Photon random number generator

Abstract

To continuously improve the performance of a quantum random number generator (QRNG) that is based on a gain-switched laser.SOLUTION: A quantum random number generator (QRNG) 1 comprises: a semiconductor laser 3 configured to emit a stream of phase randomized pulses; an optical amplifier 9 (e.g., a semiconductor optical amplifier) which is an intensity controller for modulating pairs of pulses in the stream of phase randomized pulses; a time delay interferometer 11 which serves as a phase measuring element configured to convert the phase difference between the modulated pairs of pulses into intensity modulation at the output of the phase measuring element; and an optical detector assembly optically coupled to the output of the phase measuring element and comprising optical detectors D1 and D2.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] FIELD OF THE INVENTION The embodiments described herein relate to quantum random number generators, and in particular to optical quantum random number generators. [Background technology]

[0002] Random numbers are used in a variety of applications, including cryptography, numerical simulation, or lotteries. Random numbers can be produced from a quantum random number generator (QRNG). In a QRNG, the source of randomness is physical and relies on the unpredictability of measurements; in particular, the unpredictability relies on quantum mechanical properties. A QRNG can be implemented using a gain-switched diode laser. In a gain-switched diode laser, the lasing threshold is governed by spontaneous emission, a quantum mechanical process; therefore, the phase of the emitted pulses is random. By repeatedly switching the diode laser on and off, a stream of optical pulses can be generated, each with a random phase. By measuring the random phase of each optical pulse in the stream of optical pulses, a sequence of random numbers can be obtained.

[0003] There is a need to continue improving the performance of QRNGs based on gain-switched lasers.

[0004] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: [Brief explanation of the drawings]

[0005] [Figure 1] FIG. 1 is a schematic diagram of a comparative example QRNG. [Figure 2] FIG. 2 is a schematic diagram of an exemplary QRNG according to one embodiment. [Figure 3] FIG. 3 shows the digitization and processing of the output of the differential amplifier to obtain a random number. [Figure 4]Figure 4 is a schematic diagram of the intensity profile of the laser pulse. [Figure 5] FIG. 5 is a schematic diagram of a further exemplary QRNG according to an embodiment. [Figure 6] FIG. 6 is a schematic diagram of a further exemplary QRNG according to an embodiment. [Figure 7A] FIG. 7A shows the experimental data. [Figure 7B] FIG. 7B shows the experimental data. [Figure 7C] FIG. 7C shows the experimental data. [Figure 7D] FIG. 7D shows the experimental data. [Figure 8] FIG. 8 shows the experimental data. DETAILED DESCRIPTION OF THE INVENTION

[0006] To avoid unnecessary repetition, like reference numerals will be used to denote like features in the figures.

[0007] In one embodiment, a new and useful optical quantum random number generator (QRNG) is provided that at least partially overcomes the problems of conventional devices. For example, a conventional optical QRNG may employ a gain-switched semiconductor laser to generate phase-randomized optical pulses. Two of these phase-randomized optical pulses are then interfered to generate interference pulses of random intensity. The intensity of the interference pulses is converted by a photodiode to provide an analog voltage signal, which is digitized by an analog-to-digital converter (ADC) for further processing to generate random numbers. A problem with conventional optical QRNGs (e.g., when the device is operated at a high repetition rate and / or when the device is implemented as a photonic integrated circuit) is that the voltage pulses provided by the photodiode are often too small to optimally utilize the dynamic range of the ADC (unless significant electronic amplification, which introduces undesirable classical noise, is used). This reduces the achievable random number generation rate. To this end, the present disclosure proposes a QRNG that uses an optical amplifier to amplify the intensity of the phase-randomized optical pulses (i.e., before the phase-randomized optical pulses are interfered with) so that the interference pulses become a voltage signal on the photodiode that optimally utilizes the dynamic range of the ADC. Another problem with conventional optical QRNGs employing gain-switched lasers is that each of the generated phase-randomized pulses contains an initial portion of reduced coherence (as explained in more detail below). In practice, this means that this portion of the pulse does not contribute to the desired interference signal but instead adds undesirable classical noise to the photodiode signal. To this end, the present disclosure proposes a QRNG that blocks the relevant initial (or "leading") portion of the phase-randomized optical pulse before it is interfered with (or at least reduces the relative intensity of the initial portion of the pulse compared to the later "useful portion" of the pulse).Another problem with conventional optical QRNGs is that the generated photodiode signal is often "contaminated" by classical noise (such as electrical crosstalk from the laser drive signal), which introduces undesirable correlations into the raw output. To this end, this disclosure proposes a QRNG that employs two photodetectors configured to detect complementary interfering pulses. In this case, crosstalk and classical noise are suppressed by taking the difference between the outputs of the two photodetectors and generating a random number from this difference.

[0008] In one embodiment, a quantum random number generator (QRNG) is provided that includes a laser configured to emit a plurality of streams of phase-randomized pulses, an intensity controller configured to modulate (e.g., amplify or attenuate) pairs of pulses in the plurality of streams of phase-randomized pulses, a phase measurement element configured to convert a phase difference between pairs of modulated pulses into an intensity modulation at an output of the phase measurement element, and an optical detector optically coupled to the output of the phase measurement element.

[0009] In one embodiment, the intensity controller may be configured to modulate pairs of pulses in a stream of multiple phase-randomized pulses such that, for each pulse in the pair, the intensity of a leading portion of the pulse is reduced compared to the intensity of a trailing portion of the pulse.

[0010] In one embodiment, the intensity controller may comprise an optical amplifier (eg, a semiconductor optical amplifier) ​​configured to amplify the intensity of the pair of pulses.

[0011] In one embodiment, the optical amplifier may be configured to amplify the intensity of the trailing portion of the pulse more than the intensity of the leading portion of the pulse.

[0012] In one embodiment, the intensity controller may be configured to modulate each emitted pulse by attenuating the intensity of a leading portion of the pulse more than the intensity of a trailing portion of the pulse.

[0013] In one embodiment, the intensity controller element may comprise an electro-absorption modulator, a Mach-Zehnder modulator, or an optical switch.

[0014] In one embodiment, the phase measurement element may be a time delay interferometer configured to direct light from an optical amplifier towards two arms, at least one of the arms comprising an interferometer delay, and the light from the two arms may be interfered with and directed to the output of the phase measurement element.

[0015] In one embodiment, the interferometer delay may be set to allow a time delay interferometer to interfere with pairs of modulated pulses (e.g., the interferometer delay may be equal to an integer multiple of the temporal separation between pulses in the emitted stream of pulses, or the interferometer delay may be equal to the temporal separation between pulses in the emitted stream of pulses such that consecutive optical pulses are interfered by the time delay interferometer).

[0016] In one embodiment, the QRNG may further include an optical detector assembly coupled to the first and second output ports of the time delay interferometer and configured to generate as output signals analog signals indicative of a difference between the intensity at the first output port and the intensity at the second output port of the time delay interferometer, and a processing circuit configured to generate a plurality of random numbers based on the output signals of the optical detector assembly.

[0017] In one embodiment, the optical detector assembly may include a plurality of balanced photodetectors for generating an analog signal indicative of the difference between the intensity at the first output port and the intensity at the second output port of the time delay interferometer. Alternatively, the optical detector assembly may include a first optical detector and a second optical detector coupled to the first output port and the second output port of the time delay interferometer, respectively, and a differential amplifier configured to receive the analog signals provided by the first optical detector and the analog signals provided by the second optical detector as a plurality of input signals and to generate an analog signal indicative of the difference between the plurality of input signals as an output signal.

[0018] In one embodiment, the QRNG may further comprise a semiconductor substrate, and multiple components of the QRNG may be integrated on the semiconductor substrate to form a photonic integrated circuit.

[0019] In one embodiment, the laser may be further configured to emit multiple phase-randomized pulses at a repetition rate between 1 and 2 GHz, and each emitted pulse may have a pulse duration between 200 ps and 500 ps.

[0020] In one embodiment, a quantum random number generator (QRNG) is provided that includes: a laser configured to emit a stream of multiple phase-randomized pulses; a phase measurement element configured to convert a phase difference between two pulses from the stream of multiple phase-randomized pulses into intensity modulation at first and second output ports of the phase measurement element; an optical detector assembly coupled to the first and second output ports of the phase measurement element for generating as output signals analog signals indicative of the difference between the intensity at the first and second output ports of the phase measurement element; and a processing circuit configured to generate multiple random numbers based on the output signals of the optical detector assembly.

[0021] In one embodiment, the optical detector assembly may include a plurality of balanced photodetectors for generating an analog signal indicative of the difference between the intensity at the first output port and the intensity at the second output port of the phase measurement element. Alternatively, the optical detector assembly may include a first optical detector and a second optical detector coupled to the first output port and the second output port of the phase measurement element, respectively, and a differential amplifier configured to receive the analog signals provided by the first optical detector and the analog signals provided by the second optical detector as a plurality of input signals and to generate an analog signal indicative of the difference between the plurality of input signals as an output signal.

[0022] Before describing further proposed embodiments of a quantum random number generator (QRNG), the design of an optical QRNG will now be described in detail with reference to FIG. 1. This design employs a laser that emits optical pulses into the input port of a time-delay interferometer (also called an asymmetric Mach-Zehnder interferometer (AMZI)). More specifically, FIG. 1 shows a portion of a conventional optical QRNG comprising a pulsed laser P1 driven at a fixed repetition rate by a controller P2 to output a stream of pulses. When the repetition rate is sufficiently low, each pulse from the stream of pulses can have a random phase. The pulses are coupled to a time-delay interferometer P3 via an input coupler P4. The time-delay interferometer P3 comprises a short arm and a long arm. The long arm of the time-delay interferometer P3 comprises a delay element P7, which delays the pulse by a time D relative to the pulse traveling in the short arm. In this QRNG device, the delay element P7 is configured such that the resulting delay D is such that each delayed pulse overlaps in time with the previous reference pulse in the reference arm. The delayed and reference pulses interfere at a 2x2 coupler P5 (or beam splitter) in the time-delay interferometer P3, and the interference pulse is sent to a single photodetector P6, which converts the random intensity of the interference pulse into a voltage pulse of random intensity. This electrical signal, corresponding to the intensity of the interference pulse, has a random value because the phases of the reference and delayed pulses are random. A random number (e.g., a sequence of bits with random values) can be generated from the random intensity of the interfered pulse by digitizing the electrical signal of the photodetector P6 using a digitizer P8.

[0023] 2 illustrates an exemplary optical quantum random number generator (QRNG) 1 according to one embodiment. Generally, QRNG 1 is configured to generate (and output) random numbers from the random intensities of interfered optical pulses. QRNG 1 is typically provided as an integrated device (and described as such below), i.e., the components of device 1 are integrated on a common semiconductor substrate (or on multiple semiconductor substrates assembled / connected as appropriate). However, in other embodiments, QRNG 1 may also be implemented using discrete (optical fiber pigtailed or free-space) components.

[0024] The optical device 1 comprises a semiconductor laser 3 (e.g., a distributed feedback (DFB) laser, a Fabry-Perot laser diode, etc.) and a controller 5 for controlling the operation of the semiconductor laser 3. The semiconductor laser 3 is operable to emit coherent light. The semiconductor laser 3 is operated in pulsed operation (i.e., to emit a stream of light pulses, e.g., nanosecond or picosecond long pulses). When operated in pulsed mode, the semiconductor laser 3 has a well-defined pulse duration and well-defined regular time intervals (the time interval between subsequently emitted pulses is Δ laserThe semiconductor laser 3 may be driven (by the controller 5) at a fixed repetition rate (e.g., at a fixed repetition rate selected from a range of 1 to 2 GHz) to output a stream of pulses (denoted as ). More specifically, the semiconductor laser 3 may be a gain-switched semiconductor laser driven to output a stream of phase-randomized pulses (i.e., each pulse from the stream of pulses may have a random phase). Generally, a gain-switched laser generates light when the laser is switched above the lasing threshold and generates very little light when the laser is switched below the lasing threshold. Thus, the controller 5 may control modulation of the gain of the laser 3 by modulating an electrical drive current applied to the laser 3 in a time-varying manner (as shown by reference numeral 7 in FIG. 2). For example, the semiconductor laser 3 may be periodically switched above and below the lasing threshold by application of a time-varying current. In this manner, the laser generates optical pulses. It should be understood that the controller 5 may comprise (or be connected to) an appropriate drive circuit for generating and applying such a time-varying current.

[0025] QRNG 1 further comprises an intensity controller configured to receive the phase-randomized pulses emitted by laser 3 and to modulate (e.g., amplify or attenuate) the optical intensity of the received pulses. More particularly, in the embodiment of FIG. 2 , the intensity controller is an optical amplifier 9 (e.g., a semiconductor optical amplifier). Optical amplifier 9 may be optically coupled to laser 3 via an optical waveguide, via an optical free-space path, etc. Optical amplifier 9 is further configured to amplify the intensity of the received pulses and output an “amplified pulse.” Amplifier 9 may be operated in different modes, as described in more detail below. Generally, controller 5 may be further configured to control the operation of amplifier 9.

[0026] The optical device 1 further comprises a time delay interferometer 11 configured to receive the output of the amplifier 9. The time delay interferometer 11 acts as a phase measurement element that converts the phase difference between two received pulses into an intensity modulation. The time delay interferometer 11 comprises an input coupler 13, a short arm 15, a long arm 17, and an output coupler 21. The input coupler 13 is configured to direct light from the amplifier 9 towards the short arm 15 and the long arm 17. The long arm 17 introduces a delay time Δ delay a delay element (e.g., a delay line) configured to delay the optical pulse propagating in the long arm 17 by a delay time Δ delay is the delay time Δ delay is the time interval Δ laser (e.g., the time interval Δ laser is the delay time Δ delay or the delay time Δ delay is the time interval Δ laser , etc. In this way, the pulse propagating in the long arm 17 is delayed by the delay element so as to interfere with the non-delayed pulse propagating in the short arm 15 at the output coupler 21. The phases of the delayed and non-delayed pulses are random (because these pulses are generated in different cycles), and therefore the intensities of the interfered pulses at the two output ports of the output coupler 21 have random values. A random number can be generated from the random intensities of the interfered pulses.

[0027] QRNG1 further includes an optical detector assembly coupled to an output port of output coupler 21. Generally, the optical detector assembly is configured to generate as an output signal an analog signal indicative of the difference between the light intensities at the output ports of output coupler 21 (so that a random number can be generated from this output signal of the optical detector assembly). In the embodiment of FIG. 2, the optical detector assembly includes photodetectors D1 and D2, each optically coupled to a different output port of output coupler 21. Photodetectors D1 and D2 are configured to convert the random intensities of the interfered light pulses into an electrical signal (e.g., a voltage pulse). The electrical signals provided by the detectors are generally analog signals (e.g., rather than the binary "click" signals associated with single-photon detectors). Photodetectors D1, D2 may be InP-based on-chip photodiodes.

[0028] It should be understood that the two detectors D1, D2 are configured to detect "complementary" interference pulses because the detectors are coupled to different outputs of the time delay interferometer 11. For example, interference of a particular pair of phase-randomized pulses may result in an interference pulse of increased intensity at detector D1 (due primarily to constructive interference at the corresponding output port coupled to detector D1) and a correspondingly reduced intensity interference pulse at detector D2 (due primarily to destructive interference at the corresponding output port coupled to detector D2).

[0029] QRNG1 further comprises a processing circuit configured to generate a random number from the analog signals provided by photodetectors D1 and D2. In particular, the processing circuit may be configured to generate the random number based on the difference between the analog signal provided by photodetector D1 and the analog signal provided by photodetector D2. To this end, QRNG1 comprises a differential amplifier 23 that receives the analog signals provided by photodetectors D1 and D2 and determines the difference between these signals. The differential amplifier 23 may further amplify the determined difference by a predetermined gain. The differential amplifier 23 then outputs the (amplified) difference between the analog signals to a digitizer circuit 25 that is configured to digitize the received signals and extract the random number from the digitized signals.

[0030] One embodiment of the digitizer circuit 25 is shown in Figure 3. The digitizer circuit 25 may include an analog-to-digital converter (ADC) 27 that converts the (amplified) difference between the photodetector signals into a digital signal. The ADC 27 is coupled to a post-processor 29. The digital signal generated by the ADC 27 is provided to the post-processor 29, which processes the digital signal using a randomness extractor algorithm to output a sequence of random numbers 31 with a uniform probability distribution.

[0031] Advantageously, extracting a random number from the difference between the analog signals provided by the photodetectors D1, D2 may result in suppression of undesired crosstalk and classical noise (e.g., classical noise introduced by amplifier 9), as will be further explained below with reference to FIG. 7.

[0032] It should be understood that other embodiments may not include differential amplifier 23. In these embodiments, photodetectors D1, D2 of the optical detector assembly may be configured as balanced photodetectors to directly generate an analog signal indicative of the difference between the light intensities at the output ports of output coupler 21 (e.g., photodetectors D1, D2 are configured such that the currents produced by the photodetectors are (directly) subtracted). The outputs of the balanced photodetectors may be provided to digitizer circuitry 25 (e.g., directly, or the outputs of the balanced photodetectors may be electrically amplified and the amplified signal provided to digitizer circuitry 25).

[0033] Next, the operation of the amplifier 9 will be described in more detail. The amplifier 9 can be operated in a "continuous mode" or in a "pulsed mode." For example, in a "continuous mode," the amplifier 9 is operated so that the intensity of the incoming pulse is amplified throughout the pulse duration. In this case, the amplifier 9 can be continuously switched "on." The amplification provided by the amplifier 9 can increase the intensity of the phase-randomized pulses so that the amplitude of the analog signal produced by the detectors D1, D2 is increased. More specifically, the gain of the amplifier 9 can be selected so that the dynamic range of the ADC 27 (i.e., the range of signal amplitudes that the ADC 27 can resolve) is optimally (or at least better) used. In other words, the gain of the amplifier 9 can be selected so that the distribution of the amplitude values ​​of the analog signal produced by the detectors D1, D2 (i.e., the distribution of amplitude values ​​formed by the amplitude values ​​of multiple interfered pulses) is such that the resulting distribution of difference values ​​provided by the differential amplifier 23 substantially matches the dynamic range of the ADC 27. By utilizing the full dynamic range of ADC 27, a wider (i.e., finer) distribution of digitized difference values ​​is generated (by ADC 27), which increases the random number generation rate. In some embodiments, the gain of optical amplifier 9 may be at least 10 dB, at least 20 dB, or at least 30 dB in "continuous mode." The provision of amplifier 9 is particularly advantageous in situations where QRNG 1 is implemented as a photonic integrated circuit (because in such implementations, high losses in the waveguides may result in weak signals at photodetectors D1, D2 that do not utilize the full dynamic range of the ADC).

[0034] The operation of the amplifier 9 in "pulse mode" will now be described with reference to Figure 4, which shows the temporal intensity profile of a typical optical pulse 41 generated by gain switching the laser 3. The pulse 41 typically has a pulse duration t between 200 ps and 500 ps. pulse It has.

[0035] In general, the optical output produced by a gain-switched laser does not simply follow the shape of the electrical drive signal (which is typically a rectangular or Gaussian pulse). Instead, the temporal characteristics of the optical emission depend on the interaction between photons and electrically injected carriers in the laser cavity, and carriers in semiconductor lasers typically have lifetimes of nanosecond duration. When a large current is initially applied to the laser, carriers can rapidly accumulate and temporarily overshoot the carrier density threshold. This results in a large emission of photons, which subsequently depletes the carriers. The resulting interaction between photons and carriers causes damped oscillations (and corresponding phase fluctuations) known as "relaxation oscillations." These relaxation oscillations gradually decay and reach a steady state. Therefore, gain-switched lasers typically produce rectangular-shaped pulses with an initial overshoot (as shown in Figure 4). Thus, pulse 41 may be considered to comprise a front portion 43 (also called a "leading portion") and an end portion 45 (also called a "trailing portion"), where front portion 43 of pulse 41 may have lower temporal coherence than end portion 45. Front portion 43 extends from the leading edge of pulse 41 to the midpoint of pulse 41. End portion 45 extends from the midpoint to the trailing edge of pulse 41.

[0036] As mentioned above, the leading portion 43 comprises the leading edge of the pulse 41, i.e., the initial "non-steady-state" phase of the pulse 41. Therefore, the leading portion 43 may comprise a large, sharp intensity peak caused by relaxation oscillations. In particular, the leading portion 43 may comprise a decaying oscillation of the optical intensity at the beginning of the pulse. The leading portion 43 of the pulse typically exhibits a frequency chirp due to changes in the refractive index within the laser cavity with respect to the varying carrier density. As a result of the frequency chirp, the leading portion 43 has reduced coherence and therefore results in a reduced (or completely suppressed) interference signal. In other words, the leading portion of the phase-randomizing pulse does not (significantly) affect the interference signal from which random numbers can be extracted. Instead, the leading portion 43 of the pulse 41 contributes undesirable (classical) noise to the photodetector signal. While the duration of the leading portion 43 generally depends on the specifics of the laser and drive current, in many cases the leading portion 43 lasts for at least 50 ps (e.g., between 50 ps and 200 ps). In one embodiment, the duration of the front portion 43 may be in the range of 10% to 40% of the total duration of the pulse 41 .

[0037] Trailing portion 45 comprises the "steady-state" phase following pulse 41 (i.e., trailing portion 45 comprises the remainder of pulse 41 following leading portion 43). The intensity in trailing portion 45 is generally constant. Importantly, the optical frequency of pulse 41 is generally constant in trailing portion 45, and therefore, these trailing portions of the pulse produce the desired random interference signal from which random numbers may be extracted as described above.

[0038] In "pulse mode," amplifier 9 is operated to amplify only the trailing portion 45 of pulse 41 (or to amplify trailing portion 45 (significantly) more strongly than the leading portion 43). To this end, controller 5 may appropriately synchronize operation of amplifier 9 with drive current pulses provided to laser 3. In other words, controller 5 may control amplifier 9 such that the gain provided by amplifier 9 is significantly higher during the trailing portion of the received pulse than during the leading portion. In some embodiments, the gain of optical amplifier 9 during trailing portion 45 of pulse 41 may be at least 10 dB, at least 20 dB, or at least 30 dB higher than during the leading portion 43 of pulse 41.

[0039] This time-selective amplification effectively reduces the relative influence of the initial "non-steady-state" portion of the pulse on the signal detected by photodetectors D1, D2 compared to the influence of the later "steady-state" portion of the pulse. As a result, the signal produced by photodetectors D1, D2 exhibits less classical noise. Furthermore, amplification in "pulsed mode" may result in a desirable broadening of the distribution of values ​​produced by differential amplifier 23 to better match the dynamic range of ADC 27 as described above with respect to the "continuous mode" of amplifier 9.

[0040] Although not shown in FIG. 2, it will be appreciated that QRNG1 may comprise further elements, for example (adjustable) attenuators, to compensate for differences in accumulated optical losses in the short arm 15 and the long arm 17 (to ensure that delayed and non-delayed pulses having substantially the same intensity enter the output coupler 21).

[0041] Additionally or alternatively, the input coupler 13 may have an adjustable combining ratio (i.e., the combining ratio may be changed during use, i.e., typically by employing electro-optic or thermo-optic effects). In one embodiment, the input coupler 13 may comprise a tunable Mach-Zehnder interferometer to adjust the combining ratio.

[0042] Figure 5 shows a variation of the QRNG of Figure 2 (to avoid unnecessary repetition, like reference numbers are used to indicate like features). The QRNG 51 of Figure 5 includes the same components as the QRNG 1 of Figure 2 and operates in substantially the same way, except that the QRNG 51 of Figure 5 includes an intensity modulator 53 instead of optical amplifier 9 as the intensity controller.

[0043] The intensity modulator 53 is configured to receive pulses emitted by the laser 3, modulate the intensity of the received pulses, and provide the modulated pulses to the time delay interferometer 11. Similar to the amplifier 9 described above (when operated in “pulse mode”), the intensity modulator 53 modulates each emitted pulse such that the intensity of the leading portion 43 of the pulse 41 is reduced compared to the intensity of the trailing portion 45. The intensity modulator 53 achieves this by attenuating the intensity of the leading portion 43 of the pulse 41 more than the intensity of the trailing portion 45. In other words, the intensity modulator 53 “carves” the received pulse so as to block unwanted optical signals from reaching the photodetectors D1, D2. To this end, the controller 5 may be configured to synchronize the attenuation provided by the intensity modulator 53 with the electrical drive signal provided to the laser 3. In embodiments, the intensity modulator may be implemented as an electro-absorption modulator, a Mach-Zehnder modulator, or an optical switch. In some embodiments, the optical attenuation of the intensity modulator 53 during the leading portion 43 of the pulse 41 may be at least 10 dB, at least 15 dB, or at least 20 dB higher than during the trailing portion 45 of the pulse 41 .

[0044] This selective attenuation effectively reduces the relative contribution of the initial "non-steady state" portion of the pulse to the signal detected by photodetectors D1, D2, compared to the contribution of the later "steady state" portion of the pulse. As a result, the signal produced by photodetectors D1, D2 exhibits less classical noise.

[0045] In another variation (shown in FIG. 6 ), QRNG 61 may include an intensity modulator 53 (as described with reference to FIG. 5 ) in addition to amplifier 9 (as described with reference to FIG. 2 ). In this case, amplifier 9 may be operated in “continuous mode” (as described above), and intensity modulator 53 “shapes” the front portion of the pulse (as described above). The intensity modulator may be upstream or downstream of the amplifier. Although shown in FIG. 6 as individual components, in embodiments, amplifier 9 and modulator 53 may be provided as integrated components.

[0046] 7A-7D show experimental data acquired by a first exemplary QRNG (the first exemplary device is configured as described above with reference to FIG. 2; in particular, the first exemplary QRNG is implemented as an (unpackaged) photonic integrated circuit with a semiconductor optical amplifier operated in continuous or pulsed mode). FIG. 7A shows an accumulated oscilloscope trace 73 produced by a (single) photodiode detecting an optical pulse generated by gain-switching laser 3. As described above with reference to FIG. 4, optical pulse 73 includes an initial portion with a large, sharp intensity peak and a subsequent steady-state portion. FIG. 7B shows an accumulated oscilloscope trace 75 of an optical pulse after intensity modulator 53 attenuates the leading edge of the pulse (as described above with reference to FIG. 5). It can be seen that the resulting pulse 75 no longer comprises the (unwanted) initial non-steady-state phase, while the (desired) steady-state portion remains essentially unchanged (i.e., unattenuated). Histograms 77, 79 of voltage values ​​corresponding to pulses 73 and 75 are shown in Figures 7C and 7D. It can be seen that both histograms 77, 79 generally follow an arcsine distribution as would be expected for interference pulses generated by pulses with random, uniformly distributed phases.

[0047] FIG. 8 shows additional experimental data acquired by a second exemplary QRNG configured similarly to the first exemplary QRNG used to acquire the data of FIG. 7, but implemented as a packaged photonic integrated circuit (PIC). The top row of FIG. 8 shows a heat map 81 of a signal generated by a single photodetector (e.g., photodetector D1) detecting an interference pulse generated by the interference of two phase-randomized pulses emitted by laser 3. (For the experimental data shown in FIG. 8, the amplifier was operated in continuous mode (i.e., not pulsed mode).) In addition to the signal caused by the interference pulse, the generated signal 83 includes significant crosstalk contributions from the electrical laser drive signal. This crosstalk is significantly stronger in the packaged photonic integrated circuit than in the unpackaged photonic integrated circuit (FIGS. 7A-7D) because the electrical lines / wires carrying the laser drive current and the electrical lines / wires carrying the photodetector signal are necessarily closer to each other than in the unpackaged first exemplary device. The top row of FIG. 8 also shows a histogram 87 corresponding to heat map 81. It can be seen that the two peaks in the histogram 87 are not well resolved.

[0048] The bottom row of FIG. 8 shows a heat map 89 of the signal generated by the differential amplifier 23 (i.e., the heat map 89 shows the difference between the signals of the photodetectors D1 and D2). It can be seen that the aforementioned crosstalk is strongly suppressed by amplifying the difference between the two photodetector outputs with the differential amplifier 23. Thus, undesirable crosstalk, which is particularly noticeable in highly integrated devices, can be significantly reduced by using the difference between the two photodetector outputs. A signal 91 corresponding to the interference pulse shows the expected random distribution of values. Advantageously, the width of the corresponding histogram 93 is significantly wider than that of histogram 87 (which reduces the need for further electrical amplification), and the two peaks of histogram 93 are much more clearly resolved.

[0049] 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 devices and methods described herein may be embodied in a variety of other forms, and various omissions, substitutions, and modifications of the forms of the devices, methods, and products 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 may fall within the scope and spirit of the present invention.

Claims

1. a laser configured to emit a stream of a plurality of phase-randomized pulses; an intensity controller configured to modulate pairs of pulses in the stream of phase-randomized pulses; a phase measurement element configured to convert a phase difference between the pair of modulated pulses into an intensity modulation at an output of the phase measurement element; an optical detector optically coupled to the output of the phase measurement element; A quantum random number generator (QRNG) comprising:

2. 2. The quantum random number generator of claim 1, wherein the intensity controller is configured to modulate pairs of pulses in the stream of phase-randomized pulses such that, for each pulse of the pair, an intensity of a leading portion of the pulse is reduced compared to an intensity of a trailing portion of the pulse.

3. 3. The quantum random number generator of claim 2, wherein the intensity controller comprises an optical amplifier configured to amplify the intensity of the pair of pulses.

4. 4. The quantum random number generator of claim 3, wherein the optical amplifier is configured to amplify the intensity of the trailing portion of the pulse more than the intensity of the leading portion of the pulse.

5. The quantum random number generator of claim 3 , wherein the intensity controller comprises a semiconductor optical amplifier.

6. 3. The quantum random number generator of claim 2, wherein the intensity controller is configured to modulate each emitted pulse by attenuating the intensity of the leading portion of the pulse more than the intensity of the trailing portion of the pulse.

7. 7. The quantum random number generator of claim 6, wherein the intensity controller comprises an electro-absorption modulator, a Mach-Zehnder modulator, or an optical switch.

8. 4. The quantum random number generator of claim 3, wherein the phase measurement element is a time delay interferometer configured to direct light from the optical amplifier toward two arms, at least one arm comprising an interferometer delay, wherein light from the two arms is interfered with each other and directed to the output of the phase measurement element.

9. 9. The quantum random number generator of claim 8, wherein the interferometer delay is set to allow the time delay interferometer to interfere with the modulated pulse pairs.

10. 10. The quantum random number generator of claim 9, wherein the interferometer delay is equal to an integer multiple of the temporal separation between pulses in the stream of emitted pulses.

11. 11. The quantum random number generator of claim 10, wherein the interferometer delay is equal to the temporal separation between pulses in the stream of emitted pulses such that consecutive optical pulses are interfered by the time delay interferometer.

12. an optical detector assembly coupled to a first output port and a second output port of the time delay interferometer and configured to generate as an output signal an analog signal indicative of a difference between an intensity at the first output port and an intensity at the second output port of the time delay interferometer; a processing circuit configured to generate a plurality of random numbers based on the output signal of the optical detector assembly; 9. The quantum random number generator of claim 8, further comprising:

13. 13. The quantum random number generator of claim 12, wherein the optical detector assembly comprises a plurality of balanced photodetectors for generating the analog signal indicative of the difference between the intensity at the first output port and the intensity at the second output port of the time delay interferometer.

14. 13. The quantum random number generator of claim 12, wherein the optical detector assembly comprises: a first optical detector and a second optical detector coupled to the first output port and the second output port of the time delay interferometer, respectively; and a differential amplifier configured to receive the analog signal provided by the first optical detector and the analog signal provided by the second optical detector as a plurality of input signals, and to generate, as an output signal, an analog signal indicative of a difference between the plurality of input signals.

15. 10. The quantum random number generator of claim 1, further comprising a semiconductor substrate, wherein multiple components of the QRNG are integrated onto the semiconductor substrate to form a photonic integrated circuit.

16. 2. The quantum random number generator of claim 1, wherein the laser is further configured to emit the plurality of phase-randomized pulses at a repetition rate between 1 and 2 GHz, and wherein each emitted pulse has a pulse duration between 200 ps and 500 ps.

17. a laser configured to emit a stream of a plurality of phase-randomized pulses; a phase measurement element configured to convert a phase difference between two pulses from the stream of phase-randomized pulses into intensity modulation at a first output port and a second output port of the phase measurement element; an optical detector assembly coupled to the first output port and the second output port of the phase measurement element and configured to generate as an output signal an analog signal indicative of a difference between an intensity at the first output port and an intensity at the second output port of the phase measurement element; a processing circuit configured to generate a plurality of random numbers based on the output signal of the optical detector assembly; A quantum random number generator (QRNG) comprising:

18. 18. The quantum random number generator of claim 17, wherein the optical detector assembly comprises a plurality of balanced photodetectors for generating the analog signal indicative of the difference between the intensity at the first output port and the intensity at the second output port of the phase measurement element.

19. 18. The quantum random number generator of claim 17, wherein the optical detector assembly comprises: a first optical detector and a second optical detector coupled to the first output port and the second output port of the phase measurement element, respectively; and a differential amplifier configured to receive the analog signal provided by the first optical detector and the analog signal provided by the second optical detector as a plurality of input signals, and to generate, as an output signal, an analog signal indicative of a difference between the plurality of input signals.

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