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Register a new userGeneration of polarization-entangled photon pairs in a cascade of two type-I crystals pumped by femtosecond pulses
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We report the generation of polarization-entangled photons by femtosecond-pulse-pumped spontaneous parametric down-conversion in a cascade of two type-I crystals. Highly entangled pulsed states were obtained by introducing a temporal delay between the two orthogonal polarization components of the pump field. They exhibited high-visibility quantum interference and a large concurrence value, without the need of post-selection using narrow-bandwidth-spectral filters. The results are well explained by the theory which incorporates the space-time dependence of interfering two-photon amplitudes if dispersion and birefringence in the crystals are appropriately taken into account. Such a pulsed entangled photon well localized in time domain is useful for various quantum communication experiments, such as quantum cryptography and quantum teleportation.
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We demonstrate experimentally that spontaneous parametric down-conversion in an AlGaAs semiconductor Bragg reflection waveguide can make for paired photons highly entangled in the polarization degree of freedom at the telecommunication wavelength of 1550 nm. The pairs of photons show visibility higher than 90% in several polarization bases and violate a Clauser-Horne-Shimony-Holt Bell-like inequality by more than 3 standard deviations. This represents a significant step toward the realization of efficient and versatile self pumped sources of entangled photon pairs on-chip.
We experimentally demonstrate a polarization-entangled photon source at 810 nm using a type-II phase-matched PPKTP crystal pumped by a low-cost, broadband laser diode with a central wavelength of 405 nm and a typical bandwidth of 0.53 nm. The PPKTP crystal is placed in a Sagnac-loop to realize the compact size and high stability. The downconverted biphotons, the signal and the idler, have typical bandwidths of 5.57 nm and 7.32 nm. We prepare two Bell states |Psi+> and |Psi-> with the fidelities of 0.948+-0.004 and 0.963+-0.002. In polarization correlation measurement, the visibilities are all higher than 96.2%, and in the Bell inequality test, the S value can achieve 2.78+-0.01. To our knowledge, this experiment is the first to combine a multi-mode pump laser with a Sagnac-loop configuration. This high-quality and low-cost entangled photon source may have many practical applications in quantum information processing.
Quantum state transfer between microwave and optical frequencies is essential for connecting superconducting quantum circuits to coherent optical systems and extending microwave quantum networks over long distances. To build such a hybrid `quantum Internet, an important experiment in the quantum regime is to entangle microwave and optical modes. Based on the model of a generic cavity electro-optomechanical system, we present a heralded scheme to generate entangled microwave--optical photon pairs, which can bypass the efficiency threshold for quantum channel capacity in direct transfer protocols. The parameter regime for entanglement verification is identified that is compatible with realistic experimental settings. Our scheme is feasible given the latest experimental progress on electro-optomechanics, and can be potentially generalized to various physical systems.
Optical quantum information processing needs ultra-bright sources of entangled photons, especially from synchronizable femtosecond lasers and low-cost cw-diode lasers. Decoherence due to timing information and spatial mode-dependent phase has traditionally limited the brightness of such sources. We report on a variety of methods to optimize type-I polarization-entangled sources - the combined use of different compensation techniques to engineer high-fidelity pulsed and cw-diode laser-pumped sources, as well as the first production of polarization-entanglement directly from the highly nonlinear biaxial crystal BiB3O6 (BiBO). Using spatial compensation, we show more than a 400-fold improvement in the phase flatness, which otherwise limits efficient collection of entangled photons from BiBO, and report the highest fidelity to date (99%) of any ultrafast polarization-entanglement source. Our numerical code, available on our website, can design optimal compensation crystals and simulate entanglement from a variety of type-I phasematched nonlinear crystals.
We present a quantum mechanical description of parametric down-conversion and phase-matching of Bloch-waves in non-linear photonic crystals. We discuss the theory in one-dimensional Bragg structures giving a recipe for calculating the down-converted emission strength and direction. We exemplify the discussion by making explicit analytical predictions for the emission amplitude and direction from a one-dimensional structure that consists of alternating layers of Al0.4Ga0.6As and Air. We show that the emission is suitable for the extraction of polarization-entangled photons.
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