Patent Description:
Conventional QKD systems use separate hardware components to implement different aspects, which are necessary to build an encrypted communication link. This includes, among other things, the following components: a classical transmitter and receiver for exchanging a classical optical signal to establish a communication channel, encryptors/decryptors to establish an encrypted connection, a key management infrastructure to manage the keys, and a QKD transmitter and QKD receiver that exchange a QKD signal and generate random encryption keys at both locations.

<CIT> describes a method for integrating an Optical Service Channel (OSC) with a Quantum Key Distribution (QKD) channel.

The present disclosure is based on the observation that as components in conventional QKD systems are typically at least <NUM>-<NUM> in number, the conventional QKD system becomes difficult to assemble and maintain. Also, there is a significant risk of human error during setup, configuration, and maintenance of the conventional QKD system.

In addition, if a transmission line like a fiber or freespace transmission link is to be shared by the classical and QKD transmitters and receivers, respectively, additional WDM multiplexers and demultiplexers need to be used and significantly increase the loss of the QKD signal. The loss severely decreases the transmission distance over which the QKD can be performed.

In view of the above-mentioned problems and disadvantages, the present disclosure aims to improve the conventional QKD systems, particularly QKD transmitters and receivers used in such systems. In particular, it is an object to allow for a low complexity and cost efficient implementation of a QKD transmitter, QKD receiver and thus QKD system, in which the QKD is combined with classical communication, i.e. the transmission of a QKD signal is combined with the transmission of a classical optical signal. The QKD transmitter, receiver and system should be suited for secure short reach communication applications like data center interconnects. The transmission link for the classical optical signal and the QKD signal should be implemented in a single device, ideally using the same hardware (e.g. ASIC, optics, etc.) for classical signal processing, QKD, key management, and encryption. In this way, a reduction of the risk for a successful active attack or for human error during the configuration and handling of the QKD system is obtained. In addition the QKD system as a whole is further improved, the number of components decreased, and the loss that is experienced by the QKD signal reduced.

The above object is achieved by the solution provided in the enclosed independent claims.

In particular it is proposed to combine hardware of QKD transmitters/receivers with at least parts of classical transmitters/receivers in a single device or system. In this way, a QKD system can be optimized, in particular by improving security, reducing complexity, and maximizing QKD reach. All processing related to QKD, key handling and data encryption may be combined in one processing unit (e.g. ASIC, FPGA, DSP, processor, etc.).

The present invention is defined by transmitter for QKD according to independent claim <NUM>, receiver for QKD according to independent claim <NUM> and system for QKD according to independent claim <NUM>.

A first aspect of an explicative embodiment provides a transmitter for QKD, the transmitter comprising a signal generation unit configured to generate a QKD signal and to output it onto a transmission line, and an input configured to receive an optical signal having a different wavelength than the QKD signal, the optical signal being output onto the transmission line, wherein between the input and the transmission line the optical signal is passed through a part of the signal generation unit, which is transparent to the optical signal, wherein at least one of: the signal generation unit includes a wavelength-selective attenuator configured to attenuate the QKD signal, and the optical signal is passed through the attenuator, which is transparent to the optical signal; and the signal generation unit includes an optical modulation unit configured to modulate a laser beam to generate the QKD signal, and the optical signal is passed through the optical modulation unit, which is transparent to the optical signal, wherein the optical modulation unit includes a wavelength-selective interferometer configured to pass the laser beam through both of its arms but the optical signal only through one of its arms, and/or wherein the optical modulation unit includes a phase modulator and a wavelength-selective coupler, and the wavelength-selective coupler is configured to direct a first part of the laser beam to the phase modulator and to pass a second part of the laser beam and the optical signal by the phase modulator.

In this document, the 'optical signal' refers to a classical optical signal, i.e. to a non-quantum optical signal as it is used in standard optical telecommunication. 'Transparent' means that the optical signal passes through without being significantly affected. For instance, the optical signal passes undisturbed through the part of the signal generation unit, e.g. through optics provided in this part for affecting the QKD signal. With the term undisturbed it is meant in the present disclosure that the signal is only subject to negligible disturbances that do not impact the transmission performance, for example losses due to the optical path. In this disclosure we will refer to the above described property as a component or subsystem being transparent for the optical signal. Thus, the optical signal typically experiences a small power loss when passing thorough a transparent part or component. The wavelength difference between the (classical) optical signal and the QKD signal may be at least <NUM> for low cost implementations (large manufacturing tolerances, low loss, etc.). Especially for discrete variable QKD systems that rely on single photon detectors, the QKD signal has in any case a shorter wavelength than the classical optical signal. For continuous variable QKD systems, where the wavelength selectivity relies on the use of local oscillator, this condition is not required.

When using the QKD transmitter of the first aspect, for example to build a QKD system in which quantum and non-quantum signals are exchanged, the fragile QKD signal is not (or only minimally) affected, since no extra components (other than the components used for QKD) are needed for coupling the QKD signal and the optical signal before the transmission line. In particular, extra wavelength multiplexers in the path of the QKD signal can be avoided. In other words, the losses for the QKD signal in the transmitter are minimized, although the QKD signal and the optical signal are transmitted over the same transmission line. This significantly increases the reach for the QKD signal and/or allows higher QKD key rates.

The optical modulation unit may, for instance, include an interferometer with a phase modulator in one of its two arms and/or an attenuator. Since the optical modulation unit or optical modulator is transparent to the optical signal, the losses of the optical signal are minimized. Nevertheless, the optical signal can be coupled through the optical modulation unit towards the transmission line. A possible implementation of such an optical modulator may be realized as a Mach-Zehnder interferometer with, for instance, directional couplers, multimode interference (MMI) couplers or optical filters, which passes the classical optical signal only through one arm of the interferometer. Other interferometers, such as for instance Michelson interferometers, using the same principle may alternatively be used.

The interferometer acts as interferometer only on the laser beam for the QKD signal, but not on the classical optical signal. However, the interferometer may function to couple the optical signal towards the transmission line.

For instance, the optical modulation unit is a wavelength-selective interferometer with the phase modulator provided in/on one of its arms, and with the wavelength-selective coupler provided at the interferometer input. The coupler is used to split the laser beam between the two arms, while letting the optical signal pass undisturbed through the interferometer arm without the phase modulator. The wavelength-selective coupler thus simultaneously serves as a wavelength multiplexer and splitter for the QKD signal. Accordingly, no additional multiplexer is required, thus minimizing the QKD signal loss.

In a further implementation form of the first aspect, the wavelength-selective coupler is a directional coupler or a multimode interference coupler.

These coupler components may, for instance, output the optical signal at one output port, but split the QKD signal to two output ports. Further, these components are easy to implement and are cheap.

The QKD signal may selectively be attenuated to the quantum level, while the optical signal experiences only a small loss. This implementation form is of particular advantage when photonic integration is of interest.

In a further implementation form of the first aspect, the wavelength-selective attenuator is implemented with a semiconductor optical amplifier (SOA) or an electro absorption modulator (EAM).

The bandgap of the SOA or EAM can be easily selected such that the attenuator does not influence the optical signal used. In particular, the bandgap may be selected to be much larger than the energy that corresponds to the wavelength of the optical signal.

A second aspect of an explicative embodiment provides a receiver for QKD, the receiver comprising a signal receiving unit configured to receive a QKD signal and an optical signal over a transmission line, the optical signal having a different wavelength than the QKD signal and an output for the optical signal, wherein between the transmission line and the output, the optical signal is passed through a part of the signal receiving unit, which is transparent to the optical signal, and wherein at least one of: the signal receiving unit includes a wavelength-selective detecting unit configured to detect the QKD signal, and the optical signal is passed through at least a part of the detecting unit, which is transparent to the optical signal; and the signal receiving unit includes an optical demodulation unit configured to demodulate the QKD signal, and the optical signal is passed through the optical demodulation unit, which is transparent to the optical signal, wherein the optical demodulation unit includes a wavelength-selective interferometer configured to pass the QKD signal through both of its arms but the optical signal only through one of its arms, and/or wherein the optical demodulation unit includes a phase modulator and a wavelength-selective coupler, and the wavelength-selective coupler is configured to direct a first part of the QKD signal to the phase modulator and to pass a second part of the QKD signal and the optical signal by the phase modulator.

When using the QKD receiver of the second aspect, for example to build a QKD system in which quantum and non-quantum signals are exchanged, the QKD signal is not (or only minimally affected), since no extra components (other than the components used for the QKD signal) are needed for receiving and demultiplexing the QKD signal and optical signal coming from the transmission line. In particular, extra wavelength demultiplexers in the path of the QKD signal can be avoided. In other words, the losses for the QKD signal in the receiver are minimized, although the QKD signal and the optical signal are transmitted and received over the same transmission line. This significantly increases the reach for the QKD signal. Further, wavelength multiplexers and/or demultiplexers in the path of the QKD signal can be avoided.

The optical demodulation unit may, for instance, be an interferometer with a phase modulator in one of its arms. Since the optical demodulation unit is transparent to the optical signal, the losses of the optical signal are minimized. Nevertheless, the optical signal can be coupled through the optical demodulation unit into the receiver.

The interferometer acts as interferometer only on the laser beam for the QKD signal, but not on the classical optical signal. However, the interferometer may function to couple the optical signal into the transceiver, and even to decouple the QKD signal from the optical signal.

For instance, the optical demodulation unit is a wavelength-selective interferometer with the phase modulator provided in one of its arms, and with the wavelength-selective coupler provided at the interferometer input, in order to split the QKD signal between the two arms while letting the optical signal pass undisturbed through the arm without the phase modulator. The wavelength-selective coupler thus simultaneously serves as a wavelength multiplexer and splitter for the QKD signal.

In a further implementation form of the second aspect, the wavelength-selective detecting unit includes a detector stack made of a first detector, which is configured to detect the QKD signal but is transparent to the optical signal, and a second detector, which is configured to detect the optical signal.

Advantageously, the QKD signal is detected with high efficiency in the first detector, which may be a silicon (Si) avalanche photodiode (Si-APD) detector, while the classical optical signal passes through the Si-APD to be detected in the following second detector. The second detector may be an indium gallium arsenide (InGaAs) APD or a classical receiver. Since the first detector is for the QKD signal, and is in front of the second detector for the optical signal, the loss is minimized for the QKD signal. Further, no Wavelength-Division-Multiplexing (WDM) demultiplexer is needed that would introduce additional losses, since this task is also fulfilled by the detector stack.

A third aspect of an explicative embodiment provides a system for QKD, the system comprising a transmitter according to the first aspect or any of its implementation forms, and a receiver according to the second aspect or any of its implementation forms, wherein the transmitter and the receiver are connected over the transmission line.

The transmission line may be the one mentioned above for the receiver of the first aspect, or the one mentioned above for the receiver of the second aspect, or may include both of these transmission lines connected to another. The system of the third aspect achieves all the advantages of the transmitter and receiver, in particular a low complexity and cost efficient implementation of a QKD system that combines QKD with classical communication, i.e. combines the transmission of a QKD signal and a classical optical signal.

In an implementation form of the third aspect, the QKD signal has a shorter wavelength, for example of <NUM>-<NUM>, and the optical signal has a longer wavelength, for example of <NUM>-<NUM>. This means for example, when the QKD signal e.g. has a wavelength of <NUM> suitable for shorter reaches, the wavelength of the classical signal can be chosen around e.g. <NUM> or <NUM>.

The wavelength ranges of the QKD signal depends on the transmission distance. The above-mentioned range is preferred for up to about <NUM> of transmission line. For longer distances, a QKD signal in the range of <NUM>-<NUM> may also be used. The wavelength range for the optical signal depends of course on the wavelength of the QKD signal. In general, the gap between the wavelengths of the two signals should be at least <NUM>. In a second example, if the QKD signal has a wavelength of <NUM>, the wavelength of the classical signal can be chosen e.g. around <NUM>.

A fourth aspect of an explicative embodiment provides a transmission method for QKD, the method comprising generating, with a signal generation unit, a QKD signal and outputting it onto a transmission line, and receiving, with an input, an optical signal having a different wavelength than the QKD signal, and outputting the optical signal onto the transmission line, wherein between the input and the transmission line, the optical signal is passed through a part of the signal generation unit, which is transparent to the optical signal.

The method of the fourth aspect provides the same advantages and effects as the transmitter of the first aspect. Further, it can be developed with implementation forms according to the implementation forms of the first aspect.

A fifth aspect of an explicative embodiment provides a receiving method for QKD, the method comprising receiving, with a receiving unit, a QKD signal and an optical signal over a transmission line, the optical signal having a different wavelength than the QKD signal and outputting the optical signal via an output, wherein between the transmission line and the output, the optical signal is passed through a part of the signal receiving unit, which is transparent to the optical signal.

The method of the fifth aspect provides the same advantages and effects as the receiver of the second aspect. Further, it can be developed with implementation forms according to the implementation forms of the second aspect.

The fourth and the fifth aspect are not encompassed by the wording of the claims but are considered as useful for understanding the invention.

The aspects of the explicative embodiments allow all for the integration of the electronics for QKD, key management and encryption in one QKD system, thereby reducing possible attack vectors by any potential adversary that tries to gain physical access to the system. To access the key, an adversary would have to physically probe data within the processing device (ideally an ASIC). Further, the aspects of the present invention all allow for optimization of the QKD system, particularly for minimal QKD signal excess loss in both transmitter and receiver. In the QKD receiver this is especially important as the loss reduction there directly translates into additional link budget or higher key rates. In addition, the aspect of the present invention allow for joint optimization or even photonic integration, paving the way for small form-factors and low cost. Notably, one or both of the transmitter and receiver of the invention may be implemented as transceiver(s).

The following embodiment is not encompassed by the wording of the claims but is considered as useful for understanding the invention.

<FIG> shows a transmitter <NUM> according to an embodiment. The transmitter <NUM> is configured for QKD and is thus a QKD transmitter. The transmitter <NUM> may be a transceiver.

The transmitter <NUM> includes a signal generation unit or signal generator <NUM>, which is configured to generate a QKD signal <NUM>, and to output the QKD signal <NUM> onto a transmission line <NUM>, for instance, onto an optical fiber.

Further, the transmitter <NUM> includes an input <NUM>, which is configured to receive an optical signal <NUM> (classical signal) having a different, for example longer, wavelength than the QKD signal <NUM>. The transmitter <NUM> is further configured to output the optical signal <NUM> onto the transmission line <NUM>. Notably, the optical signal <NUM> may be generated in a further (classical) signal generation unit. This further signal generation unit could be a part of the transmitter <NUM>, for instance, for low cost applications when there is no need of high performance (long reach). However, for high performance, for instance long reach, applications, an external signal generation unit may be used to generate the classical optical signal and provide it to the input <NUM> of the QKD transmitter <NUM>.

Between the input <NUM> and the transmission line <NUM> the optical signal <NUM> is passed through a part of the signal generation unit <NUM>, i.e. through optical components arranged in this part and in the path of the optical signal <NUM>, which does not or not significantly affect the optical signal <NUM>. In other words the optical components of the signal generation unit <NUM> is transparent to the optical signal.

<FIG> shows a receiver <NUM> according to an embodiment of the present invention. The receiver <NUM> is configured for QKD and is thus a QKD receiver. The receiver <NUM> may be a transceiver.

The receiver <NUM> includes a signal receiving unit <NUM> configured to receive a QKD signal <NUM> and an optical signal <NUM> over a transmission line <NUM>, the optical signal <NUM> having a different, for example longer, wavelength than the QKD signal <NUM>. That is, both signals <NUM>, <NUM> are transmitted and received over a common transmission line <NUM>, for instance, transmitted over the transmission line <NUM> by the transmitter <NUM> of <FIG>. In this case, the optical signal <NUM> is the optical signal <NUM>, and the QKD signal <NUM> is the QKD signal <NUM>.

The receiver <NUM> includes further an output <NUM> for the optical signal <NUM> for outputting it, for instance, to a further (classical) signal receiving unit. This further signal receiving unit may be external to the receiver <NUM>, particularly for high performance applications, but may also be a part of the receiver <NUM>, when lower performance is sufficient.

Between the transmission line <NUM> and the output <NUM>, the optical signal <NUM> is passed through a part of the signal receiving unit or signal receiver <NUM>, which is transparent to the optical signal <NUM>. That means, it passes through optical components arranged in this part and in the path of the optical signal <NUM>, which do not or not significantly affect the optical signal <NUM>. In other words the optical components of the signal receiving unit <NUM> is transparent to the optical signal.

Most generally, a system <NUM> according to an embodiment includes a transmitter <NUM> as shown in <FIG> and a receiver <NUM> as shown in <FIG>, wherein the transmitter <NUM> and the receiver <NUM> are connected over at least one transmission line <NUM> and/or <NUM>.

<FIG> shows a particular system <NUM> according to an embodiment, which includes a transmitter <NUM> that builds on the transmitter <NUM> shown in <FIG> and a receiver <NUM> that builds on the receiver <NUM> shown in <FIG>. Same elements in <FIG> and in <FIG> and/or <FIG> share the same reference signs and functions. The transmitter <NUM> and receiver <NUM> shown in <FIG> include all elements of the transmitter <NUM> of <FIG> and the receiver <NUM> of <FIG>, respectively, even if not explicitly shown in <FIG>. Further, even if the transmitter <NUM> and the receiver <NUM> of <FIG> are described as part of a system, these may also be standalone devices, which may be used independently for each other.

The system <NUM> shown in <FIG> combines a Mach-Zehnder interferometer implementation for QKD with a classical communication link. Here, in particular a Discrete Variable (DV) QKD system <NUM> in a double Mach-Zehnder configuration is illustrated. The applied techniques according to the invention, however, can be used in the same way on other QKD systems.

Accordingly, the transmitter <NUM> includes a signal generation unit <NUM> including a QKD laser configured to provide a laser beam <NUM> for the QKD signal <NUM>, an optical modulation unit <NUM>, and a wavelength-selective attenuator <NUM>.

The laser beam <NUM> has a different wavelength than the optical signal <NUM>, which is generated by an external (classical) signal generation unit <NUM>, and is received by the transmitter <NUM> via input <NUM>.

The optical modulation unit <NUM> is configured to modulate the laser beam <NUM> to generate the QKD signal <NUM>. However, the optical signal <NUM> is passed through the optical modulation unit or optical modulator <NUM>, which is transparent to the optical signal <NUM>. Transparency to the optical signal <NUM> may be for instance obtained by using a first and a second selective coupler <NUM> and <NUM> in the optical modulator <NUM>. The first selective coupler <NUM> and the second selective coupler <NUM> may be realized for instance, directional couplers, multimode interference (MMI) couplers or optical filters, which passes the classical optical signal only through one arm of the optical modulator <NUM>.

The optical modulator <NUM> may be implemented as a Mach-Zehnder interferometer. Other interferometers, such as for instance Michelson interferometers, using the same principle may alternatively be used. The behavior may be obtained by exploiting the wavelength dependence of directional couplers, MMIs or even optical filters. In particular due to the wavelength dependence of the optical modulator, the optical signal entering the optical modulator is redirected to pass only through one of the optical paths, while the QKD signal passes through both paths.

The optical modulation unit <NUM> here includes a first wavelength-selective coupler <NUM>, a phase modulator <NUM>, and optionally a second wavelength selective coupler <NUM>.

The first wavelength-selective coupler <NUM> is configured to direct a first part of the laser beam <NUM> to the phase modulator <NUM> and to pass a second part of the laser beam <NUM>, and the optical signal <NUM>, by the phase modulator <NUM>.

In particular, the optical modulation unit <NUM> is thus in the shown implementation a wavelength-selective interferometer, which is configured to pass the laser beam <NUM> through both of its arms - wherein the phase modulator <NUM> is provided on one of the arms - but the optical signal <NUM> only through the one of its arms without the phase modulator <NUM>. Thereby, the QKD signal <NUM> is produced from the laser beam <NUM>.

The second wavelength-selective coupler <NUM> combines the split laser beam <NUM> and the optical signal <NUM> before inputting the light (signals) into the wavelength-selective attenuator <NUM>. The wavelength-selective attenuator <NUM> is configured to attenuate the QKD signal <NUM>, for instance to quantum level. However, the optical signal <NUM> is passed through the attenuator <NUM>, which is transparent to the optical signal <NUM>. The attenuator <NUM> may be a wavelength-selective attenuator implemented with a semiconductor optical amplifier (SOA) or an electro absorption modulator (EAM). The bandgap of the SOA or EAM can be easily selected such that the attenuator does not influence the optical signal used. In particular, the bandgap may be selected to be much larger than the energy that corresponds to the wavelength of the optical signal.

A further coupler <NUM> may couple the attenuated QKD signal <NUM> and the optical signal <NUM> into a transmission line <NUM>, which may be the same as, or may be connected to, the transmission line <NUM> leading to the receiver <NUM>. The further coupler <NUM> may be implemented in the same way as the first and second couplers <NUM>, <NUM> as described above.

The receiver <NUM> includes a signal receiving unit <NUM> including an optical demodulation unit <NUM> and a wavelength-selective detecting unit <NUM>.

The optical demodulation unit or optical demodulator <NUM> is configured to demodulate the QKD signal <NUM> (which is here the same as the QKD signal <NUM>). However, the optical signal <NUM> (which is here the same as the optical signal <NUM>) is passed through the optical demodulation unit <NUM>, which is transparent to the optical signal <NUM>.

Transparency to the optical signal <NUM> may be for instance obtained by using a first and a second selective coupler <NUM> and <NUM> in the optical demodulator <NUM>. The first selective coupler <NUM> and the second selective coupler <NUM> may be realized for instance, by directional couplers, multimode interference (MMI) couplers or optical filters, which passes the classical optical signal only through one arm of the optical demodulator <NUM>.

The optical demodulator <NUM> may be implemented as a Mach-Zehnder interferometer. Other interferometers, such as for instance Michelson interferometers, using the same principle may alternatively be used. The behavior may be obtained by exploiting the wavelength dependence of directional couplers, MMIs or even optical filters. In particular due to the wavelength dependence of the optical modulator, the optical signal entering the optical modulator is redirected to pass only through one of the optical paths, while the QKD signal passes through both paths.

The optical demodulation unit <NUM> here includes a first wavelength-selective coupler <NUM>, a phase modulator <NUM>, and optionally a second wavelength selective coupler <NUM>. The first wavelength-selective coupler <NUM> is configured to direct a first part of the QKD signal <NUM> to the phase modulator <NUM> and to pass a second part of the QKD signal <NUM>, and the optical signal <NUM>, by the phase modulator <NUM>, i.e. straight to the second wavelength selective coupler <NUM> without passing through the phase modulator <NUM>.

In particular, the optical demodulation unit <NUM> is thus in this implementation a wavelength-selective interferometer, which is configured to pass the QKD signal <NUM> through both of its arms - wherein the phase modulator <NUM> is provided on one of the arms - but the optical signal <NUM> only through the one of its arms without the phase modulator <NUM>. Thereby, the QKD signal <NUM> is demodulated.

The second wavelength-selective coupler <NUM> optionally combines the split QKD signal <NUM> and the optical signal <NUM>, before inputting the light (signals) into the wavelength-selective detecting unit <NUM>. The wavelength-selective detecting unit <NUM> is configured to detect the QKD signal <NUM>. However, the optical signal <NUM> is passed through the detecting unit <NUM>, which is transparent to the optical signal <NUM>. The detecting unit <NUM> may particularly include two QKD detectors <NUM> and <NUM>, wherein the coupler <NUM> splits the QKD signal <NUM> (after first combining the QKD signal) to the two QKD detectors <NUM> and <NUM>, while it provides the optical signal <NUM> only to one of the QKD detectors <NUM>, which is transparent for the optical signal <NUM>. The optical signal <NUM> may thus pass via an output <NUM> to a classical detecting unit <NUM>.

<FIG> shows also that at the transmitter side <NUM> of the system, the system <NUM> may further include a classical receiver <NUM>, and may further include a classical transmitter <NUM> at the receiver <NUM> side of the system. The classical receiver <NUM> and transmitter <NUM> may communicate over another transmission line <NUM>.

In the above-described manner, the described optics for the QKD transmitter <NUM> and QKD receiver <NUM> are transparent to the classical optical signal <NUM>/<NUM>. As a result, the system <NUM> performance is similar to that of a QKD system without classical link. The system <NUM> of <FIG> is thus an example for a full and efficient QKD system with a classical link superimposed.

<FIG> shows a system <NUM> that differs from the system <NUM> shown in <FIG> only on the transmitter side. Accordingly, the system <NUM> also includes a receiver <NUM> as shown in <FIG>, which receiver <NUM> builds on the receiver <NUM> shown in <FIG>. Same elements in <FIG> and in <FIG> or <FIG> have same reference signs, function likewise, and are thus not be described again at this point. Also for this embodiment, even if the transmitter <NUM> and the receiver <NUM> of <FIG> are described as part of a system, these may also be standalone devices, which may be used independently for each other.

As the QKD signal loss is not as critical on the transmitter side as on the receiver side, the system <NUM> is a simplified version of the system <NUM>. The system <NUM> includes a transmitter <NUM> that uses conventional optics, while the optical signal <NUM>/<NUM> only passes through the optics used for the QKD signal <NUM> at the receiver <NUM>. Such system <NUM> still shows improved performance over a conventional system in which QKD signal and optical signal are transmitted over the same transmission line.

<FIG> shows components for a system <NUM> according to an embodiment. In particular, <FIG> shows the wavelength-selective interferometer including the coupler <NUM>, the coupler <NUM> and the phase modulator <NUM>, as it is used in the transmitter <NUM> shown in <FIG>. Further, <FIG> shows the wavelength-selective interferometer including the coupler <NUM>, the coupler <NUM> and the phase modulator <NUM>, as it is used in the receiver <NUM> shown in <FIG>. Also for this embodiment, even if the transmitter <NUM> and the receiver <NUM> of <FIG> are described as part of a system, these may also be standalone devices, which may be used independently for each other.

The components shown in <FIG> selectively affect only the QKD signal <NUM>/<NUM>, but not the classical optical signal <NUM>/<NUM>. A possible implementation is as a Mach-Zehnder interferometer with, for instance, directional couplers or multimode interference (MMI) couplers, which pass the classical optical signal <NUM>/<NUM> only through one arm of the interferometer. At least the couplers <NUM> and <NUM> selectively split only the QKD signal <NUM>/<NUM> to both of their outputs, while the classical optical signal <NUM>/<NUM> is passed to one output only. In the transmitter <NUM>, such a coupler <NUM> can simultaneously perform wavelength multiplexing of the classical optical and the QKD signal <NUM> and <NUM>.

<FIG> shows further components for a system <NUM> according to an embodiment of the invention. In particular, <FIG> shows the QKD detector <NUM> of the receiver <NUM> shown in <FIG>, which detects the QKD signal <NUM> but passes through the optical signal <NUM>. It thus acts also as a demultiplexer for the optical signal <NUM> and QKD signal <NUM>. The optical signal <NUM> could then be detected in any following detector or receiver. For instance, for higher rate systems (100Gbit/s, <NUM> Gbit/s, etc.), a complex Dense Wavelength Division Multiplexing (DWDM receiver) structure <NUM> may be used to detect the optical signal <NUM> after the QKD detector <NUM>. Thereby, the DWDM receiver <NUM> maybe external to the receiver <NUM> (as shown in <FIG>) or included in the receiver <NUM>.

In another advantageous implementation, a detector stack <NUM> made of a Si detector 601a stacked with an InGaAs detector 601b (in this example both are APDs) could be used to realize the QKD detector <NUM> in <FIG>. In this way, both signals <NUM>, <NUM> could be detected in the receiver <NUM> in a very compact manner.

<FIG> shows further a wavelength-selective attenuator <NUM> as used in the transmitter <NUM> shown in <FIG>. The wavelength-selective attenuator <NUM> may be a SOA or EAM configured to selectively attenuate the QKD signal <NUM> (symbolized by the top arrow) while leaving the classical signal <NUM> undisturbed (symbolized by the bottom arrow).

Claim 1:
Transmitter (<NUM>) for Quantum Key Distribution, QKD, the transmitter (<NUM>) comprising
a signal generation unit (<NUM>) configured to generate a QKD signal (<NUM>) and to output it onto a transmission line (<NUM>), and
an input (<NUM>) configured to receive an optical signal (<NUM>) having a different wavelength than the QKD signal (<NUM>), the optical signal (<NUM>) being output onto the transmission line (<NUM>),
wherein between the input (<NUM>) and the transmission line (<NUM>) the optical signal (<NUM>) is passed through a part of the signal generation unit (<NUM>), which is transparent to the optical signal (<NUM>),
wherein at least one of:
the signal generation unit (<NUM>) includes a wavelength-selective attenuator (<NUM>) configured to attenuate the QKD signal (<NUM>), and the optical signal (<NUM>) is passed through the attenuator (<NUM>), which is transparent to the optical signal (<NUM>); and
the signal generation unit (<NUM>) includes an optical modulation unit (<NUM>) configured to modulate a laser beam (<NUM>) to generate the QKD signal (<NUM>), and the optical signal (<NUM>) is passed through the optical modulation unit (<NUM>), which is transparent to the optical signal (<NUM>),
wherein the optical modulation unit (<NUM>) includes a wavelength-selective interferometer configured to pass the laser beam (<NUM>) through both of its arms but the optical signal (<NUM>) only through one of its arms, and/or
wherein the optical modulation unit (<NUM>) includes a phase modulator (<NUM>) and a wavelength-selective coupler (<NUM>), and the wavelength-selective coupler (<NUM>) is configured to direct a first part of the laser beam (<NUM>) to the phase modulator (<NUM>) and to pass a second part of the laser beam (<NUM>) and the optical signal (<NUM>) by the phase modulator (<NUM>).