KR-20260063211-A - Apparatus and Method for Polarized Control in Quantum Key Distribution System

KR20260063211AKR 20260063211 AKR20260063211 AKR 20260063211AKR-20260063211-A

Abstract

An apparatus and method for correcting polarization distortion in a receiver for quantum key distribution are disclosed. The present disclosure provides a polarization distortion correction module for correcting changes in polarization states occurring in different optical paths within a quantum key distribution receiver configured based on optical fibers, a quantum key distribution receiver including the same, and a method of operation thereof. Additionally, it provides an algorithm capable of detecting and correcting changes in polarization states at high speed. Through this, stable transmission and reception of polarization signals is possible, and the effects of facilitating the construction and operation of a quantum key distribution system and increasing operating time can be obtained.

Inventors

박준범
김상혁
박철우
이학순

Assignees

에스케이텔레콤 주식회사

Dates

Publication Date: 20260507
Application Date: 20241030

Claims (9)

It includes one or more processors and memory operably coupled with one or more processors, and The above memory stores instructions that cause one or more processors to perform operations in response to the execution of instructions by one or more processors, and The above operations are, A process of sweeping a first control voltage on a polarization controller to obtain a first signal strength and a second signal strength measured at a first photodetector and a second photodetector, respectively; A process of calculating a trajectory on a Poincaré sphere representing a change in polarization state based on a change in the first control voltage according to a change in the first signal strength and the second signal strength; and A process of determining a control voltage to be applied to the polarization controller based on a point on the Poincaré sphere representing the polarization state of the calculated trajectory and the reference polarization signal. A polarization distortion correction module including
In Article 1, A polarization distortion correction module characterized in that the first photodetector and the second photodetector detect optical signals of polarization that are orthogonal to each other.
In Article 1, The above calculation process is, A process of determining a first control voltage as a first comparison voltage when the first signal strength and the second signal strength are measured to be the same value; A process of determining a first control voltage as a second comparison voltage when the first signal strength and the second signal strength are measured as different values at a certain ratio; and A polarization distortion correction module further comprising a process of calculating a change amount of the first control voltage based on the difference between the first comparison voltage and the second comparison voltage.
In Article 1, The above-mentioned decision process is, A process of calculating a first point on a calculated trajectory where the first signal strength and the second signal strength are measured to be the same value; A process of calculating a vertical plane passing through a point on the Poincaré sphere that is perpendicular to the calculated trajectory and represents the polarization state of the reference polarization signal; A process of calculating a second point where the above vertical plane and the calculated trajectory intersect; A process of calculating the amount of change of a first control voltage to be applied to the polarization controller based on the rotation angle corresponding to the movement from the first point to the second point on the calculated trajectory; A process of calculating a change in a second control voltage to be applied to a polarization controller based on a rotation angle corresponding to a movement from the second point on the vertical plane to a point on the Poincaré sphere representing the polarization state of a reference polarization signal; and A polarization distortion correction module comprising a process for determining first and second control voltages to be applied to the polarization controller based on the amount of change of the first control voltage and the amount of change of the second control voltage.
In Article 1, The above operations are, A polarization distortion correction module further comprising a process of controlling the polarization controller based on a determined control voltage.
In Article 1, The above operations are, A process of sweeping a first control voltage on a second polarization controller to obtain a third signal strength and a fourth signal strength measured at a third photodetector and a fourth photodetector, respectively. A process of calculating a trajectory on a Poincaré sphere representing a change in polarization state based on a change in the first control voltage according to a change in the third signal strength and the fourth signal strength; and A polarization distortion correction module further comprising a process of determining a control voltage to be applied to the second polarization controller based on a point on the Poincaré sphere representing the polarization state of the calculated trajectory and the reference polarization signal.
A receiver for quantum key distribution comprising a first polarization controller, a second polarization controller, a beam splitter, a first polarization beam splitter, a second polarization beam splitter, first and second photodetectors, third and fourth photodetectors, and a polarization distortion correction module, The first polarization controller is configured to output a received optical signal to the beam splitter, and The first polarizing beam splitter is positioned on the optical path through which the optical signal is input to and transmitted by the beam splitter, and The second polarization controller is positioned on the optical path where the optical signal is input to and reflected by the beam splitter, and The second polarization beam splitter is positioned on the optical path between the second polarization controller and the third and fourth photodetectors, and A receiver for quantum key distribution, wherein the polarization distortion correction module outputs a signal controlling the operation of the first polarization controller based on the signal strength output from the first and second photodetectors, and outputs a signal controlling the operation of the second polarization controller based on the signal strength output from the third and fourth photodetectors.
A receiver for quantum key distribution comprising a first polarization controller, a second polarization controller, a beam splitter, a first polarization beam splitter, a second polarization beam splitter, first and second photodetectors, third and fourth photodetectors, and a polarization distortion correction module, The first polarization controller is positioned on the optical path through which the received optical signal is input to and transmitted by the beam splitter, and The first polarization beam splitter is positioned on the optical path between the first polarization controller and the first and second photodetectors, and The second polarization controller is positioned on the optical path where the optical signal is input to and reflected by the beam splitter, and The second polarization beam splitter is positioned on the optical path between the second polarization controller and the third and fourth photodetectors, and A receiver for quantum key distribution, wherein the polarization distortion correction module outputs a signal controlling the operation of the first polarization controller based on the signal strength output from the first and second photodetectors, and outputs a signal controlling the operation of the second polarization controller based on the signal strength output from the third and fourth photodetectors.
A method performed by a polarization distortion correction module, A process of sweeping a first control voltage on a polarization controller to obtain a first signal strength and a second signal strength measured at a first photodetector and a second photodetector, respectively; A process of calculating a trajectory on a Poincaré sphere representing a change in polarization state based on a change in the first control voltage according to a change in the first signal strength and the second signal strength; and A method comprising the process of determining a control voltage to be applied to a polarization controller based on a point on a Poincaré sphere representing the polarization state of a calculated trajectory and a reference polarization signal.

Description

Apparatus and Method for Polarized Control in Quantum Key Distribution System The present disclosure relates to a polarization control device and method in a quantum key distribution system. More specifically, it relates to a technique for correcting a change in the polarization state of a receiver for quantum key distribution configured based on optical fibers. The following description merely provides background information related to the present embodiment and does not constitute prior art. Quantum Key Distribution (QKD) systems serve as a means to realize quantum cryptographic communication and play a role in securely sharing cryptographic keys. Cryptographic key information is encoded using the polarization, phase, or entanglement of light (single photon) and transmitted through a wired or wireless (free space) quantum channel. A general configuration of a conventional QKD system is illustrated in FIG. 1. Referring to FIG. 1, the transmitter and receiver are configured using optical components in free space. In this case, there are limitations in that miniaturization and integration are difficult, and precise optical alignment between optical components is required for stable polarization signal transmission. The transmitter and receiver of the QKD system can be configured based on optical fibers, and a schematic diagram of the system is shown in Fig. 2. Referring to FIG. 2, the transmitter is composed of a plurality of light sources (LD, laser diode), a beam splitter (BS, beam splitter), a variable optical attenuator (VOA, variable optical attenuator), etc. Depending on which light source is operated, an optical signal in a specific polarization state is transmitted to the receiver through a free-space quantum channel. The receiver is composed of a polarization controller (PC), a beam splitter (BS), a half-wave plate (HWP), a polarization beam splitter (PBS), and multiple photodetectors (PD) connected via optical fibers. The polarization signal received through the free-space quantum channel passes through various optical components, such as a telescope (not shown) and a collimator (not shown), and is input into a polarization controller. The polarization controller corrects for changes in the polarization state caused by atmospheric influences or disturbances as the polarization signal is transmitted through the free-space quantum channel. Additionally, it corrects for all changes in the polarization state that occur as the polarization signal passes through various optical components to point A. Subsequently, the polarized signal is split into two optical paths using a beam splitter. The vertical (V) and horizontal (H) polarized signals are separated through a polarizing beam splitter connected to PD1 and PD2 and measured by PD1 and PD2. The diagonal (D) and anti-diagonal (A) polarized signals are rotated 45° while passing through a half-wave plate along a different optical path, and then separated through a polarizing beam splitter connected to PD3 and PD4 and measured by PD3 and PD4. Meanwhile, the polarization state (e.g., shape or angle of polarization) of an optical signal passing through an optical fiber can change due to external environmental factors such as vibration, pressure, and temperature changes. Therefore, additional changes in the polarization state may occur in the A-B and A-C optical paths due to different surrounding environments. These changes can be measured using a polarimeter and then corrected by a polarization controller. However, in terms of the actual operation of a quantum key distribution system, there is a problem in that it is difficult to operate a polarimeter separately. Consequently, in the receiver configuration shown in Fig. 2, there is a problem in that it is difficult to correct for additional changes in the polarization state occurring in the A-B and A-C optical paths. Generally, when correcting polarization signal distortion, dithering is performed by adjusting the three control voltages of the polarization controller for all possible scenarios while monitoring the output of the polarization meter. However, correcting polarization signal distortion by dithering the entire polarization state presents a problem of increased calibration time. Figure 1 is a schematic diagram of a conventional QKD system that transmits and receives polarized signals for quantum key distribution. Figure 2 is a schematic diagram of a wireless QKD system configured based on optical fibers. Figure 3 is a diagram illustrating the operation of a polarization controller and the change in polarization state according to the control voltage. Figure 4(a) is a diagram illustrating an exemplary polarization state trajectory during dithering using a polarization controller when there is no additional polarization change at the receiver. Figure 4(b) is a diagram illustrating an exemplary polarization state trajectory during dithering using a polarization controller when there is an additional p