GB2640907A - A quantum random number generator - Google Patents

Abstract

A quantum random number generator 1 comprises a light source 3 configured to emit a plurality of pulses having randomised intensity values, a combining unit 5 optically coupled to the light source and configured to generate an electrical analog signal so that a signal value of the electrical analog signal equals a combination of the intensity values of the plurality of intensity randomised pulses, and circuitry 7 configured to generate a random number from the electrical analog signal. The combining provides a more uniform intensity distribution (e.g. Figures 6 and 7), so the amount of entropy generated per clock cycle is increased (e.g. Figure 8).

Description

A Quantum Random Number Generator

Technical Field

Embodiments described herein relate to quantum random number generators, in particular to optical quantum random number generators.

Background

Random numbers are used in a variety of applications including cryptography, numerical simulations, or lotteries. Random numbers can be produced from Quantum Random Number Generators (QRNG). In QRNG, the source of randomness is physical and relies on the unpredictability of a measurement, and, in particular, the unpredictability relies on a quantum mechanical property. A QRNG can be implemented using a light source that employs a quantum mechanical process to emit a stream of optical pulses, each having a random intensity. By measuring the random intensity of each optical pulse in the stream of optical pulses, a sequence of random digital signals can be obtained. Subsequent digital post-processing can be used to convert these random digital signals into uniformly distributed random numbers.

There is a continuing need to improve the performance of QRNGs.

Brief Description of the Drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: Figure 1 is a schematic illustration of a QRNG according to a comparative example; Figure 2 shows simulation results; Figure 3 is a schematic illustration of a first example QRNG according to an embodiment; Figure 4 is a schematic illustration of a first example combining unit; Figure 5 is a schematic illustration of a second example combining unit; Figures 6 to 15 show simulation results; Figure 16 is a schematic illustration of a first example light source; Figure 17 is a schematic illustration of a second example light source; Figure 18 is a schematic illustration of a third example light source; Figure 19 is a schematic illustration of a fourth example light source; Figure 20 is a schematic illustration of a fifth example light source; Figure 21 is a schematic illustration of a sixth example light source, and Figure 22 is a schematic illustration of a second example QRNG according to an embodiment.

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

Detailed Description

In an embodiment, a new and useful optical Quantum Random Number Generator (QRNG) is provided which at least partially overcomes problems of conventional devices. For example, a conventional optical QRNG may employ a gain-switched semiconductor laser to generate a stream of phase-randomised optical pulses. In each clock cycle, two of these phase-randomised optical pulses are then interfered to generate an interference pulse of random intensity. The intensity of the interference pulse is converted by a photodiode to provide an analog voltage signal which is digitised by an analog-to-digital (ADC) converter to be further digitally processed to generate or "extract" random numbers. A problem of conventional optical QRNGs is that the amount of entropy (and thus the number of random bits) generated per clock cycle is low. Although the optical pulses produced by a gain-switched semiconductor laser can have a (nearly) uniformly random phase, after interference the randomised intensity of the resulting optical pulse has a strongly non-uniform distribution. As described below in more detail, the non-uniform distribution of the randomised intensity limits the amount of randomness that can be extracted per clock cycle. To improve the achievable random number generation rate, the present disclosure proposes a QRNG which combines (e.g. optically or electrically) two or more intensity-randomised pulses into a single pulse so that the resulting combined pulse has more uniformly distributed random signal values (i.e. more uniformly than the initial intensity-randomised pulses). The combined pulse is then digitally processed to extract random numbers. Because of the more uniform intensity distribution, the amount of entropy (and thus the number of random bits) generated per clock cycle is increased. As described below in more detail, a further advantage of the proposed QRNG is that the need for digital post-processing is reduced.

In an embodiment, a quantum random number generator (QRNG) is provided. The QRNG comprises a light source configured to emit a plurality of pulses having randomised intensity values (the pulses having randomised intensity values may have a non-uniform distribution of intensity values, e.g. an arcsine distribution), a combining unit optically coupled to the light source and configured to generate an electrical analog signal so that a signal value of the electrical analog signal equals a combination of the intensity values of the plurality of intensity-randomised pulses, and circuitry configured to generate a random number from the electrical analog signal.

In an embodiment, the combining unit may generate the electrical analog signal according to a combining function, the combining function comprising one or more of: a linear weighted sum of the intensity values of the intensity-randomised pulses; a linear weighted subtraction of the intensity values of the intensity-randomised pulses; a modular arithmetic function of the intensity values of the intensity-randomised pulses; and a polynomial function of the intensity values of the intensity-randomised pulses.

In an embodiment, the combining unit may comprise one or more optical attenuators to implement weights of the combining function.

In an embodiment, the combining unit may comprise an optical detector configured to detect the plurality of intensity-randomised pulses.

In an embodiment, the optical detector may comprise an optical sensor and the plurality of intensity-randomised pulses impinges upon different portions of the optical sensor.

In an embodiment, the combining unit may comprise, for each one of the plurality of intensity-randomised pulses, a respective optical detector configured to detect the corresponding intensity-randomised pulses.

In an embodiment, the combining unit may comprise circuitry to implement the combining function from the electrical signals generated by the optical detectors.

In an embodiment, the light source may comprise a coherent light source configured to emit a plurality of pulses of coherent light so that said pulses have randomised phases, and an interference module optically coupled to the coherent light source and configured to generate the plurality of intensity-randomised pulse from the plurality of phase-randomised pulses.

In an embodiment, the coherent light source may comprise, for each one of the plurality of intensity-randomised pulses, a corresponding primary laser configured to emit a respective phase-randomised pulse.

In an embodiment, the primary lasers may operate at different optical frequencies.

In an embodiment, the coherent light source may further comprise one or more secondary lasers optically coupled to the interference module, and the interference module may further be configured to generate the plurality of intensity-randomised pulse from the plurality of phase-randomised pulses by interfering the plurality of phase-randomised pulses with coherent light emitted by the one or more secondary lasers.

In an embodiment, the one or more secondary lasers may be operated in a pulsed operation mode or in a continuous operation mode.

In an embodiment, the secondary lasers may comprise, for each one of plurality of intensity-randomised pulses, a corresponding secondary laser.

In an embodiment, the coherent light source may comprise, for each one of the plurality of intensity-randomised pulses, a corresponding pair of primary lasers, each primary laser configured to emit a respective phase-randomised pulse.

In an embodiment, the coherent light source may comprise a primary laser configured to emit, for each one of the plurality of intensity-randomised pulses, a pair of phase-randomised pulses, and the interference module comprises a time delay interferometer to pairwise interfere the pulses emitted by the primary laser.

In an embodiment, the plurality of intensity-randomised pulses may consist of a first intensity-randomised pulse and a second intensity-randomised pulse, and the coherent light source comprises a first primary laser configured to emit a first pair of phase-randomised pulses and a second primary laser configured to emit a second pair of phase-randomised pulses, and the interference module may generate the first intensity-randomised pulse and the second intensity-randomised pulse so that the polarisation of the first and second intensity-randomised pulse are orthogonal.

In an embodiment, the QRNG may further comprise at least one photodetector. The interference module may comprise, for each of at least one of the plurality of intensity-randomised pulses, a corresponding 2x2-coupler comprising a first output port for providing intensity-randomised pulses to the combining unit, and a second output port coupled to the photodetector.

In an embodiment, at least one of the primary lasers may be a gain-switched semiconductor laser.

In an embodiment, the circuitry may comprise a digitising unit configured to perform an analog to digital conversion of the electrical analog signal. The circuitry may further comprise a processing unit configured to apply a randomness extraction process to the digitally converted electrical analog signal.

Before describing further proposed embodiments relating to quantum random number generators (QRNG), a design of an optical QRNG will now be described in detail with reference to Figure 1. This design employs a laser that emits optical pulses into an input port of a time delay interferometer (also referred to as an asymmetric Mach Zehnder interferometer (AMZI)). More specifically, Figure 1 shows a portion of a conventional optical QRNG that comprises 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 low enough, each pulse from the stream of pulses may have a random phase. The pulses are coupled into the time delay interferometer P3 via an input coupler P4. The time delay interferometer P3 comprises a short and a long arm. The long arm of the time delay interferometer P3 comprises a delay element P7, which delays the pulses by a time D with respect to the pulses travelling in the short arm. In this QRNG device, the delay element P7 is configured such that the delay D introduced is such that each delayed pulse temporally overlaps with a previous reference pulse in the reference arm. The delayed and reference pulses interfere in a 2x2 coupler P5 (or beam splitter) of 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 intensities of the interference pulse has a random value because the phases of the reference and delayed pulses are random. Random numbers (e.g. a sequence of bits with random values) may be generated from the random intensities of the interfered pulses. To this end, the voltage pulse generated by the photodiode P6 is digitised by an analog-to-digital (ADC) converter P8 to be further digitally processed (by a digital post-processing unit P9) to generate random numbers.

The amount of quantum random bits of information generated in a clock cycle may be estimated by the "minimum entropy Hmin" which can be derived from the distribution of ADC output values P (d), i.e. the (measured or simulated) distribution of digitised intensity values.

More specifically, the minimum entropy Hmin per output bit of the ADC may be calculated via [log2 max P(d)]

-

log2 S Where "S" denotes the digitisation resolution (i.e. length of the output bit-string) of the ADC. This means that the higher the probability of the most probable intensity value (i.e. max P (d)), the lower the resulting minimum entropy Hmin.

As noted above, the probability distribution P (d) of the intensity-randomised pulses generated by the device of Figure 1 is non-uniform which limits the amount of randomness that can be extracted per clock cycle. More specifically, the probability distribution of intensity-randomised pulses generated by interference of phase-randomised pulses may follow an arcsine distribution. This is illustrated in Figure 2 which shows a histogram P10 of (simulated) normalised intensity values detected by the photodiode P6 and digitised into an 8-bit value by the ADC 8. It can be seen that the histogram P10 generally follows an arcsine distribution. Notably, the high probabilities for intensity values of "0.0" and "1.0" limit the amount of entropy that can be generated per clock cycle. For the example of in Figure 2, the entropy minimum entropy Hmin is only 0.58 per bit or 4.64 bits of entropy (i.e. randomness) per 8-bit ADC output (i.e. per clock cycle).

It is known that the digital post-processing unit P9 may apply a mathematical smoothing function (such as a finite impulse response (FIR) filter or a Toeplitz matrix) during digital post-processing to increase uniformity of the signal (i.e. before generating the digital random number). Such digital post-processing steps are undesirable since they are computationally/resource-intensive and/or require additional components (i.e dedicated circuitry).

Figure 3 shows an example optical quantum random number generator (QRNG) 1 according to an embodiment. In general, the QRNG 1 is configured to generate (and output) random numbers from the random intensity of interfered optical pulses. In particular, the QNRG 1 is configured to generate N optical pulses (N a. 2) with randomised intensities and to combine the N optical pulses (either in the optical or in the electrical domain) into a single (electrical or optical) pulse so that the resulting combined pulse has more uniformly distributed random intensity values (i.e. more uniformly than the initial N optical pulses). Since, as described above, a more uniform intensity distribution corresponds to a higher amount of generated entropy, the combined pulse may be referred to as a high-entropy intensity-randomised pulse ("high-entropy pulse" for short hereafter). The QNRG 1 is further configured to digitise the high-entropy pulse and to digitally process the digitised high-entropy pulse to the extract random numbers. Since the intensity distribution of the high-entropy pulse is more uniform than the distributions of the initial N optical pulses, more quantum random bits can be extracted per clock cycle. Further, compared to the known QRNG of Figure 1 the need for digital post-processing is reduced.

The QRNG 1 is typically provided as an integrated device (and described as such in the following), i.e. the components of the device 1 are integrated on a common semiconductor substrate (or on multiple semiconductor substrates that are appropriately assembled/connected). However, in other embodiments, the QRNG 1 may also be implemented using discrete (optical fibre pigtailed or free-space) components.

The QRNG 1 comprises a light source 3 configured to emit a plurality of pulses having randomised intensity values. More specifically, the light source 3 may be configured to operate at a fixed clock rate (e.g. at a clock rate of at least 1 GHz) and to emit N intensity-randomised optical pulses (N 2) per clock cycle. Many possibilities of implementing the light source 3 exist (example implementations are described below with reference to Figures 16 to 22). In general, the light source 3 employs optical interference to generate the N intensity-randomised optical pulses. As described above, this means that the distribution of intensity values of the N intensity-randomised optical pulses may be non-uniform. For example, the light source 3 may be configured to generate, for each of the N intensity-randomised optical pulses, an optical pulses having a random phase and to interfere the phase-randomised optical pulse with a second optical pulses (which may or may not have a randomised phase) to convert the random phase into a random intensity of the resulting interference pulse. In this case, the distribution of intensity values of the N intensity-randomised optical pulses may generally follow an arcsine distribution.

The QRNG 1 further comprises a combining unit 5 optically coupled to the light source 3 (e.g. via optical waveguides, free-space optical channels, optical fibres or the like). The combining unit 5 is configured to generate the aforementioned high-entropy pulse from the N intensity-randomised optical pulses. More specifically, the combining unit 5 is configured to receive the N intensity-randomised optical pulses and to generate, as output signal, the high-entropy pulse in the form of an electrical analog signal (e.g. an analog voltage or current signal) which has a signal value (i.e. a voltage value or a current value) that equals a combination of the intensity values of the received intensity-randomised optical pulses. In particular, combining unit 5 may combine the intensity values of the received intensity-randomised optical pulses so that the output signal is characterised by a distribution of random intensity values which is more uniformly distributed than the corresponding distribution of the received intensity-randomised optical pulses. Many possibilities of implementing the combining unit 5 exist (example implementations are described below with reference to Figures 14 and 15). As one example, the combining unit 5 may first optically combine the received intensity-randomised optical pulses and then convert the combined optical pulse into the electrical analog output signal. As another example, the combining unit 5 may first individually convert the received intensity-randomised optical pulses into N electrical signals and then combine the N electrical signals into the electrical analog output signal. However, many other implementations are possible as described further below.

With reference to Figures 4 and 5, example implementations of the combining unit are now described. Figure 4 illustrates a first embodiment of the combining unit. In this example, the combining unit 40 comprises a (single) photodiode 42 configured to receive the N intensity-randomised optical pulses (NB the combining unit 40 of Figure 4 is an example where the combination of the input pulses is performed in the "optical domain"). The output photocurrent generated by the photodiode 42 is provided to the digitiser 7. More specifically, the combining unit 40 is configured so that N intensity-randomised pulses impinge on the photodiode 42 (substantially) at the same time. Thus, the output photocurrent generated by the photodiode 42 is equal to the sum of the N optical input pulse intensities. In this case, to avoid interference of the N input pulses when impinging on the photodiode, the N input pulses may be orthogonal (i.e. mutually exclusive) in at least one degree-of-freedom, e.g. in wavelength, polarisation (e.g. in embodiments where N=2), spatial mode, optical angular momentum, or the like. In one example, the photodiode 42 may have an active area which is large enough so that the N input pulses impinge on different portions of the active area.

Figure 5 illustrates a second embodiment of the combining unit. In this example, the combining unit 50 comprises N photodiodes 521 to 52N corresponding to the N optical input pulses (each photodiode is configured to receive a corresponding optical input pulse). The N photodiodes 521 to 52N generate N corresponding photocurrents which combined by combining circuity 54 (NB the combining unit 50 of Figure 5 is an example where the combination of the input pulses is performed in the "electrical domain"). The combining circuitry 54 is configured to implement a combining function which is described in detail further below. The combining circuitry 54 may comprise analog electrical signal combining circuits (i.e. the combining circuitry 54 may implement the combining function using analog signal processing circuitry (e.g. without using digital signal processing circuitry)). The combining circuitry 54 outputs the high-entropy pulse to the digitiser 7.

Referring back to Figure 3, the QRNG 1 further comprises a digitiser unit 7 configured to generate a random number from the electrical analog signal (i.e. the high entropy pulse) generated by the combining unit 5. The digitiser unit 7 may comprise an analog-to-digital converter (ADC) and a post-processing unit. The ADC converts the received electrical analog signal into a digital signal. The ADC is coupled to a post processor and provides the generated digital signal to the post processor which processes the digital signal using a (suitable and known) randomness extractor algorithm to generate a random number.

In general, the combining unit 5 generates the high-entropy pulse according to a combining function which is selected so that the resulting signal distribution is more uniform than the distributions of the initial N optical pulses (thus enabling more quantum random bits to be extracted per clock cycle). This means that the combining function may be selected based the non-uniform distributions of the initial N optical pulses (which can experimentally be determined for any given light source emitting intensity-randomised optical pulses). The combining function may specify a predefined rule (i.e. a mapping) which receives, as input, the intensity values of the N optical pulses and provides, as output, the signal value of the high-entropy pulse. Thus, the combining function may conveniently be characterised by a corresponding mathematical function /c(/,, ..., IN) (where /1, ..., IN denote the intensities of the N input pulses).

In a first example, the combining function may be a linear weighted sum of the intensity values of the intensity-randomised pulses, such as -J10 = Z7-1 at h Equation (1) where the weights are denoted with the symbol cti. In some embodiments, all of the weights may be positive numbers within a range of 0 to 1. In this case, the weights a1 may be implemented in optical domain or in the electrical domain. For example, the combining unit may comprise optical attenuators to implement the weights (4 of the combining function in the optical domain. For example, the above described combining unit 40 of Figure 4 may further comprise N optical attenuators so that each input pulse is attenuated by a corresponding optical attenuator to implement the weights a1 (it is to be understood that no attenuator may needed for input pulses that are associated with a weight of ai=1). Alternatively, a combining function described by Equation (1) may be implemented in the electrical domain, e.g. by the combining unit 50 of Figure 5 comprising correspondingly configured combining circuitry 54.

The inventors have realised that by summing the intensities from multiple intensity-randomised optical, the resulting probability distribution becomes flatter (for the same number of intensity bins on the ADC), i.e. more uniform. This leads to higher amount of minimum entropy / randomness. This is illustrated in Figure 6 which shows a histogram 60 of simulated signal values for a combining function that sums two input pulses (i.e. Equation (1) with N = 2 and al = a2 = 1) (in the simulations described with reference to Figures 6 to 15, it assumed that each of the input pulse has an intensity distribution that follows an arcsine distribution as described above). It can be seen that the probability of the most probable bin of the histogram is significantly reduced compared to the probability of the most probable bin of the single-pulse histogram P10 of Figure 2.

Figure 7 shows a histogram 70 of simulated signal values for a combining function that sums three input pulses (i.e. Equation (1) with N = 3 and al = a2 = a3 = 1). It can be seen that the probability of the most probable bin of the histogram 70 is further reduced compared to the histogram 60 of Figure 6.

Figure 8 shows a graph of the resulting minimum entropy Hmin as a function of the number of summed optical pulses N. In particular, it can be seen that using single pulses N=1 (corresponding to the conventional QRNG of Figure 1) generates a minimum entropy Hmin of 0.58 per bit, summing N=2 pulses generates a minimum entropy Hmin of 0.80 per bit, and summing N=3 pulses generates a minimum entropy Hmin of 0.90 per bit. For N>3, the minimum entropy Rim, per bit decreases with increasing N. The minimum entropy Hmin per bit may be further increased by selecting appropriate values for the weights a1. For example, a combining function may implement Equation (1) with ai = yi where y denote an relative attenuation factor (i.e. y < 1). For example, a combining function may sum N=2 input pulses with a relative attenuation of y = 0.5 (i.e. Equation (1) with N = 2 and al = 1 and a2 = 0.5). Figure 9 shows histograms 90 and 92 which illustrate the intensity distribution of the first and second input pulse after the weights are applied. Figure 10 shows the intensity distribution 100 after the summation. It can be seen that the probability of the most probable bin of the histogram 100 is reduced compared to the histogram 60 of Figure 6 (which corresponds to the case Equation (1) with N = 2 and al = a2 = 1).

Figure 11 shows a graph of the resulting minimum entropy Hmin as a function of the number of summed optical pulses N and for various relative attenuation factor. In particular, the reference numerals 110, 112, 114 and 116 respectively indicate the resulting minimum entropy Hmin for y = 0, 0.25, 0.5 and 0.75. In this case, the highest minimum entropy is achieved for summation of N=3 pulses and successive attenuations of y = 0.5 (i.e. al = 1,a2 = 0.5, a3 = 0.25), yielding a minimum entropy of 0.93 per bit (i.e. 7.44 bits of randomness for an 8-bit ADC output). It is to be understood that selecting the weights according to ai = yi is only one possibility and that in other embodiments the weights may be chosen differently. It is further to be understood that although the graphs of Figures 8 and 11 show that the highest minimum entropy is achieved for N=3, in some embodiments it may be attractive to sum only N=2 pulses. This is because a summation of N=2 pulses may be implemented with a simpler hardware design (i.e. in the case of N=2, the benefits of a simpler hardware design may outweigh the decrease in produced minimum entropy).

In some embodiments, the combining function may be a linear weighted subtraction of the intensity values of the intensity-randomised pulses (e.g. Equation (1) where at least one of the weights is negative, ai < 0). Such a linear weighted subtraction may be implemented in the electrical domain, e.g. by the combining unit 50 of Figure 5 comprising correspondingly configured combining circuitry 54. The use of a linear weighted subtraction may increase the minimum entropy per bit. In addition, the linear weighted subtraction may be selected such that the average of the high entropy pulse is zero voltage which may be advantageous for downstream handling of the electrical signal, for example when AC coupling is incorporated.

Figure 12 shows a histogram 120 of simulated signal values for a combining function that subtracts N=2 input pulses with equal weights (i.e. Equation (1) with N = 2 and al = -a2). It can be seen that the probability of the most probable bin of the histogram 120 is similar to the histogram 60 of Figure 6.

In some embodiments, the combining function may comprise a modular arithmetic function. Such a modular arithmetic function may be implemented in the electrical domain, e.g. by the combining unit 50 of Figure 5 comprising correspondingly configured combining circuitry 54 (e.g. the combining circuitry 54 may implement the modular arithmetic function using analog electronic circuits).

For example, the modular arithmetic function may be applied after the summation/subtraction of the input pulses as described above. As an example, Figure 13 shows a histogram 130 of simulated signal values for a combining function that sums N=2 input pulses with equal weights and subsequently applies a "mod 1" operation. It can be seen that the probability of the most probable bin of the histograms 130 is lower compared to the histogram 60 of Figure 6. As another example, Figure 14 shows a histogram 140 of simulated signal values for a combining function that sums N=3 input pulses with equal weights and subsequently applies a "mod 1" operation. It can be seen that the probability of the most probable bin of the histogram 140 is lower compared to the histogram 70 of Figure 7.

Figure 15 shows a graph of the resulting minimum entropy Hmin as a function of the number of summed optical pulses N for a combining function that sums N input pulses with equal weights and subsequently applies a "mod 1" operation. It can be seen that the minimum entropy is larger than 0.95 per bit for N a 3.

In some embodiments, the combining function may comprise a polynomial function of the intensity values of the intensity-randomised pulses. Such a polynomial function may be implemented in the electrical domain, e.g. by the combining unit 50 of Figure 5 comprising correspondingly configured combining circuitry 54.

Implementations of the light source 3 of the QRNG 1 will now be described. In an embodiment, the light source 3 of the QRNG 1 may use interference of phase-randomised optical pulses to generate the desired N intensity-randomised optical pulses. As illustrated in Figure 16, the light source 3 comprises a phase-randomised laser pulse source 160, i.e. a coherent light source configured to emit a plurality of pulses of coherent light so that the pulses have randomised phases. In particular, the phase-randomised laser pulse source 160 emits at least N phase-randomised pulses per clock cycle.

Phase-randomisation may be achieved in any suitable manner. As a first example, the coherent light source may comprise a semiconductor laser and phase-randomised optical pulses may be generated by gain-switching the semiconductor laser (further described below with reference to Figure 17). As a second example, the coherent light source may be configured so that a coherence time of the emitted light is shorter than the time between successive phase-randomised pulses. As a third example, the coherent light source may comprise elements to actively randomise the phase of the emitted pulses.

The light source 3 comprises an interference module 162 which acts as phase-to-intensity converter. The interference module 162 is optically coupled to the phase-randomised laser pulse source 160 and configured to generate the plurality of intensity-randomised pulse from the plurality of phase-randomised pulses. More specifically, the interference module 162 is configured to generate, per clock cycle, N intensity-randomised optical pulses from the at least N phase-randomised pulses. The output of the interference module 162 is optically coupled to the combining unit 5.

Example implementations of the phase-randomised laser pulse source and the interference module will now be described with reference to Figures 17 to 22. In the example of Figure 17 the phase-randomised laser pulse source 170 comprises 2N gain-switched semiconductor lasers (grouped into N pairs) configured to generate corresponding 2N phase-randomised optical pulses per clock cycle which are converted by the interference module 172 into N intensity-randomised pulses. In general, a gain-switched semiconductor laser generates light when the laser is switched above the lasing threshold and generates almost no light when the laser is switched below the lasing threshold. Thus, a controller of the QRNG 1 (not shown) may control a modulation of the gain of the semiconductor lasers by modulating an electrical drive current applied to the lasers in a time varying manner. For example, the semiconductor lasers may be periodically switched above and below the lasing threshold by application of a time varying current. In this manner, the 2N lasers may generate 2N phase-randomised pulses per clock cycle. The phase-randomised laser pulse source 170 outputs the 2N phase-randomised pulses to the interference module 172.

The interference module 172 is configured to generate the desired N intensity-randomised pulse from the received 2N phase-randomised pulses by pairwise interference. To this end, the interference module 172 comprises N 2x2-coupler (e.g. (integrated) beam splitters or the like). Each 2x2-coupler comprises two input ports and two output ports. The two input ports are coupled to different lasers of a corresponding pair of the semiconductor lasers. A first output port of each 2x2 coupler is coupled to the combining unit 5. A second output port of each 2x2 coupler is coupled to a corresponding photodetector (e.g. a photodiode). Thus, when two phase-randomised pulse interfere on a respective 2x2-coupler an intensity-randomised pulse exits the first output port towards the combining unit 5 and a "complementary" interference pulse exits the second output port and is detected by the photodetector. The signal detected by the photodetector may be processed to determine whether or not the intensity distribution of the generated interference pulse is normal or anomalous, e.g. whether or not the intensity distribution of the generated interference pulse matches an expected distribution (e.g. whether the detected distribution is sufficiently close to an expected arcsine distribution). A discrepancy between the detected histogram and the expected histogram exceeding a predefined threshold may indicate that the intensity-distribution is not random (e.g. caused by imperfect interference because of a timing mismatch of the corresponding pulses). Thus, detecting the intensity distributions via the photodetectors enables real-time device health monitoring and fault diagnosis (i.e. determining whether and which pair of the lasers is problematic).

It is to be understood that the QRNG may comprise control circuits to control (the frequency and pulse timings of) the semiconductor lasers, so that the optical pulses emitted by a corresponding pair of semiconductor lasers interfere at the corresponding 2x2-coupler.

In a variation of the light source of Figure 17, the interference module may use 2x1-couplers instead of (at least some) of the 2x2-coupler (i.e. in this case, the interference module may not comprise (at least some of) the above described photodetectors).

In another variation of the light source of Figure 17, only one laser of each of the 2N pairs of lasers may be operated to emit phase-randomised pulses (via gain-switching), and the respective other laser may be operated in a (quasi)-continuous mode.

Figure 18 illustrates a variation of the light source which has the same configuration as the light source of Figure 17 except that this variation requires only N + 1 semiconductor lasers (instead of 2N semiconductor lasers) to generate the desired N intensity-randomised optical pulses (thus for N>2, the light source of Figure 18 requires fewer lasers than the light source of Figure 17). More specifically, the phase-randomised laser pulse source 180 comprises N gain-switched semiconductor lasers configured to generate corresponding N phase-randomised optical pulses per clock cycle (i.e. each laser emits one phase-randomised optical pulses per clock cycle) and one "common" semiconductor laser ("Laser N+1" in Figure 18) which acts as phase reference (i.e. the common semiconductor laser may be configured to operate in a continuous or in a (phase-randomised or non-randomised) pulsed mode). Each of the N gain-switched semiconductor lasers is coupled to an input port of a different 2x2-coupler of the interference module 172. The common semiconductor laser is coupled, via an 1xN-coupler, to the other input port of each of the 2x2-couplers. The desired N intensity-randomised pulses are generated by pairwise interference at the N 2x2-coupler.

Figure 19 illustrates an embodiment of the light source in which the phase-randomised laser pulse source 192 comprises only a single gain-switched semiconductor laser configured to generate a stream of N phase-randomised optical pulses per clock cycle (i.e. N phase-randomised optical pulses having well-defined pulse durations and well-defined, regular temporal spacings; the temporal spacing between two consecutive pulses is denoted T). The laser is coupled to a 1xN coupler of the interference module 194. The first to Nth output ports of the 1xN coupler are respectively coupled to first to Nth delay lines configured to delay an optical pulse. The delay time Di of the i-th delay line is Di = i * T (i = 1....N). Each delay line is coupled to a corresponding time delay interferometer (also known as asymmetric MachZehnder interferometer). The phase-randomised pulses are coupled into the time delay interferometer via an input coupler. The time delay interferometer comprises a short and a long arm. The long arm of the time delay interferometer comprises a delay element, which delays the pulses by a time T (i.e. the delay time matches the temporal spacing between two consecutive pulses) with respect to the pulses travelling in the short arm. Thus, the delay element is configured such that the introduced delay is such that each delayed pulse temporally overlaps with a subsequent pulse in the short arm. The delayed and subsequent pulses interfere in a 2x2 output-coupler of the time delay interferometer, and the interfered pulses are sent to the combining unit 5. In some embodiments, other output port of the 2x2 output-coupler may be coupled to a photodetector for monitoring the intensity distribution of the complementary interference pulse (as described above with reference to Figure 17). The light source of Figure 19 enables designs of reduced complexity since only a single laser is used (i.e. no need to stabilise multiple lasers to realise interference between light emitted by different lasers). ;Figure 20 illustrates an embodiment of the light source comprises the phase-randomised laser pulse source 192 (described above with reference to Figure 19) coupled to an interference module 194. The interference module 194 comprises a time delay interferometer (the long arm of the time delay interferometer comprises a delay element, which delays the pulses by a time T (i.e. the delay time matches the temporal spacing between two consecutive pulses) with respect to the pulses travelling in the short arm) coupled to a 1xN coupler. In particular, the phase-randomised pulses emitted by phase-randomised laser pulse source 192 are coupled into the time delay interferometer via an input coupler. The delayed and subsequent pulses interfere in a 2x2 output-coupler of the time delay interferometer, and the interfered pulses are sent to the 1xN coupler. The first to Nth output ports of the 1xN coupler are respectively coupled to first to Nth delay lines configured to delay an optical pulse. The delay time Di of the i-th delay line is Di = i * T (i = 1....N). The delays lines are coupled to the combining unit 5. In some embodiments, other output port of the 2x2 output-coupler may be coupled to a photodetector for monitoring the intensity distribution of the complementary interference pulse (as described above with reference to Figure 17).

Figures 21 illustrates an embodiment of the QRNG that uses lasers emitting light at different optical frequencies. More specifically, the QRNG comprises a phase-randomised laser pulse source 212 comprising N gain-switched semiconductor lasers. Each laser is configured to generate two phase-randomised optical pulses per clock cycle (as above, the temporal spacing between the pulses is denoted T). The lasers are coupled to an interference module 214 which comprises a time delay interferometer (the long arm of the time delay interferometer comprises a delay element, which delays the pulses by a time T (i.e. the delay time matches the temporal spacing between the two pulses) with respect to the pulses travelling in the short arm). Thus, each laser is coupled (e.g. via a combination of beam splitters) to an input port of the time delay interferometer. The delayed and subsequent pulses emitted by the same laser interfere in a 2x2 output-coupler of the time delay interferometer, and the interfered pulses are sent to the digitiser 7. Pulses emitted by different lasers may not interfere since, in this embodiment, the lasers emit at different optical frequencies. Thus, in this embodiment, high-entropy pulses exit the time delay interferometer.

Figures 22 illustrates an embodiment of the QRNG that uses polarisation rotation to generate high-entropy pulses from two phase-randomised pulses (i.e. N=2). More specifically, the QRNG comprises a phase-randomised laser pulse source 222 comprising a first and a second gain-switched semiconductor laser. Each laser is configured to generate two phase-randomised optical pulses per clock cycle (as above, the temporal spacing between the pulses is denoted T). The lasers are coupled to an interference module 224 which comprises polarisation rotating element configured to rotate the polarisation of the light emitted by the second laser to be orthogonal to the polarisation of the light emitted by the first laser. The interference module 224 further comprises a time delay interferometer (the long arm of the time delay interferometer comprises a delay element, which delays the pulses by a time T (i.e. the delay time matches the temporal spacing between the two pulses) with respect to the pulses travelling in the short arm). The first laser is coupled to a first input port of the time delay interferometer and the output of the polarisation rotation element is coupled to a second input port of the time delay interferometer. The delayed and subsequent pulses emitted by the same laser interfere in a 2x2 output-coupler of the time delay interferometer, and the interfered pulses are sent to the digitiser 7. Pulses emitted by different lasers may not interfere in the 2x2 output-coupler because of these pulses have orthogonal polarisation. Thus, in this embodiment, high-entropy pulses exit the time delay interferometer.

Whilst certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel devices, and methods described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the devices, methods and products described herein may be made without departing from the spirit of the inventions.

The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.

Claims (20)

1. CLAIMS: 1. A quantum random number generator comprising: a light source configured to emit a plurality of pulses having randomised intensity values; a combining unit optically coupled to the light source and configured to generate an electrical analog signal so that a signal value of the electrical analog signal equals a combination of the intensity values of the plurality of intensity-randomised pulses; and circuitry configured to generate a random number from the electrical analog signal.
2. 2. The quantum random number generator of claim 1, wherein the combining unit generates the electrical analog signal according to a combining function, the combining function comprising one or more of: a linear weighted sum of the intensity values of the intensity-randomised pulses; a linear weighted subtraction of the intensity values of the intensity-randomised pulses; a modular arithmetic function of the intensity values of the intensity-randomised pulses; and a polynomial function of the intensity values of the intensity-randomised pulses.
3. 3. The quantum random number generator of claim 2, wherein the combining unit comprises one or more optical attenuators to implement weights of the combining function.
4. 4. The quantum random number generator of any one of the preceding claims, wherein the combining unit comprises an optical detector configured to detect the plurality of intensity-randomised pulses.
5. 5. The quantum random number generator of claim 4, wherein the optical detector comprises an optical sensor and the plurality of intensity-randomised pulses impinges upon different portions of the optical sensor.
6. 6. The quantum random number generator of any one of claims 1 to 3, wherein the combining unit comprises, for each one of the plurality of intensity-randomised pulses, a respective optical detector configured to detect the corresponding intensity-randomised pulses.
7. 7. The quantum random number generator of claim 6, wherein the combining unit comprises circuitry to implement the combining function from the electrical signals generated by the optical detectors.
8. 8. The quantum random number generator of any one of the preceding claims, wherein the light source comprises a coherent light source configured to emit a plurality of pulses of coherent light so that said pulses have randomised phases; and an interference module optically coupled to the coherent light source and configured to generate the plurality of intensity-randomised pulse from the plurality of phase-randomised pulses.
9. 9. The quantum random number generator of claim 8, wherein the coherent light source comprises, for each one of the plurality of intensity-randomised pulses, a corresponding primary laser configured to emit a respective phase-randomised pulse.
10. 10. The quantum random number generator of claim 9, wherein the primary lasers operate at different optical frequencies.
11. 11. The quantum random number generator of claim 9, wherein the coherent light source further comprises one or more secondary lasers optically coupled to the interference module, and the interference module is further configured to generate the plurality of intensity-randomised pulse from the plurality of phase-randomised pulses by interfering the plurality of phase-randomised pulses with coherent light emitted by the one or more secondary lasers.
12. 12. The quantum random number generator of claim 11, wherein the one or more secondary lasers are operated in a pulsed operation mode or in a continuous operation mode.
13. 13. The quantum random number generator of claim 11 or 12, wherein the secondary lasers comprise, for each one of plurality of intensity-randomised pulses, a corresponding secondary laser.
14. 14. The quantum random number generator of claim 8, wherein the coherent light source comprises, for each one of the plurality of intensity-randomised pulses, a corresponding pair of primary lasers, each primary laser configured to emit a respective phase-randomised pulse.
15. 15. The quantum random number generator of claim 8, wherein the coherent light source comprises a primary laser configured to emit, for each one of the plurality of intensity-randomised pulses, a pair of phase-randomised pulses, and the interference module comprises a time delay interferometer to pairwise interfere the pulses emitted by the primary laser.
16. 16. The quantum random number generator of claim 8, wherein the plurality of intensity-randomised pulses consists of a first intensity-randomised pulse and a second intensity-randomised pulse, and the coherent light source comprises a first primary laser configured to emit a first pair of phase-randomised pulses and a second primary laser configured to emit a second pair of phase-randomised pulses, and the interference module generates the first intensity-randomised pulse and the second intensity-randomised pulse so that the optical polarisations of the first and second intensity-randomised pulse are orthogonal.
17. 17. The quantum random number generator of any one of claims 8 to 16 further comprising at least one photodetector, wherein the interference module comprises, for each of at least one of the plurality of intensity-randomised pulses, a corresponding 2x2-coupler comprising a first output port for providing intensity-randomised pulses to the combining unit, and a second output port coupled to the photodetector.
18. 18. The quantum random number generator of any one of claims 8 to 17, wherein at least one of the primary lasers is a gain-switched semiconductor laser.
19. 19. The quantum random number generator of any one of the preceding claims, wherein the circuitry comprises a digitising unit configured to perform an analog to digital conversion of the electrical analog signal.
20. 20. The quantum random number generator of claim 19, wherein the circuitry further comprises a processing unit configured to apply a randomness extraction process to the digitally converted electrical analog signal.