Patent Description:
Quantum states of light, in particular single photon states allow the transmission of quantum information between users and thus enable secure communication, for example via quantum key distribution (QKD). A challenge towards the realization of quantum communication networks is the routing of quantum states to multiple users.

The majority of known quantum communication systems in networks is static and point to point from the source to a particular receiver, with the disadvantage that the system cannot be adapted or changed, for example in order to add new receivers. In static networks the routing is realized by a source with a broad wavelength spectrum, multiplexing of this broad spectrum into small wavelength-bands for the receivers and a fixed assignment of the wavelength-bands to the receivers. The broad spectrum of the source is split according to the number of receivers leading to a low single photon rate transmitted to each receiver and thus to a low communication rate in the case of multiple receivers.

In known dynamic network systems where receivers can be added or removed electrically controlled optical routers are used with the disadvantage of high losses.

Further prior art solutions are disclosed in documents <CIT> and <CIT>.

It is an object of the present invention to provide a more flexible, improved, and cost-efficient method and system for quantum communication with a high communication bandwidth.

According to the present invention this object is achieved by a method of low loss routing for quantum communication according to claim <NUM>.

The object is achieved by a method of low loss routing for quantum communication, comprising an entangled photon pair source, a router, three or more receivers, and a quantum network connecting each receiver via a quantum channel with the source, whereby the method comprises the steps.

According to the invention the source comprises a modification means, and the source is connected with the router via a multi-wavelength quantum channel, and the modification means set the wavelength of the generated quantum state, preferably of the signal and/or idler photon, in step i) in order to enable a specific allocation of the signal and idler photon in step ii) to two specific quantum channels to two specific receivers, in order to enable a quantum communication between the specific first and the specific second receivers.

According to the present invention this object is achieved by a system of low loss routing for quantum communication according to claim <NUM>.

The object is further achieved by a system of low loss routing for quantum communication, comprising.

According to the invention the source comprises a modification means in order to set the wavelength of the generated quantum state, and
the source is connected with the router via a multi-wavelength quantum channel.

Allocation means here, that the quantum state is redirected or assigned to a quantum channel by the router based on the wavelengths. This means that the spectrum of the quantum state is not split into multiple wavelength bands like in known systems in order to enable a communication between different receivers. The advantage by an allocation is the higher communication rates by the use of the whole wavelength spectrum of the quantum state for a communication between the source and the specific receivers or two specific receivers.

An advantage of the inventive method and system is the high communication rate between two receivers, or the source and one receiver. This is realized by the dynamic generation of the wavelength of the quantum states in the source in addition to the static and thus low loss allocation in the router and the use of the whole spectrum of the photon state. An advantage is, that not the network or the allocation means has to be adjusted or changed in order to enable a communication to different receivers, as in known systems, but the allocation is realized by the wavelength depending generation of the quantum state in the source by the modification means.

The inventive method and system generate a quantum state with a specific wavelength and allocates the quantum state with a specific wavelength to the specific receiver. In contrast to this in known systems a source with a broad wavelength band is used splitting the broad wavelength band of the quantum state by a wavelength multiplexer into multiple parts which leads to a drastically loss of communication rate between two specific receivers.

The object is further achieved by a method of low loss routing for quantum communication, according to one of the embodiments above, for a dynamic network with a new receiver. According to the invention before the steps i) to iv) according to one of the embodiments above, the method comprises the additional steps of:.

An advantage of the method for a dynamic network is the ability to add new receiver while not changing the components of the source and the router while still enable the high communication rate of the inventive method and system, even for the new receiver. This is realized by the inventive method and system by an allocation of a new wavelength to the new receiver and the ability of the generation of wavelength dependent quantum states by the source and the modification means. In known systems for dynamic networks either the components of the source and/or router are changed or a controllable multiplexer is used leading to high losses. An advantage of the inventive method and system is, that by adding a new receiver not the whole system for communication has to be changed and adapted, for example by changing the source and the router, but only a new wavelength is allocated to the new receivers and a quantum state with the new wavelength is generated by the source and the modification means.

In a preferred embodiment, the single photon in step i) is generated by a single photon laser, preferably a quantum dot. In a preferred embodiment, the single photon in step i) is generated by a laser and an attenuator in order to attenuate the generated laser beam to a single photon level. In a preferred embodiment, the single photon in step i) is generated by a non-linear optical effect, preferably by four-wave-mixing. In a preferred embodiment, the single photon source is a single photon laser, or a laser and an attenuator in order to generate single photons, or a single photon source by the interaction of a pump laser with a non-linear crystal, preferably by four-wave-mixing in order to generate single photons.

In a preferred embodiment, the weak coherent light pulse in step i) is generated by a laser and an attenuator in order to attenuate the generated laser beam to the weak coherent light pulse. In a preferred embodiment, the weak coherent light source is a laser and an attenuator in order to generate the weak coherent light pulse. The laser and the attenuator can be used as the single photon source and/or the weak coherent light source for example by generating single photons by driving the laser at a low power level, or by generating a weak coherent light pulse by driving the laser to various higher power levels or change the attenuation of the attenuator.

In a preferred embodiment, the entangled photon pair in step i) is generated by a non-linear optical effect, preferably by four-wave-mixing or by down-conversion, preferably by spontaneous parametric down conversion. In a preferred embodiment, the entangle photon pair source is a parametric down-conversion source or a four-wave mixing source comprising a pump laser and a non-linear crystal.

In a preferred embodiment, the modification means for the single photon source and/or the weak coherent light source is an adjustable optical cavity, whereby the source is located in or in front of the optical cavity. In a preferred embodiment, the set of the wavelength by the modification means for the single photon source and/or the weak coherent light source in step i) is realized by an adjustable optical cavity. The advantage of this embodiments is the precise wavelength modification, the small wavelength bandwidth of the generated quantum state, and that the source inside the optical cavity is excited to emit photons only in the desired wavelength leading to a higher emission rate. The small wavelength bandwidth enables the allocation of the wavelength in a very precise way without losses.

In a preferred embodiment, the modification means for the single photon source is an adjustable heater or adjustable cooler for the laser generating the single photons and/or the weak coherent light pulse. In a preferred embodiment, the set of the wavelength by the modification means for the single photon source and/or the weak coherent light source in step i) is realized by set of the temperature of the laser of the source enable the source to generate a photon with the wavelength for the specific receiver. The advantage of this embodiments is the cheap and cost-efficient implementation of the modification means.

In a preferred embodiment, the modification means for the single photon source is an adjustable pump laser for the four-wave-mixing, and/or an adjustable heater or adjustable cooler for the non-linear crystal for four-wave-mixing in order to generate single photons. In a preferred embodiment, the set of the wavelength by the modification means for the single photon source in step i) is realized by set of the pump wavelength by an adjustable pump laser for four-wave-mixing, and/or by set of the temperature of the non-linear crystal for four-wave-mixing in order to generate single photons. The advantage of this embodiments is the precise adjustment of the wavelength of the generated quantum state.

In a preferred embodiment, the modification means for the single photon source and/or the weak coherent light source is a dispersive element, preferably a reflective or transitive dispersive element, more preferably a grating, and/or a diffraction grating, and/or a filter, and/or a Bragg grating. In a preferred embodiment, the dispersive element is arranged in or behind the source. In a preferred embodiment, the set of the wavelength by the modification means for the single photon source and/or the weak coherent light source in step i) is realized by a dispersive element. The advantage of this embodiments are the low costs and the precise wavelengths modification possibilities.

In a preferred embodiment, the modification means for the entangled photon source is an adjustable pump laser of the source for pumping a non-linear crystal for down-conversion or for four-wave-mixing. In a preferred embodiment, the modification means for the entangled photon source is an adjustable heater or adjustable cooler for the non-linear crystal or for the pump laser. In a preferred embodiment, the set of the wavelength for the entangled photon source in step i) is realized by set of the pump wavelength for the non-linear crystal by an adjustable pump laser for down-conversion or four-wave-mixing or is realized by set of the temperature of the non-linear crystal for down-conversion or four-wave-mixing. Down-conversion or four-wave-mixing are nonlinear optical effects, whereby the interaction of the non-linear crystal with one or two wavelengths produces photons with new wavelengths. The wavelengths of the generated photons are dependent on the phase matching condition in these non-linear processes, for example dependent on the pump wavelength and/or on the temperature of the non-linear crystal. An advantage of this embodiment is the wavelength dependent generation of both photons of the entangled photon pair.

In a preferred embodiment, the modification means for the entangled photon source, and/or the single photon source, and/or the weak coherent light source is an acousto-optic modulator, and/or an electro-optic modulator, and/or a phase modulator, and/or a Lithium niobate (LN) phase modulator in the source in order to realize a frequency shift of a generated photon in the source by a linear phase ramp of the modulator. In a preferred embodiment, the set of the wavelength for the entangled photon source, and/or the single photon source, and/or the weak coherent light source in step i) is realized by a frequency shift of the wavelength of the quantum state due to a phase modulation of the quantum state, preferably by a linear phase ramp of the modulator. An advantage of this embodiment is the possibility of a separate wavelength change of each photon of the quantum state behind the pair production step in the source.

Set the wavelength of the generated quantum state means here, that the modification means influences the wavelength in the generation process of the quantum state in the source or influences the wavelength by a change of the wavelength of the generated quantum state in the source. The first case can for example be realized by a single photon laser with an adjustable optical cavity or a temperature means, or by an entangled photon source by changing the pump wavelength and/or the temperature of the non-linear crystal. That means the modification means is a component of the source influencing the wavelength of the quantum state in the generation process. The second case can for example be realized by one of the modulators mentioned above in a single photon source or an entangled photon pair source. That means the modification means is a component of the source influencing the wavelength of the quantum state after the generation process.

In a preferred embodiment, the wavelength bandwidth of the quantum state of the source is smaller as the wavelength bandwidth of the channels of the router, preferably smaller as the wavelength bandwidth of each channel of the router. In a preferred embodiment, the wavelength bandwidth of the generated single photons, or weak coherent light pulse, or signal photon and idler photon is smaller as the wavelength bandwidth of the channels of the router. The advantage of this embodiment is that no photons are lost in the router by the allocation of the quantum states according to the wavelength to the intended receivers for the quantum communication. In contrast to that, in known systems the bandwidth of the source is split into multiple parts for multiple receivers introducing high losses, reducing the communication bandwidth by the losses and the number of the receivers.

In a preferred embodiment, the system comprises a control means, whereas control means set the wavelength of the quantum state in order to allow a communication between the source and one specific receiver, or between two specific receivers. In a preferred embodiment, the source, and/or the modification means is controlled by the control means.

In a preferred embodiment, the whole wavelength bandwidth of the source is used for the communication between the source and one specific receiver, or between two specific receivers. The advantage of this embodiment is the higher transmission rates by the use of the whole wavelength bandwidth.

In a preferred embodiment, the allocation in step ii) is a realized by static routing. In a preferred embodiment, the router is a static router. Static means here, that the routing is not realized by a switch or a movable or changeable element, but by a fixed and/or determined spatial separation of the quantum state according to its wavelength. The advantage of that embodiment is the lower cost of the router and the higher transmission rate in the router.

In a preferred embodiment, the router is a grating, preferably a in fiber grating or a free space grating, preferably a Bragg grating.

In a preferred embodiment, the allocation in step ii) is a realized by diffraction. In a preferred embodiment, the router is a diffractive optical element.

In a preferred embodiment, the quantum channels are guided channels, preferably fiber channels or the quantum channels are unguided channels, preferably free space channels. In a preferred embodiment, in step iii) the transmission is realized by guidance of the quantum state in a guided quantum channel, preferably in a fiber. In a preferred embodiment, in step iii) the transmission is realized by the alignment of the quantum state at the router and a unguided transmission in an unguided channel, preferably in a free space channel.

In a preferred embodiment, the router is a diffractive optical element in order to split a broad spectral bandwidth into multiple spectral channel-bandwidths and a mirror or a mirror system, and/or a lens or a lens system. The diffractive optical element realizes the wavelength separation, the mirror or lens system realizes the alignment or guidance of the quantum state in order to transmit the quantum state in an un-guided channel.

Add a new channel for a guided channel means, that the router is connected with the channel. Add a new channel for an unguided channel means, that the router is adjusted in order to enable a transmission of the quantum state towards the new receivers by alignment. This adjustment can be realized for example by a diffractive optical element, and/or a mirror or mirror system, and/or lens or lens system.

In a preferred embodiment, the detection in step iv) is realized by a measurement means and a detection means, whereby the measurement means set the property to be measured and the detection means detect the photon. A property to be measured for the communication could be for example the polarization, and/or the time, and/or orbital angular momentum (OAM) or spin angular momentum (SAM).

In a preferred embodiment, the entangled photon state is entangled in the polarization, and/or the time, and/or orbital angular momentum (OAM) or spin angular momentum (SAM).

In a preferred embodiment, the measurement means comprises a polarizer, and/or a polarizing beam splitter, and/or an electro-optical modulator, and/or an acousto-optic modulator, and/or one or more waveplates, and/or an interferometer, and/or a Spatial Light Modulator.

In a preferred embodiment, the detection means is a single photon detector, preferably germanium (Ge), or silicon (Si), or germanium on silicon (Ge on Si) single-photon avalanche diode (SPAD), or Indium gallium arsenide (InGaAs/Inp) single photon detectors, or semiconductor-based single-photon avalanche diode (SPAD), or superconducting nanowire single-photon detector (SNSPD), or Silicon Avalanche Photodiodes (Si APD).

In a preferred embodiment, the detection means comprise one or more detectors to measure the photons in at least two mutually unbiased measurement bases, and/or to measure the photons in at least one or more orthogonal states.

The above and other aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

<FIG> shows the inventive system for low loss routing for quantum communication comprising a source <NUM> with a modification means <NUM>, a router <NUM>, a quantum network connecting each receiver <NUM> via a quantum channel <NUM> with the source <NUM>.

The source <NUM> in <FIG> can be a single photon source, and/or a weak coherent light source, or an entangled photon source. The source <NUM> generates a quantum state, whereby the wavelength of the quantum state is set by the modification means <NUM> and the source <NUM>. The quantum state of the single photon source is a single photon. The quantum state of the weak coherent light source is a weak coherent light pulse. The quantum state of the entangled photon source is an entangled photon pair. The quantum state is then guided via a multi-wavelength channel <NUM> from the source <NUM> to the router <NUM>.

Set the wavelength of the generated quantum state means here, that the modification means <NUM> influences the wavelength in the generation process of the quantum state in the source <NUM> or influences the wavelength by a change of the wavelength of the quantum state in the source <NUM>.

The modification means <NUM> for the single photon source and/or the weak coherent light source can be an adjustable optical cavity, or an adjustable heater or adjustable cooler for the laser generating the single photons or the weak coherent light pulse, or an adjustable pump laser for the four-wave-mixing, and/or an adjustable heater or adjustable cooler for the non-linear crystal for four-wave-mixing, or a dispersive element, or an acousto-optic modulator, and/or an electro-optic modulator, and/or a phase modulator, and/or a Lithium niobate (LN) phase modulator.

The modification means <NUM> for the entangled photon source can be an adjustable pump laser of the source for pumping a non-linear crystal for down-conversion or for four-wave-mixing, or adjustable heater or adjustable cooler for the non-linear crystal or for the pump laser, or an acousto-optic modulator, and/or an electro-optic modulator, and/or a phase modulator, and/or a Lithium niobate (LN) phase modulator.

The router <NUM> allocates the quantum state according to the wavelength of the quantum state to a specific quantum channel <NUM> in order to transmit the quantum state to a specific receiver <NUM>, or in the case of an entangled photon pair to two specific receivers <NUM>. The allocation is realized by a fixed correlation of the output of the router <NUM> with each quantum channel <NUM> and by a fixed redirection of the quantum state according to its wavelength in the router <NUM>.

As shown in <FIG> three receivers <NUM> are connected via the quantum channels <NUM> with the router <NUM> depicted by the solid lines. A new receiver <NUM> can be added to the network via a new quantum channel <NUM> depicted by dashed lines, without changing any components in the router <NUM> or the source <NUM>. The communication to the new receiver <NUM> (dashed) is realized by the generation of the quantum state in the source <NUM> with the new wavelength by the source <NUM> and the modification means <NUM>.

<FIG> shows an example of the influence of the modification means <NUM> on the wavelength of a quantum state as a single photon. The solid line shows the wavelength spectrum of a generated quantum state, in the case of <FIG> for the quantum channel C3. The wavelength bandwidth of the quantum state of the source <NUM> is smaller than the wavelength bandwidth of the channels of the router <NUM>. In a new step the communication with quantum channel C4 should be realized.

For that the modification means <NUM> and the source <NUM> generate by the influence of the modification means <NUM> the quantum states with a wavelength spectrum for quantum channel <NUM> C4 as depicted by the dashed line.

<FIG> shows an example of the influence of the modification means <NUM> on the wavelength of a quantum state as an entangled photon pair. The wavelengths of the generated photons are dependent on the phase matching condition in non-linear processes like down-conversion or four wave mixing, for example dependent on the pump wavelength and/or on the temperature of the non-linear crystal. <FIG> shows the symmetric shift by the phase matching conditions of the signal photon in quantum channel Cs<NUM> and the idler photon in quantum channel Ci<NUM> toward the quantum channels Cs<NUM> and Ci<NUM> by the influence of the modification means <NUM> on the source <NUM>.

The modification means <NUM> is influencing the generation of the entangled photon pair and thus the wavelength of both entangled photons. This can be realized by the modification means <NUM> as an adjustable pump laser of the source for pumping a non-linear crystal for down-conversion or for four-wave-mixing, or an adjustable heater or an adjustable cooler for the non-linear crystal or for the pump laser. It is also possible to set the wavelength for both photons of an entangled photon pair separate by an acousto-optic modulator, and/or an electro-optic modulator, and/or a phase modulator, and/or a Lithium niobate (LN) phase modulator for each photon.

Claim 1:
Method of low loss routing for quantum communication, comprising an entangled photon pair source (<NUM>), a router (<NUM>), three or more receivers (<NUM>), and a quantum network connecting each receiver via a quantum channel (<NUM>) with the source (<NUM>), and the source (<NUM>) is connected with the router (<NUM>) via a multi-wavelength quantum channel (<NUM>), whereby the method comprises the steps
i) generation of a quantum state in the source (<NUM>) and transmission of the quantum state to the router (<NUM>), whereby the quantum state is an entangled photon pair with a signal photon and an idler photon;
ii) allocation of the quantum state based on the wavelengths of the signal photon to a first quantum channel (<NUM>) and the idler photon to a second quantum channel (<NUM>) by the router (<NUM>);
iii) transmission of the quantum state via the quantum channels (<NUM>) to the two receivers (<NUM>), preferably a first and a second receiver (<NUM>);
iv) detection of the quantum state at the two receivers (<NUM>) in order to establish a quantum communication between the two receivers (<NUM>);
characterized in that
the source (<NUM>) comprises a modification means (<NUM>), and
the modification means (<NUM>) set the wavelength of the generated quantum state, preferably of the signal and/or idler photon, in step i) in order to enable a specific allocation of the signal and idler photon in step ii) to two specific quantum channels (<NUM>) to two specific receivers (<NUM>), in order to enable a quantum communication between the specific first and second receivers (<NUM>).