A single-fiber implementation quantum light source calibration detector structure, preparation method and measurement method

Abstract

This invention discloses a detector structure, fabrication method, and measurement method for calibrating a quantum light source using a single optical fiber. It employs a hybrid integrated structure at the fiber endface, consisting of stepped electrodes, a bottom-layer SNSPD, a top-layer SNSPD, and a reflector stacked vertically and tightly bonded to the endface of a single optical fiber. In two steps, a thin fiber electrode, a thick fiber electrode, a signal electrode, and a ground electrode are deposited on the fiber cladding. Then, through a three-step transfer process, the stepped electrodes, bottom-layer SNSPD, top-layer SNSPD, and reflector are hybridized and integrated onto the fiber endface and encapsulated. A quantum light source generates a pair of orthogonally polarized photons, which are then filtered and input to the detector via an adjustable polarization controller. The two layers of SNSPD absorb the photons, generating response pulses, which are input to two channels of a time-correlated single-photon counter (TCSPC) for reading. The coincidence count, the ratio of coincidence count to random coincidence count, and the light source brightness are calculated, thus achieving parameter calibration of the quantum light source.

Description

Technical Field

[0001] This invention belongs to the field of quantum detection technology and relates to a technology for calibrating quantum light sources using a single optical fiber. Background Technology

[0002] Quantum light sources are widely used in various quantum technologies, such as quantum information and communication (QICT), quantum optical coherence tomography (QOCT), quantum key distribution (QKD), quantum infrared sensing (QIS), and noise-resistant quantum imaging. In environments with strong background noise, the correlation characteristics of photon pairs in quantum light sources can significantly improve the signal-to-noise ratio, outperforming classical imaging schemes. This makes them suitable for high-precision imaging in scenarios such as live biological imaging and precision material detection. The imaging quality depends on the stability and accuracy of the correlation parameters of the quantum light source.

[0003] The calibration of quantum light sources, especially correlated photon pairs and entangled sources, typically involves using detectors to perform coincidence measurements on the photon pairs. Traditional detection systems often employ discrete structures, including a polarization beam splitter (PBS) and two single-photon detectors, coupled via free space or optical fibers. This structure presents significant technical bottlenecks: firstly, the optical coupling loss and insertion loss between discrete components are substantial, leading to decreased detection efficiency and calibration accuracy; secondly, the system requires high alignment accuracy, and coupling instability reduces the repeatability of calibration results, failing to meet the demands of long-term, continuous operation. Furthermore, discrete structures are ill-suited for the application requirements of fiber-integrated quantum systems.

[0004] Superconducting nanowire single-photon detectors (SNSPDs) boast advantages such as >95% efficiency, <1Hz dark count rate, and <10ps time jitter, making them the best-performing single-photon detector in the near-infrared band and the preferred choice for quantum information detectors. As quantum technology rapidly advances towards integration and miniaturization, fiber optic end-face integrated detection technology is gradually becoming a key path to overcome the aforementioned bottlenecks. Directly integrating the detector onto the fiber end-face effectively reduces coupling loss, significantly improves system stability and integration, and is suitable for engineering applications. Summary of the Invention

[0005] This invention is based on a hybrid integrated double-layer orthogonal SNSPD structure at the fiber end face. It integrates a double-layer orthogonal SNSPD inside only one input fiber and uses a quantum calibration method based on single-fiber integrated dual-channel detection to read out data using two sets of independent radio frequency output circuits.

[0006] The detector structure adopts a hybrid integrated structure of fiber endface, consisting of stepped electrodes, a bottom SNSPD, a top SNSPD, and a reflector stacked vertically and tightly bonded to the endface of a single fiber. The fiber endface includes the fiber cladding and the fiber core. The stepped electrodes include thin fiber electrodes and thick fiber electrodes. The bottom SNSPD includes a pair of on-chip electrodes, meandering nanowires, and a silica insulating layer. The top SNSPD includes a pair of on-chip electrodes, meandering nanowires, and a silica cavity. Both the thin and thick fiber electrodes include a pair of signal electrodes and a ground electrode facing opposite directions. The on-chip electrodes of both the bottom and top SNSPDs also include a pair of signal electrodes and a ground electrode facing opposite directions. The meandering nanowires of the bottom and top SNSPDs are orthogonal and located directly above the fiber core. The reflector is located directly above the top SNSPD.

[0007] Furthermore, the thickness of the silicon dioxide insulating layer of the underlying SNSPD is The thickness of the silica cavity in the top-layer SNSPD is , where λ is the wavelength of the light incident into the optical fiber, and n is the refractive index of silicon dioxide in the λ band.

[0008] Electron beam evaporation (EBE) is used to deposit thin-film and thick-film signal and ground electrodes on the fiber cladding in two steps, arranged in a cross shape centered on the fiber core. A superconducting thin film is grown on a silicon substrate with a silicon dioxide layer on its surface. Winding nanowires are etched on the superconducting thin film, and the signal and ground electrodes are fabricated on both sides of the winding nanowires. Silicon dioxide and the underlying silicon substrate are etched around the winding nanowires and the on-chip electrodes to obtain suspended bottom and top SNSPDs. A mirror is fabricated on the silicon substrate with a silicon dioxide layer on its surface. Silicon dioxide and the underlying silicon substrate are etched around the mirror to obtain a suspended mirror. Using microchip transfer technology, the stepped electrodes, bottom SNSPD, top SNSPD, and mirror are integrated and encapsulated on the fiber end face through a three-step transfer process.

[0009] Furthermore, in the first step, Ti / Ag electrodes are deposited in the fiber cladding as thin fiber electrodes, and in the second step, Ti / Ag electrodes are deposited in the fiber cladding in an orthogonal direction as thick fiber electrodes. The two sets of electrodes form a height difference in the electrical contact of the subsequent device.

[0010] Further, the bottom layer SNSPD is picked up using a PDMS stamp and transferred to a heat-release adhesive. The heat-release adhesive is then bonded to the fiber end face, aligning the signal and ground electrodes of the bottom SNSPD with the signal and ground electrodes of the thin fiber electrode, respectively. The meandering nanowires of the bottom SNSPD coincide with the fiber core, completing the first transfer step. The heat-release adhesive is then removed by heating, allowing the bottom SNSPD to adhere to the fiber end face via van der Waals forces. The top layer SNSPD is then picked up using a PDMS stamp and transferred to a heat-release adhesive. The heat-release adhesive is then bonded to the fiber end face, aligning the signal and ground electrodes of the top layer SNSPD with the signal and ground electrodes of the thick fiber electrode, respectively. The meandering nanowires of the top layer SNSPD... The nanowires overlap with the fiber core, and the meandering nanowires of the top-layer SNSPD are orthogonal to those of the bottom-layer SNSPD, achieving the second transfer step. Heating removes the heat-releasing adhesive, allowing the top-layer SNSPD to adhere to the fiber end face via van der Waals forces. A PDMS stamp picks up a reflector and transfers it to the heat-releasing adhesive, which is then adhered to the fiber end face, bonding the reflector to the top-layer SNSPD, achieving the third transfer step. Heating removes the heat-releasing adhesive again, allowing the reflector to adhere to the fiber end face via van der Waals forces. Bonding wires are used to lead the two signal electrodes of the bottom-layer and top-layer SNSPD to the SMA interface of the printed circuit board of the encapsulation box, and the two ground electrodes are connected to the ground of the printed circuit board.

[0011] A quantum light source generates a pair of photons with orthogonally polarized signal light and idler light. The input filter removes residual visible light from the pump, retaining the photon pair with the center wavelength. The input of an adjustable polarization controller adjusts the polarization state of the photon pair. The photons are coupled to a detector, where two layers of SNSPD absorb the photons and generate response pulses. These pulses are then input to two channels of a time-correlated single-photon counter (TCSPC) for reading. Each response pulse is recorded as an event carrying photon timing information. The coincidence count, the ratio of coincidence count to random coincidence count, and the brightness of the quantum light source are calculated to calibrate the parameters of the quantum light source.

[0012] Furthermore, the quantum light source is internally connected via a polarization-maintaining fiber (PMF) and uses a tunable continuous wave laser (CW) with a center wavelength of 775 nm as the pump source. A periodically polarized lithium niobate (PPLN) waveguide is injected, and under type II quasi-phase-matched quantum millimeter (QPM) conditions, a pair of photons with orthogonally polarized signal and idler light is generated via spontaneous parametric conversion (SPDC). The center wavelength is in the 1550 nm band, and the power range is set from 0.4 mW to 5.6 mW. A filter removes the residual visible light from the 775 nm pump, retaining the photon pair in the 1550 nm band. The tunable polarization controller adjusts the polarization state of the incident photon pair by compressing the arbitrary polarization state of the Poincaré sphere with fiber compression.

[0013] Furthermore, the bottom-level SNSPD and the top-level SNSPD are each connected to two channels of the time-correlated single-photon counter (TCSPC) via an RF line. The two channels have different counts. Let the channel with fewer events be t_short and the channel with more events be t_long. Iterate through all events in t_short, setting the loop start flag i=1. Iterate through the events in t_long, selecting the event with the smallest time difference from the current event in t_short. If the time difference is less than a preset adjustable threshold, record the time difference, remove the event from t_long, and increment the loop flag i. Perform Gaussian fitting on the distribution of all time differences, and calculate the full width at half maximum (FWHM). With underlying SNSPD time jitter Top-level SNSPD time jitter Quantum light source time jitter The relationship is .

[0014] Furthermore, let the pump light power of the quantum light source be... The two-layer SNSPD dark count rate The number of orthogonally polarized photon pairs generated per second is The number of idler photons is The number of signal photons is The probabilities of detecting idler photons and signal photons in the underlying SNSPD are respectively , The probabilities of the top-level SNSPD detecting idler photons and signal photons are respectively , The probability of detecting two-photon coincidence of a quantum light source is then... The probability of the underlying SNSPD detecting any photon is: The probability of the top-level SNSPD detecting any photon is: The coincidence count is The ratio of coincident counts to random coincident counts is The brightness of the light source is If the polarization directions of the incident photon pairs coincide with the directions of the detector's orthogonal nanowires, the coincidence count rate is the maximum; if they differ by 45°, the coincidence count rate is the minimum.

[0015] The alternative quantum light source utilizes visible light, near-infrared light, mid-infrared light, microwave, and terahertz light bands; photon pairs are generated using semiconductor quantum dots, four-wave mixing technology, and spontaneous parametric down-conversion; the meandering nanowires use NbN, WSi, MoSi, and NbTiN superconducting materials; the reflectors use distributed Bragg reflectors and metal reflectors made of Au and Ag high-reflectivity materials; the detector integration process employs probe transfer, micro-nano bonding, end-face thin film deposition, in-situ photolithography fabrication, and flip-chip bonding.

[0016] The structure of this invention achieves high integration, combining polarization beam splitting and dual-path single-photon detection into a single fiber endface. By integrating a double-layer orthogonally stacked SNSPD into the fiber endface, beam splitting and detection of orthogonally polarized photons can be performed simultaneously. In this structure, each layer of the device detects both the signal photon and the idler photon in the incident photon pair with a certain probability. Compared to a discrete scheme that first splits the photon pair and then uses two independent detectors to detect the signal photon and the idler photon respectively, this integrated detector achieves higher coincidence detection efficiency while maintaining the same system efficiency.

[0017] The fabrication method of this invention provides an efficient and feasible technology for the hybrid integration of multilayer detectors and optical fibers. Through the fabrication of stepped electrodes at the fiber end face and a three-step microchip transfer process, non-destructive integration of SNSPD devices at the fiber end face is achieved. This approach retains the original high-performance single-photon detection capability of the device while adding efficient photon correlation detection capability, and is expected to be applied in fields such as quantum correlation imaging and quantum key distribution (QKD). It also has the potential for large-scale fabrication, which is beneficial for engineering application.

[0018] The measurement method of this invention exhibits higher stability. Existing discrete systems have multiple fiber optic connection points and insertion loss points. In contrast, this scheme eliminates fiber coupling loss and insertion loss between discrete components, effectively improving the system's coincidence detection efficiency for quantum light sources. It is particularly suitable for high signal-to-noise ratio coincidence measurements under weak light sources and can reduce the acquisition time required for calibration. Furthermore, this scheme avoids complex optical alignment processes, significantly improving the system's integration and mechanical stability, and truly achieving all-fiber, ultra-compact quantum light source calibration. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the measurement method.

[0020] Figure 2 It is an integrated structure diagram of the bottom SNSPD, the top SNSPD, and the reflector.

[0021] Figure 3 This is a diagram of a stepped electrode structure.

[0022] Figure 4 This is an optical micrograph of the detector.

[0023] Figure 5 It is a graph showing the changes in optical count rate and dark count rate of the underlying SNSPD as a function of normalized bias current.

[0024] Figure 6 This is a graph showing the changes in optical and dark count rates of the top-level SNSPD as a function of normalized bias current.

[0025] Figure 7 It is the output timing pulse diagram.

[0026] Figure 8 This is a flowchart of the reading processing.

[0027] Figure 9 It is a distribution chart of the number of occurrences.

[0028] Figure 10 This is a distribution diagram of coincidence counts for different polarization states.

[0029] Figure 11 This is a graph showing the coincidence counts of polarization angle 1 and polarization angle 2 as a function of pump light power.

[0030] Figure 12 This is a graph showing the ratio of coincidence count to random coincidence count for polarization angle 1 and polarization angle 2 as a function of pump light power.

[0031] Figure 13 This is a graph showing the variation of light source brightness with pump light power for polarization angles 1 and 2.

[0032] Figure labels: 1-Fiber cladding, 2-Fiber core, 3-Thin fiber electrode, 4-Thick fiber electrode, 5-Bottom layer SNSPD, 6-Top layer SNSPD, 7-Reflector. Detailed Implementation

[0033] The present invention will now be described in further detail with reference to the embodiments and the accompanying drawings.

[0034] The principle of the measurement method is as follows Figure 1 As shown, a quantum light source generates a pair of photons with orthogonally polarized signal light and idler light. The input filter removes residual visible light from the pump, retaining the photon pair with the center wavelength. The input of an adjustable polarization controller adjusts the polarization state of the photon pair. The photons are coupled to a detector, where two layers of SNSPD absorb the photons and generate response pulses. These pulses are then input to two channels of a time-correlated single-photon counter (TCSPC) for reading. Each response pulse is recorded as an event carrying photon timing information. The coincidence count, the ratio of coincidence count to random coincidence count, and the brightness of the quantum light source are calculated to calibrate the parameters of the quantum light source.

[0035] Existing detection schemes employ discrete optical components, including a polarization beam splitter and two single-photon detectors. The polarization beam splitter splits photon pairs of orthogonally polarized signal light and idler light into two optical fibers, which are then input to the two single-photon detectors for detection.

[0036] The detector employs a microchip transfer method to sequentially integrate two orthogonal superconducting nanowire single-photon detectors (SNSPDs) at the bottom and top layers, along with a mirror, onto the fiber end face, and electrically connects them to the fiber electrodes. Figure 2As shown, the bottom-layer SNSPD and the top-layer SNSPD are optically coupled through a single optical fiber. Compared with existing technologies, the same function is achieved using only one optical fiber end face, making it more compact and integrated.

[0037] Microchip transfer is a promising micro / nano fabrication technology that flexibly integrates superconducting nanowire single-photon detectors (SNSPDs) with various optical components. However, the transfer of multilayer structures faces greater technical challenges. For example, the top-layer SNSPD must be connected across the bottom-layer SNSPD film, which carries the risk of poor contact or excessively high contact resistance.

[0038] Stepped electrode structure, such as Figure 3 As shown, it includes an optical fiber cladding, an optical fiber core, a thin optical fiber electrode, and a thick optical fiber electrode. Both the thin and thick optical fiber electrodes include a pair of signal electrodes and a ground electrode facing opposite directions. The four independent electrodes are deposited on the optical fiber cladding in two steps using an electron beam evaporation (EBE) process, and are arranged in a cross shape with the optical fiber core as the center.

[0039] The first step involves depositing a 100nm Ti / Ag electrode as a thin fiber electrode, and the second step involves depositing a 500nm Ti / Ag electrode as a thick fiber electrode. The two sets of electrodes form a 500nm height difference, ensuring reliable electrical contact in subsequent device layers.

[0040] The bottom layer SNSPD includes a pair of on-chip electrodes, a meandering nanowire, and a silicon dioxide insulating layer. The on-chip electrodes are aligned and bonded with the thin fiber electrode to achieve the first step of transfer. The silicon dioxide insulating layer prevents thermal coupling and electrical crosstalk between the two SNSPD layers.

[0041] The top-layer SNSPD includes a pair of on-chip electrodes, a meandering nanowire, and a silica cavity. The on-chip electrodes are aligned and bonded to the thick fiber electrode to achieve the second-step transfer, and the silica cavity is used to enhance the overall detection efficiency.

[0042] Optical microscopy of the detector, such as Figure 4 As shown, the nanowire regions of both SNSPD layers coincide with the fiber core. The signal electrode and ground electrode of the bottom SNSPD are aligned with the signal electrode and ground electrode of the thin fiber electrode, respectively. The signal electrode and ground electrode of the top SNSPD are aligned with the signal electrode and ground electrode of the thick fiber electrode, respectively. There are no air bubbles between the bottom and top SNSPD layers and the fiber end face, and they have formed a tight fit.

[0043] The thickness of the silicon dioxide insulating layer of the bottom SNSPD is twice that of the silicon dioxide cavity of the top SNSPD, at 530nm and 265nm respectively, so that the two SNSPDs have the same maximum detection efficiency.

[0044] The photocount rate and dark count rate of the underlying SNSPD at 1550nm band as a function of normalized bias current are shown in the following curves. Figure 5 As shown, the photocounting curve gradually approaches saturation as the normalized bias current increases, indicating that the quantum efficiency is close to 1, which confirms the excellent performance of the underlying SNSPD. The dark count rate d is 300Hz, which is low.

[0045] The photocount rate and dark count rate of the top-layer SNSPD at 1550nm band as a function of normalized bias current are shown in the following curves. Figure 6 As shown, the photocounting curve gradually approaches saturation as the normalized bias current increases, indicating that the quantum efficiency is close to 1, which confirms the excellent performance of the top-level SNSPD. The dark count rate d is 300Hz, which is low.

[0046] The two SNSPD layers are each connected to two channels of the time-correlated single-photon counter (TCSPC) via an RF line. They absorb photons and output response pulses, which are read by the TCSPC. Each response pulse is recorded as an event, and the event contains the timing information of the response pulse.

[0047] Output timing pulses such as Figure 7 As shown, the solid and dashed lines represent the response pulse curves of the bottom-layer SNSPD and top-layer SNSPD, respectively. The pulse half-width at half-maximum (WHM) is extremely narrow, both being 20 ns. The time corresponding to half the pulse rise height is the pulse arrival time, and the difference between the arrival times of the two pulses is... .

[0048] The reading processing flow is as follows Figure 8 As shown, all events are extracted from the file stored in TCSPC. The counts of the two channels are different. Let the channel with fewer events be t_short and the channel with more events be t_long. Traverse all events in t_short. Set the loop start flag i=1. Traverse the events in t_long and select the event with the smallest time difference from the current event in t_short. If the time difference is less than a preset adjustable threshold, record the time difference and remove the event from t_long. Increment the loop flag i by 1. Perform Gaussian fitting on the distribution of all time differences to calculate the coincidence number, the ratio of coincidence count to random coincidence count, and the brightness of the quantum light source, thereby realizing the parameter calibration of the quantum light source.

[0049] Consistent with the distribution of counts, such as Figure 9 As shown, half-width It is 108ps, compared to the underlying SNSPD time jitter. Top-level SNSPD time jitter Quantum light source time jitter The relationship is The total coincident count is the integral of the coincident count distribution.

[0050] The coincidence count distributions of different polarization states are as follows: Figure 10 As shown, the horizontal and vertical axes correspond to the tilt angles of the Poincaré sphere, respectively. Azimuth Twice that of the period in both the horizontal and vertical directions, with a period of 90°, which is half the response period of the winding nanowire to the polarization state of coherent light, confirms that the incident light has orthogonal polarization characteristics and can measure information about the polarization dimension of the light source.

[0051] Based on the system efficiency characterization results, the detection efficiencies for polarization angles one and two are shown in the table below.

[0052]

[0053] The ratio of the maximum probability to the minimum probability of two-photon coincidence detected by a two-layer SNSPD from a quantum light source is: The results are in high agreement with those of the polarization scanning test.

[0054] Assuming the system efficiency of the existing discrete two-detector system is equal to that of the bottom and top detectors of this invention, and neglecting the imperfect polarization beam splitting ratio, as well as the coupling losses of the filter, polarization beam splitter, and two single-photon detectors, the two-photon coincidence probability measured by the existing technology is: Calculations yielded This confirms that the coincidence count rate measured by the present invention is at least 5% higher than that measured by the prior art.

[0055] The coincidence count probability of a coherent pulsed light source is The ratio of the maximum probability to the minimum probability is This is significantly different from the coincidence count distribution of quantum light sources.

[0056] The coincidence counts of polarization angle 1 and polarization angle 2 as a function of pump power are shown in the following curves. Figure 11 As shown, the coincidence count is The measured results are consistent with the formula. The measured coincidence count value is proportional to the pump light power. The ratio of the coincidence count values ​​measured at polarization angle 1 and polarization angle 2 is similar.

[0057] The curves showing the ratio of coincidence count to random coincidence count at polarization angle 1 and polarization angle 2 as a function of pump power are as follows: Figure 12 As shown, the ratio of coincidence count to random coincidence count The measured results are consistent with the formula. As the pump light power increases, the probability of accidental coincidence increases, and the CAR decreases rapidly. The maximum value of CAR was measured to be about 1500. The ratio of CAR measured at polarization angle 1 and polarization angle 2 is similar.

[0058] The curves showing the variation of light source brightness with pump power for polarization angle 1 and polarization angle 2 are as follows: Figure 13 As shown, the pump light power is The brightness of the light source is The measured results are consistent with the formula, and the brightness of the light source fluctuates little as the pump light power increases.

Claims

1. A single-fiber implementation quantum light source calibration detector structure, characterized in that, include: It adopts a hybrid integrated structure of fiber end face, consisting of stepped electrodes, bottom SNSPD, top SNSPD and reflector stacked vertically and tightly attached to the end face of a single fiber. The fiber end face includes the fiber cladding and the fiber core. The stepped electrode includes a thin fiber electrode and a thick fiber electrode. The bottom SNSPD includes a pair of on-chip electrodes, a meandering nanowire, and a silicon dioxide insulating layer. The top SNSPD includes a pair of on-chip electrodes, a meandering nanowire, and a silicon dioxide cavity. Both the thin and thick fiber electrodes include a pair of signal electrodes and a ground electrode with opposite directions. The on-chip electrodes of both the bottom and top SNSPDs include a pair of signal electrodes and a ground electrode with opposite directions. The meandering nanowires of the bottom and top SNSPDs are orthogonal and located directly above the fiber core. The reflector is located directly above the top SNSPD.

2. The detector structure for quantum light source calibration using a single optical fiber according to claim 1, characterized in that, The thickness of the silicon dioxide insulating layer of the bottom SNSPD is The thickness of the silicon dioxide cavity of the top SNSPD is where λ is the wavelength of light incident into the optical fiber, and n is the refractive index of silicon dioxide at the λ band.

3. A method for preparing a single-fiber implementation quantum light source calibration detector, characterized in that, The detector structure for single-fiber quantum light source calibration according to claim 1 includes: depositing a thin fiber electrode, a thick fiber electrode, a signal electrode, and a ground electrode on the fiber cladding in two steps using electron beam evaporation (EBE) technology, arranged in a cross shape with the fiber core as the center; growing a superconducting thin film on a silicon substrate with a silicon dioxide layer on its surface, etching a meandering nanowire on the superconducting thin film, fabricating the signal electrode and ground electrode on both sides of the meandering nanowire, etching silicon dioxide and the silicon substrate below it around the meandering nanowire and the on-chip electrode to obtain a suspended bottom-layer SNSPD and a top-layer SNSPD; fabricating a mirror on a silicon substrate with a silicon dioxide layer on its surface, etching silicon dioxide and the silicon substrate below it around the mirror to obtain a suspended mirror; and using microchip transfer technology, integrating the stepped electrode, bottom-layer SNSPD, top-layer SNSPD, and mirror on the fiber end face and encapsulating them through a three-step transfer process.

4. The method for fabricating a detector for quantum light source calibration using a single optical fiber according to claim 3, characterized in that, The electron beam evaporation (EBE) process is used to deposit signal electrodes and ground electrodes on the fiber cladding in two steps: first, Ti / Ag electrodes are deposited in the fiber cladding as thin fiber electrodes; second, Ti / Ag electrodes are deposited in the fiber cladding in an orthogonal direction as thick fiber electrodes. The two sets of electrodes form a height difference that creates electrical contacts for subsequent devices.

5. The method for fabricating a detector for quantum light source calibration using a single optical fiber according to claim 3, characterized in that, The process involves a three-step transfer to integrate and encapsulate the stepped electrode, bottom SNSPD, top SNSPD, and reflector onto the fiber end face. This includes: using a PDMS stamp to pick up the bottom SNSPD and transfer it to a heat-release adhesive; attaching the heat-release adhesive to the fiber end face; aligning and attaching the signal and ground electrodes of the bottom SNSPD with the signal and ground electrodes of the thin fiber electrode, respectively; aligning the meandering nanowires of the bottom SNSPD with the fiber core to complete the first transfer step; heating to remove the heat-release adhesive, allowing the bottom SNSPD to adhere to the fiber end face via van der Waals forces; and using a PDMS stamp to pick up the top SNSPD and transfer it to a heat-release adhesive; attaching the heat-release adhesive to the fiber end face; aligning the signal and ground electrodes of the top SNSPD with the signal and ground electrodes of the thick fiber electrode, respectively. The signal and ground electrodes are aligned and bonded together. The meandering nanowires of the top-layer SNSPD coincide with the fiber core, and the meandering nanowires of the top-layer SNSPD are orthogonal to the meandering nanowires of the bottom-layer SNSPD, achieving the second transfer step. The heat release adhesive is removed by heating, allowing the top-layer SNSPD to bond to the fiber end face through van der Waals forces. The reflector is picked up with a PDMS stamp and transferred to the heat release adhesive, which is then bonded to the fiber end face, allowing the reflector to bond to the top-layer SNSPD, achieving the third transfer step. The heat release adhesive is removed by heating, allowing the reflector to bond to the fiber end face through van der Waals forces. The two signal electrodes of the bottom-layer and top-layer SNSPDs are led out to the SMA interface of the printed circuit board of the encapsulation box using bonding wires, and the two ground electrodes are connected to the ground of the printed circuit board.

6. A detector measurement method for calibrating a quantum light source using a single optical fiber, characterized in that, The detector structure for calibrating a quantum light source using a single optical fiber as described in claim 1 includes: generating a pair of photons with orthogonally polarized signal light and idler light using a quantum light source; inputting a filter to filter out residual visible light from the pump and retaining the photon pair with the center wavelength; inputting an adjustable polarization controller to adjust the polarization state of the photon pair; coupling the detector to a detector; having two layers of SNSPD absorb photons and generate response pulses; inputting the response pulses to two channels of a time-correlated single-photon counter (TCSPC) for reading; recording each response pulse as an event carrying photon timing information; calculating the coincidence count, the ratio of coincidence count to random coincidence count, and the light source brightness of the quantum light source; and thus calibrating the parameters of the quantum light source.

7. The detector measurement method for quantum light source calibration using a single optical fiber according to claim 6, characterized in that, The quantum light source is internally connected via a polarization-maintaining fiber (PMF) and uses a tunable continuous wave laser (CW) with a center wavelength of 775 nm as the pump source. A periodically polarized lithium niobate (PPLN) waveguide is injected, and under type II quasi-phase-matched quantum millimeter (QPM) conditions, a pair of orthogonally polarized signal and idler photons is generated via spontaneous parametric conversion (SPDC). The center wavelength is in the 1550 nm band, and the power range is set from 0.4 mW to 5.6 mW. The filter removes residual visible light from the 775 nm pump, retaining the 1550 nm photon pair. The tunable polarization controller adjusts the polarization state of the incident photon pair by compressing an arbitrary polarization state covering the Poincaré sphere with fiber.

8. The detector measurement method for quantum light source calibration using a single optical fiber according to claim 6, characterized in that, The step of inputting readings into two channels of the time-correlated single-photon counter (TCSPC) includes: the bottom-layer SNSPD and the top-layer SNSPD are each connected to the two channels of the TCSPC via an RF cable. The two channels have different counts. Let the channel with fewer events be t_short and the channel with more events be t_long. Iterate through all events in t_short, setting the loop start flag i=1. Iterate through the events in t_long, selecting the event with the smallest time difference from the current event in t_short. If the time difference is less than a preset adjustable threshold, record the time difference, remove the event from t_long, increment the loop flag i by 1, and perform Gaussian fitting on the distribution of all time differences. With underlying SNSPD time jitter Top-level SNSPD time jitter Quantum light source time jitter The relationship is .

9. The detector measurement method for quantum light source calibration using a single optical fiber according to claim 6, characterized in that, The calculation of the coincidence count, the ratio of the coincidence count to the random coincidence count, and the brightness of the quantum light source includes: assuming the pump power of the quantum light source is... The two-layer SNSPD dark count rate The number of orthogonally polarized photon pairs generated per second is The number of idler photons is The number of signal photons is The probabilities of detecting idler photons and signal photons in the underlying SNSPD are respectively , The probabilities of the top-level SNSPD detecting idler photons and signal photons are respectively , The probability of detecting two-photon coincidence of a quantum light source is then... The probability of the underlying SNSPD detecting any photon is: The probability of the top-level SNSPD detecting any photon is: The coincidence count is The ratio of coincident counts to random coincident counts is The brightness of the light source is If the polarization directions of the incident photon pairs coincide with the directions of the detector's orthogonal nanowires, the coincidence count rate is the maximum; if they differ by 45°, the coincidence count rate is the minimum.

10. The detector measurement method for quantum light source calibration using a single optical fiber according to claim 6, characterized in that, The quantum light source is replaced with visible light, near-infrared, mid-infrared, microwave, and terahertz light; the photon pairs are replaced with semiconductor quantum dots, four-wave mixing technology, and spontaneous parametric down-conversion generation; the meandering nanowires are replaced with NbN, WSi, MoSi, and NbTiN superconducting materials; the reflectors are replaced with distributed Bragg reflectors or metal reflectors made of Au or Ag high-reflectivity materials; and the microchip transfer technology used in the detector is replaced with probe transfer, micro-nano bonding, end-face thin film deposition, in-situ photolithography, and flip-chip bonding.

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