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Up-conversion single-photon detectors (UCSPD) are based on sum-frequency generation of the telecom band single-photons to near-infrared wavelengths at which efficient and low-noise silicon single-photon detectors are available. Moreover, because of high dynamic range of silicon single-photon detectors, UCSPD is suitable for high-speed quantum communication. UCSPDs reported to date, however, have a very narrow fixed window of detectable wavelengths, severely limiting their applications in wavelength-multiplexed quantum networks. In this work, we report a tunable UCSPD module that covers the complete telecom C band, making it suitable for quantum communication networks based on sharing wavelength-multiplexed entangled photons.

We report on the implementation of a Bennett-Brassard 1984 quantum key distribution protocol over a free-space optical path on an optical table. Attenuated laser pulses and Pockels cells driven by a pseudorandom number generator are employed to prepare polarization-encoded photons. The sifted key generation rate of 23.6 kbits per second and the quantum bit error rate (QBER) of 3% have been demonstrated at the average photon number per pulse μ = 0.16. This QBER is sufficiently low to extract final secret keys from shared sifted keys via error correction and privacy amplification. We also tested the long-distance capability of our system by adding optical losses to the quantum channel and found that the QBER remains the same regardless of the loss.

The decoy-state method allows the use of weak coherent pulses in quantum cryptography, and to date, various strategies for the decoy state have been proposed. Here, we experimentally compare the secret key generation rates between the one-decoy and two-decoy implementations of the Bennett-Brassard 1984 (BB84) quantum key distribution protocol through a 3.1-km optical fiber at 780 nm. Once the parameters of the experimental setup are optimized for the maximal secret key generation rate for each implementation, it is found that the two-decoy implementation outperforms the one-decoy implementation.

We propose and experimentally implement the delayed-choice decoherence suppression protocol. Using photonic entanglement, we successfully demonstrated that the choice to suppress decoherence can be delayed after decoherence and even after the detection of a qubit.

The depolarizing quantum operation plays an important role in studying the quantum noise effect and implementing general quantum operations. In this work, we report a scheme which implements a fully controllable input-state independent depolarizing quantum operation for a photonic polarization qubit.

Many paradoxes of quantum mechanics come from the fact that a quantum system can possess different features at the same time, such as in wave-particle duality or quantum superposition. In recent delayed-choice experiments, a quantum mechanical system can be observed to manifest one feature such as the wave or particle nature, depending on the final measurement setup, which is chosen after the system itself has already entered the measuring device; hence its behaviour is not predetermined. Here, we adapt this paradigmatic scheme to multi-dimensional quantum walks. In our experiment, the way in which a photon interferes with itself in a strongly non-trivial pattern depends on its polarisation, that is determined after the photon has already been detected. Multi-dimensional quantum walks are a very powerful tool for simulating the behaviour of complex quantum systems, due to their versatility. This is the first experiment realising a multi-dimensional quantum walk with a single-photon source and we present also the first experimental simulation of the Grover walk, a model that can be used to implement the Grover quantum search algorithm., Comment: 8 pages, 5 figure

Taming decoherence is essential in realizing quantum computation and quantum communication. Here we experimentally demonstrate that decoherence due to amplitude damping can be suppressed by exploiting quantum measurement reversal in which a weak measurement and the reversing measurement are introduced before and after the decoherence channel, respectively. We have also investigated the trade-off relation between the degree of decoherence suppression and the channel transmittance.

We experimentally demonstrated that the decoherence due to amplitude damping can be suppressed using quantum measurement reversal. By implementing two partial measurements, we succeeded in preferentially selecting the cases where states do not experience decoherence.

We report operation and characterization of a lab-assembled single-photon detector based on commercial silicon avalanche photodiodes (PerkinElmer C30902SH, C30921SH). Dark count rate as low as 5 Hz was achieved by cooling the photodiodes down to -80 C. While afterpulsing increased as the photodiode temperature was decreased, total afterpulse probability did not become significant due to detector's relatively long deadtime in a passively-quenched scheme. We measured photon detection efficiency higher than 50% at 806 nm., Comment: Fixed a broken figure, updated references. 6 pages, 7 figures, 1 table

We have studied experimentally the effect of a depolarizing quantum channel on polarization-encode weak pulse BB84 and SARG04 quantum cryptography. Experimental results show that, in real world conditions in which channel depolarization cannot be ignored, BB84 is more robust than SARG04 on the effect of the depolarizing quantum channel.

We report experimental studies on the effect of the depolarizing quantum channel on weak-pulse BB84 and SARG04 quantum cryptography. The experimental results show that, in real world conditions in which channel depolarization cannot be ignored, BB84 should perform better than SARG04., 4 pages, 4 figures

We report operation of a passively quenched silicon single-photon avalanche photodiode (SPAD) at extremely low dark count rate of 5 Hz. This was achieved by lowering the temperature of a PerkinElmer C30921S SPAD down to −77 °C. We found that the quantum efficiency at 780 nm remained constant over a wide range of cooling temperatures from −32 °C down to −77 °C. The after-pulsing characteristics, however, depended on the cooling temperature.

We report a weak-pulse implementation of SARG04 quantum cryptography protocol in free-space. The shifted key generation rate of 22 k bit per second and quantum bit error rate of 4% have been demonstrated. We have also studied the long-distance capability of the QKD system by adding optical losses to the quantum channel.

We report a bright source of polarization-entangled photon pairs using spontaneous parametric down-conversion (SPDC) in a 10 mm long type-II PPKTP crystal pumped by a broadband multi-mode diode laser with the coherence length of 330 mu m. Ordinarily, the huge mismatch between the pump coherence length and the PPKTP length would degrade the polarization entanglement completely. By employing the universal Bell-state synthesizer scheme, we remove the spectral/temporal distinguishability of the biphoton amplitudes entirely to recover high-visibility and high-fidelity two-photon polarization entanglement. The pair detection rates are 7,000 pairs/mW via single-mode fibers (with 99.2% fidelity) and 90,900 pairs/mW via multi-mode fibers (with 96.8% fidelity). We also analyze the scheme theoretically to show the effect of broadband multi-mode pumping on the phase matching condition of the type-II PPKTP. (C) 2016 Optical Society of America

SARG04 uses the same quantum states and the same measurement bases with BB84 protocol, but SARG04 can generate secret keys from two-photon pulses when BB84 cannot. We report the implementation of BB84 and SARG04 protocols using a polarization-encoded weak coherent pulse with an optical fiber quantum channel. We have experimentally investigated the secret key rate of BB84 and SARG04 protocols, and BB84 gave a higher secret key rate than SARG04.