Tunable up-conversion single-photon detector at telecom wavelengths.

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.

Deterministic Secure Quantum Communication on the BB84 System.

In this paper, we propose a deterministic secure quantum communication (DSQC) protocol based on the BB84 system. We developed this protocol to include quantum entity authentication in the DSQC procedure. By first performing quantum entity authentication, it was possible to prevent third-party intervention.

Bright source of polarization-entangled photons using a PPKTP pumped by a broadband multi-mode diode laser.

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 μm. 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.

Experimental demonstration of delayed-choice decoherence suppression.

Wheeler's delayed-choice experiment illustrates vividly that the observer plays a central role in quantum physics by demonstrating that complementarity or wave-particle duality can be enforced even after the photon has already entered the interferometer. The delayed-choice quantum eraser experiment further demonstrates that complementarity can be enforced even after detection of a quantum system, elucidating the foundational nature of complementarity in quantum physics. However, the applicability of the delayed-choice method for practical quantum information protocols continues to be an open question.

Ultra-low noise single-photon detector based on Si avalanche photodiode.

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.

Experimental demonstration of decoherence suppression via quantum measurement reversal.

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.