WO2020211950A1

WO2020211950A1 - Efficient quantum-key-secured passive optical point to multipoint network

Info

Publication number: WO2020211950A1
Application number: PCT/EP2019/060164
Authority: WO
Inventors: David Hillerkuss; Lucian COMANDAR; Fred Chi Hang FUNG; Ivan Nicolas Cano VALADEZ
Original assignee: Huawei Technologies Duesseldorf Gmbh
Priority date: 2019-04-18
Filing date: 2019-04-18
Publication date: 2020-10-22
Also published as: CN113454944A

Abstract

The present disclosure relates to a passive optical network transmitter and receiver for quantum key distribution and in particular, to an efficient implementations thereof. A transmitter includes an optical circuitry including at least one optical component shared for modulating a classical signal and a quantum signal, a switch configured to control a time 5 division for transmitting the classical signal and the quantum signal, and an optical output for transmission of the time division multiplexed classical signal and quantum signal. Correspondingly, a receiver comprises an optical input for receiving a signal including a classical signal and a quantum signal in a time division multiplex from an optical path, a switch configured to control a time division for reception of the classical signal and the 10 quantum signal, and an optical circuitry including at least one optical component shared for demodulating the classical signal and the quantum signal.

Description

EFFICIENT QUANTUM-KEY-SECURED PASSIVE OPTICAL POINT TO MULTIPOINT

NETWORK

TECHNICAL FIELD

Embodiments of the present invention relate to the field of quantum key distribution (QKD), and, in particular to passive optical networks, network components, and the corresponding methods employing QKD.

BACKGROUND

QKD is a technique that enables two distant, legitimate parties to establish a common (or shared) secret key in a way that is, considering the laws of quantum mechanics, secure against eavesdropping on the used communication channel(s).

To be specific, a shared secret key is a piece of information that is known to both the legitimate parties and unknown to anyone else. Since the shared secret (key) is known only to the legitimate parties, it plays a key role in cryptography where it and has many application, such as secure communication, e.g., encryption, decryption of messages and message authentication.

In optical data communications, an eavesdropper, conventionally called Eve, can acquire information about a signal (e.g., a key) transmitted from a sender to a receiver, conventionally called Alice and Bob, respectively, e.g., by splitting of and detecting a fraction of the information carrying light.

In non-QKD systems, the security of the key exchange between two distant parties is typically based on asymmetric cryptography, which relies on the computational complexity of certain mathematical problems, e.g., Diffie-Hellman key exchange or Rivest-Shamir- Adleman public-key cryptosystems. However, as soon as a sufficiently powerful (quantum) computer is available or mathematical progress (e.g., more efficient algorithms) has been made, such key distribution methods may become insecure. Even worse, all data that has been encrypted using keys distributed with these methods can retroactively be broken if the key exchange has been recorded by an eavesdropper. In QKD, on the other hand, the security of the key distribution is guaranteed by the laws of quantum mechanics, which allow to derive Heisenberg’s uncertainty principle and the no cloning theorem. The uncertainty principle, which states that certain variables cannot be known simultaneously with arbitrary precision, implies that measuring one variable destroys information about some other variable. Thus, when Eve performs measurements on the transmitted signal, she inevitably leaves a trace by introducing transmission errors. The no cloning theorem states that it is impossible to make a perfect copy of an unknown quantum state, e.g., of a random signal (or a fraction thereof) encoded in an optical mode. Consequently, it is also impossible to circumvent the uncertainty principle by performing measurements on perfect copies.

Thus, in short, the presence of an eavesdropper spying on communication between the sender and the recipient inevitably leaves a trace that can be detected by way of observing the amount of transmission errors or, equivalently, noise in the transmission channel. In QKD, this is exploited by calculating, based on the observed noise, an upper bound for the information accessible to any eavesdropper. If this upper bound is sufficiently small, a shared secret key can be extracted from the information shared between the sender and the recipient. Under certain conditions, this shared secret key extraction can be proven to be information theoretic secure.

QKD systems can be divided into discrete variable QKD (DV QKD) systems and continuous variable QKD (CV QKD) systems. In DV QKD systems, the information from which the shared secret key is extracted is encoded in a discrete variable, which usually is the polarization/spin degree of freedom of, ideally, single photons, as, e.g., in the BB84 protocol. In CV QKD systems, on the other hand, the information from which the shared secret key is extracted is encoded in a continuous variable. Correspondingly, CV QKD protocols are usually based on the transmission of coherent or squeezed states of light, where said information is continuously encoded in the quadratures (phase and amplitude) of the transmitted light/electromagnetic field. At the receiver, the received signal can thus be measured by means of coherent detection (e.g., homodyne, intradyne, or heterodyne detection) using a strong local oscillator (LO).

SUMMARY

Some embodiments may facilitate efficient implementation of a classical and QKD transmitter and/or receiver. The efficiency may be facilitated by sharing hardware (optical and electronic) components between the classical and quantum transmitter and receiver.

According to an embodiment, a transmitter for quantum key distribution is provided. The apparatus comprises an optical circuitry including at least one optical component shared for modulating a classical signal and a quantum signal, a switch configured to control a time division for transmitting the classical signal and the quantum signal, and an optical output for transmission of the time division multiplexed classical signal and quantum signal.

Such QKD transmitter enables hardware sharing and thus, a cost-efficient implementation.

For example, the switch may comprise a variable attenuator following the optical circuitry and switches between a first transmission power for transmitting the classical signal and a second transmission power for transmitting the quantum signal.

In particular, the switch may be a semiconductor optical amplifier. Usage of this kind of switch may facilitate fast switching and provide for easier integration on one chip, together with the other QKD and classical transmitter parts.

In one exemplary implementation, the switch is configured to change an amplitude of a modulating signal used for modulating the classical signal and the quantum signal. Modifying the amplitude provides an efficient means to distinguish between the classical and quantum transmission.

In any of the above-mentioned examples, the transmitter may further comprise an optical receiver shared for receiving a classical optical signal and for channel monitoring to be used for transmission of the classical signal and/or the quantum signal; and a controller configured to switch the optical receiver to channel monitoring when the optical transmitter is transmitting the quantum signal, wherein the optical output is, at the same time an optical input for the optical receiver.

This further extends the level of hardware sharing and provides additional efficiency for the hardware implementation.

In an embodiment, the transmitter further comprises an optical receiver for receiving a classical optical signal; and an optical input for the optical receiver and separate from the optical output. Such transmitter can thus be connected with a receiver over a single fiber or generally used a same optical transmission path, leading to an efficient and cost-saving implementation. Thus, two different optical paths may be used for the classical/quantum reception and classical transmission.

The optical circuitry in certain implementations includes intensity modulator and phase modulator. This may be employed in DV-QKD systems or also in the CV-QKD system. In other implementations, it includes dual quadrature modulator. Similarly, this may be employed in DV-QKD systems or also in the CV-QKD system.

According to an embodiment, a receiver for quantum key distribution is provided comprising: an optical input for receiving a signal including a classical signal and a quantum signal in a time division multiplex from an optical path, a switch configured to control a time division for reception of the classical signal and the quantum signal, and an optical circuitry including at least one optical component shared for demodulating the classical signal and the quantum signal.

Such receiver enables efficient construction.

In an exemplary implementation, the optical circuitry comprises at least one optical detector; and the switch is operable to adjust the operating mode of the optical detector. In particular, the optical circuitry may comprise two avalanche photo detectors (APD); and the switch is operable to adjust a bias voltage of the avalanche photo detectors.

For example, the optical circuitry is a continuous variable quantum key distribution receiver including a local oscillator; and the switch is operable to adjust the power of the local oscillator.

In another example, the optical circuitry is a continuous variable quantum key distribution receiver; and the switch is operable to adjust the receiver gain.

The receiver may comprise an optical transmitter for transmitting a classical signal including feedback related to the quantum signal transmission. This enables for bi-directional control and/or data communication.

The receiver may further comprise an optical output for the optical transmitter which is separate from the optical input. Thus, two different optical paths may be used for the classical/quantum reception and classical transmission.

The optical transmitter, in some implementations, comprises a direct modulation laser. In addition, a system for quantum key distribution may be provided, comprising: a control node comprising the receiver for QKD as described above; at least one remote node comprising the transmitter for QKD as described above; and a passive optical point-to- multipoint network for connecting the control node and the at least one remote node. Providing the QKD receiver at the control node enables for a more cost-efficient and compact construction of remote nodes.

Details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following embodiments of the invention are described in more detail with reference to the attached figures and drawings, in which:

Fig. 1 is a block diagram illustrating an exemplary passive optical network with remote nodes connected to a single central control node.

Fig. 2 is a block diagram illustrating an exemplary passive optical network with remote nodes connected to a plurality of central control nodes.

Fig. 3 is a block diagram illustrating an exemplary embodiment of a passive optical

network, PON, in which quantum signals are sent from the remote node(s) to the control node.

Fig. 4 is a block diagram illustrating an exemplary embodiment of a PON, in which

quantum signals are sent from the control node to the remote node(s).

Fig. 5 is a block diagram illustrating an exemplary PON employing dual fiber for the

reverse classical link and QKD link. Fig. 6 is a block diagram illustrating an exemplary PON with a shared transmitter and receiver for the classical and QKD transmission.

Fig. 7 is a block diagram illustrating an exemplary detailed PON embodiment featuring a single intensity and a single phase modulator in combination with a single Mach- Zehnder interferometer at the receiver, usable for DV-QKD. Fig. 8 is a block diagram illustrating an exemplary detailed PON embodiment featuring a single intensity modulator and a Mach-Zehnder interferometer with a phase modulator at the transmitter in combination with a Mach-Zehnder interferometer at the receiver, usable for DV-QKD.

Fig. 9 is a block diagram illustrating an exemplary PON embodiment featuring an IQ

Mach-Zehnder modulator, usable for CV-QKD.

In the following identical reference signs refer to identical or at least functionally equivalent features.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following description, reference is made to the accompanying figures, which form part of the disclosure, and which show, by way of illustration, specific aspects of embodiments of the invention or specific aspects in which embodiments of the present invention may be used. It is understood that embodiments of the invention may be used in other aspects and comprise structural or logical changes not depicted in the figures. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

For instance, it is understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if one or a plurality of specific method steps are described, a corresponding device may include one or a plurality of units, e.g. functional units, to perform the described one or plurality of method steps (e.g. one unit performing the one or plurality of steps, or a plurality of units each performing one or more of the plurality of steps), even if such one or more units are not explicitly described or illustrated in the figures. On the other hand, for example, if a specific apparatus is described based on one or a plurality of units, e.g. functional units, a corresponding method may include one step to perform the functionality of the one or plurality of units (e.g. one step performing the functionality of the one or plurality of units, or a plurality of steps each performing the functionality of one or more of the plurality of units), even if such one or plurality of steps are not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary embodiments and/or aspects described herein may be combined with each other, unless specifically noted otherwise. Moreover, it is noted that, in general, all numerical values are examples for the sake of a full description.

Embodiments shown herein facilitate allowing for a low complexity and cost efficient implementation of a quantum key secured passive optical point-to-multipoint network for secure short reach control and communication applications with low data rates such as ATMs (Automated Teller Machines, i.e. money automats), control applications or the like.

Encrypted communication links using QKD systems today use separate hardware components to implement the different aspects necessary to build an encrypted communication link. This includes among other things the following components: a classical transceiver to establish a communication channel, encryptors and/or decryptors to establish an encrypted connection, a key management infrastructure to manage the keys and a QKD system that generates the random encryption keys at both locations (ends). In addition, the QKD systems are not developed for point-to-multipoint applications, but rather mostly for point-to-point applications. This makes current technologies expensive for short reach networking applications with low data rate requirements. In other words, today’s systems rely on classical encryption with security being limited by current knowledge of mathematics, current QKD systems do not scale well for this application.

Here, the term a“classical transceiver¹’ refers to any transceiver, i.e. transmitter and/or receiver, which are capable of transmitting and/or receiving classical (i.e. non-quantum) signals. In general, the quantum signals differ from the classical signals in terms of signal intensity, as already mentioned above. Quantum signals do have very low intensity, mostly only one or few several photons.

The term“key management infrastructure” refers to entities for storing and retrieving of encryption keys. Generating of the keys may also belong to key management, as well as further functions such as encrypting or decrypting the keys, associating the keys with particular contents or users, or the like.

The embodiments presented herein may facilitate implementation of a bidirectional classical and a unidirectional quantum key distribution link, using the same components for both, classical and QKD communication. The switching between classical and QKD communication in a first direction may be achieved by varying the transmitted power and in a second direction by switching the detectors from quantum detection mode to a classical mode of operation. Here, the first direction is the direction in which quantum signal is transmitted and the second direction is reverse to the first direction. It is noted that the first direction is the direction from the control node to remote nodes in some embodiments, while it is the direction from the remote node to control node(s) in other embodiments. In general, the present disclosure is applicable to any two or more communication nodes.

In the first direction which may be for transmitting signal, the switching may be accomplished, for instance, with a variable optical attenuator modulator or switch. In the second direction which may be for receiving signal, the switching may be accomplished, for instance, by changing the bias voltage of single photon avalanche detectors for DV-QKD or by changing the local oscillator power or electrical gain of a coherent detector in a CV-QKD system.

The present disclosure provides several detailed embodiments with additional features which may reduce system complexity and cost by implementing a passive optical network.

For example, the costly components may be used only in the central office (a central control node) and remote functionalities can be realized with low overall complexity and cost. The following embodiments and implementations may be employed on their own or in combination:

- To minimize the optical complexity, the optical transmission and reception hardware is shared between QKD and classical communication.

- The QKD transmitter is switched between classical and QKD operation with a variable attenuator that changes the transmitted power (such a Semiconductor Optical Amplifier, SOA).

- The QKD receiver also switches between the two modes, classical and quantum. This can for example be achieved by adjusting the bias voltage to adjust the avalanche photo-diode (APD) operating mode, single photon avalanche detector (SPAD) or the like. For CV-QKD, switching can be achieved by adjusting the local oscillator power and/or the gain of the receiver amplifiers.

- The placement of QKD transmitter and receiver is decided on a cost basis. Components that are required to have high performance such as single photon detection capable detectors should be placed in the central office (control node), while lower cost components are placed in the remote nodes.

In the reverse direction the classical link may reuse a channel monitoring detector in the QKD transmission (TX) as receiving detector adding a circulator and an additional directly modulated laser next to the quantum receiver as transmitter.

- Alternatively, completely separate hardware may be added for the classical

communication link. This strongly depends on cost and performance requirements.

In other words, a combined classical and QKD passive optical network (PON) for control applications as proposed here facilitate achieving classical communication and QKD with only a small amount of additional components when compared to a pure QKD or classical link.

Figure 1 illustrates an exemplary passive optical network 100. The term“passive” here means that the network may employ passive components such as splitters and combiners but no active amplifiers. When referring to optical network in the embodiments, some examples with optical fibers as medium will be described. Here, the optical fiber may be a glass fiber or a plastic fiber. However, the present disclosure is not limited thereto: especially for short distances, free-air transmission is also possible.

It is noted that the classical and quantum signal may be transmitted for example in a time division multiplex. In general, the way of multiplexing the classical and quantum signal is not to limit the present disclosure and any other multiplexing approaches (e.g. polarization, wavelength-division, or the like) may be used in addition or alternatively to time division. Using one wire, complexity of implementation may be reduced and the common use of transmitter and/or receiver elements may be facilitated. However, it is noted that some of the embodiments may also use separate fibers for the classical and quantum signals.

The PON 100 of Figure 1 includes a control node 170, connected by means of a splitter 150 to three remote nodes 120_1 , 120_2, and 120_3. In general, there may be any number of remote nodes in the network such as 1 , 2, 4, or more. In this architecture, the control node 170 may broadcast signal to all nodes at once (point-to-multipoint) or individually transmit data to one of them (point-to-point transmission). The remote nodes may individually transmit data to the control node 170. The splitter is an optical splitter which passively divides the signal (e.g. evenly) to the respective nodes. In general, the control node may have a functionality of a scheduler for scheduling transmission of data on the classical channel.

Figure 2 includes another PON 200 with more than one control nodes 270_1 and 270_2. Even though Figure 2 shows only two control nodes, in general, there may be more than two control nodes. In the network 200, connected via splitter 250 to the remote nodes 220_1 , 220_2, and 220_3. Then, a remote node may broadcast signal to all control nodes 270_1 and 270_2 (point-to-multipoint transmission) or to one control node (point-to-point transmission). As in the network 100, the each control node 270_1 and 270_2 may broadcast data to all remote nodes or transmit data to a selected remote node. Multiple control nodes being connected to the same network may achieve redundancy and therefore increased reliability. For such applications, it may be advantageous if the control nodes are coordinated either by communicating with each other or from another central control entity.

In such a network such as PON 100 of Figure 1 or PON 200 of Figure 2, two alternative cases that can be implemented:

- A first embodiment, where quantum signals are sent from the remote nodes to the control node. Such exemplary PON 300 is shown in detail in Figure 3.

- A second embodiment, where quantum signals are sent from the control node to the remote nodes. Such case is exemplified in Figure 4.

In particular, Figure 3 illustrates the first embodiment, where quantum signals are sent from the remote node(s) to the control node. Remote nodes 310 and 315 are connected via a splitter 350 to the control node 370.

The control node 370 includes a combined classical and a quantum receiver 390 (RX) as well as a classical transmitter 380 (TX). They are connected to a circulator 360. The optical circulator 360 here is a three port device. That light entering any port exits from the next port in the circulation direction. For example, if light enters from the splitter 350, it is emitted to the classical & QKD transmitter 390 which may also perform switching between the classical and quantum mode. The light input from the classical transmitter 380 is emitted towards the splitter 350 for transmission to the remote nodes 310 and 310. In this way, a fiber-optic circulator may be used to separate optical signals that travel in opposite directions in an optical fiber, for example, as here, to achieve bi-directional transmission over a single fiber.

The classical transmitter 380 may employ any coding, modulation and multiplexing technique among the well-known transmission techniques. For example, it may modulate signals using pulse amplitude modulation (PAM). The classical transmitter 380 serves as a feedback channel for performing quantum key distribution and, in particular, the steps of sifting, reconciliation, and/or privacy amplification. However, the present disclosure is not limited to this utilization of the classical channel from the control node to one or more remote nodes. In general, this classical (feedback) link may also be used to transmit user data or configuration (signaling) data to the remote nodes in a unicast, multicast, and/or broadcast manner.

The classical and QKD receiver 390 is capable of operating in both modes, the classical and quantum mode. It receives optical signal from the remote node(s) through the splitter and detects the quantum signal depending on implementation of the QKD protocol and detects the classical signal also depending on implementation of the classical transmission. Some particular examples of the respective implementations will be described later on.

The remote device 310 (similarly a second or any further remote device 315) includes a classical and QKD transmitter 320, as well as a classical receiver 330. In particular, the classical and quantum transmitter 320 is capable of operating in both regimes (modes) namely the classical mode and the quantum mode. It is connected to a coupler 340 which passes the signal from the classical and quantum transmitter 320 to the optical channel (here exemplified as a fiber, but in general any optical channel). The remote node may further comprise a classical receiver 330. In the example of Figure 3, the classical receiver 330 is combined with a channel monitoring function.

The classical receiver and channel monitoring 330 is configured to receive the classical signal from the feedback channel. In other words, it receives the signal generated and transmitted by the classical transmitter 380 of the control node 370. The classical signal may include some reference signals based on which the classical receiver and channel monitor 330 performed channel estimation. However, the present disclosure is not limited to channel estimation using pilot signals. In general, error rate may be monitored in addition or alternatively.

The classical and QKD transmitter 320 is capable of generating quantum signal as well as the classical signal and to switch between these two modes. The coupler 340 couples the classical receiver and channel monitor 330 or the classical and quantum transmitter 320 to the output of the remote node.

As can be seen in Figure 3, the remote device (node) 310 may also include an output power monitor 325. Output power monitoring may be performed by a photo diode (in general an optical detector). Power monitoring measures a part of the power on one end of the coupler which is different from the end input /output to the link. If the coupling ratio is known, this enables monitoring of the power actually output to the link. The coupler 340 may be a static coupler or a coupler with a controllable coupling ratio.

In this example, each remote node 310, 315 transmits its individual signal to the splitter, resulting in unicast transmission to the control node. However, if there are more than one control nodes connected to the splitter 350, the remote node may be actually multicasting (broadcasting to all nodes connected to the output of the splitter) its signal.

In Figure 3, the remote devices embodied QKD transmitter while the control device(s) embodied a QKD receiver. However, for some deployments, it may be beneficial to maintain the complexity of the remote devices low. Accordingly, in some specific scenarios, it may be beneficial to implement the QKD transmitter, which typically also includes light source, in the control node. The control node 370 may be a central node, in the sense of providing a central unit for QKD generation and distribution to more remote devices 310 and 315.

Figure 4 shows such an example of a PON 400. In this example, the remote node 410 (as well as one or more further remote nodes 415) comprises a classical and QKD receiver 490 as well a classical transmitter 480 connected via a circulator 460 to the link leading to the splitter 450. The classical and QKD receiver 490 has the same functionality as the classical and QKD receiver described with reference to the control node 370 of Figure 3. The classical transmitter (TX) 480 functionality is the same as the functionality of the classical transmitter 380 described above with reference to the control node 370 of Figure 3.

The control node of Figure 4 includes a classical and QKD transmitter 420 as well as a classical receiver and channel monitor 430 coupled to the input / output of the optical link via a coupler 440. On the other side of the coupler, the output power monitoring detector 425 can be connected. This structure of the classical and QKD transmitter and classical receiver is the same as shown in Figure 3 for the remote device.

The control node 470 transmits the quantum signal in a common link to the splitter and then to the respective one or more remote devices. This may be a multicast for more remote control devices. However, as each remote node then detects its own photons, the sifting, reconciliation and/or the privacy amplification procedures are performed in a unicast, point- to-point manner. For this purpose, in addition to the link from the classical and quantum transmitter to the receiver, a classical link is established in the reverse direction, between the classical transmitter 480 in the remote node 410 and the classical receiver 430 in the control node 470. Also, the output power monitoring in the control node ensures that quantum channel output power is correctly calibrated.

Although Figures 3 and 4 show a single control node, and two remote nodes, it is noted that in general, there may be more remote nodes and/or control nodes, which applies to any embodiments described therein, including the embodiments described below. For a cost effective implementation it is typically beneficial to place the more costly components in the control node and the lower cost hardware in the remote nodes as shown in Figure 4.

There may be fiber pairs available rather than single fibers at the QKD transmitter. In general, the present disclosure is not limited to the implementation using only a single fiber for the quantum channel and classical channels in both directions. Figure 5 illustrates a dual fiber solution for a PON 500 in which a bidirectional fiber is available, avoiding the need to turn off the reverse classical connection while QKD communication occurs. In Figure 3 and 4, there is only a single optical channel available for both directions at the random node(s). Thus, in order to enable efficient quantum signal detection, it is advantageous to time- multiplex the forward communication in the direction from the QKD transmitter with the classic reverse communication in the direction from the QKD receiver.

As shown in Figure 5, there are two splitters 550 and 555, one splitter 550 for the classical reverse communication and one splitter 555 for the quantum and classical forward communication. Figure 5 shows an example in which there is one control node 570 with a classical transmitter 580 and a classical receiver and QKD receiver 590 which can work in both modes classical and quantum. Moreover, in Figure 5 there are two remote nodes 510 and 515 illustrated. However, there may in general be more than one control node and/or one or more remote node(s). Furthermore, it is noted that the structure shown in Figure 5 for the control node 570 may in fact be implemented by the remote node(s) while the structure shown for the remote nodes 510 and 515 may be implemented by one or more control nodes. Such implementation moves the complexity of implementation from the remote nodes to the control nodes, which may be beneficial for some applications.

As shown in Figure 5, a circulator and the coupler in remote and control node can be omitted to allow for an increase in data rate and allows for the purely classical link through splitter 550 to be active at all times without disturbing the quantum channel. The remote node 510 has a coupler 540. However, the coupler 540 does not couple the classical reverse received signal link with the classical and QKD forward transmitted signal link. Rather, it merely couples the classical and QKD transmitter 520 with the channel monitor 535 and the output power monitor 525. As can be seen in Figure 5, the classical transmitter 580 in the control node 570 is connected with the classical receiver 530 in the remote node 510 and/or further remote node(s) 515 over the splitter 550. On the other hand, the classical and QKD transmitter 520 in the remote node(s) 510 (and 515) is connected to the classical and QKD receiver 590 in the control node via the splitter 555 using fibers or optical channels separated from those used for the classical reverse communication.

In this case illustrated in Figure 5, down-stream (from a control node to a remote node) data rates similar to standard classical communications systems can be achieved. The up-stream (from the remote node(s) to a control node) data rates are reduced by a factor that corresponds to the ratio between classical and QKD communication in this direction.

The sharing of the components between the classical and quantum communication may be achieved by various means, for example by one or more of the following:

- The classical and quantum transmitter 520 and receiver 590 use the same link for sending and receiving both classical and quantum signals as is shown in Figures 3 to 5. This may be achieved by time multiplexing the transmitted classical and QKD signals in a predefined or predetermined way. For example, the schedule for the classical and QKD signal transmission may be fixed or may be configured before or during the transmission by some signaling.

- Correspondingly, the classical and quantum transmitter 520 may include a switch which switches the sent power and/or the modulation format between the classical levels and the quantum levels. This switching is performed in accordance with the above- mentioned predetermined or predefined schedule for the transmission of the classical and the quantum signals.

- Correspondingly, the classical and quantum receiver 590 may switch between classical operation and quantum operation. The implementation of such switching depends on the particular QKD implementation. For example: o in DV-QKD receivers, single photon avalanche detectors (APDs) are employed. In order to switch between the classical and quantum operation mode, the APDs can be operated with different bias voltages. In this way, the APDs may operate to implement single photon detection for QKD reception or to implement pulse detection for classical communication. The switching may be performed by a controller controlling the bias voltage according to the predefined or predetermined timing. o in CV-QKD receivers, the gain of the receiver can be adjusted by changing the local oscillator power or the amplifier gain to accommodate for the difference between classical and quantum signals. The adjustment may be performed by a controller controlling the oscillator power and/or the amplifier gain according to the predefined or predetermined timing. In case the receiver hardware offers a sufficiently large dynamic range, it might also be sufficient to only switch the digital signal processing between processing the quantum or classical signal.

- During the transmission of QKD signals, the classical signal in the reverse direction may be switched off, to avoid disturbances of the quantum signal due to scattering and reflections of the classical signal in the embodiments with shared fiber.

The above mentioned controller for controlling the timing in which the switching between the classical and the quantum operation mode is performed may be implemented as any microcontroller, processor, a piece of a specialized hardware or any other electric or electro optic circuitry.

An example of a shared transmitter for the classical and QKD transmission is shown in Figure 6. In particular, Figure 6 shows a PON 600 implementation for QKD operation. The control node 670 in Figure 6 includes classical transmitter 680 and classical and QKD receiver 690 connected to a circulator 660, similarly as Figure 3. In general, the control node may implement sharing of the components but does not have to. Figure 6 also shows a remote device 610 including a classical receiver and channel monitoring 630, output channel monitoring 625, a coupler 640 and the classical and QKD transmitter which shares a transmitter 620 for the classical and quantum operation. Sharing of the components at the transmitter is enabled by adding a switch 622 to switch between classical and quantum power levels in the remote node 610.

Addition of the output power monitoring detector may further help to correctly calibrate the classical and quantum states. In addition, the classical receiver 630 in the remote nodes can serve as channel monitoring device, as shown in Figure 6. Channel monitoring improves the quantum key derivation. Moreover, the remote node and the control node have to apply the same timing to switch between the classical operation and the quantum operation. This may be performed by pre-synchronization, transmission of synchronization sequences or symbols regularly over the classical channel, or in another way.

Irrespectively of whether the QKD transmitter is implemented in the control node or in the remote node, according to a more general embodiment, a transmitter for quantum key distribution is provided and comprises:

- an optical circuitry 620 including at least one optical component shared for generating a classical signal and a quantum signal,

- a switch 622 operable to or configured to control a time division for transmitting the classical signal and the quantum signal, and

- an optical output 640 for transmission of the time division multiplexed classical signal and quantum signal.

The generation of classical signal may be performed in any known way. For example, the laser is configured to output a substantially coherent light with a certain wavelength. This light is modulated by e.g. pulse amplitude modulation, pulse width modulation, polarization modulation, phase modulation, or the like in order to carry digital data. The data themselves may be coded by a forward error correcting code or inserted some parity for error detection. There may be reference symbols and/or synchronization symbols included in the classical signal. The present disclosure is not limited to any particular classical signal transmission generation. There may be different transmission technologies more or less suitable for the glass fiber, plastic fiber or free air transmission.

The quantum signal may be generated using the same light source (such as a laser or a laser diode) as the classical transmitter. In this way the classical and QKD transmitter may share the light source component to save costs. The light of the laser may be used to generate two (or more) different quantum states such that they differ in properties such as polarization or phase and/or amplitude or angular momentum. For example, in some implementations, the states may differ in a single one of these properties. In other implementations, the states may differ in more than one properties, e.g. amplitude and phase, corresponding to a quadrature amplitude modulation. Other combinations of the properties are possible.

A classical signal is in general an optical signal with a power higher than the quantum signal and carrying data and/or reference signal. The data may be a control data (signaling) or payload data. A quantum signal is an optical signal carrying raw key data and having a very low power as mentioned above. Raw key data are data out of which the key is then obtained. This data is typically randomly generated at the transmitter of the quantum signal.

The optical output may be an interface to an optical medium. For example, the optical signal formed by the classical and/or quantum signal can be injected to the fiber or transmitted via optical air-interface.

The switch 622 may be a hardware switch or a software switch. The hardware switch may be operated by a controller such as a processor or a microcontroller or any electric or optoelectronic circuitry. The software switch may be configured by a program code running on a processing circuitry.

In particular, in one exemplary implementation, the switch 622 comprises a variable attenuator following the optical circuitry and switches between a first transmission power for transmitting the classical signal and a second transmission power for transmitting the quantum signal. For example, the switch 622 is a semiconductor optical amplifier, SOA. The SOA is electronically controllable and may be switched relatively rapidly. It is a commercially available component which facilitates an efficient implementation and may even allow for on-chip integration.

Alternatively, the switch 622 is configured to change an amplitude of a modulating signal used for modulating the classical signal and the quantum signal. The modulating signal driving the modulator can of course change also other characteristics (such as phase, angular momentum or polarization) depending on whether is used for classical or the quantum signal.

The transmitter 610 in one embodiment comprises an optical receiver 630 shared for receiving a classical optical signal and for channel monitoring to be used for transmission of the classical signal and/or the quantum signal. Moreover, the transmitter also includes a controller configured to switch the optical receiver to channel monitoring when the optical transmitter is transmitting the quantum signal, wherein the optical output is, at the same time an optical input for the optical receiver. This can be seen, for example in Figure 6, in which the received classical signal and the transmitted classical and quantum signal are both coupled to the same optical fiber on the output of the coupler 640. It is noted that in absence of the channel monitor, the optical receiver may be switched off during the transmission of the quantum signal. However, it may be advantageous not to switch off the receiver as it can be used to monitor the optical fiber for possible attacks, thus improving security of the system.

It is further noted that the above examples show fibers, but, as mentioned above, the present disclosure is not limited thereto. The input /output of the coupler may lead to an air interface which may be particularly interesting for application in which short distances between the receiver and transmitter of the QKD are envisioned. This may be, for instance payment related applications in which one of the ends (remote node) can be a mobile phone or another portable device.

As already shown in Figure 5, in one embodiment, an optical receiver 530 for receiving a classical optical signal is provided in the transmitter 510. Moreover, the transmitter 510 includes an optical input for the optical receiver and the optical input is separate from the optical output. These two outputs may lead to two separate optical fibers or other optical paths such as an air interface paths as mentioned above. There may be even two distinct optical paths in the same fiber such as orthogonal optical channels (carried on orthogonal frequencies, phases, polarizations, orbital angular momentum, or the like).

In Figures 1 to 6, the receiver 670 for quantum key distribution in one embodiment comprises:

- an optical input 660 for receiving a signal including a classical signal and a quantum signal in a time division multiplex from an optical path;

- a switch configured to control a time division for reception of the classical signal and the quantum signal; and

- an optical circuitry 690 including at least one optical component shared for demodulating the classical signal and the quantum signal.

The optical input may correspond to the input from the circulator 660, 460, or 360, or directly from a separate optical path (in Figure 5 it is a separate fiber). Also here the switch can be a software switch or a hardware switch. If the receiver 690 has a sufficient dynamic range, like e.g. some CV-QKD implementations, the QKD and classical signals are received by the same hardware and the switch is implemented in software, changing the signal processing from classical processing to QKD processing.

Moreover, the optical circuitry in some implementations comprises at least one optical detector and the switch is operable to adjust the operating mode of the optical detector. It is noted that the optical detector may be a photo diode or any other optical detector such as a superconducting detector.

The receiver for QKD 670 may be further comprising an optical transmitter 680 for transmitting the classical signal including feedback related to the quantum signal transmission. As discussed above, it may also comprise an optical output (to an optical path) for the optical transmitter 680 which is separate from the optical input. This may but do not necessarily have to be separate fibers. In some embodiments, the optical transmitter comprises a direct modulation laser, DML.

In Figures 7, 8, and 9, more detailed examples of the classical and QKD transmitters and receivers are provided, suitable for various different QKD systems.

In particular, Figure 7 illustrates an example of a PON 700 which supports DV-QKD and an implementation deploying a single Mach-Zehnder interferometer. The classical and QKD signals are generated by several components which are shared between the classical and QKD operation mode. In particular, the transmitter 710 (corresponding to a remote node) includes a light source which is here a laser 724. Moreover, an intensity modulator (IM) 726 is used to modulate the laser beam output from the laser 724 in intensity. Then, a phase modulator (PM) 728 is used to modulate the laser beam output from the IM 726 in phase. It is noted that these two modulators IM 726 and PM 728 may also operate in reversed order (PM first and IM afterwards). The output power is adjusted with a variable optical attenuator (VOA, here an SOA) 722 which fulfills the role of the switch 622 shown in Figure 6. Correspondingly, the laser 724, IM 726, and PM 728 correspond to the transmitter 620 of Figure 6. The output power may be monitored with an output power monitoring photo detector 725. The output power monitor 725 is located at the output of the coupler 740. On the input of the coupler 740, the classical and QKD signal generator 720 is connected. As already mentioned, also in Figure 7, the classical communication receiver 730 and transmitter 720 may be integrated together with the QKD transmitter on a single chip which may be used in remote nodes as illustrated in Figure 7 or in control nodes (cf. Figure 4). The classical receiver may be provided together with the channel monitoring 735.

In the classical and QKD transmitter 720, the SOA serves as a gain module or attenuator to allow for quickly switching between launching classical and quantum level signals. In the receiver of the control node 770 shown in Figure 7, the interferometer 792 can be easily aligned to the transmitter while operating with classical signals. The alignment typically concerns the interferometer phase. The subsequent APDs are either operated as single photon avalanche detectors for quantum signals with a large bias voltage or as regular avalanche photo detectors with a reduced bias voltage. The switch to switch between these two operating modes is not shown in Figure 7. The classical transmitter 780 includes the DML 785. A circulator 760 separates the two signal directions namely forward direction (direction of reception of the quantum signal) and reverse direction corresponding to transmission of the classical signal by the classical transmitter 780. It is noted that the classical transmitter 780 and classical receiver 730 may communicate using PAM which may be binary or of a higher order.

In other words, the optical circuitry in this exemplary embodiment comprises two avalanche photo detectors and the switch is operable to adjust a bias voltage of the two avalanche photo detectors APD.

Figure 7 shows details of a combined QKD and classical communication network PON, with remote nodes 710, 715 (left), splitter 750 (center) and control node 770 (right). The remote nodes contain the classical receiver 730, the classical & QKD transmitter 720, the coupler 740 to connect both to the single connecting fiber and to monitor the output power, and a subsequent output power monitoring. Here, the classical and QKD link uses phase and intensity modulation in the transmitter 720 and detection based on a delay interferometer with avalanche photo detectors (APDs) in the receiver 790. In the counter propagating (reverse) direction, this embodiment shows a simple directly modulated link, but this can be easily adapted to any modulation format and transmitter/receiver desired. The control node 770 contains the receiver 790 for classical & QKD signals and the classical transmitter 780. The circulator 760 minimizes signal path loss for the QKD signal and allows to share a single fiber. The receiver 790 for classical and QKD signals contains an interferometer 792 that can be aligned to the transmitter wavelength during periods of classical communication and avalanche photo detectors (APDs) that are switched between the classical and quantum operating regimes by adjusting the bias voltage. Classical communication from the control nodes 770 to the remote nodes 710, 715 in this example is implemented using simple direct modulation of a directly modulated laser (DML) 785 and detection of the signal in the remote node using a photo detector.

In summary, the following are features which may facilitate low-cost implementation:

- Fully integratable QKD and classical transmitter 720 (potentially on-chip), at multiple remote nodes 710, 715 where controlled components such as an ATM are placed.

- The transmitter 720 switches between classical and QKD operation by changing the emitted power levels by means of the switch 722.

The QKD and classical receiver 790 is placed at a central point, so that for DV-systems, single photon detectors (APDs) and interferometer 792 can be shared among all reception systems. Typical single photon avalanche detectors APD can be operated as classical detectors by reducing the bias voltage.

- A return channel can be implemented, using a directly modulated laser 785, a circulator 760, and by using the channel monitoring detector 735 in the QKD and classical receiver 730 as receiving detector.

- The channel is shared by classical and QKD signals in a time multiplex.

Figure 8 illustrates a combined QKD and classical communication network, PON, 800 with remote nodes 810 (left), splitter 850 (center) and control node 870 (right). In particular, Figure 8 illustrates another embodiment featuring a double Mach-Zehnder modulator. In the embodiment of Figure 8, there is a slightly different transmitter scheme. Contrary to the previous schemes, an interferometer 828 with a phase modulator in one arm is used to modulate data and for the generation of the QKD signals.

In particular, the remote node(s) 810 includes a classical and QKD transmitter 820 which includes a laser 824 as a light source, an intensity modulator 826, the interferometer with the phase modulator 828 and the switch (SOA) 822. The classical receiver and channel monitoring are represented by the photo detector 835, which is also a shared element for both purposes. The classical receiver 835 is coupled together with the classical and QKD transmitter 820 through a coupler 840. The outputs of the coupler include the output power monitoring 825 and the actual signal output which is input to the splitter 850.

Control node(s) 870 include a circulator 860 to separate upstream and downstream signals. The circulator 860 is connected to the classical transmitter 885 implemented by a DML. It is further connected to the classical and QKD receiver which includes a switch 895, an interferometer 892 and the two APDs as detectors.

An exemplary CV-QKD implementation of a PON 900 is shown in Figure 9. In particular, Figure 9 shows combined QKD and classical communication network based on CV-QKD, in which the receiver 970 includes the optical circuitry which is a continuous variable quantum key distribution receiver including a local oscillator, and in which the switch is operable to adjust the power of the local oscillator (not shown in Figure 9). Alternatively or in addition, the switch is operable to adjust the receiver gain.

The classical and QKD transmitter comprises a laser 924, an optical IQ (quadrature) modulator 928, a switch 922, and a coupler 940. The laser beam is modulated in the optical IQ modulator 928, and the SOA 922 adjusts the power levels for classical or quantum operation. The remaining parts of the remote node 910 are similar as in the above-described embodiments. The coupler 940 couples optical path from the output of the SOA 922 with the optical path corresponding to the classical receiver and channel monitoring 935 (actually an output of the coupler in the reverse direction) and outputs optical path for output power monitoring 925 and an optical path with the remaining signal to the splitter 950.

In the receiver 970 the signal is received with a local oscillator laser 990 and a dual polarization (DP) 180° Hybrid with balanced photo detectors 995. To adjust the receiver to classical or QKD signals, one can either change the local oscillator power or the gain in the trans-impedance amplifiers (not shown in Figure 9) of the photo detectors. Both can be achieved by means of a control circuitry which controls the local oscillator laser 990 and in particular its power and/or the gain of the trans-impedance amplifiers that are placed after the photo detector amplifiers at the output of the DP 180° Hybrid 995.

One of the advantages of choosing CV-QKD over DV-QKD may be that the return channel in counter-propagation to the QKD signal can be operated continuously as long as both channels are more than 100 nm apart due to the inherent wavelength selectivity of the CV- QKD receiver. For such a large wavelength separation, continuous operation of the return channel in the DV-QKD cases could also be achieved if high performance wavelength filters would be used to suppress the scattered photons outside the bandwidth of the quantum signals. However, such filters typically introduce significant losses and complexity.

In summary, the optical circuitry at the QKD transmitter may include either an intensity modulator and a phase modulator (for DV-QKD) or dual quadrature modulator (for CV-QKD - could also be envisioned for DV-QKD).

As described with reference to Figures 1 to 9, the present disclosure provides the components of a PON. In addition, a system is provided for quantum key distribution comprising a control node including the receiver such as 310, 410, 510, 610, 710, 810, or 910. The system further includes at least one remote node comprising the transmitter such as 370, 470, 570, 670, 770, 870, or 970. Moreover, a passive optical point-to-multipoint network for connecting the control node and the at least one remote node may be provided as well.

However, systems provided by the present invention may have a different structure. In particular, in an alternative implementation the system may comprise a remote node including the receiver such as 310, 410, 510, 610, 710, 810, or 910. Such system further includes at least one control node comprising the transmitter such as 370, 470, 570, 670, 770, 870, or 970.

There may be more than one control nodes in any of the embodiments described with reference to Figures 1 to 9.

The present disclosure further provides methods. For example, an optical transmission method is provided for quantum key distribution comprising the steps of:

- modulating a classical signal and modulating a quantum signal performed by an optical circuitry including at least one optical component shared for the modulating of the classical and quantum signal,

- controlling a time division for transmitting the classical signal and the quantum signal by a switch, and

- (optically) transmitting (outputting) the time division multiplexed classical signal and quantum signal.

The controlling comprises controlling a variable attenuator (following the optical circuitry as illustrated in the figures) and thereby switching between a first transmission power for transmitting the classical signal and a second transmission power for transmitting the quantum signal. The switch (variable attenuator) may be a semiconductor optical amplifier.

The controlling in an embodiment includes changing an amplitude of a modulating signal used for modulating the classical signal and the quantum signal.

The reception of the classical signal may be performed by an optical receiver shared also for channel monitoring of the classical and quantum channel. The controlling step may switch the optical receiver to channel monitoring when the optical transmitter is transmitting the quantum signal, when a same fiber is used for both directions quantum/classical transmission direction and the reverse classical reception direction.

However, there are embodiments, in which the two directions are transmitted via separate respective fibers.

The transmission of the quantum and classical signal may be performed by intensity modulation combined with phase modulation or by dual quadrature modulation. Moreover an optical reception method is provided for quantum key distribution comprising: the steps of:

- receiving a signal including a classical signal and a quantum signal in a time division multiplex from an optical path;

- controlling a time division for reception of the classical signal and the quantum signal by a switch, and

- demodulating the classical signal and the quantum signal using an optical circuitry including at least one optical component shared for the demodulation of the classical signal and the quantum signal.

The controlling step may adjust the operating mode of an optical detector. The optical detector may include two avalanche photo detectors (APD), and the controlling may include adjusting a bias voltage of the avalanche photo detectors.

The transmission and reception of the quantum signal may be performed according to a continuous variable quantum key distribution approach including generating by a local oscillator a reference signal and performing the controlling step by adjusting the power of the local oscillator. Alternatively, the controlling step may be performed by adjusting the receiver gain.

The reception of the quantum signal may also include transmitting a classical signal including feedback related to the quantum signal transmission. As mentioned above for the transmission of the quantum signal, the quantum/classical forward channel and the classical backward channel may be transmitted via a common fiber or via respective separate fibers. The reverse classical channel may be modulated by a direct modulation laser.

Summarizing, the present disclosure relates to a passive optical network transmitter and receiver for quantum key distribution and in particular, to an efficient implementations thereof. A transmitter includes an optical circuitry including at least one optical component shared for modulating a classical signal and a quantum signal, a switch configured to control a time division for transmitting the classical signal and the quantum signal, and an optical output for transmission of the time division multiplexed classical signal and quantum signal. Correspondingly, a receiver comprises an optical input for receiving a signal including a classical signal and a quantum signal in a time division multiplex from an optical path, a switch configured to control a time division for reception of the classical signal and the quantum signal, and an optical circuitry including at least one optical component shared for demodulating the classical signal and the quantum signal.

Claims

1. A transmitter for quantum key distribution comprising: an optical circuitry (620) including at least one optical component shared for modulating a classical signal and a quantum signal; a switch (622) configured to control a time division for transmitting the classical signal and the quantum signal; an optical output (640) for transmission of the time division multiplexed classical signal and quantum signal.

2. The transmitter according to claim 1 , wherein the switch (622) comprises a variable attenuator following the optical circuitry (620) and switches between a first transmission power for transmitting the classical signal and a second transmission power for transmitting the quantum signal.

3. The transmitter according to claim 1 or 2, wherein the switch (622) is a

semiconductor optical amplifier.

4. The transmitter according to claim 1 , wherein the switch (622) is configured to

change an amplitude of a modulating signal used for modulating the classical signal and the quantum signal.

5. The transmitter according to any of claims 1 to 4, comprising: an optical receiver (630) shared for receiving a classical optical signal and for channel monitoring to be used for transmission of the classical signal and/or the quantum signal; a controller configured to switch the optical receiver to channel monitoring when the optical transmitter is transmitting the quantum signal, wherein the optical output is, at the same time an optical input for the optical receiver.

6. The transmitter according to any of claims 1 to 4, comprising: an optical receiver (530) for receiving a classical optical signal; an optical input for the optical receiver and separate from the optical output (540).

7. The transmitter according to any of claims 1 to 6, wherein the optical circuitry (720) includes either intensity modulator (826) and phase modulator (828) or dual quadrature modulator (928).

8. A receiver for quantum key distribution comprising: an optical input (660) for receiving a signal including a classical signal and a quantum signal in a time division multiplex from an optical path; a switch configured to control a time division for reception of the classical signal and the quantum signal; and an optical circuitry (690) including at least one optical component shared for demodulating the classical signal and the quantum signal.

9. The receiver according to claim 8, wherein the optical circuitry (690) comprises at least one optical detector (APD); and the switch is operable to adjust the operating mode of the optical detector (APD).

10. The receiver according to claim 8, wherein the optical circuitry (690) comprises two avalanche photo detectors (APD); and the switch is operable to adjust a bias voltage of the avalanche photo detectors.

1 1. The receiver according to claim 8, wherein the optical circuitry (690) is a continuous variable quantum key distribution receiver including a local oscillator (990); and the switch is operable to adjust the power of the local oscillator.

12. The receiver according to claim 8, wherein the optical circuitry (690) is a continuous variable quantum key distribution receiver; and the switch is operable to adjust the receiver gain.

13. The receiver according to any of claims 1 to 12, comprising an optical transmitter (680) for transmitting a classical signal including feedback related to the quantum signal transmission.

14. The receiver according to claim 13, comprising an optical output for the optical

transmitter which is separate from the optical input.

15. The receiver according to any of claims 1 to 14, wherein the optical transmitter (590) comprises a direct modulation laser (785).

16. A system for quantum key distribution comprising: a control node comprising the receiver according to any of claims 8 to 15; at least one remote node comprising the transmitter according to any of claims 1 to 7; a passive optical point-to-multipoint network for connecting the control node and the at least one remote node.