.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.

No abstract

We study the purity of correlated photon pairs generated in a dispersion-shifted fiber at various temperatures. The ratio of coincidence to accidental-coincidence counts greater than 100 can be obtained as the fiber is cooled to liquid-nitrogen temperature (77 K). We then generate polarization-entangled photon pairs by using a compact counterpropagating scheme. Two-photon interference with visibility >98% and Bell's inequality violation by >8 standard deviations of measurement uncertainty are observed at 77 K, without subtracting the accidental-coincidence counts due to background Raman photons.

We report measurement of co- and cross-polarized Raman gain spectra at the zero-dispersion wavelength of standard dispersion-shifted fiber for detunings down to 0.17 THz (5.7cm-1) on both Stokes and anti-Stokes sides by using a photon-counting technique. This technique separates the Raman scattering from the four-photon scattering. In addition, the use of a pulsed pump eliminates Brillouin scattering and the use of a Sagnac loop rejects the pump photons that spectrally spread into the detection band due to self-phase-modulation.

By utilizing a fiber-based indistinguishable photon-pair source in the 1.55 m telecommunications band ͓J. Chen et al., Opt. Lett. 31, 2798 ͑2006͔͒, we present the first, to the best of our knowledge, deterministic quantum splitter based on the principle of time-reversed Hong-Ou-Mandel quantum interference. The deterministically separated identical photons' indistinguishability is then verified by using a conventional Hong-Ou-Mandel quantum interference, which exhibits a near-unity dip visibility of 94± 1%, making this quantum splitter useful for various quantum information processing applications.The nascent field of quantum information science, motivated by the extraordinary computing power of a full-fledged quantum computer, naturally selects photons-the fundamental energy packets of the electromagnetic field-as the carrier of quantum information from one computing node to another, a task typically associated with quantum communication. The omnipresent telecommunication fibers constitute the quantum channels for quantum cryptography ͓1͔, quantum gambling ͓2͔, and quantum games ͓3͔, which utilize remote sharing of quantum entanglement as a resource. In addition, indistinguishable photons-photons having identical wave packets-play a major role in the arena of linear optical quantum computing ͓4͔. Therefore a telecom-band indistinguishable photon-pair source is particularly useful for the above quantum-information-processing ͑QIP͒ tasks. The traditional method of producing such photons-spontaneous parametric down-conversion in second-order ͑ ͑2͒ ͒ nonlinear crystals ͓5͔-faces formidable engineering challenges when these photons are launched into a single-mode optical fiber for long-distance transmission or mode cleansing. Large coupling losses inevitably occur due to mode mismatch ͓6͔, limiting the usefulness of such a source. Recently we demonstrated a fiber-based source of indistinguishable photon pairs at a telecom-band wavelength near 1550 nm that utilizes the third-order ͑ ͑3͒ ͒ nonlinear process of four-wave mixing ͑FWM͒ ͓7͔ in the fiber itself. This approach automatically takes care of the aforementioned mode-matching issue, since the photonic spatial mode of the generated photon pair is the same as that of standard optical fiber.Four-wave mixing is a third-order process mediated by the Kerr nonlinearity of optical fiber, wherein two pump photons annihilate to give birth to a pair of time-energy entangled daughter photons, usually denoted as signal and idler. Energy conservation as well as momentum conservation are obeyed during the FWM process: p1 + p2 = s + i , and k ជ p1 + k ជ p2 = k ជ s + k ជ i , where j and k ជ j stand for the frequency and wave vector of the jth photon, and the subscripts p1, p2, s, and i denote the two pump photons, the signal photon, and the idler photon, respectively. Our group has previously demonstrated generation ͓9,10͔, distribution ͓11͔, and storage ͓12͔ of nondegenerate photon pairs ͑s i͒ with a singlefrequency pump pulse ͑p1= p2͒. More recently, we have also achieved success...

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.

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 gate's 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.

We demonstrate accurate single-qubit control in an ensemble of atomic qubits trapped in an optical lattice. The qubits are driven with microwave radiation, and their dynamics tracked by optical probe polarimetry. Real-time diagnostics is crucial to minimize systematic errors and optimize the performance of single-qubit gates, leading to fidelities of 0.99 for single-qubit π rotations. We show that increased robustness to large, deliberately introduced errors can be achieved through the use of composite rotations. However, during normal operation the combination of very small intrinsic errors and additional decoherence during the longer pulse sequences precludes any significant performance gain in our current experiment.