Method for quantum communication

1. A method for quantum communication between at least three receivers in a network system, the network system comprising a source, a frequency multiplexer, the at least three receivers and a network of quantum channels, wherein the method comprises the following steps:

i) generating an entangled photon pair in the source with a signal photon and an idler photon, wherein the signal photon is generated in a signal wavelength range and the idler photon is generated in an idler wavelength range;

ii) assigning the signal and idler photons to the quantum channels in the frequency multiplexer on the basis of their wavelength;

iii) transmitting the photon pair to the receivers via the quantum channels;

iv) detecting the photon pair at the receivers for the quantum communication,

wherein the photons generated in step i) are generated in a signal wavelength range and an idler wavelength range that are spectrally separated from one another, and

wherein, in step ii), the photons are assigned to the quantum channels such that, in step iii), only signal photons are transmitted to a first receiver for the communication with all other receivers, and, in step iii), only idler photons are transmitted to a second receiver for the communication with all other receivers, and, in step iii), both signal photons and idler photons are transmitted to the further receivers for the communication with all other receivers, and

wherein all receivers are connected to the source via in each case one quantum channel.

2. The method for quantum communication according to claim 1 , wherein the signal wavelength range and the idler wavelength range are formed spectrally separated from one another by at least 50 nm.

3. The method for quantum communication according to claim 1 , wherein the signal wavelength range is formed between 400 nm and 900 nm.

4. The method for quantum communication according to claim 1 , wherein the idler wavelength range is formed between 950 nm and 2 μm (O band), and/or between 1360 nm and 1460 nm (E band), and/or between 1460 nm and 1530 nm (S band), and/or between 1530 nm and 1565 nm (C band), and/or between 1565 nm and 1625 nm (L band).

5. The method for quantum communication according to claim 1 , wherein the spectral width of the signal and idler photons is smaller than 5 nm.

6. The method for quantum communication according to claim 1 , wherein, in each case, two receivers of the photons of a photon pair form a communication pair, wherein each partial wavelength range for each communication pair is formed spectrally separated from the partial wavelength ranges of all other communication pairs.

7. The method for quantum communication according to claim 6 , wherein the partial signal wavelength ranges are spectrally separated such that at most 5% of the wavelength spectrum of two signal photons overlaps, and/or wherein the partial idler wavelength ranges are spectrally separated such that at most 5% of the wavelength spectrum of two idler photons overlaps.

8. The method for quantum communication according to claim 6 , wherein the source is formed for simultaneous quantum communication between several communication pairs, because in step i) in each case one photon pair is generated simultaneously for two or more communication pairs.

9. The method for quantum communication according to claim 8 , wherein, for simultaneous quantum communication, in step ii), the photons are assigned to the quantum channels such that, in step iii), only signal photons are transmitted to a first receiver for the communication with all other receivers, and, in step iii), only idler photons are transmitted to a second receiver for the communication with all other receivers, and, in step iii), both signal photons and idler photons are transmitted to the further receivers for the communication with all other receivers.

10. The method for quantum communication according to claim 6 , wherein a transmission rate of at least 1 kHz is performed simultaneously between two or more communication pairs.

11. The method for quantum communication according to claim 1 , wherein the first receiver, which receives only signal photons, is arranged at most 20 km away from the source, or is arranged in close proximity to the source, and/or wherein the second receiver, which receives only idler photons, is connected to the source via an optical fiber.

12. The method for quantum communication according to claim 1 , wherein the receiver has a detection module with at least one detector or with several detectors, and/or wherein the receiver has several detection modules with in each case at least one detector or with several detectors for the communication, wherein each detection module is formed for communication with one receiver.

13. The method for quantum communication according to claim 1 , wherein two or more network systems are connected to one another via a receiver, because the two network systems share this receiver, because, for the communication between the network systems, a Bell state measurement, swapping or quantum teleportation is carried out across this receiver; the receiver via which the two network systems are connected.

14. A network system for quantum communication between several receivers,

wherein the network system has a source, a frequency multiplexer, a network of quantum channels and the several receivers,

wherein the source generates entangled photon pairs with in each case one signal photon and one idler photon, and the signal photon has a wavelength in a signal wavelength range and the idler photon has a wavelength in an idler wavelength range, and

wherein the frequency multiplexer assigns the signal and idler photons to the quantum channels on the basis of their wavelength, and

wherein each receiver has a detection module with at least one detector for detecting the photons for the quantum communication, and

wherein the signal wavelength range and the idler wavelength range are formed spectrally separated from one another, and

wherein the network has one quantum channel between the source and each receiver for the transmission of the photons, and

wherein the quantum channels are assigned to the frequency multiplexer such that only signal photons are transmitted to a first receiver for the communication with all other receivers, and only idler photons are transmitted to a second receiver for the communication with all other receivers, and both signal photons and idler photons are transmitted to the further receivers for the communication with all other receivers.

15. The network system according to claim 14 , wherein two or more network systems according to the invention are connected to one another via a receiver, because the two network systems share this receiver.