Countermeasure for security loophole caused by asymmetric correlations of reference frame independent quantum key distribution with fewer quantum states.

One of the challenging issues in free-space quantum key distribution (QKD) is the requirement of active compensation of the reference frame between the transmitter and receiver. Reference frame independent (RFI) QKD removes active compensation, but it requires more quantum states. A recent proposal can effectively reduce the required quantum states, but this can be achieved assuming the correlations defined in RFI QKD are symmetric. In a real QKD system, such symmetric correlations cannot always be satisfied owing to the device imperfections and optical misalignment. We theoretically analyze the effect of asymmetric correlations. Consequently, we report that the asymmetry causes security loopholes and provide a countermeasure to prevent them. Furthermore, we provide the experimental results of a free-space RFI QKD system to verify the countermeasure for the aforementioned problem. In conclusion, our work provides feasibility of the practical RFI QKD system with fewer quantum states by effectively preventing the security loophole.

Cost-effective 400-Gbps micro-intradyne coherent receiver using optical butt-coupling and FPCB wirings.

We present a cost-effective and bandwidth-enhanced 64-Gbaud micro-intradyne coherent receiver based on hybrid integration of InP waveguide-photodetector (WG-PD) and silica planar lightwave circuit (PLC). InP waveguide-photodetector (WG-PD) arrays are simply chip-to-chip bonded and optically butt-coupled to a silica-based dual-polarization optical hybrid chip. Multiple flexible printed circuit boards are adapted for electrical RF and DC wirings, which provide low-cost integration and good RF performance of the receiver. A 3-dB bandwidth of the fabricated coherent receiver is extended to ~36 GHz by optimization of bondwire inductance between the WG-PD array and the transimpedance amplifier (TIA), even when commercial TIAs with a typical bandwidth of ~29 GHz are used. Through optimization of the silica hybrid integrated coherent receiver, 64-Gbaud DP-16QAM signal transmission over 1050-km standard single-mode fiber is successfully demonstrated below a bit error rate of 2 × 10-3. This is the threshold for a soft decision-based forward error correction, at the optical signal to noise ratio of 23.8 dB.

Experimental filtering effect on the daylight operation of a free-space quantum key distribution.

One of the challenges of implementing free-space quantum key distribution (QKD) systems working in daylight is to remove unwanted background noise photons from sunlight. Elaborate elimination of background photons in the spectral, temporal, and spatial domains is an indispensable requirement to decrease the quantum bit error rate (QBER), which guarantees the security of the systems. However, quantitative effects of different filtering techniques and performance optimization in terms of the secure key rate have not been investigated. In this study, we quantitatively analyze how the performance of the QBER and the key rates changes for different combinations of filtering techniques in a free-space BB84 QKD system in daylight. Moreover, we optimize the conditions of filtering techniques in order to obtain the maximum secure key rate.

High-speed waveguide photodetector for 64 Gbaud coherent receiver.

A high-speed waveguide photodetector has been successfully fabricated for an integrated coherent receiver. Dual laterally tapered structures are introduced for a spot-size converter. We optimize the responsivity and the polarization-dependent loss of the spot-size converter-integrated waveguide photodetector through the beam propagation method simulation. The waveguide photodetector is designed with electrical as well as optical optimizations. The photodetector provides sufficient alignment tolerance, high responsivity of 0.73 A/W, and low polarization-dependent loss of 0.27 dB, which is in good agreement with the simulation results. By increasing the thickness of the matching layer and the n-doped upper taper, the electrical properties of the photodetector are enhanced. The photodetector has a 3 dB bandwidth of 45 GHz, providing high-speed operation. Through the electrical and optical optimizations, we successfully obtain the high-speed waveguide photodetector for a 64 Gbaud integrated coherent receiver.

Critical side channel effects in random bit generation with multiple semiconductor lasers in a polarization-based quantum key distribution system.

Most polarization-based BB84 quantum key distribution (QKD) systems utilize multiple lasers to generate one of four polarization quantum states randomly. However, random bit generation with multiple lasers can potentially open critical side channels that significantly endangers the security of QKD systems. In this paper, we show unnoticed side channels of temporal disparity and intensity fluctuation, which possibly exist in the operation of multiple semiconductor laser diodes. Experimental results show that the side channels can enormously degrade security performance of QKD systems. An important system issue for the improvement of quantum bit error rate (QBER) related with laser driving condition is further addressed with experimental results.

Simple full-range carrier frequency offset estimation for high speed CO-OFDM.

We propose a simple, full-range carrier frequency offset (CFO) algorithm for coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. By applying the Chinese remainder theorem (CRT) to training symbol of single frequency, the proposed CFO algorithm has wide range with shorter training symbol. We numerically and experimentally demonstrate the performance of CRT-based algorithms in a 16-ary quadrature amplitude modulation (QAM) CO-OFDM system. The results show that the estimation range of the CRT-based algorithm is full-range corresponding to the sampling frequency. Also, the bit error ratio (BER) degradation of the proposed algorithm with one training symbol is negligible. These results indicate that the proposed algorithm can be used as a wide range CFO estimator with an increased data rate in high speed CO-OFDM systems.

Optimization of spot-size converter for low polarization dependent loss of waveguide photodetector.

We present an optimization of spot-size converter (SSC) of waveguide photodetector (PD) for small polarization dependent loss (PDL). Beam-propagation method simulation gives responsivity for each polarization and SSC structure. From the calculated responsivity data, optimum structure of SSC is determined that can be implemented with a sufficient process tolerance. We confirm the optimization by measuring PDL of waveguide PD designed according to the structure obtained through the simulation.

Channel estimation and synchronization for polarization-division multiplexed CO-OFDM using subcarrier/polarization interleaved training symbols.

We propose and demonstrate the use of subcarrier/polarization-interleaved training symbols for channel estimation and synchronization in polarization-division multiplexed (PDM) coherent optical orthogonal frequency-division multiplexed (CO-OFDM) transmission. The principle, the computational efficiency, and the frequency-offset tolerance of the proposed method are described. We show that the use of subcarrier/polarization interleaving doubles the tolerance to the frequency offset between the transmit laser and the receiver's optical local oscillator as compared to conventional schemes. Using this method, we demonstrate 43-Gb/s PDM CO-OFDM transmission with 16-QAM subcarrier modulation over 5,200-km of ultra-large-area fiber.