Kim Fook Lee

Generation of correlated photons in nanoscale silicon waveguides

.We experimentally study the generation of correlated pairs of photons through four-wave mixing (FWM) in embedded silicon waveguides. The waveguides, which are designed to exhibit anomalous group-velocity dispersion at wavelengths near 1555 nm, allow phase matched FWM and thus efficient pair-wise generation of non-degenerate signal and idler photons. Photon counting measurements yield a coincidence-to-accidental ratio (CAR) of around 25 for a signal (idler) photon production rate of about 0.05 per pulse. We characterize the variation in CAR as a function of pump power and pump-to-sideband wavelength detuning. These measurements represent a first step towards the development of tools for quantum information processing which are based on CMOS-compatible, silicon-on-insulator technology.

Generation of high-purity telecom-band entangled photon pairs in dispersion-shifted fiber

We generate entangled photon-pairs from dispersion-shifted fiber at various temperatures. Two-photon interference with > 98% visibility and Bellpsilas inequality violation by > 8 standard deviations are observed at 77 K, without subtracting background Raman photons.

Demonstration of a Quantum Controlled-NOT Gate in the Telecommunications Band

We present the first quantum controlled-not (cnot) gate realized using a fiber-based indistinguishable photon-pair source in the 1.55 microm telecommunications band. Using this free-space cnot gate, all four Bell states are produced and fully characterized by performing quantum-state tomography, demonstrating the gates unambiguous entangling capability and high fidelity. Telecom-band operation makes this cnot gate particularly suitable for quantum-information-processing tasks that are at the interface of quantum communication and linear optical quantum computing.

Fiber-based telecom-band degenerate-frequency source of entangled photon pairs

We demonstrate the generation of polarization-entangled photon pairs of degenerate frequency for the first time, to the best of our knowledge, in standard optical fiber using a novel dual-pump, counterpropagating configuration. Two-photon interference with >97% visibility is obtained. The purity of the photon source, as characterized by the ratio of coincidence to accidental-coincidence counts, is shown to be as high as 116 under suitable operating conditions.

Characterization of fiber-generated entangled photon pairs with superconducting single-photon detectors

We demonstrate the suitability of fiber-generated entangled photon pairs for practical quantum communications in the telecom band by measuring their properties with superconducting single-photon detectors that produce negligible dark counts. The photon pairs are created in approximately 5-ps duration windows at 50 MHz rate while the detectors are operated in ungated free running mode. We obtain a coincidence to accidental-coincidence ratio >80 with raw photon-counting data, i.e., without making any post-measurement corrections. Using a previously demonstrated counter-propagating scheme we also produce polarization-entangled photon pairs at 50-MHz rate, which in coincidence detection directly yield two-photon interference with a fringe visibility >98%.

Heterodyne measurement of Wigner distributions for classical optical fields

We demonstrate a two-window heterodyne method for measuring the x-p cross correlation, ??(*)(x)? (p)?, of an optical field ? for transverse position x and transverse momentum p. This scheme permits independent control of the x and p resolution. A simple linear transform of the x-p correlation function yields the Wigner phase-space distribution. This technique is useful for both coherent and low-coherence light sources and may permit new biological imaging techniques based on transverse coherence measurement with time gating. We point out an interesting analogy between x-p correlation measurements for classical-wave and quantum fields.

Schemes for fibre-based entanglement generation in the telecom band

We investigate schemes for generating polarization-entangled photon pairs in standard optical fibres. The advantages of a double-loop scheme are explored through comparison with two other schemes, namely, the Sagnac-loop scheme and the counter-propagating scheme. Experimental measurements with the double-loop scheme verify the predicted advantages.

Measurement of co- and cross-polarized Raman spectra in silica fiber for small detunings: erratum

We present corrected versions of Fig. 5(a) and Fig. 6(b) below with the original captions.

Accurate microwave control and real-time diagnostics of neutral-atom qubits

Department of Physics and Astronomy, University of New Mexico,Albuquerque, NM 87131(Dated: November 21, 2008)We demonstrate accurate single-qubit control in an ensemble of atomic qubits trapped in an opticallattice. The qubits are driven with microwave radiation, and their dynamics tracked by opticalprobe polarimetry. Real-time diagnostics is crucial to minimize systematic errors and optimize theperformance of single-qubit gates, leading to delities of 0:99 for single-qubit ˇrotations. We showthat increased robustness to large, deliberately introduced errors can be achieved through the use ofcomposite rotations. However, during normal operation the combination of very small intrinsic errorsand additional decoherence during the longer pulse sequences precludes any signi cant performancegain in our current experiment.

Ultra stable all-fiber telecom-band entangled photon-pair source for turnkey quantum communication applications

We demonstrate a novel alignment-free all-fiber source for generating telecom-band polarization-entangled photon pairs. Polarization entanglement is created by injecting two relatively delayed, orthogonally polarized pump pulses into a piece of dispersion-shifted fiber, where each one independently engages in four-photon scattering, and then removing any distinguishability between the correlated photon-pairs produced by each pulse at the fiber output. Our scheme uses a Michelson-interferometer configuration with Faraday mirrors to achieve practically desirable features such as ultra-stable performance and turnkey operation. Up to 91.7% two-photon-interference visibility is observed without subtracting the accidental coincidences that arise from background photons while operating the source at room temperature.