We present a high-performance reconfigurable coincidence counting unit (CCU) using a low-end field programmable gate array (FPGA) and peripheral circuits. Because of the flexibility guaranteed by the FPGA program, we can easily change system parameters, such as internal input delays, coincidence configurations, and the coincidence time window. In spite of a low-cost implementation, the proposed CCU architecture outperforms previous ones in many aspects: it has 8 logic inputs and 4 coincidence outputs that can measure up to eight-fold coincidences. The minimum coincidence time window and the maximum input frequency are 0.47 ns and 163 MHz, respectively. The CCU will be useful in various experimental research areas, including the field of quantum optics and quantum information.

Quantum key distribution (QKD) networks constitute promising solutions for secure communication. Beyond conventional point-to-point QKD, we developed 1 × N QKD network systems with a sub-nanosecond resolution optical path length compensation scheme. With a practical plug-and-play QKD architecture and compact timing control modules based on a field-programmable gate array, we achieved long-term stable operation of a 1 × 64 QKD network system. Using this architecture, 64 users can simultaneously share secret keys with one server, without using complex software algorithms and expensive hardware. We demonstrated the workings of a 1 × 4 QKD network system using the fiber network of a metropolitan area.

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We present a true random number generator (TRNG) using dark noise of a CMOS image sensor. Because the proposed TRNG is based on the dark characteristics of the CMOS image sensor, it does not require any additional hardware, such as light source and optics, for providing true randomness. Therefore, it can be a promising solution for compact and low-cost mobile application. By using NIST SP 800-90B entropy assessment suite, we evaluate the min-entropy for the raw outputs of our original noise source and the final random numbers including post-processing as well. We also adopt NIST SP 800-22 statistical randomness test suite for the evaluation of the random numbers. The test results demonstrate that the generated random numbers pass all the statistical tests and have high entropy. INDEX TERMS Random number generation, CMOS image sensors, dark current.

Reference-Frame-Independent quantum key distribution (RFI-QKD) is known to be robust against slowly varying reference frames. However, other QKD protocols such as BB84 can also provide secrete keys if the speed of the relative motion of the reference frames is slow enough. While there has been a few studies to quantify the speed of the relative motion of the reference frames in RFI-QKD, it is not yet clear if RFI-QKD provides better performance than other QKD protocols under this condition. Here, we analyze and compare the security of RFI-QKD and BB84 protocol in the presence of the relative motion of the reference frames. In order to compare their security in real world implementation, we also consider the QKD protocols with decoy state method. Our analysis shows that RFI-QKD provides more robustness than BB84 protocol against the relative motion of the reference frames.

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The quantum key distribution (QKD) research, which is drawing attention as the next secure communication, is actively expanding from point to point system to network architecture. In QKD network system, it is important to increase the number of users who can securely communicate. Up to date, a wavelength division multiplexing (WDM) architecture has successfully expanded the number of channels without significant system loss, but there is a limitation of increasing channels considering the range of telecommunication wavelength and crosstalk noise, etc. In this paper, we propose a polarizing division method that increases user channels independently of wavelengths. The proposed architecture can increase the number of wavelength multiplexed channels by a multiple of the polarization number. We identify the issues in the QKD network system that may occur when using polarization and wavelengths simultaneously, then provide solutions and optimize the system operation accordingly. Finally, we describe a field test result of a one to many QKD network system that shows successful key exchange with 3% QBER.