Byung-Seok Choi

Recent advances in quantum computing pose a potential threat to conventional public-key crypto systems such as RSA(Rivest-Shamir-Adleman), motivating the development of quantum key distribution systems. In free-space quantum key distribution, fiber-coupling efficiency is a critical factor because optical loss directly affects the secret key rate and ultimately limits the achievable transmission distance. Therefore, minimizing coupling loss through stable beam tracking is essential, particularly in free-space QKD platforms involving satellites, drones, and mobile terminals[1,2]. In fine beam-tracking systems, beacon signals for the fast-steering mirror control loop can be detected using either quadrant detectors or CMOS cameras. Quadrant detectors offer fast response times and are suitable for real-time feedback control, but their tracking performance can be strongly affected by the spatial profile, size, and symmetry of the incident beam[3]. In contrast, CMOS cameras provide direct beam-profile information and are less sensitive to beam-shape distortion, but they generally suffer from slower response times. In this work, we investigate a fine-tracking configuration in which a beacon laser is transmitted through a central aperture in an off-axis parabolic mirror. As a result, the reflected beam has a centrally obscured, non-ideal Gaussian profile, which may degrade the tracking accuracy of a quadrant-detector-based system. We compare the performance of quadrant-detector- and CMOS-camera-based feedback schemes for fast steering mirror control under non-ideal beam-detection conditions. Furthermore, we analyze the influence of beam size and beam-profile distortion on tracking performance and discuss optimization strategies for stable fiber coupling in free-space QKD systems.

Stable optical output power is essential in chip-based QKD transmitters because fluctuations in the emitted mean photon number directly affect security analysis and the secret key rate. Conventional monitoring at the laser diode output cannot directly reflect downstream drift from intensity modulators, variable optical attenuators, thermo-optic elements, and driving circuits. Here, we propose a homodyne detection-based optical output power stabilization method using the complementary port signal of a final Mach-Zehnder-based variable optical attenuator. As a preliminary step toward a chip-based near-output monitor, we demonstrate its feasibility with a bulk-optics proof-of-concept experiment.

We propose and experimentally demonstrate a passive polarization stabilization method for robust entanglement distribution using a cross-aligned pair of polarization-maintaining fibers (PMFs). Unlike standard single-mode fiber links, the proposed configuration inherently mitigates polarization and phase fluctuations without active feedback control. Using polarization-entangled photon pairs generated from a Sagnac interferometer, we compare SMF-based transmission and the proposed PMF configuration under stable and mechanically perturbed conditions. While the SMF-based system shows severe degradation under vibration and fiber bending, with the average visibility dropping to 0.444, the proposed PMF configuration maintains a high average visibility of 0.867 with minimal fluctuation. These results demonstrate that the proposed approach enables reliable entanglement distribution under field conditions without complex active compensation, providing a practical route toward scalable quantum communication systems.

Photonic integrated circuits (PICs) provide a compact and scalable platform for practical quantum key distribution (QKD), enabling the integration of multiple optical functions required for quantum state preparation [1, 2]. While PIC can provide compact structure integrating a variety of optical components in small space, the compact structure can cause that final output state can be sensitive to unintended optical coupling between the densely integrated optical components. In this work, we investigate inter-channel crosstalk induced by the VOA array in the PIC-based QKD transmitter which is recently reported in [3] as a free-space reference-frame-independent QKD transmitter including six laser diodes, six variable optical attenuators (VOAs), and a polarization beam combiner into a C Form-factor Pluggable 2 (CFP2) module. Each VOA is implemented using a 2 × 2 Mach–Zehnder interferometer (MZI), although only one input port and one output port are connected to the functional optical path between the laser-diode array and the polarization beam combiner. The remaining ports are redundant from the viewpoint of QKD transmitter operation, but they are inherently present because of the MZI structure. When a VOA is driven to provide high attenuation, only a small fraction of the optical signal is delivered to the intended output port, while the remaining optical power is routed toward the unused output port. We observe that this residual light can couple into adjacent VOA channels as stray light, even in the absence of a designed waveguide connection. Consequently, although the transmitter is intended to activate only a single path in a given time slot, unintended neighboring paths may also activate adjacent path, resulting in a mixture of polarization components at the transmitter output. The aforementioned effect directly distorts the generated polarization states. As the VOA driving current is varied to control the attenuation and mean photon number, the measured Stokes parameters deviate from the target polarization states, and the deviation becomes more pronounced at higher attenuation. Therefore, the usable attenuation range of the VOA in a QKD transmitter is not determined solely by its standalone attenuation performance, but is instead limited by the resulting polarization-state distortion. This finding reveals that VOA-induced inter-channel crosstalk is a critical design constraint for densely integrated PIC-based QKD transmitters. Our results suggest that future designs should suppress stray-light coupling from redundant MZI ports increasing physical and optical isolation between adjacent VOAs, or redesigning the attenuation structure to avoid unused high-power output paths. These considerations are essential for simultaneously achieving high attenuation of VOA and high fidelity of generated polarization state in PIC-based QKD systems.

In polarization-based Quantum Key Distribution (QKD), imperfections in the states of polarization (SoPs)—such as limited orthogonality within a basis and insufficient mutual unbiasedness between bases—can significantly increase the quantum bit error rate (QBER), undermining the reliability and security of key exchange. Reducing QBER is therefore critical for enhancing protocol robustness and extending the operational range of practical QKD systems [1]. This issue is particularly pronounced in compact and lightweight implementations using Photonic Integrated Circuits (PICs), where the high level of integration restricts the flexibility to optimize or substitute individual polarization-controlling components. Although some degree of electrical tunability is possible, dependence on active feedback and real-time correction compromises the intrinsic advantages of simplicity, stability, and low power consumption. A passive approach requiring minimal intervention is thus especially desirable. We present an analytic method for determining the optimal unitary transformation that minimizes QBER under given non-ideal SoPs. When the polarization states are not mutually orthogonal, it is unclear how to align or transform them to ensure secure and efficient operation. Our method addresses this ambiguity by introducing QBER minimization as a physically meaningful and practical criterion for optimizing polarization alignment, as exemplified by scenarios such as six-state polarization encoding in reference-frame-independent QKD [2]. For the optimal transformation, we derive a closed-form expression, which can be implemented using static optical elements such as waveplates or integrated passive structures [3]. The proposed framework identifies the transformation direction that maximizes QKD performance. While initially designed for fixed SoPs, it is also expected to outperform iterative methods in dynamic environments by enabling fast, precomputed correction. As such, it is particularly useful in passive or resource-constrained polarization-based QKD systems, where active control is undesirable or impractical.

Modern information and communications security has relied on public-key cryptosystems-such as RSA and ECC-based on computational complexity. With the accelerating global research and investment in quantum computing, however, warnings that these traditional schemes could be fundamentally broken by quantum algorithms such as Shor’s algorithm[1] are becoming increasingly realistic. As a result, quantum key distribution (QKD), which provides security independent of computational complexity by harnessing quantum-mechanical principles, has emerged as a promising alternative. QKD has advanced from laboratory experiments to inter-city trials, nationwide fiber-optic networks, and satellite links. Nevertheless, challenges in manufacturing, especially cost, size, and power consumption, continue to hinder the commercialization of QKD. To mitigate these issues, photonic integrated circuit (PIC) transmitters and receivers based on silicon, InP, SiN, and LiNbO_3 platforms have been developed[2,3,4]; to our knowledge, no commercial form-factor transmitter integrating both the light source and the modulator has been reported. In this work, we present the first QKD transmitter that hybrid-integrates a laser source, asymmetric delay interferometer, variable optical attenuator, and phase modulator into a CFP2 form-factor. We first measure and report the performance of each component, including the pulse characteristics depending on the system rate, the interferometer characteristics, the maximum optical attenuation, and the phase modulator’s half-wave voltage (V_pi). Furthermore, using the integrated transmitter, we implement a phase-encoding BB84 system, measure the interference visibility as a function of the phase modulator’s drive voltage and the sifted-key rate, then calculate the resulting quantum bit error rate (QBER) and secret-key rate (SKR).

Photonic integrated circuits (PICs) are emerging as a key enabler for compact, rugged and mass-producible quantum key distribution (QKD) terminals, dramatically reducing size, weight, power and cost while facilitating co-packaging with classical transceivers. However, inevitable fabrication asymmetries and on-chip optical components such as modulator, attenuator, and wavelength division multiplexer (WDM) introduce polarization dependent loss (PDL) of up to several decibels. In polarization-based QKD that employs multiple polarization states, such intrinsic PDL can distort those states and thereby degrade overall system performance. The impact is particularly severe in polarization-based reference frame independent (RFI) QKD, where the six polarization states must remain mutually unbiased and orthogonal within each basis so that the secret key rate depends only on the quantum bit error rate (QBER) and the security parameter. PDL skews the Jones amplitudes, breaks these state relations, inflates QBER and can drive the secret key rate to low values at moderate channel loss. To counter this impairment, post-selection methods non-optically compensating PDL have been proposed. In this work, we introduce an optical PDL compensation which is a passive optical compensator consisting of a matched PDL element followed by a half-wave plate that swaps the orthogonal polarization components. The combined transfer matrix is effectively polarization-isotropic apart from a uniform attenuation so both the security parameter and the secret key rate are preserved, at the expense of additional insertion loss. Experiments with a free-space RFI QKD link confirm the scheme’s effectiveness: while uncompensated PDL quickly degrades secret key raete, the compensator restores state and sustains secret key rate across the entire operating range explored, validating the effectiveness of the compensator and demonstrating a practical path toward PDL tolerant, PIC-based QKD systems.

Quantum Key Distribution (QKD) has been actively studied due to its unconditional security against eavesdropping, especially in the era of quantum computing. Among various QKD implementations, free-space QKD—which transmits and receives single photons through free-space channels—has gained significant attention due to its lower signal attenuation over long distances and its potential for establishing global quantum networks without the need for fiber-optic infrastructure. Recently, it has also emerged as a promising solution for secure communications on mobile platforms such as drones and autonomous vehicles. To integrate QKD systems into compact mobile platforms, it is essential to achieve system miniaturization and weight reduction, while also maintaining stable optical link in environments with motion and vibration. One effective approach is the use of gimbal-based beam tracking systems, as gimbals are well-established technologies that provide high stabilization performance with relatively low weight, making them ideal for mobile use. In this study, we investigate the performance of a lightweight gimbal-based beam tracking system in an outdoor environment. Lasers with visible-wavelength and CMOS cameras were used as beacon beams and detectors to implement the tracking system. To compensate for the limited angular resolution of the gimbal, beam expansion of the beacon beam and appropriate design of fine tracking is incorporated. The results demonstrate the feasibility of using gimbal-based tracking systems for free-space QKD applications on mobile platforms.

In this study, we report the performance of our polarization-based BB84 protocol QKD system using a silica-based polarization encoding chip and a fiber-based WDM filter, which can easily combine the quantum channel of 785nm with the synchronization channel of 1550nm. Among the optical transmission windows in the atmosphere, 785nm was selected as the wavelength of the quantum channel due to its availability for high-performance silicon-based single-photon avalanche diodes and laser diodes. Additionally, 1550nm was selected for beam tracking and synchronization signal for its availability in commercial transceivers and optical amplifiers. The operation speed of the system was 100 MHz. The sifted key rate and quantum bit error rate (QBER) were measured in real-time by a field programmable gate array(FPGA). The system was installed in an indoor laboratory, and the QBER and sifted key rate were measured while increasing channel attenuation to test the system’s performance in lossy environments. We obtained superior QKD performance with a QBER of 0.62% and a sifted key rate of 1.61 Mbps at no attenuation, and 5.64% and 5.69 kbps at 25dB attenuation, showing potential application for outdoor operation over longer distances.

Research on quantum key distribution (QKD) using discrete variables is intensively progressing towards commercialization. The development of Photonic Integrated Chips is essential for this success. Specifically, in fiber-based QKD systems utilizing time-bin and phase-encoding, the characteristics of the asymmetric delay line interferometer play a critical role in the system's performance. This paper investigates the variations in the extinction ratio of interferometer based on the power loss ratio between optical signals traversing the short and long paths and the time delay error in the interferometer. Additionally, we analyze how the characteristics of the input optical waveform affect the extinction ratio of the interferometer.

Since the first quantum key distribution (QKD) BB84 was proposed, various relevant QKD systems have been proposed. The systems mainly are implemented based on bulk-optics. The bulk-optics based implementation has large volume, heavy weight, high power consumption, and high cost, so that it is not easy to compatible to current communication system and is hard to be commercialized. In order to overcome the aforementioned, recently photonic integrated chip (PIC) based implementation of QKD systems have been proposed. Unfortunately, PIC based implementation has inevitable polarization depende loss (PDL), which can be induced by imperfect fabrication of waveguide, or plasma dispersion effect which can change absorption in case of a polarization modulator. Additionally, a QKD system currently cannot be implemented with only PIC so that it requires additional optical components that has PDL as well. PDL may cause the severe performance degradation of QKD, by destroying the mutually unbiasedness between the states in QKD and increasing quantum bit error rate (QBER). In this study, we analyze effect of PDL in QKD. First, we experimentally analyze state change of polarization qubit depending on PDL which can be emulated by a PDL emulator. Based on this, we theoretically calculate intrinsic quantum bit error rate (QBER) and the corresponding secure key rate of QKD.

In this work, we investigate the effect of the Kalman filter, an algorithm predicting future values of a system, for reducing pointing errors and improving the tracking performance of the coarse tracking system. We present the pointing error based on the angular velocity of the target when the Kalman filter is applied to the tracking algorithm. The tracking system is mounted on a fixed tripod, while the mobile platform moves around the system at a constant speed as a target. The effect of the Kalman filter on the performance of the tracking system and future work will be given.

In recent times, field trials of quantum key distribution (QKD) have been conducted using the existing optical fiber infrastructure. However, one significant challenge faced during these trials is ensuring the stability of QKD operation. The instability of QKD operation is caused by the two factors: random fluctuations in polarization of photon over time and time drift of the photon as it traverses the deployed optical fiber. These issues are unavoidable due to the inability to accurately estimate and control factors such as temperature, vibration, and stress in the deployed optical fiber. To address this instability, various solutions based on active or passive optics have been proposed. In this paper, we present an active optics-based simple polarization stabilizer utilizing an optical polarizer, an active polarization controller, and a single photon detector. For the fast operation, we utilize only 2 out of the 4 axes of the polarization controller for the stabilizer. The experimental results verify the stability of the stabilizer.

Quantum key distribution (QKD) provides capability of secure communication between two remote locations. Depending on its applications, for the surroundings that fiber connection between two remote locations becomes impossible, QKD should be performed through free-space. Such QKD is called as free-space QKD. The applications corresponds to moving objects such as vehicle, aircraft, and satellite. In such free-space QKD, one fundamental characteristic is that transmitter and receiver are moving in real time. In case of conventional BB84 like QKD protocols requiring an identical reference frame between the transmitter and receiver, its performance can be affected by the moving characteristic because the relative movement causes reference frame deviation between them. This can be alleviated with active compensation of the reference frame, but it makes QKD system complex. In the protocol point of view, one has been proposed and it is called as reference frame independent (RFI) QKD. However, RFI QKD is based on ideal situation such as symmetric channels depending on encoded quantum states. This usually cannot achieved in real QKD system due to device imperfections. In this paper, we theoretically analyze how the device imperfections affect on the performance RFI QKD. In order to verify the theoretical analysis, we implement a free-space RFI QKD system with practical devices and identify the effect of device imperfections on RFI QKD.

Quantum key distribution (QKD) has been widely studied for its inherent security against eavesdropping. Among them, free-space QKD has been actively studied for its wide range of applications. For global-scale quantum network, satellite-to-ground quantum key distribution has been studied in major countries around the world. Also, due to recent progress on drone and autonomous vehicle technologies and applications, short to intermediate-range applications for small moving platforms are gaining more interests than before. For applying QKD on these platforms, one of the most challenging requirements is reducing the size and weight of the QKD system, including beam tracking components. In this study, we report a compact beam tracking system and its tracking performance on a moving transmitter. The coarse tracking part of the system consists of pan-tilt module and a CMOS camera. The fine tracking part consists of a MEMS-based fast steering mirror (FSM) and a quadrant-cell photodetector module. By using compact MEMS-based FSM, the size of the system was reduced to 15 × 15 × 30 cm and can be further reduced by using smaller optical components. For testing the tracking performance, transmitter on a moving platform was placed 1 m away from the fixed tracking system and moved at a constant speed along a circular track around the tracking system. A diverging 650 nm laser source on the transmitter was used as a tracking target for both coarse and fine tracking. When tracking the target moving at angular speed of 20 mrad/s, angular error was less than 0.12° and beam tracking induced optical loss into a multimode fiber was measured to be lower than 2.5 dB.

Free-space quantum key distribution (QKD) is a promising solution for secure communication between two remote parties through free space. Due to the possibility of free space communication, in general, the application candidates of free space QKD are focused on secure communication between moving terminals. Such applications have characteristics such as moving position, outside operation, and limited internal space and power consumption. First, the moving position of a terminal needs active compensation of a shared reference between transmitter and receiver because general QKD protocols requires a shared reference frame, i.e., polarization reference in a QKD protocol using polarization encoding. This can be solved by using reference frame independent (RFI) QKD. Second, the outside operation brings intensive noise issue caused by the sun light which significantly degrades the performance of free-space QKD. The change of operating wavelength of QKD to 1550-nm and single mode fiber coupling can significantly alleviate the noise issue. Finally, the limitation of internal space requires to chip scale implementation. The use of 1550-nm wavelength provides comparability of integrated (silicon) photonic chips which is already studied in the fiber based QKD. In this paper, we provide a 1550-nm free-space RFI QKD system which incorporates the aforementioned solutions while the previous free-space RFI QKD is realized with visible wavelength. We also shows that our system achieves about 0.8% quantum bit error rate (QBER) without additional blocking out external light to the receiver through single mode fiber coupling. This low QBER indicates the possibility of daylight free-space QKD.

Free-space quantum key distribution (QKD) has received an increasing attention for its inherent secure communication between two remote systems. Most of the free-space QKD systems require integration of various signals such as quantum and beam tracking channels with different wavelengths into the same optical path for beam tracking. The beam tracking system, consisting of fast steering mirrors and position detectors, maintains optical beam path by adjusting the misalignments induced by moving terminals, vibrations and atmospheric turbulence in the free-space QKD system. Most QKD systems use dichroic mirrors for combining and separating quantum and beam tracking channels through free-space alignment. However, integration of such channels in free-space could require a large volume space, much alignment effort for mode overlap and can be easily affected by mechanical shock. In this paper, we report the effect of using a fiber-based wavelength division multiplexing (WDM) filter for integrating the quantum and beam tracking channels in free-space QKD systems. A custom-made WDM filter was used in the transmitter part of the free-space BB84 QKD system, combining a 785 nm signal for quantum channel and a 1550 nm signal for beam tracking channel. HI780 fiber was selected for common output port of the device to maintain the orthogonality of polarization states and beam quality of the quantum channel. Although 1550 nm signal can suffer from the insertion and bending loss caused by smaller core size of the HI780 fiber, we could reduce the loss as low as 1.4 dB by designing the fiber with length within about 15 cm, well straightened. We could also obtain good beam quality and mode overlap of the quantum and beam tracking channels by using the common output port of the WDM filter.