Photonic quantum information processing system

Abstract

Systems, devices, methods, and techniques for photonic quantum information processing are provided. In one aspect, a photonic integrated circuitry for counting one or more photons includes: a plurality of single-photon counters each including a single-photon avalanche diode (SPAD) and one or more waveguides arranged to optically couple the one or more photons to the plurality of single-photon counters. A count of the plurality of single-photon counters is greater than a count of the one or more photons to be counted. The plurality of single-photon counters and the one or more waveguides are formed on a substrate.

Description

Atorney Docket No.: 42571-0074 WO 1 / AN2025-002-WO-001PHOTONIC QUANTUM INFORMATION PROCESSING SYSTEMCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 633,859, filed April 15, 2024, U.S. Provisional Patent Application No. 63 / 643,437, filed May 7, 2024, and U.S. Provisional Patent Application No. 63 / 692,175, filed September 8, 2024, which are incorporated by reference herein in their entireties.TECHNICAL FIELD

[0002] This application relates to implementations of quantum information processing systems.BACKGROUND

[0003] Optical quantum information processing includes using photons for quantum computing, quantum communication, and quantum metrology, which requires precise control over light at the quantum level.SUMMARY

[0004] The present disclosure describes systems, devices, methods, and techniques for photonic quantum information processing, e g., using a number photon counter (NPD) implemented using a Single-Photon Avalanche Diode (SPAD) array.

[0005] One aspect of the present disclosure features a photonic integrated circuitry for counting one or more photons. The photonic integrated circuitry includes: a plurality of single-photon counters, where a single-photon counter of the plurality of singlephoton counters includes a single-photon avalanche diode (SPAD); and one or more waveguides arranged to optically couple the one or more photons to the plurality of single-photon counters. A count of the plurality of single-photon counters is greater than a count of the one or more photons to be counted. The plurality of single-photon counters and the one or more waveguides are formed on a substrate.

[0006] In some implementations, the SPAD of the single-photon counter includes a germanium-silicon (GeSi) SPAD.

[0007] In some implementations, the one or more waveguides include one or more silicon waveguides, and the substrate includes a silicon-on-insulator (SOI) substrate.Attorney Docket No.: 42571-0074 WO 1 / AN2025-002-WO-001

[0008] In some implementations, the one or more waveguides include: an input waveguide configured to support a propagation of incident light that includes the one or more photons; a plurality of output waveguides, each of the plurality of output waveguides being optically coupled to a corresponding one of the plurality of singlephoton counters; and an optical splitter configured to split the one or more photons, such that the one or more photons are optically guided to a subset of the plurality of output waveguides for photon counting.

[0009] In some implementations, the input waveguide and the plurality of output waveguides includes rib waveguides.

[0010] In some implementations, the optical splitter includes a star-coupler.

[0011] In some implementations, the one or more waveguides further include an input tapered waveguide arranged between the input waveguide and the optical splitter, the input tapered waveguide being configured to expand or shrink a mode of the incident light from the receiving waveguide.

[0012] In some implementations, each of the plurality of single-photon counters further includes: an output waveguide configured to receive a portion of a diffracted wave from the optical splitter; a laterally tapered waveguide coupled to the output waveguide, the laterally tapered waveguide being configured to shrink or expand a mode of light from the output waveguide along a direction that is parallel to a surface of the substrate; and a step coupler coupled to the laterally tapered waveguide. A height of the step coupler is higher than a height of the laterally tapered waveguide, and the step coupler of each of the plurality of single-photon counters is optically coupled to a corresponding SPAD.

[0013] In some implementations, the step coupler includes a multi-mode interferometer with light oscillating along a direction that is perpendicular to the surface of the substrate.

[0014] In some implementations, the photonic integrated circuitry further includes one or more metal reflectors coupled to one or more SPADs of the plurality of singlephoton counters.

[0015] In some implementations, the one or more waveguides include: an input waveguide and a plurality of evanescent coupler waveguides that are optically coupled to the input waveguide. Each of the plurality of evanescent coupler waveguides isAtorney Docket No.: 42571-0074 WO 1 / AN2025-002-WQ-001 optically coupled to a corresponding single-photon counter of the plurality of singlephoton counters.

[0016] In some implementations, each of the plurality of single-photon counters further includes: an output waveguide coupled to the corresponding evanescent coupler waveguide; a laterally tapered waveguide coupled to the output waveguide, the laterally- tapered waveguide being configured to expand or shrink a mode of light from the output waveguide along a direction that is parallel to a surface of the substrate; and a step coupler coupled to the laterally tapered waveguide. A height of the step coupler is higher than a height of the laterally tapered waveguide, and the step coupler of each of the plurality of single-photon counters is optically coupled to a corresponding SPAD.

[0017] In some implementations, the one or more waveguides include: an input waveguide; a laterally tapered waveguide; and a step coupler coupled to the laterally tapered waveguide. A height of the step coupler is higher than a height of the laterally tapered waveguide, and the step coupler is optically coupled to the SPADs of the plurality of single-photon counters.

[0018] In some implementations, each of the plurality of single-photon counters is configured to operate in a time-gating mode. Photon detection events occur in a single pre-defined time-grating window or multiple pre-defined time-gating windows with a temporal width that is greater than a timing jitter associated with the plurality of singlephoton counters.

[0019] In some implementations, each of the plurality of single-photon counters is configured to operate in a free-running mode.

[0020] Another aspect of the present disclosure features a photonic quantum computing system, including: quantum sources each configured to generate a photon; quantum circuits configured to manipulate a path degree of freedom of one or more photons from the quantum sources; and a quantum detector circuitry coupled to the quantum circuits and configured to count the one or more photons from the quantum circuits. The quantum detector circuitry includes: a plurality of single-photon counters, a single-photon counter of the plurality of single-photon counters including a singlephoton avalanche diode (SPAD) and one or more waveguides arranged to optically couple the one or more photons to the plurality of single-photon counters. A count of the plurality of single-photon counters is greater than a count of the one or moreAttorney Docket No.: 42571-0074 WO 1 / AN2025-002-WO-001 photons, and the plurality of single-photon counters and the one or more waveguides are formed on a substrate.

[0021] In some implementations, the quantum circuits include a field-programmable interferometer mesh.

[0022] In some implementations, each of the plurality of single-photon counters is configured to operate in a time-gating mode, and photon detection events occur in a single pre-defined time-grating window or multiple pre-defined time-gating windows with a temporal width that is greater than a timing jitter associated with the plurality of single-photon counters.

[0023] In some implementations, each of the plurality of single-photon counters is configured to operate in a free-running mode, and the photonic quantum computing system further includes a time-to-digital converter (TDC) circuitry coupled to the plurality of single-photon counters and configured to construct a detection histogram.

[0024] In some implementations, the photonic quantum computing system further includes one or more processing circuitries configured to determine a relevant time window for photon arrival. The TDC circuitry includes a TDC configured to convert a time interval between the photon arrival and a reference clock signal into a digital value, and the TDC circuitry' is configured to collect outputs from the TDC to construct the detection histogram.

[0025] The details of one or more disclosed implementations are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the description, the drawings and the claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The foregoing aspects and many of the advantages of this application will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings:

[0027] FIG. 1 shows a top- view of an example number photon counter.

[0028] FIG. 2 shows a top-view of an example number photon counter.

[0029] FIG. 3A shows a top-view of an example number photon counter.

[0030] FIG. 3B shows a side-view of an example number photon counter.

[0031] FIG. 4A shows a top-view of an example single-photon counter.Attorney Docket No.: 42571-0074 WO 1 / AN2025-002-WO-001

[0032] FIG. 4B shows a side-view of an example single-photon counter.

[0033] FIG. 5A shows a top-view of an example single-photon counter.

[0034] FIG. 5B shows a side-view of an example single-photon counter.

[0035] FIG. 6 illustrates an example photonic quantum computing system.

[0036] Like reference numbers and designations in the various drawings indicate like elements.DETAILED DESCRIPTION

[0037] A number photon counter (NPD) is a device that measures the exact number of photons present in a pulse of light. Unlike conventional photodetectors, which can only indicate the presence of light without precisely counting the number of photons, number photon counters (NPDs) provide a detailed measurement by directly counting the photons. This capability is crucial for applications that require precise control over light at the quantum level, such as quantum computing, quantum cryptography, quantum optics, quantum information processing, and various experiments in quantum mechanics.

[0038] A Single-Photon Avalanche Diode (SPAD) is a type of photodetector that can be used as a component of a number photon counter (NPD). SPADs can operate in Geiger mode, where the diode is biased above its breakdown voltage. In this highly sensitive state, a single photon striking the diode can trigger an avalanche of electronhole pairs, resulting in a measurable current pulse. This capability to detect single photons makes SPADs highly useful in low-light conditions and for applications requiring precise photon counting. However, using SPADs for NPD presents some technical challenges. First, after detecting a photon and generating an avalanche, the SPAD enters a "‘dead time” period during which it cannot detect another photon, making it difficult to count multiple photons arriving in quick succession or in a high photon flux. Second, due to potential dark currents associated with the device, SPADs can generate signals even in the absence of light. The SPAD dark count rate therefore adds noise to the photon counting measurement. Generally, an NPD that has a high single-photon detection efficiency and low dark count rate and low after-pulsing probability is desirable.

[0039] Implementations of the present disclosure provide an NPD implemented using an SPAD array, such as a germanium-silicon (GeSi) SPAD array on a silicon-Atorney Docket No.: 42571-0074 WO 1 / AN2025-002-WO-001 photonics platform. A germanium-silicon (GeSi) SPAD that can operate in room temperature has been reported (Na. N.. Lu, YC., Liu, YH. et al. Room temperature operation of germanium-silicon single-photon avalanche diode. Nature (2024), incorporated herein by reference). With its low dark count rate, such GeSi SPAD device may be used in an NPD. Moreover, a photonic integrated circuit implemented on a silicon-on-insulator (SOI) platform may further improve the single-photon detection efficiency of the NPD. Note that an SPAD array described below may also be implemented using different materials such as silicon, and passive waveguide components (e.g., waveguides, Multi-Mode Interferometer (MMI), etc.) may also be implemented using different materials such as silicon-nitride or other suitable materials.

[0040] FIG. 1 shows an example NPD implemented using a photonic integrated circuitry 100. The photonic integrated circuitry 100 includes waveguide components configured to receive N photons and to optically couple the N photons to M singlephoton counters, where each single-photon counter includes a SPAD (e.g., GeSi SPAD). In some implementations, the photonic integrated circuitry 100 splits the input power of the N photons equally to the M single-photon counters, and when M » N. the conditional probability of detecting a single-photon and multi-photons are ~ 1 and ~ 0, respectively. The photonic integrated circuitry 100 can be formed on a substrate (e.g., silicon waveguides on a SOI substrate).

[0041] Referring to FIG. 1, the waveguide components include an input waveguide 102 that receives the N photons from a light source (e.g., a pulsed laser). In some implementations, the waveguide components further include an input tapered waveguide 104 configured to laterally (e.g., along the x-direction) expand a mode (e.g., a single mode or a fundamental mode) of the incident light from the input waveguide 102. The waveguide components can further include an optical splitter 106 (e.g., a starcoupler) configured to split the N photons (e.g., convert the incident light into a diffracted wave), such that the N photons are optically guided to a subset of the M single-photon counter (M »N). In some implementations, the waveguide components may further include mode converters 108 between the optical splitter 106 and the single-photon counters to laterally convert the mode from the optical splitter 106 back into the fundamental mode.

[0042] In some implementations, the waveguide components may include rib waveguides. The dimensions of the waveguide rib and the waveguide core may beAtorney Docket No.: 42571-0074 WO 1 / AN2025-002-WO-001 designed based on the desirable waveguide characteristics. As an example, the heights of the waveguide rib and the waveguide core may be smaller than 100 nm and 250 nm, respectively. The width of the waveguide core may be smaller than 1 pm, or any suitable dimension to achieve the desirable waveguide characteristics (e.g., single mode, lower optical loss, etc ).

[0043] FIGS. 4A and 4B show an example single-photon counter 400 implemented on an SOI platform. The single-photon counter 400 includes an input waveguide 402 (e.g., a single-mode waveguide), a laterally tapered waveguide 404, a step coupler 406, and a GeSi SPAD 408. The input waveguide 402 is coupled to an output of the optical splitter 106. The laterally tapered waveguide 404 is coupled to the input waveguide 402, where the laterally tapered waveguide 404 is configured to expand a mode (e.g., single Transverse Electric (TE) or Transverse Magnetic (TM) mode) of light from the input waveguide 402 along a direction that is parallel to a surface of the substrate (e.g., along the x-direction). The laterally tapered waveguide 404 has a length of Liaper that can be optimized based on design considerations (e.g., optical loss, and / or mode conversion efficiency, etc.). As an example, the start width of the laterally tapered waveguide 404 may be smaller than 1 pm, and the end width of the laterally tapered waveguide 404 may be smaller than 5 pm over a taper length smaller than 20 pm.

[0044] The step coupler 406 is coupled to the laterally tapered waveguide 404, where the height of the step coupler 406 is higher than the height of the laterally tapered waveguide 404. In some implementations, the step coupler 406 may be a multi-mode interferometer (MMI). As light enters the MMI, higher-order mode(s) having a higher optical intensity may be generated closer to the top surface (e.g., along the y-direction) of the step coupler 406, which can help with optical coupling of the light into the GeSi SPAD for improved absorption. The step coupler 406 has a length of Lstep and a height that can be optimized based on design considerations (e.g., optical coupling efficiency, and / or MMI beat length, etc.). As an example, the height of the laterally tapered waveguide 404 may be smaller than 250 nm, and the height of the step coupler 406 may be smaller than 800 nm.

[0045] The GeSi SPAD 408 is coupled to the step coupler 406. The propagated photon from the step coupler 406 is directly coupled to the Ge absorption region, where the photon is absorbed and a photo-current is generated. Under the Geiger mode operation, the generated photo-current may be amplified by an amplification regionAttorney Docket No.: 42571-0074 WO 1 / AN2025-002-WO-001(e.g., in the Si region below the Ge absorption region) and collected by circuitry' (not shown) for further processing.

[0046] The single-photon counter 400 may further include one or more metal layers 410 (e.g., aluminum) configured to reflect the light not completely absorbed by the Ge absorption region of the GeSi SPAD 408 back to the GeSi SPAD 408.

[0047] FIGS. 5A and 5B shows another example single-photon counter 500 implemented on an SOI platform. The single-photon counter 500 includes an input waveguide 402 (e g., a single-mode waveguide), a laterally tapered waveguide 404, a step coupler 406, and a GeSi SPAD 508. Unlike the GeSi SPAD 408 (e.g., formed as a mesa on silicon) as described in reference to FIGS. 4A and 4B, the GeSi SPAD 508 is embedded in the silicon (e.g., deposited in a trench formed in silicon). In some implementations, the step coupler 406 may be laterally tapered to improve the optical coupling efficiency from the step coupler 406 to the GeSi SPAD 508.

[0048] FIG. 2 show s another example NPD implemented using a photonic integrated circuitry 200. The photonic integrated circuitry 200 includes waveguide components configured to receive N photons and to optically couple the N photons to M singlephoton counters. The photonic integrated circuitry 200 includes an input waveguide 202, and a plurality of evanescent coupler waveguides 204a-204M that are optically coupled to the input waveguide 202, and a plurality of single-photon counters 206a- 206M. The evanescent coupler waveguides 204a-204M may be designed to control the coupling efficiency of N photons to AT waveguides by the waveguide-to-waveguide gap (along x-direction) and the waveguide length (along z-direction) of each evanescent coupler. Each of the plurality of evanescent coupler waveguides 204a-204M is optically coupled to a corresponding single-photon counter of the plurality of single-photon counters 206a-206M (e.g., single-photon counter 400 / 500).

[0049] FIGS. 3A-3B show another example NPD implemented using a photonic integrated circuitry 300. The photonic integrated circuitry 300 includes waveguide components configured to receive N photons and to optically couple the N photons to M single-photon counters. The photonic integrated circuitry 300 includes an input waveguide 302 (e.g., similar to the input waveguide 402), a laterally tapered waveguide 304 (e.g., similar to the laterally tapered waveguide 404), a step coupler 306 (e.g., similar to the step coupler 406), and a plurality of SPADs 308a-308M (e g., similar to the GeSi SPAD 408 / 508). The step coupler 306 may be designed to control the verticalAttorney Docket No.: 42571-0074 WO 1 / AN2025-002-WO-001(along y-direction) coupling efficiency of N photons to M SPADs by the length of each SPAD and the waveguide length (e.g., MMI beat length) of each segmented step coupler in the step coupler 306.

[0050] FIG. 6 illustrates an example photonic quantum computing (PQC) system 600 that is operable at room-temperature. The photonic quantum computing system 600 includes quantum sources 610. quantum circuits 620, and a quantum detector circuitry that includes quantum detectors 630.

[0051] In some implementations, the quantum sources 610 may be implemented using on-demand single-photon generation from single quantum dots, defects, or impurities, weakly coupled to optical microcavities or nanocavities. In some other implementations, the quantum sources 610 may be implemented using heralded singlephoton generation from on-chip optical waveguides through nonlinear optical processes such as spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (SFWM). With the advancement of spatial, temporal, and spectral multiplexing techniques to overcome the probabilistic nature of heralded single-photon generation, quantum sources that generate single photons through on-chip optical waveguides with off-resonant optical nonlinearities have becoming practical for room-temperature PQC. For example, since SOI waveguides inherently possess the 3rd order Kerr nonlinearity, SFWM source can be implemented at room-temperature. Moreover, an increased value of Kerr index n2 can be attained if SiGe alloy is employed as the ring-waveguide material in the SFWM sources.

[0052] In some implementations, the quantum circuits 620 includes a field- programmable interferometer mesh (FPIM) implemented by cascaded Mach-Zehnder interferometers (MZIs), so that various quantum algorithms can be performed through manipulating the path degree of freedom of single photons at room temperature. In some implementations, the quantum circuits 620 may be implemented using one or more silicon waveguides (e.g., on an SOI platform). In some other implementations, the quantum circuits 620 may be implemented using one or more silicon-nitride waveguides (e.g., on an SOI platform or a silicon platform). The quantum detectors 630 may be implemented using any of the single-photon counter(s) or number photon counter(s) described in this disclosure, e.g., the NPD as described with respect to FIG. 1. FIG. 2, or FIGS. 3A-3B. The NPD can be implemented using a photonic integrated circuitry, e.g., 100 of FIG. 1, 200 of FIG. 2, or 300 of FIGS. 3A-3B. The NPD canAttorney Docket No.: 42571-0074 WO 1 / AN2025-002-WO-001 include a plurality of single-photon counters (e.g., 400 of FIGS. 4A-4B or 500 of FIGS. 5A-5B), and a single-photon counter can include an SPAD, e.g.. the SPAD 408 of FIGS. 4A-4B or 508 of FIGS. 5A-5B.

[0053] In some implementations, the photonic quantum computing system 600 may be configured to operate in a time-gating mode, where all photon detection events occur in a single or multiple pre-defined time-gating windows with a temporal width TG that is greater than the timing jitter associated with the quantum detectors 630. In some cases, in time-of-flight applications, it is not possible to know the time-of-flight of all photons reflected from a scene in advance. However, in PQC applications, the time-of- flight of single photons in pre-defined quantum circuits can be predictable. This is especially true for the FPIM. in which single photons reach the quantum detectors 630 at multiple instances separated by a constant temporal period that is associated with the delay introduced by an individual MZI. In some implementations, the pre-defined timegating windows may be stored in a memory and the detection window may be controlled by a processor or other suitable circuitry.

[0054] In some other implementations, the photonic quantum computing system 600 may be configured to operate in a free-running mode, where a time-to-digital converter (TDC) circuitry may be used to construct a detection histogram. In some cases, a TDC is used to convert the time interval between the photon arrival and a reference clock signal into a digital value, which allows for precise timing measurements for applications like time-correlated single-photon counting. Over time, the outputs from the TDC may be collected to construct a histogram, and the relevant time window for photon arrival may be determined by one or more processing circuitry'.

[0055] Unless otherwise specified, as used herein, the terms ‘“photodetector”, ‘‘optical sensor”, “optical sensing apparatus”, or other similar terms can include a device that has been designed and / or operated as a photodiode (PD), an avalanche photodiode (APD), a single-photon avalanche diode (SPAD), or a locked-in PD (LIPD).

[0056] As used herein, the terms such as “first”, “second”, “third”, “fourth” and “fifth” describe various elements, components, regions, layers and / or sections, these elements, components, regions, layers and / or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another. The terms such as “first”, “second”, “third”, “fourth” and “fifth” when used herein do not imply a sequence or order unless clearly indicated byAtorney Docket No.: 42571-0074 WO 1 / AN2025-002-WO-001 the context. The terms “photo-detecting’", “photo-sensing”, “light-detecting”, “lightsensing” and any other similar terms can be used interchangeably.

[0057] Spatial descriptions, such as “above”, "‘over,”, “under”, “top”, and "‘bottom” and so forth, are indicated with respect to the orientation shown in the figures unless otherwise specified. It should be understood that the spatial descriptions used herein are for purposes of illustration only, and that practical implementations of the structures described herein can be spatially arranged in any orientation or manner, provided that the merits of embodiments of this disclosure are not deviated by such arrangement.

[0058] As used herein and not otherwise defined, the terms “substantially” and “about"’ are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can encompass instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. For example, when used in conjunction with a numerical value, the terms can encompass a range of variation of less than or equal to ±10% of that numerical value, such as less than or equal to ±5%. less than or equal to ±4%. less than or equal to ±3%. less than or equal to ±2%. less than or equal to ±1%. less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%.

[0059] While the concepts have been described by way of examples and in terms of embodiments, it is to be understood that the disclosure is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.

Claims

Atorney Docket No.: 42571-0074 WO 1 / AN2025-002-WQ-001CLAIMS1. A photonic integrated circuitry for counting one or more photons, the photonic integrated circuitry comprising: a plurality of single-photon counters, wherein a single-photon counter of the plurality of single-photon counters comprises a single-photon avalanche diode (SPAD); and one or more waveguides arranged to optically couple the one or more photons to the plurality of single-photon counters, wherein a count of the plurality of single-photon counters is greater than a count of the one or more photons to be counted, and wherein the plurality of single-photon counters and the one or more waveguides are formed on a substrate.

2. The photonic integrated circuitry of claim 1, wherein the SPAD of the singlephoton counter comprises a germanium-silicon (GeSi) SPAD.

3. The photonic integrated circuitry of claim 2, wherein the one or more waveguides comprise one or more silicon waveguides, and wherein the substrate comprises a silicon-on-insulator (SOI) substrate.

4. The photonic integrated circuitry of claim 1, wherein the one or more waveguides comprise: an input waveguide configured to support a propagation of incident light that includes the one or more photons; a plurality of output waveguides, each of the plurality of output waveguides being optically coupled to a corresponding one of the plurality of single-photon counters; and an optical splitter configured to split the one or more photons, such that the one or more photons are optically guided to a subset of the plurality of output waveguides for photon counting.

5. The photonic integrated circuitry of claim 4, wherein the input waveguide and the plurality of output waveguides comprises rib waveguides.Atorney Docket No.: 42571-0074 WO 1 / AN2025-002-WO-0016. The photonic integrated circuitry of claim 4, wherein the optical splitter comprises a star-coupler.

7. The photonic integrated circuitry of claim 4, wherein the one or more waveguides further comprises: an input tapered waveguide arranged between the input waveguide and the optical splitter, the input tapered waveguide being configured to expand or shrink a mode of the incident light from the input waveguide.

8. The photonic integrated circuitry of claim 7. wherein each of the plurality’ of single-photon counters further comprises: an output waveguide configured to receive a portion of a diffracted wave from the optical splitter; a laterally tapered waveguide coupled to the output waveguide, the laterally tapered waveguide being configured to shrink or expand a mode of light from the output waveguide along a direction that is parallel to a surface of the substrate; and a step coupler coupled to the laterally tapered waveguide, wherein a height of the step coupler is higher than a height of the laterally tapered waveguide, and wherein the step coupler of each of the plurality' of single-photon counters is optically coupled to a corresponding SPAD.

9. The photonic integrated circuitry of claim 8, wherein the step coupler comprises a multi-mode interferometer with light oscillating along a direction that is perpendicular to the surface of the substrate.

10. The photonic integrated circuitry of claim 8, further comprising one or more metal reflectors coupled to one or more SPADs of the plurality of single-photon counters.

11. The photonic integrated circuitry of claim 1 , wherein the one or more waveguides comprise: an input waveguide; andAtorney Docket No.: 42571-0074 WO 1 / AN2025-002-WO-001 a plurality of evanescent coupler waveguides that are optically coupled to the input waveguide. wherein each of the plurality of evanescent coupler waveguides is optically coupled to a corresponding single-photon counter of the plurality of single-photon counters.

12. The photonic integrated circuitry of claim 11, wherein each of the plurality of single-photon counters further comprises: an output waveguide coupled to the corresponding evanescent coupler waveguide; a laterally tapered waveguide coupled to the output waveguide, the laterally tapered waveguide being configured to expand or shrink a mode of light from the output waveguide along a direction that is parallel to a surface of the substrate; and a step coupler coupled to the laterally tapered waveguide, wherein a height of the step coupler is higher than a height of the laterally tapered waveguide, and wherein the step coupler of each of the plurality of single-photon counters is optically coupled to a corresponding SPAD.

13. The photonic integrated circuitry of claim 1, wherein the one or more waveguides comprise: an input waveguide; a laterally tapered waveguide; and a step coupler coupled to the laterally tapered waveguide, wherein a height of the step coupler is higher than a height of the laterally tapered waveguide, and wherein the step coupler is optically coupled to the SPADs of the plurality of single-photon counters.

14. The photonic integrated circuitry of claim 1. wherein each of the plurality of single-photon counters is configured to operate in a time-gating mode, and wherein photon detection events occur in a single pre-defined time-grating window or multiple pre-defined time-gating windows with a temporal width that is greater than a timing jitter associated with the plurality of single-photon counters.Attorney Docket No.: 42571-0074 WO 1 / AN2025-002-WO-00115. The photonic integrated circuitry of claim 1. wherein each of the plurality’ of single-photon counters is configured to operate in a free-running mode.

16. A photonic quantum computing system, comprising: quantum sources each configured to generate a photon; quantum circuits configured to manipulate a path degree of freedom of one or more photons from the quantum sources; and a quantum detector circuitry coupled to the quantum circuits and configured to count the one or more photons from the quantum circuits, wherein the quantum detector circuitry comprises: a plurality of single-photon counters, wherein a single-photon counter of the plurality7of single-photon counters comprises a single-photon avalanche diode (SPAD); and one or more waveguides arranged to optically couple the one or more photons to the plurality of single-photon counters, wherein a count of the plurality of single-photon counters is greater than a count of the one or more photons, and wherein the plurality of single-photon counters and the one or more waveguides are formed on a substrate.

17. The photonic quantum computing system of claim 1 , wherein the quantum circuits comprise a field-programmable interferometer mesh.

18. The photonic quantum computing system of claim 16, wherein each of the plurality of single-photon counters is configured to operate in a time-gating mode, and wherein photon detection events occur in a single pre-defined time-grating window or multiple pre-defined time-gating windows with a temporal width that is greater than a timing jitter associated with the plurality of single-photon counters.

19. The photonic quantum computing system of claim 16, wherein each of the plurality of single-photon counters is configured to operate in a free-running mode, and wherein the photonic quantum computing system further comprises a time-to-Atorney Docket No.: 42571-0074 WO 1 / AN2025-002-WQ-001 digital converter (TDC) circuitry coupled to the plurality of single-photon counters and configured to construct a detection histogram.

20. The photonic quantum computing system of claim 19, further comprising one or more processing circuitries configured to determine a relevant time window for photon arrival, and wherein the TDC circuitry comprises a TDC configured to convert a time interval between the photon arrival and a reference clock signal into a digital value, and the TDC circuitry is configured to collect outputs from the TDC to construct the detection histogram.

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