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Abstract
We developed a mobile superconducting strip photon detector (SSPD) system operated in a liquid-helium Dewar. By adopting highly disordered NbTiN thin films, we successfully enhanced the detection performance of superconducting strips at higher operation temperatures and realized SSPDs with nearly saturated detection efficiency at 4.2 K. Then we customized a compact liquid-helium Dewar and a battery-based electronic module to minimize the SSPD system. A mobile SSPD system was integrated, which showed a system detection efficiency of 72% for a 1550 nm wavelength with a dark count rate of 200 cps and a timing jitter of 67.2 ps. The system has a weight of 40 kg and a power consumption of 500 mW, which can work continuously for 20 hours. The metrics can be further optimized in accordance with the various practical application platforms, such as aircraft, drones, etc.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High efficiency superconducting strip photon detectors (SSPDs) have been widely applied in broad fields for both quantum and classical realms, such as loophole free tests for local realism [1], quantum key distribution (QKD) [2–4], optical quantum computing [5,6]. To achieve the maximum system detection efficiency (SDE) as high as 98% [7–10], the operation temperatures of SSPDs are usually 0.7-2.2 K, which were obtained by using two or three-stage mechanical cryocoolers. The size, weight, and power consumption (SWaP) are evident drawbacks of SSPD systems compared with their semiconducting counterparts [11]. Despite several successful attempts have been carried out by adopting space-compatible miniatured cryocoolers, the resulting cryocoolers are rather expensive and yet not commercially available [12–15].
Indeed, there are some interesting applications, such as aviation platform-based lidar and QKD [16,17], which need a mobile detection system for limited operation time. One option is to adopt liquid-helium Dewar instead of the mechanical cryocooler. By getting rid of heavy cryocooler, high efficiency SSPDs at 4.2 K (boiling point of liquid helium at ambient pressure) would enable vast potential applications in a series of fields [13,18–24]. However, the typical SSPDs had a much lower efficiency at 4.2 K. To improve the efficiency, SCONTEL has applied Dewar-based superfluid Helium to obtain a base temperature of 1.7 K. Despite this solution improves the detection efficiency of SSPDs, it also makes the system applicable for lab usage only [25]. To enhance the detection performance, Gourgues et al. has investigated the detection performance of SSPDs based on thicker superconducting NbTiN thin films, and a detection efficiency of 64% is realized at 4.2 K in a Gifford–McMahon (GM) refrigerator [26]. More recently, He et al. has also fabricated 7-nm-thick NbN-based SSPDs for 1064 nm photons. However, the devices only possess a limited detection performance, with an intrinsic detection efficiency (IDE) of 50% at 4.2 K for 1064 nm photons in a GM cryocooler [27].
Therefore, the main challenges to construct a mobile SSPD system relies on the fabrication of high efficiency SSPDs with an operation temperature at 4.2 K and the customization of compacted liquid helium Dewar. Moreover, to maximally reduce the power consumption, it also necessitates solid battery-based electronic modules. In this express, we present the design and manufacture of a mobile SSPD system with high detection efficiency. The SSPDs with high SDE and high operation temperature are realized with extremely disordered NbTiN films, showing an SDE more than 70% at 4.2 K. To realize a cryostat with limited SWaP and reliability in complex environment, we manufactured a miniature liquid helium Dewar and lithium-ion battery-based electronic modules. The constructed mobile SSPD system shows a SDE of 72% and a DCR of 280 cps.
2. High efficiency SSPDs with operation temperature of 4.2 K
To constitute a liquid helium based mobile SSPD system, the key is to fabricate high efficiency SSPDs with operation temperature up to 4.2 K. Intuitively, by adopting thick superconducting strips with higher critical temperature, it would improve the transition temperature of SSPD [26]. However, since the photon energy converting efficiency of superconducting strips is highly dependent on film thickness [28], the saturation plateau for 1550 nm photons of such detectors is actually not enhanced. Recently it has been experimentally demonstrated that superconducting strips with higher disorder level (larger resistivity or sheet resistance) exhibited excellent photon-response performance [29–32]. Moreover, superconducting strips with narrow strip width will further facilitate single photon detection performance of SSPDs. SSPDs with narrower strip width, however, are susceptible to the constrictions introduced by the nano-fabrication process, which in turn suppresses the detection performance and increases the dark count rate of SSPDs [33,34]. We herein fabricated SSPDs based on a highly disordered 7-nm-NbTiN films, with a normal state sheet resistance of 618 Ω. Compared with NbN, The NbTiN based SSPDs had higher critical current density but lower dark count rate [35,36], guaranteeing the NbTiN SSPDs can be operated at higher temperatures. The fabricated NbTiN SSPDs had a strip width and period of 55 nm and 120 nm, respectively, leading to a fill factor of 46%, as it is shown in the inset of Fig. 1(a). The nanowires show an excellent uniformity of strip width, and the inner corners of the 180-degree turns are optimally rounded to maximally reduce the current crowding effect [37–40]. The current-voltage characteristic of the resulting device is presented in Fig. 1, showing a switching current ${I_{SW}}$ and returning current ${I_r}$ of 9.86 µA and 1.46 µA, respectively. Due to the higher disorder level, the ${I_{SW}}$ is lower than our previous publications [29].
Fig. 1. (a) The current-voltage characteristic for the 55-nm-wide SSPD at 2.2 K. Inset: a scanning electron microscopy image of the SSPD. (b) The SDE and DCR as a function of bias current at temperatures of 2.2 K, 3 K, and 4.2 K, respectively. The red solid lines are based on sigmoid curve fitting for SDE curves at 2.2 K and 4.2 K, respectively.
Figure 1(b) shows the detection performance of the resulting SSPD working in a GM refrigerator at temperatures of 2.2 K, 3 K, and 4.2 K, respectively. At 2.2 K, the SDE curve shows a maximum SDE around 80% for 1550 nm photons, presenting a saturated intrinsic detection efficiency with a broad plateau of approximately 2.5 µA. The SDE curve terminates at a bias current ${I_b}$ of 9.2 µA, where the dark count rate (DCR) is still dominated by the stray photons. At higher operation temperature of 3 K, the ${I_{SW}}$ is lowered to around 8 µA. and the SDE curve shows a similar bias current dependence with T = 2.2 K, with a reduced saturation plateau around 1 µA, however. At liquid helium temperature, the ${I_{SW}}$ is suppressed around 6.4 µA, and the SDE curve shows a weak saturation behavior, with a maximum SDE more than 75%. Based on the sigmoid curve fitting for SDE curve, the IDE (neglecting the photon coupling losses) of the 55-nm-wide detector is found to be around 95.7%. Despite the SDE can still reach around 79% at a bias current of 6.1 µA, the DCR is then beyond 1000 cps, which is therefore not shown in the figure.
3. Manufacture of the miniature liquid helium Dewar system
To realize a cryogenic system with high reliability, long operation life time and limited SWaP, we here designed and manufactured a miniature liquid helium Dewar to construct the mobile SSPD system. To achieve a relatively long operation time, we firstly designed and manufactured a three-layer miniature liquid helium Dewar, namely the outer vessel, the intercalation shielding layer, and the inner liquid helium vessel. The materials of the inner vessel and outer vessel are 316 L stainless steel and 304 stainless steel, respectively. The intercalation shielding layer is connected to the neck of the inner liquid helium vessel, where the temperature is estimated to be below 100 K. To reduce the thermal radiation from the outer vessel, a radiation shield of thin aluminum foil was further added between the inner vessel and outer vessel on the intercalation shielding layer. The inner liquid helium vessel is designed to withstand a pressure of 1.5 bar and the bending stress. Therefore, the inner vessel shell was constituted by a cylindrical vessel with a minimum thickness of 0.5 mm and hemispherical heads. With respect to the outer vessel, a wall thickness of 3 mm and two hemispherical heads are adopted due to the collapse pressure. The vacuum between inner and outer shell was designed to be lower than 10−5 mbar, and a one-way vacuum valve was used to sustain the high vacuum inside, which is able to keep the vacuum for more than three months. The manufactured liquid helium Dewar is 800 mm in height and 400 mm in diameter, in which the total designed helium volume is 10 L, and a 10 percent ullage volume was considered, as it is shown in Fig. 2(a). The evaporation time of liquid helium for the current system is measured to be around 31 h, and the operation time with loaded samples is measured to be more than 20 h.
Fig. 2. (a) The manufactured liquid helium Dewar, dipstick for sample-loading, and the loaded sample. (b) The manufactured lithium-ion battery based electric component.
With respect to the sample installation, in order to reduce the heat load from the room temperature end, a dipstick made of glass fiber reinforced plastic tube with low thermal conductivity is adopted, as it is shown in Fig. 2(a). Moreover, to further decreasing the evaporation of liquid helium, several baffles are applied near the neck of the inner vessel to distribute the helium vapor flow, and the polyurethane foam is inserted between the baffles to reduce heat leak due to the convection of helium gas. Currently, the SSPD enclosed in the homemade sample holder is directly fixed on to dipstick, with the optical fiber and the coaxial cable lying inside the glass fiber reinforced plastic tube, as it is shown in Fig. 2(a). In the future, by removing the dipstick, more than ten SSPDs with different characteristic wavelength will be directly installed on the bottom of the liquid helium vessel to further facilitate the applications of the mobile SSPD system. To reduce the power consumption of the system, we here designed the bias-tee and amplification modules by using a lithium-ion battery.
The bias current is generated by the ${V_{\textrm{bias}}}$ through an adjustable resistor. The amplification module is constituted by two low noise high gain amplifiers, with a total gain more than 45 dB. The total power consumption of the two-stage amplification module is of 500 mW. Both the ${V_{\textrm{bias}}}$ and the ${V_{\textrm{CC}}}$ are fed through an isolated voltage regulator chip to maximally reducing the noise level, which are all enclosed in an aluminum box. Figure 2(b) shows manufactured electric module, which is power supplied by a single NCR18650B lithium-ion battery. With fully charged lithium-ion battery, the bias-tee and amplification module are able to sustain for more than 48 hours. The total weight of the manufactured electric module is 115 mm in length and 60 mm in width, with a total weight around 0.3 kg.
4. Detection performance of the mobile SSPD system
Based on the homemade Dewar, we here characterized the detection performance of the whole mobile SSPD system. The detectors, enclosed in a cooper holder and coupled with optical fiber, are directly merged into the liquid helium. To reduce the dark counts from the black-body radiation from the room temperature terminal of the optical fiber, the fiber is circled in diameter around 50 mm, as it is shown in Fig. 2(a). Figure 3(a) shows the bias current dependence of SDE and DCR for a 50-nm-wide SSPD with a switching current of 4.4 µA, in which an SEM image of the detector is presented. Due to the slightly lower filling factor, the maximal SDE of the device is limited around 72%. Owing to the narrower strip width, the detector shows a more saturated SDE curve as compared with the 55-nm-wide device shown in Fig. 1(b). According to the maximum SDE values and the saturated SDE value obtained from the sigmoid curve fitting, the intrinsic detection efficiency is estimated to be around 98%. It is also worth noting that the DCR of the detector is still out of the intrinsic exponential growth region, with a near constant DCR around 200 cps, which is due to the stray photons that coupled through the fiber.
Fig. 3. The bias current dependence of the system detection efficiency and dark count rate for a 50-nm-wide SSPD measured with the liquid helium Dewar. Inset: a scanning electron microscopy image of the SSPD.
Furthermore, we also characterized the timing jitter and the recovery time of the detector at 4.2 K. The timing jitter for the device is found to be around 67.2 ps at 4.2 K, with a normalized bias current of ${I_b} = 0.9{I_{SW}}$ for 1550 nm photons [41]. Despite of an extremely disordered NbTiN strip and a comparatively low operation current, a relatively low timing jitter is still realized, which will further facilitate the real applications of our mobile SSPD system. With respect to the recovery time, by capturing detection signal with an oscilloscope, we have compared the recorded voltage signals as a function of time at various temperatures. Despite the amplitude at different temperatures is bias current dependent, by normalizing the curves, it was found that both the rising time of the pulse and the recovery time of the device are not temperature dependent. The normalized signal pulses coincide with each other, resulting in a recovery time of 43.6 ns.
5. Discussion
It is noteworthy that the SDE of the involved devices can be further improved. Currently the SDE of the devices are partially suppressed by the relatively low filling factor and small sensitive area. For a strip width of 50 nm and a pitch of 120 nm, the calculated absorption efficiency is 85.8%. However, the measured SDE of 72% is still lower than this value, which might be attributed to the non-ideal material and fabrication induced photon losses. Moreover, the sensitive area of the devices is 15 µm, which is lower than the previously applied high efficiency detectors of 23 µm [7]. Despite narrower nanowires below 50 nm may further improve the sensitivity of SSPDs, it will then also introduce more constrictions on the nanowire. Moreover, the nanofabrication for such narrow wires with high filling factor is also rather challenging. By further increasing the sensitive area and optimizing the geometry of detectors, SSPDs with SDE up to 90% for 1550 nm photons would be achieved at 4.2 K.
With respect to the size and weight of the Dewar, note that the currently manufactured Dewar is relatively large and heavy, this is due to a designing consideration that the position of the intercalation shielding is adjustable to figure out the optimal location for further reducing the evaporation rate of liquid helium. Currently, the Dewar shown in Fig. 2(a) is able to be disassembled through these clamps, and sufficient spaces are left for adjusting the positions of the intercalation layer. Through the removal of these unnecessary parts from the helium Dewar, the total weight of a miniature system can be further reduced to 26 kg, as listed in Table 1. Moreover, the diameter of the Dewar can also be reduced from 400 mm to 325 mm. Based on the current experimental data, a relationship among the size, weight, and operation time can be estimated based on simulations from ANSYS Fluent 18.0, as summarized in Table 1. Aiming at different application scenarios, we are able to flexibly design the size and weight of the Dewar based on the necessitated operation time, and customize compact and miniature high efficiency SSPD system for specific applications.
Table 1. The estimated size, weight, and liquid helium volume dependent sustaining time and operation time
6. Conclusion
In summary, we successfully demonstrated the design and manufacture of liquid helium Dewar based mobile SSPD system, which shows a system detection efficiency more than 72% and timing jitter of 67.2 ps for 1550 nm photons. By further optimizing the SSPDs and customize miniature liquid size with specified size, more compact mobile SSPD systems with SDE more than 90% can be manufactured. Such a mobile SSPD system is highly promising for the avionic applications based on airborne platform, such as single photon quantum Lidar or mapping of complex landform.
Funding
Shanghai Sailing Program (21YF1455700).
Acknowledgments
The fabrication was performed in the Superconducting Electronics Facility (SELF) of SIMIT, CAS.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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.
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