Quantum random number generator

Abstract

A system for generating random numbers based on quantum mechanical properties of photons. A photon source generates one or more photons, which are guided to a working optical path where a splitting component probabilistically guides the one or more photons to one of a plurality of heterogeneous optical paths. The photon is then detected at a detecting means, based on which a random number may be determined according to the heterogeneous optical path taken by the photon.

Description

QUANTUM RANDOM NUMBER GENERATORField of the Invention

[0001] The present invention relates generally to a random number generator, more specifically to a system which utilises the quantum properties of photons to generate random numbers.Background

[0002] Generation and usage of random numbers is essential in the modern world. For example, random number generation is used for encrypted communications, computer simulations, and many other purposes. Many random number generators rely on software-based approaches, however these are generally recognised as producing only pseudorandom numbers, and thus are not perfect solutions, which can result in security flaws, imperfect results or other sub-optimal results.

[0003] Hardware-based approaches for generating random numbers do exist, and are generally seen as providing random numbers with a greater degree of randomness than software-based solutions. However, even these hardware-based approaches are, in general, not truly random. Furthermore, existing hardware-based random number generators generally have low generation rates, making them unsuitable for many modern applications. The present invention identifies a new approach for generating random numbers which provides truly random numbers in a scalable manner.Summary of the Invention

[0004] The invention is defined by the appended claims.

[0005] According to a first aspect of the invention, there is provided: a system for generating a random number, the system comprising: a photon source configured to generate one or more photons each having a predefined wavelength; guiding means configured to deterministically guide the one or more generated photons to a working optical path according to the wavelength of the one or more photons; one or more splitting components arranged on the working optical path and configured to probabilistically guide the photon to one of a plurality of heterogeneous optical paths; and detecting means configured to detect the photon after it has travelled along a particular optical path of the plurality of heterogeneous optical paths, wherein the detecting means is configured to output a signal indicating a random number, wherein the indication of the random number comprises an indication of the particular optical path taken by the photon. Accordingly, a random number generator is provided which generates random numbers based on quantum mechanical properties of photons, and therefore provides numbers which are truly random.

[0006] In some examples, the guiding means may be further configured to supress photons having wavelengths other than the predefined wavelength(s). As such, random numbers may be generated as discussed herein with imperfect photon sources which produce additional photons other than the photon(s) of a desired predetermined wavelength(s).

[0007] According to some examples, the photon source may be configured to generate two or more photons each having a respective predefined wavelength, wherein the two or more photons include a first photon having a first predefined wavelength, and one or more second photons each having a respective second predefined wavelength, wherein the first and the one or more second predefined wavelengths are different; wherein the guiding means is a multiplexer or demultiplexer configured to deterministically guide each of the two or more photons to either a control optical path or one or more working optical paths according to the wavelength of the respective photon,wherein the guiding means is configured to guide the first photon to one of the one or more working optical paths, and the guiding means is configured to guide the second photon to the control optical path; and wherein the 1-to-N splitters are arranged on the one or more working optical paths. Advantageously, the detecting means may be further configured to detect photons that have travelled on the control optical path, and wherein the detecting means is configured to output a signal indicative of detection of the second photon from the control optical path. As such, it is possible to determine, using the control photon, that a photon has been generated by the photon source such that a photon can be expected to be detected within a particular time period.

[0008] In some examples, the two or more photons generated by the photon source are wavelength multiplexed. As such, the two or more photons may be generated concurrently as part of a single emission process by the photon source, and random numbers can be generated at a high rate based his single emission process.

[0009] Advantageously, the photon source may be further configured to generate an additional photon having a particular predefined wavelength, and wherein the guiding means is configured to deterministically guide a first photon having a first predetermined wavelength to a first working optical path, and deterministically guide the additional photon to a second working optical path according to the respective wavelengths of the first and additional photons. As such, multiple random numbers may be generated based on a single photon source emission process, thereby increasing the rate of random number generation.

[0010] According to certain examples, each of the plurality of heterogeneous optical paths may be arranged to terminate at the detecting means in different spatial locations; and optionally wherein the indication of the random number comprises an indication of the spatial location at which the photon is detected by the detecting means. As such, the random numbers may be generated based on the spatial location at which a photon is detected, thereby providing a reliable system which provides an unambiguous indication of a random number.

[0011] In some cases, the system may further comprise: an N-to-1 combiner configured to guide photons on any of the plurality of heterogeneous optical paths to a common spatial termination location at the detecting means, wherein each of the plurality of heterogeneous optical paths are arranged to terminate at the common spatial termination location, wherein each of the plurality of heterogeneous optical paths has a different length; and optionally wherein the indication of the random number comprises an indication of a time at which the photon is detected by the detecting means. As such, the random numbers may be generated based on the time at which a photon is detected, thereby providing a comparatively compact system, which only detects photons on the working optical path at a single spatial locations.

[0012] In some cases, the 1-to-N splitter, the plurality of heterogeneous optical paths, and the N- to-1 combiner may form an optical loop, and wherein the system further comprises: a coupler configured to permit photons on the working optical path to enter the optical loop, and to permit photons on the optical loop to exit the optical loop towards the common spatial termination location. The coupler may be a deterministic coupler configured to deterministically cause photons on the working optical path to enter the optical loop, and to deterministically cause photons on the optical loop to exit the optical loop towards the common spatial termination location.

[0013] Alternatively, the coupler may be a probabilistic coupler configured to probabilistically permit photons on the working optical path to enter the optical loop, and to probabilistically permit photons on the optical loop to exit the optical loop towards the common spatial terminationlocation. Accordingly, the range of random numbers that may be generated may be increased as photons may perform any number of passes / laps of the optical loop, thereby increasing the number of possible optical paths (and hence detection times) for the photon. In some cases, the coupler may be tuneable to alter a probability with which photons are permitted to enter and exit the optical loop, thereby providing flexibility and an ability to adapt the distribution of random numbers which may be generated.

[0014] In some examples, the detecting means may comprise one or more photodetectors. As such, photons may be reliably detected. Furthermore, in some cases the photon source is configured to generate two or more photons via spontaneous parametric down-conversion of spontaneous four-wave mixing. As such, two or more photons of different predetermined wavelengths may be generated concurrently as part of a single emissions process.

[0015] According to some examples, the system may comprise a controller configured to receive the signal indicating a random number and determine a random number according to the signal. As such, the random number may be utilised and / or processed in substantially any manner so as to be suitable for a range of different applications.Brief Description of the Drawings

[0016] Embodiments of the invention will now be described, by way of example only, with reference to the following figures.

[0017] In accordance with one (or more) embodiments of the present invention the Figures show the following:

[0018] Figure 1 illustrates a quantum random number generator configured to generate a random number according to a spatial location at which a photon is detected.

[0019] Figure 2 illustrates a quantum random number generator configured to generate a random number according to a spatial location at which a photon is detected.

[0020] Figure 3 illustrates a quantum random number generator configured to generate a random number according to a time at which a photon is detected.

[0021] Figure 4 illustrates a quantum random number generator configured to generate a random number according to a time at which a photon is detected.

[0022] Figure 5 illustrates a quantum random number generator configured to generate multiple random numbers according to the spatial locations at which multiple photons are detected.

[0023] Figure 6 illustrates a quantum random number generator configured to generate multiple random numbers according to the times at which multiple photons are detected.

[0024] Figure 7 illustrates a general quantum random number generator based on quantum mechanical properties of photons.

[0025] Any reference to prior art documents in this specification is not to be considered an admission that such prior art is widely known or forms part of the common general knowledge in the field. As used in this specification, the words “comprises”, “comprising”, and similar words, are not to be interpreted in an exclusive or exhaustive sense. In other words, they are intended to mean “including, but not limited to”. The invention is further described with reference to the following examples. It will be appreciated that the invention as claimed is not intended to be limitedin any way by these examples. It will also be recognised that the invention covers not only individual embodiments but also combination of the embodiments described herein.

[0026] The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and / or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and / or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the spirit and scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc, other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in future.Detailed Description

[0027] Random number generators are utilised for a variety of modern applications. However many random number generators do not produce numbers which are truly random. As such, the applications in which said numbers are used may be subject to (security) vulnerabilities, inaccuracies, or other drawbacks. Only random number generators which utilise truly random processes can produce truly random numbers. Quantum mechanics is an inherently probabilistic physical theory and as such random number generators which rely upon quantum mechanical properties may produce numbers which are truly random. The present invention provides a random number generator which produces truly random numbers based on the quantum mechanical properties of individual photons.

[0028] Figure 1 illustrates an example of a system 100 for generating a random number according to the present disclosure. The system includes a photon source 110 which is configured to generate one or more photons each having a respective predefined wavelength. For example, the photon source may be a so-called ‘single-photon source’, or may generate a predetermined number of photons, such as 2 photons, 3 photons, or more, and as such may be referred to as an n-photon source. Each of the photons generated by such an n-photon source has a different predetermined wavelength. As such, the photon source 110 is configured to generate a predefined number of individual photons (rather than being a coherent light source), and as such photons emitted by the photon source 110 exhibit quantum mechanical characteristics. Photon sources such as photon source 110 which generate a predefined number of individual photons exhibiting quantum mechanical properties are known in the art, as discussed in [1], the entire contents of which are hereby incorporated by reference. It should also be appreciated that in some cases the photon source 110 may produce additional unwanted photons having wavelengths other than the predefined wavelength(s). Such a photon source may still be referred to as single-photon sources or n-photon sources, as they produce a predetermined number of photons of respective predetermined wavelengths (e.g. one photon at each of one or more predetermined wavelengths), from a single emission process (e.g. pumping cycle).

[0029] The photon source 110 of Figure 1 is optically connected to a guiding means 120 configured to guide the generated photons to a working optical path 121. In other words, photons generated by the photon source 110 may pass to the guiding means 120 from the photon source 110 (e.g. via an optical pathway). The guiding means 120 may include one or more optical fibres, or any other optical pathway suitable for guiding photons. The guiding means 120 may also include one or more additional components. For example, in the example of Figure 1 , the photonsource 110 is only required to generate a single photon in order to generate a random number. Therefore, the guiding means 120 may include filtering means configured to filter the n photons in order to provide only one photon to the working optical path 121. For example, a filtering means (i.e. suppressing means) may filter the photons according to wavelength, i.e. using a wavelength division multiplexer or demultiplexer (WDM) (referred to as a (de)multiplexer for brevity), such that a single photon of a predetermined wavelength is provided to the working optical path 121. The working optical path 121 may include one or more optical fibres or any other alternative or additional components suitable for propagation of photons.

[0030] The working optical path 121 guides the photons to a splitting component (hereinafter referred to as a 1-to-N splitter) 130, which may be considered to be arranged on the working optical path 121. The 1-to-N splitter 130 probabilistically guides a photon to one of N heterogeneous optical paths 131 (1)-131 (N). That is, due to the quantum mechanical properties of individual photons, an individual photon passing though the 1-to-N splitter 130 passes to only one of the N heterogeneous optical paths 131 (1)-131 (N) according to a probability distribution. As such, each of the N heterogeneous optical paths 131 (1)-131 (N) has an associated probability of receiving the photon. The associated probability of each of the heterogeneous optical paths 131 (1 )-131 (N) may be equal, or the associated probability of each of the heterogeneous optical paths 131 (1)-131 (N) may follow a different distribution, such as a Gaussian distribution or any other non-equal probability distribution. The properties / configuration of the 1-to-N splitter 130 may be adjustable (e.g. via an external controller) in order to adjust the probability distribution associated with the heterogeneous optical paths 131 (1)-131 (N).

[0031] The heterogeneous optical paths 131 (1)-131 (N) terminate at a detecting means 140 which is configured to detect photons, such as one or more photodetectors. In the example of Figure 1 , each of the heterogeneous optical paths 131 (1 )-131 (N) are heterogeneous as they each terminate at the detecting means 140 at a different spatial location 141 (1)-(N). For example, a photodetector may be located at each of the different spatial locations 141 (1)-(N). As such, according to the particular heterogeneous optical path 131 (1 )-131 (N) on which the photon travels from the 1-to-N splitter 130, the photon is detected by the detecting means 140 at a particular spatial location 141 (1)-(N). In other words, the spatial location 141(1)-(N) at which the photon is detected is determined by the heterogeneous optical path 131 (1)-131 (N) to which the photon passes from the 1-to-N splitter 130, which is probabilistic (due to the quantum mechanical properties of the photon) and therefore random. Moreover, as the spatial location 141 (1)-(N) which the photon is detected is determined by quantum mechanical properties, the spatial location 141 (1 )-(N) at which the photon is detected is truly random.

[0032] The detecting means 140 is further configured to output a signal indicative of the optical path 131 (1 )-131 (N) taken by the detected photon, and therefore indicative of a random number. In the example of Figure 1 , the optical path 131 (1)-131 (N) taken by a photon is determined (e.g. by the detecting means 140 or an external controller) based on the spatial location 141 (1 )-(N) at which the photon is detected. Therefore, according to this example the detecting means 140 is configured to output a signal indicative of the spatial location 141 (1)-(N) at which the photon is detected. The signal may be an output to a controller or other processing means which may process the signal using classical or quantum processing means. The signal may be an electrical signal, an optical signal, a radio frequency (RF) signal, or substantially any other form of signal capable of indicating the spatial location 141(1)-(N) at which the photon is detected. The controller or other processing means may determine a random number according to the signal indicating the spatial location 141(1)-(N) at which the photon is detected. The number of possible random numbers corresponds to the number of heterogeneous optical paths 131 (1)-131 (N). Accordingly,information within the signal output by the detecting means may itself be considered to be a random number. In other examples, the controller or other processing means may be considered to form part of the detecting means, and the detecting means may therefore output a random number (based on the heterogeneous optical path 131 (1 )-(N) taken by the photon) for use by one or more other entities.

[0033] Figure 2 shows another example of a system 101 for generating a random number according to the present disclosure. Unless otherwise stated, like reference numerals refer to the same components as illustrated and discussed with respect to the other figures shown herein. The photon source 110 according to the example of Figure 2 is configured to generate two or more photons having different predefined wavelengths. In other words, the photon source 110 generates a pair of photons through a single generation process, where the photons have predetermined wavelengths which are different from one another. The pair of photons may be generated by the photon source concurrently as part of a single emission mechanism, for example via spontaneous parametric down-conversion of spontaneous four-wave mixing. It should also be appreciated that in some cases the photon source 110 may produce additional photons having wavelengths other than the predefined wavelengths. Such a photon source may still be referred to as an n-photon source, as it produces n photons at respective predetermined wavelengths (e.g. one photon at each of two or more predetermined wavelengths), from a single emission process (e.g. pumping cycle).

[0034] The guiding means 120 is configured to guide each of the pair of photons to a particular optical path. In particular, the guiding means 120 is configured to guide a first (i.e. a signal / working) photon having a first wavelength to the working optical path 121 , and to guide a second (i.e. control) photon having a second wavelength to a control optical path 122. While this example generally refers to a signal / working photon as a first photon, it should be appreciated that this example is also applicable to multi-photon systems utilising more than one signal / working photon. As such, reference to the first photon should generally be understood to refer to signal / working photons in general. The control optical path 122 may comprise an optical fibre or any other suitable means for propagation of individual photons, and the control optical path 122 may terminate at the detecting means 140 at a particular spatial location 142 different from the spatial locations 141 (1)-(N) at which the heterogeneous optical paths 131 (1)-131 (N) terminate. Accordingly, the detecting means 140 may detect the second photon (i.e. the control photon) independent of the first photon. The detection of the second photon may be used for calibration of one or more components of the system 101 such as the photon source 110 or guiding means 120. Additionally or alternatively, the detection of the second photon may be used for timing purposes in order to provide an indication that a photon can be expected to be detected on one of the heterogeneous optical paths 131 (1)-131 (N) (i.e. at one of the spatial locations 141(1)-(N)) within a particular period of time. The detecting means 140 may therefore output a signal (such as an electrical signal, an optical signal or substantially any other form of signal) indicating that a photon has been detected on the control optical path 122 (i.e. at the particular spatial location 142). This signal may be a different signal to the signal indicating the detection of the first photon on a particular heterogeneous optical path 131 (1)-131 (N), or may be part of the same signal.

[0035] As with Figure 1 , the detecting means 140 detects the photon on one of the heterogeneous optical paths 131 (1)-(N) and outputs a signal indicative of a random number, based on the heterogeneous optical path 131(1)-(N) taken by the photon. The number of possible random numbers N corresponds to the number of heterogeneous optical paths 131 (1)-(N). As an example, the detecting means 140 may output a signal indicating that a photon has been detected at a particular location 141 (1 )-(N). For example, the signal may be an electrical current transmitted toa particular port of a controller (which may be the case for any of the examples described in the present disclosure), however other types of signal are envisaged. The signal may in some cases indicate a label for the particular location 141 (1)-(N) or the heterogeneous optical path 131(1)- (N). Furthermore, in some examples, the detecting means 140 may itself determine a random number according to the particular location 141 (1 )-(N) at which the photon is detected, and output the random number within the signal. The same signal may be used for indicating the detection of signal / working photons (e.g. the first photon) on a heterogeneous optical path 131 (1)-(N) and for indicating the detection of the control photon on the control optical path 122. Alternatively, different signals may be used for indicating the detection of the signal / working photons on a heterogeneous optical path 131(1)-(N) and for indicating the detection the control photon on the control optical path 122. For example, the detecting means 140 may output a signal indicating the detection the control photon on the control optical path 122 upon detection of the second photon, and subsequently output a signal indicating the detection of one or more signal / working photons upon detection of the signal / working photons.

[0036] In addition to guiding the first photon to the working optical path 121 and the second photon to the control optical path 122, the guiding means may also filter (i.e. suppress) additional photons not having the first or second wavelength. For example, as discussed above the guiding means 120 may include a WDM which deterministically guides the first photon to the working optical path 121 and deterministically guides the second photon to the control optical path 122, and which absorbs or otherwise suppresses photons of other wavelengths. It should, however, be appreciated that the guiding means 120 may include other components for filtering photons of other wavelengths, and or other means for guiding the first and second photons to their respective optical paths. In addition, while the working optical paths 121 shown in Figures 1 and 2 each include only a single 1-to-N splitter 130, it should be appreciated that one or more additional splitting components (e.g. 1-to-N splitters) may be located on any of the heterogeneous optical paths 131 (1)-(N) to further increase the range of random numbers that may be produced. This is generally applicable to all of the examples discussed in the present disclosure.

[0037] Figures 1 and 2 discussed above therefore provide systems for generating random numbers based on the spatial location at which a photon is detected, which is ultimately determined by the quantum mechanical properties of the photon at a 1-to-N splitter, and where the number of possible random numbers is determined by the number of spatial locations at photons may be detected (i.e. the number of heterogeneous optical paths). It should be appreciated that the while the systems of Figures 1 and 2 will produce random numbers in the range of 1-N, additional processing may be employed by a controller or other processing means (using classical or quantum computing hardware) to increase the range of the random numbers produced, alter the distribution of random numbers procedure, utilise the random number produced as a seed for a further process, or perform any other form of processing or mathematical operations using the random number.

[0038] In addition to the examples of Figures 1 and 2 which utilise spatial differences in the detection of photons, the present disclosure additionally conceives of systems in which a random number is generated based on the time at which a photon is detected. Figure 3 shows an example of such a system 200. The system 200 includes a photon source 210 configured to generate two or more photons (i.e. an n photon source such as the photon source 110 discussed above in relation to Figures 1 and 2) having different predetermined wavelengths. The system 200 additionally includes guiding means 220 configured to guide a first photon having a first predetermined wavelength to a working optical path 221 and to guide a second photon having a second (different) predetermined wavelength to a control optical path 222.

[0039] A 1-to-N splitter 230 is located on the working optical path 230. The 1-to-N splitter 230 is (in a similar manner to the 1-to-N splitter 130 discussed above in relation to Figures 1 and 2) configured to probabilistically guide the first photon to one of a plurality of heterogeneous optical paths 231 (1)-231(N). The system 200 additionally includes an N-to-1 combiner 235 (i.e. an N-to- 1 splitter or multiplexer) configured to guide photons on any of the plurality of heterogeneous optical paths 231(1)-231 (N) to a common termination location 241 at a detecting means 240. The detecting means 240 may be substantially similar to the detecting means 140 described above and may include any suitable means (such as one or more photodetectors) for detecting photons at the common spatial location 241 at which the heterogeneous optical paths 231(1)-231(N) terminate, and for detecting photons at a separate spatial location 242 at which the control optical path 222 terminates. The N-to-1 combiner 235 may additionally include an optical fibre or any other suitable means for propagation of photons in order to guide the photon to the common termination location 241. Accordingly, the first photon is detected at the detecting means 240 at the common termination location 241 regardless of the particular heterogeneous optical path 231 (1)-231 (N) on which the first photon travels. However, it should be appreciated that in some cases the example of Figure 3 may be modified such that each of the heterogeneous optical paths 231 (1)-231 (N) terminates at a different spatial location at the detecting means 240, in a similar manner to Figures 1 and 2.

[0040] In the example of Figure 3, the heterogeneous optical paths 231(1)-231(N) are heterogeneous as each of the heterogeneous optical paths 231(1)-231(N) has a different length (i.e. different physical length). As such, the first photon takes a different amount of time to reach the common termination location 241 based on the particular optical path 231(1)-231 (N) taken. As such, the first photon is detected at the detecting means at a particular time based on the particular optical path 231 (1)-231 (N) taken by the first photon. Accordingly, the optical path 231 (1)-231 (N) taken by the first photon may be determined based on a time window in which the first photon is detected. Each of the optical paths 231 (1)-231(N) may be arranged such that the first photon will always be detected by the detecting means 240 after the second photon has been detected on the control optical path 222. Accordingly, the control optical path 222 and the detection of the second photon on the control optical path 222 may be used for timing of the first photon. For example, a preconfigured time window after the detection of the second photon may be assigned to each of the optical paths 231(1)-231(N). Based on the time window in which the first photon is detected, it may be determined that the photon travelled on a particular one of the optical paths 231 (1)-231(N), and as such a random number may be determined.

[0041] The detecting means 240 may output a signal indicative of the optical path 231(1)-231 (N) taken by the first photon. For example, the detecting means 240 may output respective signals upon detection of the second and first photons, where the time of receipt of the signal by a controller or other processing means is taken as an indication of the time at which the respective photon is taken. The controller or other processing means may then, based on the times at which the respective signals are received, determine the optical path 231 (1)-231 (N) taken by the first photon. In some examples the signal may be electrical currents transmitted upon detection of the respective photons, however the signals may take substantially any form and may include substantially any quantity of information. For example, in some cases the detecting means 240 may output one or more signals indicating absolute times or relative times at which the first and second photons are detected. Alternatively, the detecting means 240 may itself determine a time window (after detection of the second photon) at which the first photon is detected and output a signal indicative of said time window. Alternatively, the detecting means 240 may determine for itself the optical path 231 (1 )-231 (N) taken by the first photon, based on the time window in whichthe first photon is detected, and output a signal indicative of the determined optical path 231(1)- 231 (N) taken by the first photon. A controller or other processing means may determine a random number according to the optical path 231 (1)-231(N) taken by the first photon (i.e. according to a time window after detection of the second photon in which the first photon is detected), as indicated by the signal output by the detecting means. Alternatively, the detecting means 240 may itself determine the random number according to the optical path 231 (1)-231 (N) taken by the first photon, and may output a signal indicating the random number for use by one or more other entities. While this example of Figure 3 generally refers to a signal / working photon as a first photon, it should be appreciated that this example is also applicable to multi-photon systems utilising more than one signal / working photon. As such, reference to the first photon should generally be understood to refer to signal / working photons in general.

[0042] Therefore, the example of Figure 3 provides a system whereby a range of random numbers may be produced based on the time at which a photon is detected, where the number of possible random numbers corresponds to the number of heterogeneous optical paths. It should be appreciated that the while the system of Figure 3 will produce random numbers in the range of 1-N, additional processing may be employed by a controller or other processing means (using classical or quantum computing hardware) to increase the range of the random numbers produced, alter the distribution of random numbers procedure, utilise the random number produced as a seed for a further process, or perform any other form of processing using the random number.

[0043] Figure 4 shows a system 201 which is a modification to the system 200 of Figure 3, which increases the number of heterogeneous optical paths, and hence increases the range of random numbers that can be produced. The photon source 210 and guiding means 220 may be substantially similar to those described above with respect to Figure 3. In particular, the photon source 210 generates two or more photons and the guiding means guides a first photon to a working optical path 221 and guides a second photon to a control optical path 222.

[0044] Arranged on the working optical path 221 is an optical coupler 225. The coupler 225 may permit photons to pass from the working optical path 221 to an optical loop 250. In the optical loop 250, photons pass through a 1-to-N splitter 230 to one of a plurality of heterogeneous optical paths 231 (1)-231(N), each having a different length, as in the example of Figure 3. An N-to-1 combiner 235 then combines the heterogeneous optical paths 231 (1)-231(N) to provide a single optical pathway which returns the photon to the coupler 225, regardless of the heterogeneous optical path 231(1)-231 (N) on which the photon travelled. The coupler may then permit the photon to exit the optical loop 250 towards the detecting means 240, where it can be detected as a particular spatial location 241.

[0045] The coupler 225 may be a deterministic coupler, such that 100% of photons which enter the coupler 225 from the working optical path 221 will pass to (i.e. enter) the optical loop 250, and 100% of photons which enter the coupler 225 from the optical loop 225 will exit the optical loop 250 towards the detecting means 240. However, in other examples the coupler 225 may be a probabilistic coupler which permits photons which enter the coupler 225 from the working optical path 221 will pass to the optical loop 250 with a probability of less than 100%, and which permits photons which enter the coupler 225 from the optical loop 225 to exit the optical loop 250 towards the detecting means 240 with a probability of less than 100%. In particular, photons entering the coupler 225 from the working optical path 221 may in some cases pass directly to the detecting means 240, without entering the optical loop 250. Furthermore, photons in the optical loop 250 entering the coupler 225 (which have already traversed one of the heterogeneous optical paths231 (1 )-231 (N) may remain in the optical loop 250 and traverse one of the heterogeneous optical paths 231 (1)-231(N) again. Accordingly, photons may perform no laps / passes of the optical loop 250, may perform one lap / pass of the optical loop 250, or may perform two or more laps / passes of the optical loop 250.

[0046] The coupler 225 may be probabilistic in that whether the photon enters or exits the optical loop 250 may be based on the quantum mechanical properties of the photon. As such, whether the photon enters or exits the optical loop 250 is truly random. Therefore, the number of laps / passes of the optical loop 250 made by each photon is also random, as well as the heterogeneous optical path 231(1)-231(N) taken by the photon on each loop (the same photon may take a different optical path 231(1)-231(N) on different laps / passes of the optical loop 250). Therefore, the distance travelled by the photon is random, as is the time said photon is detected. Accordingly, a random number may be determined based on the time at which the first photon is detected at the detecting means 240. For example, predetermined time windows may be defined and a number assigned to each time window, such that a random number is identified based on the time window in which the first photon is detected. In some examples, the time windows may be defined for times after the second photon has been detected on the control optical path 222.

[0047] The coupler 225 may be configured such that the probability of a photon entering or exiting the optical loop 250 may be any suitable value, such as 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90%. In addition, the probability of a photon entering the optical loop 250 may be the same as the probability of a photon exiting the optical loop 250, or the probability of a photon entering the optical loop 250 may be different to the probability of a photon exiting the optical loop 250. For example the probability of a photon entering the optical loop 250 may be higher than the probability of a photon exiting the optical loop 250, or the probability of a photon entering the optical loop 250 may be lower than the probability of a photon exiting the optical loop. In addition, the configuration of the coupler 225 may be adjustable (e.g. by a controller or other entity) such that the probability of a photon entering the optical loop 250 and / or the probability of a photon exiting the optical loop 250 may be adjusted. This adjustment of the configuration of the coupler 225 may be performed prior to the generation of the two or more photons by the photon source 210, or may be performed after the generation of the two or more photons but before the detection of the first photon (i.e. mid-run or mid-cycle). As such, the coupler’s 225 associated probabilities (the probability of a photon entering the optical loop 250 and the probability of a photon exiting the optical loop 250) may be adjusted, for example to increase or decrease the expected number of laps / passes of the optical loop 250 made by a photon. This may be useful for calibration purpose, for example to maintain a comparatively low possibility that the first photon is absorbed within the system 201.

[0048] In some cases, the lengths of each of the heterogeneous optical paths 231 (1)-231 (N) may be chosen such that it is possible, based on the time the first photon is detected, to determine the number of times a photon has passed through each of the heterogeneous optical paths 231 (1)- 231 (N). As such, each of the possible paths taken by a photon may have a unique length, such that the time window in which the photon is detected is specific to that path. For systems using a probabilistic coupler 225, the number of possible paths is Nm+ 1, where N is the number of heterogeneous optical paths 231 (1)-231 (N) in the optical loop, and m is the number of laps / passes of the optical loop 250 made by the photon. As such, the number of possible paths, and hence the range of random numbers produced, may scale exponentially. Accordingly, the range of random numbers can be greatly increased without a substantial increase in footprint, therefore providing a compact design.

[0049] While Figure 4 illustrates an example for increasing the range of random numbers produced, the example of Figures 5 and 6 illustrate approaches for increasing the rate of production of random numbers. Figure 5 illustrates a system 300 which generates random numbers based on the spatial location at which a photon is detected, in a similar manner to the examples shown in Figures 1 and 2. The photon source 310 of this example is configured to generate two or more photons have different predetermined wavelengths (and may be substantially similar to the photon sources 110 and 210 of Figures 2-4). The guiding means 320 receives the two photons from the photon source 310 and directs a first photon having a first predetermined wavelength to a first working optical path 321 , and directs a second photon having a second predetermined wavelength to a second working optical path 322 (in substantially the same manner as the guiding means 120 and 220 of Figures 2-4).

[0050] A first 1-to-N splitter 330 (substantially similar to 1-to-N splitters 130 and 230 discussed above in relations to Figures 1-4) is located on the first working optical path 321 and is configured to probabilistically guide the first photon to one of a plurality of first heterogeneous optical paths 331 (1)-331 (N) each arranged to terminate at a detecting means 340 at a different spatial location. The detecting means may be substantially similar to the detecting means 140 and 240 discussed above in relation to Figures 1-4). A second 1-to-N splitter 335 (substantially similar to 1-to-N splitters 130, 230, and 330 discussed above) is located on the second working optical path 322 and is configured to probabilistically guide the second photon to one of a plurality of second heterogeneous optical paths 332(1 )-332(M) each arranged to terminate at the detecting means 340 at a different spatial location. As such, the detecting means 340 is configured to detect the first photon on one of N heterogeneous optical paths 331(1)-331 (N), and to detect the second photon on one of M heterogeneous optical paths 332(1 )-332(M). The number N of first heterogeneous optical paths 331(1)-331 (N) may be equal to or different to the number M of second heterogeneous optical paths 332(1)-332(M). Random numbers can be determined based on the spatial locations at which the first photon and second photon are detected, in a similar manner as discussed above in relation to Figures 1 and 2.

[0051] As the first and second photons are generated from a single emission process from the photon source 310, it is therefore possible to generate two random numbers from a single emission process from the photon source 310. This effectively doubles the rate of random number generation. Moreover, it should be noted that in some examples the photon source may generate one or more additional photons as part of the emission process, each of the one or more additional photons having a different predetermined wavelength to the first and second photons. The additional photons may be guided to additional working optical paths having a 1-to-N splitter and arranged to terminate at the detecting means 340 in a similar manner to the working optical paths 321 , 322 shown in Figure 5. Accordingly, the rate of random number generation may be further increased. Additionally or alternatively, the additional photons may be guided to a control optical path and utilised in a similar manner to the control optical path 122 discussed in relation to Figure 2.

[0052] Figure 6 illustrates a system 400 which generates random numbers based on measuring the time at which a photon is detected (in a similar manner to the examples shown in Figures 3 and 4), and which generates random numbers at an increased rate. Photon source 410 may be substantially similar to photon sources 110, 210, 310, discussed above except that the photon source 410 generates three or more photons as part of a single emission process, where each of the three photons has a different predetermined wavelength. The system 400 includes guiding means 420 which may be substantially similar to the guiding means 120, 220, 320 discussed above. The guiding means 420 guides a first photon having a first predetermined wavelength toa first working optical path 421 based on the wavelength of the first photon. The guiding means 420 guides a second photon having a second predetermined wavelength to a second working optical path 422 based on the wavelength of the second photon. The guiding means 420 guides a third photon having a third predetermined wavelength to a control optical path 423 based on the wavelength of the third photon. The first and second working optical paths 421 , 422 each include one or more 1-to-N splitters 430A, 430B, one or more N-to-1 combiners 435A, 435B and a plurality of heterogeneous optical paths 431(1)-(N), 432(1)-(M) arranged therebetween, in a similar manner to the working optical paths 221 discussed in relation to Figures 3 and 4. In particular, each of the heterogeneous optical paths 431 (1)-(N) has a different length, and each of the heterogeneous optical paths 432(1 )-(M) has a different length. The number N of first heterogeneous optical paths on the first working optical path 421 may be the same as or different to the number M of second heterogeneous optical paths on the second working optical path 422. Furthermore, the lengths of the heterogeneous optical paths 431 (1)-(N) may be the same as or different to the lengths of the heterogeneous optical paths 432(1)-(M).

[0053] Each of the first, second and third photons are detected at the detecting means 440, which is substantially similar to the detecting means 140, 240, 340 discussed above in relation to Figures 1-5. In particular, the detection of the third photon on the control optical path 423 may be used for timing purposes in order to determine a time window in which the first and second photons are detected (in a similar manner as discussed above in relation to Figured 3 and 4). Random numbers are determined (e.g. by the detecting means 440 or by an external entity such as a controller) based on the time window in which the first and second photons are detected. As the detecting means 440 detects both the first and second photons, two random numbers may be generated (based on the time at which both the first and second photons are detected). Accordingly, two random numbers may be generated based on a single emission process from the photon source 410. Moreover, it should be noted that in some examples the photon source may generate one or more additional photons as part of the emission process, each of the one or more additional photons having a different predetermined wavelength to the first, second and third photons. The additional photons may be guided to additional working optical paths having a 1-to-N splitter and arranged to terminate at the detecting mean in a similar manner to the working optical paths 421 , 422 shown in Figure 6. Accordingly, the rate of random number generation may be further increased.

[0054] Figure 7 illustrates a general system 500 for generating random numbers in accordance with the examples disclosed herein. In particular, the system includes a controller 550 which may transmit signals / data to and receive signals / data from any of a photon source (such as photon source 110, 210, 310, 410 discussed herein), 1-to-N splitter 530 (such as 1-to-N splitters 130, 230, 330, 430A, 430B discussed herein), and / or detecting means 540 (such as detecting means 140, 240, 340, 440 discussed herein). For example, the controller 550 may control the photon source 510 so as to cause the photon source 510 to generate one or more photons. As another example, the controller 550 may control the 1-to-N splitter 530 so as to adjust the configuration of the 1-to-N splitter 530 by adjusting the probability with which a photon travels on a particular heterogeneous optical path coupled to the 1-to-N splitter 530. As a further example, the controller 550 may receive the one or more signals from the detecting means 540 indicating that a photon has been detected, and / or indicating a time or time window at which a photon is detected, and / or indicating a random number and / or indicating a heterogeneous optical path on which a photon has travelled and / or indicating a spatial location at which a photon is detected. The controller 550 may additionally or alternatively transmit signals / data to and receive signals / data from a coupler 525 (such as coupler 225 discussed herein). For example, the controller 550 may control thecoupler 525 to adjust the probability with which photons enter and / or exit an optical loop (such as optical loop 250 shown in Figure 4). It should be appreciated that the controller 550 may be considered as a single logical entity, but may in practice include multiple distinct entities which may or may not be configured to communicate with one another (e.g. via one or more networks). The controller may comprise classical and / or quantum computing means.

[0055] Accordingly, in view of the foregoing disclosure, there has been described a system for generating random numbers based on quantum mechanical properties of photons. A photon source generates one or more photons, which are guided to a working optical path where a splitting component probabilistically guides the one or more photons to one of a plurality of heterogeneous optical paths. The photon is then detected at a detecting means, based on which a random number may be determined according to the heterogeneous optical path taken by the photon.References[1] Caspani, L., Xiong, C., Eggleton, B. et al. Integrated sources of photon quantum states based on nonlinear optics. Light Sci Appl 6, e17100 (2017).

Claims

CLAIMS1. A system (100, 101 , 200, 201) for generating a random number, the system comprising: a photon source (110, 210) configured to generate one or more photons each having a predefined wavelength; guiding means (120, 220) configured to deterministically guide the one or more generated photons to a working optical path (121 , 221) according to the wavelength of the one or more photons; one or more 1-to-N splitters (130, 230) arranged on the working optical path and configured to probabilistically guide the photon to one of a plurality of heterogeneous optical paths (131 , 231); and detecting means (140) configured to detect the photon after it has travelled along a particular optical path of the plurality of heterogeneous optical paths, wherein the detecting means is configured to output a signal indicative of a random number based on the particular optical path taken by the photon.

2. The system according to claim 1 , wherein the guiding means is further configured to supress photons having wavelengths other than the predefined wavelength.

3. The system according to claim 1 or claim 2, wherein the photon source is configured to generate two or more photons each having a respective predefined wavelength, wherein the two or more photons include a first photon having a first predefined wavelength, and one or more second photons each having a respective second predefined wavelength, wherein the first and the one or more second predefined wavelengths are different; wherein the guiding means is a multiplexer or demultiplexer configured to deterministically guide each of the two or more photons to either a control optical path (122, 222) or one or more working optical paths according to the wavelength of the respective photon, wherein the guiding means is configured to guide the first photon to one of the one or more working optical paths, and the guiding means is configured to guide the second photon to the control optical path; and wherein the 1-to-N splitters are arranged on the one or more working optical paths.

4. The system according to claim 3, wherein the detecting means is further configured to detect photons that have travelled on the control optical path, and wherein the detecting means is configured to output a signal indicative of detection of the second photon from the control optical path.

5. The system according to claim 3 or claim 4, wherein the two or more photons generated by the photon source are wavelength multiplexed.

6. The system according to any preceding claim, wherein the photon source is further configured to generate an additional photon having a particular predefined wavelength, and wherein the guiding means is configured to deterministically guide a first photon having a first predetermined wavelength to a first working optical path, and deterministically guide the additional photon to a second working optical path according to the respective wavelengths of the first and additional photons.

7. The system according to any preceding claim, wherein each of the plurality of heterogeneous optical paths are arranged to terminate at the detecting means in different spatial locations (141 (1)-(N)); and wherein the indication of the random number comprises an indication of the spatial location at which the photon is detected by the detecting means.

8. The system according to any of claims 1-6, wherein the system further comprises: an N-to-1 combiner configured to guide photons on any of the plurality of heterogeneous optical paths to a common spatial termination location (241) at the detecting means, wherein each of the plurality of heterogeneous optical paths are arranged to terminate at the common spatial termination location, wherein each of the plurality of heterogeneous optical paths has a different length; and wherein the indication of the random number comprises an indication of a time at which the photon is detected by the detecting means.

9. The system according to claim 8, wherein the 1-to-N splitter, the plurality of heterogeneous optical paths, and the N-to-1 combiner form an optical loop (250), and wherein the system further comprises: a coupler (225) configured to permit photons on the working optical path to enter the optical loop, and to permit photons on the optical loop to exit the optical loop towards the common spatial termination location.

10. The system according to claim 9, wherein the coupler is a deterministic coupler configured to deterministically cause photons on the working optical path to enter the optical loop, and to deterministically cause photons on the optical loop to exit the optical loop towards the common spatial termination location.

11. The system according to claim 9, wherein the coupler is a probabilistic coupler configured to probabilistically permit photons on the working optical path to enter the optical loop, and to probabilistically permit photons on the optical loop to exit the optical loop towards the common spatial termination location.

12. The system according to any of claims 9-11 , wherein the coupler is tuneable to alter a probability with which photons are permitted to enter and exit the optical loop.

13. The system according to any preceding claim, wherein the detecting means comprises one or more photodetectors.

14. The system according to any preceding claim, wherein the photon source is configured to generate two or more photons via spontaneous parametric down-conversion of spontaneous four- wave mixing.

15. The system according to any preceding claim, further comprising: a controller configured to receive the signal indicating a random number and determine a random number according to the signal.

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