Optical transmitter for quantum key distribution system or quantum random number generator

Abstract

To provide an optical transmitter that accelerates a pulse generation rate, a quantum key distribution (QKD) system, and a quantum random number generator (QRNG), which are means for generating optical pulses with random phases.SOLUTION: A system including a QKD transmitter comprises: a laser source 103 that generates a plurality of pulses of light for output by a transmitter 101; a pump for the laser source; and a light source 117 that emits a plurality of photons via spontaneous emission. The light source is coupled to a cavity of a distributed feedback (DFB) laser 103 such that the plurality of photons emitted by the light source will seed the plurality of pulses of light generated by the laser. Each of successive pulses has a random phase.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The embodiments described herein relate to optical transmitters for quantum key distribution systems or quantum random number generators. [Background technology]

[0002] The generation of optical pulses with random phase at high speed is a key component for quantum communication systems. Most protocols in quantum communication, including decoy-state quantum key distribution (QKD), measurement device independent (MDI) QKD, twin-field (TF) QKD, and asynchronous MDI-QKD, require that the transmitted optical pulses have random phases. The generation of optical pulses with random phases is also the basis for phase-noise-based quantum random number generators (QRNGs).

[0003] Both conventional QKD transmitters and QRNGs rely on the spontaneous emission process in gain-switched distributed feedback (DFB) diode lasers for phase randomization. During gain switching, the DFB laser is periodically driven below and above the lasing threshold to generate a train of pulses. When the laser is driven below its threshold, oscillation is suppressed and spontaneous emission becomes the dominant emission process. When the laser is again driven above threshold, the generation of optical pulses is seeded by spontaneously emitted photons present in the laser cavity. As a result of each optical pulse being seeded by spontaneous emission, the pulses are all given a random phase.

[0004] A problem that arises in conventional systems is that when a laser is driven below the lasing threshold, photons from a previous pulse generation event may still be present in the laser cavity. If the next pulse generation event is seeded by these residual photons, the newly generated pulse will inherit the phase from the previous pulse, thus resulting in phase correlation between the pulses. To mitigate this, it is necessary to provide sufficient "downtime" for the laser between pulses; in other words, the laser must remain below the lasing threshold long enough to ensure that all residual photons have escaped the laser cavity and that spontaneously emitted photons have become the dominant photons in the cavity. However, this extended downtime results in a limit to the maximum clock rate at which the laser can be operated, which limits the pulse generation rate and therefore performance.

[0005] Conventional approaches also require careful optimization of the laser diode's driving conditions due to its complex dynamics. For example, if the laser's bias current is too high, phase correlation between pulses can also occur. For long-term operation, the laser can also deviate from the optimal driving conditions, reducing the phase randomization effect.

[0006] Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 illustrates a system comprising a QKD transmitter according to one embodiment. [Figure 2] FIG. 2 illustrates a system including a quantum random number generator (QRNG), according to one embodiment. [Figure 3] FIG. 3 shows the intensity distribution seen when successive pulses output by a conventional laser diode are interfered with one another by passing the pulses through an asymmetric Mach-Zehnder interferometer. [Figure 4] Figure 4A shows the intensity distribution obtained during an experiment in which a standard laser diode was gain-switched at 8 GHz and the output pulses were sent to an asymmetric Mach-Zehnder interferometer. Figure 4B shows the autocorrelation of the pulses for the same experimental setup as in Figure 4A. [Figure 5] Figure 5A shows an intensity distribution obtained during an experiment in which pulses are seeded by spontaneous emission of photons from an external light source at 8 GHz, according to one embodiment. Figure 5B shows the autocorrelation of the pulses for the same experimental setup as in Figure 5A. [Figure 6] Figure 6A shows the intensity distribution obtained during an experiment in which a standard laser diode was gain-switched at 10 GHz and the output pulses were sent to an asymmetric Mach-Zehnder interferometer. Figure 6B shows the autocorrelation of the pulses for the same experimental setup as in Figure 6A. [Figure 7] Figure 7A shows an intensity distribution obtained during an experiment in which pulses are seeded by spontaneous emission of photons from an external light source at 10 GHz, according to one embodiment. Figure 7B shows the autocorrelation of the pulses for the same experimental setup as in Figure 7A. [Figure 8] Figure 8A shows a zoomed-in view of the autocorrelation results of Figure 5B. Figure 8B shows a zoomed-in view of the autocorrelation results of Figure 7B. DETAILED DESCRIPTION OF THE INVENTION

[0008] According to a first embodiment, there is provided an optical transmitter, a laser source configured to generate a plurality of pulses of light output by the optical transmitter; a pump for the laser source; a light source configured to emit a plurality of photons via spontaneous emission, the light source being coupled to a cavity of the laser source such that the plurality of photons emitted by the light source seed a plurality of pulses of light generated by the laser source, each successive pulse having a random phase; An optical transmitter is provided, comprising:

[0009] The laser source may be a gain-switched laser source.

[0010] The light source may be an amplified spontaneous emission (ASE) light source. The light source may be any one of a superluminescent diode, a semiconductor optical amplifier, and a fiber amplifier.

[0011] The optical transmitter may further comprise an optical filter that selects one or more wavelengths of light from the light source to be coupled into the laser source cavity.

[0012] The optical transmitter may further comprise an optical amplifier that amplifies the light output by the light source before the light source is coupled into the laser source cavity.

[0013] The optical transmitter may further include an optical filter that filters the plurality of pulses generated by the laser source.

[0014] The laser source may be a solid state laser source. The laser source may be a distributed feedback (DFB) laser.

[0015] According to a second embodiment, there is provided a quantum key distribution (QKD) system including the optical transmitter according to the first embodiment.

[0016] The QKD system may include an interferometer having a first arm and a second arm. Each pulse output by the optical transmitter may be input to the interferometer. The first arm of the interferometer may include a delay line. The second arm of the interferometer may include a phase modulator.

[0017] The QKD system may include an intensity modulator that modulates the intensity of the pulses output by the optical transmitter.

[0018] According to a third embodiment, there is provided a quantum random number generator (QRNG) including the optical transmitter according to the first embodiment. The QRNG may include an interferometer including a first arm and a second arm. Each pulse output by the optical transmitter may be input to the interferometer. The first arm of the interferometer may include a delay line. The QRNG may include a photodiode that detects the multiple pulses traveling along the first and second arms of the interferometer.

[0019] The embodiments described herein provide a simple means of transmitting a train of optical pulses with random phase, which can facilitate robust and high-speed operation of both QKD and QRNG.

[0020] 1 shows an example of an optical transmitter 101 for use in a QKD system, according to one embodiment. The transmitter 101 comprises a gain-switched laser 103 that is used to generate a train of phase-randomized pulses. The pulses are output through an intensity modulator 105 and an asymmetric Mach-Zehnder interferometer 107 with a delay line 109 and a phase modulator 111.

[0021] In this embodiment, the gain-switched laser 103 comprises a solid-state diode laser in the form of a distributed feedback (DFB) laser. The gain-switched laser 103 is electronically pumped and has a DC bias 113 and an AC input 115 that can be used to modulate the gain of the laser 103 and to drive the laser 103 above and below the lasing threshold.

[0022] Transmitter 101 also includes a light source 117 that serves as a spontaneous light emission source. The light source comprises an amplified spontaneous emission (ASE) light source, such as a superluminescent diode (it will be appreciated that other forms of ASE may also be used, including, for example, a semiconductor optical amplifier or a fiber amplifier). Photons emitted by light source 117 are injected into the cavity of gain-switched laser diode 103. If high injection power is required, the spontaneous emission from source 117 may be amplified using an optical amplifier, such as an erbium-doped optical fiber (not shown in FIG. 1 ); alternatively, an amplified spontaneous emission light source, such as a superluminescent diode, may be used as light source 117. Optical filters may be used to prevent unwanted wavelengths of light from entering the laser cavity.

[0023] By injecting spontaneously emitted photons from source 117 into the cavity of gain-switched laser 103, the number of spontaneously emitted photons present in the cavity can be increased so that they dominate the laser cavity even in the presence of residual (stimulated) photons from earlier pulse-generation events. Thus, each time the gain-switched laser is driven above the lasing threshold, the subsequent laser pulse will be seeded by the spontaneously emitted photons injected into the cavity from source 117, rather than by residual photons from earlier pulse-generation events. Accordingly, the downtime required before each drive of the gain-switched laser above the lasing threshold can be reduced, allowing the gain-switched laser to operate at higher pulse repetition rates. In this way, embodiments enable the generation of phase-randomized pulse trains at clock rates exceeding current state-of-the-art QKD systems.

[0024] The optical transmitter 101 may be embodied as an on-chip optical source that produces a train of pulses with random phase, and may be used in a QKD system that implements BB84, MDI QKD, or any one of other QKD protocols that require phase-randomized coherent states.

[0025] In addition to QKD systems, the transmitter components shown in Figure 1 can be used to improve the operation of quantum random number generators (QRNGs). Conventional QRNGs rely on detecting fluctuations in the intensity of light output by a spontaneous emission source. The intensity fluctuations caused by spontaneous emission have a quantum mechanical origin and follow a Gaussian distribution. However, intensity fluctuations resulting from classical noise also have a Gaussian distribution. Therefore, conventional QRNGs that utilize intensity fluctuations due to spontaneous emission require additional precautions to ensure that the extracted randomness arises from quantum mechanical effects rather than classical effects. Such precautions complicate the real-world operation of QRNGs. For example, current techniques quantify the level of electronic noise when a spontaneous emission source is switched off, but this is difficult to monitor during real-time operation.

[0026] In contrast to conventional approaches, the embodiments described herein facilitate the implementation of QRNGs by using randomness in the phase of spontaneously emitted photons rather than intensity fluctuations. That is, the embodiments exploit the inherent randomness in the phase of photons rather than their intensity fluctuations. By ensuring that the phase of each successive pulse in a train of pulses is randomized relative to the other pulses, it is possible to employ conventional methods of measuring phase in a QRNG based on phase noise. Figure 2 shows an example of how such a QRNG can be achieved in practice. The QRNG includes a transmitter 201, whose components are identical to those of Figure 1, except that the transmitter 201 does not include the intensity modulator 105. Pulses output by a gain-switched DFB laser 103 are input to an interferometer 107, and the intensity distribution at the output of the interferometer 107 is measured on a photodiode 203. Here, the intensity distribution follows an arcsine distribution, which differs from distributions arising from electronic noise. As a result, it is possible to actively monitor the quantum nature of the randomness generated by a system during its operation in real time. Moreover, the described embodiments improve the long-term operational stability of the QRNG, providing a system that is less sensitive to fluctuations in laser dynamics and external disturbances.

[0027] The reduction in laser downtime provided by the embodiments described herein can be further appreciated with reference to FIGS.

[0028] Figure 3 shows the intensity distribution observed when successive pulses output by a conventional laser diode are interfered with by passing them through an asymmetric Mach-Zehnder interferometer. If two interfering coherent pulses have a constant phase difference ΔΦ, the output intensity I will be proportional to (1 + cos ΔΦ). Accordingly, if the downtime between pulses is long enough and the phase distribution is uniform, the intensity distribution should exhibit an arcsine distribution with characteristic double peaks at both ends. This occurs when the interpulse spacing is 125 ps (line 301) and, to a lesser extent, when the interpulse spacing is 83 ps (line 302). In contrast, at high repetition rates with short interpulse downtimes, residual photons from the preceding pulse do not have enough time to escape the laser cavity. Here, the next pulse to be generated is seeded on the residual photons from the previous pulse. Line 303 in FIG. 3 shows the intensity distribution for a much shorter inter-pulse spacing of 42 ps, where the arcsine distribution is no longer seen, indicating that the phases of successive pulses are not random but are instead correlated with each other.

[0029] Figure 4A shows the intensity distribution obtained during an experiment in which a standard laser diode was gain-switched at 8 GHz, the optical pulses so generated were sent to an asymmetric Mach-Zehnder interferometer, and the output of the interferometer was measured by a photodiode. As shown in Figure 4A, the intensity distribution does not follow a typical arcsine distribution, suggesting that the phase is not random between interfering pulses. Figure 4B shows the autocorrelation of pulses output by the laser diode. Here, a clear correlation between successive pulses can be observed.

[0030] Figure 5A shows an intensity distribution obtained during an experiment in which pulses, again at 8 GHz, according to an embodiment described herein, are seeded by spontaneous emission of photons from an external light source. In contrast to the distribution shown in Figure 4A, the intensity distribution in Figure 5A follows an arcsine distribution, as expected from interference between phase-randomized pulses. Figure 5B shows the autocorrelation of pulses for the same experimental setup as in Figure 5A; in contrast to the autocorrelation shown in Figure 4B, in Figure 5B the correlation between pulses is removed.

[0031] Figure 6A shows an intensity distribution obtained during an experiment in which a standard laser diode was gain-switched at 10 GHz, the optical pulses so generated were sent to an asymmetric Mach-Zehnder interferometer, and the output of the interferometer was measured with a photodiode. As in Figure 4A, the intensity distribution in Figure 6A does not follow a typical arcsine distribution, suggesting that the phase is not random between interfering pulses. Figure 6B shows the autocorrelation of pulses output by the laser diode. Here, a clear correlation between successive pulses can be observed.

[0032] Figure 7A shows an intensity distribution obtained during an experiment in which pulses, again at 10 GHz, according to an embodiment described herein, are seeded by spontaneous emission of photons from an external light source. In contrast to the distribution shown in Figure 6A, the intensity distribution in Figure 7A follows an arcsine distribution, as expected from interference between phase-randomized pulses. Figure 7B shows the autocorrelation of pulses for the same experimental setup as in Figure 7A; in contrast to the autocorrelation shown in Figure 6B, in Figure 7B the correlation between pulses is removed.

[0033] Figure 8A shows a close-up of the autocorrelation results of Figure 5B (8 GHz pulse rate), and Figure 8B shows a close-up of the autocorrelation results of Figure 7B (10 GHz pulse rate). In both cases, the results show that the samples are well within the 99% confidence limits, indicating the absence of correlation in the described embodiment.

[0034] Implementations of the subject matter and operations described herein can be realized in digital electronic circuitry, or in computer software, firmware, or hardware, or in combinations of one or more of them, including the structures disclosed herein and their structural equivalents.

[0035] While several embodiments have been described, these embodiments are presented by way of example only and are not intended to limit the scope of the present invention. Indeed, the novel methods, devices, and systems described herein may be embodied in a variety of forms, and various omissions, substitutions, and modifications of the forms of the methods and systems described herein may be made without departing from the spirit of the present invention. The appended claims and their equivalents are intended to cover such forms or modifications as fall within the scope and spirit of the present invention.

Claims

1. An optical transmitter, a laser source configured to generate a plurality of pulses of light output by the optical transmitter; a pump for the laser source; a light source configured to emit a plurality of photons via spontaneous emission, the light source coupled to a cavity of the laser source such that the plurality of photons emitted by the light source via spontaneous emission seed a plurality of pulses of light generated by the laser source, each successive pulse having a random phase; An optical transmitter comprising:

2. 2. The optical transmitter of claim 1, wherein the laser source is a gain-switched laser source.

3. 10. The optical transmitter of claim 1, wherein the light source is an amplified spontaneous emission (ASE) light source.

4. The light source is Superluminescent diodes, a semiconductor optical amplifier; Fiber amplifiers and 4. The optical transmitter of claim 3, wherein:

5. 10. The optical transmitter of claim 1, further comprising an optical filter that selects one or more wavelengths of light from the light source to be coupled into the cavity of the laser source.

6. 10. The optical transmitter of claim 1, further comprising an optical amplifier that amplifies the light output by the light source before the light source is coupled into the cavity of the laser source.

7. 10. The optical transmitter of claim 1, further comprising an optical filter that filters the plurality of pulses generated by the laser source.

8. 10. The optical transmitter of claim 1, wherein the laser source is a solid-state laser source.

9. 9. The optical transmitter of claim 8, wherein the laser source is a distributed feedback (DFB) laser.

10. A quantum key distribution (QKD) system comprising the optical transmitter of claim 1.

11. 11. The QKD system of claim 10, wherein each pulse output by the optical transmitter is input to an interferometer having a first arm and a second arm.

12. 12. The QKD system of claim 11, wherein the first arm of the interferometer comprises a delay line and the second arm of the interferometer comprises a phase modulator.

13. The QKD system of claim 10, further comprising an intensity modulator that modulates the intensity of the plurality of pulses output by the optical transmitter.

14. A quantum random number generator (QRNG) comprising the optical transmitter of claim 1.

15. 15. The quantum random number generator of claim 14, wherein each pulse output by the optical transmitter is input to an interferometer comprising a first arm and a second arm, the first arm comprising a delay line.

16. 16. The quantum random number generator of claim 15, comprising a photodiode that detects a plurality of pulses traveling along the first arm and the second arm of the interferometer.

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