Planar lightwave circuit-based polarized signal transmission device for quantum key distribution and transmission method thereof

Abstract

Disclosed are a device and a method for transmitting a polarized signal for a quantum key distribution (QKD) system. A transmission device according to embodiments of the present disclosure can stably generate and transmit a plurality of polarization states only by controlling the operation of a laser light source by using an optical integrated structure composed of a planar lightwave circuit and a passive element. In particular, a transmission device according to another embodiment of the present disclosure is designed to be suitable for reference frame independent quantum key distribution (RFI-QKD), and thus generates light of different polarization states (vertical, horizontal, diagonal, anti-diagonal, right circular, left circular) through a transmission unit optical integrated module including a plurality of polarization-maintaining optical fibers, a junction unit, and an optical coupler, combines the light into a single output, and transmits the single output at a single photon level. According to the present disclosure, a transmission unit for QKD can be miniaturized and integrated without active element control, and stable polarization signal generation and highly reliable quantum key distribution can be simultaneously implemented.

Description

Planar optical waveguide-based polarization signal transmitter for quantum key distribution and transmission method thereof

[0001] The present disclosure relates to a quantum key distribution system. More specifically, it relates to a polarization signal transmitting device and a transmitting method that stably generate and transmit a plurality of polarization states using an optical integrated structure based on a planar optical waveguide.

[0002] The following description merely provides background information related to the present embodiment and does not constitute prior art.

[0003] Quantum Key Distribution (QKD) is a technology based on quantum mechanics that securely shares cryptographic keys (symmetric keys) between a sender and a receiver, and it is attracting attention as a core element of quantum communication and quantum cryptography systems.

[0004] Figure 1 is a schematic diagram illustrating the general configuration of a conventional QKD system. Referring to Figure 1, the conventional configuration can be briefly described as follows: the transmitting unit may be composed of a laser light source, a polarization coupler, a half-wave plate, a beam coupler, a variable optical attenuator (VOA), etc., and the receiving unit may be composed of a beam splitter, a half-wave plate, a polarization splitter, a single photon detector (SPD), etc.

[0005] For example, in QKD methods such as the BB84 protocol, it is essential for the transmitter to reliably generate and transmit multiple polarization states that are orthogonal to each other (e.g., vertical, horizontal, +45°, -45°). However, conventional technology has limitations in that miniaturization and integration of the transmitter and receiver are difficult due to the use of discrete optical components, and precise optical alignment between optical components is required for stable polarization signal transmission.

[0006] Meanwhile, traditional polarization-based QKD protocols (e.g., BB84) rely on the alignment and sharing of polarization reference axes (reference frames) between the sender and receiver. However, due to various factors in real-world environments, the continuous maintenance and correction of reference frames entails costs and complexity, and this burden can be further increased in multi-party network configurations.

[0007] Reference-Frame-Independent Quantum Key Distribution (RFI-QKD) is known as a method to alleviate these constraints. RFI-QKD can reduce dependence on the reference frame by generating a key on a basis insensitive to rotation of the reference frame and ensuring security by evaluating the correlation on a separate non-commutative basis. However, implementing a transmitter optical system that operates with miniaturization and low power while stably generating and maintaining six polarization states remains a challenge.

[0008] Meanwhile, Planar Lightwave Circuit (PLC) technology is a passive integrated optical system fabricated by integrating silica (SiO2)-based waveguides using semiconductor processes or equivalent microfabrication processes; it can ensure stable performance, ease of fabrication, and mass production, and is advantageous for the miniaturization and integration of QKD transmitters.

[0009] Therefore, a transmitting device for quantum key distribution and a transmitting method thereof are required, which can stably generate and transmit multiple polarization states using an optical integrated structure composed of a Planar Lightwave Circuit (PLC) and a Polarization-Maintaining Fiber (PMF).

[0010] The present disclosure aims to solve the aforementioned problems by providing a transmitting device and a transmitting method for quantum key distribution (QKD) that can stably generate and transmit multiple polarization states using a passive device based on a planar optical waveguide and a polarization-maintaining optical fiber.

[0011] In addition, another objective of the present disclosure is to improve the stability and scalability of the system by providing a transmission structure that can also be applied to a reference frame-independent quantum key distribution (RFI-QKD) protocol.

[0012] The problems that the present invention aims to solve are not limited to those mentioned above, and other unmentioned problems will be clearly understood by a person skilled in the art from the description below.

[0013] According to one aspect of the present disclosure, a transmitting device for quantum key distribution is provided, comprising: a plurality of light sources; a plurality of polarization-maintaining optical fibers connected one-to-one to the plurality of light sources to transmit light generated from the plurality of light sources; an optical coupler that combines light of different polarization states input from the plurality of polarization-maintaining optical fibers into a single optical path; and a variable optical attenuator for reducing the intensity of light output from the optical coupler, wherein the optical coupler is formed using a planar optical waveguide, and the plurality of polarization-maintaining optical fibers each have their slow-axis rotated at different angles and connected to a plurality of input terminals of the optical coupler.

[0014] The slow axes of the above plurality of polarization-maintaining optical fibers can be aligned according to a quantum key distribution protocol.

[0015] The output terminal of the above optical coupler can be connected to the variable optical attenuator through a single-mode optical fiber.

[0016] The above-described transmitting device may further include a transmitting control unit that generates a random number, determines a polarization corresponding to the random number, and controls the operation of the plurality of light sources so that light is generated in only one of the plurality of light sources based on the determined polarization.

[0017] According to another aspect of the present disclosure, a transmitter optical integrated module for reference frame-independent quantum key distribution is provided, comprising: an optical fiber block that fixes and supports a plurality of polarization-maintaining optical fibers; a junction that is optically joined to the optical fiber block; and a planar optical waveguide-based optical coupler that combines light of different polarization states input through the junction into a single optical path.

[0018] The above joint may be configured such that an isotropic film or a quarter-wave delay plate is disposed corresponding to each polarization-maintaining optical fiber, and may further include an anti-reflection layer formed at least one of the output end of the optical fiber block or the input end of the optical coupler.

[0019] The above-described transmitting optical integration module is configured such that when linearly polarized light is input to the junction through the plurality of polarization-maintaining optical fibers, the light input through the polarization-maintaining optical fiber with an isotropic film is transmitted to the optical coupler while maintaining a linear polarization state, and the light input through the polarization-maintaining optical fiber with a quarter-wave delay plate is converted into right-circular polarization or left-circular polarization and transmitted to the optical coupler.

[0020] According to another aspect of the present disclosure, a transmitting device for reference frame-independent quantum key distribution comprises: a plurality of light sources; a transmitting optical integration module; and a variable light attenuator for controlling the intensity of light output from the transmitting optical integration module, wherein the plurality of light sources includes a first light source, a second light source, a third light source, a fourth light source, a fifth light source, and a sixth light source, and the light output from the plurality of light sources is input to the transmitting optical integration module through the plurality of polarization-maintaining optical fibers.

[0021] According to the embodiments of the present disclosure, the miniaturization and integration of the QKD transmitter are facilitated, and mass production and price competitiveness can be secured by adopting a planar optical waveguide based on a semiconductor process. In addition, a stable polarization signal can be provided without control through active components, and since optical alignment during manufacturing or periodic realignment during operation is not required, the burden on manufacturing and operation can be significantly reduced.

[0022] In addition, according to another embodiment of the present disclosure, the implementation of the RFI-QKD transmitter is easy, and stable quantum key distribution is possible even in long-distance or multi-party network environments.

[0023] The effects of the present disclosure are not limited to those mentioned above, and other unmentioned effects will be clearly understood by a person skilled in the art from the description below.

[0024] Figure 1 is a schematic diagram illustrating the transmitting and receiving optical systems of a conventional quantum key distribution (QKD) system.

[0025] FIG. 2 is a block diagram of a transmitting device for QKD according to one embodiment of the present disclosure.

[0026] FIG. 3 is a plan view of a flat optical waveguide according to one embodiment of the present disclosure.

[0027] Figure 4 is a cross-sectional view in the A-A' direction of Figure 3.

[0028] FIG. 5 is an exemplary diagram showing the polarization state of light output from an optical coupler according to the rotation angle of the polarization axis of a polarization-maintaining optical fiber according to one embodiment of the present disclosure.

[0029] FIG. 6 is a flowchart of a polarization signal transmission method performed by a transmission device for QKD according to one embodiment of the present disclosure.

[0030] FIG. 7 is a schematic diagram illustrating the transmitter and receiver optical systems of a conventional reference frame-independent quantum key distribution (RFI-QKD) system.

[0031] FIG. 8 is a block diagram of a transmitter for RFI-QKD according to another embodiment of the present disclosure.

[0032] FIG. 9a is a plan view of a transmitting optical integrated module according to another embodiment of the present disclosure.

[0033] FIG. 9b is a cross-sectional view in the A-A' direction of FIG. 9a, illustrating a state in which a plurality of polarization-maintaining optical fibers are fixed to an optical fiber block and the input and output polarization states of a transmitting optical integration module.

[0034] FIG. 9c is a plan view showing an embodiment in which an anti-reflective layer is additionally formed on the joint shown in FIG. 9a.

[0035] FIG. 10a is a plan view of a transmitting optical integrated module according to another embodiment of the present disclosure.

[0036] FIG. 10b is a cross-sectional view in the A-A' direction of FIG. 10a, illustrating a state in which a plurality of polarization-maintaining optical fibers are fixed to an optical fiber block and the input and output polarization states of a transmitting optical integration module.

[0037] FIG. 11 is a flowchart illustrating the operation method of a transmitter for RFI-QKD according to another embodiment of the present disclosure.

[0038] Some embodiments of the present disclosure are described in detail below with reference to exemplary drawings. It should be noted that in assigning reference numerals to the components of each drawing, the same components are given the same reference numeral whenever possible, even if they are shown in different drawings. Furthermore, in describing the present disclosure, if it is determined that a detailed description of related known components or functions could obscure the essence of the present disclosure, such detailed description is omitted.

[0039] In describing the components of the embodiments according to the present disclosure, symbols such as first, second, i), ii), a), b), etc., may be used. These symbols are intended only to distinguish the components from other components, and the essence, order, or sequence of the components is not limited by the symbols. When a part in the specification is described as 'comprising' or 'having' a component, this means that, unless explicitly stated otherwise, it does not exclude other components but may include additional components.

[0040] The detailed description set forth below, together with the accompanying drawings, is intended to describe exemplary embodiments of the present disclosure and is not intended to represent the only embodiment in which the present disclosure may be practiced.

[0041] In this specification, 'polarization signal' refers to an optical signal to which a specific polarization has been applied.

[0042] Quantum Key Distribution (QDB) is a communication technology based on the principles of quantum mechanics in which a transmitter and a receiver exchange light signals at the single-photon level through a quantum channel to share a quantum cryptographic key. In this QDB, '0' and '1' signals are represented using the polarization state of photons, and information is transmitted based on this. Therefore, it is crucial for the transmitter to reliably generate various polarization states.

[0043] The present disclosure provides a transmitting device and a transmitting method for quantum key distribution (QKD) capable of stably generating and transmitting multiple polarization states using passive components based on planar optical waveguides and polarization-maintaining optical fibers. The transmitting device according to the embodiments of the present disclosure can transmit stable polarization information solely through the operation control of a laser light source. Furthermore, the miniaturization and integration of a quantum key distribution system are facilitated by using the transmitting device according to the embodiments of the present disclosure.

[0044] FIG. 2 is a block diagram of a transmitting device for QKD according to one embodiment of the present disclosure.

[0045] Referring to FIG. 2, the transmitting device (10) includes a plurality of laser sources (110), a plurality of polarization maintaining fibers (PMF, 120), a transmitter-side optical integrated module (130), a single-mode fiber (140), and a variable optical attenuator (VOA, 150). The transmitting device (10) may further include a transmission control unit (160).

[0046] A plurality of laser light sources (110) are configured to generate single photons or coherent light. Each laser light source may be configured to generate light of the same linear polarization (e.g., vertical polarization), and the linear polarization may be aligned with the slow-axis or fast-axis of a polarization-maintaining optical fiber (120) connected to the corresponding laser light source. The plurality of laser light sources (110) may include, but are not limited to, a laser diode (LD), an external cavity laser (ECL), a fiber laser, a helium-neon (He-Ne) laser, etc.

[0047] The number of laser light sources may vary depending on the QKD protocol being implemented. For example, if based on the BB84 protocol using four polarization states, the transmitter (10) may include four laser light sources. For example, if based on the B92 protocol using two polarization states, the transmitter (10) may include two laser light sources. For example, if based on the RFI protocol using six polarization states, the transmitter (10) may include six laser light sources.

[0048] Since a transmitting device for QKD according to one embodiment of the present disclosure implements a QKD protocol using four polarization states, the transmitting device (10) may be configured to include four laser light sources.

[0049] A plurality of polarization-maintaining optical fibers (120) are connected one-to-one to a plurality of laser light sources (100) to form optical paths that transmit light generated from the plurality of laser light sources (100) to a transmitting optical integration module (130). Polarization-maintaining optical fibers of various structures, such as panda type, bow-tie type, and elliptical, can be used.

[0050] The number of polarization-maintaining optical fibers may vary depending on the QKD protocol implemented. For example, since a QKD transmitter according to one embodiment of the present disclosure implements a QKD protocol using four polarization states, the transmitter (10) may be configured to include four polarization-maintaining optical fibers.

[0051] Polarization-maintaining optical fibers are optical fibers that possess the characteristic of maintaining a specific polarization state as electromagnetic waves propagate along the fiber. While light in conventional optical fibers can change to various polarization directions as it passes through the fiber, polarization-maintaining optical fibers are designed to minimize such changes in polarization.

[0052] The cross-section of a polarization-maintaining optical fiber generally consists of an asymmetric structure formed by two polarization axes with different refractive indices—namely, a slow axis and a fast axis—around the optical fiber core due to the birefringence effect. Due to this asymmetric structure, light within the polarization-maintaining optical fiber propagates at different speeds along the slow and fast axes, preventing interference and allowing each to maintain its own polarization state. Linear polarization is typically maintained aligned with the slow axis of the polarization-maintaining optical fiber.

[0053] A plurality of polarization-maintaining optical fibers (120) are precisely fixed to the optical fiber block (131) of the transmitting optical integration module (130) with each slow axis or fast axis rotated at a different specific angle. At this time, the rotation angle of the plurality of polarization-maintaining optical fibers (120) can be determined based on N polarization states used by the QKD protocol. For example, since a transmitting device for QKD according to one embodiment of the present disclosure implements a QKD protocol using four polarization states, the first polarization-maintaining optical fiber (120-1) can be fixed to the optical fiber block (131) with the slow axis rotated to 0°, the second polarization-maintaining optical fiber (120-2) with the slow axis rotated to 45°, the third polarization-maintaining optical fiber (120-3) with the slow axis rotated to 90°, and the fourth polarization-maintaining optical fiber (120-4) with the slow axis rotated to 135°.

[0054] When the slow axis or fast axis of each of the multiple polarization-maintaining optical fibers (120) rotates, the polarization state of the light transmitted through it also rotates, and light with a polarization state identical to the rotation angle is output.

[0055] The transmitting optical integration module (130) combines light of different polarization states input from a plurality of polarization-maintaining optical fibers (120) into a single optical path.

[0056] FIG. 3 illustrates a transmitting optical integrated module (130) according to one embodiment of the present disclosure.

[0057] Referring to FIG. 3, the transmitting optical integrated module (130) includes a fiber block (131), an optical junction (132), and a planar optical waveguide-based optical coupler (133).

[0058] The optical fiber block (131) fixes and supports a plurality of polarization-maintaining optical fibers (120). A plurality of polarization-maintaining optical fibers (120) are precisely fixed in the V-groove of the optical fiber block (131), and each polarization-maintaining optical fiber is fixed in a state rotated at a predetermined angle with respect to linear polarization (e.g., vertical polarization) output from a plurality of laser light sources (110).

[0059] The junction (132) is located between the output end of the optical fiber block (131) and the input end of the optical coupler (133) to optically combine them and perform the function of maintaining the polarization state of the light. Depending on the embodiment, an anti-reflection coating may be formed on at least one of the output end of the optical fiber block (131) or the input end of the optical coupler (133) of the junction (132) to minimize light loss and improve coupling efficiency.

[0060] The optical coupler (133) combines light of different polarization states input through the junction (132) into a single optical path.

[0061] The optical coupler (133) is formed based on a planar lightwave circuit (PLC) and may adopt a structure such as a Y-junction type or a multi-mode interference (MMI) type.

[0062] The optical coupler (133) is a passive component and can be formed in a 4x1 structure that combines four inputs into one output. Through this, light of different polarization states (e.g., vertical, horizontal, diagonal, opposite angle) generated from four laser light sources (110) and transmitted through the polarization-maintaining optical fiber (120) and the junction (132) can be stably output to a single output port.

[0063] Planar optical waveguides are generally fabricated using semiconductor processes, which eliminates optical alignment issues found in conventional technology. In other words, ultra-small planar optical waveguides that do not require optical alignment can be fabricated using semiconductor process technology. Planar optical waveguides can be fabricated using materials such as silica (SiO2), silicon nitride (Si3N4), indium phosphide (InP), and lithium niobate.

[0064] Meanwhile, regarding the stability of a planar optical waveguide, if the refractive index of the medium is isotropic when light passes through the waveguide, N input polarizations can be combined without variation and output. If the temperature distribution of the planar optical waveguide is constant, the refractive index can be considered isotropic; however, if there is a gradient in the temperature distribution, a spatial gradient in the refractive index occurs, which can cause variations in polarization. Therefore, to maintain a constant temperature of the planar optical waveguide, temperature control can be performed using a thermoelectric element or insulation treatment can be applied.

[0065] The output terminal of the optical coupler (133) is connected to a variable optical attenuator (150) through a single-mode optical fiber (140). The single-mode optical fiber (140) has the advantage of having very little loss and almost no signal distortion because the core is very small (less than 10 μm) and light propagates only in a single mode.

[0066] FIG. 4 is a cross-sectional view in the A-A' direction of FIG. 3, showing the rotation angles of four panda-type polarization-maintaining optical fibers (120-1, 120-2, 120-3, 120-4). That is, it can be seen that the first polarization-maintaining optical fiber (120-1) is fixed to the optical fiber block (131) with its slow axis rotated to 0°, the second polarization-maintaining optical fiber (120-2) is fixed with its slow axis rotated to 45°, the third polarization-maintaining optical fiber (120-3) is fixed with its slow axis rotated to 90°, and the fourth polarization-maintaining optical fiber (120-4) is fixed with its slow axis rotated to 135°.

[0067] FIG. 5 is an exemplary diagram showing the polarization state of light output through an optical coupler (133) according to the rotation angle of a polarization-maintaining optical fiber in a transmitting device for QKD according to one embodiment of the present disclosure.

[0068] Referring to FIG. 5, when the slow axis or fast axis of each of the four polarization-maintaining optical fibers (120-1, 120-2, 120-3, 120-4) is rotated to a specific angle, it can be seen that the light output through the optical coupler (133) is in a polarization state identical to the rotation angle of each polarization-maintaining optical fiber. That is, when the slow axis of the first polarization-maintaining optical fiber (120-1) is rotated 0°, vertically polarized light is output through the optical coupler (133); when the slow axis of the second polarization-maintaining optical fiber (120-2) is rotated 45°, 45° polarized light is output through the optical coupler (133); when the slow axis of the third polarization-maintaining optical fiber (120-3) is rotated 90°, horizontally polarized light is output through the optical coupler (133); and when the slow axis of the fourth polarization-maintaining optical fiber (120-4) is rotated 135°, 135° polarized light is output through the optical coupler (133).

[0069] The variable optical attenuator (150) controls the intensity of the polarization signal output from the optical coupler (133). The variable optical attenuator (150) can reduce the intensity of the polarization signal output from the optical coupler (133) to a single photon energy level. In quantum cryptography or quantum key distribution systems, since single photons are used instead of multiple photons, it is necessary to control the intensity of the transmitted polarization signal.

[0070] The transmission control unit (160) controls the operation of the plurality of laser light sources (110) so that light is generated in only one of the plurality of laser light sources (110).

[0071] The transmission control unit (160) can generate random data, that is, a random number. The transmission control unit (160) can generate a random number using a Random Number Generator (RNG). The transmission control unit (160) can generate a quantum random number using a Quantum Random Number Generator (QRNG).

[0072] The transmission control unit (160) can control the operation of the plurality of laser light sources (110) so that light is generated in only one of the plurality of laser light sources (110) based on a generated random number.

[0073] For example, based on the BB84 protocol using four polarization states, the transmission control unit (160) can determine the polarization corresponding to the generated random number and control the operation of the multiple laser light sources (110) so that light is generated in only one of the multiple laser light sources (110) based on the determined polarization. For example, the transmission control unit (160) can control the operation so that only the first laser light source (110-1) operates when the determined polarization is vertical polarization. For example, the transmission control unit (160) can control the operation so that only the second laser light source (110-2) operates when the determined polarization is 45° polarization. For example, the transmission control unit (160) can control the operation so that only the third laser light source (110-3) operates when the determined polarization is horizontal polarization. For example, the transmission control unit (160) can control the operation of only the fourth laser light source (110-4) when the determined polarization is 135° polarization.

[0074] FIG. 6 is a flowchart of a polarization signal transmission method performed by a transmission device for QKD according to one embodiment of the present disclosure.

[0075] Referring to Fig. 6, the method generates random data, i.e., a random number (S610).

[0076] The method operates a laser light source corresponding to the generated random number to output a light signal (S620). The transmitting device (10) can control the operation of multiple laser light sources so that light is generated in only one of the multiple laser light sources based on the generated random number.

[0077] The output optical signal passes through an optical path composed of polarization-maintaining optical fibers and is incident with a specific polarization at the coupling point where it meets a planar waveguide. In other words, the polarization state of the optical signal is defined through the corresponding optical path and combined into a single optical path through an optical coupler based on the planar waveguide.

[0078] The method attenuates the intensity of the polarization signal output from the optical coupler to a single photon energy level through a variable optical attenuator (S630).

[0079] The method transmits the attenuated polarization signal through a quantum channel to a receiver (not shown) within a quantum key distribution system (S640).

[0080] The configuration of the transmitting device (10) may vary depending on the various QKD protocols.

[0081] For example, in the case of a BB84 protocol based on four polarization states, the transmitting device (10) may include four laser light sources, four polarization-maintaining optical fibers, and a planar waveguide-based 4 x 1 optical coupler that combines four optical paths into one optical path.

[0082] For example, in the case of an RFI protocol based on six polarization states, the transmitting device (10) may include six laser light sources, six polarization-maintaining optical fibers, and a planar optical waveguide-based 6 x 1 optical coupler that combines six optical paths into one optical path.

[0083] For example, in the case of a B92 protocol based on two polarization states, the transmitting device (10) may include two laser light sources, two polarization-maintaining optical fibers, and a planar waveguide-based 2 x 1 optical coupler that combines two optical paths into one optical path.

[0084] Meanwhile, according to another embodiment of the present disclosure, a transmitter optical system suitable for RFI-QKD is provided.

[0085] The configuration of the transmitting and receiving optical systems of a conventional RFI-QKD system is shown in FIG. 7.

[0086] Referring to FIG. 7, a conventional polarization-based RFI-QKD transmitter uses a total of six light sources (LDs), and these light sources form pairs of two to generate and transmit light in vertical / horizontal, diagonal / opposite diagonal, and right-circular / left-circular polarization states.

[0087] Vertical / horizontal polarization signals transmit linearly polarized light output from the corresponding light source as is. Diagonal / opposite polarization signals are transmitted after vertically / horizontally polarized light output from the corresponding light source is converted into diagonal / opposite directions as it passes through a half-wave plate (HWP). Right-circular / left-circular polarization signals are transmitted after vertically / horizontally polarized light output from the corresponding light source is converted into a circularly polarized state (right-turned, left-turned) as it passes through a quarter-wave plate (QWP).

[0088] Conventional technology has limitations in that the miniaturization and integration of the transmitter and receiver are difficult due to the use of discrete optical components, and precise optical alignment between optical components is required for stable polarization signal transmission.

[0089] Meanwhile, the existing BB84 protocol distributes quantum keys based on linear polarization (vertical / horizontal, diagonal / opposite). However, in this case, there was a problem that if there was a change in the reference frame between the transmitter and the receiver, the polarization information extracted at the receiver could be distorted.

[0090] In contrast, the RFI-QKD protocol utilizes both right-circle polarization and left-circle polarization, which has the advantage of enabling the receiver to stably extract quantum key information even if the reference frame changes. Therefore, the transmitter must be able to stably generate six polarization states (vertical polarization, horizontal polarization, diagonal polarization, opposite-diagonal polarization, right-circle polarization, and left-circle polarization), which is a key element for the stable sharing of quantum cryptographic keys.

[0091] A transmitting device according to another embodiment of the present disclosure can provide stable polarization information of a total of six polarization states, including vertical polarization, horizontal polarization, diagonal polarization, opposite diagonal polarization, right circular polarization, and left circular polarization, without control through active elements. In addition, a transmitting device according to another embodiment of the present disclosure facilitates the miniaturization and integration of the RFI-QKD transmitting unit.

[0092] FIG. 8 is a block diagram of a transmitter for RFI-QKD according to another embodiment of the present disclosure.

[0093] Referring to FIG. 8, the transmitting device (10) includes a plurality of laser sources (110), a plurality of polarization maintaining fibers (120), a transmitter-side optical integrated module (130), a single-mode fiber (140), and a variable optical attenuator (150). The transmitting device (10) may further include a transmitting control unit (160).

[0094] A plurality of laser light sources (110) are configured to generate a single photon or coherent light, having the same components as the plurality of laser light sources (110) shown in FIG. 2.

[0095] The number of laser light sources may vary depending on the implemented QKD protocol.

[0096] For example, since an RFI-QKD transmitter according to another embodiment of the present disclosure implements an RFI protocol using six polarization states, the transmitter (10) may be configured to include six laser light sources.

[0097] A plurality of polarization-maintaining optical fibers (120) are components identical to the plurality of polarization-maintaining optical fibers (120) shown in FIG. 2, and are connected one-to-one to a plurality of laser light sources (110) to form optical paths that transmit light generated from a plurality of laser light sources (100) to a transmitting optical integration module (130). Various polarization-maintaining optical fibers of different structures, such as panda type, bow-tie type, and elliptical, can be used.

[0098] The number of polarization-maintaining optical fibers may vary depending on the QKD protocol being implemented. For example, since an RFI-QKD transmitter according to another embodiment of the present disclosure implements an RFI protocol, the transmitter (10) may be configured to include six polarization-maintaining optical fibers.

[0099] A plurality of polarization-maintaining optical fibers (120) are precisely fixed to the optical fiber block (131) of the transmitting optical integration module (130) with each slow axis or fast axis rotated at a different specific angle. For example, the first polarization-maintaining optical fiber (120-1) may be fixed to the optical fiber block (131) with the slow axis rotated to 0°, the second polarization-maintaining optical fiber (120-2) with the slow axis rotated to 45°, the third polarization-maintaining optical fiber (120-3) with the slow axis rotated to 90°, the fourth polarization-maintaining optical fiber (120-4) with the slow axis rotated to 135°, the fifth polarization-maintaining optical fiber (120-5) with the slow axis rotated to 90°, and the sixth polarization-maintaining optical fiber (120-6) with the slow axis rotated to 0°.

[0100] When the slow axis or fast axis of each of the multiple polarization-maintaining optical fibers (120) rotates, the polarization state of the light transmitted through it also rotates, and light with a polarization state identical to the rotation angle is output.

[0101] The transmitting optical integration module (130) is a component for processing light transmitted from a plurality of polarization-maintaining optical fibers (120) to stably generate light of different polarization states and combine them into a single output.

[0102] FIG. 9a illustrates a transmitting optical integrated module (130) according to another embodiment of the present disclosure.

[0103] Referring to FIG. 9a, the transmitting optical integrated module (130) includes a fiber block (131), an optical junction (132), and a planar optical waveguide-based optical coupler (133).

[0104] The optical fiber block (131) fixes and supports a plurality of polarization-maintaining optical fibers (120). A plurality of polarization-maintaining optical fibers (120) are precisely fixed in the V-groove of the optical fiber block (131), and each polarization-maintaining optical fiber is fixed in a state rotated at a predetermined angle with respect to linear polarization (e.g., vertical polarization) output from a plurality of laser light sources (110).

[0105] The junction (132) is located between the output end of the optical fiber block (131) and the input end of the optical coupler (133) to optically combine them and perform the function of converting or maintaining the polarization state of the light. Specifically, the junction (132) is configured such that an isotropic film (132-1) or a quarter-wave plate (132-2) is placed corresponding to each polarization-maintaining optical fiber (120). Accordingly, light input through the path where the isotropic film (132-1) is placed is transmitted to the optical coupler (133) while maintaining a linear polarization state, and light input through the path where the quarter-wave plate (132-2) is placed is converted into right-circular polarization or left-circular polarization and transmitted to the optical coupler (133). Additionally, depending on the embodiment, the joint (132) may have an anti-reflection coating (132-3) formed on at least one of the output end of the optical fiber block (131) or the input end of the optical coupler (133) to minimize optical loss and improve coupling efficiency.

[0106] The optical coupler (133) combines light of different polarization states input through the junction (132) into a single optical path. The optical coupler (133) is formed based on a planar lightwave circuit (PLC) and may adopt a structure such as a Y-junction type or a multi-mode interference (MMI) type. The optical coupler (133) is a passive component and may be formed in a 6x1 structure that combines six inputs into one output. Through this, light generated from six laser light sources (110) passes through the polarization-maintaining optical fiber (120) and the junction (132) to be converted into different polarization states (e.g., vertical, horizontal, diagonal, opposite angle, right circle, left circle) and then can be stably output to a single output port.

[0107] Planar optical waveguides are generally fabricated using semiconductor processes, which eliminates optical alignment issues found in conventional technology. In other words, ultra-small planar optical waveguides that do not require optical alignment can be fabricated using semiconductor process technology. Planar optical waveguides can be fabricated using materials such as silica (SiO2), silicon nitride (Si3N4), indium phosphide (InP), and lithium niobate.

[0108] Meanwhile, regarding the stability of a planar optical waveguide, if the refractive index of the medium is isotropic when light passes through the waveguide, N input polarizations can be combined without variation and output. If the temperature distribution of the planar optical waveguide is constant, the refractive index can be considered isotropic; however, if there is a gradient in the temperature distribution, a spatial gradient in the refractive index occurs, which can cause variations in polarization. Therefore, to maintain a constant temperature of the planar optical waveguide, temperature control can be performed using a thermoelectric element or insulation treatment can be applied.

[0109] The output end of the optical coupler (133) is connected to a variable optical attenuator (150) through a single-mode optical fiber (SMF, 140). The single-mode optical fiber (140) has the advantage of having very little loss and almost no signal distortion because the core is very small (less than 10 μm) and light propagates only in a single mode.

[0110] FIG. 9b is a cross-sectional view in the AA' direction of FIG. 9a, illustrating the state in which each polarization-maintaining optical fiber (120-1 to 120-6) is fixed to the optical fiber block (131) and the polarization states input and output through these channels. For example, light input through the first to fourth polarization-maintaining optical fibers maintains a linear polarization state by passing through an isotropic film (132-1), and linear polarization input through the fifth to sixth polarization-maintaining optical fibers is converted into right-circular polarization and left-circular polarization, respectively, and output through a quarter-wave delay plate (132-2). In this way, the transmitting optical integration module (130) stably provides six polarization states required for Reference Frame Independent Quantum Key Distribution (RFI-QKD) as a single output.

[0111] FIG. 9c shows a modified example in which an anti-reflective layer is further formed on the joint (132) shown in FIG. 9a, which can minimize optical signal loss.

[0112] FIG. 10a illustrates a transmitting optical integrated module (130) according to another embodiment of the present disclosure. The basic configuration is the same as FIG. 9a, but differs in that the optical coupler (133) is formed in an 8x1 structure.

[0113] An optical coupler (133) based on an 8x1 planar optical waveguide is designed so that the length of each branch path is equal, and all input signals are configured to be output with equal temporal delays. Therefore, it prevents inter-channel time delay inconsistencies that may occur in the 6x1 structure of FIG. 9a and ensures synchronization of the output signals. Accordingly, it has the effect of improving the key generation rate and reducing the transmission error rate. This is an important advantage that improves the key distribution rate and reliability in reference frame-independent quantum key distribution.

[0114] In the case of an 8x1 planar optical waveguide-based optical coupler (133), any two input ports can be left empty without being connected to a polarization-maintaining optical fiber. This allows for the utilization of extra channels, thereby increasing design flexibility.

[0115] FIG. 10b is a cross-sectional view in the AA' direction of FIG. 10a, illustrating a plurality of polarization-maintaining optical fibers (120-1 to 120-6) fixed in a V-groove and the input and output polarization states. Likewise, a specific channel is transmitted in a linear polarization state as is, while another channel is converted to right-circular polarization or left-circular polarization through a quarter-wave delay plate (132-2).

[0116] The variable optical attenuator (150) controls the intensity of the polarization signal output from the transmitting optical integration module (130). The variable optical attenuator (150) can reduce the intensity of the polarization signal output from the transmitting optical integration module (130) to a single photon energy level. In quantum cryptographic communication or quantum key distribution systems, since single photons are used instead of multiple photons, it is necessary to control the intensity of the transmitted polarization signal.

[0117] The transmission control unit (160) controls the operation of the plurality of laser light sources (110) so that light is generated in only one of the plurality of laser light sources (110).

[0118] The transmission control unit (160) can generate a random number. The transmission control unit (160) can generate a random number using a random number generator (RNG, not shown). The transmission control unit (160) can generate a quantum random number using a quantum random number generator (QRNG).

[0119] The transmission control unit (160) can determine the polarization corresponding to the generated random number and control the operation of the plurality of laser light sources (110) so that light is generated in only one of the plurality of laser light sources (110) based on the determined polarization.

[0120] For example, to implement an RFI-QKD protocol using six polarization states, the transmission control unit (160) can control the operation of only the first laser light source (110-1) when the determined polarization is vertical polarization. The transmission control unit (160) can control the operation of only the second laser light source (110-2) when the determined polarization is 45° polarization. The transmission control unit (160) can control the operation of only the third laser light source (110-3) when the determined polarization is horizontal polarization. The transmission control unit (160) can control the operation of only the fourth laser light source (110-4) when the determined polarization is 135° polarization. The transmission control unit (160) can control the operation of only the fifth laser light source (110-5) when the determined polarization is right-circular polarization. The transmission control unit (160) can control the operation of only the sixth laser light source (110-6) when the determined polarization is left-circular polarization.

[0121] FIG. 11 is a flowchart illustrating the operation method of a transmitter (10) for RFI-QKD according to another embodiment of the present disclosure.

[0122] Referring to FIG. 11, the transmission control unit (160) generates a random number using a random number generator (RNG) or a quantum random number generator (QRNG) (S1101).

[0123] The transmission control unit (160) determines the target polarization state based on the generated random number and selects and drives a laser light source corresponding to the determined polarization (S1102). At this time, the transmission control unit (160) controls the light to be output from only one of the multiple laser light sources (110) according to the random number value. The linearly polarized light output from the selected laser light source is transmitted to the transmission unit optical integration module (130) through the corresponding polarization-maintaining optical fiber (120).

[0124] At the junction (132) of the transmitting optical integration module (130), the input light passes through an isotropic film (132-1) or a quarter-wave delay plate (132-2) for each channel, thereby maintaining or changing the polarization state (S1103). Specifically, the light passing through the isotropic film (132-1) maintains the linear polarization state, while the light passing through the quarter-wave delay plate (132-2) is converted into right-circular polarization or left-circular polarization. Additionally, an anti-reflection layer (132-3) may be further formed at the junction (132) to minimize light loss.

[0125] Subsequently, the light converted to different polarization states is combined into a single output in a planar optical waveguide-based optical coupler (133) (S1104). The optical coupler (133) can be formed, for example, in a 6x1 structure, and in other embodiments, implemented in an 8x1 structure so that the optical signals of all input paths are transmitted along the same length, thereby preventing time delay mismatch between channels.

[0126] The light combined into a single output through the optical coupler (133) is transmitted to the variable light attenuator (150) via the single-mode optical fiber (140).

[0127] The variable optical attenuator (150) attenuates the intensity of the optical signal to a single photon level and adjusts it to a signal intensity suitable for quantum key distribution (QKD). Afterward, the optical signal is transmitted to the receiving side through a quantum channel (S1105).

[0128] Each component of the device or method according to the present invention may be implemented in hardware or software, or in a combination of hardware and software. Additionally, the function of each component may be implemented in software, and a microprocessor may be implemented to execute the function of the software corresponding to each component.

[0129] Various embodiments of the systems and techniques described herein may be realized as digital electronic circuits, integrated circuits, field programmable gate arrays (FPGAs), application-specific integrated circuits (ASICs), computer hardware, firmware, software, and / or combinations thereof. These various embodiments may include being implemented as one or more computer programs executable on a programmable system. A programmable system comprises a storage system, at least one input device, and at least one programmable processor (which may be a special-purpose processor or a general-purpose processor) coupled to receive data and instructions from and transmit data and instructions to at least one output device. Computer programs (which are also known as programs, software, software applications, or code) include instructions for the programmable processor and are stored on a "computer-readable recording medium."

[0130] Computer-readable recording media include all types of recording devices in which data that can be read by a computer system is stored. Such computer-readable recording media may be non-volatile or non-transitory media such as ROM, CD-ROM, magnetic tape, floppy disk, memory card, hard disk, magneto-optical disk, and storage device, and may also include transitory media such as data transmission media. Additionally, computer-readable recording media may be distributed across networked computer systems, and computer-readable code may be stored and executed in a distributed manner.

[0131] Although the flowcharts and timing diagrams in this specification describe each process as being executed sequentially, this is merely an illustrative explanation of the technical concept of one embodiment of the present disclosure. In other words, a person skilled in the art to which one embodiment of the present disclosure belongs may modify and adapt the flowcharts and timing diagrams in various ways, such as changing the order described in the flowcharts and timing diagrams or executing one or more of the processes in parallel, without departing from the essential characteristics of one embodiment of the present disclosure; therefore, the flowcharts and timing diagrams are not limited to a chronological order.

[0132] The above description is merely an illustrative explanation of the technical concept of the present embodiment, and a person skilled in the art to which the present embodiment belongs would be able to make various modifications and variations within the scope of the essential characteristics of the present embodiment. Accordingly, the present embodiments are intended to explain, not limit, the technical concept of the present embodiment, and the scope of the technical concept of the present embodiment is not limited by these embodiments. The scope of protection of the present embodiment shall be interpreted by the claims below, and all technical concepts within an equivalent scope shall be interpreted as being included within the scope of rights of the present embodiment.

[0133] Statements regarding sponsored research or development

[0134] This invention is the result of research conducted in 2024 with funding from the Ministry of Science and ICT and support from the Korea Institute of Information and Communication Technology Planning and Evaluation (Project No.: 2710008212, Project Title: Development of 30km-class long-distance wireless quantum key distribution transmission technology for satellite quantum secure communication and core element technology for satellite quantum key distribution (QKD)).

[0135] CROSS-REFERENCE TO RELATED APPLICATION

[0136] This patent application claims priority to Korean patent application No. 10-2024-0144592, filed on October 22, 2024, and Korean patent application No. 10-2025-0139638, filed on September 26, 2025, the entirety of which is incorporated herein by reference.

Claims

1. Multiple light sources; A plurality of polarization-maintaining optical fibers connected one-to-one to the plurality of light sources and transmitting light generated from the plurality of light sources; An optical coupler that combines light of different polarization states input from the plurality of polarization-maintaining optical fibers into a single optical path; and It includes a variable light attenuator for reducing the intensity of light output from the above-mentioned optical coupler, The above optical coupler is formed using a planar optical waveguide, and A transmitting device for quantum key distribution, wherein the plurality of polarization-maintaining optical fibers each have their slow-axis rotated at different angles and connected to the plurality of input terminals of the optical coupler.

2. In Paragraph 1, A transmitting device characterized in that the slow axes of the plurality of polarization-maintaining optical fibers are aligned according to a quantum key distribution protocol.

3. In Paragraph 1, A transmitting device in which the output end of the above optical coupler is connected to the above variable optical attenuator through a single-mode optical fiber.

4. In Paragraph 1, The above plurality of light sources includes a first light source, a second light source, a third light source, and a fourth light source, and A transmitting device characterized by the optical coupler having a 4 x 1 structure that combines four inputs into one output.

5. In Paragraph 4, The above plurality of polarization-maintaining optical fibers are, A first polarization-maintaining optical fiber connected to the optical coupler with a slow axis rotated to 0˚; A second polarization-maintaining optical fiber with a slow axis rotated 45˚ and connected to the optical coupler; A third polarization-maintaining optical fiber with a slow axis rotated 90˚ and connected to the optical coupler; and A transmitting device comprising a fourth polarization-maintaining optical fiber connected to the optical coupler with a slow axis rotated 135˚.

6. In Paragraph 4, The transmission control unit further includes generating a random number, determining a polarization corresponding to the random number, and controlling the operation of the plurality of light sources so that light is generated in only one of the plurality of light sources based on the determined polarization. The above transmission control unit is, When the determined polarization is vertical polarization, control is made so that only the first light source operates, and When the determined polarization is 45˚ polarization, control is made so that only the second light source operates, and If the determined polarization is horizontal polarization, control is made so that only the third light source operates, and A transmitting device characterized by controlling only the fourth light source to operate when the determined polarization is 135˚ polarization.

7. In a transmitting optical integrated module for reference frame-independent quantum key distribution, Optical fiber block that fixes and supports multiple polarization-maintaining optical fibers; A joint optically bonded to the above optical fiber block; and It includes a planar waveguide-based optical coupler that combines light of different polarization states input through the junction into a single optical path, and The above junction is configured such that an isotropic film or a quarter-wave delay plate is disposed corresponding to each polarization-maintaining optical fiber, and When linearly polarized light is input to the junction through the plurality of polarization-maintaining optical fibers, Light input through a polarization-maintaining optical fiber with an isotropic film arranged thereon maintains a linear polarization state and is transmitted to the optical coupler, and A transmitting optical integration module configured such that light input through a polarization-maintaining optical fiber having a quarter-wave delay plate is converted into right-circular polarization or left-circular polarization and transmitted to the optical coupler.

8. In Paragraph 7, The above joint is, A transmitting optical integrated module further comprising an anti-reflection coating formed on at least one of the output terminal of the optical fiber block or the input terminal of the optical coupler.

9. In Paragraph 7, The above optical fiber block includes a V-groove substrate formed through a semiconductor process, and The above plurality of polarization-maintaining optical fibers are fixed in the V-groove, in a transmitting optical integration module.

10. In Paragraph 7, The above plurality of polarization-maintaining optical fibers are, A first polarization-maintaining optical fiber fixed to the optical fiber block with the slow axis rotated to 0°; A second polarization-maintaining optical fiber fixed to the optical fiber block with the slow axis rotated to 45°; A third polarization-maintaining optical fiber fixed to the optical fiber block with the slow axis rotated 90°; A fourth polarization-maintaining optical fiber fixed to the optical fiber block with the slow axis rotated to 135°; A fifth polarization-maintaining optical fiber fixed to the optical fiber block with the slow axis rotated 90°; and It includes a sixth polarization-maintaining optical fiber fixed to the optical fiber block with the slow axis rotated to 0°, and A transmitting optical integrated module characterized by the optical coupler having a 6x1 structure that combines six inputs into one output.

11. In Paragraph 10, At the above joint, An isotropic film is disposed corresponding to the first to fourth polarization-maintaining optical fibers, and A transmitting optical integrated module characterized by having a quarter wave delay plate arranged in correspondence with the 5th to 6th polarization-maintaining optical fibers.

12. As a transmitting device for reference frame-independent quantum key distribution, Multiple light sources; The transmitting optical integrated module of claim 7; and It includes a variable light attenuator for adjusting the intensity of light output from the above-mentioned transmitting optical integration module, The above plurality of light sources includes a first light source, a second light source, a third light source, a fourth light source, a fifth light source, and a sixth light source, and A transmitting device characterized in that light output from the plurality of light sources is input to the transmitting unit optical integration module through the plurality of polarization-maintaining optical fibers.

13. In Paragraph 12, The output terminal of the above-mentioned transmitting optical integration module is a transmitting device connected to the above-mentioned variable optical attenuator through a single-mode optical fiber.

14. In Paragraph 12, It further includes a transmission control unit that generates a random number, determines polarization based on the generated random number, and controls the selection of one of the plurality of light sources to generate light. The above transmission control unit is, When the determined polarization is vertical polarization, control is made so that only the first light source operates, and When the determined polarization is 45˚ polarization, control is made so that only the second light source operates, and If the determined polarization is horizontal polarization, control is made so that only the third light source operates, and When the determined polarization is 135˚ polarization, control is made so that only the fourth light source operates, and If the determined polarization is right-circular polarization, control is made so that only the fifth light source operates, and A transmitting device characterized by controlling only the sixth light source to operate when the determined polarization is left-circular polarization.

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