Systems and methods for quantum key distribution - Patents.com

Abstract

To provide a system and a method for quantum key distribution.SOLUTION: A system includes a first data processing device (10), a second data processing device (11), and a transmission line (12) between the first data processing device (10) and the second data processing device (11). The transmission line (12) includes an internal optical fiber (20), a shield layer (21) surrounding the internal optical fiber (20), and an external optical fiber (22) surrounding the shield layer (21). The first data processing device (10) and the second data processing device (11) determine a shared key by quantum key distribution via a quantum signal along the internal optical fiber (20). The system determines external signal loss along the external optical fiber (22).SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a system for quantum key distribution. Additionally, a method for quantum key distribution is disclosed. [Background technology]

[0002] When using optical fiber as a quantum information channel to establish a shared key by quantum key distribution (QKD), there are generally two types of losses that can appear: natural signal loss and local (possibly artificial) leakage. The former are caused by light scattering on impurities and density variations in the optical fiber and are homogeneous throughout the optical fiber, while the latter can be deliberately introduced locally by an eavesdropper.

[0003] In general, it would be difficult for an eavesdropper to exploit natural signal losses for two main reasons. First, intercepting such losses would generally require large antennas covering a significant portion of the optical fiber line, which would be difficult to provide undetected, and currently available measurement equipment would cover at most about one centimeter of the optical fiber line. Second, scattering would fundamentally change parameters such as the phase and shape of the transmitted wave package. Thus, to decipher a scattered signal, an eavesdropper would need to transform the state of the intercepted photons, which transformation could be considered as comparable to Maxwell's demon's operations in terms of difficulty to perform.

[0004] Nevertheless, the risk remains that an eavesdropper can intercept and decode the natural signal losses from standard optical fiber.

[0005] US Pat. No. 5,399,633 relates to an optical fiber with concentrically arranged cores separated by a cladding layer for trapping light and preventing crosstalk between the cores. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] European Patent Application Publication No. 0105461 Summary of the Invention [Problem to be solved by the invention]

[0007] It is an object of the present disclosure to provide improved techniques for securely transmitting data via quantum key distribution, in particular to reduce the risk of successfully intercepting signals transmitted over optical fibers. [Means for solving the problem]

[0008] To achieve that object, a system and a method for quantum key distribution are provided as set forth in the independent claims. Further embodiments are disclosed in the dependent claims.

[0009] According to one aspect, there is provided a system for quantum key distribution, comprising a first data processing device, a second data processing device, and a transmission line between the first data processing device and the second data processing device. The transmission line includes an inner optical fiber, a shielding layer surrounding the inner optical fiber, and an outer optical fiber surrounding the shielding layer. The first data processing device and the second data processing device determine a shared key by quantum key distribution via a quantum signal along the inner optical fiber. The system determines an external signal loss along the outer optical fiber.

[0010] According to another aspect, a method for quantum key distribution is provided, which can be implemented in a system comprising a first data processing device, a second data processing device, and a transmission line between the first and second data processing devices, the transmission line including an inner optical fiber, a shielding layer surrounding the inner optical fiber, and an outer optical fiber surrounding the shielding layer. The method includes determining a shared key between the first and second data processing devices by quantum key distribution via a quantum signal along the inner optical fiber, and determining an external signal loss along the outer optical fiber.

[0011] The claimed system and method may allow for a step of determining the misalignment and / or damage done to the external or internal optical fiber. In contrast to normal optical fibers, the particular design of the transmission line may significantly reduce the probability of undetected interception of scattering losses. In general, any local intrusion may provide an eavesdropper with only a very small number of scattered photons. To intercept a sufficient amount of scattering losses, the eavesdropper would need to break through a large portion of the controlled external optical fiber. Before such a breakthrough could be effectively performed, the protocol may be terminated (e.g. by the first and second data processing devices corresponding to the legitimate user). A further layer of security may be provided by a step of determining the external signal loss along the external optical fiber. The claimed system may therefore tolerate a significant signal leakage rate without the need to immediately terminate the key exchange as soon as a channel breakthrough / intrusion event is detected.

[0012] In the present disclosure, a layer surrounding a fiber should be understood as the inner surface of the layer surrounding the outer surface of the fiber (along the radial / lateral direction). Correspondingly, a fiber surrounding a layer should be understood as the inner surface of the fiber surrounding the outer surface of the layer. Surrounding may include surrounding along the entire perimeter of the side.

[0013] The inner optical fiber may be in physical contact with the shielding layer. The shielding layer may be in physical contact with the inner optical fiber and / or the outer optical fiber. The outer optical fiber may be in physical contact with the outer jacket. In particular, an outer surface of the inner optical fiber may be in physical contact with an inner surface of the shielding layer, preferably along the entire periphery. An outer surface of the shielding layer may be in physical contact with an inner surface of the outer optical fiber, preferably along the entire periphery. An outer surface of the outer optical fiber may be in physical contact with an inner surface of the outer jacket, preferably along the entire periphery.

[0014] The inner optical fiber may include an inner fiber core and preferably an inner cladding layer. The inner fiber core and the inner cladding layer may be in physical contact. The outer optical fiber may include an outer fiber core and preferably two outer cladding layers. The outer fiber core may be in physical contact with the outer cladding layers. The outer optical fiber may surround a first outer cladding layer. The outer optical fiber may be surrounded by a second outer cladding layer.

[0015] The cladding layers may each include a cladding material having a lower refractive index than the respective fiber core material of the fiber core. The fiber core material and / or the cladding material may be, for example, silica.

[0016] It may be provided that quantum key distribution is not performed via external optical fibers, in other words the shared key is determined only via internal optical fibers, in which case the external optical fibers may only serve to determine (external) signal loss and / or intrusion events.

[0017] The transmission line may include an optical amplifier, preferably an erbium-doped fiber amplifier or a Raman amplifier. The transmission line may include a plurality of optical amplifiers. It may be specified that the amplifier distance between two adjacent amplifiers is 30 km to 200 km, preferably 30 km to 60 km. The plurality of optical amplifiers may be equally spaced along the transmission line.

[0018] Where the system performs steps / determines amounts, such steps may be performed / such amounts may be determined in at least one of the first data processing device, the second data processing device and the third data processing device.

[0019] The inner optical fiber and the outer optical fiber may be arranged concentrically. Preferably, the inner optical fiber, the shielding layer, and the outer optical fiber are arranged concentrically. In particular, at least two, and preferably each, of the inner fiber core, the inner cladding layer, the shielding layer, the first outer cladding layer, the outer fiber core, the second outer cladding layer, and the outer jacket may be arranged concentrically.

[0020] The shielding layer may laterally shield signals within the inner optical fiber, in other words, the shielding layer may prevent signals along the inner optical fiber from escaping laterally.

[0021] In particular, the shielding layer may be made of a metal, which may be, for example, one of copper, aluminum, copper, gold, nickel, silver, bronze, and nichrome.

[0022] In general, the shielding layer may include or consist of a conductive material.

[0023] Alternatively, the shielding layer may be made of quartz and / or have a concentration of 10 15 cm -3 ~10 18 cm -3 , or 0.75 ppm to 75 ppm of the shield layer dopant material. The shield layer dopant material may be one of erbium (Er), neodymium (Nd), europium (Eu), terbium (Tb), ytterbium (Yb), and thulium (Tm). The shield layer dopant material concentration may be substantially constant along the length of the transmission line and / or the perimeter of the shield layer. The shield layer dopant material concentration may also vary along the length of the transmission line and / or the perimeter of the shield layer.

[0024] The shielding layer may have a shielding layer thickness of 10% to 300%, preferably 25% to 200%, more preferably 50% to 100% of the outer optical fiber thickness. The inner fiber core thickness and / or the outer fiber core thickness may be 5 μm to 10 μm. The inner cladding thickness and / or the outer cladding thickness may be 100 μm to 250 μm. The shielding layer thickness may be approximately constant along the length of the transmission line and / or the periphery of the shielding layer.

[0025] The internal and / or external optical fibers have a concentration of 10 12 cm -3 ~10 14 cm -3 The inner optical fiber (particularly the inner fiber core) may contain an optical fiber dopant material having a concentration of 10 12 cm -3 ~10 14 cm -3 In addition, the outer optical fiber (particularly the outer fiber core) may include an inner optical fiber dopant material having a concentration of 10 12 cm -3 ~10 14 cm -3 The internal optical fiber dopant material and the external optical fiber dopant material may be different.

[0026] The (internal and / or external) optical fiber dopant material may be one of Al, P, N, and Ge. The (internal and / or external) optical fiber dopant material concentration may be substantially constant along the length of the transmission line and / or the circumference of the (internal / external) optical fiber. The (internal and / or external) optical fiber dopant material concentration may also vary along the length of the transmission line and / or the circumference of the (internal / external) optical fiber.

[0027] The inner optical fiber and / or the outer optical fiber may also be free of optical fiber dopant material.

[0028] The system may determine the external signal loss as a function of a position in the transmission line. In other words, the external signal loss may be determined as a function of a position along the transmission line. In particular, for each position along the transmission line, a corresponding external signal loss may be determined. Thus, the external signal loss may correspond to an (external) signal loss profile.

[0029] The system may determine an internal signal loss along the internal optical fiber, and preferably further determine the intrusion event based on the internal loss. The internal signal loss may be determined as a function of position within the transmission line.

[0030] The internal signal loss and / or the external signal loss may be determined by at least one of the first data processing device, the second data processing device and the third data processing device.

[0031] The system may repeatedly determine the external signal loss and / or the internal signal loss. For example, the external signal loss and / or the internal signal loss may be repeatedly determined at a time interval of 1 ms to 50 s, preferably 20 ms to 10 s, in particular one of 10 ms to 100 ms, 50 ms to 500 ms, 100 ms to 1000 ms, 0.5 s to 5 s, and 5 s to 10 s. The time interval may depend on the length of the transmission line. The time interval may increase with the length of the transmission line.

[0032] The system may determine external signal loss and / or internal signal loss during the determination of the shared key, in particular during / in temporal association with the transmission of a key signal pulse for determining the shared key.

[0033] The external signal loss and / or the internal signal loss may further be determined after determining the shared key.

[0034] The system may determine an initial external signal loss along an outer optical fiber and / or determine an initial internal signal loss along an inner optical fiber.

[0035] The initial external signal loss and / or the initial internal signal loss may be determined prior to determining the shared key, in particular prior to sending a key signal pulse for determining the shared key.

[0036] The first data processing device and / or the second data processing device may transmit a plurality of test signals via a transmission line, preferably an internal optical fiber and / or an external optical fiber, and determine the external signal loss and / or the internal signal loss from backscattered test signal components of the plurality of test signals.

[0037] In particular, the first data processing device and / or the second data processing device may transmit a plurality of inner test signals via an inner optical fiber and determine the inner signal loss from backscattered inner test signal components of the plurality of inner test signals. Further, the first data processing device and / or the second data processing device may transmit a plurality of outer test signals via an outer optical fiber and determine the outer signal loss from backscattered outer test signal components of the plurality of outer test signals.

[0038] The (internal and / or external) test signals may for example be test pulses, in particular coherent optical pulses. The average test pulse strength of the test pulses is, for example, 10 3 ~10 10 times, preferably 10 5 ~10 8 Each of the key signal pulses may be 10 times larger. 2 ~10 5 Each test pulse may contain 10 8 ~10 12 may include photons.

[0039] The first data processing device and / or the second data processing device may receive backscattered test signal components, in particular backscattered inner test signal components and / or backscattered outer test signal components.

[0040] The outer test pulse may be transmitted before, during and / or after the transmission of the key signal pulse, and the inner test pulse may be transmitted before, alternating with and / or after the transmission of the key signal pulse.

[0041] The internal signal loss and / or the external signal loss may be determined by optical time domain reflectometry. The external signal loss may include an (external optical fiber) reflectogram. The internal signal loss may include an internal optical fiber reflectogram.

[0042] In addition to, or as an alternative to, determining the internal / external signal loss from the backscattered test pulse components, the system may also determine the external signal loss and / or the internal signal loss from the transmitted components of the multiple test pulses.

[0043] In particular, the system may determine an outer signal loss from a transmitted outer test signal component of a plurality of test pulses. The system may also determine an inner signal loss from a transmitted inner test signal component of a plurality of test pulses.

[0044] The internal and / or external signal loss may be determined by transmittometry, i.e., by determining the internal / external signal loss from the transmitted signal pulses and / or test pulses. In particular, the internal and / or external signal loss may be determined by comparing the input power to the output power of a test pulse transmitted through the entire transmission line.

[0045] The system may determine an intrusion event based on an external signal loss and / or abort (terminate) the determination of the shared key based on an external signal loss. Additionally or alternatively, the system may determine an intrusion event based on an internal signal loss and / or abort the determination of the shared key based on an internal signal loss.

[0046] The system may revoke the shared key based on internal signal loss and / or external signal loss.

[0047] The step of determining the intrusion event may further comprise classifying the internal signal loss (depending on the transmission line position) and / or the external signal loss by a signal loss type (depending on the transmission line position). The signal loss type may be at least one of: deviation of exponential power decay, poor quality splice, physical connector, angled physical connector, and fiber bend. The classifying step may be performed, for example, via a machine learning method, for example an artificial neural network. The classifying step may be performed in at least one of the first data processing device, the second data processing device, and the third data processing device. The internal signal loss and / or the external signal loss may correspond to reflectogram features and / or transmitgram features from transmittometry.

[0048] The step of determining the intrusion event may further comprise the step of determining a signal loss type length indicative of a length of the signal loss type along the transmission line. The step of determining the intrusion event may be based on the signal loss type, preferably the signal loss type length, exceeding a threshold. The threshold may be, for example, between 1 m and 10 km, preferably between 100 m and 2 km, more preferably between 0.5 km and 1.5 km.

[0049] Thus, determining an intrusion event may be based on the loss of signal type and / or the loss of signal type length.

[0050] The step of determining the intrusion event may further comprise the step of determining an initial signal loss type, preferably an initial signal loss type length indicating a length of the initial signal loss type along the transmission line. The step of determining the intrusion event may further be based on a determined difference between the signal loss type at the transmission line location / region and the initial signal loss type (and / or between the signal loss type length and the initial signal loss type length).

[0051] The system may determine a severity index for the intrusion event. Determining the severity index may include determining a magnitude and / or probability of an information leak.

[0052] It may be provided that in response to a determination of an intrusion event, and in particular a severity index of the intrusion event, the determination of the shared key is aborted.

[0053] The protocol may be terminated based on a determination that the total amount of loss from the inner fiber core (including natural scattering loss and local leakage) exceeds a predetermined limit, which may depend on one of the target key distribution rate, the length of the transmission line, the distance between two optical amplifiers, the number of optical amplifiers, and the optical amplifier gain.

[0054] The transmission line may include a second shielding layer surrounding the external optical fiber, and a second external optical fiber surrounding the second shielding layer.

[0055] The characteristics relating to the shielding layer and the outer optical fiber may apply mutatis mutandis to the second shielding layer and / or the second outer optical fiber. The inner optical fiber, the shielding layer, and the outer optical fiber, the second shielding layer and the second outer optical fiber may be arranged concentrically. The system may determine a second outer signal loss along the second outer optical fiber. The step of determining an intrusion event may (additionally or alternatively) be based on the second outer signal loss.

[0056] It may be specified that the quantum key distribution is not performed via the second external optical fiber. In other words, it may be specified that the shared key is determined only via the internal optical fiber and the (first) external optical fiber. Alternatively, it may be specified that the quantum key distribution is not performed via the (first) external optical fiber and the second external optical fiber. In other words, it may be specified that the shared key is determined only via the internal optical fiber.

[0057] Such a design may enable high key distribution rates while retaining high security against eavesdropping in the case of quantum key distribution using the inner optical fiber and a (first) outer optical fiber, with even higher security if only the inner optical fiber is used for quantum key distribution.

[0058] The transmission line may also include a plurality of second shield layers and a plurality of second external optical fibers alternately disposed about the external optical fiber.

[0059] The embodiments described above relating to a system for quantum key distribution may correspondingly be provided for a method for quantum key distribution, and vice versa.

[0060] Embodiments will now be described, by way of example only, with reference to the drawings in which: [Brief description of the drawings]

[0061] [Figure 1] FIG. 1 is a graphical diagram illustrating a system for quantum key distribution and an arrangement of eavesdropping devices. [Diagram 2] FIG. 2 is a graphical diagram showing a segment of a transmission line. [Diagram 3] FIG. 3 is a graphical diagram illustrating a method for quantum key distribution. [Figure 4] FIG. 4 is a diagram showing a reflectogram. [Diagram 5] FIG. 5 is a detailed view showing the reflectogram. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0062] 1 shows a graphical diagram of a system for quantum key distribution and the placement of an eavesdropping device 13 ("Eve"). The system comprises a first data processing device 10 ("Alice") and a second data processing device 11 ("Bob"). The first data processing device 10 may include a first processor 10a and a first memory 10b, and the second data processing device 11 may include a second processor 11a and a second memory 11b.

[0063] The first data processing device 10 and the second data processing device 11 can exchange classical signals and / or quantum signals, for example, via a transmission line 12. For this purpose, the first data processing device 10 and the second data processing device 11 are connected to the transmission line 12. The transmission line 12 may include a quantum channel for transmitting a quantum signal. Furthermore, a classical channel for transmitting a classical signal may be provided. The classical channel may be provided within the transmission line 12 or separately. The transmission line 12 may further include a plurality of optical amplifiers (not shown). The optical amplifiers may be provided at equal intervals along the transmission line 12, for example, with an amplifier interval of 30 km to 200 km, preferably 30 km to 100 km, more preferably 30 km to 60 km.

[0064] The system may include a number of further data processing devices, in particular a third data processing device (not shown) having a third processor and a third memory. The third data processing device may be connected to the transmission line 12. Additionally or alternatively, the third data processing device may exchange classical and / or quantum signals with the first data processing device 10 and / or the second data processing device 11 via further communication channels. If the system executes a step, such step may be executed in at least one of the first data processing device 10, the second data processing device 11 and the third data processing device.

[0065] An eavesdropping device 13 having an eavesdropping processor 13a and an eavesdropping memory 13b represents a device external to the system that may have access to the transmission line 12. The eavesdropping device 13 may be placed on the transmission line 12 such that at least one of the classical and / or quantum signals transmitted over the transmission line 12 is received and / or retransmitted by the eavesdropping device 13. The eavesdropping device 13 may also have access to further communication channels.

[0066] The first memory 10b, the second memory 11b, the third memory, and the eavesdropping memory 13b may each include a quantum memory for storing quantum signals and a classical memory for storing classical signals. The quantum memory may be implemented using optical delay lines, controlled reversible inhomogeneous broadening (CRIB), Duan-Lukin-Cirac-Zoller (DLCZ) type schemes, regeneration with echo suppression (ROSE), and / or hybrid photon echo rephasing (HYPER).

[0067] The first data processing device 10, the second data processing device 11, the third data processing device and the eavesdropping processing device 13 may each comprise means for transmitting and / or receiving quantum states.

[0068] 2 shows a graphical view of a segment of a transmission line 12. The transmission line 12 includes an inner optical fiber 20, a shielding layer 21 surrounding the inner optical fiber 20, and an outer optical fiber 22 surrounding the shielding layer 21. As such, the outer optical fiber 20 represents a hollow optical fiber. Additionally, an outer jacket 23 (preferably including a coating layer and / or a strengthening layer) may surround the outer optical fiber 22. The strengthening layer may include at least one of a resin layer, a polymer buffer layer, and a plastic layer. The strengthening layer may increase the strength of the optical fiber and / or provide isolation from the environment.

[0069] Thus, the transmission line 12 includes a transmission line segment in the radial order of (from inside to outside) an inner optical fiber 20 , a shielding layer 21 , an outer optical fiber 22 , and an outer jacket 23 .

[0070] The inner optical fiber 20 includes an inner fiber core 20a and an inner cladding layer 20b. The outer optical fiber 22 includes an outer fiber core 22a and two outer cladding layers 22b, 22c, with the outer fiber core 22a radially disposed between the outer cladding layers 22b, 22c. The cladding layers 20b, 22b, 22c include a cladding layer material having a lower refractive index than the fiber core material of the fiber cores 20a, 22a.

[0071] Thus, the transmission line 12 includes a transmission line segment in radial order (from inside to outside): an inner fiber core 20a, an inner cladding layer 20b, a shielding layer 21, a first outer cladding layer 22b, an outer fiber core 22a, a second outer cladding layer 22c, and an outer jacket 23.

[0072] The transmission line 12 may further include a second shielding layer surrounding the outer optical fiber 22 and a second outer optical fiber surrounding the second shielding layer, while the outer jacket 23 surrounds a second outer optical fiber (not shown).

[0073] The shielding layer 21 can be made of metal. Alternatively, it can be made of quartz and can be highly doped, for example with erbium. The shielding layer 21 therefore shields the scattering losses escaping from the inner optical fiber 20 as the scattered light waves undergo inelastic secondary scattering and are converted into heat. Essentially, the resulting radiation losses cannot be decoded, since they do not contain any information from the signal in the inner optical fiber 20.

[0074] To remove the shielding layer 21 (e.g. to intercept scattering losses or to create artificial leakage from the inner fiber core 20a), an eavesdropper would first need to pass through the outer fiber core 22a. However, such an act would not go unnoticed, and the intrusion event would be detected since the external signal losses along the optical fiber 22, in particular the external (signal) loss profile of the outer optical fiber 22 and / or the outer fiber core 22a, are constantly updated (physical loss control).

[0075] Importantly, the inner fiber core 20 may also be subjected to physical loss control, i.e., internal signal loss may be determined and further used for intrusion detection. Physical loss control may enable quantitative estimation and localization of damage inflicted on the outer fiber core 22 or the inner fiber core 20.

[0076] Fig. 3 shows a graphical illustration of a method for quantum key distribution. The method may be implemented in a system comprising a first data processing device 10, a second data processing device 11 and a transmission line 12 between them. The first data processing device 10 and the second data processing device 11 are connected via an authenticated (public) classical channel, and the (internal) optical fiber 20 of the transmission line 12 serves as a quantum channel.

[0077] In a first step 30, an initial internal loss profile of the inner optical fiber 20 (particularly the fiber core 20a) and an initial external signal loss profile of the outer optical fiber 22 (particularly the fiber core 22a) are determined, said loss profiles essentially representing the natural signal loss of the transmission line 12. In this preliminary step, it must be ensured that no eavesdropper has access to the transmission line 12. The initial loss profiles may be shared between the first data processing device 10 and the second data processing device 11 via an authenticated classical communication channel.

[0078] In a first step 31, a physical loss control of the inner fiber core 20a is performed (by the first data processing device 10 and the second data processing device 11). In particular, an internal loss profile is determined and preferably shared between the first data processing device 10 and the second data processing device 11 via a classical channel.

[0079] By comparing the updated internal loss profile with the initial internal loss profile, the proportion of signals that could have been captured by an eavesdropper, r E For example, in a section of the transmission line 12 that does not contain an optical amplifier, the natural signal loss of this section may be denoted as r0, and the probability of an eavesdropper intercepting the signal may be determined as r E If a message is intercepted, the interception rate r E The total loss r t can be derived from

[0080] (1-r t )=(1-r E )(1-r0) (1)

[0081] Interception rate r E The protocol terminates when becomes too large, so that the legitimate user loses its information advantage over the eavesdropper. The termination of the protocol depends on the length of the transmission line and the distance between the two of the optical amplifiers. In particular, the protocol may terminate if the effective key rate is less than the target key rate value.

[0082] In a second step 32, a bit sequence of sequence length L is determined in the first data processing device 10 using a random number generator.

[0083] In a third step 33, the bit sequence is encoded into a quantum signal comprising a series of L key signal pulses (coherent light pulses), which are transmitted (via the internal optical fiber 20) to the second data processing device 11. The signal bits 0 and 1 correspond to the coherent states |γ0> and |γ1>, respectively. The specific manner of encoding the 0 and 1 signal bits into the parameters of the coherent states |γ0> and |γ1> may be different. For example, the signal bits may be encoded into coherent light pulses of different intensities and the same phase, or into coherent light pulses of the same intensity but different phase.

[0084] The complex amplitude of the coherent state is a determined fraction r of the signal intercepted by an eavesdropper. E It is therefore possible for an optimized coherent state to be transmitted by the first data processing device 10, which corresponds to the maximum key generation rate for a given loss in the quantum channel.

[0085] In a fourth step 34, the quantum signal / coherent light pulse is amplified by a number of optical amplifiers arranged along the transmission line 12. The optical amplifiers may be arranged at equal intervals along the transmission line. Each optical amplifier of the number of optical amplifiers compensates for the natural signal loss along the transmission line 12 from the previous optical amplifier to the respective optical amplifier. As a result, the output signal strength at the second data processing device 11 is equal to the initial signal strength as prepared by the first data processing device 10. Although such signal amplification does not compensate for the artificial losses, they are relatively small since the information advantage of the legitimate users is usually ensured if the artificial losses only account for a few percent and otherwise the protocol is terminated.

[0086] In a fifth step 35, the quantum signal is received and measured by the second data processing device 11. The corresponding received bit sequence is determined in the second data processing device 11. All determined artificial losses give rise to a signal interception rate r E may be considered to represent.

[0087] In a sixth step 36, uncertain signal bits corresponding to quantum measurements at the second data processing device 11 determined as uncertain are discarded from the bit sequence at the first data processing device 10 and from the received bit sequence at the second data processing device 11. For this purpose, the bit positions of the uncertain signal bits are transmitted from the second data processing device 11 to the first data processing device 10 via a classical channel.

[0088] In a seventh step 37, an error rate may be determined by disclosing a part of the bit sequence and / or the received bit sequence via the classical channel and an error correction may be performed by the first data processing device 10 and the second data processing device 11 on the bit sequence and the received bit sequence, respectively. The error correction may be performed, for example, by means of a Low Density Parity Check (LDPC) code. As a result, an error-corrected bit sequence is determined in the first data processing device 10 and the second data processing device 11.

[0089] In an eighth step 38, an amplified key sequence is determined from the error-corrected bit sequence using privacy amplification. The amplified key sequence is shorter than the error-corrected bit sequence, such that any potential eavesdropper has no or negligibly small information about the amplified key sequence. The amplified key sequence represents the shared key sequence between the first data processing device 10 and the second data processing device 11 as a result of the quantum key distribution.

[0090] The first step 31 through the eighth step 38 may then be repeated (arrow 39) and the amplified key sequences may be concatenated to the total shared key until the total length of the shared key is as long as required by the current application.

[0091] During all steps 31 to 38 the external optical fiber is continuously controlled, 30a. That is, an external signal loss profile is determined and shared between the first data processing device 10 and the second data processing device 11, preferably via a classical channel. When the integrity of the external fiber core 22a is compromised to a degree of substantial risk that an eavesdropper would decipher the scattering losses, i.e. when the total length of the damaged transmission line segment is of the order of 1 km, the protocol may be terminated.

[0092] The physical loss control (step 31) of the inner fiber core 20a may also be performed alternating with the transmission of quantum signals of the shared key sequence, for example key signal pulses (of the shared key sequence) may be transmitted alternating with test (signal) pulses for physical loss control.

[0093] The selection of the encoding and measurement of parameters, and the selection of whether the protocol should be terminated, is based on an evaluation of the loss profile of the line, the outer fiber core 22a and the inner fiber core 20a being monitored separately.

[0094] The physical loss control is based on the analysis of the scattered components of a test (signal) pulse transmitted through the optical fibers 20, 22. The test pulse has a relatively high intensity.

[0095] Optical time domain reflectometry is used to measure the backscattered test pulse components in a time series of test pulses. The response delay of the backscattered test pulse components corresponds to the distance / transmission line location of a particular fiber discontinuity, while their magnitude reflects the respective signal loss.

[0096] Figure 4 shows an example reflectogram obtained from optical time-domain reflectometry. Basic measurements were performed using a 2 μs, 1550 nm pulsed laser with a power of less than 100 mW. The experimental data was averaged over 16000 measurements.

[0097] The reflectogram shows the logarithmic power of the backscattered test pulse components as a function of the distance between the reflectometer and the corresponding discontinuity. The reflectometer may be located within or adjacent to the first data processing device 10 or the second data processing device 11.

[0098] Natural signal loss along the transmission line 12 is due to uniform scattering, resulting in an exponential decay of power corresponding to a linear region 40. Reflectogram features 41-44 including deviations from the exponential decay of the reflectogram curve, especially sharp peaks and / or dips in the reflectogram curve, allow classification of signal loss at corresponding locations of the transmission line 12. This is particularly useful in the initial step 30 during key exchange where identification and mitigation of local losses are important for comparison with the determined losses.

[0099] Reflectogram features 41-44 generally correspond to imperfections in transmission line 12 and may represent poor quality splices, bends, and various connectors. Scattering losses from such regions are localized relative to transmission line 12. The peaks in reflectogram features 41-44 may result from excessive scattering by the test pulse undergoing Fresnel reflections in the case of physical connectors. The noisy region 45 on the right side of the reflectogram represents the end of the backscattered signal.

[0100] 5 shows a detailed view of a reflectogram depicting respective reflectogram features 41-44. Reflectogram feature 41 corresponds to a poor quality splice of optical fibers 20, 22, reflectogram feature 42 corresponds to an angled physical connector of optical fibers 20, 22, reflectogram feature 43 corresponds to a bend in optical fibers 20, 22, and reflectogram feature 44 corresponds to another physical connector.

[0101] Additionally or alternatively, the physical loss control (transmission line control) may comprise transmittometry, i.e. the strength of the test pulses transmitted by the first data processing device 10 and received by the second data processing device 11 is analyzed in order to classify the signal loss at each position of the transmission line 12. Classification by analysis of the test pulses received by the second data processing device 11 may also be performed in the second data processing device 11. The first data processing device 10 may also perform classification, in particular by combining measurements of backscattered test pulse components with measurements of test pulses received by the second data processing device 11.

[0102] Each of the interior loss profile, the exterior signal loss profile, the initial interior loss profile, and the initial exterior signal loss profile may include a respective reflectogram and / or transmitgram.

[0103] To distinguish between intrinsic and artificial signal loss, the initial internal / external signal loss profile is used as a reference. To provide a non-reproducible profile of the transmission line 12, i.e. a physically unclonable structure, the fiber cores 20a, 22a may be lightly doped, for example with Al, P, N or Ge. The most common eavesdropping attacks correspond to unitary transformations of the quantum state of the composite system, including the propagating signal and the auxiliary system. However, the only way to redirect photons is to introduce significant changes in the optical fiber medium, which will necessarily result in changes from the (initial) reflectogram and thus be detectable.

[0104] Preferably, the physical loss control is continuous and does not stop even during pauses in the quantum key distribution. For example, it is expected that a test pulse length of 10 ns to 50 ms will allow any real-time mechanical interference to the line to be immediately detectable.

[0105] For example, the separation time between the test pulses may be less than 10 ms, in particular less than one of 1 ms, 100 μs, and 10 μs. The separation time between the test pulses may be less than the round trip time of each test pulse through a transmission line, in particular optical fibers and optical connections.

[0106] If the external signal loss deviation of the outer fiber core 22a is determined at the same transmission line location as the internal loss deviation of the inner fiber core 20a is determined, an intrusion event is determined. In this case, the loss may be considered to have been intercepted by an eavesdropper. Such internal / external signal loss deviations could have been classified as a splice or a fiber bend.

[0107] For a transmission line length of 20,000 km, optical amplification is required for efficient signal transmission. However, the accuracy of physical loss control is not hindered by using optical amplifiers for the following reasons. Rather, the test pulse may be prevented from fading out in the same way as the key signal pulse for quantum key distribution.

[0108] If the optical amplifiers are spaced equally along the transmission line 12 with an amplifier distance d=50 km between each two optical amplifiers, the optical power drops by a factor of about 10 (corresponding to a transmission probability T=0.1). The required amplification factor of a single amplifier is therefore G=10.

[0109] When the test pulse is transmitted from the first data processing device 10, the initial photon number

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[0110] By passing through a series of M amplifiers, each of which independently adds further variation, the total variation is

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[0111] The features disclosed in this specification, the drawings and / or the claims may, alone or in various combinations thereof, be material for realizing various embodiments.

Claims

1. 1. A system for quantum key distribution, comprising: A first data processing device (10); A second data processing device (11); a transmission line (12) between the first data processing device (10) and the second data processing device (11); The transmission line (12) An internal optical fiber (20); a shielding layer (21) surrounding the inner optical fiber (20); and an outer optical fiber (22) surrounding the shielding layer (21), the first data processing device (10) and the second data processing device (11) determine a shared key by quantum key distribution via a quantum signal along the internal optical fiber (20); The system determines an external signal loss along the external optical fiber (22). system.

2. The inner optical fiber (20), the shielding layer (21), and the outer optical fiber (22) are arranged concentrically. The system of claim 1 .

3. The shielding layer (21) laterally shields signals within the internal optical fiber (20). The system of claim 1 .

4. The shield layer (21) is made of a metal or has a concentration of 10 １５ cm －３ ~10 １８ cm －３ The shield layer includes a dopant material. The system of claim 1 .

5. The inner optical fiber (20) and / or the outer optical fiber (22) have a concentration of 10 １２ cm －３ ~10 １４ cm －３ The optical fiber dopant material includes The system of claim 1 .

6. The system determines the external signal loss as a function of position within the transmission line (12). The system of claim 1 .

7. The system determines an internal signal loss along the internal optical fiber (20), and preferably further determines the intrusion event based on the internal loss. The system of claim 1 .

8. The system repeatedly determines the external signal loss. The system of claim 1 .

9. The system determines the external signal loss during the determination of the shared key. The system of claim 1 .

10. The system determines an initial external signal loss along the external optical fiber (22) and / or determines an initial internal signal loss along the internal optical fiber (20). The system of claim 1 .

11. The first data processing device (10) and / or the second data processing device (11) Transmitting a plurality of test signals over the transmission line (12); determining said external signal loss from backscattered test signal components of said plurality of test signals; The system of claim 1 .

12. The system further determines the external signal loss from a transmit component of the plurality of test pulses. The system of claim 11.

13. The system determines an intrusion event based on the external signal loss, and / or aborts the determination of the shared key based on the external signal loss. The system of claim 1 .

14. The transmission line (12) includes a second shielding layer surrounding the external optical fiber (22), and a second external optical fiber surrounding the second shielding layer. A system according to any one of claims 1 to 13.

15. 1. A method for quantum key distribution operable in a system, comprising: The system comprises: A first data processing device (10); A second data processing device (11); a transmission line (12) between the first data processing device (10) and the second data processing device (11); The transmission line (12) An internal optical fiber (20); a shielding layer (21) surrounding the inner optical fiber (20); and an outer optical fiber (22) surrounding the shielding layer (21), The method comprises: determining a shared key between the first data processing device (10) and the second data processing device (11) by quantum key distribution via a quantum signal along the internal optical fiber (20); determining an external signal loss along said external optical fiber (22). method.

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