-matching, and the inherent stability provided by a Sagnac interferometer. With a modest average pump power (300 microW), we create all four Bell states with a detected two-photon coincidence rate of 7 kHz per bandwidth of 0.9 nm, in a spectral range of more than 20 nm. To characterize the purity of the states produced by this source, we use quantum-state tomography to reconstruct the corresponding density matrices, with fidelities of 95 % or more for each Bell state.

Phase-sensitive four-wave mixing and Raman suppression in a microstructure fiber with dual laser pumps.

On coupling two phase-coherent frequency-conjugate laser pulses into a microstructure fiber, we observe phase-sensitive photon generation via four-wave mixing, and suppression of Raman emission at the middle frequency.

Generation of cross-polarized photon pairs in a microstructure fiber with frequency-conjugate laser pump pulses.

We propose and experimentally demonstrate the generation of cross-polarized photon pairs via four-wave mixing with cross-polarized frequency-conjugate laser pump pulses. This method can be used for various quantum information applications such as the preparation of Bell-states.

Spatial and spectral mode selection of heralded single photons from pulsed parametric down-conversion.

We describe an experiment in which photon pairs from a pulsed parametric down-conversion (PDC) source were coupled into single-mode fibers with heralding efficiencies as high as 70%. Heralding efficiency or mode preparation efficiency is defined as the probability of finding a photon in a fiber in a definite state, given the detection of its twin. Heralding efficiencies were obtained for a range of down-conversion beam-size configurations. Analysis of spatial and spectral mode selection, and their mutual correlation, provides a practical guide for engineering PDC-produced single photons in a definite mode and spectral emission band. The spectrum of the heralded photons were measured for each beam configuration, to determine the interplay between transverse momentum and spectral entanglement on the preparation efficiency.

Efficient generation of correlated photon pairs in a microstructure fiber.

We report efficient generation of correlated photon pairs through degenerate four-wave mixing in a microstructure fiber. With 735.7 nm pump pulses producing correlated signal (688.5 nm) and idler (789.8 nm) photons in a 1.8 m microstructure fiber, we detect photon pairs at a rate of 37.6 kHz with a coincidence/ accidental contrast of 10:1 with deltalambda = 0.7 nm. This is the highest rate to our knowledge reported to date in a fiber-based photon source. The light source is highly nonclassical as defined by the Zou-Wang-Mandel inequality, which is violated by 1100 times the uncertainty.

Measuring absolute infrared spectral radiance with correlated visible photons: technique verification and measurement uncertainty.

An experimental system in which correlated photons for radiometric measurements were used has been set up at the National Institute of Standards and Technology. We use visible-IR pairs of correlated photons produced by means of optical parametric downconversion to measure the radiance of a high-temperature IR source at 3.415 and 4.772 mum in an intrinsically absolute manner (i.e., without requiring any externally calibrated radiometric standard). To our knowledge, this is the only radiometric method with which one measures radiance directly, instead of using radiant power and aperture geometry measurements to deduce radiance indirectly. This technique has an additional unusual characteristic: It allows absolute radiometric measurements of IR radiation to be made with high-quality visible detectors. We compare measurements made with this technique with radiance measurements made with conventional means tied to existing radiometric standards. These comparisons show an average agreement to within ~3% between the two methods. The results demonstrate an accuracy consistent with the estimated uncertainty of the currentmeasurements. This is the first time to our knowledge that this method has been used to provide absolute radiance measurements of a source that has been calibrated conventionally, revealing unexpected systematic effects and allowing estimates of the ultimate accuracy of this method. In addition, these measurements are further into the IR than any previous measurements of this process and have produced the highest thermally stimulated downconversion signal yet seen.

Calibration of a pyroelectric detector at 10.6 microm with the National Institute of Standards and Technology high-accuracy cryogenic radiometer.

The National Institute of Standards and Technology (NIST) is establishing an infrared detector calibration facility to improve radiometric standards at infrared wavelengths. The absolute response of the cryogenic bolometer that serves as the transfer standard for this facility is being linked to the NIST high- accuracy cryogenic radiometer (HACR) at a few laser wavelengths. At the 10.6-microm CO(2) laser line, this link is being established through a pyroelectric detector that has been calibrated against the HACR. We describe the apparatus, methods, and uncertainties for the calibration of this pyroelectric detector.