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Abstract
The single-photon detector is an essential technology in photonic quantum information science and technology. Large-scale photonic quantum computers and quantum networks inevitably require numerous high-performance single-photon detectors. Superconducting nanostrip single-photon detectors (SNSPDs) using around 100-nm-wide nanostrips are promising technologies with high detection efficiency, low dark count, and low jitter, but there has been room for evolution in terms of polarization dependence and productivity. Using wide strips with widths of tens of micrometers provides polarization-independent high detection efficiency and high-yield fabrication using high-throughput photolithography with submicron resolution. However, detecting photons with such wide strips has been challenging due to rapidly increasing intrinsic dark counts caused by the uneven distribution of the superconducting current in the strip. Here, we present a novel superconducting wide strip photon detector (SWSPD) with a high critical current bank (HCCB) structure. This new strip structure suppresses the intrinsic dark counts and provides highly efficient photon detection in the wide strips. We have simultaneously achieved a polarization-independent detection efficiency of over 78% for 1550-nm wavelength photons, a low dark count rate (DCR) of ∼80 cps, and a low jitter of 29.8 ps using a 20-µm-wide SWSPD with the HCCB structure. This result paves the way for a new class of photon detectors using ultra-wide superconducting strips. These photon detectors with excellent productivity and polarization-independent high detection performances would boost the advance of large-scale photonic quantum technologies.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The single-photon detector is an essential technology in photonic quantum information science and technology. Simultaneously achieving high detection efficiency, low dark count rate (DCR), and low jitter without depending on the polarization state of the photon is desirable for many photonic quantum applications. Moreover, high-yield and high-throughput productivity is required to accelerate the development of large-scale photonic quantum computers and quantum networks, which require numerous photon detectors. Although various detectors—including photomultiplier tubes, avalanche photodiodes, and superconducting detectors—have been developed to detect single photons efficiently, a single-photon detector with a current-carrying superconducting strip is one of the most promising candidate devices. Superconducting nanostrip single-photon detectors (SNSPDs), which use superconducting strips with widths of approximately 100 nm, were first demonstrated in 2001, and these detectors have seen rapid growth over the past two decades [1,2]. Excellent detection performances have been achieved, including a system detection efficiency (SDE) of more than 98% [3,4], a low DCR of less than 1 × 10−5 Hz [5], low timing jitter of less than 3 ps [6], and a high maximum counting rate exceeding 1 GHz [7]. Moreover, single-photon imaging has been demonstrated using arrayed SNSPDs and/or specific readout techniques [8–15]. In addition, quasi-photon-number-resolving detection with a multiplexed array detector [16–22] and intrinsic photon-number-resolving performance [23–25] have also been demonstrated. These features have made SNSPDs essential devices not only for quantum technologies [26–30], but also for a broad range of applications from bioimaging [31,32] to remote sensing [33–35] and space communications [36]. In recent years, superconducting microstrip single-photon detectors (SMSPDs) with strip widths of 1 µm or more have offered new potential for the development of superconducting strip single-photon detectors. For many years, narrow superconducting strips with widths of around 100 nm have been considered essential for single-photon detection, particularly in the infrared wavelength range. In 2018, Korneeva et al. demonstrated near-infrared single-photon detection using superconducting strips with widths of a few micrometers [37], with experimental results that were consistent with predictions based on the vortex model [38]. Their innovative research has excited interest in the physics inside the current-carrying superconducting strip and has raised a fundamental question about the upper limit to the strip width that allows a single photon to be detected. Moreover, the SMSPD has the advantages that it can be fabricated with photolithographic process and can form an extra-large photo-receiving area while maintaining low kinetic inductance and a high temporal response. To make the most of these advantages, which are difficult to achieve with SNSPDs, SMSPDs based on various superconducting materials have been developed actively over the past few years [39–47]. In addition, the geometries of these superconducting strips have been studied to reduce the geometrical current-crowding effect, which reduces the switching bias current and limits detection efficiency [48–51]. As a result of these challenges, SDEs of more than 90% have been achieved using superconducting strips with widths in the vicinity of 1 µm [50,51].
However, it is still highly challenging to detect single photons with wider superconducting strips with widths of tens of micrometers, despite their practical advantages in photonic quantum technologies. Such an extra-wide strip can form a photo-receiving area with a simple straight strip, with the advantage of strip widths much wider than the optical spot diameter. This allows an ultimate improvement in detection efficiency based on perfect coupling efficiency with a 100% filling factor, which is desirable for applications requiring extremely high detection efficiency. In addition, the wider strips can be fabricated more easily by photolithography with submicron resolution improving fabrication throughput and yield, which accelerates the development of large-scale photonic quantum computers requiring numerous photon detectors [27]. Also, improving yields increases the number of pixels in single-photon imagers and quasi-photon-number-resolving detectors, and photolithographic fabrication helps their integration with the integrated quantum photonic circuits [52] due to the consistency of the fabrication process. Moreover, the unpatterned geometric shape of the wide strip photo-receiving area provides natural polarization insensitivity in their detection efficiency, without using complex strip structures [53,54] or special optical coatings or cavities [55,56]. This provides a practically high SDE in practical photonic quantum systems where the polarization is not always ideally controlled and determined. Also, the naturally achieved polarization insensitivity, coupled with the perfect filling factor, provides polarization insensitive high detection efficiency in a broad wavelength range, which is desirable for applications using broadband quantum light sources, such as quantum optical coherence tomography [57]. Furthermore, polarization insensitivity fundamentally solves problems caused by polarization-dependent detection efficiency mismatch, such as a security loophole in quantum key distribution [58]. Therefore, ultra-wide superconducting strip detectors with widths of tens of micrometers are expected to impact the advance of quantum information science and technology if they achieve photon detection with excellent performance.
One of the major difficulties of achieving photon detection using ultra-wide superconducting strips is the detector's rapidly increasing intrinsic dark counts due to the non-uniform distribution of the superconducting current in the strip, which becomes severe with increasing strip width and bias current. To reduce these intrinsic dark counts, we propose a novel superconducting strip structure that has two different critical currents across the strip, with the edge regions having higher critical currents than the center region. We call this structure the high critical current bank (HCCB) structure. Using this new strip structure, we demonstrate single-photon detection using a 20-µm-wide niobium-titanium nitride (NbTiN) superconducting wide strip photon detector (SWSPD) operating in the near-infrared wavelength range. The proposed strip structure successfully suppresses increases in the intrinsic dark counts while also supplying increased bias current to the wide strip. As a result, a high SDE is achieved together with a low DCR. In addition, we also demonstrate the polarization insensitivity and low timing jitter of the SWSPD under high bias current conditions. These results represent a major step toward ultra-wide SWSPDs with high SDE, low DCR, and low timing jitter. Furthermore, these results provide valuable insights for investigations into the dynamics of photon detection and dark count generation in current-carrying superconducting strips.
2. Concept of the SWSPD with the HCCB Structure
High-efficiency single-photon detection using wider superconducting strips requires a sufficiently high bias current to be supplied to the strips while the intrinsic dark count must be suppressed. However, in practice, the distribution of the bias currents is concentrated near the edges of the strip because of the influence of the magnetic flux density distribution [59]. Because SNSPDs and SMSPDs are usually designed to have uniform thickness and superconductivity across the widths of their strips, the concentration of the bias current induces vortex penetration from the side edges that leads to a rapid increase in the intrinsic dark counts with increasing bias current. As a result, the strip switches permanently to the normal conducting state before a sufficient bias current can be supplied to the central region of the strip.
To solve this problem, we propose a superconducting strip with an HCCB structure, as drawn schematically in Fig. 1. In the proposed device, both side edge regions in a strip are structured to have higher critical currents per unit width than the central region. These side strip regions suppress vortex penetration into the strip, thus reducing the intrinsic dark count. As a result, a sufficiently high bias current can be applied more evenly to the central region along the strip width. In addition, because the central area acts as the photon detection area and the side regions are not involved, the sensitivity limitations caused by defects on the side edges of the strip, e.g., from the manufacturing process, can be suppressed to some extent.
Fig. 1. Concept of the novel SWSPD with the HCCB structure. (a) Schematic of the SWSPD device with a center strip area acting as the photo-receiving area and side strip areas formed outside the center strip area. (b) Cross-sectional view of the SWSPD device and schematic image showing the distribution of the critical current per unit strip width of the wide strip and the bias current flowing in the wide strip. The bias current flowing in the superconducting strip is concentrated near the edges, which induces vortex penetration from the side edges and leads to a rapid increase in the intrinsic dark counts. The novel superconducting strip structure has side strip areas that have higher critical currents per unit strip width than the center strip area to suppress both vortex penetration into the strip and the increase in the intrinsic dark count. We call this the HCCB structure. In this work, we realized this structure by using ion beam irradiation treatment on the center strip area to reduce the critical current per unit strip width, as detailed later in Subsection 3.B.
3. Device Fabrication and Experimental Results
3.1 Detection Performance of Conventional SMSPD with Wide Width
Before testing our device concept, we investigated the photon detection characteristics of an SMSPD that was designed to have uniform superconductivity across the width of its strip. Figure 2 shows the bias current dependencies of the SDE and DCR of the approximately 4.9-nm-thick NbTiN straight strip detector with both a strip width and length of 20 µm measured at temperatures of (a) 0.76 K, and (b) 2.2 K. The detector shape was the same as that used for the novel detector concept (which will be described later) except for the structure of the side strip areas. The wavelength of the photons irradiating the detector was 1550 nm, and the average photon number was tuned to 1 × 105 photons/s. The 20-µm-wide SMSPD device successfully detected single photons at a near-infrared wavelength. However, as mentioned in the concept described above, the rapid increase in the intrinsic dark count limited device operation in the high bias current region. As a result, the device was permanently switched to the normal conducting state before reaching SDE saturation. Specifically, at a temperature of 2.2 K, the intrinsic dark counts rose from much lower bias currents, causing device operation in the high bias current region to be strongly limited. Therefore, suppression of the intrinsic dark counts is a significant requirement to produce higher detection efficiency when using the wider superconducting strip.
Fig. 2. Bias current dependencies of the SDE and DCR of the conventional SMSPD with broad width. (a) SDE (red filled circles) and DCR (red open circles) curves measured at a temperature of 0.76 K. The NbTiN microstrip thickness was approximately 4.9 nm. The strip width and length were both 20 µm. The photon wavelength at which the strip was irradiated was 1550 nm. The rapid increase in the intrinsic dark count prevented device operation before the SDE curve reached a plateau. (b) SDE and DCR curves when measured at a temperature of 2.2 K. The intrinsic dark count increased from much lower bias currents and limited device operation severely.
3.2 Fabrication Process of the SWSPD with the HCCB Structure
In this work, we realized the proposed strip structure by using argon (Ar) ion beam irradiation to reduce the critical current per unit strip width in the central region. Ion beam irradiation effectively reduces the critical temperature of the thin superconducting strip and the maximum superconducting current that can be applied to that strip [60,61]. For example, the critical temperature of the 10-µm-wide NbTiN strip with a thickness of approximately 5.9 nm was degraded from 8.2 K to 6.4 K after 5 s of irradiation with the Ar ion beam (see Fig. S3 in Supplement 1 for details of the preliminary experiments). Note that the mechanisms of the effects of Ar ion beam irradiation on NbTiN film have not been clarified at this time. Possible mechanisms include the formation of vacancies as suggested by Zhang et al. in Ref. [61], implantation of ions, thinning of the superconducting film due to the etching effect, and their combined contributions, and elucidating these mechanisms is an important future research subject. Figure 3 shows schematics and micrographs of SWSPD devices that were irradiated using the Ar ion beam. We fabricated two SWSPD device types for comparison that were irradiated with Ar ion beams over different areas on their strips. Figure 3(a) shows the SWSPD device that was irradiated by the Ar ion beam over the central area of the strip to form side strip areas with higher critical currents. The area irradiated by the Ar ion beam is bounded by the red dashed line in the micrograph. Figure 3(b) shows that the SWSPD device that was irradiated with the Ar ion beam over the entire width of the strip has uniform superconductivity across the strip width. Note that the devices shown in Fig. 3 do not have optical cavity structures to increase photon absorption. To fabricate these devices, an NbTiN thin film with a thickness of approximately 5.9 nm was first deposited on a silicon wafer with a thermally oxidized layer using DC magnetron reactive sputtering. The NbTiN film was then patterned to form a wide strip with a length of 20 µm, a width of 20 µm, and a coplanar waveguide (CPW) structure using maskless photolithography and reactive ion etching. Subsequently, the Ar ion beam was used to irradiate the wide strip over a length of 15 µm to reduce the critical currents in the strips. For the SWSPD device with the HCCB structure, the width of the area irradiated by the Ar ion beam was 18 µm, meaning that the width of the side strip area was 1 µm for each side region. The fabricated devices were cooled down to the 0.76 K to 2.2 K temperature range using an He-4 sorption refrigerator, and the detection performance for the 1550-nm wavelength photons was then evaluated.
Fig. 3. Schematics and micrographs of the SWSPD devices that were irradiated using the Ar ion beam. (a) SWSPD device with the HCCB structure. The Ar ion beam irradiated the central area of the wide strip to form side strip areas with high critical current per unit strip width. The area irradiated by the Ar ion beam is bounded by the red dashed line shown in the micrograph. (b) SWSPD device without the HCCB structure. The Ar ion beam irradiated the entire width of the wide strip, causing the wide strip to have uniform superconductivity across its width.
3.3 Detection Performance of the SWSPD with the HCCB Structure
Figure 4 shows the bias current dependencies of the SDE and DCR of the two types of SWSPD device at temperatures of 0.76 K and 2.2 K. The photons emitted from a continuous wave (CW) laser source were attenuated to an intensity of 1 × 105 photons/s and propagated into a refrigerator via a single-mode optical fiber after passing through a polarization control module. The spot size on the wide strip caused by photon illumination from the back side of the device was tuned to a 1/e2 diameter of approximately 8.2 µm, and the spot center was aligned with the center of the photo-receiving area. To prevent a permanent transition into the normal conducting state and to ensure stable operation of the SWSPD devices, a series-connected 220-nH inductor and a parallel-connected 12.5-Ω shunt resistor were used on the outside of the radio-frequency (RF) input/output port of the refrigerator (see Fig. S1 in Supplement 1 for details of the experimental setup). The logarithmically displayed dark count curves have two regions that have different slopes. The counts that increase slowly toward saturation in the low bias current range are caused by the extrinsic dark count, which is mainly blackbody radiation, while the counts that increase steeply within the high bias current range are caused by the intrinsic dark count that is inherent to these devices. The intrinsic dark count of the SWSPD without the side strip areas increased from a relatively low bias current, resulting in the device being switched into the normal conducting state before its detection efficiency approached saturation. In contrast, the SWSPD with the HCCB structure suppressed the rise of the intrinsic dark count to the higher bias current range successfully and had a longer extrinsic dark count region. As a result, the detection efficiency of the device was clearly saturated with a broad plateau region, indicating that the internal detection efficiency reached 100%. These results clearly demonstrate the effect of the HCCB structure in suppressing the intrinsic dark count and thus increasing the maximum bias current that can be supplied to the wide strip, resulting in improved detection efficiency.
Fig. 4. Bias current dependencies of the SDE and DCR of the SWSPD devices that were irradiated using the Ar ion beam. (a) SDE (red filled circles) and DCR (red open circles) curves of the SWSPD with the HCCB structure when measured at a temperature of 0.76 K. The wide strip was 20 µm wide, including an 18-µm-wide center strip area and 1-µm-wide side strip areas. The irradiating photon wavelength was 1550 nm. The increase in the intrinsic dark count was suppressed to the higher bias current region and a long extrinsic dark count region was observed. The SDE curve achieved saturation before the intrinsic dark count increased. (b) SDE and DCR curves of the SWSPD without the HCCB structure. The intrinsic dark count increased from a relatively low bias current and limited device operation in the high bias current region.
Next, to improve the SDE, the SWSPD with the HCCB structure was integrated into a double-sided optical cavity [62] designed to enhance the optical absorption at a wavelength of 1550 nm. The NbTiN film thickness was reduced slightly to 5.6 nm in expectation of it having increased sensitivity to near-infrared photons. Figure 5 shows the bias current dependencies of the SDE and the DCR of the SWSPD with the HCCB structure when integrated inside the optical cavity. The inset shows a schematic of the cross-sectional structure of the SWSPD with the double-sided optical cavity. An SDE of ∼78% with a broad plateau region and a DCR of ∼80 cps were achieved at a temperature of 0.76 K. Because the SDE curve saturated at bias currents that were sufficiently lower than the intrinsic dark count rises, the system dark counts could be reduced further by reducing the extrinsic factor via appropriate filtering, e.g., using a cold optical filter [63]. In addition, the broad plateau region in the SDE curve indicates that the detector has high sensitivity, even for photons at much longer wavelengths. When the temperature increases, the intrinsic dark count rises at lower bias currents, thus limiting the maximum bias current at which the device can operate stably. However, even at a temperature of 2.2 K, the detector approaching saturation of the SDE curve achieved an SDE of ∼76% and a DCR of less than 100 cps.
Fig. 5. Bias current dependencies of the SDE and DCR of the SWSPD device with the HCCB structure when integrated inside the optical cavity. (a) SDE curves measured at temperatures of 0.76 K (red filled circles) and 2.2 K (blue filled triangles). The inset shows a cross-sectional view of the optical cavity structure that was designed to enhance absorption at the optical wavelength of 1550 nm. Photons with a wavelength of 1550 nm irradiated the device from the back side. (b) DCR curves measured at temperatures of 0.76 K (red open circles) and 2.2 K (blue open triangles). The SDE of ∼78% with the broad plateau region and the DCR of ∼80 cps were achieved at a temperature of 0.76 K. Although the intrinsic dark count begins to increase from the lower bias current region, the SDE curve approached saturation even at the temperature of 2.2 K, and an SDE of ∼76% and a DCR of less than 100 cps were achieved.
Figure 6(a) shows the SDE’s wavelength dependence within the range 1548 to 1552 nm when measured at a bias current of 2.7 mA and a temperature of 0.76 K. The variation in the SDE with a period of approximately 0.7 nm is due to optical interference induced between the back and front surfaces of the silicon wafer with the thermally oxidized layer. At wavelengths within the vicinity of the top of the variation curve, SDEs of ∼80% were achieved. We believe that further improvements in the SDE can be achieved via appropriate fine tuning of the optical cavity design and fabrication process. Figure 6(b) shows the SDE measured at a bias current of 2.7 mA and a temperature of 0.76 K while the polarization state of the photon input to the refrigerator system was varied. The polarization state of the photon was manipulated using a polarization control module consisting of a polarizer, a half-wave plate, and a quarter-wave plate. The SDE varied from a maximum of 78.7% to a minimum of 78.0% as the result of rotation of the half-wave plate from 0 to 90° and the quarter-wave plate from 0 to 45° in 5° steps. These results indicate that the detector has quite low polarization sensitivity.
Fig. 6. Wavelength and polarization dependencies of the SDE, and system timing jitter. (a) Optical wavelength dependence of the SDE when measured at a bias current of 2.7 mA and at a temperature of 0.76 K. The variation in the SDE was caused by optical interference induced between the back and front surfaces of the device wafer. (b) Input photon polarization state dependence of the SDE when measured at a bias current of 2.7 mA and at a temperature of 0.76 K. The polarization state of the input photon was manipulated by rotating the half-wave plate from 0 to 90° and the quarter-wave plate from 0 to 45° in 5° steps. (c) Timing jitter histogram measured at a bias current of 2.8 mA and a temperature of 0.76 K. The experimental data are plotted as red open circles and the red solid line shows the fitting curve plotted with the Gaussian function.
Finally, we evaluated the timing jitter performance of the SWSPD with the HCCB structure using a femtosecond pulsed laser operating at a wavelength of 1550 nm with a pulse duration of 100 fs and a 10-MHz repetition rate. The number of photons per pulse was set at 0.001. Figure 6(c) shows the timing jitter histogram when measured at a bias current of 2.8 mA and a temperature of 0.76 K. The full width at half maximum timing jitter of 29.8 ps was achieved using the 20-µm-wide SWSPD with the HCCB structure.
4. Conclusion
We have demonstrated a novel superconducting wide strip detector with side strip areas that have higher critical currents per unit width than the central area of the wide strip. Using this HCCB structure, the intrinsic dark count was suppressed effectively even in photon detectors with wider superconducting strips. As a result, by supplying sufficiently high bias currents, an SDE of ∼78% and a DCR of ∼80 cps were achieved using the SWSPD with the HCCB structure, which has a total strip width of 20 µm and is integrated into an optical cavity that is designed to enhance optical absorption at a wavelength of 1550 nm. The bias current dependence of the SDE showed a broad plateau region, indicating that the internal detection efficiency reached 100% at a 1550-nm wavelength and that the detector has high sensitivity, even for photons with wavelengths longer than 1550 nm. Because the strip width was sufficiently wider than the optical spot diameter, the detection efficiency was independent of the polarization state of the input photons. The SDE could be improved further by optimizing the optical cavity design and fabrication process. The system’s dark count could also be reduced further by reducing the extrinsic factor via appropriate filtering. We also demonstrated timing jitter of 29.8 ps using the 20-µm-wide SWSPD with the HCCB structure. This result suggests that by applying a sufficiently high bias current, low timing jitter could be achieved even for photon detectors with wider superconducting strips. These results open up the possibility of the development of ultra-wide SWSPDs with high detection efficiencies, low dark counts, and high temporal resolution, and will help to reveal the origin of the intrinsic dark count in current-carrying superconducting strips. These ultra-high performance SWSPDs, which can be mass-fabricated with high yields via photolithography, would accelerate the realization of future large-scale quantum information and communication technologies, which will require enormous numbers of ultimate-performance single-photon detectors.
Funding
Moonshot Research and Development Program (JPMJMS2066); Japan Society for the Promotion of Science (22H01965).
Acknowledgments
The authors thank Tomoya Minami for his assistance in performing device fabrication and measurements.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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