Photonic integrated circuit and method

Abstract

To provide a photonic integrated circuit (PIC) used to generate a random number.SOLUTION: The PIC includes on a substrate: a light source 2; a first photodetector 4 for outputting an electrical signal used when generating a random number in response to receiving light from the light source 2; and a light guidance system 6 for directing light from the light source 2 to the photodetector 4.EFFECT: An genuine random number is generated.SELECTED DRAWING: Figure 1

Description

[Background technology]

[0001] Random number generators based on the inherently random behavior of photons caused by quantum mechanical effects are known: for example, the behavior of a single photon can be determined and used to generate truly random numbers.

[0002] It would be desirable to improve random number generators based on photon detection. [Brief explanation of the drawings]

[0003] [Figure 1] FIG. 1 shows a schematic diagram of a system for generating random numbers according to an example. [Figure 2] 10a and 10b show plots relating to current readout of a photodetector according to an example; [Figure 3] 2 is a schematic cross-sectional view of an example photonic integrated circuit (PIC) (e.g., the example of FIG. 1). [Figure 4] FIG. 1 is a flow diagram illustrating a method for generating random numbers based on an example. [Figure 5] 1A-1C illustrate an example manufacturing method. DETAILED DESCRIPTION OF THE INVENTION

[0004] The examples described herein relate to photonic integrated circuits (PICs) used to generate random numbers based on the behavior of photons due to quantum mechanical effects. Specifically, a PIC can be considered a monolithic PIC when the optical components for generating random numbers are formed on a single substrate (i.e., on a common substrate). The substrate can be, for example, a compound of elements from groups III and V of the periodic table, such as a so-called III-V semiconductor compound, e.g., indium phosphide (InP). Such PICs can be housed entirely without optical input or output and are compact enough to be integrated within devices such as computers and smartphones. In practice, such PICs are more compact and simpler in construction and operation than known random number generators. For example, a PIC can be 100 microns by 1000 to 4000 microns by 4000 microns or smaller.

[0005] Figure 1 illustrates a schematic diagram of an example system for generating random numbers. The system is implemented, at least in part, as a PIC on a common substrate, e.g., a III-V semiconductor compound (e.g., InP). Figure 4 illustrates a flow diagram of a method for generating random numbers.

[0006] The PIC used to generate random numbers includes a light source 2 on a substrate (illustrated later using FIG. 3), a photodetector 4 on the substrate, and a light guidance system 6 on the substrate. The light guidance system is configured to direct light from the light source to the photodetector. A waveguide 8 (schematically illustrated as an arrow) of the light guidance system guides the light from the light source to the photodetector 4. When the light from the light source is incident on, or received by, the photodetector 4, the photodetector outputs an electrical signal used to generate random numbers. This will be described in more detail later. The light source is, for example, a semiconductor optical amplifier (SOA) or a laser. The light detector is, for example, a photodiode capable of detecting low light intensities. Such photon detectors are simpler than known examples known as "single-photon detectors," which use more complex or design-limited detectors, such as silicon photomultiplier (SiPM) avalanche photodiodes. Furthermore, the photodiodes in the examples are monolithically integrated photodiodes with sampling rates of at least gigahertz (GHz) frequencies. This allows for faster operation and therefore faster random number generation than conventional photodetectors. Furthermore, with GHz or higher sampling rates, such photodiodes can detect photocurrents of larger magnitudes than known photodetectors. As a result, background electronic noise is more effectively overcome, and fluctuations associated with quantum effects become even more dominant.

[0007] In examples such as those shown in FIGS. 1-3, an optical attenuation system is provided on the common substrate. The attenuation system is configured to attenuate the intensity of the light output from the light source to an intensity used by the photodetector 4 used when generating random numbers. Thus, between the light source and the photodetector 4, the attenuation system reduces the intensity of the light. In examples such as those shown in FIGS. 1-3, the attenuation system includes one or more optical splitters 10 (e.g., passive optical splitters) on the common substrate. While FIG. 1 shows two such splitters, this is a schematic. A first optical splitter 10a receives light from the light source before another of the optical splitters (e.g., a second optical splitter 10b) receives light from the light source (via the first optical splitter). While FIG. 1 is a schematic and shows first and second optical splitters, in some other examples there may be only one such splitter, and in other examples there may be more than two splitters, as needed, to reduce the light intensity to a level appropriate for the photodetector 4 (and overall system setup) to be used when generating random numbers.

[0008] The preferred level is low enough that any fluctuations in the light intensity detected by the photodetector 4 are dominated by quantum effects and therefore follow Poisson statistics. Otherwise, if the intensity were higher, and therefore a larger number of photons per sample were detected by the photodetector, any fluctuations caused by quantum effects would be hidden and therefore negligible compared to the total number of photons detected per sample, making them undetectable and unusable for random number generation. Those skilled in the art are familiar with Poisson statistics and, therefore, Poisson probability distribution. Poisson probability distribution describes the probability of multiple events occurring during a given period of time. Events occur at a known, constant average rate and are independent of the time since the last event. The application of Poisson distribution in random number generation is further described below.

[0009] Typically, the standard deviation (or intrinsic fluctuation) of a Poisson distribution is the square root of the mean sample size. Therefore, the fluctuation relative to the mean sample size decreases as the number of photons per sample increases. On the other hand, low photon counts (especially single-photon counting) require optimized photodiodes and electronics. InP PICs are characterized by high sample rates and low electronic background noise of the photodiode. Therefore, at photon counts of 1000 photons per sample, the intrinsic quantum fluctuations still far exceed the electronic background current (by at least about 10 times). At high sample rates, such 1000 photons per sample still generate a significant photodiode current that can be measured using conventional readout electronics.

[0010] The light source, light detector, and light splitter of the optical attenuation system are optically connected to one another, as needed, by a light guidance system (e.g., waveguide 8). Thus, the input of a first optical splitter is optically connected by a waveguide to receive light from the light source (directly, in some examples), and the output of the first optical splitter is optically connected by a waveguide to either the input of a subsequent optical splitter or to the light detector. Although each splitter may be thought of as guiding or redirecting a portion of the light from its input to the splitter's output, the splitters are described herein as part of the optical attenuation system because their function is to reduce the intensity of light propagating from the light source.

[0011] In some examples, a light attenuation system is provided between the light source and the light detector 4. It is configured to attenuate the intensity by a factor of at least 1000. Therefore, for example, the intensity of the light incident on the photodetector 4 is at most 0.1% of the intensity output by the light source.

[0012] As will be appreciated by those skilled in the art, such attenuation can be achieved in different ways. In the example described herein, optical splitters are used. Each optical splitter splits the light generated by the light source into a first portion of light for propagation by the light guidance system to the photodetector 4 and a second portion of light that is not guided to the photodetector 4. Multiple optical splitters may be arranged in series with one another. The first portion of light output from one optical splitter is guided forward and input to the next optical splitter in the series, which then outputs its first portion of light to guide forward to any subsequent optical splitters in the series. This continues until the photodetector 4 is reached.

[0013] Each optical splitter in the attenuation system is, for example, an optical filter, e.g., a multimode interferometer (MMI) familiar to those skilled in the art. In an MMI, for example, the waveguide width is widened to transition the light from single-mode propagation to multimode propagation. The spatial distribution of the optical modes changes as a function of the MMI length, and numerical calculations can be used to calculate the appropriate distribution required for a particular splitter application. In the case of a 1x2 splitter, two waveguides are fabricated along the length from the beginning of the MMI (where the two modes are formed and sufficiently spatially separated).

[0014] Depending on the specifications of each splitter, in some instances each splitter reduces the light intensity by 50% (50% light in the first portion of the light input to the splitter and 50% light in the second portion), and in other instances each splitter reduces the light intensity by 70% (30% light in the first portion of the light input to the splitter and 70% light in the second portion). In the former case, in some instances ten so-called 50:50 splitters are used in series to reduce the intensity to the photodetector 4. In the latter case, in other instances six so-called 30:70 splitters are used in series to reduce the intensity to the photodetector 4. In further instances, different splitting ratios are possible.

[0015] The MMI is an example of a passive optical splitter. Passive optical splitters do not require voltage drive to function (as do active components). Using a passive optical splitter in an optical attenuation system means that higher power can be used for the light source, thereby reducing the need for and / or power requirements of circuitry to drive active components elsewhere in the PIC. Furthermore, as explained further below, such a passive optical splitter means that the intensity of the light output by the light source can be monitored, which can aid in the calibration and adjustment of the system for generating random numbers.

[0016] The photodetector 4 described thus far is also referred to herein as a first photodetector. Additionally, in some examples, such as the example of FIG. 1, there are one or more second photodetectors 12, each optically connected by a waveguide 8 to receive the second portion of the optical output from a corresponding optical splitter 10. Each of the second photodetectors is the same type of photodiode as the first photodetector, but in other examples, may be a different type. For example, the first photodetector may be a balanced photodiode or another type of optimized high-precision photodiode, and the second photodetector may be the same or a different type of photodetector.

[0017] The PIC includes a first circuit for processing the electrical signal output from each of one or more second photodetectors. Such second photodetectors may be used, for example, to monitor the intensity of light output by the light source. If the second photodetector detects a deviation from a target intensity, this indicates that the intensity of the first portion of light guided onward to the photodetector has also deviated, and appropriate action can be taken. For example, one or more of the second photodetectors may For each of the second photodetectors, the first circuit receives an electrical signal from the corresponding second photodetector. A characteristic (e.g., current) of the electrical signal is measured to determine a magnitude of the characteristic indicative of the actual intensity of the light output by the light source. If the actual intensity of the light output by the light source is determined to differ from the target intensity of the light that the light source should output (e.g., based on the fact that the second portion of light should be 50% of the intensity of the light output by the light source), the first circuit can take appropriate action to maintain the system's ability to generate random numbers. Such a response may be to adjust the current applied to the light source to adjust the intensity of the light output by the light source. Thus, if the intensity of the second portion is less than the target intensity, the current may be increased to increase the intensity of the light output by the light source and reduce the difference between the actual intensity and the target intensity. Additionally or alternatively, such a response may be to calibrate the process for generating random numbers (described further below) (by adjusting at least one of the threshold value or the magnitude of the characteristic (e.g., current) measured from the electrical signal output by the first photodetector 4) to compensate for any deviation in the light intensity from the light source. Otherwise, such deviations could cause the system to malfunction, such that any supposedly random numbers generated are not in fact truly random. Such monitoring is performed by at least a second photodetector receiving the second portion of light from the first optical splitter in series, and in some examples, multiple second portions of light from the optical splitters in series are used to monitor attenuation. For example, the second portion of light from each optical splitter is output to a different second photodetector to monitor each attenuation step by the splitter, and any deviations from the target intensity are adjusted in the current applied to the light source and / or in the processing of the electrical signal output by the first photodetector 4.

[0018] Next, we will discuss random number generation. The PIC includes a second circuit configured to use the electrical signal output by the first photodetector 4 to generate random numbers. Typically, the second circuit receives the electrical signal output by the photodetector 4 and determines a value to use when generating the random number based on a characteristic (e.g., current) of the electrical signal. In these examples, such values ​​are binary values ​​(otherwise known as bits) with a value of 1 or 0. A string of binary values ​​may be generated based on a series of measurements of the current (one binary value per measurement). The characteristic of the measured electrical signal changes (or fluctuates) over time. This occurs due to quantum effects that affect the passage of photons from the light source to the first photodetector 4. This means that the number of photons incident on the photodetector 4 is different at different times, resulting in fluctuations in the characteristic of the output electrical signal. These fluctuations, which are truly random because they are determined by quantum effects, can be used as the basis for determining each binary value, thereby generating a truly random string of binary values. Such a string can then be used to generate truly random numbers, such as decimal integers.

[0019] Various methods are possible for using the fluctuations in the characteristics of the electrical signal output by the first photodetector 4 when generating random numbers. For example, because the fluctuations are determined by quantum effects, the measurements fit a Poisson distribution. Therefore, each measurement of the characteristics of the electrical signal can be compared to a Poisson distribution or parameters derived therefrom to determine the value (e.g., 1 or 0) obtained for each measurement to generate a random number.

[0020] Figure 2a shows an example plot of many current measurements of the electrical signal output by photodetector 4. The plot assumes that 1000 photons are detected per measurement (referred to in the figure as a readout). The x-axis is time, and the y-axis is the magnitude of the current for each readout. Each readout can be thought of as a sample of the current, with subsequent samples taken periodically at intervals of equal duration spaced apart from one another. As can be seen in Figure 2a, the current per readout fluctuates around a mean magnitude, exemplified by the concentration of data points along horizontal bands on the plot. Figure 2b shows the same readout, now plotted as a histogram, with the magnitude of the current for each readout shown on the x-axis and the frequency of each measurement on the y-axis. 2b. The histogram plot is a Poisson distribution. An average reading (in these examples, the average current magnitude) is determined. This average is, for example, an average corresponding to the most commonly measured current magnitude, as indicated by vertical line 14 in FIG. 2b. It is contemplated that such an average value can be set as a threshold to determine which value to assign for use in generating random numbers, or in other examples, a different statistical value can be calculated and used to set the threshold. After determining the average value, each subsequent measurement of a characteristic, such as current, is mapped to the average value. For example, the difference between the measurement and the threshold is calculated, or whether the measurement is greater or less than the threshold. The magnitude of the difference, or simply whether the measurement is greater or less than the threshold, can be used to determine the value assigned from the measurement to generate a random number. For example, FIG. 2b shows that if the measurement is less than threshold 14, a bit value of 0 is determined and recorded in the bit string, while if the measurement is greater than threshold 14, a bit value of 1 is determined and recorded in the bit string. This method is repeated for each subsequent measurement, generating a string of randomly generated bit values ​​given their dependence on quantum-effect-based fluctuations in light intensity on the photodetector 4. The interval separating each subsequent measurement from the previous one (and thus the sampling frequency) may be set by the particular photodetector used or may be controlled by the second circuit based on a clock signal. Thus, the second circuit in the example receives the electrical signal output by the first photodetector 4, measures a characteristic (e.g., current) of the electrical signal at a first time point, and, based on the measured characteristic, determines that the magnitude of the measured characteristic is less than a threshold value. Based on this, the second circuit determines a first value (e.g., a bit value, e.g., 0 in FIG. 2b) to use in generating the random number. At a second time point immediately following the first time point but separated by the sampling period, a similar process is performed for the next measurement. However, if the second measurement of the characteristic is instead greater than the threshold value, the second value (e.g., a bit value, e.g., 1 in FIG. 2b) is determined to be different from the first value.

[0021] In some examples, the threshold is determined before the circuitry begins generating random numbers. Measurements of a characteristic of the electrical signal output by photodetector 4 may be sampled over a given time period long enough to determine the mean (e.g., average) of a Poisson distribution of the measurements. This mean value is then set as the threshold value used to determine values ​​(e.g., bits) based on future variations in the characteristic. In other examples, the threshold may be determined and stored during a calibration process during manufacturing before shipping to a consumer. This is again done by measuring the characteristic over a time period long enough to determine the mean value for the Poisson distribution.

[0022] As previously mentioned, the intensity of the light output by the light source and / or any second portion of the light output by the optical splitter 10 may deviate from the target intensity. Such deviations may be greater than any fluctuations caused by quantum effects. By monitoring any such deviations using one or more second photodetectors 12, the process for determining the value (e.g., bit value) for the random number generation may be adjusted accordingly. For example, a corresponding offset may be applied to the magnitude of the measured characteristic to compensate for the deviation and ensure a correct comparison against the threshold. Alternatively, the threshold may be adjusted to accommodate any such deviations so that a correct comparison of the measured magnitude against the threshold is again achieved. Alternatively, if any such deviations are sufficiently large or long-lasting, the threshold may be reset by repeating the calibration process to determine a mean value according to the Poisson distribution.

[0023] In a specific example, consider a light source that outputs light with an intensity of 200 microwatts (μW), an attenuation factor of 1000 provided by the attenuation system, a sampling rate of 1 GHz, a threshold of 0.16 microamperes (μA), and a conversion efficiency of 100% for the first photodetector 4 to convert photons to electrons (and thus a measurable current). Figure 3 shows an example of the layers of a PIC. Cross-sectional views taken along lines AA, BB, CC, and DD, respectively, in FIG. 1 are shown from left to right. The light source, waveguide 8, and photodiode 4 are each shown between an N-InP compound substrate and a p-type compound (e.g., P-InP, P-InGaAs, P-InAlAs, P-InAlGaAs, or P-InGaAsP (Ga is gallium, As is arsenic, Al is aluminum, and P is phosphorus)). The semiconductor materials on which the photodetector and photodiode are based are the same in some instances and different in other instances. Optional materials secondary to the functions described herein (e.g., electronic conductivity or environmental passivation) are not illustrated for clarity.

[0024] Based on this, the light intensity measured by the first photodetector 4 can fluctuate by up to 3.1%. Because these fluctuations are due to quantum effects, the range of readout magnitude values ​​(of the current) covered by the Poisson distribution is 3.1%. Such a sufficiently large fluctuation range is useful for determining whether each measurement value is greater or less than a threshold value. This determination can become more difficult with smaller fluctuation ranges. The 3.1% fluctuation range is particularly larger than the range of fluctuation obtained by systematic fluctuations in a PIC without the optical attenuation system described herein. Note that the fluctuation range can increase with a decrease in the intensity of the light output by the light source, a decrease in the conversion efficiency, an increase in the attenuation rate, and / or an increase in the sampling rate. In contrast, the fluctuation range can decrease with an increase in the intensity of the light output by the light source, an increase in the conversion efficiency, a decrease in the attenuation rate, and / or a decrease in the sampling rate.

[0025] The electronic circuits (e.g., the first and second circuits described herein) may be implemented using at least one processor and at least one memory storing instructions. The instructions, when executed on the at least one processor, perform any example method described herein in connection with at least the first or second circuits. The at least one processor may be, for example, a general-purpose processor, a microprocessor, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA) or other programmable logic circuit, discrete gate or transistor logic, discrete hardware components, or any suitable combination thereof designed to perform the functions described herein. The first and second circuits may be provided together on a single printed circuit board (PCB), or each of the first and second circuits may be implemented on one or more boards and connected to the PIC accordingly. Those skilled in the art will recognize that suitable power circuits for powering the PIC and the first and second circuits are also contemplated.

[0026] Those skilled in the art will appreciate that various techniques may be used to deposit and pattern layers of semiconductor material according to the examples described herein. Such techniques may include chemical vapor deposition (e.g., metalorganic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE)). Those skilled in the art will appreciate that etching techniques may be used to remove portions of material as part of the patterning.

[0027] Thus, based on an example and with reference to 501-504 of FIG. 5 , an example PIC described herein is fabricated by a method including: providing a substrate; forming a light source on the substrate; forming a first photodetector on the substrate, the first photodetector configured to output an electrical signal for use in generating a random number in response to receiving light from the light source; and forming a light guidance system on the substrate configured to direct light from the light source to the first photodetector. Further, the example method includes forming a light attenuation system on the substrate, the light attenuation system configured between the light source and the photodetector to attenuate the intensity of light from the light source so that fluctuations in the intensity of light detected by the photodetector are governed by Poisson statistics. Furthermore, in an example, the method includes forming a plurality of second photodetectors as described above, the light attenuation system being a series of passive The system includes an optical splitter, wherein each of the series of passive optical splitters is configured to split light from the light source into a first portion of light for propagation by the light guidance system to a first optical detector and a second portion of light for propagation to a second optical detector of the plurality of second optical detectors, the second portion of light being used to perform at least one of: adjusting a current applied to the light source to adjust the intensity of light output by the light source to reduce a difference between an actual intensity output by the light source and a target intensity to be output by the light source; or calibrating a process for generating random numbers based on the difference between the actual intensity and the target intensity.

[0028] It should be understood that the foregoing examples are illustrative. It should be understood that any feature described in connection with any one example may be used alone or in combination with other features described, or in combination with one or more features of any other example (or any combination of any other examples). Furthermore, equivalents and modifications not described above may be used without departing from the scope of the appended claims.

Claims

1. i) a light source on the substrate; ii) a photodetector on the substrate, the photodetector configured to output an electrical signal for use in generating the random number in response to receiving light from the light source; and iii) a light guidance system on the substrate, the light guidance system being configured to direct light from the light source to the light detector; Including, A photonic integrated circuit used to generate random numbers.

2. The substrate is At least one of a III-V semiconductor compound or indium phosphide, InP; The photonic integrated circuit of claim 1 .

3. iv) a light attenuation system on said substrate, comprising: the light attenuation system configured between the light source and the light detector to attenuate the intensity of light from the light source so that fluctuations in the intensity of light detected by the light detector are governed by Poisson statistics; Including, 3. The photonic integrated circuit according to claim 1 or 2.

4. The photonic integrated circuit of claim 3 , wherein the optical attenuation system is configured to attenuate the intensity of light from the light source by at least a factor of 1000 between the light source and the photodetector.

5. a passive optical splitter on the substrate, the passive optical splitter configured to split light from the light source into a first portion of light for propagation by the light guidance system to the light detector and a second portion of light; Including, The photonic integrated circuit according to any one of claims 1 to 4.

6. a series of passive optical splitters on the substrate, each of the series of passive optical splitters configured to split light from the light source into a first portion of light and a second portion of light for propagation by the light guidance system to the light detector; Including, The photonic integrated circuit according to any one of claims 1 to 4.

7. each said passive optical splitter includes a multimode interferometer (MMI); 7. A photonic integrated circuit according to claim 5 or 6.

8. the photodetector is a first photodetector, and each second portion of light is directed to a corresponding second photodetector; The photonic integrated circuit according to any one of claims 5 to 7.

9. a first circuit, receiving an electrical signal output by one of the corresponding second photodetectors; determining a magnitude of the electrical signal output by said one of said corresponding second photodetectors, said magnitude indicating an actual intensity of light output by said light source; To determine determining that the actual intensity of light output by the light source differs from a target intensity of light to be output by the light source; at least one of adjusting a current applied to the light source to adjust the intensity of the light output by the light source to reduce the difference between the actual intensity and the target intensity, or calibrating the process for generating the random number based on the difference between the actual intensity and the target intensity; the first circuit configured to perform Including, 9. The photonic integrated circuit of claim 8.

10. a second circuit, a) receiving the electrical signal output by the photodetector of ii); b) determining a value to be used when generating the random number based on characteristics of the electrical signal received in a); the second circuit configured to perform Including, The photonic integrated circuit according to any one of claims 1 to 9.

11. a second circuit, a) receiving the electrical signal output by the photodetector of ii); b) measuring a characteristic of the electrical signal received in a) at a point in time; and c) determining, based on the characteristic measured in b), that the magnitude of the characteristic measured in b) is less than a threshold value; and d) determining a value to be used when generating the random number based on c); the second circuit configured to perform The photonic integrated circuit according to any one of claims 1 to 9, comprising:

12. The value used when generating the random number is a binary value.

12. The photonic integrated circuit of claim 11.

13. the time point is a first time point, the value used when generating the random number is a first value, and the second circuit f) measuring said characteristic of said electrical signal received in a) at a second time; and g) determining, based on the characteristic measured in f), that the magnitude of the characteristic measured in f) is greater than the threshold value; h) determining a second value, different from the first value, to be used when generating the random number based on g); configured to:

13. A photonic integrated circuit according to claim 11 or 12.

14. the second value is a binary value; 14. The photonic integrated circuit of claim 13.

15. the property is a current; The photonic integrated circuit according to any one of claims 10 to 14.

16. the threshold is an average magnitude of the characteristic of the electrical signal; The photonic integrated circuit according to any one of claims 10 to 15.

17. outputting light from a light source on the substrate; guiding the light from the light source to a photodetector on the substrate by a light guidance system on the substrate; outputting an electrical signal used to generate a random number by the photodetector; Including, How to generate random numbers.

18. The substrate is 18. The method of claim 17, wherein the material is at least one of a III-V compound, or indium phosphide, InP.

19. a light attenuation system on the substrate, attenuating the intensity of the light from the light source between the light source and the light detector so that fluctuations in the intensity of the light detected by the light detector are governed by Poisson statistics.

19. The method of claim 17 or 18.

20. the light attenuation system is configured to attenuate the intensity of light from the light source by at least a factor of 1000 between the light source and the light detector.

20. The method of claim 19.

21. The light from the light source splitting, by a series of passive optical splitters respectively, a first portion of the light for propagation to said photodetector and a second portion of the light for reception by a corresponding second photodetector; 21. The method of claim 20.

22. each of the at least one passive optical splitter is an MMI; 22. The method of claim 21.

23. determining a magnitude of an electrical signal output by one of the corresponding second photodetectors, the magnitude indicating an actual intensity of light output by the light source; determining that the actual intensity of light output by the light source differs from a target intensity of light to be output by the light source; at least one of adjusting a current applied to the light source to adjust the intensity of the light output by the light source to reduce the difference between the actual intensity and the target intensity, or calibrating the process for generating the random number based on the difference between the actual intensity and the target intensity; Including, 23. The method of claim 21 or 22.

24. a) determining a value to be used when generating the random number based on characteristics of the electrical signal output by the photodetector of ii); The method according to any one of claims 17 to 23.

25. a) measuring a characteristic of the electrical signal received in a) at a point in time; b) determining, based on the characteristic measured in b), that the magnitude of the characteristic measured in a) is less than a threshold value; and c) determining a value to be used when generating the random number based on b); Including, The method according to any one of claims 17 to 23.

26. The value used when generating the random number is a binary value.

26. The method of claim 25.

27. the time point is a first time point, the value used when generating the random number is a first value, and the method includes: f) measuring said characteristic of said electrical signal received in a) at a second time; and g) determining, based on the characteristic measured in f), that the magnitude of the characteristic measured in f) is greater than the threshold value; h) determining a second value, different from the first value, to be used when generating the random number based on g); Including, 27. The method of claim 25 or 26.

28. the second value is a binary value; 28. The method of claim 27.

29. the property is a current; The method according to any one of claims 24 to 28.

30. the threshold is an average magnitude of the characteristic of the electrical signal; 30. The method according to any one of claims 25 to 29.

31. Preparing the substrate; forming a light source on the substrate; forming a photodetector on the substrate, the photodetector configured to output an electrical signal for use in generating the random number in response to receiving light from the light source; forming a light guidance system on the substrate configured to direct light from the light source to the light detector; Including, A method for fabricating a photonic integrated circuit for use in generating random numbers.

32. 32. The method of claim 31 , comprising forming an optical attenuation system on the substrate, the optical attenuation system being configured between the light source and the light detector to attenuate the intensity of light from the light source so that variations in the intensity of light detected by the light detector are governed by Poisson statistics.

33. The optical detector is a first optical detector, the method includes forming a plurality of second optical detectors, and the optical attenuation system includes a series of passive optical splitters, each of which splits light from the light source into: a first portion of light for propagation by the light guidance system to the first light detector; a second portion of light for propagation to a second photodetector of the plurality of second photodetectors, adjusting the current applied to the light source to adjust the intensity of the light output by the light source to reduce the difference between the actual intensity output by the light source and the target intensity that the light source should output; or generating the random number based on a difference between the actual intensity and the target intensity; calibrating the process; the second portion of light used in performing at least one of each configured to divide 33. The method of claim 31 or 32.

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