Young-Ho Ko

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.

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.

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.

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.

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.