Eavesdrop-resistant single-photon detector for optical signal detection

Abstract

The single photon detection device (10) of the present invention is a single photon detection device (10) for detecting an optical signal, comprising an optical waveguide (12) and at least two nanowires (16, 18). The optical waveguide (12) is designed to guide the optical signal along an optical axis (14), the at least two nanowires (16, 18) are arranged along the optical axis (14), at least one second nanowire (18) is arranged in front of the first nanowire (16) with respect to the optical axis (14), the at least two nanowires (16, 18) are designed to be superconducting at a predetermined temperature and are configured to generate an output signal in a superconducting state when a threshold intensity of the optical signal is exceeded, the at least two nanowires (16, 18) are designed to have a different threshold intensity from each other. The use of the single photon detection device of the present invention is also a use for recognizing an attack during transmission of a signal encrypted with quantum key distribution.

Description

Technical Field

[0001] The present invention relates to a single photon detector for optical signal detection, including an optical waveguide and at least two nanowires.

[0002] The present invention further relates to the use of the device for recognizing attacks during signal transmission encrypted by quantum key distribution.

Background Art

[0003] Due to the emergence of quantum computers, efficient attacks using quantum computers have become possible, so conventional encryption methods are at risk. In contrast, the method of quantum key distribution (QKD) itself is secure when using a quantum computer. In QKD, due to the characteristics of quantum mechanics, a shared random number is provided to both parties. This number is used as a secret key for signal transmission in a form protected from eavesdropping by a symmetric encryption method in cryptography.

[0004] QKD enables verifiable and secure communication based on physical principles, because the security achieved is not based on assumptions regarding the performance of computers and algorithms or the reliability of trusted persons, but on known physical laws. The security of QKD results from detecting eavesdropping on the key transmission by an attacker.

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the implementation of QKD in a physical system, the physical components used (single photon detectors) also need to meet certain criteria. One of the important prerequisites is that the physical components must function probabilistically without being affected by external attackers.

[0006] The assumed attacks on QKD in single-photon detectors generally aim to move the single-photon detector from the probabilistic operating region to the operating region controlled by the attacker. Such attacks are called detector-blinding attacks. Due to this attack, the detector no longer satisfies the prerequisite of probabilistic behavior required for QKD, rendering the verifiable security ineffective.

[0007] The approach known so far for recognizing detector-blinding attacks in single-photon detectors is to split a part of the light incident on the detector and check whether it has a high optical power that may have been generated by the attacker. This requires additional components such as beam splitters and detectors separated from the actual detection setup. Thus, due to the additional components, the complexity and spatial requirements in the setup increase. Also, new security gaps are masked by the additional components.

[0008] Therefore, an object of the present invention is to provide a single-photon detector that can surely recognize detector-blinding attacks and has reduced complexity.

Means for Solving the Problem

[0009] This object is achieved by the features of the independent claims. The dependent claims relate to preferred developments.

[0010] Thus, the single-photon detection device of the present invention is a single-photon detection device for detecting an optical signal, including an optical waveguide and at least two nanowires, wherein the optical waveguide is designed to guide the optical signal along an optical axis, the at least two nanowires are arranged along the optical axis, and at least one second nanowire is arranged in front of a first nanowire with respect to the optical axis. the at least two nanowires are designed to be superconducting at a predetermined temperature and configured to generate an output signal of an optical signal in a superconducting state above a threshold intensity; The single-photon detection device, wherein the at least two nanowires are designed to have different threshold intensities.

[0011] This object is achieved by the use of said single photon detection device for recognizing attacks during quantum key distribution encrypted signal transmission. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 shows a schematic diagram of a single-photon detection device according to a preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] The core of the present invention is the use of at least two nanowires in the same optical waveguide, which have different threshold intensities and are capable of generating an output signal. That is, in a single-photon detection device, at least two superconducting nanowire single-photon detectors (SNSPDs) are used in the same optical waveguide. SNSPDs have nanowires that are superconducting at sufficiently low temperatures as the actual detection elements. SNSPDs allow output signal generation only when the intensity of the optical signal exceeds a certain threshold (threshold intensity).

[0014] At least two nanowires are arranged along the optical axis of the optical waveguide, so that the optical signal guided in the optical waveguide passes through each nanowire sequentially. Because the nanowires have different threshold intensities, each nanowire generates a different output signal when the optical signal guided in the optical waveguide has a different intensity. Therefore, statistical evaluation of the nanowire output signals allows for drawing conclusions about the intensity of the incident optical signal. In this way, detector-blinding attacks against single-photon detectors can be reliably recognized.

[0015] Therefore, the single-photon detection device has reduced complexity compared to known devices in which additional components, such as beam splitters and detectors, are used to draw conclusions about the strength of the optical signal. In this way, security is improved because the second nanowire is located in the same waveguide as the first nanowire, eliminating the need for additional photonic components that may themselves have strong wavelength-dependent properties that an attacker could exploit. Additionally, because the nanowires are very small, the space requirements are very low. The low space requirements also make the single-photon detection device inexpensive to manufacture. Furthermore, the low complexity allows the single-photon detection device to be easily scaled up. Therefore, the single-photon detection device does not require additional detectors and / or optical components, and the at least two nanowires can recognize a detector-blinding attack against the single-photon detection device. This provides a single-photon detection device that is protected against eavesdropping and can be used in QKD.

[0016] In principle, a single-photon detection device may have multiple nanowires, each arranged along the optical axis of an optical waveguide. In this arrangement, the nanowire to which an optical signal guided along the optical axis in the optical waveguide finally reaches corresponds to the detector element of the SNSPD of the single-photon detection device used as the primary detector. This nanowire is hereinafter also referred to as the first nanowire. Therefore, further nanowires (second nanowire, third nanowire, etc.) are arranged in front of the first nanowire with respect to the optical axis. As a result, the optical signal guided in the optical waveguide first passes through the further nanowire before reaching the first nanowire. Preferably, the first nanowire is designed so that its output signal can be generated based on very few photons, particularly preferably on a single photon. In this way, the primary detector of the single-photon detection device is highly sensitive and can even detect individual photons.

[0017] The additional nanowire corresponds to a detector element of the SNSPD used as a secondary detector, ensuring the recognition of attacks against the single-photon detection device. Because the single-photon detection device has at least one secondary detector, the single-photon detection device includes at least two nanowires, along with the first nanowire. As a result, the single-photon detection device can recognize optical signals at the single-photon level, as required for QKD, while also recognizing optical signals whose intensities deviate from expected intensities and may result from a detector-blinding attack.

[0018] The detection mechanism of SNSPDs is based on the following principle: an external DC current is applied to a nanowire, the absolute value of which is slightly lower than the critical current at which the nanowire's superconductivity is destroyed. One or more photons incident on the nanowire locally reduce the absolute value of the critical current below the absolute value of the applied DC current due to the decay of Cooper pairs. This creates a local non-superconducting region or hot spot with finite electrical resistance. Due to the finite electrical resistance, a measurable signal (output signal) is generated in a readout amplifier.

[0019] The optical waveguide is preferably a planar optical waveguide structure used in an integrated optical unit. The first nanowire can be applied to the optical waveguide in principle. The first nanowire is preferably integrated into the waveguide. It is particularly preferable that the first nanowire is directly integrated into the waveguide structure during the fabrication of the waveguide molded on the chip.

[0020] According to a preferred embodiment of the present invention, the second nanowire is designed to have a higher threshold intensity than the first nanowire. As mentioned above, an optical signal guided in the optical waveguide first passes through the second nanowire. Since the latter preferably has a higher threshold intensity, the second nanowire does not generate an output signal for optical signals below this threshold intensity. That is, if the optical signal intensity is low, the secondary detector is not triggered. However, the second nanowire generates an output signal for optical signals with a high intensity. In this way, the second nanowire can be used to reliably recognize detector-blinding attacks on single-photon detection devices. If there are two or more nanowires, i.e., two or more secondary detectors, all further nanowires preferably have a higher threshold intensity than the first nanowire, and the threshold intensities of the further nanowires are different from each other. Therefore, a statistical evaluation of the output signals of the nanowires allows conclusions to be drawn about the intensity of the incident optical signal.

[0021] The second nanowire is preferably positioned not only in front of the first nanowire in the direction in which the optical signal is guided by the optical waveguide, but also in close spatial proximity to the first nanowire. The second nanowire is preferably positioned at a distance of 100 nm or less from the first nanowire in relation to the optical axis. In particular, in relation to the fabrication of single-photon detection devices in integrated optical units, the spatial proximity has the advantage of avoiding non-uniformities during fabrication that may result in different yields.

[0022] The threshold intensity of the nanowire can be specified, among other things, by the absorption coefficient of the nanowire. In this regard, according to a preferred development of the present invention, the absorption coefficients of at least two nanowires are different from each other. Particularly preferably, the second nanowire is designed such that only a part of the optical signal guided in the optical waveguide is absorbed by the second nanowire. In contrast, it is preferable to design the first nanowire such that all of the optical signal guided in the optical waveguide is absorbed by the first nanowire. Since most of the optical signal can pass through the second nanowire due to the different absorption coefficients, the optical signal can reach the first nanowire acting as the detector element of the primary detector with little change.

[0023] Regarding the different absorption coefficients of the first and second nanowires, according to a preferred development of the present invention, the second nanowire is shorter than the first nanowire. Since the length of the nanowire affects the absorption coefficient of the nanowire, the absorption coefficient can be specified corresponding to the length of the nanowire. The first nanowire preferably has a length that can completely absorb the light guided in the optical waveguide. In contrast, the second nanowire is preferably shorter than the first nanowire. Particularly preferably, the length of the second nanowire is at most one-fifth, more preferably at most one-tenth of the length of the first nanowire. By shortening the length of the second nanowire, when the intensity of the optical signal is low, it is ensured that only the first nanowire generates an output signal and the second nanowire does not generate an output signal. In addition, by shortening the length of the second nanowire, it is ensured that only a very small part of the optical signal guided in the waveguide is absorbed. Also, since the second nanowire has a short length, its detection rate is very high and its dead time is short. Thus, the second nanowire can recognize very short optical signals at a high repetition rate. This further simplifies the recognition of detector blind spot attacks.

[0024] Not only the length of the nanowire, but also the orientation of the nanowire with respect to the propagation direction of the optical signal guided in the optical waveguide affects the absorption coefficient of the nanowire. In principle, the nanowire can extend in any desired orientation with respect to the propagation direction of the optical signal. For example, the nanowire can meander along or perpendicular to the optical axis. However, in connection with this, according to a further preferred development of the present invention, the first nanowire extends along the optical axis of the waveguide. The absorption of the optical signal by the first nanowire does not depend on the width of the first nanowire by extending along the optical axis, but depends on its length which is many times longer than the width. Furthermore, the absorption can be set by the length of the first nanowire. It is preferable that the length of the first nanowire is such that all of the optical signal guided in the optical waveguide is absorbed by the first nanowire.

[0025] Since the first nanowire acts as a detection element of the primary detector, as the length of the first nanowire along the optical axis increases, the detection rate of the single-photon detection device decreases. On the contrary, as the length of the first nanowire along the optical axis increases, the sensitivity of the single-photon detection device increases. Regarding a good balance, according to a preferred development of the present invention, the first nanowire is U-shaped. That is, it preferably includes two regions in which the first nanowires extend parallel to each other along the optical axis so that the first nanowire is U-shaped, and the two regions are connected by a third region at their ends. This U-shape combines a high detection rate and high sensitivity of the single-photon detection device, and in addition, the manufacturing is simple. A further advantage of the U-shape is that the two ends of the first nanowire are substantially at the same point with respect to the optical axis. This simplifies the attachment of the electrodes and thus the manufacture of the single-photon detection device. Instead of the U-shape, a double U-shape or a W-shape may be used. In this case, the two ends of the "W" are likewise substantially at the same point with respect to the optical axis. Also, in the case of the W-shaped first nanowire, the length is longer in the same range along the optical axis compared to the case of the U-shape. As a result, the absorption by the first nanowire also increases.

[0026] Regarding the orientation of the second nanowire, according to a further preferred development of the present invention, the second nanowire extends laterally with respect to the optical axis of the waveguide. Since the second nanowire extends laterally with respect to the optical axis, the absorption of the optical signal by the second nanowire depends on its width, which is many times shorter than its length, rather than on its length. Thus, only a very small part of the optical signal is absorbed by the second nanowire, so that the optical signal can reach the first nanowire with little change. In addition, this simplifies the connection of the end of the second nanowire to the electrode, particularly preferably to the electrode of the first nanowire, without increasing the length of the second nanowire.

[0027] In this regard, according to a preferred development of the present invention, the second nanowire is in the shape of an I. It is particularly preferred that the second nanowire extends in a straight line across the optical axis of the waveguide in the shape of an I rather than in a serpentine shape. This makes it possible to make the second nanowire as short as possible. When the length of the second nanowire is short, the detection rate is high and the dead time is short, which is particularly advantageous for recognizing detector blind spot attacks.

[0028] Regarding the lengths of the first and second nanowires, it is preferable that the length of the first nanowire is from 1 μm to 500 μm, particularly preferably from 20 μm to 500 μm, and / or it is preferable that the length of the second nanowire is from 100 nm to 50 μm, particularly preferably from 100 nm to 10 μm. The specific lengths have proven to be particularly suitable for reliably recognizing detector blind spot attacks. The length corresponds to the range from one end to the other end of the nanowire. Since the nanowire, particularly the first nanowire, may be in a serpentine shape, a U shape, or any other desired shape, the length of the nanowire does not necessarily correspond to the range of the nanowire along the optical axis.

[0029] According to a further preferred embodiment of the present invention, the at least two nanowires can be operated electrically in series, in parallel, or independently. As already mentioned, the principle of SNSPDs used as primary and secondary detectors is based on the fact that the nanowires are supplied with an external direct current. Controlling the at least two nanowires electrically independent of one another has the advantage that each nanowire can be supplied with an individual amount of direct current. In this way, the intensity threshold at which the nanowire begins to generate an output signal can be influenced individually for each nanowire by the amount of direct current. However, the disadvantage of electrically independent control is the increased space requirements and higher manufacturing costs. Electrical series control and / or electrical parallel control have the advantage of being inexpensive and very space-saving to implement. Furthermore, electrical series control and / or electrical parallel control do not require additional contacts and readout electronics, which means that the number of electrical lines to the single-photon detection device is independent of the number of additional nanowires.

[0030] In a preferred development of the invention, with regard to generating an output signal, the single-photon detection device includes at least two electrodes, with the ends of the nanowires connected to the electrodes. In principle, it is possible for the single-photon detection device to include exactly two electrodes, with all nanowire ends connected to these two electrodes (i.e., the first end of each nanowire is connected to the first electrode and the second end of each nanowire is connected to the second electrode). In this way, one electrode is used for multiple nanowires. Alternatively, the single-photon detection device may have three or more electrodes, with each nanowire connected to two dedicated electrodes. Furthermore, a mixed configuration is also possible for a large number of nanowires. The electrodes allow an external direct current to be supplied to the nanowires. The electrodes are preferably made of a metallic material, such as chromium and / or gold. In principle, the electrodes can be positioned at any desired location on the optical waveguide, as long as they are in contact with the nanowires.

[0031] As already mentioned, the first nanowire is preferably a waveguide-integrated nanowire. In this regard, according to a further preferred development of the invention, the second nanowire is applied to the optical waveguide, arranged in the spatial vicinity of the optical waveguide, or integrated into the optical waveguide. Therefore, the second nanowire may be a waveguide-integrated nanowire. In this way, the single-photon detection device can recognize a detector-blinding attack using only an integrated optical unit. Alternatively, the second nanowire can be applied to the top surface of the waveguide or the bottom surface of the waveguide. In addition, the second nanowire can also be applied to the side of the waveguide. Similarly, the second nanowire may be arranged in the spatial vicinity of the optical waveguide, preferably in the optical near-field of the waveguide, particularly preferably at a distance of 1 μm or less from the optical waveguide.

[0032] More preferably, the nanowires have a substantially rectangular cross section and a thickness that allows a superconducting current to be conducted through the nanowires. Preferably, the nanowires have a thickness between 3 nm and 20 nm. More preferably, the nanowires have a width between 10 nm and 500 nm. The thickness of the nanowires can be influenced by the thickness of the superconducting layer deposited during the manufacturing process. After deposition, the layer can be structured, for example, by electron beam lithography and plasma-assisted etching, and the width of the nanowires can be determined during the process.

[0033] Regarding the raw materials of the nanowires, the nanowires are Nb, NbN, NbTi, NbTiN, Nb3Ge, Nb3Sn, SmFeAsO 1-x F x , CeOFeAs, MgB2, W x Si 1-xPreferably, the nanowires comprise at least one of the materials selected from the group comprising: MoRe, MoSi, TaN, graphene, iron-based high-temperature superconductors (iron pnictides), and high-temperature superconductors with copper oxide (in particular YBCO and / or BSCCO). The nanowires may be composed of one of the above materials or of any desired combination of several materials. Alternatively or additionally, the nanowires may be composed of further superconducting materials.

[0034] The invention will now be described by way of example using preferred embodiments and with reference to the drawings, in which:

[0035] FIG. 1 shows a schematic diagram of a single-photon detection device according to a preferred embodiment of the present invention.

[0036] FIG. 1 shows a schematic diagram of a single-photon detection device 10 for detecting an optical signal. The single-photon detection device 10 includes an optical waveguide 12. The waveguide 12 is designed to guide an optical signal along an optical axis 14. The single-photon detection device 10 further includes at least two nanowires 16, 18. The nanowires 16, 18 are designed to be superconducting at sufficiently low temperatures and are configured to generate an output signal in their superconducting state when the optical signal exceeds a threshold intensity. The nanowires 16, 18 are the actual detector elements of a superconducting nanowire single-photon detector (SNSPD), and are used on the waveguide 12 as a primary detector 20 and a secondary detector 22.

[0037] The first nanowire 16 is part of the primary detector 20 of the single-photon detection device 10. To that end, the first nanowire 16 is connected by its ends 24a, 24b to electrodes 26a, 26b, respectively, which in this case are located on the left and right sides of the waveguide 12. The first nanowire 16 is U-shaped and extends along the optical axis 14 of the waveguide 12 so that its two ends 24a, 24b are at the same height.

[0038] The second nanowire 18 is part of the secondary detector 22 of the single-photon detection device 10. The second nanowire 18 is arranged in front of the first nanowire 16 with respect to the optical axis 14 of the waveguide 12. Therefore, the optical signal guided in the waveguide along the optical axis 14 first reaches the second nanowire 18 and then the first nanowire 16. The second nanowire 18 extends in a lateral direction with respect to the optical axis 14 of the waveguide 12 and is in a straight I shape. The two ends 28a, 28b of the second nanowire 18 are similarly connected to the electrodes 26a, 26b. For this purpose, the two electrodes 26a, 26b each have extensions 30a, 30b.

[0039] In principle, the single-photon detection device 10 may include two or more secondary detectors 22, and thus there may be another nanowire in addition to the second nanowire 18.

[0040] The nanowires 16, 18 of the primary detector 20 and the secondary detector 22 are designed to have different threshold intensities from each other. In this case, since the second nanowire 18 has a higher threshold intensity, the output signal is generated by the secondary detector 22 only from optical signals of higher intensity. In this case, in addition to being shorter than the first nanowire 16, the second nanowire 18 can have a higher threshold intensity by being oriented in a lateral direction with respect to the optical axis 14, in contrast to the first nanowire 16 extending along the optical axis 14. The length of the first nanowire 16 is preferably 180 μm, and the length of the second nanowire 18 in this case is 5 μm.

Description of Reference Numerals

[0041] 10 Single-photon detection device 12 Waveguide 14 Optical axis 16 First nanowire 18 Second nanowire 20 Primary detector 22 Secondary detector 24 End of the first nanowire 26 Electrode 28 End of the second nanowire 30 Extension of the electrode

Claims

1. A single-photon detection device (10) for detecting an optical signal, comprising: an optical waveguide (12); and at least two nanowires (16, 18), wherein the optical waveguide (12) is designed to guide the optical signal along an optical axis (14); the at least two nanowires (16, 18) are arranged along the optical axis (14), and at least one second nanowire (18) is arranged in front of a first nanowire (16) with respect to the optical axis (14); the at least two nanowires (16, 18) are designed to be superconducting at a predetermined temperature and are configured to generate an output signal in a superconducting state when the intensity of the optical signal exceeds a threshold intensity; the at least two nanowires (16, 18) are designed to have different threshold intensities from each other, the single-photon detection device (10).

2. The single-photon detection device (10) according to claim 1, wherein the second nanowire (18) is designed to have a higher threshold intensity than the first nanowire (16).

3. The single-photon detection device (10) according to any one of the preceding claims, wherein the absorption coefficients of the at least two nanowires (16, 18) are different from each other.

4. The single-photon detection device (10) according to any one of the preceding claims, wherein the second nanowire (18) is shorter than the first nanowire (16).

5. The single-photon detection device (10) according to any one of the preceding claims, wherein the first nanowire (16) extends along the optical axis (14) of the optical waveguide (12).

6. The single-photon detection device (10) according to any one of the preceding claims, wherein the first nanowire (16) is U-shaped.

7. The single-photon detection device (10) according to any one of the preceding claims, wherein the second nanowire (18) extends in a direction transverse to the optical axis (14) of the optical waveguide (12).

8. The single-photon detection device (10) according to any one of the preceding claims, wherein the second nanowire (18) is I-shaped.

9. The single-photon detection device (10) according to any one of the preceding claims, wherein the at least two nanowires (16, 18) are operable electrically in series, electrically in parallel, or electrically independently of each other.

10. The single-photon detection device (10) according to any one of the preceding claims, wherein the second nanowire (18) is applied to the optical waveguide (12), arranged in a spatial vicinity of the optical waveguide (12), or integrated with the optical waveguide (12).

11. Use of the single-photon detection device (10) according to any one of the preceding claims for recognizing an attack during signal transmission encrypted by quantum key distribution.

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