Franco N. C. Wong

Efficient single-photon counting at 1.55 µm by means of frequency upconversion

We demonstrate efficient single-photon detection at 1.55 microm by means of sum-frequency mixing with a strong pump at 1.064 microm in periodically poled lithium niobate followed by photon counting in the visible region. This scheme offers significant advantages over existing InGaAs photon counters: continuous-wave operation, higher detection efficiency, higher counting rates, and no afterpulsing. We achieved single-photon upconversion efficiency of 90% at 21.6 W of circulating power in a resonant pump cavity with a 400-mW Nd:YAG laser. We observed high background counts at strong circulating pump powers due to efficient upconversion of pump-induced fluorescence photons.

Phase-stable source of polarization-entangled photons using a polarization Sagnac interferometer

We demonstrate a simple, robust, high-flux source of polarization-entangled photons based on a polarization Sagnac interferometer, measuring 5000 polarization-entangled pairs/s/mW of pump power in a 1-nm bandwidth with 96.8% quantum-interference visibility.

First-Photon Imaging

Computing an Image Firing off a burst of laser pulses and detecting the back-reflected photons is a widely used method for constructing three-dimensional (3D) images of a scene. Kirmani et al. (p. 58, published online 29 November) describe an active imaging method in which pulsed laser light raster scans a scene and a single-photon detector is used to detect the first photon of the back-reflected laser light. Exploiting spatial correlations of photons scattered from different parts of the scene allows computation of a 3D image. Importantly, for biological applications, the technique allows the laser power to be reduced without sacrificing image quality. A computational imaging method based on photon timing enables three-dimensional imaging under low light flux conditions. Imagers that use their own illumination can capture three-dimensional (3D) structure and reflectivity information. With photon-counting detectors, images can be acquired at extremely low photon fluxes. To suppress the Poisson noise inherent in low-flux operation, such imagers typically require hundreds of detected photons per pixel for accurate range and reflectivity determination. We introduce a low-flux imaging technique, called first-photon imaging, which is a computational imager that exploits spatial correlations found in real-world scenes and the physics of low-flux measurements. Our technique recovers 3D structure and reflectivity from the first detected photon at each pixel. We demonstrate simultaneous acquisition of sub–pulse duration range and 4-bit reflectivity information in the presence of high background noise. First-photon imaging may be of considerable value to both microscopy and remote sensing.

Long-term femtosecond timing link stabilization using a single-crystal balanced cross correlator

We demonstrate a self-aligned balanced cross correlator based on a single type-II phase-matched periodically poled KTiOPO4 crystal. The birefringence of the crystal generates a walk-off between the two orthogonally polarized pulses. This enables the balancing of the cross correlator with input pulses at the same center wavelength. As a first application of this single-crystal balanced cross correlator, we stabilized a 310 m long optical fiber link for timing distribution with long-term stable 10 fs precision.

Generation of ultrabright tunable polarization entanglement without spatial, spectral, or temporal constraints

The need for spatial and spectral filtering in the generation of polarization entanglement is eliminated by combining two coherently driven type-II spontaneous parametric down-converters. The resulting ultrabright source emits photon pairs that are polarization entangled over the entire spatial cone and spectrum of emission. We detect a flux of ,12 000 polarization-entangled pairs/s per mW of pump power at 90% quantuminterference visibility, and the source can be temperature tuned for 5 nm around frequency degeneracy. The output state is actively controlled by precisely adjusting the relative phase of the two coherent pumps.

High-flux source of polarization-entangled photons from a periodically poled KTiOPO~4 parametric down-converter (5 pages)

We have demonstrated a high-flux source of polarization-entangled photons using a type-II phase-matched periodically poled KTiOPO{sub 4} parametric down-converter in a collinearly propagating configuration. We have observed quantum interference between the single-beam down-converted photons with a visibility of 99% together with a measured coincidence flux of 300 s{sup -1}/mW of pump. The Clauser-Horne-Shimony-Holt version of Bells inequality was violated with a value of 2.711{+-}0.017.

Time-bin-modulated biphotons from cavity-enhanced down-conversion.

We have generated a new type of biphoton state by cavity-enhanced down-conversion in a type-II phase-matched, periodically poled KTiOPO4 crystal. By introducing a weak intracavity birefringence, we obtained signal and idler photons whose quantum interference was modulated between singlet and triplet signatures according to their arrival-time difference. This cavity-enhanced biphoton source is spectrally bright, yielding a single-mode fiber-coupled coincidence rate of 0.7 pairs/s per mW of pump power per MHz of down-conversion bandwidth.

Extended phase matching of second-harmonic generation in periodically poled KTiOPO4 with zero group-velocity mismatch

Under extended phase-matching conditions, the first frequency derivative of the wave-vector mismatch is zero and the phase-matching bandwidth is greatly increased. We present extensive three-wave mixing measurements of the wave-vector mismatch and obtain improved Sellmeier equations for KTiOPO 4 . We observed a type-II extended phase-matching bandwidth of 100 nm for second-harmonic generation in periodically poled KTiOPO 4 , centered at the fundamental wavelength of 1584 nm. Applications in quantum entanglement and frequency metrology are discussed.

Exploiting sparsity in time-of-flight range acquisition using a single time-resolved sensor.

Range acquisition systems such as light detection and ranging (LIDAR) and time-of-flight (TOF) cameras operate by measuring the time difference of arrival between a transmitted pulse and the scene reflection. We introduce the design of a range acquisition system for acquiring depth maps of piecewise-planar scenes with high spatial resolution using a single, omnidirectional, time-resolved photodetector and no scanning components. In our experiment, we reconstructed 64 × 64-pixel depth maps of scenes comprising two to four planar shapes using only 205 spatially-patterned, femtosecond illuminations of the scene. The reconstruction uses parametric signal modeling to recover a set of depths present in the scene. Then, a convex optimization that exploits sparsity of the Laplacian of the depth map of a typical scene determines correspondences between spatial positions and depths. In contrast with 2D laser scanning used in LIDAR systems and low-resolution 2D sensor arrays used in TOF cameras, our experiment demonstrates that it is possible to build a non-scanning range acquisition system with high spatial resolution using only a standard, low-cost photodetector and a spatial light modulator.

Deterministic Controlled-NOT Gate For Single-Photon Two-Qubit Quantum Logic

We demonstrate a robust implementation of a deterministic linear-optical controlled-not gate for single-photon two-qubit quantum logic. A polarization Sagnac interferometer with an embedded 45 degrees -oriented dove prism is used to enable the polarization control qubit to act on the momentum (spatial) target qubit of the same photon. The optical controlled-not gate requires no active stabilization because the two spatial modes share a common path, and it is used to entangle the polarization and momentum qubits.