WO2019053463A1

WO2019053463A1 - Apparatus and method for generating a quantum and classical signal

Info

Publication number: WO2019053463A1
Application number: PCT/GB2018/052638
Authority: WO
Inventors: Rupesh Kumar Parapatil SUBRAMANIAN; Adrian Wonfor; Richard Penty; Ian H. White
Original assignee: University Of York
Priority date: 2017-09-15
Filing date: 2018-09-14
Publication date: 2019-03-21
Also published as: GB201714843D0

Abstract

A transmitter system is proposed which is operative to generate pulses which each encode both quantum signals and classical signals, by splitting laser pulses generated into pump and signal pulses. The pump pulses are modulated with classical information (amplitude and/or phase), and the signal pulses are modulated with quantum information. The modulated pump and signal pulses are then allowed to interfere to generate a pulsed displaced signal in which each pulse encodes both the classical information in its phase and amplitude, and also the quantum information. The modulation may be performed on a loop path along which the pump and signal pass in opposite directions. A single beam-splitter both generates the pump and signal pulse, and causes the interference between them following the modulation.

Description

APPARATUS AND METHOD FOR GENERATING A QUANTUM AND CLASSICAL

SIGNAL

Technical Field of the invention

The present invention relates to quantum communication, and more precisely to a transmitter apparatus for generating a single pulse carrying both quantum-encoded information and classically-encoded information. It further relates to a communication system incorporating the transmitter apparatus, and a method employing the transmitter apparatus.

Background of the invention

Exchanging information between a transmitter and a receiver requires that the transmitter and receiver both know the signal format, timing, amplitude and phase, etc. In many cases, the signal is used to transmit this information. For example, the receiver is able to evaluate the timing of the signal (e.g. a clock signal embedded within the signal) and phase of the signal by continuous monitoring of the signal itself. However, this requires that the signal power is high enough: of the order of a few thousand photons per pulse, so that each signal contributes to a detectable event at the receiver.

In quantum communication, exchange of information is carried out at a much lower signal power: of the order of less than one or a few photons per pulse, not all of which may reach the receiver, especially in the case of long distance transmission. This is true for quantum key distribution (QKD) where information is encoded at quantum level using few photons and decoded by a rigorous quantum measurement involving photon detection. A quantum transmitter and receiver have to be synchronized for proper measurement of the quantum signal and decoding of the information. In most QKD systems, it is a difficult task to synchronise the receiver's measurement and also the detection with the transmitter by monitoring the signal. As an example, in Discrete Variable quantum key distribution (DV-QKD), the photons that carry information in their polarization, phase, etc. may not contribute a detectable event at the receiver due to transmission loss and low detector efficiency. However, this difficulty can be overcome by using an additional intense synchronization signal at a different wavelength or polarization or time than that of the quantum signal. The receiver separates this sync signal from the transmission channel, converts it to electrical pulses and then uses it as the clock for the measurement and subsequent detection and data acquisition.

In Continuous Variable quantum key distribution (CV-QKD), information is encoded in amplitude and phase, otherwise called quadrature, of the signal. The weak signal, consisting of a few or a few tens of photons per signal, is decoded by homodyne/heterodyne detection at the receiver against a strong reference pulse. This reference pulse is called the local oscillator, and is normally sent by the transmitter at a different polarization and/or time from that of the signal. At the receiver, part of the local oscillator is converted into clock signals and then used to synchronize measurements and detection. The local oscillator also serves as a phase reference for the signal measurement.

In a CV-QKD system with a "local local oscillator" (LLO-CVQKD) the local oscillator signal is generated at the receiver, and a low intensity reference signal of few thousand photons is sent for synchronizing the receiver with the transmitter. Here the reference signal is used to lock the phase and clock of the local local oscillator.

Thus, all the above quantum signal transmission methods are assisted by using, in addition to the quantum pulse, a comparatively intense "classical pulse" for receiver synchronization, to realize efficient quantum communications. The classical synchronization pulse consumes additional resources such as wavelength, polarization, timing, etc., so it reduces the bandwidth of the overall quantum transmission. Moreover, the classical pulse carries no useful information other than clock or phase information.

Quantum signal displacement using an intense pump signal applied to an asymmetric beam splitter has theoretically been described by Matteo G. A. Paris in Physics Letters A 217 78 (1996). This operation preserves the information encoded on the quantum signal, but "displaces" it (adjusts its amplitude and/or phase) by an amount decided by the amplitude and phase of the pump signal.

Simultaneous transmission of quantum and classical information has been theoretically proposed for continuous variables by Bing Qi in Physical Review A 94, 042340 (2016), where displacement of the quantum signal is used to transmit classical information. At the receiver, the displaced signals are measured using vacuum noise limited coherent receivers. Quantum signal information is extracted from the measurement outcome by subtracting the magnitude of the displacement which is publically disclosed by the transmitter. However, this paper gives no information about how the transmitter should be constructed, and a transmitter for such signals has not been proposed to date.

Summary of the invention

In general terms, the present invention proposes a transmitter system which is operative to generate pulses which each encode both quantum signals and classical signals, by splitting pulses generated by a laser into pump and signal pulses, modulating the pump pulse with classical information (amplitude and/or phase), and modulating the signal pulse with quantum information. The modulated pump and signal pulses are then allowed to interfere to generate a pulsed displaced signal in which each pulse encodes both the classical information in its phase and amplitude, and also the quantum information.

Encoding the pulses in this way makes it unnecessary also to transmit a "classical pulse" as in the known techniques described above. This reduces the resources required to perform continuous variable quantum communication and increases the available bandwidth.

For example, the invention makes it possible to use a single laser source to transmit both the classical information and the quantum information.

Furthermore, as compared to a conceivable quantum communication system (e.g. a continuous variable quantum communication system using a "local local oscillator" configuration), in which the transmitter transmits classical and quantum information multiplexed by time and/or polarization (e.g. by using alternate pulses to transmit the classical or quantum information), an embodiment of the present invention may substantially double the bandwidth.

A specific expression of the invention is a transmitter apparatus for a communication system, the transmitter apparatus comprising:

a laser source for generating laser pulses; a pump-signal beam splitter unit arranged to split received pulses into a pump pulse and a signal pulse, and to direct the pump pulse and the signal pulse in opposite directions along a loop path; and

a modulation system on the loop path for performing classical modulation on the pump pulse, and quantum modulation on the signal pulse;

the pump-signal beam splitter unit being arranged to receive the modulated pump pulse and the modulated signal pulse, and to cause interference between the modulated pump pulse and the modulated signal pulse to generate a pulsed displaced signal in which each pulse encodes both quantum and classical information;

the transmitter apparatus further comprising:

an interface for transmitting the displaced signal along a communication path to a receiver apparatus; and

a circulator unit for directing laser pulses to the pump-signal beam splitter unit, and the displaced signal from the pump-signal beam splitter unit to the interface.

Since the signal pulse and the pump pulse travel the same distance around the loop path, when they interfere they have a constant phase relation (specifically, their relative phase difference is zero). In other words, the loop path automatically compensates for any random variations in the components on the loop path, such as variations due to temperature fluctuations. Providing a loop path to do this is straightforward, reliable and stable.

Optionally, the transmitter apparatus may also include a supplementary beam splitter arranged to receive laser pulses from the laser, and split each laser pulse into a first pulse which is transmitted to the pump-signal beam splitter, and a second pulse which has a higher intensity than the displaced signal. The second pulse is sent to the interface, which transmits the second pulse to the receiver to act as a phase reference or amplitude reference. The modulation system preferably includes a phase modulator which modulates the phase of the pump signal under the control of a phase modulation signal. It further includes an amplitude modulation unit. The amplitude modulation unit may comprise one or more amplitude modulators for modulating the pump pulse and signal pulse as the pump and signal pulses pass through them. The modulation system preferably modulates each of a plurality of electromagnetic wavelengths (that is, wavelength ranges) of the electromagnetic radiation of the pump pulse and/or the signal pulse to encode different respective information. Thus, it may implement wavelength divisional multiplexing (WDM), substantially increasing the bandwidth of the communication to the receiver.

The transmitter apparatus may also include a feedback based control system, which may also facilitate calibration of the transmitter apparatus. The control system generates control signal(s) to modify the operation of the transmitter apparatus based on measurement signal(s) generated by at least one monitor unit. Each monitor unit is operative to receive a pulse or signal generated within the transmitter apparatus. From it, the monitor unit generates a measurement signal, which may be indicative of a property of at least one of the pump pulse and signal pulse. For example, the interference in the interference unit may generate, in addition to the displaced signal, a monitor pulse which is transmitted to a monitor unit. The monitor unit may measure the amplitude and/or phase of the monitor pulse, and send a measurement signal to the control system. In another example, the amplitude modulation unit may comprise a monitor unit which monitors measurement signal(s) based on signals generated by the amplitude modulators, which may be components of the modulated pulses.

The pump-signal beam splitter may be provided as a variable beam splitter controlled by the control system. For example, it may be controllable to vary the relative amplitudes of the pump pulse and signal pulse when they are generated. It may also be controllable to vary the amplitude of the displaced signal.

Conveniently, at least one optical delay unit is provided on the loop path (e.g. a variable delay unit controlled by the control system). Each delay unit may control the control time at which one of the signal pulse or pump pulse arrives at the modulation system. For example, one optical delay unit may ensure that the signal pulse does not reach the modulation unit or the amplitude modulation unit while that unit is operating on the pump pulse. The intensity of the modulated signal pulse is preferably much less than that of the modulated pump pulse. To facilitate this, the transmitter apparatus preferably comprises at least one directional attenuator (isolator) on the loop path. Each attenuator is arranged to attenuate the signal pulse to a greater degree than a pump pulse travelling in the opposite direction along the loop path. One or more of the directional attenuators may be implemented as a tuneable directional attenuator, which is controllable by a control signal generated by the control system.

For example, in one form, the tuneable directional attenuator may be implemented as at least one light blocking optical element (e.g. a combination of a polarizer and a Faraday rotator) located on the loop path, and a magnetic field generation element arranged to generate a controllable magnetic field inside the light blocking optical element (in a portion of the loop path which is on a side of the polarizer on which the signal pulse approaches the polarizer). By controlling the magnetic field, the polarization of the signal pulse can be modified, so as to modify the proportion of the signal pulse which passes through the light blocking optical element. Conversely, the pump pulse, which approaches the light blocking optical element from the other side, may have a polarization equal to that of the polarizer, so it is substantially completely passed by the light blocking optical element. This is true irrespective of the strength of the controllable magnetic field, which the pump pulse first encounters after passing through the light blocking optical element. Note that the tuneable directional attenuator may have other applications than in the transmitter apparatus defined above, and constitutes an independent inventive concept. The invention may alternatively be expressed as a communication system including the transmitter apparatus and a receiver apparatus, or as a method carried out by the transmitter apparatus or by the communication apparatus.

Brief description of the drawings

Preferred embodiments of the invention are described in the following reference to the drawings, which illustrate the preferred embodiments of the invention without limiting the same. In the drawings:

Fig. 1 shows a transmitter apparatus according to an embodiment of the invention for generating simultaneous quantum and classical signal on a single pulse; Fig. 2 shows an amplitude modulation unit of the transmitter apparatus of Fig. 1 ; and

Fig. 3 illustrates a variable directional attenuator of the transmitter apparatus of

Fig. 1 .

Description of preferred embodiments

Referring firstly to Fig. 1 , a transmitter apparatus is shown which is an embodiment of the invention. The transmitter comprises a pulsed laser source (10) operative to generate pulsed light and transmit it along a light transmittal path. The term "light" is not to be understood as requiring that the light is necessarily of a frequency within the visible spectrum.

A beam splitter (1 1 ) is provided in the light transmittal path, and splits each light pulse into a high intensity pulse (R1 ) and a low intensity pulse (L1 ). The direction in which each of the pulses is transmitted is indicated by arrows in Fig. 1 . The high intensity pulse (R1 ) is transmitted to an interface 19 of the transmitter apparatus where the transmitter apparatus interfaces with a communication path 20 having two signal channels 21 , 22. The communication path 20 leads to a remote receiver apparatus (not shown). The interface 19 feeds the high intensity pulse (R1 ) to the signal channel 21 for transmission over the communication path 20 the receiver apparatus. The receiver apparatus uses the high intensity pulse (R1 ) for power monitoring or phase reference estimation. The high intensity pulse (R1 ) may be used to lock the phase of a second laser at the receiver.

Meanwhile, the low intensity pulse (L1 ) passes to an circulator (12). The circulator (12) re-directs the low intensity pulse (now referenced as L2 in Fig. 1 ) to a pump-signal beam splitter, implemented as a 2x2 beam splitter, and in particular as a 2x2 variable beam splitter (13). The pump-signal beam splitter (13) splits the pulse L2 into comparatively high intensity pulse called a pump pulse (P1 ), and comparatively low intensity pulse called a signal pulse (S1 ). The splitting ratio of the pump-signal beam splitter (13) may be set to greater than 99 percent for the pump pulse (P1 ) and less than 1 percent for the signal pulse (S1 ). The pump-signal beam splitter (13) is located on a loop path 17. The loop path is light signal path which is arranged in a loop, as in a Sagnac interferometer. The pump- signal beam splitter (13) directs the pump pulse P1 and the signal pulse S1 to propagate along the loop path in anticlockwise and clockwise directions, respectively.

A tuneable directional attenuator (B) allows the pump pulse (P1 ) to pass substantially without any attenuation, following which the pump pulse is designated P2. The amplitude of the pump pulse (P2) is modulated by the amplitude modulation unit (A), as described below with reference to Fig. 2. The resultant amplitude-modulated pulse (P3) then passes through a phase modulator (15). In this way, the amplitude modulation unit (A) and the phase modulator (15) encode classical information respectively on the amplitude and phase of the pump pulse (P2).

The signal pulse S1 passes first through an optical delay unit (14). Following that, quantum information is encoded on the phase and amplitude of the signal pulse (S1 ). First, phase information is encoded on the signal pulse by the phase modulator (15), to form a phase modulated pulse (S2). Then, the phase modulated pulse S2 passes through the amplitude modulation unit A for amplitude modulation. In this way, the amplitude modulation unit (A) and the phase modulator (15) encode quantum information respectively on the amplitude and phase of the signal pulse (S1 ), to form a modulated signal pulse (S3).

As mentioned above, the optical delay unit (14) gives a selected delay to the signal pulse (S1 ). This ensures that the signal pulse does not reach the phase modulator (15) while the phase modulator (15) is phase modulating the pump pulse (P3), or the amplitude modulation unit while the amplitude modulation unit is amplitude modulating the pump pulse (P2). Thus, the signal pulse is not affected by the phase and amplitude modulation of pump pulse. Thus, this allows the pump pulse and signal pulse to be modulated in separate respective time windows.

The tunable directional attenuator (B) attenuates the signal pulse (S3) to a desired level. Similarly, the amplitude modulation unit provides high amplitude extinction of the signal pulse (S2). In fact, the amplitude modulation unit (A) and tuneable directional attenuator (B) together attenuate the signal pulse down to a quantum distribution of a few photons per pulse. By contrast, the amplitude modulation unit (A) may apply low amplitude extinction when modulating the pump pulse.

The modulated signal pulse (S4) is called the quantum signal pulse and the modulated pump pulse (P4) is called the classical pump pulse. At the variable beam splitter (13), the classical pump pulse (P4) displaces the quantum signal pulse (S4) and generates a displaced signal (D1 ). The amount of displacement is very much greater than the quantum signal variance. The displaced signal (D1 ) therefore includes classical and quantum signal information. The direction and amplitude of the displacement indicates the classical information and is set by the phase and amplitude of the classical pump pulse (P4).

For example, setting the phase of the classical pump pulse (P4) selectively to either 0 or 180 degrees, generates displaced signals which are displaced in opposite respective phase directions. These can be used respectively to encode bit 0 and bit 1 of the classical information. More generally, multi-level phases (in which the phase of the pump pulse is selected from a subset of two of more phase values) and/or multi-level amplitude values (in which the amplitude of the pump pulse is set to one of a set of multiple amplitude values) of the classical pump pulse can used to represent multiple bits of information.

The amplitude and/or phase modulation of the signal pulse, and/or the amplitude and/or modulation of the pump pulse, may encode different respective information for each of a plurality of wavelength ranges within the wavelength range of the laser pulses. Thus, wavelength divisional multiplexing is performed.

The circulator (12) directs the displaced signal (D1 ), now labelled D2, to the interface 19, where the interface transfers it to the signal channel 22. The output pulse D2 is then transmitted to the receiver through the signal channel 22.

As well as the displaced signal (D1 ) the variable beam splitter generates a monitor pulse M1 . The monitor (16) is used to measure the monitor pulse M1 producing a result which is sent to a control system (not shown) for calibration and/or feedback purposes. Fig. 2 shows the configuration of the amplitude modulation unit (A). It includes a first amplitude modulator (A1 ) and a second amplitude modulator (A2). The overall extinction ratio, which is the difference between the maximum and minimum of the transmittance, of the amplitude modulators (A1 , A2) is greater than 60dB. Both amplitude modulators (A1 , A2) may have identical characteristics. The amplitude modulation unit (A) further comprises a signal monitor (A3). The reason for providing two amplitude modulators (A1 , A2) is that it is difficult (albeit possible) to provide 60dB modulation with a single amplitude modulator. Moreover, providing two modulators gives more flexibility in providing modulation. For example, one of the amplitude modulators (e.g. A1 ) may be used to set the pump and signal attenuation levels, while the other amplitude modulator (A2) is used to set the modulation pattern.

At a given time the pump pulse (P2) or the phase modulated signal pulse (S2) arrives at the amplitude modulation unit A. For each pulse, identical modulation patterns are applied to it by the amplitude modulators (A1 , A2). For each amplitude modulator, a portion of the modulated light is sent to the signal monitor (A3). The signal monitor (A3) is used for characterising the classical pump pulse (P3) and quantum signal pulse (S3) and is also used for evaluating feedback information for amplitude modulation correction. For example, the control system may compare an output of the signal monitor (A3) to that of the monitor (16).

In a variation of the embodiment, more than two amplitude modulators in series can be used for attaining a higher extinction ratio. Similarly, a single amplitude modulator of extinction ratio greater than 60dB can be also used in place of multiple amplitude modulators.

Figure 3 shows the construction of the tuneable directional attenuator (B). The directional attenuator achieves an attenuation of 30dB or higher isolation in one direction (the direction in which the signal pulse (S3) is travelling), and negligible attenuation in the opposite direction (the direction in which the pump pulse P1 is travelling).

The tuneable directional attenuator includes a light blocking optical element (31 ) of the type that is based on the effect of a magnetic field on the polarization of light (a Faraday optical isolator), and a magnet (32) (a permanent magnet or an electromagnet) which is mounted on a translational stage (33). The translational stage (33) can move the magnet (32) (horizontally and/or vertically) under the control of the control system to vary the magnetic field inside the light blocking element on the loop path. This modifies the internal magnetic field of the light blocking element (32) and thereby modifies the isolation in a range from zero to a maximum isolation value. The magnetic field applied by the magnet 32 has no effect on the negligible attenuation in the opposite direction.

Finally, the figure of merit of the displaced signal (D2) can be evaluated, if required, with the help of the high intensity pulse (R1 ) with proper quantum noise limited detections. This can be done by providing the transmitter apparatus with another calibrated receiver unit (not shown) which is able (e.g. selectively) to receive the displaced signal (D2), and optionally the high intensity signal (R1 ), other than over the communication path. This optional additional receiver unit is a quantum noise limited detector, and produces calibrated value(s) (a figure of merit) which are compared with the values received by the remote receiver, to identify channel parameters such as channel propagation error, channel attenuation etc. The remote receiver cannot determine these parameters on its own, since the figure of merit has to be measured without a channel.

Although only a single embodiment has been described in detail, many variations are possible within the scope of the invention as defined in the claims. For example, the optical delay unit (14) may be provided as a tuneable optical delay unit which is controllable by the control system. This enables tuneability in pulse repetition rate. Furthermore, as noted above, the high intensity pulse (R1 ) is used to lock the phase of a laser at the receiver, and in such cases classical information can be encoded on the amplitude of R1 . Doing this may slightly increase the error transmission rate of the quantum information. Furthermore, although in the embodiment the pulse (R1 ) sent over channel 21 is described as a "high intensity pulse" in contrast to the "low intensity pulse" (L1 ) sent to the circulator (12), the intensity of the former pulse (R1 ) is not necessarily higher than that of the latter pulse (L1 ). The amplitude of pulse (R1 ) determines the sensitivity of the remote receiver if it is directly used for measurement, but if the pulsed laser (10) has a sufficiently high intensity, the ratio of the intensities of (R1 ) and (L1 ) can be of any value. Particularly in the case that the remote receiver includes a second laser, so that (R1 ) is mainly used for locking the phase of that second laser, (L1 ) can have a higher amplitude than (R1 ).

Claims

CLAIMS:

1. A transmitter apparatus for a communication system, the transmitter apparatus comprising:

a laser source for generating laser pulses;

a pump-signal beam splitter unit arranged to split received pulses into a pump pulse and a signal pulse, and to direct the pump pulse and the signal pulse in opposite directions along a loop path; and

the transmitter apparatus further comprising:

2. A transmitter apparatus according to claim 1 in which the loop path comprises at least one optical delay unit operative to modify the time at which one of the signal pulse and pump pulse arrives at the modulation system.

3. A transmitter apparatus according to claim 1 or claim 2, further comprising one or more directional attenuators arranged on the loop path, and each arranged to attenuate the signal pulse to a greater degree than the pump pulse.

4. A transmitter apparatus according to any preceding claim in which the modulation system comprises a phase modulation unit operative to modify a phase of the pump signal based on a phase control signal.

5. A transmitter apparatus according to any preceding claim in which the modulation system includes an amplitude modulation unit which comprises one or more amplitude modulators for the signal pulse and pump pulse.

6. A transmitter apparatus according to any preceding claim, further comprising at least one monitor unit operative to generate a measurement signal indicative of a property of at least one of the pump pulse and signal pulse, and a control system operative to modify the operation of the transmitter based on the measurement signal.

7. A transmitter apparatus according to claim 6 in which the pump-signal beam splitter unit is a variable beam splitter, and the control system is operative to modify the operation of the variable beam splitter.

8. A transmitter apparatus according to claim 6 or claim 7, when dependent upon claim 3, in which at least one of the directional attenuators is a tuneable direction attenuator which is operative to modify the degree of attenuation it applies to the signal pulse based on a control signal generated by the control system.

9. A transmitter apparatus according to claim 8 in which the tuneable directional attenuator comprises at least one light blocking optical element located on the loop path and based on magnetic field effect on polarization, and a magnetic field generation element external to the light blocking optical element and arranged to generate a controllable magnetic field in the light blocking optical element.

10. A transmitter apparatus according to any of claims 6 to 9, in which the interference unit is operative, upon interference between the modulated pump pulse and the modulated signal pulse, to generate a monitor pulse, and to transmit the monitor pulse to a said monitor unit.

1 1. A transmitter apparatus according to any of claims 6 to 10, when dependent upon claim 5, in which the respective amplitude monitors are operative to generate respective monitor signals, and the system further comprises a said monitor unit arranged to receive the monitor signals.

12. A transmitter apparatus according to any preceding claim further comprising a supplementary beam splitter arranged to receive laser pulses from the laser, and split each laser pulse into a first pulse which is transmitted to the pump-signal beam splitter unit, and a second pulse, the interface being additionally arranged to transmit the second pulse to the receiver, the second pulse having a higher intensity than the displaced signal.

13. A transmitter apparatus according to any preceding claim which the modulation system modulates each of a plurality of electromagnetic wavelengths of the pump pulse and/or the signal pulse to encode different respective information, thereby transmitting information by wavelength divisional multiplexing (WDM).

14. A communication system comprising a transmitter apparatus according to any preceding claim and a receiver apparatus operative to receive the displaced signal, and to extract from the displaced signal the classically encoded information and the quantum-encoded information.

15. A communication system according to claim 14 in which the receiver apparatus uses the classically encoded information to determine at least one of a clock signal and a phase reference, and extracts quantum-encoded information from the displaced signal based on at least one of the clock signal and the phase reference.

16. A communication system according to claim 14 or claim 15 in which the receiver apparatus generates a local oscillator signal.

17. A communication system according to any of claims 14 to 16, when dependent on claim 12 in which the receiver apparatus is operative to use the second pulse for phase reference or amplitude reference.

18. A method according of generating a signal, the method comprising:

using a laser source to generate laser pulses;

splitting the pulses into a pump pulse and a signal pulse;

performing classical modulation on the pump pulse, and quantum modulation on the signal pulse;

causing interference between the modulated pump pulse and the modulated signal pulse, and to generate a pulsed displaced signal in which each pulse encodes both quantum and classical information; and

transmitting the displaced signal along a communication path to a receiver apparatus.