Multi-pass photon detector

Abstract

A photonic integrated circuit (PIC) has a photon number resolving detector that has a superconducting nanowire and a waveguide. The waveguide has a plurality of waveguide segments, and each of the waveguide segments are connected through a curved waveguide portion, and a reflector is positioned at the end of the waveguide. The waveguide is arranged proximate to portions of the superconducting nanowire, and one or more photons that propagate through a given waveguide segment have an absorption probability to be absorbed by any one of the portions of superconducting nanowire. The absorption probability distribution for light in the waveguide to be absorbed by the portions of the superconducting nanowire is substantially uniform across the portions of the superconducting nanowire.

Description

MULTI-PASS PHOTON DETECTORCLAIM OF PRIORITY

[0001] This application claims the benefit of priority to U.S. Provisional Patent Application No. 63 / 724,862, filed on November 25, 2024, which is incorporated by reference herein in its entirety.TECHNICAL FIELD

[0002] The present disclosure generally relates to optical devices, and more particularly to optical detectors.BACKGROUND

[0003] Optical detectors can be implemented in optical and electrical devices to detect light for signal processing. It is difficult to implement photodetectors that are sensitive enough to detect individual photons. Single photon detectors can be used in a variety of applications including optical communications and quantum information processing systems.BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0004] The following description includes discussion of figures having illustrations given by way of example of implementations of embodiments of the disclosure. The drawings should be understood by way of example, and not by way of limitation. As used herein, references to one or more "embodiments" are to be understood as describing a particular feature, structure, or characteristic included in at least one implementation of the inventive subject matter. Thus, phrases such as "in one embodiment" or "in an alternate embodiment" appearing herein describe various embodiments and implementations of the inventive subject matter, and do not necessarily all refer to the same embodiment. However, they are also not necessarily mutually exclusive. To easily identify the discussion of any particularelement or act, the most significant digit or digits in a reference number refer to the figure ("FIG.") number in which that element or act is first introduced.

[0005] FIG. 1 shows an example of a qubit entangling system, in accordance with some example embodiments.

[0006] FIG. 2 shows a photon number resolving superconducting detector, in accordance with some example embodiments.

[0007] FIG. 3A, FIG. 3B, and FIG. 3C show operations of a superconducting detector, in accordance with some example embodiments.

[0008] FIG. 4A and FIG. 4B show an example superconducting detector with an integrated waveguide, in accordance with some example embodiments.

[0009] FIG. 5A and FIG. 5B show an example superconducting detector with a multi-pass integrated waveguide, in accordance with some example embodiments.

[0010] FIG. 6 shows a functional diagram for a superconducting detector having a multi-pass integrated waveguide, in accordance with some example embodiments.

[0011] FIG. 7 shows an example flow diagram for designing a superconducting detector with a multi-pass integrated waveguide, in accordance with some example embodiments.

[0012] FIG. 8 shows an example graph of an absorption probability distribution per unit wire for two examples of detectors with a multi-pass integrated waveguide, in accordance with some example embodiments.

[0013] FIG. 9 shows an example graph of an absorption probability distribution per unit wire with contributions from multiple passes through an integrated waveguide, in accordance with some example embodiments.

[0014] FIG. 10 shows an example of a superconducting detector having a four-pass integrated waveguide, in accordance with some example embodiments.

[0015] FIG. 11 shows an example of a four-pass integrated waveguide for use with a superconducting detector, in accordance with some example embodiments.

[0016] FIG. 12 shows an example superconducting detector with an eightpass integrated waveguide, in accordance with some example embodiments.

[0017] FIG. 13 shows an example of an eight-pass integrated waveguide for use with a superconducting detector, in accordance with some example embodiments.

[0018] FIG. 14 shows an example of an eight-pass integrated waveguide with confinement structures for use with a superconducting detector, in accordance with some example embodiments.DETAILED DESCRIPTION

[0019] In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide an understanding of various embodiments of the inventive subject matter. It will be evident, however, to those skilled in the art, that embodiments of the inventive subject matter may be practiced without these specific details. In general, well-known instruction instances, structures, and techniques are not necessarily shown in detail.

[0020] As discussed, it is difficult to implement photodetectors that are sensitive enough to detect individual photons. Applications that can depend on single photons, such as optical communication systems and quantum information processing (e.g., photonic quantum circuits, photonic quantum computers), can generate a class of specific types of photons that are to be tracked or processed across the entire optical system. That is, for example, in optical communication systems that depend on single photons, the single photons may contain optical information that is lost if the single photons are not detected and processed correctly. As an additional example, in a quantum information processing systems that utilize single photons, the photons can be used as qubits (e.g., quantum information units of a quantum circuit or quantum computer) which can undergo one or more entanglement and measurement operations to process information.

[0021] The described examples relate to multi-pass photon number resolving detectors, which address challenges in achieving high-efficiency single photon detection and photon number resolution for applications such as quantum information processing and optical communications.

[0022] Photonic integrated circuits that include superconducting nanowire single photon detectors (SNSPDs) face limitations in achieving the high detection efficiencies used in quantum computing applications. These detectors SNSPDs include a superconducting nanowire positioned above an optical waveguide. When a photon is absorbed by the nanowire, it creates a resistive hotspot, causing a detectable voltage pulse. The nanowire can be arranged in a serpentine fashion, so that each straight portion of the nanowire is a photo-sensitive wire segment.

[0023] The multi-pass designs aim to improve detection efficiency by allowing light to interact with the superconducting nanowire having photosensitive segments multiple times. This is achieved through a waveguide structure that guides light through several passes, increasing the probability of photon absorption. The multi-pass design consists of a multi-pass waveguide comprising multiple waveguide segments (e.g., connected by curved portions). It guides light through multiple passes, increasing interaction length with the nanowire. A reflector is positioned at the end of the waveguide structure to reflect light back through the waveguide, effectively doubling the number of passes. The superconducting nanowire remains positioned above the waveguide, and the length of the photosensitive segments can be increase such that a photo-sensitive segment of the superconducting nanowire is positioned above multiple waveguide segments. The multi-pass design can improve overall detection efficiency and can achieve a uniform absorption probability distribution across the photonsensitive wire segments.

[0024] FIG. 1 illustrates an example of a qubit entangling system 100 in accordance with some embodiments. Such a system can be used to generate qubits (e.g., photons) in an entangled state. The qubit entangling system 100 can include a photon source module 120 that is optically connected to an entangled state generator 110. Both the photon source module 120 and the entangled state generator 110 can be coupled to a classical processing system 101 using classical channels 130 (e.g., classical channels 130-a to 130-d). The classical processing system 101 can use the classical information channel 130-a to communicate or to control the photon source module 120. The classical processing system 101 can use the classical informationchannel 130-b to communicate with or to control the entangled state generator 110. In some example embodiments, the qubit entangling system 100 can include classical channels 130 (e.g., classical information channel 130-a through 130-d) for interconnecting and providing classical information between components.

[0025] In some example embodiments, classical processing system 101 can include memory 104, one or more processor(s) 102, a power supply, an input / output (I / O) subsystem, and a communication bus or interconnecting these components. The processor(s) 102 can execute applications, programs, or instructions stored in memory 104 and thereby perform processing operations.

[0026] In some example embodiments, the memory 104 stores one or more programs (e.g., sets of instructions) or data structures. For example, in some example embodiments, the entangled state generator 110 generates an entangled state over successive stages, any one of which may be successful in producing the entangled state. In this example, the memory 104 can store one or more programs for determining that a given stage of the successive stages was successful, and can additionally configure the entangled state generator 110 accordingly. In a similar example, the memory 104 stores detection patterns, which the classical processing system 101 can use to determine that a stage was successful. In some example embodiments, the memory 104 stores settings that are provided to configurable components.

[0027] The photon source module 120 can include a collection of singlephoton sources that output photons to entangled state generator 110 by way of interconnecting waveguides 132. The entangled state generator 110 can receive the output photons and can convert them to one or more entangled photonic states. The entangled state generator 110 can output the entangled photonic states into a plurality of output waveguides 134. In some example embodiments, one or more of the output waveguides 134 are coupled to one or more downstream circuits (not depicted in FIG. 1) that use the entangled states for performing a quantum information processing (e.g., the entangled states are used as resource states for a downstream quantum optical circuit).

[0028] In some example embodiments, some or all of the above-described functions can be implemented with hardware circuits on photon source module 120 or entangled state generator 110. For example, in some example embodiments, the photon source module 120 includes one or more controllers 112 (e.g., control circuitry, logic controllers) which can comprise field programmable gate arrays (FPGAs), application specific integrated circuits (ASICS), a "system on a chip" that includes classical processors and memory, etc. In some example embodiments, controller 112 can determine that photon source module 120 was successful and can output a reference signal indicating that photon source module 120 was successful. In some example embodiments, the output of controller 112 can be used to configure hardware in controller 122.

[0029] Similarly, in some example embodiments, the entangled state generator 110 includes one or more controllers 122 (e.g., logical controllers) -which may be implemented using field programmable gate arrays (FPGAs) or application specific integrated circuits (ASICS)- that determine whether a respective stage of entangled state generator 110 has succeeded.

[0030] In some example embodiments, a system clock signal can be provided to the photon source module 120 and the entangled state generator 110 via an external source (not shown) or by the classical processing system 101 generating the signal via classical channels 130-a or 130-b. In some example embodiments, the system clock signal that is provided to the photon source module 120 triggers the photon source module 120 to attempt to output one photon per waveguide. In some example embodiments, the system clock signal that is provided to the entangled state generator 110 triggers, or gates, multiple sets of detectors in the entangled state generator 110 to attempt to detect photons. For example, in some example embodiments, triggering a set of detectors in the entangled state generator 110 to attempt to detect photons includes gating the set of detectors.

[0031] FIG. 2 shows a photon number resolving superconducting (PNRS) detector 200 comprising a plurality of photosensitive superconducting segments that are connected in an electrical series, in accordance with some example embodiments. The PNRS detector 200 is an example photondetector that is used in the photon source module 120 and the entangled state generator 110 to detect single photons (e.g., heralded photons) and to perform entangled state measurements, in accordance with some example embodiments.

[0032] As illustrated, a current source 202 is electrically coupled to a superconducting wire 203 by via metal layer 205 and provides a current to the wire. In some example embodiments, the provided current is below a superconducting transition current level such that the superconducting wire 203 maintains superconductivity. That is, for example, the level of electrical current supplied by the current source 202 is selected or otherwise provided so as to maintain the superconducting wire 203 in a superconductive state (e.g., zero resistance) absent other conditions (e.g., incident photons). As illustrated, the superconducting wire 203 comprises a plurality of alternating narrow and wide portions, such as a narrow portion 250 and a wide portion 252. In some example embodiments, an optical waveguide is coupled to the plurality of narrow portions of the superconducting wire 203, such as narrow portion 250. In some example embodiments, one or more photons in the optical waveguide impinge one or more of the narrow portions of superconducting wire 203, which results in detection of the one or more photons. In some example embodiments, the narrow portions of the superconducting wire 203 have smaller widths than the wider portions (e.g., the narrow portion 250 has a width of less than lOOnm and the wider portion has a width greater than 150 nm).

[0033] In some example embodiments, multiple photons can be detected in multiple individual narrow portions of the superconducting wire (e.g., a first narrow portion detects a first photon from the optical waveguide, a second narrow portion detects a second photon, where the first and second narrow portions may be separated by one or more narrow portions or wider portions of the superconducting wire 203). In the example illustrated in FIG. 2, the narrow portions are connected to the wider portions in series using one or more bends in the wider portions, such as the U-shaped bend portion 204, such that direct current flow in superconducting material of the superconducting wire 203 can change directions (e.g., 90 degree directionchange, greater than 90 degree direction change in the U-shaped bend portion 204 of a wider portion of the superconducting wire 203).

[0034] In some example embodiments, to mitigate current crowding in the superconducting wire 203 (e.g., reduce or avoid current crowding), the narrow portions of the superconducting wire 203 are formed straight (e.g., vertically straight, up-and-down, from the perspective of FIG. 2). In this way, the device is efficient because current changing bends are implemented in the wider portions of the superconducting wire 203 and the narrow portions of the superconducting wire 203 are straight and narrow such that they can be implemented as efficient thin segmented photon sensitive regions that can absorb single photons without causing the entire superconducting wire 203 to lose superconductivity (e.g., transition to a normal non-superconducting state). Additionally, and in accordance with some example embodiments, an absorption of a single photon in one narrow portion does not cause an avalanche effect (e.g., whereby current from a given narrow portion that detects a given photon can bleed or otherwise be diverted to other remaining narrow portions that are still in superconductive states).

[0035] Additionally, in this embodiment, the superconducting wire 203 can be formed having a meandering geometry, such that the narrow segments are arrayed along a longitudinal direction (z-direction) of the detector.

[0036] In some example embodiments, a metal layer 205 is disposed along an edge of the superconducting wire 203 to provide addressability of each the narrow portion during detection events. In some example embodiments, the metal layer 205 is in parallel with the superconducting wire 203 and forms one or more electrical contacts along the edge of the superconducting wire 203. For instance, in the example of FIG. 2 the metal layer 205 partially overlaps with each U-shaped wider bottom wider portions of the superconducting wire 203; though it is appreciated that in alternative embodiments a metal layer can interface with the top U-shaped wider portions of the superconducting wire 203 (e.g., U-shaped bend portion 204), or a first metal layer can interface with bottom wider portions of thesuperconducting wire 203 and a second metal layer can interface with the top wider portions of the superconducting wire 203.

[0037] In some example embodiments, the metal layer 205 provides parallel electrical resistance across each of the wider leg portions (e.g., in each U-shaped segment), thereby providing a shunting path for current to flow should any of the narrow segments absorb a photon and transition to a non-superconductive state. Additionally, the current can shunt should one or more of the narrow portions become damaged or permanently resistive for any reason.

[0038] Thus, using the architecture shown in FIG. 2, the resistance of the parallel combination of the metal layer 205 and the meandering superconducting wire 203 varies based on how many narrow segments are in the non-superconducting state (e.g., how many single photons have been absorbed). This effective parallel resistance can then be measured using readout circuit 207, e.g., by measuring a corresponding voltage drop across the length of the metal layer 205 (e.g., the readout circuit is configured to measure the voltage across the electrical contact, which comprises metal layer 205 and is coupled in parallel with the superconducting wire 203).

[0039] In some example embodiments, the PNRS detector 200 can be an integrated device with one or more components being disposed on, or in, one or more layers of material, e.g., the device can be formed as a planar integrated circuit. For example, the superconducting wire 203 can be formed from a superconductor layer (e.g., a thin film) that itself can be patterned onto the surface of a substrate layer . As already noted above, the superconductor layer can be formed from any superconducting material that has been deposited onto the substrate layer via any suitable process, e.g., by physical vapor deposition, chemical vapor deposition, atomic layer deposition, etc. After deposition, the meander geometry can be subsequently patterned via any suitable patterning process, e.g., dry, or wet etching, reactive ion etching, etc. Likewise, the substrate layer can include any suitable substrate material, e.g., a silicon substrate, a silicon-based substrate (such as a silicon nitride (SiN) substrate) etc. In some exampleembodiments, the substrate does not include an oxide layer, so as to reduce or avoid oxidation of the superconducting material layer.

[0040] Returning to the example shown in FIG. 2, the meandering shape can be formed from an array of bent (e.g., U-shaped) superconducting structures having alternating orientations and that are each distributed along the longitudinal direction (z-direction) of the PNRS detector 200. Each U- shaped portion of the PNRS detector 200 includes two leg portions that are connected to a base portion. For example, in the embodiment shown in FIG. 2, leg portions 203a and 203b abut a base portion 203c to form a U-shaped bend portion 204 (and any number of similar U-shaped portions can be distributed along the z-direction to provide for increased dynamic range, or reliability, of the detector). Furthermore, each leg portion can include a relatively narrow, photon-sensitive central wire segment, each end of which abuts a wider end segment. For example, the leg portion 203a of inverted U- shaped bend portion 204 includes central narrow wire segment 204a, whose upper end abuts wider leg portion 203d and whose lower end abuts wider leg portion 203 e.

[0041] In the illustrative example shown in FIG. 2, the relatively narrow and straight photosensitive wire segments serve as a series array of single photon detectors that each operate in a manner that is similar to the wire. The narrow segments are electrically coupled to each other via the larger bend segments of the superconducting wire, and these lager bend segments can serve as photon-insensitive buffer regions of the superconducting wire. While the transition in thickness between each narrow central segment and wide arm of each bend segment is shown to be abrupt in FIG. 2, any gradual, tapered transition can be used without departing from the scope of the present disclosure. For example, in some example embodiments, the superconducting wire is tapered between the narrow portions and the wide portions to reduce current crowding effects. In some example embodiments, curved transitions can be employed for the upper / lower bends of the larger U-shape bend regions (e.g., bend region 211), thereby reducing current crowding phenomena that can be present in superconducting wires having sharp bends.

[0042] In addition, the relative sizes of the narrow central segments and the larger buffer regions are chosen such that, at the design wavelength, a single photon can provide enough energy to cause the narrow segment to transition to a non-superconducting state, thereby raising the series resistance of that segment of the superconducting wire. In contrast, any photon that impinges one of the larger buffer regions may not cause that entire region to transition to the non-superconducting state, but rather will create merely a local hotspot. For example, the transverse width (e.g., transverse to the current flow direction) of the thin segments can be approximately, 100 nm or greater and the transverse width of the larger buffer regions can be approximately 150 nm or less. Due to the relatively large area in the buffer region, e.g., buffer region 213, there is sufficient space for the superconducting current 209 to be redistributed around any local hotspots 210 caused by an errant photon absorption. Although in the example of FIG. 2, the hotspots are displayed in a thick bend to illustrate, it is appreciated the hotspots 210 can likewise form in the thin detection area that overlaps the waveguide.

[0043] Continuing, thus, even if a photon is absorbed by the buffer regions, the superconducting current can still proceed generally unimpeded down the remaining portion of the device. Furthermore, any local heating present in the central narrow segment can be generally localized within that segment (or can intrude minimally into the larger width buffer regions), leaving the other, downstream portions of the wire in the superconducting state.

[0044] In view of the above, the PNRS detector geometry shown in FIG. 2 can provide for a spatially selective detector that can detect photons which impinge along the length of the device only (along the z-direction) and can be insensitive to photons that are incident on the superconducting wire outside of the length of the photo-sensitive narrow segments. The active region 215 for photon detection in a device having this geometry can be an elongated rectangle, as shown.

[0045] FIG. 3A to FIG. 3C illustrate the operation of a portion of the PNRS detector 200 under the case of two incident photons, in accordance with some example embodiments. FIG. 3 A shows the state of the detector just before a first photon 303 is absorbed by a first narrow segment 305. Beforethe photon 303 is absorbed, the entire length of the superconducting wire 307 is in the superconducting state, and therefore the entire (or majority of) current I that is output from the current source 309 passes through the superconducting wire 307 and completely bypasses the resistors Ri and R2. In FIG. 3A, the resistors are shown schematically but in the actual device, these resistors would be formed by a continuous metal layer, e.g., metal layer 205 shown in FIG. 2. As shown, the PNRS detector 200 detector includes a readout circuit 311 electrically coupled to the superconducting wire 307. The readout circuit 311 is configured to measure an electrical property of the superconducting wire (e.g., a voltage across the superconducting wire 307 or an impedance of the superconducting wire 307). For example, in the state shown in FIG. 3A the output voltage measured by the readout circuit 311 is low, e.g., close to 0 V.

[0046] The narrow portions of the superconducting wire 307 are configured to transition from a superconducting (zero resistance) state to a non- superconducting (non-zero resistance) state in response to an incident photon from an optical waveguide (e.g., optical waveguide 403, FIG. 4A to FIG. 4B). After the photon 303 is absorbed by the first narrow segment 305, the narrow segment 305 transitions to a resistive normal state (also referred to as a non-superconducting state), and therefore can be represented as a resistor -R3, as shown in FIG. 3B. When this occurs (e.g., while narrow segment 305 of the superconducting wire is in a non-superconducting state), the electrical contact has a resistance Ri that is less than a resistance R3 of the superconducting wire 307. Because 3 is much greater than Ri, the current I is now forced through Ri and then back into the remaining U- shaped meandering portion 308 of the superconducting wire 307, which is still in a superconducting state. At this point, the voltage drop measured by the readout circuit 311 in this state is V = I. Shortly after the photon 303 is absorbed, or even simultaneously with the absorption of photon 303, a second photon 312 can be absorbed by a narrow segment 313. As before, this narrow segment 313 will transition to the non-superconducting state forming resistor -R4as shown schematically in FIG. 3C. Now, current is forced to pass through both resistors Ri and R2 and thus, the voltage drop measured by the readout circuit 311 in this state is=Ix(-^i + ^2).

[0047] In view of the above, the PNRS detector can discriminate between a single photon absorption or multiple photon absorptions because the voltage drop at the readout circuit 311 depends on whether one of the narrow segments of a particular U-shape meander has absorbed a photon, transitioned to the normal state, and forced its respective current through its associated shunt resistor. More generally, a PNRS can have a number “m” of U-shaped meander portions, and approximately identically valued resistors Ri = R2 = Rm= R . Then, when “n” photons are absorbed, each at a different photon sensitive segment of the U-shaped meanders, the current is re-routed into n resistors, resulting in an output voltage of approximately Vn= I x n x R which is proportional to the number of absorbed photons. In some example embodiments, the number of U-shaped meanders m can be made large compared to the number of photons n that are expected to be detected. This can reduce multi-photon detection which would, in turn, exceed a dynamic range of the detector. In this manner, the electrical property of the superconducting wire 307 measured by the readout circuit 311 (e.g., voltage across the superconducting wire 307) is indicative of a number of photons incident to the superconducting wire 307.

[0048] Note that, in some example embodiments, the wide portions are sized so as to remain in the superconducting state regardless of a state of the narrow portions. For example, U-shaped meandering portion 308 remains in a superconducting state regardless of the state of narrow segment 313.

[0049] FIG. 4A shows a PNRS detector 400 that has an integrated waveguide 403, in accordance with some example embodiments. In this example, the PNRS detector 400 can be used to detect photons that are travelling in the waveguide 403. In this example, the waveguide 403 is disposed within a central region of the superconducting wire 405, directly underneath (or above) the array of photo-sensitive wire segments. In some example embodiments, the input of the waveguide can be coupled to a photon source (not shown). Photons generated by the photon source are coupled into the waveguide and eventually arrive at the detector region shown in FIG. 4A.

[0050] In some example embodiments, the waveguide 403 and the meandering superconducting wire 405 are positioned such that a photon that is propagating in one of the guided modes of the waveguide can couple out of the waveguide and be absorbed by one of the narrow segments. After absorption, the resulting voltage is detected by a readout circuit (not shown), as described above.

[0051] As shown in the cross section along A- A' of FIG. 4B (not to scale), in some example embodiments, the optical waveguide 403 is vertically stacked with the superconducting layer 407 (e.g., with respect to base substrate 409). Superconducting layer 407 corresponds to superconducting wire 405 in FIG. 4B (e.g., superconducting wire 405 is formed from superconducting layer 407). The separation s between the waveguide 403 and any given narrow segment of superconducting layer 407 is approximately 100 nm or less. This allows for a weak, but non-zero electromagnetic coupling (e.g., evanescent coupling) between the guided photons and the photo-sensitive segments of superconducting wire. For example, in some cases the coupling can be on the order of 10% or less per photo-sensitive segment (e.g., the probability of a photon absorption by any given segment is 10%). In such an arrangement, a superconducting wire having 10, 20, 30, 40, 50, or even more photo-sensitive segments may be employed to increase the overall single / multi photon detection efficiency. In addition, because the probability of detecting more than one photon at the same photo-sensitive segment is relatively small, a device such as this can also serve as a PNRS as described above (the photon number is proportional to the number of individual narrow segments that absorb a photon).

[0052] The waveguide integrated device shown in FIG. 4A can also be fabricated as a planar integrated circuit, as shown in the cross-section of FIG. 4B. In one embodiment, the base substrate 409 can be a flat silicon wafer. This silicon substrate can be covered by a high-quality silicon oxide layer 411. A waveguide 403 can be formed from silicon and disposed on the surface of the high-quality silicon oxide layer 411, and covered by one or more capping layers, e.g., by a lower-quality silicon oxide layer 413. The superconducting layer 407 that forms the superconducting nanowire is disposed on top of the lower-quality silicon oxide layer 413. A metal layer415 (e.g., an electrical contact) is disposed on the upper layer of the device, partially overlapping with both the lower-quality silicon oxide layer 413 and the superconducting layer 407.

[0053] In the PNRS shown in FIG. 4A, eight photon-sensitive photosensitive segments are used as individual photon detection regions but any number of photo-sensitive segments can be used having any desired coupling efficiency to the waveguide without departing from the scope of the present disclosure.

[0054] In some examples, as shown in FIG. 4A , the waveguide 403 can have a constant width w. In some examples, the waveguide can have any other suitable geometry, such as tapered waveguide 403a. That is, the waveguide width can increase along a propagation direction which can improve the coupling efficiency of downstream photo-sensitive segments of the superconducting wire 405 relative to upstream photo-sensitive segments. Tapered waveguide 403a shows one such example (e.g., Tapered waveguide 403a has width w that changes along the length of the Tapered waveguide 403a, e.g., Tapered waveguide 403a has a narrow end 419a and a wide end 419b, which is wider than the narrow end 419a).

[0055] In some example embodiments, PNRS detector 400 includes a mirror 417 disposed at an end of waveguide 403 (e.g., either a straight waveguide 403 or Tapered waveguide 403a, although the mirror 417 is shown disposed at an end of Tapered waveguide 403a).

[0056] In some example embodiments, the mirror 417 is a Bragg mirror. In some example embodiments, the mirror 417 is a retroreflective mirror. In some example embodiments, the mirror 417 is a loop mirror. When the PNRS detector 400 includes the mirror 417, PRNS detector can be considered a "multi-pass" detector, meaning photons may travel along the waveguide 403 more than once (e.g., travel through the waveguide in one direction, reflect off of mirror 417, and then travel in the opposite direction).

[0057] In some applications of PNRS detector 400, a total photon detection efficiency of greater than 99% is desired. Increasing a number of photosensitive segments can increase the total photon detection efficiency, however, several hundreds of nanowires can be used to achieve this. Such ahigh quantity of nanowires can limit the overall operation of the detector (such as limiting the reset rate). Additionally, as seen in FIG. 8, a two-pass design such as PNRS detector 400 including the mirror 417 can have a large variation in the probability of absorbing a photon across the propagation direction of the detector. Thus, additional designs for multi-pass detectors can be advantageous.

[0058] FIG. 5A and FIG. 5B show an example of a multi-pass PNRS detector 500 with a multi-pass integrated waveguide. In particular, the multipass PNRS detector 500 of FIG. 5 A and FIG. 5B can be a "four-pass" detector. The multi-pass PNRS detector 500 comprises a superconducting wire 505 and a waveguide 510.

[0059] In some examples, a substantially uniform absorption probability distribution across the superconducting wire 505 can be a design goal of the multi-pass PNRS detector 500. As shown in FIG. 5 A, the waveguide 510 includes a tapered geometry of the waveguide segments 512 and 514, which can allow for tuning the absorption probability at each photo-sensitive segment of superconducting wire 505. The tapering can involve varying the width of the waveguide segments from approximately 1.6 microns to 400 nanometers. The waveguide 510 can have any suitable geometry, such as a constant width, a tapered width, or any other suitable variation of width.

[0060] The waveguide 510 includes multiple waveguide segments 512, 514 that are connected by curved portions 516. Each waveguide segment is arranged below the superconducting wire 505 such that light propagating through a given waveguide segment has an absorption probability to be absorbed by the photo-sensitive segments of superconducting wire 505.

[0061] A reflector 518 is positioned at an end of one of the waveguide segments. The reflector 518 is configured to reflect light back through the waveguide 510. In some examples, the reflector 518 comprises a loop mirror, which can include a waveguide bend and a directional coupler. Alternatively, the reflector 518 can be implemented as a Bragg mirror or other reflective structure. In some example embodiments, the waveguides are isolated from one another to avoid cross-coupling of light between them. For example, the length of waveguide segment 514 is horizontally separated(in the perspective of FIG. 5 A) from the waveguide segment 512 to ensure light does not couple between them midway through their respective forward or backward propagations down a respective segment. In some example embodiments, additional structures are included to further prevent crosscoupling between waveguide segments, as discussed in further detail below with reference to FIG. 14.

[0062] With reference to FIGs. 5A and 5B, light propagation through the waveguide 510 comprises four passes: i) a forward propagation 520 through waveguide segment 512, ii) a forward propagation 522 through waveguide segment 514, iii) a backward propagation 524 through waveguide segment 514, and iv) a backward propagation 526 through waveguide segment 512.

[0063] The backward propagation 524 and 526 can occur after light is reflected by the reflector 518.

[0064] The superconducting wire 505 is configured to transition from a superconducting state to a non-superconducting state upon absorption of a photon. This transition can be detected as a change in electrical properties of the wire, such as resistance or current flow, as described above in connection with FIG. 3 A through FIG. 3C.

[0065] In some examples, the waveguide 510 comprises silicon nitride. An aluminum nitride seed layer can be positioned between the waveguide 510 and the superconducting wire 505 in some examples to improve the properties of the superconducting material.

[0066] The four-pass configuration of multi-pass PNRS detector 500 can allow for high single-photon detection efficiencies, in some examples exceeding 99.5%, while maintaining a substantially uniform absorption probability distribution across the photo-sensitive segments of superconducting wire 505. The four-pass configuration of multi-pass PNRS detector 500 can allow for reduced total wire length compared to single-pass designs, improving reset times and in some examples, enabling operation at repetition rates exceeding 1 GHz. This design can also improve multi-photondetection efficiencies, potentially achieving two-photon detection efficiencies greater than 99%.

[0067] FIG. 6 shows a functional diagram for a superconducting detector having a multi-pass integrated waveguide. The multi-pass PNRS detector 600 comprises a multi-pass waveguide 602 and detection components 612.

[0068] The multi-pass waveguide 602 includes waveguide segments 604, waveguide segment connectors 606, and a reflector 608.

[0069] The waveguide segments 604 are arranged in a series of parallel segments connected by the waveguide segment connectors 606. This arrangement allows light to propagate through multiple passes of the waveguide structure, increasing the interaction length between the light and the detection components. In some examples, the waveguide segments 604 have a tapered geometry to control light absorption along the propagation direction. The tapering can involve varying the width of the waveguide segments from approximately 1.6 microns to 400 nanometers.

[0070] The waveguide segment connectors 606 provide curved portions that connect adjacent waveguide segments. These connectors allow the light to change direction and enter the next waveguide segment, enabling the multi-pass configuration. The curved portions of the waveguide segment connectors 606 are similar to the curved portions 516 shown in FIG. 5 A and FIG. 5B.

[0071] The reflector 608 is positioned at an end of the multi-pass waveguide 602. It is configured to reflect light back through the waveguide structure, effectively doubling the number of passes. In some examples, the reflector 608 comprises a loop mirror, which can include a waveguide bend and a directional coupler. Alternatively, the reflector 608 can be implemented as a Bragg mirror or other reflective structure. The reflector 608 serves a similar function to the reflector 518 shown in FIG. 5 A and FIG. 5B.

[0072] The Detection components 612 include a superconducting nanowire array 610, resistors 616, a current source 614, and readout electronics 618.

[0073] The superconducting nanowire array 610 is arranged in proximity to the multi-pass waveguide 602, allowing for the absorption of photonspropagating through the waveguide. This arrangement is similar to the superconducting wire 505 shown in FIG. 5A and FIG. 5B.

[0074] The superconducting nanowire array 610 comprises multiple superconducting nanowires that can transition from a superconducting state to a non-superconducting state upon photon absorption. This transition results in a detectable change in the electrical properties of the nanowires.

[0075] The current source 614 provides a bias current to the superconducting nanowire array 610. This bias current is typically set below the transition current of the superconducting nanowires to maintain their superconducting state in the absence of photon absorption. The current source 614 in FIG. 6 serves a similar purpose to the current source 309 shown in FIG. 3A-3C.

[0076] The resistors 616 are connected in parallel with the superconducting nanowires. When a nanowire transitions to a non-superconducting state upon photon absorption, the current is redirected through the corresponding resistor, producing a voltage pulse. The resistors 616 shown in FIG. 6 are similar in function to the resistors Rl, R2, R3, and R4 shown in FIG. 3A-3C.

[0077] The readout electronics 618 are configured to detect and process the voltage pulses generated when photons are absorbed by the nanowires. By analyzing these pulses, the readout electronics 618 can determine the number of photons detected, enabling photon number resolution. The readout electronics 618 in FIG. 6 perform a similar function to the readout circuit 311 in FIG. 3A-3C.

[0078] In some examples, multi-pass PNRS detector 600 can reduce the overall footprint and power consumption of the detector system compared to designs using multiple separate detector units.

[0079] FIG. 7 illustrates an example method 700 for designing a superconducting detector with a multi-pass integrated waveguide. Although the method 700 depicts a particular sequence of operations represented by blocks 702 through loop 714, the sequence may be altered without departing from the scope of the present disclosure. For example, some of the operations depicted may be performed in parallel or in a different sequence that does not materially affect the routine. In other examples, differentcomponents of an example device or system that implements the routine may perform functions at substantially the same time or in a specific sequence.

[0080] According to some examples, at block 702, the method includes designing waveguide structure. This can include selecting a number of waveguide segments, a geometry (e.g., straight, tapered, amount of taper) for each waveguide segment, and the relative arrangement of waveguide segments. In some examples, the waveguide structure can incorporate multiple waveguide segments connected by curved portions, similar to the arrangement shown in FIG. 5 A and FIG. 5B. The waveguide segments can be designed with tapered geometries to control light absorption along the propagation direction.

[0081] The waveguide structure can include any suitable number k of waveguide sections. The waveguide structure can use any suitable waveguide geometries to connect the waveguide sections, such as a bend with any suitable bend radius. In some examples, the geometry of waveguide segment connectors can be selected to reduce radiation losses in the bend, to preserve polarization of light through the bend, or to achieve any other suitable photonic outcome.

[0082] The waveguide structure can include one or more waveguide sections that are tapered. The waveguide structure can include one or more waveguide sections that are of uniform width. The waveguide structure can separate adjacent waveguide segments by any suitable amount.

[0083] According to some examples, at block 704, the method 700 includes adding a reflector to the waveguide structure. This reflector can be positioned at an end of the waveguide structure to reflect light back through the waveguide, effectively doubling the number of times light propagates through the number s of waveguide sections. That is, the number of passes S through the waveguide can be S = 2 x k. In some examples, the reflector can be implemented as a loop mirror, comprising a waveguide bend and a directional coupler, similar to the reflector 1008 shown in FIG. 10. Alternatively, the reflector can be implemented as a Bragg mirror or other reflective structure.

[0084] According to some examples, at block 706, the method 700 includes designing a superconducting nanowire array. The nanowire array can include any suitable number of U-shaped meanders m. The nanowire array can have any suitable length in the overall meander, and in some examples, can lengthen the narrow portion to ensure that photo-sensitive segments of the nanowire array are positioned over the waveguide sections arranged at block 702.

[0085] According to some examples, at block 708, the method 700 includes calculating an absorption probability distribution. The absorption probability distribution can be a probability per unit cell (e.g., per wire or photosensitive segment of the superconducting nanowire array) of absorbing a single photon for the given waveguide segment geometry and nanowire geometry of blocks 702-706. The method includes calculating a contribution to the absorption probability distribution for each of a forward propagation and a backward propagation in each waveguide segment. At block 708, additional calculations can also be determined, such as a probability of absorbing a second (or Nth) photon at a given photo-sensitive segment of the superconducting nanowire array after a first photon is absorbed at a different photo-sensitive segment. At block 708, a total absorption probability (for one photon, two photons, N photons, etc.) can additionally be calculated. A photon can strike the nthwire at any of the K waveguides with a probability Pin. The total probability of absorption in the nthwire is given by Eqn. 1 :

[0086] where, the expression for pjnis given by Eqn. 2 when j is even and Eqn. 3 when j is odd:(Eqn. 2)* (1 - e~aJndn>) Vmod(j, 2) =A 0(Eqn. 3)

[0087] In some examples, at block 708, any additional suitable characteristics of the detector can be determined, such as a reset rate (e.g., determined from the given number of U-shaped meanders m.)

[0088] In some examples, the method 700 can include loop 714, indicating that the design process can involve multiple iterations and refinements to any of block 702 through block 706 in order to achieve performance characteristics of the design goals.

[0089] According to some examples, at block 710, the method 700 can include adding confinement structures. In some examples, as discussed below in FIG. 14, any additional structures or geometrical features can be added to the waveguide sections to reduce cross-coupling between adjacent waveguide segments.

[0090] According to some examples, at block 712, the method 700 includes designing readout circuits. Using the example of multi-pass PNRS detector 600, based on a geometry of the superconducting nanowire array 610 determined at block 706 (e.g., length of nanowires, number of nanowires in the array), a value for the resistors 616 can be selected.

[0091] FIG. 8 shows a graph 800 illustrating absorption probability distributions for multi-pass photon detector designs. The graph comprises an x-axis representing the position along a superconducting nanowire and a y- axis representing the absorption probability per unit wire.

[0092] The graph displays an absorption probability distribution for a two- pass waveguide 802 802, and an absorption probability distribution for a four-pass waveguide 804.

[0093] The distribution for the two-pass waveguide 802 (dashed line) represents the probability of photon absorption along a meandering 1superconducting nanowire in a two-pass detector design, as shown in FIG. 4A. Absorption across the detector varies, with the highest probability for absorption (0.9%) at a first region of the detector around wire #20, then decreasing in a continuous decay. The cumulative absorption probability for the two-pass design is 98.7%.

[0094] The distribution for the four-pass waveguide 804 (solid line) represents the probability of photon absorption along a meandering superconducting nanowire in a four-pass design as seen in FIG. 5A and FIG. 5B. Absorption across a detector of this design is between 0.5%-0.6% along the entire length of the detector. The corresponding total detection probability for this curve is 99.8%.

[0095] The improved uniformity of distribution for the four-pass waveguide 804 compared to the distribution for the two-pass waveguide 802 illustrates a benefit of increasing the number of passes in the detector design. This improvement in uniformity, along with the higher cumulative absorption probability, can lead to additional improvements in performance characteristics of the photon detector, such as improved photon number resolution. Additionally, a shorter length of the overall meandering superconducting nanowire can be used to achieve the higher cumulative absorption probability. Additionally, instead of adding additional passes to a given waveguide segment (which may increase kinetic inductance and degrade reset rate), a given waveguide segment’s length is reduced and additional nanowire passes are managed by additional waveguide segments, thereby achieving high performance while mitigating kinetic inductance.

[0096] The absorption probability distributions shown in FIG. 8 can be used to compare and adjust the design of multi-pass photon detectors with various configurations. For example, a graph such as graph 800 can be output while performing a method such as method 700, to aid in adjusting parameters such as waveguide geometry, nanowire dimensions, and reflector properties to achieve the desired absorption probability distribution for different numbers of passes.

[0097] FIG. 9 shows a graph 900 illustrating the contribution of individual passes through a waveguide having a four-pass photon detector design. Themulti-pass design represented in FIG. 9 can be implemented using a waveguide structure that incorporates multiple waveguide segments connected by curved portions and a reflector, allowing light to make four passes through the waveguide, similar to the design shown in FIG. 5A and FIG. 5B.

[0098] The graph comprises an x-axis representing the position along a superconducting nanowire and a y-axis representing the absorption probability. The graph displays five curves: four individual pass distributions and one cumulative distribution. The curve 910 represents the cumulative absorption probability distribution, combining the effects of all four passes (curves 902-908), and in this example, is the same as the distribution of the four-pass waveguide 804 shown in FIG. 8.

[0099] The curve 902 represents the absorption probability distribution for the first pass through the four-pass design, such as forward propagation 520 of multi-pass PNRS detector 500 shown in FIG. 5A. This distribution shows the initial interaction of light with the superconducting nanowire.

[0100] The curve 904 represents the absorption probability distribution for the second pass through the four-pass design, such as forward propagation 522 of multi-pass PNRS detector 500.

[0101] The curve 906 represents the absorption probability distribution for the third pass through the four-pass design, such as backward propagation 524 of multi-pass PNRS detector 500.

[0102] The curve 908 represents the absorption probability distribution for the fourth and final pass through the four-pass design, such as backward propagation 526 of multi-pass PNRS detector 500.

[0103] This structure can be adjusted using the design process outlined in FIG. 7, particularly by blocks 702 (designing the waveguide structure), 704 (adding a reflector), and 708 (determining the absorption probability distribution).

[0104] FIG. 10 shows another example of a four-pass design 1000 for a superconducting detector with a multi-pass integrated waveguide, while FIG. 11 shows the same design with superconducting nanowire 1010 arranged over the waveguide segments 1002 and 1004. The four-pass design 1000comprises two waveguide segments 1002, 1004, connected by waveguide segment connectors 1006, with a reflector 1008.

[0105] The waveguide segment connectors 1006 provide curved portions that connect adjacent waveguide segments. These connectors allow the light to change direction and enter the next waveguide segment, enabling the multi-pass configuration.

[0106] The reflector 1008 is positioned at one end of the multi-pass waveguide structure. In particular, in some examples, the reflector 1008 can be configured to receive light at the end of a waveguide segment (such as waveguide segment 1004), and to reflect light back through the waveguide segments, effectively doubling the number of passes from two to four. In some examples, as shown in FIG. 10 and FIG. 11, the reflector 1008 can be implemented as a loop mirror, which includes a waveguide bend and a directional coupler.

[0107] The superconducting nanowire 1010 is positioned in close proximity to the waveguide segments. As shown, the narrow segments of superconducting nanowire 1010 are elongated relative to superconducting wire 405. In some examples, the geometry of the superconducting nanowire 1010 can be adjusted to position photo-sensitive segments of the superconducting nanowire 1010 over waveguide segments in the four-pass design 1000. In some examples, any suitable number of photo-sensitive segments can be used.

[0108] FIG. 12 shows an example of an eight-pass design 1200 for a superconducting detector with a multi-pass integrated waveguide, while FIG. 13 shows the same design with superconducting nanowire 1218 arranged over the waveguide segments 1202, 1204, 1206, and 1208. The four-pass eight-pass design 1200 comprises four waveguide segments 1202, 1204, 1206, and 1208 connected by waveguide segment connectors 1210, 1212, and 1214, and a reflector 1216.

[0109] The waveguide segment connectors 1210, 1212, and 1214 provide curved portions that connect adjacent waveguide segments. These connectors allow the light to change direction and enter the next waveguide segment, enabling the eight-fold multi-pass configuration.

[0110] The reflector 1216 is positioned at one end of the multi-pass waveguide structure. In particular, in some examples, the reflector 1216 can be configured to receive light at the end of a waveguide segment (such as waveguide segment 1208), and to reflect light back through the waveguide segments 1202, 1204, 1206, and 1208, effectively doubling the number of passes from four to eight. In some examples, as shown in FIG. 12 and FIG. 13, the reflector 1216 can be implemented as a loop mirror, which includes a waveguide bend and a directional coupler.[OHl] The superconducting nanowire 1218 is positioned in close proximity to the waveguide segments. As shown, the narrow segments of superconducting nanowire 1218 are elongated relative to superconducting wire 405. In some examples, the geometry of the superconducting nanowire 1218 can be adjusted to position photo-sensitive segments of the superconducting nanowire 1218 over waveguide segments in the eight-pass design 1200. In some examples, any suitable number of photo-sensitive segments can be used.

[0112] FIG. 14 shows an example of an eight-pass design 1400 with confinement structures for use with a superconducting detector, in accordance with some example embodiments. The eight-pass design 1400 can include the waveguide segments and waveguide segment connectors discussed in FIG. 12 and FIG. 13.

[0113] When multiple waveguides are positioned adjacent to each other, such as waveguide segment 1202 and 1204, there can be cross-coupling wherein light propagating in waveguide segment 1202 can couple to waveguide segment 1204, bypassing the waveguide segment connector 1210. This can be undesirable for coupling light into the superconducting nanowire 1218, as there is reduced coupling (in the vertical direction, in the perspective of FIG. 12) between the waveguide and the nanowire.

[0114] In some examples, additional structures can be included in a multipass waveguide design, such as confinement structure 1402, 1404, 1406, 1408, or 1410. In some examples, one or more confinement structures can be included. For example, confinement structure 1402 and confinement structure 1404 can be included on the outer edges of the first and lastwaveguide segment(s), respectively. In some examples, the confinement structures can be fabricated from an oxide material. In some examples, the confinement structures can have any suitable geometry to decrease crosscoupling between adjacent waveguide segments. For example, when the waveguide segments are tapered, the confinement structures can be of uniform width, to decrease cross-coupling by creating non-parallel surface between adjacent waveguide segments.

[0115] Example 1 is a photonic integrated circuit (PIC) having a photon number resolving detector, the photon number resolving detector comprising: a superconducting nanowire; and a waveguide comprising: a plurality of waveguide segments, each waveguide segment in the plurality of waveguide segments connected to another waveguide segment in the plurality of waveguide segments with a curved waveguide portion, each waveguide segment being arranged proximate to portions of the superconducting nanowire such that light propagating through a given waveguide segment has an absorption probability to be absorbed by any one of the portions of the superconducting nanowire, the light comprising one or more photons; and a reflector positioned at an end of a waveguide segment in the plurality of waveguide segments, the reflector configured to reflect the light through the waveguide; wherein an absorption probability distribution for light in the waveguide to be absorbed by the portions of the superconducting nanowire is substantially uniform across the portions of the superconducting nanowire.

[0116] In Example 2, the subject matter of Example 1 includes, wherein the reflector comprises a loop mirror.

[0117] In Example 3, the subject matter of Examples 1-2 includes, wherein the reflector comprises a waveguide bend that receives light from the superconducting nanowire and bends to direct the light towards the superconducting nanowire.

[0118] In Example 4, the subject matter of Examples 1-3 includes, wherein the reflector comprises a waveguide bend and a directional coupler.

[0119] In Example 5, the subject matter of Examples 1-4 includes, wherein the light propagation having the absorption probability comprises a forwardpropagation through the waveguide and a backward propagation after reflection by the reflector.

[0120] In Example 6, the subject matter of Examples 1-5 includes, wherein the waveguide has a tapered geometry to control light absorption along a propagation direction.

[0121] In Example 7, the subject matter of Examples 1-6 includes, % based on a substantially uniform single photon absorption probability distribution across the portions of the superconducting nanowire.

[0122] In Example 8, the subject matter of Example 7 includes, wherein the plurality of waveguide segments have tapered geometries configured to achieve the substantially uniform single photon absorption probability distribution.

[0123] In Example 9, the subject matter of Examples 7-8 includes, wherein the portions of the superconducting nanowire pass the waveguide at respective positions configured to achieve the substantially uniform single photon absorption probability distribution.

[0124] In Example 10, the subject matter of Examples 7-9 includes, wherein the superconducting nanowire comprises respective dimensions configured to achieve the substantially uniform single photon absorption probability distribution.

[0125] In Example 11, the subject matter of Examples 7-10 includes, % is reduced compared to a photon number resolving detector configured without the reflector positioned at the end of the waveguide segment of the plurality of waveguide segments.

[0126] In Example 12, the subject matter of Examples 1-11 includes, %.

[0127] In Example 13, the subject matter of Examples 1-12 includes, wherein the photon number resolving detector is configured to reduce coupling between adjacent waveguide segments by positioning optical confinement structures between adjacent waveguide segments.

[0128] In Example 14, the subject matter of Example 13 includes, wherein the optical confinement structures comprise an oxide material.

[0129] In Example 15, the subject matter of Examples 1-14 includes, wherein the photon number resolving detector is configured to reduce coupling between adjacent waveguide segments by including non-parallel features in adjacent waveguide segments.

[0130] In Example 16, the subject matter of Examples 1-15 includes, wherein the waveguide comprises silicon nitride.

[0131] In Example 17, the subject matter of Examples 1-16 includes, an aluminum nitride seed layer between the waveguide and the plurality of superconducting nanowires.

[0132] In Example 18, the subject matter of Examples 1-17 includes, readout circuitry configured to resolve a number of photons that comprise light in the waveguide based on at least one of a current and a voltage, wherein the current is produced by a resistive portion connected to at least one of the portions of the superconducting nanowire, wherein the resistive portion results from absorption of at least one photon from light in the waveguide by the at least one of the portions of the superconducting nanowire.

[0133] In Example 19, the subject matter of Example 18 includes, GHz.

[0134] Example 20 is a met hod for detecting a number of photons in a photonic integrated circuit, the method comprising: providing a superconducting nanowire; and arranging a waveguide beneath the superconducting nanowire, the waveguide comprising a plurality of waveguide segments connected by a curved waveguide portion, each waveguide segment being arranged proximate to portions of the superconducting nanowire; tapering at least one waveguide segment such that light propagating through a given waveguide segment has an absorption probability to be absorbed by any one of portions of the superconducting nanowire, the light comprising one or more photons, wherein an absorption probability distribution for light in the waveguide to be absorbed by the plurality of superconducting nanowires is substantially uniform across the plurality of superconducting nanowires; reflecting the light back through the waveguide using a reflector positioned at an end of the waveguide; detecting absorption of one or more photons by the superconducting nanowire; anddetermining a number of photons detected based on a number of portions of the superconducting nanowire that each absorb one photon.

[0135] In Example 21, the subject matter of Example 20 includes, wherein reflecting the light back through the waveguide using a reflector comprises: directing the light through a waveguide bend and a directional coupler configured to split the light into two paths and recombine the light to form a low-loss mirror.

[0136] In Example 22, the subject matter of Examples 20-21 includes, wherein the light propagation having the absorption probability comprises a forward propagation through the waveguide and a backward propagation after reflection by the reflector.

[0137] Example 23 is a waveguide for use in a photodetector, the waveguide comprising: a plurality of waveguide segments, each waveguide segment in the plurality of waveguide segments connected to another waveguide segment in the plurality of waveguide segments with a curved waveguide portion, each waveguide segment being arranged proximate a light detection structure such that light propagating through a given waveguide segment has an absorption probability to be absorbed by any one portion of the light detection structure, the light comprising one or more photons; and a reflector positioned at an end of a waveguide segment in the plurality of waveguide segments, the reflector configured to reflect the light through the waveguide; wherein an absorption probability distribution for light in the waveguide to be absorbed by the light detection structure is substantially uniform across the light detection structure.

[0138] In Example 24, the subject matter of Example 23 includes, wherein the light detection structure comprises a superconducting nanowire.

[0139] Example 25 is at least one machine-readable medium including instructions that, when executed by processing circuitry, cause the processing circuitry to perform operations to implement of any of Examples 1-24.

[0140] Example 26 is an apparatus comprising means to implement of any of Examples 1-24.

[0141] Example 27 is a system to implement of any of Examples 1-24.

[0142] Example 28 is a method to implement of any of Examples 1-24.

[0143] Other technical features and example embodiments may be readily apparent to one skilled in the art from the figures, descriptions, and claims herein.

[0144] As used herein, a computer-readable storage medium refers, for example, to both machine-storage media and transmission media. Thus, the terms include both storage devices / media and carrier waves / modulated data signals. The terms “machine-readable medium,” “computer-readable medium” and “device-readable medium” mean the same thing and may be used interchangeably in this disclosure.

[0145] As used herein, a machine storage medium refers, for example, to a single or multiple storage devices and media (e.g., a centralized or distributed database, and associated caches and servers) that store executable instructions, routines and data. The term shall accordingly be taken to include, but not be limited to, solid-state memories, and optical and magnetic media, including memory internal or external to processors. Specific examples of machine-storage media, computer-storage media and devicestorage media include non-volatile memory, including by way of example semiconductor memory devices, e.g., erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), FPGA, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD- ROM and DVD-ROM disks. The terms "machine-storage medium," "devicestorage medium," "computer- storage medium" mean the same thing and may be used interchangeably in this disclosure. The terms "machine-storage media," "computer- storage media," and "device-storage media" specifically exclude carrier waves, modulated data signals, and other such media.

[0146] As used herein, a non-transitory computer-readable storage medium refers, for example, to a tangible medium that is capable of storing, encoding, or carrying the instructions for execution by a machine.

[0147] It will also be understood that, although the terms first, second, etc. are, in some instances, used herein to describe various elements, these elements should not be limited by these terms. These terms are used only todistinguish one element from another. For example, a first tuner could be termed a second tuner, and, similarly, a second tuner could be termed a first tuner, without departing from the scope of the various described embodiments. The first tuner and the second tuner are both tuners, but they are not the same tuner.

[0148] The terminology used in the description of the various described embodiments herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used in the description of the various described embodiments and the appended claims, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term "and / or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms "includes," "including," "comprises," and / or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0149] As used herein, the term "if" is, optionally, construed to mean "when" or "upon" or "in response to determining" or "in response to detecting" or "in accordance with a determination that," depending on the context.

[0150] The foregoing description, for purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the scope of the claims to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen in order to best explain the principles underlying the claims and their practical applications, to thereby enable others skilled in the art to best use the embodiments with various modifications as are suited to the particular uses contemplated.

Claims

CLAIMSWhat is claimed is:

1. A photonic integrated circuit (PIC) having a photon number resolving detector, the photon number resolving detector comprising: a superconducting nanowire; and a waveguide comprising: a plurality of waveguide segments, each waveguide segment in the plurality of waveguide segments connected to another waveguide segment in the plurality of waveguide segments with a curved waveguide portion, each waveguide segment being arranged proximate to portions of the superconducting nanowire such that light propagating through a given waveguide segment has an absorption probability to be absorbed by any one of the portions of the superconducting nanowire, the light comprising one or more photons; and a reflector positioned at an end of a waveguide segment in the plurality of waveguide segments, the reflector configured to reflect the light through the waveguide; wherein an absorption probability distribution for light in the waveguide to be absorbed by the portions of the superconducting nanowire is substantially uniform across the portions of the superconducting nanowire.

2. The PIC of claim 1, wherein the reflector comprises a loop mirror.

3. The PIC of claim 1, wherein the reflector comprises a waveguide bend that receives light from the superconducting nanowire and bends to direct the light towards the superconducting nanowire.

4. The PIC of claim 1, wherein the reflector comprises a waveguide bend and a directional coupler.

5. The PIC of claim 1, wherein the light propagation having the absorption probability comprises a forward propagation through the waveguide and a backward propagation after reflection by the reflector.

6. The PIC of claim 1, wherein the waveguide has a tapered geometry to control light absorption along a propagation direction.

7. The PIC of claim 1, wherein the photon number resolving detector is configured to achieve a single photon detection efficiency of greater than 99.5% based on a substantially uniform single photon absorption probability distribution across the portions of the superconducting nanowire.

8. The PIC of claim 7, wherein the plurality of waveguide segments have tapered geometries configured to achieve the substantially uniform single photon absorption probability distribution.

9. The PIC of claim 7, wherein the superconducting nanowire comprises respective dimensions configured to achieve the substantially uniform single photon absorption probability distribution.

10. The PIC of claim 1, wherein the photon number resolving detector is configured to reduce coupling between adjacent waveguide segments by positioning optical confinement structures between adjacent waveguide segments.

11. The PIC of claim 10, wherein the optical confinement structures comprise an oxide material.

12. The PIC of claim 1, wherein the photon number resolving detector is configured to reduce coupling between adjacent waveguide segments by including non-parallel features in adjacent waveguide segments.

13. The PIC of claim 1, wherein the waveguide comprises silicon nitride and the PIC further comprises an aluminum nitride seed layer between the waveguide and the superconducting nanowire.

14. The PIC of claim 1, further comprising readout circuitry configured to resolve a number of photons that comprise light in the waveguide based on at least one of a current and a voltage, wherein the current is produced by a resistive portion connected to at least one of the portions of the superconducting nanowire, wherein the resistive portion results fromabsorption of at least one photon from light in the waveguide by the at least one of the portions of the superconducting nanowire.

15. The PIC of claim 14, wherein the photon number resolving detector is configured to operate at a repetition rate of greater than 1 GHz.

16. A method for detecting a number of photons in a photonic integrated circuit, the method comprising: providing a superconducting nanowire; arranging a waveguide beneath the superconducting nanowire, the waveguide comprising a plurality of waveguide segments connected by a curved waveguide portion, each waveguide segment being arranged proximate to portions of the superconducting nanowire; tapering at least one waveguide segment such that light propagating through a given waveguide segment has an absorption probability to be absorbed by any one of portions of the superconducting nanowire, the light comprising one or more photons, wherein an absorption probability distribution for light in the waveguide to be absorbed by the portions of the superconducting nanowire is substantially uniform across the portions of superconducting nanowire; reflecting the light back through the waveguide using a reflector positioned at an end of the waveguide; detecting absorption of one or more photons by the superconducting nanowire; and determining a number of photons detected based on a number of portions of the superconducting nanowire that each absorb one photon.

17. The method of claim 16, wherein reflecting the light back through the waveguide using a reflector comprises: directing the light through a waveguide bend and a directional coupler configured to split the light into two paths and recombine the light to form a low-loss mirror.

18. The method of claim 16, wherein the light propagation having the absorption probability comprises a forward propagation through the waveguide and a backward propagation after reflection by the reflector.

19. A waveguide for use in a photodetector, the waveguide comprising: a plurality of waveguide segments, each waveguide segment in the plurality of waveguide segments connected to another waveguide segment in the plurality of waveguide segments with a curved waveguide portion, each waveguide segment being arranged proximate a light detection structure such that light propagating through a given waveguide segment has an absorption probability to be absorbed by any one portion of the light detection structure, the light comprising one or more photons; and a reflector positioned at an end of a waveguide segment in the plurality of waveguide segments, the reflector configured to reflect the light through the waveguide; wherein an absorption probability distribution for light in the waveguide to be absorbed by the light detection structure is substantially uniform across the light detection structure.

20. The waveguide of claim 19, wherein the light detection structure comprises a superconducting nanowire.

Images