METHOD FOR MULTIPLE RECEIVER QUANTUM KEY GENERATION

Description

[0001] The invention relates to a method for quantum key generation between at least four receivers according to the features of the preamble of claim 1 and a system for quantum key generation between at least four receivers according to the features of the preamble of claim 15.

[0002] Networks for quantum key generation using entangled photon pairs are known. In these networks, multiple receivers are connected to each other, for example, via a single source. In such a network, the number of physical connections, i.e., the quantum channels between the receivers, increases only according to the relation i(i - 1) / 2, where i is the number of receivers. In contrast, networks with direct connections between all receivers require a large number of additional quantum channels.

[0003] In quantum key generation networks with multiple receivers connected via a source, several entangled photon pairs are generated in the source and assigned to the individual receivers, for example, according to their wavelength. Based on these entangled photon pairs, quantum keys can be generated between all receivers.

[0004] For quantum key generation, the connection between the two receivers of the entangled photon pairs must be aligned with respect to the chosen entanglement property. This means that both receivers agree on a common frame of reference for the entanglement property. This entanglement property can, for example, represent the polarization, time, and / or mode of the photons, where, in the case of polarization, the alignment of the connection is achieved, for example, in a fiber optic cable via polarization control.

[0005] In known systems, such a comparison is always performed between two receivers. In known systems with multiple receivers, all receivers are compared to a single reference frame. The problem here is that simultaneous comparison between multiple receivers interferes with each other, and this is only possible through multiple comparison steps between all receivers to gradually approximate each other, since the multiple receivers sometimes use the same fiber. In known networks, this mutual interference is accepted, leading to a lower quality of connection between all receivers, which significantly reduces the quantum key generation rate. Furthermore, such simultaneous comparison between multiple receivers is very time-consuming and resource-intensive. FITZKE ERIK ET AL: "Scalable Network for Simultaneous Pairwise Quantum Key Distribution via Entanglement-Based Time-Bin Coding", PRX QUANTUM, Vol. 3, No.2, May 1, 2022 (2022-05-01), XP093237782, ISSN: 2691-3399, DOL: 10.1103 / PRXQuantum.3.020341 Found on the Internet: URL: https: / / arxiv.org / pdf / 2110.13795 describes a method and system for generating quantum keys between at least 4 participating parties by means of channel synchronization.

[0006] The present invention is based on the objective of providing an improved, more efficient, faster and more robust method for

[0007] To provide quantum key generation between at least four receivers and a corresponding device.

[0008] The object of the invention is achieved by a method for

[0009] Quantum key generation between at least four receivers according to the features of claim 1 is solved.

[0010] According to the invention, a method for quantum key generation with entangled photon pairs between at least four receivers is proposed, wherein two receivers of an entangled photon pair each establish a connection In the Train for quantum key generation, with the following steps: i) Generation of entangled photon pairs in a source, wherein each photon pair comprises a signal photon and an idler photon, which are entangled with each other in an entanglement property; ii) Splitting the signal photons and the idler photons according to their wavelength onto quantum channels of the multiple receivers and transmitting the signal photons and the idler photons via the connections In the iii) Forming quantum channels from the source to the receivers; iii) Detection of the signal photons and the idler photons at the respective receivers; iv) Quantum key generation between the receivers of the entangled photon pairs.

[0011] The essential point is that several time intervals are created between all receivers for quantum key generation, with steps i) to iii) being carried out in each time interval, and that in each time period before and / or during step ii) only between two or more connections In the A comparison of the entanglement properties is performed, which can be compared independently of each other in order to determine the compatibility of these two or more connections. In the to enable quantum key generation, and that the time spans differ in such a way that at least one connection is possible in each case. In the through another connection In the exchanged, in which quantum key generation was not yet possible, in order to carry out quantum key generation between all receivers over the multiple time periods, and / or that in the last time period a connection In the, or several of the connections In the , or all connections In the , are repeated, which were already formed in previous time periods, in order to perform a renewed quantum key generation.

[0012] Furthermore, the problem is solved according to the invention by a system for quantum key generation with entangled photon pairs between at least four receivers according to the features of claim 15.

[0013] According to the invention, a system for quantum key generation with entangled photon pairs between at least four receivers is proposed, wherein the system comprises a source, a frequency multiplexer, multiple quantum channels and multiple receivers, wherein the multiple receivers are each connected to the source via a quantum channel, wherein the source is configured to generate entangled photon pairs, each consisting of a signal photon and an idler photon, which are entangled with each other in an entanglement property, wherein the quantum channels of each pair of receivers of an entangled photon pair are connected In thefor quantum key generation, wherein the frequency multiplexer is arranged in or after the source, which is configured to divide the signal photons and idler photons according to their wavelength onto the quantum channels of the multiple receivers, wherein each receiver has a detection module which has a measuring module and at least one detector, which are configured to detect the entanglement property of the photons for quantum key generation, wherein the system has matching devices which are arranged in the receivers or in the quantum channels to the receivers and are configured to perform a matching of the entanglement property of the entangled photon pairs with respect to two receivers.

[0014] It is essential that the system additionally has a control device and that the control device is connected to each matching device, and that the control device is configured to control the matching devices in several time intervals for quantum key generation between all receivers, preferably for carrying out the quantum key generation method according to steps i) to iv). wherein the control device is designed to only adjust the balancing devices between two or more connections at any given time interval In the to target which can be independently synchronized in order to be used in these two or more connections In the to enable quantum key generation, wherein the control device is configured to control the alignment device of at least one connection at any given time interval In the by controlling the adjustment device of another connection In the to replace, in which quantum key generation was not yet possible, in order to carry out quantum key generation between all receivers over the multiple time periods, and / or that in the last time period a connection In the , or several of the connections In the , or all connections In the , are repeated, which were already formed in previous time periods, in order to perform a renewed quantum key generation.

[0015] An advantage of the method and system according to the invention lies in the fact that, through the temporal rotation of the matching process and thus the quantum key generation, a supply of quantum keys is generated in the time intervals between all receivers. After the several time intervals have elapsed, all receivers are served with entangled photon pairs, wherein, according to the invention, in at least one time interval each connection between each receiver was matched at least once for quantum key generation.

[0016] In the inventive method and the inventive system, the connections are rotated. In the Quantum keys are generated for all recipients so that after several time periods, each recipient has generated a quantum key with every other recipient.

[0017] The advantage here is that by comparing the entanglement properties only between independent connections within specific time intervals, the method and the system are optimized in such a way that the temporal division of the comparison according to the invention results in a better generation rate for quantum key generation between all receivers. The improved generation rate is achieved because the inventive method and system optimize the connections In the They can be aligned more precisely and quickly. This is because, according to the invention, several connections can be made simultaneously, but always independently of each other. In the They are compared, so that the comparisons do not interfere with each other.

[0018] In the event that a connection was established in the recent period In the , or several of the connections In the , or all connections In theTo perform a renewed quantum key generation, repeating processes that were already formed in previous time periods, a faster adjustment can preferably be carried out in the last time period by using the known adjustment values ​​of the previous connections. In the used as initial values ​​for the comparison.

[0019] Furthermore, it is also advantageous that the quantum key generation according to the invention can be carried out faster and thus environmental effects, such as temperature fluctuations or vibrations, can be compensated for more precisely and quickly, since the adjustments do not interfere with each other. Due to the lack of mutual interference in the adjustment of the multiple connections... In the are not iterative comparison steps between multiple connections In the necessary.

[0020] Additionally, it is advantageous that the system according to the invention can be provided cost-effectively, since fewer balancing devices are necessary, as for each connection In the Only one calibration device is required.

[0021] A further advantage of the inventive method and network is the direct connection of the source to each receiver via a quantum channel. This increases the number of physical connections, i.e., quantum channels, only through the relationship between the sources and receivers. i ( i - 1) / 2. In contrast, a network with a direct connection between the individual receivers would require a large number of additional quantum channels.

[0022] Another advantage of the method and network according to the invention is that the last period of time can be used to establish a connection. In the , or several of the connections In the , or all connections In the, to repeat those already formed in previous time periods. This makes it possible to generate multiple quantum keys in a single pass. Preferably, this allows, after the complete traversal of the multiple time periods per connection, In the At least two quantum keys can be generated. This enables the rapid generation of multiple quantum keys.

[0023] The method and system for quantum key generation comprise at least four receivers, preferably i Recipient E p with the number i equal to a natural number equal to or greater than 4 and with the numbering p equal to a natural number.

[0024] Two receivers E p a pair of entangled photons form a connection In the with the number j being a natural number. In total, there are between all recipients. s Connections In the possible, with s ≥ i i − 1 2 . In any time period tk become r Connections In the matched with the number r equal to a natural number greater than 2 and less than s.

[0025] The quantum key generation method and system use multiple time spans. tk , preferably q Time periods tk with the number q equal to a natural number equal to or greater than 2 and with the numbering k equal to a natural number. This means that within the several, preferably the q Time periods tk A quantum key generation process is performed between all recipients. This quantum key generation process can be carried out using the... q Time periods tk also referred to as a complete rotation.

[0026] The inventive method and system result in an improved temporal efficiency factor for generating quantum keys with s q > 1 The temporal efficiency factor describes the increase in the generation rate compared to a method and a system in which a quantum key is generated only between two receivers at any given time. A comparison with a method and a system in which all connections are synchronized simultaneously reveals a significant difference in the quality of the synchronization between all connections.

[0027] Advantageously, the inventive method and system thus reduce the number of compounds V required for a number s. j required number q in time periods tk .

[0028] It is essential that quantum key generation takes place within a specific time period. tk only the connections V j to be used in which the matching has taken place.

[0029] Among at least four recipients, preferably between i recipients E p with i equal to a natural number equal to or greater than 4 and with p The phrase "equal to a natural number" here means that the quantum key generation is carried out between four receivers, or five receivers, or six receivers, or seven receivers, or more receivers.

[0030] Quantum key generation between all four receivers means that each receiver generates a quantum key with every other receiver. For example, with four receivers, between the first receiver E 1 and the second recipient E 2, and between the first recipient E 1 and the third recipient E 3, and between the first recipient E1 and the fourth recipient E 4, and between the second recipient E 2 and the third recipient E 3, and between the second recipient E 2 and the fourth recipient E 4, and between the third recipient E 3 and the fourth recipient E 4 each generates a quantum key.

[0031] Furthermore, quantum key generation between all four receivers means that a quantum key is generated between these four receivers. Additional receivers may be connected to the source in the system, who do not generate a quantum key between the four receivers at the time of the initial quantum key generation, but may participate in a subsequent quantum key generation event.

[0032] The generation of multiple entangled photon pairs in the source and in step i) means that several entangled photon pairs can be generated simultaneously and / or sequentially in the source. Simultaneously means that several entangled photon pairs are generated at the same or approximately the same time, with the wavelengths of these entangled photon pairs differing from one another. Sequence means that several entangled photon pairs can be generated successively over time, with these photon pairs having the same or different wavelengths. The successive generation of entangled photon pairs with the same wavelength increases the length of the quantum key between two specific receivers.Entangled photon pairs with different wavelengths enable quantum key generation between several different receivers of the multiple connections. In the This is enabled. This applies equally to steps ii), iii) and iv), wherein in step ii) several signal photons and idler photons from different entangled photon pairs are simultaneously and / or successively distributed to and transmitted into the quantum channels, and in step iii) the photons are detected simultaneously at several receivers or successively at one or more receivers, and in step iv) several quantum keys are generated simultaneously and / or successively between several receivers.

[0033] The term quantum key generation method using entangled photon pairs means that steps i), ii) and iii) are performed sequentially for each entangled photon pair, but steps i), ii) and iii) can also be performed simultaneously for several photon pairs with different wavelengths.

[0034] Alignment of the entanglement property means that the entanglement reference frame used to measure a photon pair in the detection module during detection in step iii) is aligned at the two corresponding receivers before and / or during step ii). For example, in the case of photon pairs entangled in polarization, the polarization at the respective detection modules is aligned by transmission over the corresponding link. In theThe reference frames are aligned, whereby any polarization rotation, for example due to the transmission of photons in fibers or the arrangement of the system elements, can be compensated for by so-called polarization controllers. For this purpose, a laser beam with a defined polarization in one basis, e.g., horizontally polarized, is transmitted from a first receiver to a second receiver, and the polarization of the laser beam in this basis is measured at the second receiver. The alignment of the quantum channel in the first basis can be achieved, for example, by minimizing the laser beam at the second receiver during a measurement in the vertical polarization. This minimization is then performed in a further step for a second basis, which is orthogonal to the first basis, for example, for the + / - 45° horizontally polarized laser beam.When photon pairs are entangled over time, phase alignment is performed in the respective interferometers of the receivers. Furthermore, it is essential that in the inventive method and the inventive system, in each individual time interval... tk only connections In the The matching process must involve elements that can be independently matched, meaning they do not interfere with or influence each other. This allows for a much more precise and faster matching process, resulting in a higher quantum key generation rate. Only between two or more connections In the means that at least two connections In the and at most all other possible connections In the They can be compared within a time period, and these comparisons can be made independently of each other.

[0035] Connection check In the independently of each other over a period of time tkThis means that the alignment of the receiver's reference system and the connections V j during this period tk They do not interfere with or influence each other. This significantly improves the quality of photon transmission and detection, leading to a higher quantum key generation rate. For example, with four receivers, the connection can be V 1 between the first recipient E 1 and the second recipient E 2 regardless of the connection V 2 between the third recipient E 3 and the fourth recipient E 4 can be compared. For example, with four receivers, the connection can be... V 1 between the first recipient E 1 and the second recipient E 2 regardless of the connection V 2 between the first recipient E 1 and the third recipient E3 will be checked if the connection check V 1 at the second recipient E 2 takes place and the connection is checked. V 2 at the third recipient E 3 takes place, whereas a comparison of the connection V 1 at the second recipient E 2 and the alignment of the connection V 2 at the first recipient E 1. the alignment of the connection V 1 would be disruptive.

[0036] At least one connection In the through another connection In j+x , Replacing x as an integer means that there is only one connection. In the the preceding period tk is exchanged and through a connection In j+x is replaced, in which quantum key generation was not yet possible, or that two connections V j , V+ 1 the preceding period tk be exchanged and through two connections V j+x 1 , In j+x2to be replaced in which quantum key generation was not yet possible, and so on for three, four and more connections, or that all connections of the preceding time period tk be exchanged and replaced by the corresponding number of connections in which quantum key generation was not yet possible.

[0037] A connection In j , in which quantum key generation was not yet possible means that for this connection In the Steps i) to iii) have not yet been completed while this connection was in place. In the before and / or during step ii) was independently verified. This means that, for the purposes of the procedure and system described here, quantum key generation between two receivers is only considered possible if the corresponding connection In theThe reference frames were and / or are aligned before and / or during the transmission of photons in step ii). If no alignment takes place, photons can still be measured, but a common key cannot be generated due to the different reference frames of the receivers.

[0038] To navigate through the multiple time periods tk Performing quantum key generation between all recipients means that after several time periods tk for all possible connections between all recipients, steps i), ii), iii) and iv) were performed, and that for all connections In the before and / or during step ii) in at least one time period tk A comparison was made during which time a comparison was carried out.

[0039] It can be provided that the multiple time periods are repeated several times, preferably several times consecutively, preferably a second, third, fourth, or more. This means that a complete rotation is repeated successively. This allows quantum keys to be generated between all receivers over an extended period.

[0040] It may be planned that the time spans for quantum key generation are tk The process must be carried out sequentially. This means that the time intervals for quantum key generation are... tk They must be formed directly consecutively. Directly consecutive means that no other connections are used in between. Directly consecutive also means that there is no time interval between the time periods. tk No connections within a given time window In theThey can be operated, for example, to carry out maintenance work. This advantageously results in a simple generation of quantum keys between all recipients.

[0041] It may be planned that the time spans for quantum key generation are tk This cannot be done sequentially. This means that the time intervals for quantum key generation are... tk They cannot be formed directly consecutively. Between two time periods tk This allows other connections to be used. These other connections could include one or more connections. In the from one of the several time periods tk to be understood, or several connections that cannot be compared independently of each other. A non-sequential approach advantageously results in certain connections In theThey can be used multiple times, for example, if there is an increased need for quantum keys between two specific receptions. In the case of connections In j , which can still be compared independently, the higher quality and production rate remain advantageous. Thus, in a non-sequential process, after one of several time periods... tk one of the time periods tk be repeated or a new time period can be inserted in which connections In the Connections can be operated that can be synchronized independently. If necessary, connections between two time periods that cannot be synchronized independently can also be operated, although these connections may require more adjustment or have a lower generation rate.

[0042] It can be provided that in step i) the entangled photon pairs are generated by a non-linear process, preferably by parametric fluorescence (down-conversion), spontaneous parametric fluorescence (spontaneous parametric down-conversion), or four-wave mixing. It can be provided that the source comprises one or more non-linear crystals configured to generate entangled photon pairs by a non-linear process, preferably by parametric fluorescence (down-conversion), spontaneous parametric fluorescence (spontaneous parametric down-conversion), or four-wave mixing. These non-linear processes allow entangled photon pairs in different wavelength ranges to be generated easily.

[0043] It is possible to envision entangled photon pairs in time, and / or polarization, and / or orbital angular momentum, and / or spin angular momentum. The advantage of using time-entangled photon pairs is robust entanglement. The advantage of using polarization-entangled photon pairs is their ease of generation and alignment, as well as the potential for automation of the alignment process. The advantage of using orbital angular momentum-entangled photon pairs and / or spin angular momentum is the potential for high photon pair dimensions.

[0044] It may be provided that, in order to align photon pairs entangled in polarization, a polarization rotation in the connection In theThe alignment device for entangled photon pairs in polarization can be designed to include one or more wavelength plates, and / or fiber squeezers, and / or polarization controllers, and / or liquid crystals. These components can also be combined, enabling even more precise adjustment. The advantage of this design lies in the easily controllable alignment, which can also be automated, and the cost-effective components.

[0045] It may be provided that, for the purpose of aligning photon pairs entangled in time, a time interval in the connection In the The alignment is compensated for. It may be provided that the alignment device for time-entangled photon pairs includes one or more optical delay devices. The advantage of such a design lies in the very precise and simple control of the alignment.

[0046] It may be provided that, in order to align photon pairs entangled in orbital angular momentum and / or spin angular momentum, a change in angular momentum in the connection In the The balancing mechanism for entangled photon pairs in orbital angular momentum and / or spin angular momentum can be designed to include one or more wavelength plates and / or a spatial light modulator (SLM). The advantage of such a design lies in the easily controllable balancing process, which can also be automated.

[0047] It can be provided that each receiver or each quantum channel of a receiver has a matching device. The advantage of this is that, for the method and system according to the invention, not all matching devices are required to match all possible connections. In theThis is required, thus providing a fail-safe system in which the adjustment in a failed adjustment device can be replaced by an adjustment device that has not yet been used. It is essential that the inventive method and system for adjusting all connections In the only i - 1 balancing devices are necessary in a procedure and a system with i Recipients. It may be intended that the system only i - It has one balancing device. This allows for a cost-effective system to be provided.

[0048] It may be planned that the comparison will be performed for each connection. In theBefore and / or during step ii), the adjustment is performed only by a matching device located at one of the two receivers or in one of the two quantum channels. Advantageously, the arrangement of the source, the receivers, and the quantum channels results in the matching of the entire connection being possible in such a configuration. In the This can be accomplished using a single alignment device located somewhere between the two receivers. This simplifies the alignment process. An alignment device located directly at each receiver is particularly well protected against external influences, i.e., attempted manipulation and interference, since no additional communication between the receiver and the alignment device via public channels is necessary for adjustment.

[0049] It may be provided that the adjustment before and / or during step ii) is performed, monitored, and / or controlled by a control device connected to all adjustment devices. The control device may be located at one of the receivers. The control device may be configured as a computer or integrated circuit (IC), preferably as an FPGA (Field Programmable Gate Array). The computer or integrated circuit (IC), preferably as an FPGA (Field Programmable Gate Array), may include a storage medium containing instructions that, when executed by the computer or integrated circuit (IC), preferably as an FPGA (Field Programmable Gate Array), cause it to perform the adjustment before and / or during step ii). Advantageously, such a configuration allows for the automation of the process.

[0050] It may be provided that in step i) entangled photon pairs are generated with wavelengths that are randomly distributed over a broad spectrum, or that entangled photon pairs with specific wavelengths are generated in a targeted manner. It may be provided that in step i) entangled photon pairs are generated at any time interval tk for all possible connections In the are generated, or that entangled photon pairs are generated in step i) in each time interval tk only for the connections also matched therein In the The advantage of generating photons across a broad spectrum is the cost-effective provision of entangled photon pairs to a large number of receivers. The advantage of generating photons with specific wavelengths is that for each connection In the during the relevant time period tk photons specifically for these connections In thecan be generated, thereby increasing the generation rate for quantum key generation.

[0051] It can be provided that the photons generated in step i) are produced in spectrally separate signal wavelength ranges and idler wavelength ranges. "Spectrally separate" here means that the wavelengths of the signal photon and the idler photon of the pairs differ, and that the wavelengths of the pairs differ from each other. This results in the advantage of a simpler and less lossy splitting of the photons in step ii).

[0052] It may be provided that the photon pairs generated in step i) are used for each connection In theare spectrally separated from each other. Spectrally separated entangled photon pairs, in this context, mean that the signal and idler photons of a photon pair have different wavelengths; that is, the following applies: λ sx ≠ λ ix. This is also referred to as a non-degenerate photon pair. Additionally, for multiple spectrally separated entangled photon pairs, the wavelengths of the signal photons of the photon pairs differ spectrally from each other; that is, the following holds true: λ sx ≠ λ s ( x+1 ) . This also applies accordingly to the idler photons; that is, the wavelengths of the idler photons also differ spectrally from each other. λ ix ≠ λ i ( x+1) .The essential point is that several of the spectrally separated photon pairs can also be generated successively in time in order to create a longer quantum key.

[0053] It may be provided that the frequency multiplexer is configured to perform the splitting of the signal photons and the idler photons in step ii).

[0054] Preferably, the assignment of signal and idler photons to the quantum channels according to their wavelength in step ii) and by the frequency multiplexer means that the source generates signal and idler photons with different wavelengths and transmits the photons, preferably in a spatial mode, to the frequency multiplexer. The frequency multiplexer then distributes the signal and idler photons to the different quantum channels according to their wavelength.

[0055] It is possible for one, several, or all quantum channels to be configured as fiber optic channels and / or to incorporate fiber optic links. Fiber optic links in this context mean that not the entire transmission occurs through a single fiber optic cable, but that free-space paths are also possible over certain sections. The advantage of such a design lies in the cost-effective and simple network construction.

[0056] It may be provided that the detection in step iii) is performed at each receiver by a detection module. It may be provided that each detection module comprises one or more detectors and a measurement module. Using the measurement module and the detectors, the entanglement property of the photons for quantum key generation, as well as the time of photon detection, can be determined.

[0057] It may be provided that the at least one detector or the several detectors are designed as single-photon detectors, preferably as germanium or silicon detectors, or single-photon avalanche diodes or indium gallium arsenite detectors, or superconducting nanowire single-photon detectors, or silicon avalanche photodiodes.

[0058] The measuring module may include a polarizer, and / or an asymmetrical interferometer, and / or a spatial light modulator (SLM).

[0059] The receiver may have multiple detection modules. The multiple detection modules of a receiver may detect photons in different wavelength ranges. In step iii), one or more receivers may simultaneously detect multiple signal photons and / or idler photons with different wavelengths. This allows the receiver to simultaneously generate quantum keys with multiple other receivers, as photons with different wavelengths can be detected at the same time. Frequency filters may be arranged upstream of the multiple detection modules to enable simultaneous quantum key generation with multiple receivers by distributing the photons among the detection modules based on their wavelength. This allows each detection module of a receiver to be assigned a corresponding partner receiver.The frequency filter can be designed as a dichroic mirror, or as a grating, or as a filter.

[0060] It may be stipulated that in step iv) of the quantum key generation process, a raw key is generated from the detected photons in step iii) at both receivers. Furthermore, it may be stipulated that after the raw key is generated, a sifting process is performed. For this sifting, information is exchanged between the two receivers after the photons are measured. This information includes, for example, the timestamps of the measured photons. Each receiver then performs the sifting process on its own raw key. It may be stipulated that, after the sifting, additional steps such as error detection and / or correction and / or privacy amplification are carried out. Through the sifting process and any further steps, a shared key can be generated at both receivers via quantum key generation.

[0061] It can be provided that a transmission rate of entangled photons between two or more receivers of at least 1kHz, preferably at least 100kHz, most preferably at least 10MHz, takes place.

[0062] It may be provided that the quantum key generation in step iv) is for the connections of a time span. tk each time period tkThis occurs. "While" means that steps i) to iv) are performed in each time interval. "Afterwards" means that steps i) to iii) are performed in each time interval, and step iv) occurs after the respective time interval. The generation of the quantum key from the raw key can also start during the respective time interval and only be completed in a subsequent time interval. Crucially, the detection of photons in step iii) already allows for the generation of a raw key for quantum key generation, which is then used to generate the shared key. Performing step iv) enables complete quantum key generation within the respective time interval, whereby a quantum key can be generated from the already detected photons while further photons are being generated.

[0063] It may be provided that the transmission in step ii) takes place via a splitter and / or switch in the quantum channel. It may be provided that two or more receivers are connected to the source via a splitter and / or switch over a common quantum channel. Crucially, in such an embodiment, these two or more receivers connected to the source via a splitter and / or switch over a common quantum channel cannot generate a quantum key among themselves. In this case, quantum key generation between all receivers means that each of these two or more receivers connected to the source via a splitter and / or switch over a common quantum channel can generate quantum keys with all other receivers, but not with the receiver that also receives photons via the splitter and / or switch.This advantageously results in a simpler and more cost-effective network, since only a single quantum channel is needed to connect these two or more receivers to the source. A splitter can, for example, be designed as a beam splitter, which randomly divides the photons to one of its output channels. A switch can, for example, be designed as a movable mirror or a pluggable connection, whereby the photons are directed to one of the output channels depending on the position of the mirror or the plugged-in connection. The splitters and / or switches can represent an access node (or service node), a relay node, or a user node. Such a design enables a more cost-effective connection of multiple receivers.For example, multiple user nodes and their associated access nodes can form a QKD access network (QAN) suitable for covering densely populated areas. And multiple relay nodes can form a QKD backbone network (QBN) to connect multiple QANs for wide-area coverage.

[0064] It may be planned that in any time period tk for at least two connections In the Steps i) to iii) are carried out with the adjustment, preferably steps i) to iv). This means that in each time interval tk for two connections In the Steps i) to iii), preferably steps i) to iv), are carried out with the comparison, or for three connections In the Steps i) to iii), preferably steps i) to iv), are carried out with the comparison, or for four connections In theSteps i) to iii), preferably steps i) to iv), are carried out with the comparison, and so on. The more connections In the The more frequently the quantum keys are synchronized in each time interval tk, the greater the generation rate of the quantum keys for all receivers.

[0065] It may be planned that in any time period tk A receiver can only be used once in the connections. In the the respective time period tk This is included. This makes it easy to ensure that the connections are correctly aligned. In the this period tk This was done independently of each other.

[0066] It may be planned that in any time period tk a receiver in several of the connections In the the respective time period tk is included. In other words, it can be provided that in any given time period tk Each of the recipients is in several of the connections In the the respective time period tk It is included. The essential point is that all connections are compared. In the this period tk This continues to occur independently of each other. However, this allows a receiver with high communication needs to be served multiple times with a higher rate of quantum key generation.

[0067] It may be planned that in any time period tk that one of the recipients is included in all connections of the respective time period, and / or that in different time periods tk a different recipient is included in all connections during the respective time period, and / or that within a time period t k+ 1 at least one connection In the the time period tk is included, where k is a natural number, and / or that in each time interval t k+ 1 at least one connection In thethe time period tk is included, whereby k is a natural number. This allows individual connections, multiple connections, or all connections to be traversed twice, for example, and thus used twice or multiple times for quantum key generation.

[0068] In particular, it may be stipulated that the number of time periods is equal to or greater than the number of recipients.

[0069] It may be provided that each time period tk at least 1 second, preferably at least 1 minute, preferably at least 5 minutes or more. It may be provided that the length of the time intervals varies. tk from each other. This advantageously means that for connections In the which have a higher or lower key requirement the time spans tk can be adapted to the respective needs.

[0070] In the following examples, the recipients are referred to as "recipients" for easier understanding. E1, E 2 etc. are referred to as receivers A, B etc.

[0071] As a non-exclusive first example, a quantum key generation method and a quantum key generation system with four receivers (A, B, C, D) are presented here. In this system, all receivers perform quantum key generation among themselves.

[0072] Due to the four recipients (A, B, C, D) and the communication possibility, the following results s = i i − 1 2 = 6 Connections In the with AB, AC, AD, BC, BD and CD.

[0073] In this non-exclusive first example, steps i) to iii) of these connections are shown in q = divided into 3 time periods, with each time period tkr = 2 connections In the features: t 1 = AB CD t 2 = AC BD t 3 = AD BC

[0074] For example, the first time period contains t1 the first connection V 1 = AB, i.e., a quantum key generation between the first receiver A and the second receiver B, and the second connection V 2 = CD, i.e., quantum key generation between the third receiver C and the fourth receiver D. The time intervals t 2 and t The three lines are to be read in the same way.

[0075] The time efficiency factor is s q = 2 , that is, that by the inventive method and the inventive system between the four receivers according to the non-exclusive first example, quantum key generation between all receivers is possible twice as fast compared with quantum key generation carried out successively for each connection between four receivers.

[0076] This is possible by simultaneously performing steps i) to iii), preferably i) to iv), for two connections in the inventive method and system, while simultaneously aligning these connections without mutual interference, thus also ensuring a high transmission rate and quality.

[0077] As a non-exclusive second example, a quantum key generation method and a quantum key generation system with four receivers (A, B, C, D) are presented here. In this system, all receivers perform quantum key generation among themselves.

[0078] Due to the four recipients (A, B, C, D) and the communication possibility, the following results s = i i − 1 2 = 6 Connections In the with AB, AC, AD, BC, BD, and CD.

[0079] In this non-exclusive second example, steps i) to iii), preferably steps i) to iv), of these compounds are used in q = 4 time periods divided according to the number of recipients, with each time period tk r = 3 connections In the features: t 1 = AB AC AD t 2 = BA BC BD t 3 = CA CB CD t 4 = DA DB DC

[0080] In contrast to the second example presented previously, here every time period is considered. tk a connection from one receiver to all other receivers is served, whereby the one receiver in each case is in the next time period t k+ 1 is exchanged for another receiver. This means that after all processes have been completed... q = 4 time intervals each connection between two receivers is traversed twice for quantum key generation, as illustrated in the table below. AB AC AD BC BD CD 2 2 2 2 2 2

[0081] This allows twice as many quantum keys to be generated as in the first example shown previously.

[0082] After going through all q For 4 time periods, 12 keypools or 12 quantum keys are now available, compared to the 6 keypools or 6 quantum keys according to the first example shown above.

[0083] As a non-exclusive third example, a quantum key generation method and a quantum key generation system with five receivers (A, B, C, D, E) are presented here. In this system, all receivers perform quantum key generation among themselves.

[0084] Due to the five recipients (A, B, C, D, E) and the communication possibility, the following results s = i i − 1 2 = 10 Connections In the with AB, AC, AD, AE, BC, BD, BE, CD, CE and DE.

[0085] In this non-exclusive third example, steps i) to iii), preferably steps i) to iv), of these compounds are used in q = divided into 5 time periods, with each time period tkr = 2 connections In the features: t 1 = AB CD t 2 = BC DE t 3 = AE BD t 4 = AC BE t 5 = AD CE

[0086] The time efficiency factor is s q = 2 .

[0087] It should be noted that there are also other classifications of connections. In the into the time spans that are possible.

[0088] Furthermore, it is also possible to divide the connections into more than five time periods, thereby creating one or more connections. In the are contained in several time periods. This reduces the time efficiency factor, for example, with six groups to s q = 1.67 However, it is advantageous that this is possible, for example, for one or more connections. In the, which are more frequently included in the time spans, a longer quantum key can be generated if there is increased demand for this one or more connections.

[0089] The non-exclusive third example described above shows a sequential implementation for quantum key generation.

[0090] As a variation, the following time spans are possible in a non-sequential execution, for example: t 1 = AB CD t 2 = BC DE t 2 = BC DE t 3 = AE BD t 4 = AC BE t 5 = AD CE

[0091] In this variation, the second time period t 2. This is performed a second time. This is advantageous if there is a high demand for quantum keys between recipients B and C and recipients D and E.

[0092] In another variation, it is also possible to make connections between two time windows. In theto operate which were not yet grouped into a time window, but which can still be compared independently of each other, as shown below: t 1 = AB CD t 2 = BC DE t 6 = BC AD t 3 = AE BD t 4 = AC BE t 5 = AD CE

[0093] In this variation, the compounds BC and AD are shown as an example after the second time interval t 2 in a time interval t 6. This is advantageous if there is a high demand for quantum keys between receivers B and C and receivers A and D.

[0094] As a further non-exclusive fourth example, a quantum key generation method and a quantum key generation system with five receivers (A, B, C, D, E) are presented here. In this system, all receivers perform quantum key generation among themselves. This fourth example is an extension of the previously described second example with four receivers by one receiver, thus resulting in five receivers. Analogously, this method can be extended to multiple receivers according to the second or fourth example. In particular, to six receivers, or to seven receivers, or to eight or more receivers.

[0095] Due to the five recipients (A, B, C, D, E) and the communication possibility, the following results s = i i − 1 2 = 10 Connections In the with AB, AC, AD, AE, BC, BD, BE, CD, CE and DE.

[0096] In this non-exclusive fourth example, steps i) to iii), preferably steps i) to iv), of these compounds are used in q = 5 time periods divided according to the number of recipients, with each time period tk r = 4 connections In the features: t 1 = AB AC AD AE t 2 = BA BC BD BE t 3 = CA CB CD CE t 4 = DA DB DC DE t 5 = EA EB EC ED

[0097] Unlike the third example presented previously, here every time period is considered. tk a connection from one receiver to all other receivers is served, whereby the one receiver in each case is in the next time period t k+ 1 is exchanged for another receiver. This means that after all processes have been completed... q = 5 time intervals each connection between two receivers is traversed twice for quantum key generation, as illustrated in the table below. AB AC AD AE BC BD BE CD CE DE 2 2 2 2 2 2 2 2 2 2

[0098] This allows twice as many quantum keys to be generated in any given time period as in the third example shown previously.

[0099] After going through all q For 5 time periods, 20 keypools or 20 quantum keys are now available, compared to the 10 keypools or 10 quantum keys according to the third example shown previously.

[0100] As a non-exclusive fifth example, a quantum key generation method and a quantum key generation system with six receivers (A, B, C, D, E, F) are presented here. In this system, all receivers perform quantum key generation among themselves.

[0101] Due to the six recipients (A, B, C, D, E, F) and the communication possibility, the following results s = i i − 1 2 = 15 Connections with AB, AC, AD, AE, AF, BC, BD, BE, BF, CD, CE, CF, DE, DF and EF.

[0102] In this non-exclusive fifth example, steps i) to iii), preferably steps i) to iv), of these compounds are described in q = divided into 5 time periods, with each time period tkr = 3 connections In the exhibits, for example, the following classification: t 1 = AB CD EF t 2 = AC BE DF t 3 = AD BF CE t 4 = AE BD CF t 5 = AF BC DE

[0103] Another classification is given here as an example: t 1 = AB CE DF t 2 = AC BD EF t 3 = AD BE CF t 4 = AE BF CD t 5 = AF BC DE

[0104] In both cases, the time efficiency factor is s q = 3 .

[0105] It should be noted that there are also other classifications of connections. In the into which time periods are possible, and that it is also possible to divide the connections into more than five time periods, whereby one or more connections are included in several time periods.

[0106] As a non-exclusive sixth example, a quantum key generation method and a quantum key generation system with seven receivers (A, B, C, D, E, F, G) are presented here. In this system, all receivers perform quantum key generation among themselves.

[0107] Due to the seven recipients (A, B, C, D, E, F, G) and the communication possibility, the following results s = i i − 1 2 = 21 Connections with AB, AC, AD, AE, AF, AG, BC, BD, BE, BF, BG, CD, CE, CF, CG, DE, DF, DG, EF, EG and FG.

[0108] In this non-exclusive sixth example, steps i) to iii), preferably steps i) to iv), of these compounds are performed in q = divided into 7 time periods, with each time period tkr = 3 connections In the exhibits, for example, the following classification: t 1 = AB CD EF t 2 = AC BD EG t 3 = AD BC FG t 4 = AE BF CG t 5 = AF BE DG t 6 = AG CE DF t 7 = BG CF DE

[0109] Another classification is given here as an example: t 1 = AB CG EF t 2 = AC BE DG t 3 = AD FG CE t 4 = AE BC DF t 5 = AF BD EG t 6 = AG BF CD t 7 = DE CF BG

[0110] In both cases, the time efficiency factor is s q = 3 .

[0111] It should be noted that there are also other classifications of connections. In the into which time periods are possible, and that it is also possible to divide the connections into more than seven time periods, whereby one or more connections are contained in several time periods.

[0112] As a non-exclusive seventh example, a quantum key generation method and a quantum key generation system with eight receivers (A, B, C, D, E, F, G, H) are presented here. In this system, all receivers perform quantum key generation among themselves.

[0113] Due to the eight recipients (A, B, C, D, E, F, G, H) and the communication possibility, the following results s = i i − 1 2 = 28 Connections with AB, AC, AD, AE, AF, AG, AH, BC, BD, BE, BF, BG, BH, CD, CE, CF, CG, CH, DE, DF, DG, DH, EF, EG, EH, FG, FH and GH.

[0114] In this non-exclusive seventh example, steps i) to iii), preferably steps i) to iv), of these compounds are performed in q = divided into 7 time periods, with each time period tkr = 4 connections In the exhibits, for example, the following classification: t 1 = AB CD EF GH t 2 = AC BD EG FH t 3 = AD BC EH FG t 4 = AE BF CG DH t 5 = AF BE CH DG t 6 = AG BH CE DF t 7 = AH BG CF DE

[0115] In this case, the time efficiency factor is s q = 4 .

[0116] It should be noted that there are also other classifications of connections. In theinto which time periods are possible, and that it is also possible to divide the connections into more than eight time periods, whereby one or more connections are contained in several time periods.

[0117] As a non-exclusive eighth example, a method and system for quantum key generation with four receivers (A₁, A₂, B₁, B₂) is presented here. In this method and system, all A i Receiver with all B i Perform quantum key generation between the receivers. A quantum key generation between the two receivers. A 1 and A 2 and between the two recipients B 1 and B But 2 is not possible.

[0118] Due to the four recipients (A1, A2, B1, B2) and the communication possibility, the following results: s = 4 connections divided by the number of receivers A i multiplied by the number of recipients B i with A1 B1, A1 B2, A2 B1 and A2 B2.

[0119] In this non-exclusive eighth example, steps i) to iii), preferably steps i) to iv), of these compounds are used in q = divided into 2 time periods, where each time period tkr = 2 connections In the exhibits, for example, the following classification: t 1 = A 1 B 1 , A 2 B 1 t 2 = A 1 B 2 , A 2 B 2

[0120] Two further classifications are given here as examples: t 1 = A 1 B 1 , A 1 B 2 t 2 = A 2 B 1 , A 2 B 2 t 1 = A 1 B 1 , A 2 B 2 t 2 = A 1 B 2 , A 2 B 1

[0121] In all cases, the time efficiency factor is s q = 2 .

[0122] As a non-exclusive ninth example, a method and system for quantum key generation with five receivers (A₁, A₂, A₃, B₁, B₂) is presented here. In this method and system, all A i Receiver with all B i Perform quantum key generation between the receivers. A1,A2 and A 3 and between the two recipients B 1 and B 2 is not possible.

[0123] Due to the five recipients (A1, A2, A3, B1, B2) and the communication possibility, the following results: s = 6 connections divided by the number of receivers A i multiplied by the number of recipients B i with A1 B1, A1 B2, A2 B1, A2 B2, A3 B1 and A3 B2.

[0124] In this non-exclusive ninth example, steps i) to iii), preferably steps i) to iv), of these compounds are described in q = divided into 2 time periods, where each time period tkr = 3 connections In the exhibits, for example, the following classification: G 1 = A 1 B 1 , A 2 B 1 , A 3 B 1 G 2 = A 1 B 2 , A 2 B 2 , A 3 B 2

[0125] The time efficiency factor is s q = 3 .

[0126] It should be noted that in this case, the matching of the entanglement property can be carried out solely by the receivers with the larger number, i.e., here by A. i Recipient. This means that in the case of B i Receivers do not necessarily need to be equipped with a device for comparing the entanglement properties.

[0127] Further embodiments of the invention are illustrated in the figures and described below. The figures show one possible embodiment of the invention by way of example. This embodiment serves to illustrate a possible implementation of the invention and is not intended to be limiting. The figures show: Fig. 1: a schematic representation of a system according to the invention with four receivers; Fig. 2: a schematic representation of the system made of Fig. 1 with the possible connections In theFig. 3: a schematic representation of a system according to the invention with six receivers; Fig. 4: a schematic representation of a system according to the invention with four receivers and two splitters in the quantum channels; Figs. 5 to 7: a schematic representation of the system made of Fig. 4 with the possible connections In j ; Fig. 8: a schematic representation of a system according to the invention with six receivers, two splitters and two switches in the quantum channels; Fig. 9: a schematic representation of a system according to the invention with six receivers, three splitters and one switch in the quantum channels; Fig. 10: a schematic representation of the system made of Fig. 1 with a control device; Fig. 11: a schematic representation of the system made of Fig. 4 with a control device.

[0128] Fig. 1Figure 1 shows a schematic representation of a first embodiment of a system 1 according to the invention for quantum key generation with entangled photon pairs between four receivers A, B, C and D.

[0129] System 1 for quantum key generation from Fig. 1 The device has a source 2 configured to generate entangled photon pairs. In this embodiment, a frequency multiplexer 3 is arranged in the source 2, configured to distribute the signal photons and idler photons of the entangled photon pairs to the quantum channels 5 of the multiple receivers 4 based on their wavelength.

[0130] In the exemplary embodiment of the Fig. 1Receivers 4A, B, and C each have an alignment device 8. Optionally, receiver 4D also has an alignment device 8, as shown by the dashed line. The alignment devices 8 align the reference systems with respect to the entanglement properties of the different receivers 4.

[0131] Fig. 2 The system 1 according to the invention for quantum key generation is shown. Fig. 1 , whereby in Fig. 2 For clarity, the balancing devices 8 are not shown. Fig. 2 are the possible connections In the between the receivers 4 for quantum key generation and their division into the several time periods tk depicted.

[0132] In an initial period t 1 In this embodiment, quantum key generation is performed between receivers A and C and receivers B and D, as shown in Fig. 2Indicated by the dashed arrows. For this purpose, entangled photon pairs are generated in the source 2 and transmitted to all receivers, with the connections via the quantum channels 5 between receivers A and C and between receivers B and D in this first time interval. t 1. The comparison can be carried out during this initial period. t 1 for example by means of a matching device 8 at receiver A and at receiver B.

[0133] Essentially, the alignment device 8 at receiver A adjusts the reference frames of receivers 4 A and C with respect to their entanglement properties. This means that any changes in the entanglement properties caused by the transmission of photons in this connection, such as a polarization rotation in the quantum channels 5, are compensated for by the alignment device 8 at receiver A.

[0134] In a second time interval t 2, in this embodiment, quantum key generation is carried out between receivers A and B and receivers C and D, as shown in Fig. 2 as indicated by the dotted arrows. The alignment of the connections can be carried out in this second time interval t 2, for example, by an alignment device 8 at receiver A and at receiver C.

[0135] In a third time period t 3 In this embodiment, quantum key generation is performed between receivers A and D and receivers C and B, as shown in Fig. 2 Indicated by the dash-dot arrows. The comparison of the connections can take place during this second time period. t 3 for example by means of a matching device 8 at receiver A and at receiver C.

[0136] Fig. 3shows a schematic representation of a second embodiment of a system 1 according to the invention for quantum key generation with entangled photon pairs between six receivers A, B, C, D, E and F.

[0137] The quantum key generation system 1 has a source 2 configured to generate entangled photon pairs. In this embodiment, a frequency multiplexer 3 is arranged in the source 2, which is configured to distribute the signal photons and idler photons of the entangled photon pairs to the quantum channels 5 of the multiple receivers 4 based on their wavelength.

[0138] In the exemplary embodiment of the Fig. 3Receivers 4A, B, C, D, and E each have an alignment device 8. Optionally, receiver 4F also has an alignment device 8, as shown by the dashed line. The alignment devices 8 align the reference systems with respect to the entanglement properties of the different receivers 4.

[0139] Fig. 4Figure 1 shows a schematic representation of a third embodiment of a system 1 according to the invention for quantum key generation with entangled photon pairs between four receivers A1, A2, B1, and B2. Receivers A1 and A2 (or B1 and B2) are each connected via their own quantum channel 5 to a splitter 6 and via this splitter to a common quantum channel 5 with the source 2. Crucially, in this embodiment, the two receivers 4 A1 and A2 (or B1 and B2), which are connected to the source 2 via a splitter 6 and a common quantum channel 5, cannot generate a quantum key among themselves. In this case, quantum key generation between all receivers 4 means that each of these two receivers A1 and A2 (or B1 and B2), which are connected to the source 2 via a splitter 6 over a common quantum channel 5, can communicate with all other receivers 4 B1 and B2 (or B2).A1 and A2 can generate quantum keys, but not among themselves.

[0140] In the exemplary embodiment of the Fig. 4 Receivers 4A1 and A2 each have an alignment device 8. Optionally, receivers 4B1 and B2 may also have an alignment device 8, as shown by the dashed lines. The alignment devices 8 align the reference systems with respect to the entanglement properties of the different receivers 4.

[0141] In the Fig. 5, 6 and 7 The system 1 according to the invention for quantum key generation consists of Fig. 4 depicted, whereby in the Fig. 5, 6 and 7 For clarity, the balancing devices 8 are not shown. Fig. 5, 6 and 7 show the possible connections between the receivers 4 for quantum key generation and their division into the several time spans. tk .

[0142] In Fig. 5This presents a first possible classification of the connections, by considering them within an initial time period. t 1. A quantum key generation takes place between receivers A1 and B2 and receivers A2 and B2 (dashed arrows), with verification of the connection. In a second time period... t 2. A quantum key generation takes place between receivers A1 and B1 and receivers A2 and B1 (dotted arrows) with comparison of the connection.

[0143] In Fig. 6 A second possible classification of the connections is presented, by considering a first time period. t 1. A quantum key generation takes place between receivers A2 and B1 and receivers A2 and B2 (dashed arrows), with verification of the connection. In a second time period... t2. A quantum key generation takes place between receivers A1 and B1 and receivers A1 and B2 (dotted arrows) with connection verification. The difference to the connections of Fig. 5 The reason is that for Fig. 5 The matching devices 8 at receivers A 1 and A 2 are sufficient, and for Fig. 6 The balancing devices 8 can be arranged at receivers B 1 and B 2.

[0144] In Fig. 7 A third possible classification of the connections is presented by considering, in a first time period, t 1. A quantum key generation takes place between receivers A1 and B2 and receivers A2 and B1 (dashed arrows), with verification of the connection. In a second time period... t 2. A quantum key generation takes place between receivers A1 and B1 and receivers A2 and B2 (dotted arrows) with comparison of the connection.

[0145] The Fig. 8 and 9Two further embodiments of the system 1 according to the invention for quantum key generation with six receivers 4 are shown.

[0146] In these embodiments, the source 2 is connected to the six receivers 4 via several quantum channels 5, splitters 6, and switches 7. The following aspects differ: Figures 8 and 9 only in that in Fig. 8 The receivers A1, A2, A3 and A4 are first connected to the source 2 via a splitter 6, and then a further division takes place via two switches 7 and in Fig. 9 Receivers A1, A2, A3, and A4 are first connected to source 2 via a switch 7, and then further split by two splitters 6. Quantum key generation is performed analogously in both embodiments as in the embodiment of the Fig. 4 to 7 .

[0147] Fig. 10 The system 1 for quantum key generation from the exemplary embodiment of the Fig. 1 , whereby in Fig. 10Additionally, the control device 9 (key management system) is shown. In this embodiment, the control device 9 is connected to the calibration devices 8 via lines and the respective receiver 4.

[0148] Fig. 11 The system 1 for quantum key generation from the exemplary embodiment of the Fig. 4 where in Fig. 11 Additionally, the control device 9 is shown. In this embodiment, the control device 9 (key management system) is connected to the calibration devices 8 via lines and the respective receiver 4. Reference symbol list

[0149] 1 Quantum key generation system 2 Source 3 Frequency multiplexer 4 Receiver 5 Quantum channel 6 Splitter 7 Switch 8 Calibration device 9 Control device

Claims

1. Method for quantum key generation with entangled photon pairs between at least four receivers (4), wherein in each case two receivers (4) of an entangled photon pair form a connection Vj for quantum key generation, with the following steps: i) generating entangled photon pairs in a source (2), each photon pair comprising a signal photon and an idler photon which are entangled with each other in an entanglement property; ii) distributing the signal photons and the idler photons on the basis of their wavelength to quantum channels of the multiple receivers (4) and transmitting the signal photons and the idler photons via the quantum channels forming connections Vj from the source to the receivers (4); iii) detection of the signal photons and idler photons at the respective receivers (4); iv) quantum key generation between the receivers (4) of the entangled photon pairs; wherein for quantum key generation several time spans are formed between all receivers (4), characterized in that the steps i) to iii) are carried out in each time span, and that, in each time span before and / or during step ii), an entanglement property alignment is performed only between two or more connections Vj which can be aligned independently of one another in order to enable quantum key generation in these two or more connections Vj , and that the time spans differ in such a way that in each case at least one connection Vj is replaced by another connection Vj in which quantum key generation was not yet possible in order to carry out quantum key generation between all receivers (4) by means of the several time spans, and / or that in the last time span one connection Vj , or several of the connections Vj , or all connections Vj , which have already been formed in previous time spans are repeated in order to carry out quantum key generation again.

2. Method according to claim 1, characterized in that the alignment per connection Vj before and / or during step ii) is performed by only one alignment device (8) arranged at one of the two receivers (4) or in one of the two quantum channels.

3. Method according to claim 2, characterized in that the alignment is carried out, monitored and / or controlled before and / or in step ii) by a control device (9) which is connected to all alignment devices (8).

4. The method according to any one of the preceding claims, characterized in that in step iii) one or more receivers (4) simultaneously detect multiple signal photons and / or idler photons with different wavelengths.

5. The method according to any one of the preceding claims, characterized in that the quantum key generation in step iv) for the connections of a time span tk takes place during and / or after the corresponding time span tk.

6. The method according to any one of the preceding claims, characterized in that the transmission in step ii) takes place via a splitter (6) and / or switch (7) in the quantum channel (5).

7. The method according to any one of the preceding claims, characterized in that the several time spans are repeated several times, and / or that in each time span tk a receiver (4) is included at most once in the connections Vj of the respective time span tk.

8. The method according to any one of the preceding claims, characterized in that in each time span tk a receiver (4) is contained in several of the connections Vj of the respective time span tk, and / or that in each time span tk in each case one of the receivers (4) is contained in all connections of the respective time span, and / or that in different time spans tk in each case another one of the receivers (4) is contained in all connections of the respective time span, and / or in that in a time span tk+1 at least one connection Vj of the time span tk is contained, k being a natural number, and / or in that in each time span tk+1 at least one connection Vj of the time span tk is contained, k being a natural number.

9. The method according to any one of the preceding claims, characterized in that the number of time spans is equal to or greater than the number of receivers (4).

10. The method according to any one of the preceding claims, characterized in that in step i) entangled photon pairs are generated in each time span tk for all possible connections Vj , or that in step i) entangled photon pairs are generated in each time span tk only for the connections Vj also aligned therein.

11. The method according to any one of the preceding claims, characterized in that in step i) entangled photon pairs with wavelengths are generated which are randomly distributed over a broad spectrum, or entangled photon pairs with specific wavelengths are generated in a targeted manner.

12. The method according to any one of the preceding claims, characterized in that the entangled photon pairs are entangled in time, and / or polarization, and / or orbital angular momentum, and / or spin angular momentum.

13. Method according to claim 12, characterized in that a polarization rotation in the connection Vj is compensated for the alignment of photon pairs that are entangled in the polarization, and / or that a time span in the connection Vj is compensated for the alignment of photon pairs that are entangled in time, and / or that a change in angular momentum in the connection Vj is compensated for the alignment of photon pairs entangled in orbital angular momentum and / or spin angular momentum.

14. The method according to any one of the preceding claims, characterized in that in step i) the entangled photon pairs are generated by a non-linear process.

15. System (1) for quantum key generation with entangled photon pairs between at least four receivers (4), wherein the system (1) comprises a source (2), a frequency multiplexer (3), a plurality of quantum channels and the multiple receivers (4), each of the multiple receivers (4) being connected to the source (2) via a quantum channel (5), wherein the source (2) is designed to generate entangled photon pairs each comprising a signal photon and an idler photon which are entangled with each other in an entanglement property, wherein the quantum channels of two receivers (4) of an entangled photon pair respectively form a connection Vj for quantum key generation, wherein the frequency multiplexer (3) is arranged in or after the source (2), wherein the frequency multiplexer (3) is designed to distribute the signal photons and idler photons to the quantum channels of the multiple receivers (4) on the basis of their wavelength, wherein each receiver (4) has a detection module which has a measuring module and at least one detector which are designed to detect the entanglement property of the photons for quantum key generation, wherein the system (1) has alignment devices (8) which are arranged in the receivers (4) or in the quantum channels to the receivers (4) and are designed to perform an alignment of the entanglement property of the entangled photon pairs with respect to two receivers (4), wherein the system (1) additionally has a control device (9) and the control device (9) is connected to each alignment device (8), and the control device (9) is designed to control the alignment devices (8) in several time spans for quantum key generation between all receivers (4), wherein the system (1) is designed to carry out the method for quantum key generation according to one of claims 1 to 14 in accordance with steps i) to iv), characterized in that the control device (9) is designed to control in each time span only the alignment devices (8) between two or more connections Vj which can be aligned independently of each other in order to enable quantum key generation in these two or more connections Vj , wherein the control device (9) is designed to replace in each time span the control of the alignment device (8) of at least one connection Vj by controlling the alignment device (8) of another connection Vj in which a quantum key generation was not yet possible, in order to carry out a quantum key generation between all receivers (4) by means of the several time spans, and / or wherein in the last time span one connection Vj , or several of the connections Vj , or all connections Vj , which have already been formed in previous time spans are repeated in order to carry out a quantum key generation again.

16. System (1) according to claim 15, characterized in that each receiver (4) or each quantum channel (5) of a receiver (4) has an alignment device (8), or that the system (1) has only i - 1 alignment devices (8), wherein i is the number of receivers.