HK1118652B - System and method for quantum key distribution over wdm links - Google Patents

Description

System and method for quantum key distribution over WDM links

Technical Field

The present invention relates to a communication system and method for transmitting encrypted data. In particular, the invention relates to a technique known as quantum key distribution over Wavelength Division Multiplexed (WDM) links.

Background

The aim of cryptography is to exchange messages with perfect secrecy between a transmitter and a receiver by using a secret random bit sequence, called a key. Subsequent messages may be securely transmitted over the conventional channel after the key is established. For this reason, secure key distribution is a fundamental problem in cryptography. In principle, however, classical signals are passively monitored, and thus conventional cryptography does not provide a tool for ensuring the security of key distribution. The transmitter and receiver do not know when the eavesdropping occurred.

However, secure key distribution may be achieved by using Quantum Key Distribution (QKD) techniques. Quantum key distribution is considered a natural alternative to traditional key distribution because it can provide fundamental security through the uncertainty principle of quantum mechanics, i.e. any eavesdropping behavior by an eavesdropper will inevitably modify the quantum state of the system. Thus, although an eavesdropper can get the information in the quantum channel by measurement, the transmitter and receiver will detect the eavesdropping and can change the key accordingly.

Various systems have been developed for performing QKD on optical fiber systems. Quantum cryptography has been used to achieve point-to-point distribution of quantum keys between two users. As shown in fig. 1, the prior art quantum cryptography system uses two different links. One of which is used to transmit the quantum cryptography over optical fiber, and the other of which transmits the entire data over the internet or another optical fiber.

However, it is desirable to apply quantum cryptography in commercial optical networks currently deployed. However, only a few studies on quantum key distribution over 1,300nm networks have been reported so far. One problem with the reported system is that it is difficult to transmit 1,300nm signals over long distances in standard single mode optical fiber. Therefore, in long-distance transmission, quantum key distribution with a wavelength of around 1,550nm is preferable. Further, it is believed that strong signals (e.g., legacy data) should not be present in a network with quantum channels, or that a large wavelength separation is required between quantum channels and legacy channels to reduce interference from strong signals.

However, this is not the case in established commercial optical networks, since in current optical fiber communication networks using WDM transmission, there are many strong signals that can cause severe interference to the quantum channels.

Disclosure of Invention

It is an object of the present invention to provide a communication system for quantum key distribution, wherein quantum key distribution can be achieved in current commercial optical links by simply adding a wavelength for a quantum channel as the quantum key distribution.

The invention provides a method of quantum key distribution between a plurality of transmitting units and a plurality of receiving units over a Wavelength Division Multiplexed (WDM) link, comprising 1) providing a plurality of WDM channels on the WDM link for respectively coupling the transmitting units with the receiving units, the WDM channels comprising a plurality of quantum channels and a plurality of legacy channels; 2) assigning a different wavelength to each of the WDM channels; 3) transmitting a single photon signal on each of said quantum channels; and 4) transmitting data on each of the conventional channels, the data comprising conventional data and a trigger signal for synchronizing the transmission of the single photons on the quantum channels.

In a preferred embodiment of the invention, the wavelength assigned to the WDM channels is around 1,550 nm.

The present invention further provides a communication system for distributing quantum keys over a Wavelength Division Multiplexed (WDM) optical link at wavelengths around 1,550nm, the system comprising: a plurality of emission units including a plurality of quantum emission units and a plurality of conventional emission units; a plurality of receiving units comprising a plurality of quantum receiving units and a plurality of legacy receiving units; and a WDM link linking the transmitting unit to the receiving unit. Furthermore, the WDM link includes a plurality of WDM channels, and the WDM channels may further include a plurality of quantum channels for transmitting single photon signals between the quantum emission unit and the quantum reception unit, respectively, and a plurality of legacy channels for transmitting data between the legacy emission unit and the legacy reception unit, respectively.

In some embodiments of the invention, the data transmitted over the conventional channel comprises conventional data or a trigger signal for synchronizing the transmission of the single photons over the quantum channel. Further, each of the WDM channels is assigned a wavelength that is different from each other such that the WDM channels are multiplexed in wavelength over the WDM link.

According to an aspect of the invention, quantum key distribution between specific users (e.g., transmitters and receivers) on a WDM link can be achieved by using WDM technology. The transmitter may include one or more quantum transmission units and one or more conventional transmission units. The receiver may include one or more quantum receiving units corresponding to the one or more quantum transmitting units, respectively, and one or more legacy receiving units corresponding to the one or more legacy transmitting units, respectively. Further, the WDM link linking the transmitter and the receiver may include one or more quantum channels for communicating single photon signals between the one or more quantum transmission units and the one or more quantum reception units, respectively, and one or more legacy channels for communicating data between the one or more legacy transmission units and the one or more legacy reception units, respectively. The data comprises conventional data or trigger signals for synchronizing the transmission of the single photons on the quantum channels. Further, each of the legacy channels and the quantum channels may be assigned a different wavelength from each other, such that the legacy channels and the quantum channels may be multiplexed in wavelength over the WDM link.

According to another aspect of the invention, the WDM link of the communication system may be a 3-channel WDM link comprising two quantum channels and one conventional channel. The data communicated over the legacy channel may include a trigger signal for synchronizing the quantum channel. Thus, the legacy channel may also function as a trigger channel to synchronize the system. Each of the legacy channels and the quantum channels are assigned a wavelength different from each other, and the legacy channels and the quantum channels are multiplexed by a wavelength of around 1,550nm suitable for long-distance transmission on the WDM link.

The quantum key distribution can be easily handled in current commercial fiber links by sharing a common fiber with conventional communication signals based on WDM technology that combines multiple different wavelengths into a single fiber provided by the WDM link.

In addition, the present invention uses a differential phase modulation (differential phase modulation) technique to overcome the influence of temperature offset and phase offset in the system, so as to stabilize the system.

Drawings

Fig. 1 illustrates a prior art communication system for quantum key distribution;

fig. 2 shows a schematic diagram of a communication system for quantum key distribution over a multi-user WDM network according to the present invention;

FIG. 3 shows a schematic diagram of quantum key distribution over a WDM link in accordance with the present invention;

FIG. 4 shows a schematic diagram of an embodiment of quantum key distribution over a 3-channel WDM link in accordance with the present invention;

FIG. 5 illustrates an auto-compensation architecture using differential phase modulation techniques for use in the quantum channel of the present invention; and

fig. 6a and 6b show a detailed architecture for implementing quantum key distribution on the 3-channel WDM link shown in fig. 4.

Detailed Description

The present invention will be described in detail below with reference to the accompanying drawings.

WDM is a key technology employed in the present invention that uses the parallel nature of light to combine multiple different wavelengths into a single optical fiber, thereby enabling quantum key distribution over multi-user WDM links according to the present invention. By means of WDM the system can establish simultaneously an allowed number of different secret keys depending on the number of wavelengths supported by the WDM.

For example, a communication system for quantum key distribution over a multi-user WDM network according to one embodiment of the present invention is shown in fig. 2. The communication system includes a code assigned with lambda₁-λ_NN quantum channels for connecting the N quantum transmitting units 130 and the N quantum receiving units 140 via WDM links; and is assigned λ_N+1To lambda_N+MFor connecting M conventional transmit units 330 and M conventional receive units 340 (where N and M are positive integers) via a WDM link. The WDM link includes Arrayed Waveguide Gratings (AWGs) 402 and 401 and a single optical fiber 500. In this embodiment, by using AWG401 and AWG 402, the wavelength (λ) is made different₁-λ_N+M) The quantum channel and the conventional channel are multiplexed in a single optical fiber 500. In this way, quantum key distribution between a particular quantum transmitting unit and quantum receiving unit can be achieved by using WDM techniques.

Fig. 3 illustrates an embodiment of quantum key distribution among particular ones of a plurality of users (e.g., between a transmitter and a desired receiver) over a WDM link in accordance with the present invention. As shown in fig. 3, the transmitter 711 has one or more quantum transmission units and one or more conventional transmission units. The receiver 721 has one or more quantum receiving units respectively corresponding to the one or more quantum transmitting units, and one or more conventional receiving units respectively corresponding to the one or more conventional transmitting units.

The WDM link linking the transmitter 711 and the receiver 721 comprises an AWG401, an optical fiber 501, and another AWG 402. The WDM link is for multiplexing one or more quantum channels between the quantum transmitting units and the corresponding quantum receiving units to transmit single photon signals and for multiplexing one or more conventional channels between the conventional transmitting units and the conventional receiving units to transmit data. In this embodiment, the data further comprises a trigger signal for synchronizing the transmission of the single photon signal on the quantum channel. Further, each of the conventional channel and the quantum channel is assigned with a wavelength different from each other, and the conventional channel and the quantum channel are multiplexed by a wavelength of about 1,550nm on the WDM link.

Fig. 4 shows an embodiment for quantum key distribution between a transmitter and a receiver over a 3-channel WDM link. Transmitter 712 includes a first quantum transmission unit 110, a second quantum transmission unit 210, and a conventional transmission unit 310. The receiver 722 includes a first quantum receiving unit 120 corresponding to the first quantum transmitting unit 110, a second quantum receiving unit 220 corresponding to the second quantum transmitting unit 210, and a legacy receiving unit 320 corresponding to the legacy transmitting unit 310. A 3-channel WDM link includes an AWG401, an optical fiber 502, and another AWG 402 for multiplexing two quantum channels 100 and 200 and one conventional channel 300. Quantum channels 100 and 200 are provided between the two quantum transmitting units 110 and 210 and the two quantum receiving units 120 and 220, respectively, for transmitting single photons (quantum keys). A legacy channel 300 is provided between the legacy transmitting unit 310 and the legacy receiving unit 320 for transmitting data. In this embodiment, the data includes the trigger signal S1 sent to the quantum emission units 110 and 210 and the trigger signal S2 sent to the quantum reception units 120 and 220, thereby synchronizing the single photon transmission on the quantum channels 100 and 200. The two quantum channels 100 and 200 and the conventional channel 300 are each assigned a different wavelength λ₁、λ₂And λ₃The wavelengths are all around 1,550nm compatible with standard optical links.

Thus, quantum key distribution can be conveniently implemented in current commercial optical links by simply adding another wavelength for the quantum channel as the quantum key distribution in the current commercial optical link. Furthermore, the fiber loss at 1,550nm optical wavelength is 0.2dB/km, which translates into a large increase in transmission distance compared to the fiber loss at 1,300nm wavelength for quantum cryptography systems of the same bit rate.

In quantum channels 100 and 200, the BB84 protocol may be used. To implement the BB84 protocol, it is necessary to have four states in two non-orthogonal bases, each with two orthogonal states. For example, four phases {0, π/2, π or 3 π/2} can be taken as four states. Furthermore, {0, π } corresponds to a basis, which can be achieved by choosing the measurement basis phase shift to be 0. Likewise, { π/2, 3 π/2} is another radical, which corresponds to a measurement choice with a phase shift of π/2. These four states can be represented by:

for "0",l '0'>＝1(|0>+|π/2>)

For "1",l '1'>＝1(|π>+|3π/2>)

It is clear from the above wave function that for a logical 0, the phase shifts 0 and pi/2 have a probability of 50% respectively. The same is true for a logic 1.

In the quantum channel of the present invention, an automatic compensation structure using a differential phase modulation technique is used. For example, as shown in fig. 5, in quantum channel 100 (which is similar to quantum channel 200), the phase shift Δ a provided by phase modulator 112 at transmitter 712 is added to the first of two adjacent pulses that propagate from receiver 722 to transmitter 712. When both pulses return to the receiver side after being reflected by the faraday rotator mirror 111, the phase shift Δ B provided by the phase modulator 122 at the receiver 722 is added to the second pulse. When the first and second pulses after being delayed by delay device 127 arrive at beam splitter 123, interference will occur and the phase difference will be Δ A- Δ B. Thus, only the phase difference remains. This arrangement allows the structure to compensate for errors experienced by two pulses propagating in the interference region due to temperature variations, polarization variations and path variations, since each of the two pulses interfering at the receiver side in each quantum channel experience the same change when propagating the same distance. Here we assume that another phase shift δ caused by temperature changes, polarization changes and distance changes is applied to two pulses in the same channel.

The phase shift δ typically changes at different times due to variations in the above factors. However, when the above-mentioned factors vary relatively slowly in the time interval between two adjacent pulses, the phase shift δ is almost equal for the two adjacent pulses because they undergo similar variations in the channel. For the first pulse, the phase shift is Δ a + δ, but for the second pulse, there is a phase shift δ + Δ B. Therefore, in the interference region on the receiver side, the phase difference between the two return pulses is Δ a- Δ B because the phase shift δ caused by the above-described factors can be cancelled out. Since quantum channel 200 is similar to quantum channel 100, the configuration of quantum channels 100 and 200 of the present invention can overcome fluctuations caused by changes in temperature, polarization, and distance. In theory, ideal interference can be achieved in this configuration.

The detailed structure of the principle of quantum key distribution over a 3-channel WDM link is explained below with reference to fig. 6a and 6 b.

In quantum channel 100, laser 124 at receiver 722 transmits a burst of 0dBm power into the WDM link through circulator 125. Each pulse in the pulse train will be split into two pulses, a first pulse and a second pulse, by the beam splitter 123 of 50/50. The first pulse passes through upper path 1231 with a 26ns delay set by delay device 127 (e.g., a delay line of optical fiber) before hitting polarizing beam splitter 121. The phase modulator 122 in the upper path 1231 is not used until the second pulse returns from the transmitter. The second pulse travels through a lower (shorter) path 1232 directly to the input port of the beam splitter 121.

After passing through the beam splitter 121, two pulses with orthogonal polarizations and a delay of 26ns with respect to each other are obtained. These two pulses then enter an Arrayed Waveguide Grating (AWG)402, propagate through, for example, 8.5km unicast fiber 502, enter another arrayed waveguide grating 401, and then exit the AWG401 in the channel 100 at transmitter 712.

The pulse is then again split by the beam splitter 115 of 90/10 and photons emitted from 90% of the port of the beam splitter 115 are detected by detector 113 for controlling the variable attenuator 114 to attenuate the return pulse to obtain a single photon pulse. Two pulses exiting the 10% port of the beam splitter 115 will pass first through the attenuator 114 without attenuation. It will then pass through the phase modulator 112 to the faraday mirror 111. After reflection by the faraday rotator mirror 111, the polarization of the two pulses is rotated by 90 °.

The random phase shift 0, pi/2, pi or 3 pi/2 generated by a random number signal generator (not shown) is then inserted into the first of the two return pulses by the phase modulator 112. The two return pulses are then attenuated to generate single photons in the pulses as they pass through the attenuator 114 again. The trigger signal S1 generated by the detector 313 is used to synchronize the phase modulator 112 to adjust the first return pulse from the faraday mirror 111 and to synchronize the attenuation control signal from the detector 113 to attenuate the two return pulses into single photons. The trigger signal S1 from the detector 313 should here have a suitable delay to synchronize the phase shifted signal from the data signal generator with the first return pulse. Likewise, the signal from detector 113 used to control attenuator 114 has an electrical delay to attenuate two light pulses as they pass through attenuator 114 in their return paths. Finally, after traversing AWG401, 8.5km of standard single mode fiber 502 and AWG 402, the two pulses return to receiver 722 through the opposite path between polarizing beam splitter 121 and beam splitter 123. Thus, both pulses can arrive at the beam splitter 123 simultaneously and produce interference of the holographic structure or destructive interference at the beam splitter 123 to enable the single photon detector 126 to detect single photons.

By setting the phase shift to 0 or pi/2 in the phase modulator 122, the receiver 722 can randomly and independently select the measurement basis that is synchronized by the trigger signal S2 derived from the pulse returned by the mirror 311 in the conventional data channel 300. The results are stored in the computer 600. All the fibres on the receiver side are polarization maintaining fibres, which are required in the system to ensure that the polarization of the two single photon pulses to be interfered is unchanged after they have passed through different paths of the interferometer.

Similar to the first quantum channel 100, the second quantum channel 200 includes a faraday mirror 211, a phase modulator 212, a detector 213, a variable attenuator 214, a 90/10 beam splitter 215, an AWG401, an optical fiber 500, an AWG 402, a polarization beam splitter 221, a phase modulator 222, a beam splitter 223, a laser 224, a circulator 225, a single photon detector 226, and a delay device 227. Since the configuration and principle of the channel 200 are similar to those of the quantum channel 100 except that the time delay set by the delay means 227 of the quantum channel 200 is 21ns, and an independent measurement basis and an independent random phase shift from the channel 100, a detailed description of the quantum channel 200 is omitted.

In the conventional channel 300, the common laser 324 launches a pulse with a power of 2dBm into 50/50 splitter 321 on the receiver side. After passing through 50/50 beam splitter 321, the pulse enters AWG 402, propagates in 8.5km single mode fiber 500, then passes through AWG401, and then half of the pulse is detected by detector 313. The detected pulses are applied as a first trigger signal S1 to synchronize the phase modulators 112 and 212 with their respective pulses in the quantum channels 100 and 200 with appropriate delays. The other half of the pulse will be reflected by mirror 311 to be returned to the receiver. The detector 326 will also detect the other half of the pulses to generate a second trigger signal S2 for triggering the single photon detectors 126 and 226, respectively, to measure the interference of the quantum signals and for triggering the phase modulators 122 and 222, respectively, to select the basis of the measurement on the receiver side.

The data communication channel 300 may also function as a normal optical communication channel with a high laser power, such as 2dBm emitted by the laser 324 in this embodiment. The wavelengths and pulse widths for the three channels are listed in table 1.

TABLE 1 wavelength and pulse Width

Channel with a plurality of channels	100	200	300
				Wavelength (nm)	1549.33	1551.18	1557.35
Pulse width (ns)	2.5	2.5	2.

The BB84 protocol is executed in the system. We use a 100kHz signal for phase modulation and synchronization. As shown in fig. 1, the pulse width is 2.5ns for quantum channels 100 and 200 and 20ns for conventional channel 300. To reduce crosstalk between channels, especially between weak quantum channels 100 and 200 and strong signal channels (legacy channel 300), the wavelengths must be carefully arranged. Here, the interval between the quantum channel 100 and the conventional channel 300 is about 8nm, and the interval between the quantum channel 200 and the conventional channel 300 is about 6 nm.

In this embodiment, single photon detectors 126 and 226 are used to measure single photons. The dark counts of the single photon detectors 126 and 226 are 40Hz in the gate mode at 100kHz with a measurement width of 2.5ns, and therefore the probability of measuring dark counts is 4.0X 10^-4. The efficiency of the single photon detectors 126 and 226 is greater than 10%. On the transmitter side, the average photon count per pulse should be less than 0.1 to ensure that in this embodiment a single photon is available from each pulse when the pulse passes through the variable attenuator 114 again. For a total transmission loss of 17dB, about 2% of single photons can be detected. After considering the 3dB loss due to the BB84 protocol, theoretically about 1% of single photons can be used for quantum key distribution.

TABLE 2 results of the experiment

Channel with a plurality of channels	100	200
			Key Rate (kb/s)	0.75	0.49
Error probability (%)	2.2	4.396

Experimentally, the count rate of the single photon detectors 126 and 226 is 100k/s and its efficiency is greater than 10%. To guarantee a single photon in a pulse, the average photon count per pulse should be less than 0.1 in embodiments of the present invention. Therefore, the count rate in the variable attenuators 114 and 214 should be below 10 k/s. According to this embodiment, the experimentally obtained count rate is 7.67 k/s. Considering the transmission efficiency, error rate, and detector efficiency, a 0.75kbps quantum key is available in channel 100, with 2.2% error probability due to crosstalk. Since the single photons in channel 200 are very weak, most of the error probability comes from channel 300 and a small part from channel 200. Also, in channel 200, the quantum key rate is 0.49kbps, and crosstalk causes an error probability of 4.396%. Because the wavelength of channel 200 is closer to the wavelength of a legacy communication channel than the wavelength of channel 100, the crosstalk in channel 200 is greater than the crosstalk in channel 100.

While the invention has been described in conjunction with embodiments thereof, it will be understood by those skilled in the art that the invention may be embodied in various other forms. It is intended that the scope of the invention be defined by the claims and not by the descriptions in the abstract and/or the detailed description in the specific embodiments.

Claims (17)

1. A method of distributing quantum keys among a plurality of transmit units and a plurality of receive units over a wavelength division multiplexed link, comprising:

providing a plurality of wavelength division multiplexed channels on a wavelength division multiplexed link for coupling the transmit unit and the receive unit, respectively, the plurality of wavelength division multiplexed channels comprising a plurality of quantum channels and a plurality of legacy channels;

assigning a different wavelength to each of the plurality of wavelength division multiplexed channels;

transmitting a single photon signal on each of said quantum channels; and

transmitting data on each of the conventional channels, the data comprising conventional data or trigger signals for synchronizing the transmission of the single photons on the quantum channels.

2. A communication system for quantum key distribution, comprising:

a plurality of emission units including a plurality of quantum emission units and a plurality of conventional emission units;

a plurality of receiving units comprising a plurality of quantum receiving units and a plurality of legacy receiving units; and

a wavelength division multiplexed link linking the transmitting unit to the receiving unit,

wherein the wavelength division multiplexing link comprises a plurality of wavelength division multiplexing channels, the wavelength division multiplexing channels comprise a plurality of quantum channels and a plurality of legacy channels, the plurality of quantum channels are respectively used for transmitting single photon signals between the quantum transmitting unit and the quantum receiving unit, the plurality of legacy channels are respectively used for transmitting data between the legacy transmitting unit and the legacy receiving unit,

wherein the data comprises legacy data or a trigger signal for synchronizing the transmission of the single photons on the quantum channels,

wherein each of the wavelength division multiplexing channels is assigned a different wavelength from each other so that the wavelength division multiplexing channels are multiplexed in wavelength on the wavelength division multiplexing link.

3. A method of distributing quantum keys between a transmitter and a receiver over a wavelength division multiplexed link, comprising:

providing two quantum channels and a legacy channel on a wavelength division multiplexed link;

assigning a different wavelength to each of the two quantum channels and the one legacy channel;

transmitting a single photon signal on each of the two quantum channels; and

transmitting data over the one conventional channel, the data including conventional data and a trigger signal for synchronizing the transmission of the single photons over the quantum channel.

4. The method of claim 3, wherein the wavelength range assigned to the two quantum channels and the one legacy channel is 1,475nm to 1,590 nm.

5. The method of claim 4, wherein the wavelengths assigned to the two quantum channels are 1,549.33nm and 1,551.18nm, respectively, and the wavelength assigned to the one legacy channel is 1,557.35 nm.

6. The method of claim 3, wherein the step of transmitting single photon signals on each of the two quantum channels comprises:

pulsing from a laser;

dividing the pulse into a first pulse and a second pulse;

delaying the first pulse;

transmitting the orthogonally polarized first and second pulses formed by the polarizing beam splitter through the wavelength division multiplexed link through a second arrayed waveguide grating, an optical fiber, and a first arrayed waveguide grating;

detecting a portion of the first pulse and a portion of the second pulse to control an attenuator for attenuating the first pulse and the second pulse in a backhaul;

reflecting said first and second pulses with a polarization rotation of 90 °;

modulating the first pulse by phase modulation;

attenuating, by the attenuator, the first pulse and the second pulse into a first single photon and a second single photon, respectively;

transmitting pulses of said first and second single photons through said first arrayed waveguide grating, said optical fiber and said second arrayed waveguide grating over said wavelength division multiplexed link;

modulating and delaying the second single photon pulse; and

interference between the first modulated single photon pulse and the second modulated single photon pulse is detected.

7. The method of claim 6, wherein the optical fiber is a standard single mode optical fiber.

8. The method of claim 6, wherein the step of transmitting data on the one legacy channel comprises:

pulsing from a laser;

transmitting a forward data pulse over the wavelength division multiplexed link through the second arrayed waveguide grating, the optical fiber, and the first arrayed waveguide grating;

detecting a portion of the forward data pulse as a first trigger signal;

reflecting another portion of the forward data pulse and transmitting the reflected data pulse over the wavelength division multiplexed link through the first arrayed waveguide grating, the optical fiber, and the second arrayed waveguide grating; and

the reflected data pulse is detected as a second trigger signal.

9. The method of claim 8, wherein said first trigger signal is used to synchronize said modulation of said first pulse on each of said quantum channels and to synchronize said attenuation of said first and second pulses into first and second single photons, respectively; the second trigger signal is used for synchronizing the modulation of the second single photon and synchronizing the detection of the interference result between the modulated first single photon and the second single photon.

10. The method of claim 3, wherein the quantum channel uses a BB84 protocol.

11. A communication system for quantum key distribution, comprising:

a transmitter comprising one or more quantum transmission units and one or more conventional transmission units;

a receiver including one or more quantum receiving units respectively corresponding to the one or more quantum transmitting units, and one or more legacy receiving units respectively corresponding to the one or more legacy transmitting units; and

a wavelength division multiplexed link for linking the transmitter and the receiver,

wherein the wavelength division multiplexed link comprises:

two quantum channels for transmitting single-photon signals between two quantum transmitting units and two quantum receiving units, respectively, and

a legacy channel for transmitting data between the legacy transmitting unit and the legacy receiving unit,

wherein the data comprises legacy data or a trigger signal for synchronizing the transmission of the single photons on the quantum channels,

wherein each of the legacy channel and the quantum channel is assigned a different wavelength from each other such that the legacy channel and the quantum channel are multiplexed in wavelength on the wavelength division multiplexed link.

12. The communication system of claim 11 wherein said single photon signal transmitted on each of said quantum channels is generated from a laser having a wavelength range of 1,475nm to 1,590 nm.

13. The communication system of claim 12, wherein the data has a wavelength in the range of 1,475nm to 1,590 nm.

14. The communication system of claim 13, wherein each of the quantum emission units comprises an attenuator for attenuating a laser signal into a single photon signal and a modulator for modulating the laser signal.

15. The communication system of claim 14 wherein each of said quantum receiving units comprises a modulator for modulating said single photon signal and a single photon detector for detecting said single photon signal.

16. The communication system of claim 15, wherein the legacy transmitting unit comprises a detector for detecting a portion of the data as a first trigger signal to synchronize the modulator of each of the quantum transmitting units.

17. The communication system of claim 16 wherein said legacy receiving units include detectors for detecting another portion of said data as a second trigger signal to synchronize said modulator and said single photon detector of each said quantum receiving unit.