Transmitting device for providing an encoded optical output signal, and transmitting station and communication system comprising same

Abstract

The invention relates to a transmitting device (2a) for providing an encoded optical output signal (L3), in particular for a quantum key exchange, to a transmitting station (7a), in particular a satellite (9b), having a corresponding transmitting device (2a), and to a communication system (1) for providing encoded optical communication, wherein the transmitting device (2a) comprises a light source (110), designed as a double pulse source, for providing an optical input signal (L0) comprising double pulse pairs (D) and / or individual pulses (P), and a modulation unit (120) for modulating the phase components of the polarisation components of the optical input signal (L0) during an encoding of the optical output signal (L3), and wherein the modulation unit (120) is designed to selectively enable polarisation, phase-based encoding (X, Y) and / or time-bin encoding of the pulses (P) of the optical output signal (L3).

Description

[0001] TRANSMITTING DEVICE FOR BREAKING A CODED OPTICAL OUTPUT SIGNAL AS WELL AS TRANSMITTING STATION AND COMMUNICATION SYSTEM WITH THE SAME

[0002] Technical field

[0003] The present disclosure relates to optical signal generation and transmission, in particular for the transmission of coded information using quantum key exchange. Specifically, the disclosure relates to a transmitting device for providing a coded optical output signal, in particular for quantum key exchange, a transmitting station, in particular a satellite, and a communication system for providing coded optical communication, in particular using quantum key exchange, at least along a section of a communication path between a transmitter and a receiver.

[0004] Technical background

[0005] Optical signal transmission is increasingly used, at least on sections of communication paths between transmitters and receivers, for example, by transmitting data from satellites to ground stations and vice versa. In this process, information is transmitted using appropriate technical means by assigning a unit of information to a specific state of a carrier signal. The carrier signal is typically an electromagnetic wave from a specific spectral range, such as laser light or a laser beam. To imprint information onto the carrier signal, a property of the carrier signal is changed. The change itself, or the state of the carrier signal after the change, corresponds to the information to be transmitted. The carrier signal is usually changed at intervals to transmit multiple units of information.

[0006] Depending on the carrier signal, various physical characteristics of the carrier signal can serve as information carriers, such as amplitude, frequency, phase, and / or polarization. Changing one of these characteristics over time is called modulation. Various technical components are used along the signal processing path to process the carrier signal and insert the desired information before it is transmitted over the transmission path (e.g., wired or fiber-based via fiber optic cable or wirelessly via free beam). The components used in preparing and processing the carrier signal modulate it according to specific requirements, ensuring that the information to be transmitted is correctly embedded and transmitted over the transmission path with minimal interference and loss.

[0007] Furthermore, it is desirable, especially in the case of free-space optical communication (FSOC), to protect signals transmitted as free beams from unauthorized access or hacking attacks. Quantum cryptographic methods can increasingly be used for this purpose. Such methods, for example, utilize quantum key distribution (QKD) to encode keys for accessing encrypted information embedded on carrier signals, for which modulation techniques can again be employed.

[0008] Quantum key exchange is relatively secure because the quantum states used for it are not clonable and the photons used for it are not divisible. Various methods and devices for signal modulation and encoding are known in the prior art. DE 10 2022 131 465 B3 describes a modulator unit comprising a light source, a phase modulator, a polarization component separator, and a reflector. The light source emits an optical signal with two polarization components, each with its respective polarization direction, towards the polarization component separator. The polarization component separator transmits the polarization components via different optical paths with different signal propagation times, resulting in a relative time offset between the polarization components. The phase modulator modulates a phase of the first polarization component in the first polarization direction and transmits it to the reflector.The reflector retroreflects both polarization components, thereby changing their respective polarizations. The polarization components then pass through the phase modulator again. The phase modulator now modulates one phase of the second polarization component in the first polarization direction. The polarization component separator then eliminates the time offset between the polarization components. The modulator unit outputs the modulated optical signal as a polarization-modulated output signal.

[0009] DE 10 2022 121 510 B3 relates to a modulator unit for modulating the phase of a polarization component of an optical signal, comprising a polarizing beam splitter and a phase shifter. The polarizing beam splitter splits an input signal into a first and a second polarization component with different polarization directions and sends the polarization components in opposite directions via an optical ring containing the phase shifter. A polarization rotator is arranged in the optical ring, which changes the polarization direction of passing optical signals so that the polarization components arriving at the phase shifter are polarized in the same direction. The phase shifter changes the polarization directions of the incoming optical signals so that they are orthogonally polarized to each other within the phase shifter. One polarization component is modulated.When the optical signals leave the phase shifter, their polarization direction is restored to its original polarization direction. Now, one polarization component contains a modulation, but has the same polarization direction as the other polarization component within the optical ring.

[0010] DE 10 2022 119 077 B3 deals with a modulator unit for modulating the polarization of an optical signal, comprising a light source, a polarization-dependent phase modulator, and a reflector. The light source outputs an optical signal as an input signal towards the phase modulator. The optical signal contains a first polarization component with a first polarization direction and a second polarization component with a second polarization direction. The phase modulator modulates a first phase of the first polarization component in the first polarization direction and forwards the modulated input signal to the reflector. The reflector retroreflects the received optical signal towards the phase modulator, thereby changing its polarization so that the first polarization component acquires the second polarization direction and the second polarization component acquires the first polarization direction.The phase modulator modulates a second phase of the second polarization component of the retroreflected optical signal in the first polarization direction. The modulator unit outputs the modulated optical signal as a polarization-modulated output signal.

[0011] US 12 028 113 B2 describes a method for polarization modulation of photonic pulses, in particular for generating quantum cryptographic keys, which is intended to ensure optimal stability of the outgoing polarization states and comprises the following steps: generating a plurality of photonic pulses with an unspecified polarization state obtained by overlapping their horizontal and vertical polarization modes, and guiding these pulses in a first polarization-preserving fiber; splitting the horizontal and vertical polarization modes and guiding these pulses to respective terminals of a second polarization-preserving fiber forming a ring, traversing this ring clockwise and counterclockwise, respectively;Inducing a respective phase modulation of both polarization modes at a point on the ring, spaced from the terminals by optical paths of different lengths along the ring, thereby determining a polarization-preserving fiber delay line; and recombining the polarization modes into a single photonic pulse beam and guiding the resulting beam through the first polarization-preserving fiber, obtaining at its output a polarization state of the pulses that depends on the difference between the phase modulations.

[0012] CN 105897413 A The invention discloses a polarization-coded QKD system with phase modulation based on a Sagnac ring. Horizontally polarized light and vertically polarized light propagate clockwise and counterclockwise, respectively, in an optical fiber that maintains polarization.counterclockwise; the horizontally polarized light reaches a polarization beam splitter; the vertically polarized light reaches the polarization beam splitter; after combination, the horizontally polarized and vertically polarized light exit via an optical circulator to an optical attenuator and are attenuated to single-photon levels by an optical attenuator; thus, safe transmission on one channel is ensured; then, the horizontally polarized and vertically polarized light are combined with light sent from a synchronous optical laser via a wavelength multiplexer to a public channel; after reference selection via a polarization controller, two reference sets orthogonal in one polarization direction are separated via a polarization beam splitter; and detection counting is performed via a single-photon detector. Agnesi, C., Avesani, M.Stance, A., Villoresi, P. & Vallone, G., in their paper "All-fiber self-compensating polarization encoder for quantum key distribution," state that quantum key distribution (QKD) enables remote parties to exchange cryptographic keys with unconditional security by encrypting information about the degrees of freedom of photons. Polarization coding has been widely used in QKD implementations along free-space, fiber-optic, and satellite-based links. However, the polarization encoders used in such implementations are unstable, expensive, complex, and can even exhibit side channels that undermine the security of the implemented protocol. A self-compensating polarization encoder based on a lithium niobate phase modulator in a Sagnac interferometer is proposed and is designed to be implemented using only off-the-shelf (COTS) telecommunications components.The polarization encoder is designed to combine a simple design with high stability and achieve an intrinsic quantum bit error rate of only 0.2%. Since its implementation is possible using COTS in the 800 nm to 1550 nm range, the polarization modulator is expected to offer a promising solution for free-space, fiber-optic, and satellite-based QKD.

[0013] Li, Y. et al., “High-speed robust polarization modulation for quantum key distribution”, Opt. Lett., OL 44, 5262-5265 (2019), https: / / doi.org / 10.1364 / QL.44.005262, note that polarization modulation plays a key role in polarization-encoding quantum key distribution (QKD). They report a novel polarization modulation scheme, which, to their knowledge, is based on an inherently stable Sagnac interferometer. The presented scheme is reportedly free of polarization mode dispersion and calibration, as well as being insensitive to environmental influences. Successful experiments were conducted at a repetition rate of 1.25 GHz to demonstrate the feasibility and stability of the scheme. The measured average qubit error rate of the four polarization states is reportedly 0.27% for 80 consecutive minutes without any adjustments.This fast, intrinsically stable polarization modulation should be usable in many QKD systems with polarization encoding, such as BB84, MDI, etc.

[0014] Xu, H. & Wang, S. “An intrinsic-stabilization polarization encoder for quantum key distribution, in Sixth Symposium on Nove!” Optoelectronic Detection Technology and Applications vol. 11455 1359-1362 (SPIE, 2020), propose an intrinsically stabilizing polarization encoder based on a PBS-Sagnac loop with a polarization-preserving fiber arm for quantum key distribution, which is completely immune to polarization perturbations and can be used for quantum satellite communication.

[0015] Avesani, M., Agnesi, C., Stanco, A., Vallone, G. & Villoresi, P. “Stable, low-error and calibration-free polarization encoder for free-space”, quantum communication. arXiv:2004.11877 [quant-ph] (2020), note that polarization-encoded free-space quantum communication requires a quantum state source with fast polarization modulation, long-term stability, and a low intrinsic error rate. They present a source based on a Sagnac interferometer, consisting of polarization-maintaining fibers, a fiber polarization beam splitter, and an electro-optic phase modulator. The system generates predefined polarization states with a fixed free-space reference frame that requires no calibration at either the transmitter or receiver. In this way, they achieve long-term stability and low error rates.A proof-of-concept experiment is also reported, demonstrating a quantum bit error rate of less than 0.2% over several hours without active recalibration of the devices.

[0016] Wang, J. et al., “Experimental demonstration of polarization encoding quantum key distribution system based on intrinsically stable polarization-modulated units,” Opt. Express, OE 24, 8302-8309 (2016), present a feasibility study for a one-way polarization encoding quantum key distribution (QKD) system. This approach can automatically compensate for birefringence and phase drift. This is achieved by constructing intrinsically stable polarization-modulated units (PMUs) for encoding and decoding, which can be used with the four-state protocol, the six-state protocol, and the instrument-independent (MDI) scheme. A polarization extinction ratio of approximately 30 dB was maintained for several hours over a 50 km long optical fiber without any modifications to its setup, demonstrating its potential for use in practical applications.

[0017] Davide Scalcon, Elisa Bazzani, Giuseppe Vallone, Paolo Villoresi & Marco Avesani, “Low-error encoder for time-bin and decoy states for quantum key distribution”, arXiv:2311.02059, November 2023, note that time-tap encoding is frequently used for implementing quantum key distribution (QKD) on fiber optic channels due to its robustness against optical fiber-induced variations. However, achieving a stable and low intrinsic qubit error rate (QBER) in time-tap systems can be challenging due to the use of interferometric structures. A key element for QKD preparation and decoy-state measurement is the state encoder, which must generate low-error and stable states with varying mean photon counts. They propose the MacZac (Mach-Zehnder-Sagnac), a time-code encoder with extremely low intrinsic QBER (< 2 x 10 -5) and high stability. The device is based on nested Sagnac and Mach-Zehnder interferometers and uses a single-phase modulator for both decoy and state preparation, significantly simplifying the optical setup. The encoder requires no active compensation or feedback system and can be scaled to generate states of arbitrary dimension. Yan-Lin Tang et al., “Time-bin phase-encoding quantum key distribution using Sagnac-based optics and compatible electronics,” Research Article Vol. 31, No. 167 / 31 Jul 20231 Optics Express 26335, introduce a novel time-bin phase-encoding quantum key distribution (QKD) in which the transmitter uses an inherently stable Sagnac interferometer and has comparable electrical requirements to existing polarization or phase-encoding schemes.This approach requires no intensity calibration and is insensitive to environmental disturbances, making it both flexible and powerful. Experiments were conducted with a compact QKD system to demonstrate the stability and safe key rate of the presented scheme. The results show a typical safe key rate of 6.2 kbps at 20 dB and 0.4 kbps at 30 dB with channel loss emulated by variable optical attenuators. A continuous test of a 120 km fiber reel shows a stable time-bin-based quantum bit error rate within 0.4% to 0.6% over a period of 9 consecutive days without any adjustments. This inherently stable and compatible time-division multiplexing phase-coding scheme is intended to be widely applicable in various QKD experiments, including BB84 and instrument-independent QKD.

[0018] M. Sabatini, T. Bertapelle, P. Villoresi, G. Vallone, and M. Avesani, “Hybrid encoder for discrete and continuous variable QKD,” arXiv:2408.17412v1 [quant-ph], August 10, 2024, note that quantum key distribution is evolving into an innovative application of quantum technology, gradually integrating into the industrial landscape. Over time, many protocols have been developed that utilize either discrete or continuous variables. While the former are generally superior at bridging greater distances, the latter are typically superior at generating higher secret key rates over short distances. Current efforts aim to create systems that can leverage both strengths and anticipate the future challenge of realizing a quantum network consisting of multiple and heterogeneous interconnected nodes.In such a context, systems capable of efficiently switching between discrete and continuous variable operating modes using hybrid quantum state encoders represent a potential solution. Therefore, this study introduces a novel hybrid encoder based on an iPOGNAC modulator, designed to ensure compatibility with both DV and CV QKD systems, and which can be constructed entirely from commercially available standard components. The proposed scheme is claimed to be the first to support DV polarization protocols, making it a promising candidate for space nodes in a future quantum network, as polarization-based protocols are well-suited for space-based connections.

[0019] A disadvantage of prior art methods and devices for signal modulation or coding is that they are generally limited to a specific coding method. Such limitations are reflected in particular in the different technical approaches for polarization- and phase-based coding. Prior art methods and devices that allow for a large number of different coding methods are, in our experience, quite complex and therefore costly, expensive, and potentially prone to errors.

[0020] Description

[0021] Based on this, the objective can be considered to be to provide a system that is as versatile as possible, yet not excessively complex, and that allows for a multitude of coding methods. In particular, the objective can be considered to be to enable the combined preparation of common coding methods with a single setup. This objective is achieved by the subject matter of the independent claim and the dependent claims.

[0022] Further embodiments are described in the dependent claims and in the following description. Features described in relation to the method and corresponding method steps can be implemented as transmitting features, or vice versa. Sections of the description relating to the method therefore also apply analogously to a transmitting device. In particular, method steps and related components can be implemented as functions of the transmitting device and corresponding computer programs, and any functions of the transmitting device can be implemented as method steps.

[0023] According to one aspect, a transmitting device for providing a coded optical output signal, in particular for quantum key exchange, is provided, comprising a light source designed as a double pulse source for providing an optical input signal comprising double pulse pairs and / or single pulses, and a modulation unit for modulating the phase components of the polarization components of the optical input signal during encoding of the optical output signal, wherein the modulation unit is designed to enable optional polarization-, phase-based encoding and / or time-bin encoding of the pulses of the optical output signal.

[0024] According to one aspect, a transmitting station, in particular a satellite, is provided, which includes a corresponding transmitting device.

[0025] According to one aspect, a communication system for providing coded optical communication, in particular using quantum key exchange, at least along a section of a communication path between a sender and a receiver, is provided, comprising a corresponding transmitting station, for example a satellite or a ground station, for transmitting a transmit signal based on the coded optical output signal and / or a receiving station, for example a ground station, with a receiving device that is set up to receive and / or decode the transmit signal.

[0026] The transmitted signal can be present as a received signal at the receiving device after it has traversed a given transmission path. The optical signals used can always consist of a superposition of two orthogonal polarization components. This makes it possible, for example, to represent four states using horizontal and vertical polarization coding, as employed in common QKD (Quick Diagnosis).

[0027] The proposed solution provides a novel transmitter architecture that enables the transmission of all three currently predominant QKD encodings—polarization, phase, and / or time-bin encoding—with a single, relatively simple optical and electrical setup. This solution allows, for example, the preparation of all three common encoding schemes for BB84-based Prepare & Measure protocols within a single optical design, both for discrete and expandable to include variable QKD. In contrast, previous approaches require different optical modules and often different electronic control systems.

[0028] The advantages of the proposed solution over the prior art lie primarily in the fact that, in addition to phase and time-bin coding, polarization coding is also enabled with a single setup in a transmitter device and therefore with separate control electronics. The solution allows for fast, stable, and reliable modulation of pulsed coherent light for optionally polarization-coded, phase-coded, or time-bin-coded quantum states of QKD protocols. The entire system can be designed in such a way that both thermally and mechanically induced phase fluctuations of passive components are compensated, thus eliminating the need for time stabilization of the system.

[0029] According to one embodiment, the light source can be configured to generate optical input signals comprising pairs of two successive, essentially identical pulses with a predetermined time interval and phase difference, particularly in the form of an encoded quantum state. Ideally, two identical pulses offset from each other in time can be generated. The generation of double pulses enables phase-modulation-based encoding and / or time-bin encoding, which helps to perform multiple encoding types with a single setup and corresponding control, thus increasing the flexibility and application range of the transmitter.

[0030] According to one embodiment, the light source can be configured as a phase-randomized double-pulse source to generate double-pulse pairs with initial pulses exhibiting a random phase shift. In other words, a phase-randomized double-pulse source can be provided. Each double-pulse source, or corresponding quantum state, can exhibit a randomized phase change relative to the previous one, for example, by restarting the laser or switching the laser to continuous light mode with an additional intensity modulator and a random number generator-coupled phase modulator. Generating such phase-randomized double-pulse pairs simplifies the implementation of phase-modulation-based and / or time-bin encodings, thus further facilitating the implementation of multiple encoding types with a single setup and corresponding control, thereby increasing the flexibility and application range of the transmitter.According to one embodiment, the modulation unit can be configured to use double-pulse pairs for phase-based coding, for example, phase-based QKD (phase-QKD). Two identical pulses at a predetermined interval and with a fixed phase difference can be used for phase-QKD. Each subsequent double pulse should start with a random phase. In this way, the transmitting device can be used reliably and reproducibly for phase-QKD and other phase-based encryption methods.

[0031] According to one embodiment, the modulation unit can be configured to use at least one pulse of a possible double pulse for polarization-based coding and / or for coding by temporal variance, in particular time-bin coding. For example, a pulse with an initially random phase can be used for polarization-based QKD (polarization QKD). Alternatively or additionally, it is possible to use one of the two double pulses as a single pulse for time-bin coding, whereby the transmitted data or information is encoded by temporal variance. In this way, the transmitting device can be used reliably and reproducibly for polarization QKD and other polarization- and / or time-variance-based encryption methods.

[0032] According to one embodiment, the modulation unit can be configured, at least partially, as a Sagnac interferometer. Sagnac interferometers, or encoders, can be implemented in various ways, with optical signals typically traversing the same optical path in different directions or with different polarizations. By having the optical signal traverse the same optical path twice, for example, with reversed polarization directions, parasitic influences on the phase of the optical signal, as well as error phases that may occur in the modulation unit and that can manifest as an additional, unknown phase component, are eliminated. Such phase errors or unknown phase components can arise from relatively slow thermal and / or mechanical fluctuations in the components of the modulation unit.By having the components of a Sagnac interferometer traverse a loop on both the forward and return paths, recombination of individual signal components can help to cancel out phase errors or unknown phase components. For example, on the forward path, a component of the optical signal with horizontal polarization, or a first polarization direction, can be modulated (a first phase is modulated onto this component). A component with vertical polarization, or a second polarization direction, can pass through a polarization-dependent phase modulator without any targeted changes being made to the phase of this component.

[0033] If the optical signal is subsequently looped back or reflected by a reflector and its polarization is rotated by 90°, the component with horizontal polarization is now vertically polarized, while the component with vertical polarization is now horizontally polarized. On the return path, the optical signal again passes through the polarization-dependent phase modulator, and the now horizontally polarized component (which corresponds to the vertically polarized component of the outbound path) has its phase modulated, whereas the now vertically polarized component passes through the polarization-dependent phase modulator without any further change in its phase. Thus, the polarization-dependent phase modulator has only a single modulation axis, which remains the same on both the outbound and return paths, i.e., it acts on the same polarization direction.

[0034] If an error phase (or phase shift) is introduced into the optical signal on the outward path, for example by parasitically affecting the phase of the horizontally polarized component differently than the phase of the vertically polarized component, this error phase cancels out on the return path. This is because the optical signal, with a polarization rotated by 90°, travels the same optical path, and the same error phase is now applied to the opposite polarization axis. This ensures that all relative phase errors are introduced equally into both polarization axes.

[0035] The phase applied to the horizontally polarized component of the return path is called the second phase because, on the return path, the first phase is contained in the vertically polarized component of the optical signal, and the first phase can differ from the second phase. In the example given here, reference is made to horizontally and vertically polarized components of the optical signal, specifically that the horizontally polarized component is phase-modulated by the polarization-dependent phase modulator on both the forward and return paths. This example is not to be understood as restrictive. A polarization-dependent phase modulator can phase-modulate the vertically polarized component or any other component of the optical signal instead of the horizontally polarized component.Crucially, between the two modulation processes, in which the phase of a polarization component of the optical signal is modulated, the polarization of the optical signal is rotated, for example by 90°.

[0036] As a result, in such a setup, both the vertically polarized and the horizontally polarized components of the optical signal are modulated in a single optical path within a modulation unit. The resulting optical signal achieves the desired polarization modulation by modulating the phase of the first polarization component (with a first polarization direction) on the forward path and the second polarization component (with the first polarization direction) on the return path. However, the polarization of the optical signal is shifted by 90° on the return path compared to the forward path.

[0037] The polarization-dependent phase modulator mentioned as an example is a phase-changing modulator that acts on a polarization component of an optical signal; that is, the polarization-dependent phase modulator changes the phase of two orthogonal polarization components of the optical light. For example, the polarization-dependent phase modulator is implemented as an electro-optical modulator (EOM). Any reference to an EOM is therefore merely exemplary, and corresponding explanations apply analogously to any type of polarization-dependent or polarization-independent phase modulator.

[0038] According to one embodiment, an intensity modulator for modulating the intensity of the optical input signal can be arranged after the light source and before the modulation unit. For example, the intensity modulator can be designed, at least in part, as a Sagnac interferometer. It can modulate the input signal with a real-valued prefactor for general intensity and physical and cryptographic security. This can serve, in particular, to provide additional security for the data or information to be transmitted, since photon output from the light source typically follows a Poisson distribution, meaning that multiple photons could be emitted, one of which could be emitted and / or intercepted by unauthorized persons. The intensity modulator thus helps to protect the overall signal or information.The ability to modulate and demodulate the output and / or transmit signal according to user specifications, thereby utilizing so-called lock or decoy states for (further or additional) security, thus further increasing overall data security. According to one embodiment, a quantum attack protection module arranged downstream of the modulation unit along a signal path of the transmitting device can be designed to detect and / or intercept unauthorized attacks, such as those related to information technology, on the output signal and / or a transmit signal based on the output signal. A quantum attack protection module can be implemented in various ways and should preferably be arranged downstream of the modulation unit along the signal path.The use of a quantum attack protection module helps to protect the transmitting device from unauthorized intrusions and / or accesses, which might be aimed in particular at obtaining information about the transmitted signal.

[0039] Brief description of the characters

[0040] The following section describes exemplary embodiments with reference to the accompanying drawings. The illustrations are schematic and not to scale. Identical reference numerals refer to identical or similar elements. The drawings show:

[0041] Fig. 1 shows a schematic representation of a communication system.

[0042] Fig. 2 shows a schematic representation of a transmitting device.

[0043] Fig. 3 shows another schematic representation of an optical path of the transmitting device.

[0044] Fig. 4 shows a schematic representation of part of the optical path of the transmitter. Fig. 5 shows a schematic representation of polarization-based and phase-based coding by the transmitter.

[0045] Detailed description of implementation examples

[0046] Fig. 1 shows a schematic representation of a communication system 1 comprising a number of transceivers 2 and corresponding control modules 3, which may be equipped with interface modules 4 to couple the transceivers 2 and / or control modules 3 with control elements 5. These control modules 5 may be interconnected via corresponding transmission lines 6, which may be configured for the transmission of any type of information, data, light, current, and / or energy. These transmission lines 6 may be photonic links, free-space components, fiber optic components, photonically integrated circuits, or hybrids of several technologies. Therefore, the transmission lines 6 may include any suitable wired, wireless, and / or optical communication means, including optical and / or radio-based lines, cables, transmission links, transceivers, antennas, satellite dishes, and the like.

[0047] The transmitter-receivers 2 can each comprise transmitters 2a and receivers 2b to transmit data or information at least segmentally via respective communication paths C using light signals L from a transmitter A to a receiver B and vice versa. In the present example, the transmitter 2 and the respective control module 3 can be provided for communication devices 7, such as ground stations 8 on a ground G and / or vehicles 9, including aircraft 9a and / or satellites 9b. Therefore, the devices 7 can comprise corresponding computer systems 10, which may include transmitter-receivers 2, control modules 3, interface modules 4, control elements s and / or transmission lines 6, as desired or required for the respective application, e.g., for quantum-based computations, secure communication and / or precision acquisition of certain parameters and / or values, such as...to enable the quantum measurement of acceleration, gravity, magnetic effects, photonic effects, radiation, rotation, or similar phenomena using the control elements 5. The transmitter-receivers 2 can thus serve to establish a quantum communication network Q and / or a classical or conventional network N via the communication paths C using respective transmission lines 6 and / or light signals L.

[0048] A computer program 11 or corresponding machine-readable instructions for controlling the computing devices 10, for example, computers, FPGAs, or other ICs, can be stored on a computer-readable data carrier 12, which can take the form of a computer-readable medium 13 and / or a data carrier signal 14. The computer system 10 can comprise the transceivers 2, the control modules 3, the interface modules 4, the control elements 5, and / or the transmission lines 7, the computer program 11, and computer-readable data carriers 12, which can be configured for data exchange between the respective components mentioned above. Control elements 5 can be any type of data source, such as...

[0049] Measuring elements, classical sensors, a quantum sensor array and / or actuators of the communication devices 7.

[0050] Fig. 2 shows a schematic representation of a transmitter 2a as part of a transceiver 2 of the communication system 1, in particular for the construction of the quantum communication network Q. The transmitter 2a comprises a light source 100, an intensity modulator 110, a modulation unit 120, and a quantum attack protection module 130. The light source 100 is configured to generate an input signal L0, which is converted by the intensity modulator 110 into a first intermediate signal L1 and subsequently by the modulation unit 120 into a second intermediate signal L2, which in turn can be converted by the quantum attack protection module 130 into an output signal L3. The output signal L3 can, for example, be transmitted as a transmit signal L4 by a transmitter arrangement 140 (see Fig. 3), which may, for example, include transmission lenses and / or mirrors with corresponding control elements.

[0051] Fig. 3 shows a schematic representation of an optical path 200 of the transmitting device 2a. The light source 100 can be supplied with electrical energy and electrical and / or light signals via a suitable transmission line 6, which may be connected, for example, to the control module 3, in order to generate the input signal L0. The input signal L0 can comprise two pulses P, for example, laser pulses, as a double pulse pair D. The two pulses P of the double pulse pair D can thus comprise an initial pulse P1 and a subsequent pulse P2.

[0052] The transmitting device 2a, or its optical path 200, can further comprise a polarizing filter element 101, a beam splitter 102, and a beam absorber 103, which are arranged between the light source 100 and the modulation unit 120 and can be traversed by the input signal L0, whereby signals exiting the beam splitter 102 that are not fed to the transmitting arrangement 140 can be absorbed in the beam absorber 103. Pulses of the input signal L0 can then be appropriately prepared as an input vector with property values, for example, as a doubly complex input vector. The output signal L3 can be fed to the modulation unit 120 according to Jones calculus. From the modulation unit 120, the output signal L3 can be calculated, for example, as a doubly complex output vector as follows. exit, in order to then be directed to transmission order 140: The modulation unit 120 can, for example, comprise a lambda-half-wave plate 121, a collimator 122, a polarization-dependent phase modulator or electro-optic modulator 123, a polarization beam splitter (PBS) 124, and a connector 125. A connector 125, for example in the form of a fiber coupler or a wave plate, can be arranged in a loop 126 to reverse or rotate the polarization. The loop can have a forward path 127 and a return path 128, between which the connector 125 can be arranged. While corresponding light signals L pass through the forward path 127 as forward signals e (early), they pass through the return path 128 as return signals I (late).

[0053] Fig. 4 shows a schematic representation of part of the optical path 200 of the transmitting device 2a. According to the exemplary setup of the modulation unit 120, it can function as a Sagnac interferometer or encoder. During operation of the modulation unit 120, the half-wave plate 121 rotates corresponding phases of the input signal L0 so that they are transmitted diagonally, thus horizontally and vertically, with corresponding horizontal H and vertical components V. Horizontal H and vertical components V can exhibit corresponding horizontal |H> and vertical |V> quantum states or classical states. The collimator 122 can feed the light signal L as a free beam into a transmission line 6, for example in the form of a polarization-preserving fiber, so that it can subsequently be fed into the polarization beam splitter 124 and then into the electro-optic modulator 123. An unknown phase orPhase shifts can be imposed, for example due to mechanical / thermal variation over a relatively large time invariance.

[0054] The phase modulator 125 ultimately shapes the desired phase. <t>The unknown phase or phase shift θ is eliminated by the recombination of the forward signal e with the return signal I after passing through the polarization beam splitter 124. After the electro-optic modulator 123, the light signal L is thus reintroduced into the polarization beam splitter 124, which recombines the polarization of the light signal L. Any phase shifts between the horizontal and vertical components can be applied, as illustrated by equations a. to f. below. Equation c. describes the split pulses between the polarization beam splitter 124 and the phase modulator 123, which propagate through the loop 126 clockwise (i. sc) and counterclockwise (i. scc), respectively. Equation d. analogously describes the pulses after the phase modulator 123 and before the polarization beam splitter 124.

[0055] Fig. 5 shows a schematic representation of a polarization-based coding X and a phase-based coding Y by the transmitter 2a. Polarization-based coding X can utilize individual pulses P, each of which can represent a qubit q. Phase-based coding Y can utilize double pulses D, each of which can represent a qubit q. Phase randomness between qubits q is necessary for the security of the respective encryption method, for example, QKD. Pulse carving (additional intensity modulation for the double-pulse source) can be used with the intensity modulator 110 to achieve non-random phases, and / or active phase randomization can be employed with the electro-optic modulator or phase modulator 123. Alternatively, pulsed operation and / or gain switching, as a special form of pulsed operation, are conceivable.The intensity modulator 110 can also be used to modulate and / or prepare decoy states. The pulses P can each have a random phase relationship with, for example, a uniform distribution U to other pulses P.

[0056] The transmitted signal L4, after having crossed a suitable transmission path as a free signal, can be received and processed as a received signal L5 by a receiving station, for example a receiving station 7b in the form of a ground station 8 (see Fig. 1). The receiving station can either be the receiver B or the received signal L5 can be processed according to the respective requirements and forwarded to the receiver B, for example using a conventional network M. The receiver B can be a computer system 10.

[0057] According to the present example, the light source 100, or a corresponding module, can be a phase-randomized coherent (double) pulse source capable of generating either phase-randomized coherent single pulses P or a phase-randomized double pulse D, where the phase relationship within the double pulse D is known and fixed. There are various possibilities for the concrete implementation of this light source 100, which will not be discussed further here. The intensity modulator can be able to adjust the intensity of the single and / or double pulses P or D as needed. This enables the use of so-called decoy states, for example, to securely and efficiently implement BB84 protocol-compliant encryption based on coherent states.The modulation unit 120 can be implemented as a Sagnac encoder, whereby within the encoder the incident light or the input signal L0 is typically split into two orthogonal light components. These two light components then traverse the loop 126 in opposite directions. Thus, each light component passes through every part of this loop, but at partially different times. This ensures that possible thermally or mechanically induced phase fluctuations are compensated.

[0058] The phase modulator 125 can be placed within loop 125 and act asymmetrically in the optical path 200. This means that the phase modulation (|)(t) can act exclusively on the different light components at different times (t1, t2). Two pulses are necessary to implement time-bin or phase coding, each with A <t>= 0 and (p + 0) can be modulated. This makes it possible to impose a defined phase between two pulses P without changing the polarization. If this is combined with the light source 100 in the form of a phase-randomized dual-pulse source, then it is possible to switch between polarization, phase, and time-bin encoding. Several options can be used for the concrete implementation of the Sagnac loop.

[0059] The Quantum Hacking Protection Module 130 is designed to intercept and detect potential quantum hacking attacks. Numerous implementation options exist for this as well. A combination of all four components mentioned above—the light source 100, the intensity modulator 110, the modulation unit 120, and / or the Quantum Hacking Protection Module 130—can enable the preparation of coherent phase-, time-bin-, and / or polarization-encoded quantum states. These states can be securely implemented using a BB84-based decoy-state protocol and quantum hacking protection. Due to the intrinsic stability of the Sagnac encoder, active encoder stabilization is unnecessary. The entire transmitter 2a, or...A suitable transmitter can be based on free-space components, fiber optic components, a photonically integrated circuit, or a hybrid of several technologies. Furthermore, the use of the transmitter 2a as a module with a suitable light source 100 for CV-QKD protocols is not excluded. It should also be noted that "comprehensive" or "comprising" does not exclude other elements or steps, and "a" or "an" does not exclude a plurality. It should also be noted that features or steps described with reference to one of the above embodiments can also be used in combination with other features or steps of other embodiments described above. Reference numerals in the claims are not to be considered as limitations.

[0060] Reference symbol list

[0061] 1 Communication system 123 Phase modulator /

[0062] 2 transceiver electro-optical modulator

[0063] 2a Transmitting device 124 Polarization beam splitter

[0064] 2b Receiving device 35 125 Connecting piece

[0065] 3 Control module 126 Loop

[0066] 4 Interface module 127 Lead time

[0067] 5 Control element 128 Return

[0068] 6 transmission line 130 quantum attack protection module

[0069] 7 Communication equipment 40 140 Transmitting arrangement / telescope

[0070] 7a Transmitting station 200 optical path /

[0071] 7b Receiving station signal transmission link

[0072] 8 Ground stations

[0073] 9 Vehicle e Leading signal

[0074] 9a Aircraft 45 I Return signal

[0075] 9b satellite q qubit / quantum state

[0076] 10 computer systems

[0077] 11 Computer / Control Program A Source / Sender

[0078] 12 computer-readable data carrier B Destination / Receiver

[0079] 13 computer-readable medium 50 C communication path

[0080] 14 Data carrier signal D Double pulse pair

[0081] 100 Light source G floor

[0082] 101 Polarizing filter element H Horizontal component

[0083] 102 Beam splitter L Light signal

[0084] 103 Beam absorber 55 L0 Output signal

[0085] 110 Intensity modulator L1 first intermediate signal

[0086] 120 Modulation unit / Sagnac- L2 second intermediate signal Interferometer L3 output signal

[0087] 121 Lambda-half-wave plate L4 transmit signal

[0088] 122 Collimator 60 L5 Received signal N Conventional network U Uniform distribution

[0089] P Pulse V Vertical component

[0090] P1 Initial pulse X Polarization coding

[0091] P2 follow-up pulse 10 Y phase encoding Q quantum communication network< / t> < / t>

Claims

29 Patent claims 1. Transmitting device (2a) for providing a coded optical output signal (L3), in particular for quantum key exchange, - with a light source (110) designed as a double pulse source to provide an optical input signal (L0) comprising double pulse pairs (D) and / or single pulses (P), and - with a modulation unit (120) for modulating the phase components of the polarization components of the optical input signal (L0) during encoding of the optical output signal (L3), wherein the modulation unit (120) is designed to optionally enable polarization-, phase-based encoding (X, Y) and / or time-bin encoding of the pulses (P) of the optical output signal (L3).

2. Transmitter device (2a) according to claim 1, wherein the light source (110) is configured to generate optical input signals (L0) comprising pairs of two successive substantially identical pulses (P) with a predetermined time interval and phase difference to each other, in particular in the form of two successive coded quantum states (q).

3. Transmitter device (2a) according to claim 1 or 2, wherein the light source (110) is configured as a phase-randomized double pulse source to generate double pulse pairs (D) with initial pulses (P1) which have a random phase rotation.

4. Transmitting device (2a) according to at least one of claims 1 to 3, wherein the modulation unit (120) is configured to use double pulse pairs (D) for phase-based coding (Y).

5. Transmitting device (2a) according to at least one of claims 1 to 4, wherein the modulation unit (120) is configured to transmit at least one pulse (P) 30 of a double pulse (D) for polarization-based coding (X) and / or coding by temporal variance, in particular time-bin coding.

6. Transmitting device (2a) according to at least one of claims 1 to 5, wherein the modulation unit (120) is configured at least section by section as a Sagnac interferometer.

7. Transmitter device (2a) according to at least one of claims 1 to 6, wherein an intensity modulator (110) for modulating an intensity of the optical input signal (L0) is arranged after the light source (110) and before the modulation unit (120).

8. Transmitting device (2a) according to at least one of claims 1 to 7, wherein a quantum attack protection module (130) arranged after the modulation unit (120) along a signal path (200) of the transmitting device (2a) is configured to detect and / or intercept unauthorized attacks on the output signal (L3) and / or a transmit signal (L4) based on the output signal (L3).

9. Transmitting station (7a), in particular satellite (9b), characterized by a transmitting device (2a) according to at least one of claims 1 to 8.

10. Communication system (1) for providing coded optical communication, in particular using quantum key exchange, at least along a section of a communication path (C) between a transmitter (A) and a receiver (B), comprising a transmitting station (7a), for example a satellite (9b), according to claim 9 for transmitting a transmit signal (L5) based on the coded optical output signal (L4) and / or a receiving station (7b), for example a ground station (8), with a Receiving device designed to receive and / or decode the transmitted signal (L5).

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