Superconducting single-photon detector with photon number resolution

Abstract

The single photon number discriminator provides photon number resolution. The discriminator includes a waveguide that receives photons for detection. A plurality of nanowires are disposed in proximity to the waveguide. Each nanowire is connected in series with an electrical resistive element to provide a branch. The plurality of branches are electrically connected in parallel with each other. A power source is connectable to provide a current flowing through the branches. A current monitor is connected to monitor the amplitude of the total current in the plurality of branches. At the operating temperature of the discriminator, the nanowires are superconducting and each branch has an electrical time constant small enough to latch the branch into a latched state when the nanowire of the branch absorbs a photon from the waveguide. The number of photons detected can be determined from the amplitude of the current, which depends on the number of branches in the latched state.

Description

[Technical field]

[0001] The present invention relates to superconducting single photon detectors, and in particular to photon detectors capable of discriminating between multiple photons. The present invention has exemplary applications in quantum photonics and quantum computing.

[0002] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims priority from U.S. Application No. 63 / 203478, entitled “SUPERCONDUCTING SINGLE PHOTON DETECTOR WITH PHOTON NUMBER RESOLUTION,” filed July 23, 2021, which is incorporated by reference herein for all purposes. For U.S. purposes, this application claims the benefit under 35 U.S.C. § 119 of U.S. Application No. 63 / 203478, entitled “SUPERCONDUCTING SINGLE PHOTON DETECTOR WITH PHOTON NUMBER RESOLUTION,” filed July 23, 2021, which is incorporated by reference herein for all purposes. [Background technology]

[0003] There are a variety of applications in which it is desirable to detect individual photons. These applications include quantum information technology.

[0004] One type of photon detector is the superconducting nanowire single-photon detector (SNSPD). SNSPDs offer high detection efficiency for visible and near-infrared photons with high count rates, very small timing jitter, and low dark count rates. SNSPDs are described, for example, by G.N. Goltsman et al. in Picosecond superconducting single-photon optical detector, Appl. Phys. Lett. 79, 705 (2001), https: / / doi.org / 10.1063 / 1.1388868.

[0005] An SNSPD comprises a nanowire positioned at a location where it can absorb photons of interest, and a potential source connected to pass a bias current through the superconducting nanowire, which typically follows a serpentine path to increase the chances that the photon will be absorbed.

[0006] In operation, with the nanowire at a temperature at which it is superconducting, the bias current is controlled so that the current density in the nanowire has a value close to but below the superconducting critical current density of the nanowire, which is the current density threshold above which the nanowire becomes non-superconducting or "normal."

[0007] When a nanowire absorbs a photon (e.g., a photon with a wavelength in the visible or infrared portion of the spectrum), the energy of the photon is delivered to the nanowire. This causes the nanowire to heat up locally, forming a hotspot. Because the nanowire is very thin, the energy of the photon is sufficient to make the material of the nanowire normal (non-superconducting) at the location of the hotspot. Some of the bias current passing through the hotspot is diverted around the hotspot. This increases the current density outside of the hotspot. The increased current density on either side of the hotspot can exceed the critical current density and make the strips across the nanowire normal (electrically resistive). When this occurs, the bias current dissipates additional energy in the resistive strips, which can increase the size of the hotspot.

[0008] As mentioned above, each time a photon is absorbed by a superconducting nanowire, the voltage across the nanowire will contain a pulse. Single photons can be detected by monitoring the voltage to detect such pulses. As long as the heat from the nanowire can be dissipated quickly enough into the environment, the nanowire will become superconducting again.

[0009] It would be desirable to have a photon detector capable of photon number discrimination that shares at least some of the desirable properties of SNSPDs. Various attempts have been made to provide such photon detectors. However, these suffer from various drawbacks.

[0010] Mohsen K. Akhlaghi et al., Nonlinearity in single photon detection: modeling and quantum tomography, Optics Express Vol.19, Issue 22, pp.21305-21312 (2011) https: / / doi.org / 10.1364 / OE.19.021305, showed that the voltage pulse caused by the simultaneous absorption of multiple photons at the same nanowire site in a partially biased SNSPD can be analyzed to obtain some information about the number of absorbed photons. This very limited intrinsic photon number resolution (PNR) capability of SNSPDs, which comes from nonlinear effects, is not useful for typical practical applications.

[0011] One can imagine that the amplitude of the voltage pulse across the long nanowire of an SNSPD depends on the number of photons absorbed at different positions along the nanowire (each of which may form its own resistive part). However, to obtain sufficient voltage resolution, very high input impedance amplifiers are required. Anthony J. Annunziata et al., Reset dynamics and latching in niobium superconducting nanowire single-photon detectors Journal of Applied Physics 108,084507(2010);doi:10.1063 / 1.3498809, showed that it is impractical to use readout circuits with high input impedance. For the nanowire to return to its superconducting state after photon detection, the nanowire must have an electrical time constant τ r =L K / R L must be cooled rapidly relative to the Kis the mechanical inductance of the nanowire, which is proportional to its length, and R L is the load resistance of the readout circuit. Otherwise, the SNSPD will be latched into a finite voltage state where it is less sensitive to photons.

[0012] Di Zhu et al., Resolving Photon Numbers Using a Superconducting Nanowire with Impedance-Matching Taper, Nano Lett. 2020, 20, 5, 3858-3863, 9 April 2020, https: / / doi.org / 10.1021 / acs.nanolett.0c00985, describe a photon detector with an SNSPD connected to an impedance-matching transmission line and show that the SNSPD has intrinsic photon number resolution. However, the device can only detect a small number of photons and calibration of the device is problematic because the detection probability does not depend on the time distribution of the incident photons.

[0013] Aleksander Divochiy et al., Superconducting nanowire photon-number-resolving detector at telecommunication wavelengths, Nature Photonics volume 2, pages 302-306 (2008) describes an array of serpentine nanowire discriminators electrically connected in parallel. S. Jahanmirinejad, Photon-number resolving detector based on a series array of superconducting nanowires, Appl. Phys. Lett. 101, 072602 (2012); https: / / doi.org / 10.1063 / 1.4746248, discloses an array of serially connected nanowire discriminators. These discriminators showed PNR, but the compact nanowire layout and high frequency readout caused crosstalk and scalability problems.

[0014] There remains a need for practical methods and devices that can detect photons with photon number resolution. Summary of the Invention

[0015] FIELD OF THE DISCLOSURE This application relates to an apparatus for detecting photons and a method for detecting photons. The method and apparatus have non-exclusive application to detecting photons in quantum information systems.

[0016] One aspect of the invention provides an apparatus in the form of a single photon detector that provides photon number resolution. Such a detector is referred to as a "PNRD" or photon number resolving detector. The detector includes a waveguide that receives photons for detection. Nanowires extend transversely to the waveguide. Each nanowire is connected in series with an electrical resistive element to provide a branch. The branches are connected in parallel with each other. For example, the branches may be electrically connected in parallel by first and second superconducting conductors that extend along opposite sides of the waveguide.

[0017] A power supply can be connected to provide a current flowing through the branch. A current monitor is connected to monitor the amplitude of the current. At the operating temperature of the discriminator, the nanowires are superconducting and each branch has an electrical time constant that is small enough to latch the branch into a latched state when the nanowire of the branch absorbs a photon.

[0018] In some embodiments, the branches of the PNRD are divided into a number of groups, with the branches of each group connected in parallel. The PNRD comprises means for supplying current to each group and means for monitoring the current of each group. For example, in some embodiments, the PNRD comprises a power supply and a current monitor for each of the multiple groups.

[0019] In some embodiments, the identifier has nanowires in the range of 5 to 1000 or more. In some embodiments, the spacing between the nanowires is in the range of ½ μm to 2 μm. The nanowires may be equally spaced or not equally spaced. In some embodiments, adjacent ones of the nanowires are separated by a distance in the range of about 200 nm to about 4 μm. The nanowires may extend perpendicular to the waveguide, may intersect the waveguide at an angle, or may extend transversely to the waveguide at different angles from each other. In some embodiments, the nanowires are curved adjacent to the waveguide or follow a non-linear path. In some embodiments, the nanowires are perpendicular to the waveguide or at an angle in the range of +60 degrees to −60 degrees to perpendicular.

[0020] In some embodiments, the nanowires include nanowires located on different sides of the waveguide and / or formed to extend on different sides of the waveguide, for example, one or more of the plurality of nanowires may be formed to surround two or three sides of the waveguide.

[0021] In some embodiments, the waveguide has a length in the range of 0.1 mm to 6 mm, hi some embodiments, the waveguide has a width in the range of 0.2 μm to 3 μm.

[0022] In some embodiments, the nanowires are evanescently coupled to the waveguides sufficiently strongly that a photon propagating in the waveguide has a greater than 65% probability of being absorbed by one of the nanowires of the identifier, hi some embodiments, each of the nanowires is evanescently coupled to the waveguides sufficiently weakly that a photon propagating in the waveguide has a less than 5% or less than 1% or less than 0.1% probability of being absorbed by any individual one of the nanowires.

[0023] The nanowires include a superconducting material. In some embodiments, the nanowires are comprised of one or a combination of niobium, niobium nitride, niobium titanium nitride, and tungsten silicide. In some embodiments, the nanowires may have a flattened cross-section with an aspect ratio (width:height) ranging from 5:1 to 20:1. In some embodiments, the nanowires have a width ranging from 20 nm to 200 nm. In some embodiments, the nanowires have a thickness ranging from 4 nm to 10 nm. In some embodiments, the nanowires have a length ranging from less than 1000 nm to 5000 nm.

[0024] In some embodiments, the critical currents of the multiple nanowires are the same within ±5%.

[0025] In some embodiments, the waveguide is composed of a material selected from the group consisting of silicon, silicon nitride, glass, polymer, gallium arsenide, and aluminum gallium arsenide.

[0026] The apparatus may include an optical fiber coupled to an optical input of the waveguide by an optical coupling. In some embodiments, the optical fiber is a single mode optical fiber. In some embodiments, the coupling comprises a tapered section of the waveguide. In some embodiments, the coupling comprises a bandpass filter. In some embodiments, the coupling, the waveguide and / or the optical fiber are wavelength selective.

[0027] In some embodiments, the waveguide and the nanowire are supported on a substrate. The substrate may be formed with an alignment feature for the optical fiber, and the optical fiber may be engaged with the alignment feature. In some embodiments, the nanowire is disposed between the waveguide and the substrate.

[0028] In some embodiments, the waveguide is configured to form a resonant structure.

[0029] Photon detection may be increased within a given wavelength band by patterning the waveguide with a resonant structure, incorporating an optical filter in the optical fiber, patterning an on-chip filter in the waveguide between the optical fiber taper and the first nanowire, tuning the bias current, or tuning the operating temperature. Patterning the waveguide may include etching holes in the waveguide or machining the waveguide with wavy edges that provide suitable resonant characteristics.

[0030] In some embodiments, the discriminator is wavelength specific.In some embodiments, the coupling of the optical fiber to the waveguide is wavelength specific.

[0031] The electrical resistance element may include, for example, a metal, a conductive polymer, carbon, or a doped semiconductor film. In some embodiments, the electrical resistance element has an impedance in the range of about 0.1 kΩ to about 10 kΩ. In some embodiments, the electrical resistance element includes an active element, such as a field effect transistor (FET), and / or a device that provides a reactive impedance.

[0032] The apparatus may include, or be used in conjunction with, a refrigerator operable to cool the branches to a temperature at which the nanowires are superconductors. For example, the identifier may operate at an operating temperature in the range of 1.5K to 4.5K. In some embodiments, the operating temperature is less than half the critical temperature of the nanowires (critical temperature is expressed in Kelvin). In some embodiments, the refrigerator is a cryogen-free refrigerator.

[0033] In some embodiments, the power source is an alternating current (AC) power source. In some embodiments, the power source is operable to function as a pulse generator configured to output pulses separated by reset periods. In some embodiments, the power source includes a digital-to-analog converter (DAC) controlled by the controller and one or more filters, including a passive filter, that may filter the output of the DAC. In some embodiments, the power source is disposed outside of a volume cooled to an operating temperature of the branch.

[0034] In some embodiments, the current monitor comprises a current-to-voltage converter and a voltage monitor. The voltage monitor may comprise, for example, an analog-to-digital converter (ADC) coupled to monitor a voltage output by the current-to-voltage converter. In some embodiments, the current-to-voltage converter comprises one or more of a shunt resistor and a transimpedance amplifier.

[0035] In some embodiments, the electrical time constant τ for the branch r is τ r =L K / R L (where L K is the mechanical inductance of the nanowire, and R L is the load resistance of the power supply).

[0036] The controller may be connected to obtain current measurements from the current monitor at first and second times, respectively before and after the photons are delivered to the photon number identifier, and to calculate the number of photons N by processing the difference between the current measurements.

[0037] In some embodiments, the controller:

number

[0038] In some embodiments, the controller is configured to obtain additional measurements of the current at one or more times during which the photons are detected, and to calculate the number of photons based at least in part on the additional measurements.

[0039] In some embodiments, the controller is configured to reset the photon number discriminator by interrupting the supply of current from the power source to the branch for a time long enough for all of the nanowires to cool to a temperature at which they are superconducting.

[0040] In some embodiments, the controller is configured to calibrate the photon number discriminator. Calibration by the controller or otherwise includes: a) applying a series of bias / reset pulses to the branches and monitoring the bias current just prior to resetting with no photons incident on the discriminator; b) gradually increasing the bias voltage and monitoring the resulting current; c) determining the critical bias voltage; d) setting the bias voltage to a value less than the critical bias voltage and applying a series of bias / reset pulses; e) measuring a histogram of the bias current measured immediately prior to reset; f) transmitting a weak laser pulse calibrated to have an average photon count per pulse much less than 1; g) measuring a histogram of the measured bias current measured immediately before the reset; h) gradually increasing the average photon number of the laser pulse and measuring the histogram for each setting; This may include i) processing the histogram to identify well-separated peaks and identifying threshold currents corresponding to different numbers of photons based on the location of the peaks.

[0041] Another aspect of the invention provides a method for detecting and counting photons. In some embodiments, the method comprises: a) cooling a waveguide having a plurality of nanowires extending transversely to the waveguide at spaced apart locations to a temperature at which the nanowires become superconducting; b) passing a bias current through each of the nanowires, the bias current being lower than a critical current for the nanowire; c) directing at least one photon into the waveguide and allowing the photon to be evanescently absorbed by one of the nanowires; d) latching the nanowires into a resistive state when one of the nanowires absorbs a photon; e) measuring the total current drawn by the nanowire; f) processing the measured total current to count photons.

[0042] In the case of multiple photons, the method may direct multiple photons into a waveguide to allow the photons to be evanescently absorbed by different nanowires, thereby latching the corresponding nanowires into a resistive state.

[0043] In some embodiments, the bias current in each branch ranges from about 2 μA to about 30 μA. The bias current of each nanowire is less than the critical current. In some embodiments, the bias current is at least 70% or 80% or 90% of the critical current. The bias current in an individual nanowire may be adjusted by adjusting the value of a resistor connected in series with the nanowire. The resistor may be fabricated by patterning a film of a non-superconducting conductor. The non-superconducting conductor may include, for example, a thin metal film, a carbon conductive polymer film, or a doped semiconductor film. The bias current of the nanowire may be adjusted by adjusting the voltage that provides the bias current.

[0044] The devices and methods described herein may incorporate several techniques to reduce errors and / or increase detection efficiency. These include: Reducing thermal crosstalk between the nanowires and the waveguide, for example by increasing the spacing between the nanowires and the waveguide; Reducing electrical noise in the voltages applied by the DAC or other power supplies by isolating the digital and analog ground connections and / or applying filtering; -Measure the total current multiple times; Splitting the nanowire into multiple circuits and measuring the total current in each circuit separately; and / or Adjusting the bias current.

[0045] Further aspects and example embodiments are illustrated in the accompanying drawings and / or described in the following description.

[0046] It is emphasized that the invention relates to all combinations of the above described features even if these are recited in different claims. [Brief description of the drawings]

[0047] The accompanying drawings illustrate non-limiting exemplary embodiments of the present invention.

[0048] [Figure 1] 1 is a schematic diagram of a photon detector in accordance with an example embodiment;

[0049] [Figure 1A] 1 is a schematic diagram showing an alternative arrangement of nanowires.

[0050] [Diagram 2] 1 is an electrical schematic diagram illustrating an exemplary equivalent electrical circuit for an apparatus in accordance with an exemplary embodiment.

[0051] [Figure 2A] FIG. 2 illustrates an example waveform of a power supply for a photon detector.

[0052] [Figure 2B] 2 is a graph showing a schematic example of total current as a function of time for a photon detector of the type shown in FIG. 1; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0053] In the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these details. In other instances, well-known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.

[0054] 1 illustrates a photon number discriminator 10 according to an exemplary embodiment. The discriminator 10 comprises a plurality of nanowires 12 disposed adjacent to an optical waveguide 14. The waveguide 14 is constructed of a suitable optically transparent material, such as, for example, Si3N4. In some embodiments, the waveguide 14 is constructed of silicon, silicon nitride, glass, polymer, gallium arsenide, aluminum gallium arsenide, or another material that is transparent at the target operating wavelength and has a higher refractive index than the surrounding medium.

[0055] The waveguide 14 supports an optical mode. Photons passing through the waveguide in the optical mode are internally reflected within the waveguide 14 and evanescently coupled into the nanowire 12.

[0056] The waveguide 14 has a length sufficient to accommodate a desired number of nanowires 12 with a desired spacing between the nanowires 12. For example, the length of the array of nanowires 12 along the waveguide may be on the order of 1 mm. The waveguide 14 may have a length of the array of nanowires plus any extra length desired to support the waveguide 14 and / or couple the waveguide 14 to other optical structures. In some embodiments, the waveguide 14 has a length in the range of 0.1 mm to 6 mm.

[0057] In preferred embodiments, the waveguide 14 has a sufficient length such that there is a high probability that any photon entering the waveguide 14 at the light input at the first end 14A of the waveguide 14 will be absorbed by the nanowires 12 before the photon reaches the end 14B of the waveguide 14. In some embodiments, that high probability is at least 65% (e.g., 65% or 70% or 80% or 90% or 95% or 99%). As described below, the strength of evanescent coupling into any individual nanowire 12 is sufficiently high such that there is a high probability that a photon entering the waveguide 14 will be absorbed in any one of the nanowires 12, but the probability that the photon will be absorbed in any particular one of the nanowires 12 is relatively low.

[0058] The strength of evanescent coupling of photons into nanowire 12 depends on factors such as the material and dimensions of waveguide 14, the material and dimensions of nanowire 12, the spacing between nanowire 12 and waveguide 14, the properties of the medium between waveguide 14 and nanowire 12, and the geometry of nanowire 12 and waveguide 14. In designing identifier 10, these factors may be adjusted to provide a desired strength of evanescent coupling between photons in waveguide 14 and nanowire 12.

[0059] For example, to increase the evanescent coupling of photons having a target frequency into the nanowire 12, using a material for the nanowires 12 that more strongly absorbs photons of the target frequency; Fabricating the nanowires 12 in a geometry that provides a larger area for the waveguide 14 (e.g., making the nanowires 12 wider and / or intersecting the waveguide 14 at an angle and / or The discriminator 10 can be fabricated by placing the nanowire 12 closer to the waveguide 14. Evanescent coupling of photons at the target frequency into the nanowire 12 can be reduced by making the discriminator 10 have the opposite properties as above.

[0060] The strength of the evanescent coupling of the nanowire into the waveguide can be increased by increasing the dimensions of the nanowire so that the cross-sectional area of ​​the nanowire facing the waveguide is increased, selecting a material for the nanowire that absorbs photons at the target frequency more strongly than the currently selected material, decreasing the spacing between the nanowire and the waveguide, adjusting the angle of the nanowire with respect to the normal to the waveguide so that the cross-sectional area of ​​the nanowire facing the waveguide is increased, providing a resonant structure in the waveguide, and / or modifying the geometry of the nanowire so that the cross-sectional area of ​​the nanowire facing the waveguide is increased.

[0061] The strength of the evanescent coupling of the nanowire into the waveguide can be reduced by reducing the dimensions of the nanowire so that the cross-sectional area of ​​the nanowire facing the waveguide is increased, selecting a material for the nanowire that absorbs photons at the target frequency more weakly than the currently selected material, increasing the spacing between the nanowire and the waveguide, adjusting the angle of the nanowire with respect to the normal to the waveguide so that the cross-sectional area of ​​the nanowire facing the waveguide is reduced, or changing the geometry of the nanowire so that the cross-sectional area of ​​the nanowire facing the waveguide is reduced.

[0062] The waveguide 14 may, for example, have a width on the order of 1 μm (eg, 0.5 μm). The nanowire 12 is in intimate contact with the waveguide 14 and extends across the width of the waveguide 14.

[0063] Nanowires 12 are made from a material that is superconducting at the operating temperature of device 10. Nanowires 12 may be made from, for example, niobium (Nb), niobium nitride (NbN), niobium titanium nitride (NbTiN), and / or tungsten silicide (WSi).

[0064] Nanowire 12 may be flattened in cross section. In some embodiments, the cross-sectional aspect ratio (width:height) of nanowire 12 ranges from 5:1 to 20:1, where "height" is the dimension measured perpendicular to the surface of waveguide 14 to which the nanowire is adjacent, and "width" is the dimension measured parallel to the surface of waveguide 14 and transverse to nanowire 12.

[0065] In some embodiments, nanowires 12 have a width on the order of about 100 nm. In some embodiments, nanowires 12 have a width in the range of 20 nm to 200 nm. In some embodiments, nanowires 12 have a width less than 150 nm.

[0066] As described in more detail below, if nanowire 12 is too wide for given operating conditions, some photons absorbed by nanowire 12 may not be detected. The optimal width of nanowire 12 (i.e., the width at which there is a high probability that an absorbed photon will be detected) may be determined experimentally and / or by modeling. The optimal width may depend on the material of nanowire 12, the operating temperature, and the wavelength of the photons to be detected.

[0067] Nanowires 12 may have a thickness (height) on the order of, for example, about 4 nm to 10 nm, hi some embodiments, nanowires 12 have a thickness of less than 10 nm.

[0068] Nanowires 12 may be short, in some embodiments, for example having a length not exceeding 1000 nm, or not exceeding 5000 nm.

[0069] In the illustrated embodiment, some or all of the nanowire 12 is formed by a narrower portion 12A of a superconducting conductor 12B that is adjacent to the waveguide 14. In this configuration, for a given current passing through the conductor 12B, the current density in the narrower portion 12A of the conductor 12B that forms the nanowire 12 is higher than the current density in other portions of the conductor 12B.

[0070] The nanowires 12 may extend linearly across the waveguide 14 at any desired angle (e.g., perpendicular) to the waveguide 14. In some embodiments, the nanowires 12 extend at a non-perpendicular angle to the waveguide 14 (e.g., an angle ranging from +60° to −60° with respect to the vertical).

[0071] The angle of the nanowires 12 is one parameter that may be selected to achieve a desired coupling strength between photons of a target wavelength and the nanowires 12. In some embodiments, different nanowires 12 in the discriminator 10 may be transverse to the waveguide 14 at different angles. For example, in the discriminator 14, the first nanowire 12 closest to the light inlet 14A may be perpendicular to the waveguide 14. Each subsequent nanowire 12 along the waveguide 14 may be slightly more angled. This is shown diagrammatically in FIG. 1A. Other arrangements are possible.

[0072] The nanowires 12 are spaced from adjacent nanowires 12 by a distance sufficient that there is essentially no thermal crosstalk between adjacent nanowires 12. For example, adjacent nanowires 12 may be spaced apart by a distance on the order of 1 μm (e.g., a distance in the range of about 200 nm to about 4 μm).

[0073] The identifier 10 may include a large number of nanowires 12. For example, the identifier 10 may include at least 5, at least 50, at least 100, at least 150, at least 200, at least 600, at least 800, or at least 950 nanowires 12. In one embodiment, the identifier 10 includes between 500 and 2000 nanowires 12. In some embodiments, the identifier 10 includes more than 1000 nanowires 12. There is no fixed upper limit to the number of nanowires 12 that may be included in the identifier 10.

[0074] Photons to be detected are coupled into the waveguide 14. In the illustrated embodiment, the photons are carried in an optical fiber 16 that is coupled to the waveguide 14 by a suitable optical coupling 17. The optical fiber 16 may be, for example, a single mode optical fiber. The coupling 17 may include, for example, a taper in the waveguide 14. In some embodiments, the waveguide 14 is coupled to the optical fiber 16 in the manner described in T. Zhu et al., Ultrabroadband High Coupling Efficiency Fiber-to-Waveguide Coupler Using Si3N4 / SiO2 Waveguides on Silicon, IEEE Photonics Journal, vol. 8, no. 5, pp. 1-12, Oct. 2016, Art no. 7102112, doi:10.1109 / JPHOT.2016.2600037, and the fiber 16 delivers the photons to the desired mode of the waveguide 14.

[0075] The manner in which the waveguide 14 is terminated at the interface with the fiber 16 or another optical structure that provides the photons to be detected may be selected to achieve a desired coupling efficiency of the counted photons from the fiber 16 into the waveguide 14. In general, a high coupling efficiency of the counted photons is desirable. In some embodiments, the coupling from the fiber 16 to the waveguide 14 is wavelength selective, such that photons within a target wavelength band are coupled into the waveguide 14 more strongly than photons outside the target wavelength band.

[0076] The discriminator 10 may be optimized for maximum efficiency for photons within a target wavelength band and / or target polarization state. The mode shape of the waveguide 14 and the strength of the evanescent coupling may depend on the photon polarization. In some embodiments, the waveguide 14 and the nanowires 12 are designed to optimize photon detection and reduce dark counts at a particular polarization. For example, the discriminator 10 may be optimized to detect photons of a particular polarization by adjusting design parameters of the discriminator 10, such as the number of nanowires 12, the spacing of the nanowires 12, the length of the nanowires 12, the type of optical fiber in the waveguide 14, and / or the design of the interface to the waveguide 14. Some or all of these parameters may differ between discriminators 10 optimized to detect photons with different polarizations, and may also differ between discriminators 10 optimized to detect photons with a particular polarization and discriminators 10 optimized to efficiently detect photons of multiple polarization states.

[0077] 1, the waveguide 14 and nanowire 12 are supported on a substrate 18. In this example, the substrate 18 is formed with features such as a groove 19, and the optical fiber 16 is engaged with the features (e.g., engaged with the groove 19) for alignment with the waveguide 14. In some embodiments, the waveguide 14 is part of a network of photonic elements (e.g., elements such as waveguides, resonators, optical resonators, etc.) supported on the substrate 18. The network of photonic elements may include a quantum bit that is coupled to the waveguide 14 by the network of photonic elements and emits a photon that can be detected by the identifier 10.

[0078] The physical location of the waveguide 14 and the nanowire 12 may be varied. For example, in the illustrated embodiment, the nanowire 12 is located "under" the waveguide 14 (i.e., between the waveguide 14 and the substrate 18). The nanowire 12 may additionally or alternatively be located on the other side of the waveguide 14. For example, the nanowire 12 may be located "over" the waveguide 14 (i.e., such that the waveguide 14 is interposed between the nanowire 12 and the substrate 18). In some embodiments, the nanowire 12 is formed to surround two or three sides of the waveguide 14.

[0079] As another example, the nanowires 12 or the waveguides 14 need not be straight. Any of these structures may be curved. For example, the identifier 10 may be configured with a curved waveguide 14. In some such embodiments, the nanowires 12 radiate from a central pad. Also, the individual nanowires 12 may be curved, U-shaped (e.g., such that both ends of the nanowire 12 are on one side of the waveguide 14), or other shapes. In general, the nanowires 12 and the waveguides 14 may have any shape that can be achieved by the tools (e.g., lithography systems) used to fabricate the identifier 14 that are compatible with the operating principles described herein.

[0080] In some embodiments, the waveguide 14 is patterned to form a resonant structure. Such patterning may help to enhance the detection of photons in a given wavelength band and reject photons in other wavelength bands. The resonant structure may modify the exponential decay behavior of photons traveling in the waveguide 14 so that nanowires further away from the optical fiber 16 can efficiently detect the photons. The exponential decay of photons occurs when the photons interact with the nanowire 12 along the waveguide 14. The resonant patterning may be created, for example, using lithographic methods. For example, photonic crystal resonators may be created in the material of the waveguide 14 or in adjacent materials by etching holes in the material, or the waveguide 14 may be created with wavy edges.

[0081] Resonant patterning of the waveguide 14 may improve the signal-to-noise ratio by increasing the probability that a nanowire 12 located at a given distance from the optical fiber 16 will absorb and detect a photon without affecting the generation of dark counts by the nanowire. By providing a resonant structure in or adjacent to the waveguide 14, the exponential decay behavior of photons in the waveguide 14 may be modified so that nanowires 12 at different distances along the waveguide 14 contribute more equally to photon detection than they would otherwise. This may improve the fidelity of photon detection, especially for discriminators 10 having a smaller number of nanowire sections.

[0082] A resonant structure that produces sharper resonances may also increase light-matter interactions, thereby facilitating the use of shorter nanowires 12 in the identifier 10. Shorter nanowires may facilitate easier fabrication as well as provide faster and quieter detection of photons.

[0083] Each nanowire 12 is connected in series with a resistor 15. The combination of one nanowire 12 and a corresponding resistor 15 may be referred to as a "branch" 20. The identifier 10 includes first and second conductors 22A and 22B. The branches 20 are coupled between the conductors 22A and 22B, respectively. Thus, the branches 20 are connected in parallel to each other by the conductors 22A, 22B. The conductors 22A and 22B may include superconducting traces, wires, conductors, strips, etc.

[0084] Resistor 15 has an electrical impedance (e.g., resistance) at operating temperature such that when a selected potential (voltage) is applied between conductors 22A and 22B, a bias current flows through each nanowire 12. The bias current has an amplitude smaller than the critical current of nanowire 12. Ideally, the bias current is only slightly smaller than the critical current of nanowire 12. Choosing a lower bias current reduces the efficiency of photon detection, but can also reduce "dark counts." Dark counts are unintentional counts that occur when stray photons are absorbed by nanowire 12 or due to non-ideal internal dynamics of nanowire 12.

[0085] In some embodiments, the bias current in each branch 20 ranges from about 2 μA to about 30 μA.

[0086] Resistor 15 is sufficiently thermally isolated from nanowire 12 so that heat resulting from energy dissipated in resistor 15 does not significantly affect the temperature of nanowire 12. Thermal isolation can be achieved, for example, by locating resistor 15 sufficiently far from nanowire 12. This may be achieved while keeping the conductors making up each branch 20 short enough to avoid excessive pickup of electrical noise (e.g. resulting from parasitic electrical effects such as stray capacitance and inductance).

[0087] Resistor 15 may be fabricated, for example, by patterning a film of a "normal" (i.e., non-superconducting) electrical conductor (e.g., a metal film, a carbon conductive polymer film, or a doped semiconductor film). Resistor 15 has dimensions to provide a desired electrical resistance at the operating temperature of identifier 10. Resistor 15 may have any suitable geometric shape (e.g., an elongated rectangle, a serpentine line, etc.).

[0088] In some embodiments, resistor 15 has an impedance in the range of about 0.1 kΩ to about 10 kΩ.

[0089] In some embodiments, resistor 15 is provided by an active element, such as a field effect transistor (FET), which may be controlled to adjust the bias current of the corresponding branch 20. The active element may be formed in substrate 18, for example, by any known semiconductor fabrication technique.

[0090] In some embodiments, the bias voltage is an alternating current (AC) voltage. In such embodiments, the resistor 15 may comprise or consist of a device providing a reactive impedance (e.g., a capacitor and / or an inductor) that limits the maximum current to less than the critical current of the nanowire 12 of the branch 20.

[0091] In the ideal case, each nanowire 12 has the same critical current. In such an ideal case, achieving a bias current that is slightly less than the critical current of each nanowire 12 may be achieved by making each resistor 15 provide the same resistance.

[0092] In some embodiments, the critical currents of the nanowires 12 are close enough to be the same so that resistors 15 of the same design will provide an acceptable bias current (i.e., a bias current that is less than the critical current for the nanowire and within a desired range of the critical current for the nanowire) at each nanowire 12. For example, the identifier 10 may be made such that the bias current for each nanowire 12 is less than the critical current and is at least 40%, or 50%, or 60%, or 70%, or 80%, or 90%, or 95%, or 98% of the critical current.

[0093] In some embodiments, the bias current in the individual nanowires is adjusted, which may be accomplished, for example, by adjusting the value of resistor 15 (e.g., by removing material from individual as-fabricated resistor 15 by laser ablation or other suitable techniques to reduce the bias current, and / or by controlling an active element that provides all or a portion of resistor 15).

[0094] The identifier 10 includes a refrigerator 24 operable to cool the branches 20 and the conductors 22A and 22B to a temperature at which they are superconducting (e.g., liquid helium temperature or up to a few Kelvin). For example, the refrigerator 24 may be connected to cool the entire chip shown in FIG. 1. The substrate 18 is connected to the elements of the identifier 10 that generate heat (e.g., the nanowires 12 that absorb photons or resistive losses (I 2 The substrate 18 may have a sufficiently low thermal resistance to carry heat away from the resistor 15) which dissipates energy in the form of R. Good thermal conductivity of the substrate 18 may be achieved by making the substrate 18 from materials such as silicon or sapphire.

[0095] In some embodiments, the refrigerator 24 is a cryogen-free refrigerator. In some embodiments, the operating temperature of the identifier 10 is greater than 1.5 K. In some embodiments, the identifier 10 is maintained at a temperature of 4.2 K or less, where lower temperatures are desired for better detection. In some embodiments, the operating temperature of the identifier 10 (in Kelvin) is less than half the critical temperature of the nanowires 12.

[0096] In operation, the nanowire 12 is superconducting. A power supply 30 is connected to apply a potential difference between the conductors 22A and 22B such that a bias current flows in each branch 20. The power supply 30 may include a variable voltage power supply. The voltage is adjusted to provide an appropriate bias current I BIAS may be adjusted to provide

[0097] 2 shows an example in which power supply 30 includes a digital-to-analog converter (DAC) 30A that is controlled by controller 38 to output an appropriate voltage. One or more filters may be connected to filter the output of DAC 30A that is applied between conductors 22A and 22B. In some embodiments, the filters are passive filters.

[0098] To minimize electrical noise in the voltage applied by DAC 30A between conductors 22, DAC 30A may have separate digital and analog ground connections connected to separate ground conductors. The filter may include a filter that operates to reduce electronic noise transmission at the analog ground and analog output of DAC 30A to conductors 22A and 22B. In some embodiments, the filter includes a filter that operates to reduce or eliminate ringing or overshoot of pulses generated by controlling DAC 30A. The filter may include a low pass filter and / or a band pass filter. FIG. 2 shows examples of filters 32A, 32B, and 32C. In some embodiments, at least a portion of the filters operate at cryogenic temperatures (e.g., filters 32B and 32C of FIG. 2).

[0099] Power supply 30 is optionally located outside the volume cooled by refrigerator 24. For example, power supply 30 may be at room temperature. Power supply 30 may be connected to conductors 22A and 22B by electrical connection 31, which may provide a low frequency (MHz range) electrical connection. For example, electrical connection 31 may include electrical conductors 31A, 31B electrically connected to conductors 22A and 22B at pads 23A and 23B, respectively. Electrical connection 31 may include, for example, a coaxial cable.

[0100] The amplitude of the bias current in each branch 20 depends on the voltage applied by power supply 30 and the resistance provided by resistors 15 in the branch 20. These parameters are selected or controlled to maintain the bias current at a level below but close to the critical current of the superconducting nanowire 12. In some embodiments, the applied bias current is at least 40%, or at least 60%, or at least 80%, or at least 90% of the critical current of the nanowire 12. In some embodiments, the bias current is in the range of 40%-90% of the critical current of the nanowire 12.

[0101] Any photon traveling along the waveguide 14 is weakly evanescently coupled to the nanowires 12. The probability that any individual nanowire 12 will absorb any individual photon is low. For example, the probability of absorption of a particular photon by any particular nanowire 12 may be less than 2%, or less than 1%, or less than ½%, or less than 0.1%. However, when there are multiple nanowires 12, the probability that any individual photon propagating in the waveguide 14 will be absorbed by one of the nanowires 12 may be high (e.g., at least 80%, at least 90%, at least 95%, at least 98%, or at least 99%).

[0102] Because the probability of any particular nanowire 12 absorbing a photon is low, the probability that the nanowire 12 will absorb more than one photon in one measurement cycle can be negligibly small for a reasonable number of photons (e.g., less than 10 or less than 15 or less than 20 or less than 25 photons at a time).

[0103] The power supply 30 may be operated at a constant voltage, so that the current drawn from the power supply 30 in the absence of detected photons is equal to the sum of the bias currents of all branches 20.

[0104] FIG. 2 illustrates an exemplary current measurement circuit 36 ​​operable to measure the current drawn from the power supply 30. The circuit 36 ​​may include, for example, a current-to-voltage converter 36A and a voltage monitor 36C, such as an analog-to-digital converter (ADC), connected to monitor the voltage output by the current-to-voltage converter 36A. The analog-to-digital converter has sufficient resolution to distinguish different current levels corresponding to different numbers of absorbed photons. For example, the voltage monitor 36C may include an ADC having an 8, 12 or 16 bit digital output. The current-to-voltage converter 36A may include a low noise amplifier 36B. The current-to-voltage converter 36A may include, for example, a shunt resistor, a transimpedance amplifier, or the like.

[0105] Each branch 20 is designed and operative such that when a photon is absorbed by the branch 20, the energy from the photon causes the branch 20 to latch into a state in which all or part of the corresponding nanowire 12 is resistive (non-superconducting) upon absorption of the photon. This may be achieved, for example, by shortening the nanowire 12, thereby reducing the electrical time constant τ r =L K / R L (Here, L K is the mechanical inductance of the nanowire, and R L where I is the load resistance of the power supply 30) becomes small relative to the time required for the nanowire to dissipate heat resulting directly or indirectly from the absorption of photons. The latch may be implemented, for example, by absorbing a photon, causing a portion of the nanowire 12 to become electrically resistive, resulting in resistive losses (I 2 R) may occur if the nanowire 12 subsequently cools sufficiently to prevent it from resuming being superconducting.

[0106] In some embodiments, a majority of nanowires 12 are non-superconducting when nanowires 12 are in the latched state. For example, at least 70%, 80%, 90%, or 99% of nanowires 12 may be non-superconducting in the latched state.

[0107] The branch 20 that absorbs a photon and is in a latched resistive state draws a bias current I BIAS Less than current I LATCH Therefore, the total current delivered by the power supply after absorption of a pulse of photons is reduced proportionally to the number of branches 20 that absorbed one of the photons (which is equal to the number of photons in the pulse of photons if each of the photons is absorbed by a different one of the nanowires 12). This current may be measured (e.g. by a current measurement circuit 36) and used as a readout signal.

[0108] FIG. 2B is a graph including a curve 50 showing the total current as a function of time. The measurement begins at time t0. Photons are captured from time t1 to t8. Each time a photon is captured, the total current increases by ΔI=I BIAS -I LATCH where I BIAS is the bias current of one nanowire 12, and I LATCH is the current in the nanowire 12 in the latched state. Since the nanowire 12 remains latched until it is reset, the total current can be measured any time after a photon is captured. The number N of captured photons can be determined by:

number

[0109] One limit on the number of photons that can be accurately detected by the discriminator 10 is ΔI=I BIAS -I LATCH may not be the same for all branches 20, and I START -I MEASURE This arises from the uncertainty associated with the determination of N. In particular, when a large number of photons are absorbed, there may be cases where the value of N has one or more uncertainties.

[0110] The above restrictions are measuring the total current multiple times, and / or The nanowires 12 of the identifier 10 may be addressed in various ways, including splitting them into multiple separate circuits, each containing multiple nanowires 12, and measuring the total current for each separate circuit separately.

[0111] For example, in an identifier 10 including M nanowires 12, the nanowires 12 may be grouped into two or more circuits and the total current may be measured separately for each of the circuits. In some embodiments, the total current may be measured at a rate high enough to provide time-resolved counting of the arriving photons. For example, measurements may be made at a rate high enough that it may be unlikely that more than a few photons are absorbed by the nanowire 12 between any two adjacent measurements. If the light pulse containing the photons is wide enough and the readout is fast enough, individual latch events may be identified.

[0112] 2B shows an example where one or more measurements of the total current are made while the photon is being detected, in addition to a measurement made after the photon is absorbed in the nanowire 12. FIG 2B shows an example where measurements of the total current are made at times Ma and / or Mb, in addition to a measurement at time M. These measurements may be used to achieve a measurement of N with lower uncertainty than could be achieved by a single measurement made at time M.

[0113] In some embodiments, the nanowires are grouped into circuits based on distance from the end 14A of the waveguide 14. For example, a first circuit may include a first group of nanowires 12 closest to the end 14A, a second circuit may include a group of nanowires 12 adjacent to the first group, and so on. In some embodiments, the group closest to the end 14A includes fewer nanowires 12 than the groups located further from the end 14A.

[0114] The latched branches 20 may be reset so that the corresponding nanowires 12 become superconducting again by interrupting the supply of current from the power supply 30 for a period of time sufficient for all nanowires 12 to cool to a temperature at which they become superconducting again. For example, heat may be conducted away from the latched nanowires 12 through the substrate 18.

[0115] The discriminator 10 may be reset, for example, by applying the power supply 30 acting as a pulse generator to temporarily reduce the bias voltage to zero or another low value. The pulse generator may output, for example, a waveform 40 as shown in FIG. 2A including pulses 41 separated by reset periods 42. The pulses 41 have an amplitude below a level 43 corresponding to the critical current of the nanowire 12. The amplitude of the waveform 40 in the reset period 42 is zero or low enough that resistive heating in the latched nanowire can cool to a temperature at which the latched nanowire is again superconducting by the end of the reset period 42. It is not essential that the pulses 41 are equally spaced. In some embodiments, the pulses 41 are triggered simultaneously with an event that can emit a photon to be counted by the discriminator 10. It is not essential that the pulses 41 are all of the same duration. The pulses 41 should be long enough that the photons of interest can be detected and the total current can be measured.

[0116] The pulse generator may be provided, for example, by controlling the output voltage of ADC 30A by controller 38 to generate a waveform such as waveform 40. Alternative circuits may be used for power supply 30. For example, the waveform that drives the discriminator 10 may be generated by an analog pulse generator circuit or by switching the output of a fixed voltage power supply using a switching circuit.

[0117] In some embodiments, the reset period may be short. For example, the time required to reset the latched nanowire 12 may be very short (e.g., less than 10 ns) such that the nanowire 12 is reset even if the reset period has a duration not exceeding 10 microseconds or not exceeding 10 nanoseconds. The practical minimum duration of the reset period 42 depends on various factors, including the thermal conductivity of the connection between the nanowire 12 and the cold portion of the identifier 10, which acts as a heat sink to cool the nanowire 12, and the electrical properties of the power source 30, the conductor 31, and the identifier 10.

[0118] The controller 38 is processing the readout signal to obtain photon counts; calibrating the discriminator 10, Setting the discriminator 10 to provide an appropriate base current; Additional functionality may be provided, such as triggering a reset of the discriminator 10.

[0119] To calibrate the identifier 10, the controller 38 may be configured to perform the following steps. 1- A series of bias / reset pulses are applied to the discriminator 10 and the bias current is monitored just before the reset with no photons incident on the discriminator 10. 2- The bias voltage is gradually increased and the current is monitored. The current should rise linearly at lower bias currents and then the slope of the current change or the current itself should decrease. The onset of this change indicates the bias voltage that corresponds to the critical current in the nanowire 12. This is sometimes called the critical bias voltage. 3- Set the bias to a value less than the critical bias voltage and apply a series of bias / reset. 4-Measure the histogram of the bias current measured just before the reset. 5- Send a weak laser pulse calibrated so that the average number of photons per pulse is much less than one. 6-Measure the histogram of the bias current measured just before the reset. 7- Gradually increase the average photon number and measure the histogram for each setting. The set of 8-histograms should show well-separated peaks. The vertical line separating the peaks is the threshold set corresponding to the number of photons detected. Comparing the known statistics of photons in the calibrated laser pulse with the measured photon statistics allows the determination of detection parameters such as detection efficiency and detection uncertainty. By repeating this procedure with different bias currents and different photon wavelengths, other sets of threshold and performance measurements are obtained.

[0120] The controller 38 may be configured to store the calibration information determined by the calibration procedure in a data store. For example, the calibration information may be stored in a look-up table.

[0121] It can be understood that the apparatus described herein is operable to "store" a number of single photon detection events in the form of latched branches 20. The number of latched branches 20 can be determined at any time by reading the current provided by the power supply 30. The branches 20 may then be reset as described herein. The storage function makes high frequency readout unnecessary (although high frequency readout may be performed in some embodiments). Reading the output of the discriminator 10 at a lower frequency (e.g., once after each pulse of photons is delivered to the waveguide 14) helps to eliminate electrical crosstalk as a source of error. In some embodiments, the discriminator 10 is operable at a repetition rate of at least 1 MHz.

[0122] In some embodiments, the current flowing through the discriminator 10 is sampled to detect changes in current due to the absorption of individual photons. The sampling rate may be high enough to determine the detection time of individual photons to a desired level of accuracy. This mode of operation may provide information about the shape of the optical pulse or may be useful in reducing accumulated uncertainty.

[0123] The design of the identifier 10 also facilitates a relatively large distance between adjacent nanowires 12. This feature helps to avoid thermal crosstalk between different nanowires 12 as a source of error.

[0124] In some embodiments, the discriminator 10 described herein is applied to count photons, including photons that arrive at the waveguide 14 in a pulsed manner with a high degree of confidence (e.g., a confidence level of at least 80%, at least 85%, at least 90%, or at least 95%).

[0125] In some embodiments, the waveguide 14 does not have strong wavelength selectivity such that the discriminator 10 is effective for counting photons having a reasonably wide range of wavelengths. For example, the discriminator 10 according to some embodiments is operable to count photons having a wavelength of 800 nm and photons having a wavelength of 1550 nm (e.g., the discriminator 10 may have a detection efficiency of 90% or more for photons having wavelengths in the ranges of 1550 nm ± 20 nm and 800 nm ± 10 nm, or 800 ± 50 nm and 1550 ± 100 nm). In some embodiments, the discriminator 10 described herein is operable to count photons having wavelengths in the range of about 5000 nm to about 400 nm or less.

[0126] In some embodiments, the waveguide 14 is made more wavelength selective by incorporating an optical filter in the optical fiber 16 and / or by patterning an on-chip filter in the portion of the waveguide 14 between the taper of the optical fiber 16 and the first nanowire 12. Such optical filters can be effective in eliminating stray light and help to increase the signal-to-noise ratio (i.e., reduce unwanted photons delivered to the nanowire).

[0127] The wavelength range to which the discriminator 10 is most sensitive may be adjusted within small ranges by varying the bias current and the operating temperature.

[0128] To facilitate broadband operation, each of the waveguide 14, optical fiber 16, connector 17, and nanowire 12 may be broadband (i.e., the waveguide 14, optical fiber 16, and connector 17 may efficiently transmit photons having wavelengths within a desired band, and the nanowire 12 may collectively efficiently absorb photons within the desired band by evanescent absorption). In some embodiments of broadband operation, the fiber 16 is selected for optimal performance at the target wavelength and / or the coupling mechanism 17 is optimized for the target wavelength.

[0129] To facilitate non-broadband operation, the waveguide 14 may be further patterned to manipulate light. For example, holes may be etched into the portions of the waveguide 14 that are between adjacent nanowires 12. The number, location and dimensions of the holes may be designed to tailor the interaction of the light in the waveguide 14 with the nanowires 12. Another exemplary way to achieve non-broadband operation is to form the waveguide 14 to have a width that is periodically modulated to filter or slow the propagation of the light. Such a structure typically provides a much narrower effective bandwidth.

[0130] A single substrate 18 may optionally carry multiple photon counting discriminators as described herein, for example a 20×20 mm chip can accommodate up to several hundred such discriminators.

[0131] In designing the discriminator 10 and its operating conditions, the material of the nanowire 12, the operating wavelength and the operating temperature may be taken as the basic design parameters. From these, the maximum width of the nanowire 12 for efficient photon detection may be determined. The material and dimensions of the waveguide 14, as well as the spacing from the nanowire 12 to the waveguide 14 may be designed based on the width of the nanowire 12 to obtain the desired coupling strength of the nanowire 12 to the photons in the waveguide 14. (Interpretation of terms)

[0132] Unless the context clearly requires otherwise, throughout the description and claims, "comprises," "comprising," and the like are to be construed in an inclusive sense, i.e., "including but not limited to," as opposed to an exclusive or exhaustive sense. "Connected," "coupled," or variations thereof, means a connection or coupling between two or more elements, directly or indirectly. The coupling or coupling between the elements may be physical, logical, or a combination thereof. The words "herein," "above," "below," and words of similar import, when used to describe this specification, shall refer to this specification as a whole and not to any particular portions of this specification. "Or," in reference to a list of two or more items, includes all interpretations of the word any item in the list, every item in the list, and any combination of items in the list. The singular forms "a," "an," and "the" also include the appropriate plural meanings.

[0133] Directional terms such as "vertical," "lateral," "horizontal," "upper," "lower," "front," "rear," "inner," "outer," "left," "right," "front side," "rear," "top," "bottom," "lower," "upper," "lower," and the like, used in this specification and the appended claims (if any), are dependent upon the particular orientation of the device as described and illustrated. The subject matter described herein may assume various alternative orientations. Thus, these directional terms are not precisely defined and should not be interpreted in a precise manner.

[0134] In some embodiments, the apparatus described herein includes a controller. The controller may be implemented using specially designed hardware, configurable hardware, a programmable data processor configured by provision of software (which may optionally comprise "firmware") executable on a data processor, a special-purpose computer or data processor specifically programmed, configured, or constructed to perform one or more steps in the methods detailed herein, and / or a combination of two or more thereof. Examples of specially designed hardware are logic circuits, application specific integrated circuits ("ASICs"), large scale integrated circuits ("LSIs"), very large scale integrated circuits ("VLSIs"), etc. Examples of configurable hardware are one or more programmable logic devices, such as programmable array logic ("PALs"), programmable logic arrays ("PLAs"), and field programmable gate arrays ("FPGAs"). Examples of programmable data processors are microprocessors, digital signal processors ("DSPs"), embedded processors, graphics processors, mathematical co-processors, general purpose computers, server computers, cloud computers, mainframe computers, computer workstations, etc. For example, one or more data processors in the control circuitry of the device may implement the methods described herein by executing software instructions in program memory accessible to the processors.

[0135] Some aspects of the invention may be provided in the form of a program product. The program product may comprise any non-transitory medium carrying a set of computer readable instructions that, when executed by a data processor, causes the data processor to carry out the methods of the invention (e.g., methods of controlling an apparatus described herein and / or methods of calibrating an apparatus described herein and / or methods of processing readouts from an apparatus described herein). A program product according to the invention may be in any of a wide variety of forms. The program product may comprise, for example, a hard disk drive, a magnetic data storage medium including a floppy disk, an optical data storage medium including a CDROM, a DVD, an electronic data storage medium including a ROM, a flash RAM, an EPROM, a hardwired or pre-programmed chip (e.g., an EEPROM semiconductor chip), a nanotechnology memory, etc. The computer readable signals in the program product may optionally be compressed or encrypted.

[0136] In some embodiments, certain aspects of the invention are implemented in software. For clarity, "software" includes any instructions executed on a processor, and may include, but is not limited to, firmware, resident software, microcode, etc. Both the processing hardware and software may be centralized or distributed (or a combination thereof), in whole or in part, as known to those skilled in the art. For example, the software and other modules may be accessible via local memory, over a network, via a browser or other application in a distributed computing context, or via other means suitable for the purposes described above.

[0137] Where a component (e.g., a software module, a processor, a waveguide, a resistor, a power supply, an assembly, a device, a circuit, etc.) is referred to above, unless otherwise indicated, any reference to that component (including a reference to a "means") should be interpreted as including any component that performs the function of the described component (i.e., functionally equivalent) as an equivalent of that component, including components that are not structurally equivalent to structures of the present disclosure that perform functions in exemplary embodiments of the invention.

[0138] Specific examples of systems, methods, and devices are described herein for illustrative purposes. These are merely examples. The techniques provided herein may be applied to systems other than the exemplary systems described above. Many changes, modifications, additions, omissions, and permutations are also possible within the practice of the invention. The invention includes variations of the described embodiments that are apparent to those skilled in the art, including variations obtained by replacing features, elements, and / or operations with equivalent features, elements, and / or operations, mixing and matching features, elements, and / or operations of different embodiments, combining features, elements, and / or operations of the embodiments described herein with features, elements, and / or operations of other technologies, and / or omitting to combine features, elements, and / or operations of the described embodiments.

[0139] Various features are described herein as being present in "some embodiments." Such features are not required and may not be present in all embodiments. An embodiment of the invention may include zero, any one, or any combination of two or more of such features. All possible combinations of such features are contemplated by this disclosure, even if such features are shown in different drawings and / or described in different sections or paragraphs. This is limited to the extent that certain of such features are incompatible with other of such features, in the sense that one of ordinary skill in the art would not be able to construct a practical embodiment combining such incompatible features. Thus, a statement that "some embodiments" have feature A and "some embodiments" have feature B should be interpreted as clearly indicating that the inventors also contemplate embodiments combining feature A and feature B (unless otherwise stated in the specification or unless feature A and feature B are fundamentally incompatible).

[0140] It is therefore intended that the following appended claims and the claims introduced below be interpreted to include all such modifications, permutations, additions, omissions, and subcombinations as may reasonably be inferred. The claims should not be limited by the preferred embodiments set forth in the examples, but should be accorded the broadest interpretation consistent with the description as a whole.

Claims

1. In a photon number discriminator (PNRD), a waveguide having an input connected to receive photons for detection, a plurality of nanowires extending laterally with respect to the waveguide at spaced positions, each of the nanowires including a material that is superconducting at the operating temperature of the photon number discriminator, each of the nanowires being connected in series with an electrical resistance element so as to provide a branch, and the branches being electrically connected in parallel with each other; and comprising a photon number discriminator in which, at the operating temperature, the electrical time constant of the nanowire is small enough to cause a latch to a latched state when the nanowire absorbs a photon.

2. The photon number discriminator according to claim 1, wherein different ones of the plurality of nanowires extend laterally with respect to the waveguide at different angles.

3. The evanescent coupling between the plurality of nanowires and the waveguide is strong enough such that the probability that any photon entering the waveguide at the input is absorbed by one of the plurality of nanowires is at least 65%, and weak enough such that the probability that any photon entering the waveguide at the input is absorbed by any particular one of the plurality of nanowires is less than 5%, the photon number discriminator according to claim 1.

4. The plurality of nanowires have a length not exceeding 1000 nm, the photon number discriminator according to claim 1.

5. The nanowire extends across the waveguide at an angle in the range of +60 degrees to -60 degrees with respect to the vertical, the photon number discriminator according to claim 1.

6. The photon number discriminator according to claim 1, comprising a wavelength-selective optical fiber coupled to the input of the waveguide by an optical coupling section.

7. The photon number discriminator according to claim 1, comprising an optical fiber coupled to the input of the waveguide by an optical coupling section, the optical coupling section including a bandpass filter.

8. The waveguide is patterned to form a resonant structure, the photon number discriminator according to claim 1.

9. The electrical resistance element is thermally isolated from the nanowire, the photon number discriminator according to claim 1.

10. The branches are connected in parallel by first and second superconducting conductors extending along opposite sides of the waveguide, the photon number discriminator according to claim 1.

11. The photon number discriminator according to any one of Claims 1 to 10, further comprising a power supply connected to apply a voltage to the branch.

12. The power supply is an alternating current (AC) power supply, The photon number discriminator according to Claim 11, wherein the electrical resistance element includes a device that provides a reactive impedance.

13. further comprising a refrigerator operable to cool the branch to a temperature that is a superconductor, The photon number discriminator according to Claim 11, wherein the power supply is disposed outside a volume cooled by the refrigerator.

14. comprising one or more filters, the power supply comprising a digital-to-analog converter (DAC) controlled by a controller, The photon number discriminator according to Claim 11, wherein the one or more filters are connected to filter an output of the digital-to-analog converter.

15. The power supply is configured to output a pulsed voltage, and each electrical resistance element is controllable to adjust a bias current in a corresponding branch, the photon number discriminator according to Claim 11.

16. The photon number discriminator according to Claim 11, further comprising a current monitor connected to monitor a current drawn from the power supply by the branch.

17. obtaining measured values of the current at first and second times before and after photons are delivered to the photon number discriminator from the current monitor, and calculating the number N of photons by processing a difference between the measured values of the current, the photon number discriminator according to Claim 16, further comprising a controller connected to perform the calculation.

18. The controller is 【Number 1】 configured to calculate the number of photons by calculating, where I ＳＴＡＲＴ is the measured current at a first time, and I ＭＥＡＳＵＲＥ is the measured current at a second time, and ΔI is ΔI = I ＢＩＡＳ − I ＬＡＴＣＨ is given by, where I ＢＩＡＳ is the bias current for one of the nanowires in the non-latched state, and I ＬＡＴＣＨ is the current in one of the nanowires when the nanowire is in the latched state, the photon number discriminator according to claim 17.

19. The branch of the photon number discriminator is divided into a plurality of groups, the branches of each group are connected in parallel, and the photon number discriminator includes a power supply and a current monitor for each of the plurality of groups, the photon number discriminator according to Claim 1.

20. In a method for detecting and counting photons, cooling a waveguide having a plurality of nanowires extending laterally with respect to the waveguide at spaced positions to a temperature at which the nanowires become superconducting; passing a bias current through each of the nanowires, the bias current being lower than a critical current for the nanowires; Inducing at least one photon into the waveguide and enabling the photon to be evanescently absorbed by one of the nanowires; Latch the nanowire into a resistive state when one of the nanowires absorbs the photon; Measuring the total current drawn by the nanowire and processing the measured total current to count the photons A method comprising the steps of.

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