System and method for quantum key distribution via hybrid quantum channels

Abstract

A quantum key distribution (QKD) system comprising: a transmitter (110) adapted to generate a QKD free-space signal, a transmitter station (220) adapted to receive the free-space signal from the transmitter (110), and a remote QKD receiving station (250) supporting a QKD receiver (160) located at a different location from the transmitter station, wherein the transmitter station is adapted to receive the free-space signal from the transmitter and forward the signal to the QKD receiver (160) in the remote QKD receiving station (250) via an optical fiber link (400).

Description

[0001] The present invention relates to an apparatus and method for performing enhanced free-space quantum key distribution, and more particularly, to an apparatus for performing secure QKD between a satellite or a high-altitude platform and a ground-based quantum key distribution receiver. Background Art

[0002] Quantum cryptography or quantum key distribution - also referred to as QKD in the following - is a method that allows the distribution of secret keys with provable absolute security between two distant parties - an emitter called "Alice" and a receiver called "Bob". Quantum key distribution relies on the principles of quantum physics and encoding information in quantum states or quantum bits (qubits) instead of bits as in classical communication. Typically, photons are used for these quantum states. Quantum key distribution exploits certain properties of these quantum states to ensure its security.

[0003] More specifically, the security of this method comes from the fact that the measurement of the quantum state of an unknown quantum system modifies the system itself. In other words, a spy called "Eve" eavesdropping on the quantum communication channel cannot obtain information about the key without introducing errors in the key exchanged between the transmitter and the receiver, thereby notifying the user of an eavesdropping attempt.

[0004] The cryptographic device enables secure transmission of useful payloads by performing some symmetric encryption using keys exchanged via quantum key distribution. A specific quantum key distribution system is described, for example, in US 5,307,410.

[0005] QKD is a protocol that allows the exchange of secret keys in an active scenario. In the QKD protocol, the communication channel between two users is called a quantum channel. A quantum channel is a communication channel that transmits quantum particles, usually photons, in a way that preserves their quantum properties. There are two sets of parameters for quantum coding. One is the polarization of the photons, and the second is the phase, which requires the use of an interferometer. Both have their advantages and disadvantages, depending on the physical layer of the quantum channel and the type of QKD protocol.

[0006] The basic idea of ​​QKD is to allow an eavesdropper to intercept a signal and process it in any way that is compatible with quantum mechanics. Nevertheless, the legitimate users, called Alice and Bob, can still exchange secure keys.

[0007] The most famous QKD protocol is the BB84 protocol, which is based on four different quantum states and was explained in Bennett & Brassard in 1984. Several other protocols have been invented, such as for example:

[0008] E91, which is based on entanglement;

[0009] B92 is based on only two quantum states, but it requires interferometric detection; and

[0010] • COW, which uses a variation of the phase parameter and uses time-of-detection for encoding.

[0011] Commercial systems for terrestrial QKD distributed over optical fiber have been developed, notably by ID Quantique SA. In all practical implementations of terrestrial QKD, the parameter used for quantum encoding is the phase, or related timing parameter for the COW protocol. The reason is that, since polarization is not preserved in optical fiber, polarization schemes require complex and expensive components. On the other hand, interferometric detection is easier to implement in single-mode optical fiber, which is the preferred medium for terrestrial QKD.

[0012] One of the most stringent limitations of terrestrial QKD is the distance limit. Due to the inevitable losses in optical waveguides and the fact that optical amplifiers cannot be used in quantum channels, the distance between Alice and Bob is limited to about one hundred kilometers in commercial setups and up to four hundred kilometers in academic experiments.

[0013] The first solution set to increase the distance between Alice and Bob is the implementation of a trusted node (TrustedNode, TN). The principle of a trusted node is shown in Figure 1. In this figure, we can see that a trusted node is an intermediate element between Alice and Bob, which communicates with each of them and acts as a key relay. More specifically, a trusted node includes two complete QKD nodes, such as Bernard and Amelie. Bernard receives a QKD signal from Alice and processes it to generate a first security key. Amelie generates a new QKD signal and sends it to Bob to generate a second independent key. Then the two independent keys are processed by all actors in collaboration to generate a final security key between Alice and Bob. This means that a trusted node includes a key management system, a QKD receiver for exchanging keys with Alice, and a QKD transmitter for exchanging keys with Bob. Since information is processed in a trusted node and keys are available there, a trusted node needs to be secure and trusted by both parties. By integrating multiple trusted nodes in a chain, the trusted node QKD model can be used to design long-distance QKD networks - potentially spanning an entire country. However, as explained above, the distance between trusted nodes is limited to about a hundred kilometers. The trusted node model described above cannot cross oceans and cannot provide intercontinental key distribution.

[0014] To further increase the distance range, the solution is to rely on free-space optics (FSO) QKD, where the quantum channel is in free space, which does not have the same loss limitations as optical fiber.

[0015] Free space optics (FSO) is an optical communications technology that uses light propagating in free space to transmit data wirelessly for telecommunications or computer networking. "Free space" refers to air, a vacuum, or something similar in which light propagates in straight lines. This is in contrast to guided optics, such as optical fibers or more general optical waveguides, in which light is guided and directed by waveguides. Free space technology is useful in situations where physical connections are impractical due to high cost or other considerations.

[0016] Like any other type of communication, free space optical communications require security to prevent eavesdropping. When one surveys the different means of security for free space optical communications, one can see that several solutions have been studied to provide a solution that enables transmitters and receivers to share secret information over FSO. Common solutions are based on the exchange of secret keys over the FSO channel. After their exchange, those keys are used to exchange messages in a secure way (e.g., by encryption).

[0017] Recently, FSO QKD has been studied to securely exchange keys between a transmitter and a receiver in free space—typically between a satellite or flying drone and a ground station.

[0018] Although the principle of FSO QKD has been demonstrated in academic institutions, it remains a challenging demonstration. Phase is more difficult to use in free space than terrestrial QKD. In fact, due to atmospheric distortion, the wavefront of the wave is distorted during propagation, which leads to poor interference at the receiver. This can be improved by using adaptive optical mirrors. However, this significantly increases the cost and complexity of the system. In free space, the polarization is maintained, which makes polarization-based systems even more attractive. However, due to the movement of the receiver relative to the transmitter, the polarization of the photons changes during the passage of the satellite, which requires polarization compensation components. Two types of protocols are currently being studied, phase-based or polarization-based.

[0019] Since we accept that free-space QKD - in particular satellite or high-altitude platform QKD - offers a solution for long-distance QKD, we note that, due to the above considerations, in many cases it is best to install the QKD receiver station - called an optical ground station (OGS) - in a remote location, for example in a mountainous area to reduce atmospheric absorption, or at least not close to a city center to reduce background noise due to stray light. In order to provide keys to end users, which are usually located in a city center, a second QKD link, usually based on optical fiber, must be added. Therefore, placing the OGS in such a location requires that it is a trusted node, which needs to be protected: therefore, the trusted OGS must include expensive and complex security measures to prevent intrusion, and must ensure tampering detection.

[0020] FIG2 schematically illustrates a free space QKD system according to the prior art, preferably deployed on a satellite or high altitude platform, where the OGS is a trusted node that provides keys to a terrestrial QKD network. The system 100 comprises a satellite or more generally a high altitude platform 110, which is linked via a free space channel 300 to a trusted optical ground station 150, which is physically protected to prevent tampering. Inside the optical ground station, a telescope 130 receives the signal transmitted by the satellite 110, which is then processed to a QKD receiver 160. The other elements are similar to those in FIG1, which shows an optical fiber based trusted node.

[0021] Examples of free-space QKD implementations can be found in "Progress insatellite quantum key distribution" by R. Bedington et al., https: / / arxiv.org / abs / 1707.03613v2 or in JP Bourgoin et al., “A comprehensive design and performance analysis of LEOsatellite quantum communication”, https: / / arxiv.org / abs / 1211.2733 Found

[0022] Alternatively, according to the prior art, in order to overcome the need for expensive and complex security measures for an OGS located far from the end user, such an OGS is installed at a QKD receiving station. In this case, it is usually inside a city center, where the keys will be used directly. However, this configuration reduces the quality of the free space channel through which the signal is delivered from the satellite and reduces the number of secret keys that can be distributed during each pass of the satellite.

[0023] Therefore, there is a need for a free-space QKD system and method, preferably deployed on a satellite, or alternatively a high altitude platform, which ensures good quality of the transmitted signal and a large number of keys, while avoiding that the OGS is a trusted node.

[0024] In fact, the trusted node requirement of OGS implies expensive and complex security measures to ensure tamper-security, which is extremely important for the correct use of QKD systems. Summary of the invention

[0025] The present invention is based on the general approach of a free-space QKD arrangement utilizing a hybrid quantum channel comprising both a free-space segment and an optical fiber coupled via a fiber coupling element.

[0026] The general idea of ​​the invention is that the OGS is separated from the final QKD receiving station containing the QKD receiver, so that the OGS itself does not have to be a trusted node. We will now call the OGS a transmitter station. Its role is to receive the free-space optical signal and transmit it to the QKD receiving station.

[0027] With the hybrid quantum channel system of the present invention, the transmitter station can be placed at a desired location to maximize signal quality, such as at high altitude, and the QKD receiver can be located inside a city center where the key will be used directly.

[0028] In particular, with this system, a quantum channel extends from a satellite or high-altitude platform via a free-space link to a transmitter station, which in turn transmits the signal via optical fiber to a QKD receiver, where a secure key is generated.

[0029] In any case, this system does not modify the paradigm of QKD, since an eavesdropper along the hybrid channel (ie, free link plus fiber link) will still be detected since it will modify the quantum state.

[0030] Thanks to the invention, the transmitter station no longer has to be a trusted node, so the transmitter station can even be positioned in a better location. Typically, the transmitter station is located in a more remote and / or higher altitude location without the added complexity of associating with an additional trusted node, and further improving the quality of the QKD performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The present invention will be described with reference to the drawings, wherein like reference numerals refer to like features.

[0032] Specifically,

[0033] - Figure 1 is a schematic representation of the principle of a trusted node;

[0034] - Figure 2 is a schematic representation of a conventional free-space QKD system, where the OGS is a trusted node; and

[0035] - Figure 3 is a schematic representation of a free-space QKD system according to the present invention. DETAILED DESCRIPTION

[0036] For a better understanding, the invention will be described with reference to a specific embodiment. However, it should be understood that the invention is not limited to the embodiments described herein, but is defined by the claims and includes all embodiments within the scope of the claims.

[0037] Figure 3Schematically illustrated is a preferred embodiment of the present invention, which is a quantum key distribution (QKD) system 200, wherein a transmitter, preferably a high altitude platform 110, more preferably a satellite or the like, is linked to a transmitter station, preferably an optical ground station (OGS) 220, via a free space link 300. In this embodiment, the OGS 220 is preferably located in an optimal location such as a high altitude and away from a city center to maximize signal quality. The transmitter station 220 includes a telescope 130 and a fiber coupler 140 for guiding the optical signal received from the free space into an optical fiber 400 without processing it. As shown, the transmitter station 220 is connected to a remote QKD receiving station 250 supporting a QKD receiver 160 via an optical fiber link 400, which is directly connected to the QKD receiver 160, wherein the QKD receiver is preferably located away from the transmitter station, such as 30 km or more. In this regard, the optical fiber 400 has a predetermined length that permits avoiding the trusted node requirements for the transmitter station 220 and ensures the security and high quality of the quantum key.

[0038] With this system 200, light from a free space channel 300 is directed to a fiber coupler 140 so as to be directly coupled into a low-loss optical fiber 400 within a transmitter station 220 through the fiber coupler 140 without a QKD process and then transmitted from the transmitter station 220 to a QKD receiver 160 through the optical fiber.

[0039] Typically, to achieve long-distance distribution, optical fiber 400 should be a single-mode fiber (SMF) and the light should be at a wavelength corresponding to a low-loss window in the fiber, typically the O-band (about 1310 nm) or the C-band (about 1550 nm).

[0040] Due to atmospheric disturbances, the wavefront of the light arriving at the transmitter station 220 is distorted. The distortion also evolves in time. Therefore, in order to couple it into the SMF, adaptive optics is preferred.

[0041] The light coupled into the SMF is then transmitted to a final QKD receiving station 250 hosting a QKD receiver 160, which may be several kilometers away, preferably ranging from a few hundred meters - corresponding to, for example, the transmitter station 220 being located on top of a building - to tens of kilometers - corresponding to the transmitter station being located away from an urban location.

[0042] Thus, the overall key distribution channel is a hybrid channel consisting of a free space segment 300 from the satellite 110 to the transmitter station 220 and a fiber-based segment 400 that carries light from the transmitter station 220 to the final QKD receiving station 250. Typically, the final receiving station 250 should be located at the location of the end user, who uses the key for encryption purposes, while the transmitter station 220 should be located at an optimal location in terms of signal quality, for example, at high altitude and away from urban interference.

[0043] In this way, the key is not generated at the transmitter station 220, but is generated at the QKD receiving section 250 only after it has been passed through the entire mixing channel.

[0044] As a result, the transmitter station 220 does not need to be a trusted node, and at the same time, the system is secure against attacks because any eavesdropper attempting to measure the data will disturb the quantum state and will be revealed by the QKD protocol.

[0045] Furthermore, this embodiment allows for the selection of a better location for the transmitter station 220, which may result in the following advantages:

[0046] 1. It increases the availability of channels by selecting locations with less cloud coverage.

[0047] 2. It improves the key rate by reducing the attenuation of the free space channel (higher altitude and / or less polluted air)

[0048] 3. It reduces the bit error rate in the channel by reducing the background noise due to stray light.

[0049] All three of these effects combine to increase the amount of secret keys available per satellite / high altitude platform pass, thus improving the performance of QKD.

[0050] Although embodiments have been described in conjunction with a number of embodiments, it is apparent that many alternatives, modifications, and variations will be or will be apparent to those of ordinary skill in the applicable art. Therefore, the present disclosure is intended to encompass all such alternatives, modifications, equivalents, and variations within the scope of the present disclosure. This is particularly true, for example, with respect to different devices that may be used and different types of protocols that may be run.

Claims

1. A quantum key distribution (QKD) system, comprising: a transmitter (110) adapted to generate a QKD free-space signal, a transmitter station (220) adapted to receive the QKD free-space signal from the transmitter (110), and a remote QKD receiving station (250) supporting a QKD receiver (160) located at a different location from the transmitter station, wherein the transmitter station is adapted to receive the QKD free-space signal from the transmitter and forward the QKD free-space signal to the QKD receiver (160) in the remote QKD receiving station (250) via an optical fiber link; wherein the transmitter station (220) transmits the QKD free-space signal to the QKD receiver (160) without processing the same; as well as The transmitter station (220) is an optical ground station, which includes a telescope (130) and a fiber coupler (140) for guiding an optical signal received in free space into an optical fiber (400).

2. The quantum key distribution (QKD) system of claim 1, wherein the transmitter (110) is a high altitude platform (HAP).

3. A quantum key distribution (QKD) system according to claim 1, wherein the transmitter station (220) is not a trusted node, and wherein no setup message needs to be sent to the transmitter station before the QKD free-space signal.

4. The quantum key distribution (QKD) system of claim 2, wherein the transmitter (110) is a satellite.

5. The quantum key distribution (QKD) system according to claim 1, wherein the optical fiber (400) is a single mode fiber (SMF) capable of long-distance distribution.

6. A quantum key distribution (QKD) system according to claim 5, wherein the light from the free space channel (300) and guided to the fiber coupler (140) is at a wavelength corresponding to a low loss window in the optical fiber, 1310 nm or 1550 nm.

7. The quantum key distribution (QKD) system of claim 5, wherein the remote QKD receiving station (250) is at a distance from the transmitter station (220) ranging from a few hundred meters to tens of kilometers.

8. The quantum key distribution (QKD) system of claim 5, wherein the transmitter station (220) is located at a high altitude and / or a non-urban location.

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