Patent Publication Number: US-2022231845-A1

Title: Quantum key distribution method, device, and system

Description:
TECHNICAL FIELD 
     The present disclosure relates to a quantum key distribution method, a quantum key distribution device, and a quantum key distribution system and, more particularly, to a quantum key distribution method, a quantum key distribution device, and a quantum key distribution system for controlling a polarization-dependent element provided in a receiver of the quantum key distribution system using a one-time continuous control signal to drive the same independently of polarization. 
     BACKGROUND ART 
     Recently, individuals or countries have suffered damage caused by wiretapping and eavesdropping therebetween, and there is a significantly growing interest in security. However, typical secure communication has a serious risk that communication content may be exposed by external attacks. To compensate for the shortcomings of the typical secure communication method, quantum cryptography communication has been introduced and spotlighted as a next-generation security technology. The quantum cryptography can theoretically guarantee very high security. 
     Accordingly, many studies have been actively conducted on quantum key distribution among quantum cryptography communication technologies. Quantum key distribution (QKD) is a technology of distributing and sharing a cryptographic key between distant users using the quantum mechanical properties of photons. If an attacker attempts to take cryptographic key information distributed between the users, the cryptographic key information may be changed due to the quantum mechanical properties. Accordingly the users exchanging the cryptographic key can detect the presence of the attacker. 
     Specifically, quantum key distribution (QKD) transmits information using a single photon (or a quasi-single photon at a similar level to a single photon) having quantum properties to distribute a cryptographic key using a quantum state and shares the cryptographic key safely from eavesdropping using polarization and a phase among various quantum properties of photons. 
     Accordingly, a quantum key distribution (QKD) system includes various optical elements, and the various optical elements included in the quantum key distribution (QKD) system generally have polarization-dependent operating characteristics. Therefore, a change in polarization of photons occurring in the quantum key distribution (QKD) system has a significant impact on the performance of the quantum key distribution (QKD) system. 
     For example, a quantum key distribution (QKD) system distributes a cryptographic key using a phase generally. The quantum key distribution (QKD) system employs a phase modulator (PM) to load cryptographic key information on a photon. The phase modulator (PM) generally has a polarization-dependent characteristic that a phase modulation characteristic varies according to the polarization state of a photon. 
     That is, the phase modulator (PM) is a device that controls the phase of a photon using a voltage or current and changes a phase control value by input polarization according to the polarization-dependent characteristic. Accordingly, when the polarization of a photon input to the phase modulator (PM) is changed, it is difficult to accurately control phase. Furthermore, because of a phase error caused by the polarization, the quantum key distribution (QKD) system does not properly generate/distribute the cryptographic key. 
     An intensity modulator (IM) is used to generate decoy data for detecting the presence of an attacker in a quantum key distribution (QKD) system. Such an intensity modulator is also typically polarization-dependent. Accordingly, when the polarization of a photon entering the intensity modulator (IM) is changed, the intensity modulator (IM) has difficulty in generating accurate decoy data (decoy state), which restricts a stable operation of the quantum key distribution (QKD) system. 
     A one-way quantum key distribution (QKD) system includes a transmitter generating a single photon and a receiver detecting the single photon. Accordingly, both the transmitter and the receiver are generally configured with an optical link and an element capable of maintaining proper polarization in order to control the characteristics of a polarization-dependent device. Furthermore, the receiver has a separate polarization correction function to correct polarization distortion that occurs in a quantum channel (optical link through which the single photon is transmitted). 
     A two-way quantum key distribution (QKD) system includes a transmitter generating, transmitting a single photon, and detecting the photon reflected by a receiver. The two-way quantum key distribution (QKD) system is configured to transmit an optical signal generated by the transmitter to the receiver and return the optical signal back to the transmitter through the same link in order to correct polarization distortion that occurs in a quantum channel. Accordingly, polarization distortion occurring in the transmission from the transmitter to the receiver is corrected to the same polarization as that initially occurring in the transmitter during the transmission to the transmitter through the same link. 
     In the two-way quantum key distribution (QKD) system, the stable operation of an interferometer disposed in the transmitter can be ensured by polarization correction using the same optical link. However, the receiver needs to correct an error caused by the polarization in using a phase modulator (PM) and an intensity modulator (IM) to generate a single photon in which cryptographic key information is encoded in the phase of the optical signal using the optical signal received from the transmitter. 
     In order to correct the error in the two-way system, the receiver repeats control on the element twice with respect to an optical signal entering the receiver and an optical signal existing from the receiver by using an element that generates an error in proportion to the degree of polarization distortion of the input optical signal in order to control the polarization distortion. 
     Such a typical method as described above needs to generate two control signals operating at high speed and needs to control the polarization-dependent device twice at high speed at the time when the entering optical signal and the exiting optical signal do not overlap. 
     DISCLOSURE OF INVENTION 
     Technical Problem 
     Therefore, the present disclosure has been made in view of the above-mentioned problems, and an aspect of the present disclosure is to provide a quantum key distribution method, a quantum key distribution device, and a quantum key distribution system for correcting an error due to a polarization-dependent characteristic by controlling a polarization-dependent element using a one-time continuous control signal when a device is configured using the polarization-dependent element in the quantum key distribution (QKD) system in which an optical signal travels back and forth through the same optical link as in a two-way structure. 
     Other specific aspects of the present disclosure will be clearly identified and understood by an expert or a researcher in this technical field through the detailed content to be described below. 
     Solution to Problem 
     According to an embodiment of the present disclosure, a quantum key distribution system  100  for distributing a quantum cryptographic key to a transmitter  110  and a receiver  120  includes: the transmitter  110  configured to split an optical signal into a first optical signal passing through a first path P 1  and a second optical signal passing through a second path P 2 , which is longer than the first path P 1 , and to sequentially transmit the first optical signal and the second optical signal; and the receiver  120  configured to receive the first optical signal and the second optical signal incident through a quantum channel  130  and transmit the first optical signal and the second optical signal back to the transmitter  110  through the quantum channel  130  after passing through a polarization-dependent element  123 , being reflected by a Faraday mirror  124 , and pass through the polarization-dependent element  123  again, wherein a difference (dL=L P2 −L P1 ) between a length L P1  of the first path P 1  and a length L P2  of the second path P 2  in the transmitter  110  is equal to or greater than twice a distance (=D) between the polarization-dependent element  123  and the Faraday mirror  124  (dL≥2D). 
     The transmitter  110  may include a light source  111  to generate an optical signal and a beam splitter  113  to split the optical signal into the first optical signal and the second optical signal, to output the first optical signal to the first path P 1 , and to output the second optical signal to the second path P 2 . 
     A delay line  115  extending a propagation path of the second optical signal may be provided in the second path P 2 . 
     A polarized beam splitter  116  may be provided in the transmitter  110  to polarize the first optical signal proceeding through the first path P 1  to have first polarization and to polarize the second optical signal proceeding through the second path P 2  to have second polarization perpendicular to the first polarization. 
     The polarized beam splitter  116  may enable the first optical signal transmitted from the receiver  120  to proceed to the second path P 2  and may enable the second optical signal to proceed to the first path P 1 . 
     In the receiver  120 , while the first optical signal or the second optical signal passes through the polarization-dependent element  123 , is reflected on the Faraday mirror  124 , and then passes through the polarization-dependent element  123  again, a one-time continuous control signal may be applied as a control signal for controlling an operation of the polarization-dependent element  123 . 
     The receiver  120  may include a plurality of polarization-dependent elements  123 , and the difference (dL=L P2 −L P1 ) between the length L P1  of the first path P 1  and the length L P2  of the second path P 2  may be equal to or greater than twice a distance (=D) between one of the polarization-dependent elements  123 , which is distant from the Faraday mirror  124 , and the Faraday mirror  124  (dL≥2D). 
     The plurality of polarization-dependent elements  123  may include a first phase modulator  123   a  and an intensity modulator  123   b,  and the first phase modulator  123   a  may modulate only a phase of the second optical signal. 
     The plurality of polarization-dependent elements  123  may include a first phase modulator  123   a  and an intensity modulator  123   b,  and the intensity modulator  123   b  may modulate an intensity of the first optical signal and an intensity of the second optical signal. 
     According to another embodiment of the present disclosure, a transmitter  110  provided in a quantum key distribution system  100  to distribute a quantum cryptographic key with a receiver  120  includes: a light source  111  configured to generate an optical signal; a beam splitter  113  configured to split the optical signal into a first optical signal and a second optical signal, to output the first optical signal to a first path P 1 , and to output the second optical signal to a second path P 2 ; and a delay line  115  provided in the second path P 2  to extend a propagation path of the second optical signal, wherein a difference (dL=L P2 −L P1 ) between a length L P1  of the first path P 1  and a length L P2  of the second path P 2  is equal to or greater than twice a distance (=D) between a polarization-dependent element  123  and a Faraday mirror  124  provided in the receiver  120  (dL≥2D). 
     According to still another embodiment of the present disclosure, a receiver  120  provided in a quantum key distribution system  100  to distribute a quantum cryptographic key with a transmitter  110  includes: a polarization-dependent element  123  having an operation characteristic changes depending on polarization of an optical signal; and a Faraday mirror  124  configured to reflect an entering optical signal by rotating polarization of the optical signal by 90 degrees, wherein, when the transmitter  110  generates an optical signal, splits into a first optical signal passing through a first path P 1  and a second optical signal passing through a second path P 2 , which is longer than the first path P 1 , and transmits the first optical signal and the second optical signal sequentially , and the receiver receives the first optical signal and the second optical signal incident through a quantum channel  130  and transmits the first optical signal and the second optical signal back to the transmitter through the quantum channel  130  after passing through the polarization-dependent element  123 , being reflected on the Faraday mirror  124 , and passing through the polarization-dependent element  123  again, and a difference (dL=L P2 −L P1 ) between a length L P1  of the first path P 1  and a length L P2  of the second path P 2  in the transmitter  110  is equal to or greater than twice a distance (=D) between the polarization-dependent element  123  and the Faraday mirror  124  (dL≥2D). 
     Advantageous Effects of Invention 
     By a quantum key distribution method, a quantum key distribution device, and a quantum key distribution system according to an embodiment of the present disclosure, an error caused by a polarization-dependent characteristic is corrected by controlling a polarization-dependent element using a one-time continuous control signal in the two-way quantum key distribution (QKD) system in which an optical signal travels back and forth through the same optical link as in a two-way structure. 
     That is, according to the present disclosure, a polarization-dependent element used in a two-way quantum key distribution (QKD) system may be controlled not to be affected by polarization distortion occurring in a period for transmitting light inside and outside a device. 
     Accordingly, in the present disclosure, it is possible to control an optical signal regardless of polarization distortion in a two-way quantum key distribution (QKD) system using a polarization-dependent element. . 
     Specifically, the quantum key distribution (QKD) system according to the present disclosure includes a receiver  120  that controls a polarization-dependent element, such as a phase modulator (PM) and an intensity modulator (IM). Accordingly, it is possible to control the polarization-dependent element independently of polarization using a one-time continuous control signal. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are included as a part of the detailed description to help the understanding of the present disclosure, provide embodiments of the present disclosure and describe the technical spirit of the present disclosure in conjunction with the detailed description, in which: 
         FIG. 1  and  FIG. 2  illustrate a quantum key distribution system  100  according to an embodiment of the present disclosure; 
         FIG. 3  illustrates a quantum key distribution system  100  in detail according to an embodiment of the present disclosure; 
         FIG. 4A  and  FIG. 4B  are graphs illustrating a control signal for controlling a polarization-dependent element in a quantum key distribution system  100  according to an embodiment of the present disclosure; 
         FIG. 5A  and  FIG. 5B  illustrate arrangement of a polarization-dependent element  123  of a receiver  120  in a quantum key distribution system  100  according to an embodiment of the present disclosure; and 
         FIG. 6  and  FIG. 7  illustrate a transmitter  110  and a receiver  120  in a quantum key distribution system  100  according to an embodiment of the present disclosure. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The present disclosure may be variously modified and may include various embodiments. Hereinafter, specific embodiments will be described in detail with reference to the accompanying drawings. 
     The following embodiments are provided for a comprehensive understanding of methods, devices, and/or systems described herein. However, these embodiments are provided merely for illustration, and the present disclosure is not limited thereto. 
     When detailed descriptions about a known technology related to the present disclosure are determined to make the gist of the present disclosure unclear in describing embodiments of the present disclosure, the detailed descriptions will be omitted herein. Furthermore, terms used below are defined in view of functions in the present disclosure and may thus be changed depending on a user, the intent of an operator, or the custom. Accordingly, the terms should be defined on the basis of the following overall description of this specification. The terminology used in the detailed description is for describing embodiments of the present disclosure only and is not intended to limit the present disclosure. Unless clearly used otherwise, singular forms are intended to include plural forms. It will be understood that the expression “include” or “comprise,” when used in this description, specify the presence of stated features, integers, steps, operations, elements, or some or combinations thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, or some or combinations thereof. 
     Although the terms “first”, “second”, and the like may be used to describe various components, these components should not be limited by these terms. These terms are used only to distinguish one component from another component. 
     Hereinafter, exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. 
       FIG. 1  illustrates a quantum key distribution system  100  according to an embodiment of the present disclosure. 
     As illustrated in  FIG. 1 , the quantum key distribution system  100  according to the embodiment of the present disclosure may include a transmitter  110 , a receiver  120 , and a quantum channel  130 . The transmitter  110  and the receiver  120  generate and share a quantum cryptographic key by exchanging optical signals through the quantum channel  130   
     Here, the transmitter  110  and the receiver  120  may be servers, clients or terminals connected to a server, communication devices, such as gateways or routers, or portable devices having mobility. The transmitter  110  and the receiver  120  may be configured using various devices capable of communication by generating and sharing a quantum key. 
     The quantum channel  130  is provided between the transmitter  110  and the receive  120  to transmit an optical signal. While the quantum channel  130  may be configured using an optical fiber, the present disclosure is not necessarily limited thereto, and any medium capable of transmitting an optical signal may be used to configure the quantum channel  130 . 
     Accordingly, the transmitter  110  and the receiver  120  may exchange information necessary to generate the quantum cryptographic key using the phase, polarization, and the like of the optical signal according to various protocols, such as the BB84 protocol. Further, the transmitter  110  and the receiver  120  may generate and share the quantum cryptographic key, thereby effectively preventing an attacker  140  from stealing the quantum cryptographic key and attempting to hack the quantum cryptographic key. 
     Furthermore, a quantum cryptographic communication system may perform communication while performing encryption and decryption using the quantum cryptographic key generated in the quantum key distribution system  100 , thereby enhancing the security of the communication system. 
       FIG. 2  is a block diagram illustrating a quantum key distribution system  100  according to an embodiment of the present disclosure. As illustrated in  FIG. 2 , the quantum key distribution system  100  according to the embodiment of the present disclosure distributes a quantum cryptographic key to a transmitter  110  and a receiver  120 . The quantum key distribution system  100  may include i) the transmitter  110  that divides an optical signal into a first optical signal passing through a first path P 1  and a second optical signal passing through a second path P 2 , which is longer than the first path P 1 , and sequentially transmits the optical signals and ii) the receiver  120  that transmits the first optical signal and the second optical signal to the transmitter  110  through a quantum channel  130  after the first optical signal and the second optical signal incident through the quantum channel  130 , pass through a polarization-dependent element  123 , are reflected on a Faraday mirror  124 , and then pass through the polarization-dependent element  123  again. 
     Here, in the quantum key distribution system  100  according to the embodiment of the present disclosure, the difference (dL=L P2 −L P1 ) between the length L P1  of the first path P 1  and the length L P2  of the second path P 2  in the transmitter  110  is equal to or greater than twice the distance (=D) between the polarization-dependent element  123  and the Faraday mirror  124  (dL≥2D). 
     In the quantum key distribution system  100  in which an optical signal travels back and forth through the same optical link as in a two-way structure, according to an embodiment of the present disclosure, the receiver  120  is configured using the polarization-dependent element  123 , and the polarization-dependent element  123  may be controlled using a one-time continuous control signal. Accordingly, it is not necessary to perform a non-continuous control signal two or more times. Therefore, the quantum key distribution system  100  according to an embodiment of the present disclosure corrects an error caused by a polarization-dependent characteristic. 
     Hereinafter, the quantum key distribution system  100  according to the embodiment of the present disclosure will be described in detail with reference to  FIG. 3 . 
     First, as illustrated in  FIG. 3 , the transmitter  110  may include a light source  111  and a beam splitter BS  113 . The light source  111  generates an optical signal. The beam splitter BS  113  splits the optical signal into a first optical signal and a second optical signal, outputs the first optical signal to a first path P 1 , and outputs the second optical signal to a second path P 2 . 
     A delay line DL  115  extending the propagation path of the second optical signal may be provided in the second path P 2 . 
     Further, as illustrated in  FIG. 3 , a polarized beam splitter PBS  116  may be provided in the transmitter  110  such that the first optical signal proceeding through the first path P 1  may be polarized to have first polarization, and the second optical signal proceeding through the second path P 2  may be polarized to have second polarization perpendicular to the first polarization. 
     Here, the polarized beam splitter  116  may enable the first optical signal transmitted from the receiver  120  to proceed to the second path P 2  and enables the second optical signal to proceed to the first path P 1 . 
     As illustrated in  FIG. 3 , in the receiver  120 , a one-time continuous control signal may be applied as a control signal for controlling the operation of the polarization-dependent element  123  while the first optical signal or the second optical signal passes through the polarization-dependent element  123  (e.g., a first phase modulator  123   a  and an intensity modulator  123   b ), is reflected on the Faraday mirror  124 , and then passes through the polarization-dependent element  123  again. 
     As illustrated in  FIG. 3 , the receiver  120  may include a plurality of polarization-dependent elements  123 . In this case, the difference (dL=L P2 −L P1 ) between the length L P1  of the first path P 1  and the length L P2  of the second path P 2  may be equal to or greater than twice the distance (=D) between one (e.g., the first phase modulator  123   a ) of the polarization-dependent elements  123 , which is distant from the Faraday mirror  124 , and the Faraday mirror  124  (dL≥2D). 
     The receiver  120  may include the first phase modulator  123   a  and the intensity modulator  123   b  as the plurality of polarization-dependent elements  123 , and the first phase modulator  123   a  may modulate only the phase of the second optical signal. 
     In addition, referring to  FIG. 3 , when a second phase modulator PMB  114  is positioned on the first path P 1 , the first phase modulator  123   a  in the receiver  120  may modulate only the phase of the first optical signal. 
     The receiver  120  may include the first phase modulator  123   a  and the intensity modulator  123   b  as the plurality of polarization-dependent elements  123 , and the intensity modulator  123   b  may modulate the intensity of the first optical signal and the intensity of the second optical signal. 
     Hereinafter, regarding the quantum key distribution method, the quantum key distribution device, and the quantum key distribution system  100  according to the embodiment of the present disclosure, the operation of each individual component will be described in detail with reference to  FIG. 3 . 
     First, as illustrated in  FIG. 3 , in the transmitter  110  according to an embodiment of the present disclosure, an optical signal generated in the light source  111 , such as a laser generation device, may be input to the beam splitter BS  113  via a circulator CIR  112 . 
     The beam splitter  113  may split the input optical signal into a first optical signal and a second optical signal, transmit the first optical signal to the first path P 1  that is a short path ({circle around ( 3 )} of  FIG. 3 ), and transmit the second optical signal to the second path P 2  having the delay line DL ({circle around ( 2 )} of  FIG. 3 ). 
     The first optical signal and the second optical signal are input to the quantum channel  130  via the polarized beam splitter PBS  116 . 
     Here, due to the difference between the length L P1  of the first path P 1  and the length L P2  of the second path P 2  including the delay line  115 , the second optical signal via the second path P 2  is output to the quantum channel  130  at an interval of the difference (dL=L P2 −L P1 ) between the length L P2  of the second path P 2  and the length L P1  of the first path P 1 . 
     Further, the polarized beam splitter PBS  116  enables the output of the first optical signal having via the first path P 1  to have vertical (V) polarization, and the output of the second optical signal via the second path P 2  to have horizontal (H) polarization perpendicular to the polarization of the first optical signal. 
     The first optical signal and the second optical signal reaching the receiver  120  through the quantum channel  130  are distorted to have arbitrary polarizations V′ and H′, respectively, while passing through the quantum channel  130 . The first optical signal and the second optical signal input to the receiver  120  reach the Faraday mirror FM  124  via a storage line SL, the first phase modulator PM A    123   a,  and the intensity modulator IM  123   b.    
     Accordingly, since the Faraday mirror FM  124  reflects the polarizations of the incident first optical signal and second optical signal by rotating the same by 90 degrees, the polarization V′ of the first optical signal, which is incident first, is reflected as being changed to H′, and the polarization H′ of the second optical signal, which is subsequently incident, is reflected as being changed to V′. 
     The first optical signal and the second optical signal reflected from the Faraday mirror FM  124  are input to the intensity modulator IM  123   b  and the first phase modulator PM A    123   a.  Accordingly, the intensity modulator IM  123   b  adjusts the strength of the optical signals to generate decoy data (decoy state). 
     Here, the intensity modulator IM  123   b  may modulate both the intensity of the first optical signal and the intensity of the second optical signal to generate the decoy data (decoy state). 
     The first phase modulator PM A    123   a  may change only the phase of the second optical signal among the optical signals incident to generate a cryptographic key. Accordingly, the phase of the second optical signal via the first phase modulator PM A    123   a  is changed from V′ to V′a. 
     Subsequently, the first optical signal and the second optical signal are converted into single photons (or pseudo-single photons) via a variable attenuator VA  121 , which are then transmitted back to the transmitter  110  through the quantum channel  130 . 
     Accordingly, since the first optical signal and the second optical signal at a single photon level transmitted to the transmitter  110  are transmitted back through the same path as the optical channel used when first transmitted to the receiver  120 , polarization distortion occurs to the same degree of polarization distortion in the transmission from the transmitter  110  to the receiver  120 . Consequently, the polarization distortion occurring in the quantum channel  130  is offset to disappear. 
     As a result, the first optical signal reaches the transmitter  110  again with the polarization H′ corrected to H, and the second optical signal reaches the transmitter  110  again with the polarization V′a corrected as Va. 
     Subsequently, the second optical signal, which is first transmitted through the second path P 2  having the delay line DL of the transmitter  110 , has the polarization rotated by 90 degrees by the Faraday mirror FM  124  and thus travels through the first path P 1  by the polarized beam splitter PBS  116 , while the first optical signal, which is transmitted through the first path P 1 , travels via the delay line DL  115  through the second path P 2  by the polarized beam splitter PBS  116 . As a result, the first optical signal and the second optical signal travel by the same distance with respect to the beam splitter BS  113  of the transmitter  110 , thus reaching the beam splitter BS  113  at the same time, and cause interference according to a phase change by the first phase modulator PM A    123   a  and the second phase modulator PMB  114 . 
     Here, in the receiver  120  of the quantum key distribution system  100 , the first phase modulator PM A    123   a  and the intensity modulator (IM)  123   b,  which are polarization-dependent, perform two phase controls at each of entrance and exit to correct a polarization-dependent characteristic. Accordingly, since an optical signal entering the first phase modulator PM A    123   a  and the intensity modulator (IM)  123   b  and an optical signal exiting therefrom have a polarization difference of 90 degrees by the Faraday mirror (FM)  124 , a phase control error due to polarization may be corrected by two phase controls. 
     However, in the quantum key distribution system  100 , the first optical signal and the second optical signal transmitted from the transmitter  110  enter the receiver  120  at the interval of the difference (dL=L P2 −L P1 ) between the length L P1  of the first path P 1  and the length L P2  of the second path P 2 . 
     As illustrated in  FIG. 3 , a circuit of the receiver  120  may be configured by disposing the storage line SL  122 , the first phase modulator PM A    123   a,  the intensity modulator IM  123   b,  and the Faraday mirror FM  124  in order. 
     Here, the polarization-dependent element  123  may be disposed regardless of order. That is, when the polarization-dependent element  123  and the Faraday mirror FM  124  are successively configured after the storage line SL  122  ({circle around ( 1 )} of  FIG. 3 ), if the distance including the first phase modulator PM A    123   a,  the intensity modulator IM  123   b,  and the Faraday mirror FM  124  of the receiver  120  in this configuration is defined as D, and D is configured to be ½ of dL or shorter, the second optical signal does not reach the first phase modulator PM A    123   a  until the first optical signal passes through the first phase modulator PM A    123   a,  is reflected on the Faraday mirror FM  124 , and then passes through the first phase modulator PM A    123   a  again. 
     Here, the positions of the first phase modulator PM A    123   a  and the intensity modulator IM  123   b  may be changed. Accordingly, when the intensity modulator IM  123   b  is positioned first, the second optical signal does not reach the intensity modulator IM  123   b  until the first optical signal passes through the intensity modulator IM  123   b,  is reflected on the Faraday mirror FM  124 , and then passes through the intensity modulator IM  123   b  again. 
     In the receiver  120 , the first phase modulator PM A    123   a  may operate only for the second optical signal. Accordingly, when D is configured to be ½ of dL or shorter, the first optical signal is not positioned between the first phase modulator PM A    123   a  and the Faraday mirror FM  124  until the second optical signal passes through the first phase modulator PM A    123   a,  is reflected on the Faraday mirror FM  124 , and then enters the first phase modulator PM A    123   a  again. 
     Accordingly, to control the phase of the second optical signal and correct an error caused by polarization distortion using a one-time continuous control signal, when a control signal is applied to the first phase modulator PM A    123   a  for a time of T after the first optical signal leaves the first phase modulator PM A    123   a,  the second optical signal is subjected to phase control twice while entering the first phase modulator PM A    123   a,  being reflected on the Faraday mirror FM  124 , and passing through the first phase modulator PM A    123   a  again for the time of T, thus offsetting a phase error due to polarization. 
     Here, T may be calculated by Equation 1. 
         T= 2 N/c+E (c: speed of light in optical link)   [Equation 1]
 
     Further, to generate decoy data (decoy state) in the receiver  120 , the intensity modulator IM  123   b  needs to operate for both the first optical signal and the second optical signal. Accordingly, when a one-time continuous control signal is applied while the first optical signal and the second optical signal pass through the intensity modulator IM  123   b,  are reflected on the Faraday mirror FM  124 , and then pass through the intensity modulator IM  123   b  again, the first optical signal and the second optical signal are subjected to control twice, thus offsetting a phase error due to polarization. 
     Mode for Carrying out the Invention 
     More specifically,  FIG. 4A  and  FIG. 4B  illustrate an optical signal and a control signal for controlling a polarization-dependent element in the quantum key distribution method, the quantum key distribution device, and the quantum key distribution system according to the embodiment of the present disclosure. 
     First, as illustrated in  FIG. 4A , in a related art, since the first optical signal passes through the polarization-dependent element  123  (S 12 ) between i) a time S 21  when the second optical signal is input to the receiver  120  and passes through the polarization-dependent element  123  and ii) a time S 22  when the second optical signal is reflected on the Faraday mirror  124  and passes through the polarization-dependent element  123  again, it is required to individually control S 21  and S 22  using separate high-speed control signals C 11  and C 12 . 
     However, in the quantum key distribution method, the quantum key distribution device, and the quantum key distribution system according to the embodiment of the present disclosure, as illustrated in  FIG. 4B , the first optical signal is not positioned between i) the time S 21  when the second optical signal is input to the receiver  120  and passes through the polarization-dependent element  123  and ii) the time S 22  when the second optical signal is reflected on the Faraday mirror  124  and passes through the polarization-dependent element  123  again. Therefore, it is possible to control S 21  and S 22  in combination using a single low-speed control signal C 11  according to the embodients. 
     Hereinafter, the quantum key distribution method, the quantum key distribution device, and the quantum key distribution system according to the embodiment of the present disclosure will be described in more detail with reference to  FIG. 3 ,  FIG. 4A , and  FIG. 4B . 
     Hereinafter, although a case where the phase modulator PM A    123   a  is provided as the polarization-dependent element  123  in the receiver  120  is described for illustration, the present disclosure is not necessarily limited thereto. 
     First, in the related art, when the second phase modulator PMB  114  is disposed on the second path P 2  as in  FIG. 3 , as illustrated in  FIG. 4A , after the first optical signal enters the receiver  120  (S 1  of  FIG. 4A ), a first control signal is applied at a time when the second optical signal passes through the first phase modulator PM A    123   a,  thereby modulating the phase of the second optical signal (S 21  of  FIG. 4A ). 
     In addition, the first control signal is applied again at a time when the second optical signal is reflected on the Faraday mirror FM  124  and then passes through the first phase modulator PM A    123   a  again, thereby modulating the phase of the second optical signal (S 22  of  FIG. 4A ). Accordingly, the second optical signal is subjected to phase control twice while passing through the first phase modulator PM A    123   a,  thereby correcting a phase error by polarization. 
     However, as illustrated in  FIG. 4A , the second optical signal may reach the first phase modulator PM A    123   a  before the first optical signal reaches the first phase modulator PM A    123   a  again after passing through the first phase modulator PM A    123   a  and being reflected on the Faraday mirror FM  124  (S 21  of  FIG. 4A ). 
     Accordingly, in the related art, it is required, with respect to the first phase modulator PM A    123   a,  to apply a first control signal for the second optical signal at the time S 21  and then remove the first control signal, and to apply a second control signal for the second optical signal at the time S 22  and then remove the second control signal. Thus, to properly control the first optical signal and the second optical signal, it is required to accurately control the first optical signal and the second optical signal while quickly converting the control signals for the first phase modulator PM A    123   a.    
     However, in the quantum key distribution method, the quantum key distribution apparatus, and the quantum key distribution system according to the embodiment of the present disclosure, the difference (dL=L P2 −L P1 ) between the length L P1  of the first path P 1  and the length L P2  of the second path P 2  in the transmitter  110  is equal to or greater than twice the distance (=D) between the polarization-dependent element  123  and the Faraday mirror  124  (dL≥2D). Therefore, the polarization-dependent element  123  may be controlled using a one-time continuous control signal, and an error caused by the polarization-dependent characteristic may be corrected. 
     More specifically, as illustrated in  FIG. 4B , in the present disclosure, the second optical signal cannot reach the phase modulator PM A    123   a  until the first optical signal entering the receiver  120  passes through the first phase modulator PM A    123   a  (S 11  of  FIG. 4B ), is reflected on the Faraday mirror FM  124 , and then pass through the first phase modulator PM A    123   a  again (S 12  of  FIG. 4B ) 
     Accordingly, since the first optical signal has already passed through the first phase modulator PM A    123   a  at the time when the second optical signal enters the receiver  120 , the first phase modulator PM A    123   a  may be controlled using a one-time continuous control signal for the second optical signal as illustrated in  FIG. 4B , thereby effectively correcting an error by the polarization-dependent characteristic (C 11  of  FIG. 4B ). 
     Hereinafter, although a case where the intensity modulator IM  123   b  is provided as the polarization-dependent element  123  in the receiver  120  is described for illustration, the present disclosure is not necessarily limited thereto. 
     First, in the related art, as illustrated in  FIG. 4A , when the first optical signal enters the receiver  120  (S 1  of  FIG. 4A ), a first control signal is applied at a time when the first optical signal passes through the intensity modulator IM  123   b,  thereby modulating the intensity of the first optical signal (S 11  of  FIG. 4A ). 
     In addition, the first control signal is applied again at a time when the first optical signal is reflected on the Faraday mirror FM  124  and then passes through the  3   0  intensity modulator IM  123   b  again, thereby modulating the intensity of the first optical signal (S 12  of  FIG. 4A ). Accordingly, the first optical signal is subjected to phase control twice while passing through the intensity modulator IM  123   b,  thereby correcting a phase error by polarization. 
     However, as illustrated in  FIG. 4A , the second optical signal may reach the intensity modulator IM  123   b  before the first optical signal reaches the intensity modulator IM  123   b  again after passing through the intensity modulator IM  123   b  and being reflected on the Faraday mirror FM  124  (S 21  of  FIG. 4A ). 
     Accordingly, in the related art, it is required, with respect to the intensity modulator IM  123   b,  to apply a first control signal for the first optical signal at the time S 11  of  FIG. 4A  and then remove the first control signal, and to apply a second control signal for the second optical signal at the time S 21  of  FIG. 4A  and then remove the second control signal. Thus, to properly control the first optical signal and the second optical signal, it is required to accurately control the first optical signal and the second optical signal while quickly converting the control signals for the intensity modulator IM  123   b.    
     However, in the quantum key distribution method, the quantum key distribution apparatus, and the quantum key distribution system according to the embodiment of the present disclosure, the difference (dL=L P2 −L P1 ) between the length L P1  of the first path P 1  and the length L P2  of the second path P 2  in the transmitter  110  is equal to or greater than twice the distance (=D) between the polarization-dependent element  123  and the Faraday mirror  124  (dL≥2D). Therefore, the polarization-dependent element  123  may be controlled using a one-time continuous control signal, and thus an error caused by the polarization-dependent characteristic may be corrected according to the embodiment. 
     Accordingly, in the two-way quantum key distribution system  100  according to the present disclosure, the receiver  120  and the transmitter  110  are designed such that, for example, by adjusting the length of the delay line DL  115  of an interferometer disposed in the transmitter  110  and the interval between the Faraday mirror FM  124  and the first phase modulator PM A    123   a  and the intensity modulator IM  123   b  disposed in the receiver  120 , the first optical signal among the first optical signal and the second optical signal transmitted by the transmitter  110  is reflected by the Faraday mirror FM  124 , and then can exit from the first phase modulator PM A    123   a  of the receiver  120  before the second optical signal reaches the first phase modulator PM A    123   a  of the receiver  120  so that the first optical signal is not affected while the second optical signal is affected by the first phase modulator PM A    123   a  and the intensity modulator IM  123   b.    
     Although the case of controlling the first optical signal and the second optical signal, for example, using the first phase modulator  123   a  and the intensity modulator  123   b  has been described, the present disclosure is not necessarily limited thereto, and various other polarization-dependent elements  123  may be employed. 
     Furthermore, in the quantum key distribution system  100  according to the embodiment of the present disclosure, although the plurality of polarization-dependent elements  123  of the receiver  120  may be successively disposed as illustrated in  FIG. 3  (the first phase modulator  123   a  and the intensity modulator  123   b  of  FIG. 3 ), the plurality of polarization-dependent elements  123  may be configured in various forms, such as being disposed apart using an optical element, for example, a beam splitter BS ( FIG. 5A ) or a circulator ( FIG. 5B ), to separate an optical link or to change a direction as illustrated in  FIG. 5A  and  FIG. 5B . Here, each polarization-dependent element  123  may individually modulate the first optical signal and the second optical signal depending on purposes or may modulate both the first optical signal and the second optical signal. 
       FIG. 6  illustrates the transmitter  110  of the quantum key distribution system  100  according to an embodiment of the present disclosure, and  FIG. 7  illustrates the receiver  120  of the quantum key distribution system  100  according to an embodiment of the present disclosure. 
     Since the transmitter  110  and the receiver  120  have been explained in detail in the foregoing description of the quantum key distribution system  100  according to the embodiment of the present disclosure, the gist of the configuration and operation of the transmitter  110  and the receiver  120  of the quantum key distribution system  100  according to an embodiment of the present disclosure will be described hereinafter. 
     First, as illustrated in  FIG. 6 , the transmitter  110  may distribute a quantum cryptographic key with the receiver  120 . The transmitter  110  may include i) a light source  111  that generates an optical signal, ii) a beam splitter  113  that splits the optical signal into a first optical signal and a second optical signal, outputs the first optical signal to a first path P 1 , and outputs the second optical signal to a second path P 2 , and iii) a delay line  115  provided in the second path P 2  to extend the propagation path of the second optical signal. 
     Here, the difference (dL=L P2 −L P1 ) between the length L P1  of the first path P 1  and the length L P2  of the second path P 2  is equal to or greater than twice the distance (=D) between a polarization-dependent element  123  and a Faraday mirror  124  provided in the receiver  120  (dL≥2D). 
     As illustrated in  FIG. 7 , the receiver  120  may distribute a quantum cryptographic key with the transmitter  110 . The receiver  120  may include i) a polarization-dependent element  123  of which an operation characteristic changes depending on polarization of an optical signal and ii) a Faraday mirror  124  that reflects an entering optical signal by rotating polarization of the optical signal by 90 degrees. Here, when an optical signal generated in the transmitter  110  is split into a first optical signal passing through a first path P 1  and a second optical signal passing through a second path P 2 , which is longer than the first path P 1 , and the first optical signal and the second optical signal are sequentially transmitted, the first optical signal and the second optical signal incident through a quantum channel  130  are reflected on the Faraday mirror  124  via the polarization-dependent element  123 , pass through the polarization-dependent element  123  again, and are then transmitted to the transmitter  110  through the quantum channel  130 . 
     Here, the difference (dL=L P2 −L P1 ) between the length L P1  of the first path P 1  and the length L P2  of the second path P 2  in the transmitter  110  is equal to or greater than twice the distance (=D) between a polarization-dependent element  123  and a Faraday mirror  124  (dL≥2D). 
     According to the embodiment, the receiver  120  is configured using the polarization-dependent element  123  in the quantum key distribution system  100  in which an optical signal travels back and forth through the same optical link as in a two-way structure. Therefore, the quantum key distribution system  100  according to the embodiment may control the polarization-dependent element  123  using a one-time continuous control signal, thereby correcting an error caused a polarization-dependent characteristic. 
     The foregoing description is provided merely to explain the technical idea of the present disclosure, and it will be apparent to those having ordinary skill in the art to which this disclosure belongs that various modifications and variations can be made in the present disclosure without departing from the essential characteristics of the present disclosure. The embodiments disclosed herein are provided not to limit but to describe the technical idea of the present disclosure and do not limit the scope of the present disclosure. The scope of the present disclosure should be construed as being defined by the appended claims, and any technical ideas within the appended claims and their equivalents should be construed as being included in the scope of the present disclosure.