Superconducting nanowire single-photon detectors (SNSPDs), composed of meandering nanowires with widths under 100 nm and thicknesses on the order of a few nanometers, have been widely applied in various fields due to their superior detection performance.1^–10 However, owing to the relatively low superconducting critical temperature of conventional superconducting thin films, high-efficiency SNSPDs typically operate at temperatures below 2.5 K. Therefore, these SNSPD systems are commonly incorporated with Gifford–McMahon (GM) cryocoolers or dilution refrigerators. Due to the relatively large size, weight, and high power consumption (SWaP) of the cooling systems, it is difficult to deploy high-efficiency SNSPDs on airborne or spaceborne platforms, let alone on a drone-based versatile platform.11^–15 In addition to the cryocooler, the operation of SNSPDs on avionic platforms also requires reliable and stable electrical bias modules, amplification modules, and fast pulse counters with low power dissipation.15 Due to these requirements, currently, it is still out of reach to launch an SNSPD system on airborne or avionic platforms.

Recently, there has been a growing demand for high-efficiency SNSPDs that can be integrated into mobile systems.16^–20 For instance, SNSPDs have been successfully applied in light detection and ranging (lidar) systems, providing extraordinary spatial and temporal resolutions20^–22 due to their high detection efficiency and low timing jitter. Once the SNSPDs can be deployed on drone or airborne platforms, it would significantly facilitate recent airborne single-photon lidar systems.23 In addition, SNSPDs are highly desirable for deep space optical communication (DSOC) applications.16^,17^,24^–27 Traditionally, the data transmission between deep space and Earth is transmitted by radio waves, which in turn limits the communications and data transmission rate due to the bandwidth of radio waves. By encoding data in photons at near-infrared wavelengths rather than radio waves, it is expected to increase the data transmission rate by 1 or 2 orders of magnitude, completely revolutionizing the DSOC technology and facilitating deep space explorations. In the DSOC demonstration of the Psyche mission, an ultrahigh-definition streaming video was sent from Mars with a maximum data downlink rate of 267 Mbps, which is based on a 64-pixel SNSPD array.28 Due to the lack of SNSPDs that can be equipped on a spacecraft, the uplink rate, however, was significantly limited, leading to asymmetric space-to-ground communication.29

A feasible way to realize these avionic applications is to apply miniature liquid dewar to provide the necessary low-temperature operation environment, thereby overcoming the SWaP limitations. Therefore, high-efficiency SNSPDs that can be operated at liquid helium temperature become the keystone for drone-based applications.15 Generally, the operation temperature of SNSPDs is an overall effect between the critical temperature Tc and the photoresponse performance of superconducting nanowires. Despite the higher Tc of superconducting nanowires with lower disorder levels (a slight deviation from the crystalline peers with relatively low sheet resistance) or larger film thicknesses, the single-photon detection performance of these nanowires is actually suppressed by the relatively low photon energy transformation efficiency.30^–32 In superconducting nanowires with the low disorder, the critical temperature and the critical current can be significantly enhanced, but the photon energy is quickly transferred into the substrate due to the strong electron–phonon interactions, leading to relatively poor intrinsic detection efficiency (IDE).32^,33 To simultaneously enhance the IDE and operation temperature of SNSPDs, an effective way is to reduce the nanowire width.34 However, the fabrications of constriction-free narrow nanowires (sub-50 nm) with relatively large sensitive areas (with a diameter larger than 15μm) still remain challenging. Another method to improve the IDE at finite operation temperature is to fabricate more sensitive nanowires by increasing the disorder level.35 Anyway, the strong disorder can significantly suppress the superconducting properties of two-dimensional superconducting films, which may even drive the superconductor into insulators at the quantum critical disorder level.36 Consequently, the key to improving the operation temperature and detection efficiency of SNSPDs is to balance the disorder level and superconducting properties of superconducting thin films.

In this paper, we successfully demonstrated the fabrications of high-efficiency SNSPDs based on highly disordered NbTiN films. The NbTiN-SNSPD shows a system detection efficiency (SDE) of more than 90% operated at liquid helium temperature. To get rid of the conventional cryocoolers and build a lightweight detection system, we also designed and manufactured a miniature liquid helium dewar that is compatible with commercial drones and other airborne platforms. Incorporating with homemade electric setup for biasing and data storage, we successfully constructed a high-efficiency drone-based SNSPD system that is applicable for remote imaging and sensing, or long-distance quantum communications.

Based on the recently reported disorder-tuning method,35^,37 we further optimized the deposition conditions of NbTiN films to realize more sensitive superconducting nanowires. To characterize the highly disordered NbTiN films, we also characterized the NbTiN films with a transmission electron microscope (TEM). The NbTiN films on the substrate were first thinned to around 100 nm by a focused Ga ion beam. Then, the thinned film was characterized with a 200-keV TEM facility.38Figure 1(a) shows a TEM image for the 7-nm-thick granular NbTiN films, in which the average granule size of the resulting NbTiN films is around 5 nm. Moreover, some of the NbTiN granules are even close to the noncrystalline states [inset of Fig. 1(a)]. After the deposition of highly disordered NbTiN films on an alternative SiO2/Ta2O5 dielectric mirror (optimized for the telecom wavelength photons),39 the narrow nanowires are defined by a 100-keV electron beam lithography system using the positive ZEP520 e-beam resist, and the pattern was then transferred by dry etching using reactive ions of CF4 plasma, as shown in Figs. 1(c) and 1(d). To guarantee a sufficient coupling efficiency of the devices, the sensitive area of the detector is designed to be 18μm in diameter (which is 2 times larger than the beam waist of the fiber). To assure the sensitivity and yields of the resulting detectors, the nanowire width and pitch are still, respectively, designed to be 55 and 120 nm, leading to a filling factor of around 46%. The inner corners of meander turns are elliptically optimized to reduce the dark count rate,40^,41 as is shown in Fig. 1(d). Figure 1(b) presents the temperature dependence of resistance R(T) for the fabricated devices. The R(T) dependence of the NbTiN devices can be well described with the one-dimensional superconducting fluctuation mode,42 and the zero-resistivity critical temperature is around 8 K.

To characterize the detection performance of the fabricated SNSPDs at liquid helium temperature, we here first measured the SDE as a function of temperature in a GM cryocooler. The SDE was precisely determined with a standard optical attenuator and high-precision power meter, as described in detail in previous publications.4^,39Figure 2 presents the measured bias current dependence of SDE at temperatures of 2.2, 3, 3.5, and 4.2 K. Due to the increased disorder level, the critical current at 2.2 K is slightly lower than the previously reported devices with the same device configuration.15^,35 At 2.2 K, the detector shows a saturated SDE, with a maximum SDE of more than 95% for 1550-nm photons. With the increasing temperature, due to the gradually decreasing switching current, the maximum SDE is suppressed to 91.8% at 4.2 K. Compared with the previously reported SNSPDs operating at liquid helium temperature,15^,35^,43^,44 although the critical current is suppressed, the SDE, more than 90% of the time, demonstrates that the SNSPD system can be applied for building a drone-based or avionic high-efficiency single-photon detection platform without significantly degrading its sensitivity.

To realize a drone-based superconducting nanowire single-photon detection system that can be easily deployed, a commercial drone with finite size and power consumption was adopted as the airborne platform. Due to the finite space for mounting the whole SNSPD system, the SWaP of the cryostat that supports the necessitated low-temperature working environment for SNSPDs must be low enough. To this end, a miniaturized liquid helium dewar was designed and manufactured based on our previously developed mobile SNSPD system.15 For easy attachment to the drone, the dewar’s height and diameter were specifically designed to be 450 and 250 mm, respectively, with a maximum helium capacity of 3 L and an overall weight of ∼12kg. To ensure a relatively long operational time, an intercalation shielding layer made of thin aluminum is connected to the neck of the inner liquid helium vessel and is positioned in the vacuum space between the inner and outer vessels, despite the limited space. In addition, 45 layers of multilayer insulation were wrapped around the inner vessel wall to further enhance insulation. To minimize conduction heat loss along the neck of the inner vessel, a specially designed bellows tube was welded between the bottom and top of the neck. The combined weight of the inner vessel and the intercalation shielding layer is less than 2 kg.

For the installation of SNSPDs, we designed a dipstick made from a glass fiber-reinforced plastic tube with baffles and polyurethane foam that is able to install two SNSPDs simultaneously. It is noteworthy that the baffles and polyurethane foam were strategically positioned at a height that aligns with the neck of the inner vessel. This arrangement helps to effectively distribute the helium vapor and minimize radiation and convection heat loss. To reduce the heat load from the room temperature end and maintain constant pressure inside the inner vessel, a leakage capillary tube for the evaporated helium gas was welded onto the dipstick.

With respect to the bias and readout circuit of the SNSPD system, we here designed and manufactured an integrated electrical setup that combines the bias circuit, amplification module, pulse-counting module, and data storage module. Figure 3 shows the manufactured electric setup, which is power supplied by two NCR18650A lithium-ion batteries. The bias current on the SNSPD is generated by the Vbias through an adjustable resistor (100 kΩ), where the programmable Vbias is modulated by the main control unit (MCU) and a digital-to-analog converter (DAC). The detection signal from the SNSPDs is amplified by two low-power amplifiers, with a total gain of more than 50 dB. Then, these pulse voltage signals are converted into transistor–transistor logic signals by a high-speed comparator, which either can be output to a standard counter or be directly counted by the timer module of the MCU. Finally, the bias current dependence of the SDE, or the counts at a fixed bias current, is saved in a text file. Figure 3(b) shows the manufactured electric setup, which is 150 mm in length and 100 mm in width. With fully charged lithium-ion batteries, the electric setup is able to sustain the bias and readout setup for more than 30 h, with a total weight of around 1 kg.

Based on the manufactured miniature liquid helium dewar, we here constructed the drone-based SNSPD system based on the T40 drone (AGRAS T40, DJI agricultural drone), as shown in Fig. 4(a). First, two aluminum arms were fixed on the drone, and the dewar was then mechanically mounted on the two arms. The electrical setup for testing and data recording was also fixed on the aluminum arm. After the establishment of the whole system, we then compared the measured bias current dependence of SDE on the ground for the drone-based SNSPD system with the measured SDE from standard measurement setups, as shown in Fig. 4(b). The SDE in the liquid helium dewar is very consistent with that measured in the GM cryocooler, demonstrating the robust detection performance of the NbTiN SNSPDs. Moreover, the completely consistent optical and electric response also demonstrates the reliable electrical supports of the homemade SNSPD electrical measurement setup.

To characterize the detection performance of the drone, we here measured the intrinsic detection performance of the system because, currently, a receiver is absent from the drone. The SNSPDs are illuminated with a semiconductor laser diode with a fixed attenuation factor, where the laser source is also mounted on the drone. To exclude the impact of high environment noise count rate (ENCR) on the IDE, the attenuation factor was set to ensure that the photon count rate (PCR) exceeded 1.5 million counts per second, roughly 3 orders of magnitude more than the ENCR, ensuring the accuracy of the IDE, which is defined by IDE=(PCR−ENCR)/PCR. The drone-based SNSPD system was then launched and hovered at an altitude of 30 m from the ground (which is limited by the local statutory regulations in Shanghai), and the IDE was simultaneously measured under both flight conditions and hovering conditions. The measured IDE as a function of bias current is presented in Fig. 4(b), which also coincides well with the measured data on the ground, further demonstrating the reliability and stability of the NbTiN detector and the drone-based SNSPD system.

The most significant challenge for the operation of the drone-based SNSPD system is the relatively high environmental noise. Figure 4(c) compares the bias current dependence of the noise count rate under different operating conditions. In a dark operation environment, the ENCR of the drone-based SNSPD system is consistent with the dark count rate (DCR) operated in a standard GM cryocooler, where the ENCR was measured by blocking the fiber and turning off the laser source. The ENCR is around 1 cps when the bias current is relatively low and increases exponentially in the intrinsic regime. However, under a bright sunlight environment with a ground temperature of more than 50°C, the ENCR was increased to around 10,000 cps, which is due to the strong stray photons and the blackbody radiation from the high-temperature terminal of the optical fiber. It is also interesting to note that when hovering the drone in a shadow environment (where the bright sunlight is shaded occasionally by the cloud), the ENCR was then effectively reduced by an order of magnitude. As a consequence, by optimizing the design of the receiving and coupling components of the drone-based SNSPD system, the ENCR can be significantly lowered, even when operating the drone-based SNSPD system at a relatively high altitude with strong sunlight.

Finally, to further improve the detection performance of a drone-based SNSPD system, one possible way is to continuously optimize the nanowire geometrics, where the superconducting properties can be optionally tuned. Generally, the system detection efficiency is the overall product of photon coupling efficiency, absorption efficiency, and intrinsic detection efficiency. By slightly increasing the sensitive area to 23μm in diameter, the photon coupling efficiency is expected to be near unity.4 Moreover, by slightly increasing the film thickness to improve the absorption efficiency, the system detection efficiency and the operation temperature are expected to be further improved. Beyond, by optimizing the nanowire width and the filling factor of SNSPDs, the intrinsic detection efficiency and the photon absorption efficiency can be simultaneously enhanced, which in turn refines the overall system detection efficiency. With respect to the relatively high environmental noise counts, it is necessary to technically filter out the strong stray photons and the blackbody radiation from the high-temperature terminal of the optical fiber. To lower the coupling of stray photons, an effective way is to replace the applied cladding layer-free SM-28E fiber with armored fiber at the receiving and coupling components. Moreover, the transmission of blackbody radiation-coupled photons can be significantly suppressed by applying a narrowband pass filter. Namely, only the signal photons are able to transmit the applied narrowband pass filter, with a typical passband width of ∼40nm in the 1550-nm telecom band and a peak transmittance of over 0.98.45 Such a narrowband pass filter can be deposited either directly on the SNSPD chips or on the end face of the fiber. Concerning the miniature liquid helium dewar, currently, most of the weight is still concentrated on the steel outer vessel, which provides the necessary strength against harsh operating environments. The weight can be further reduced by applying spaceflight aluminum–titanium alloy.

In summary, we successfully constructed a commercial drone-based superconducting nanowire single-photon detection system. By optimizing the deposition condition and nanofabrication process, we fabricated NbTiN-SNSPDs with near-saturated system detection efficiency. At liquid helium temperature, the system detection efficiency is as high as 91.8%. Furthermore, a specially designed miniature liquid helium dewar is manufactured, which can be directly incorporated into the T40 drone. To resolve the readout and data storage of SNSPDs, we also designed and manufactured special electrical setups. The whole system except for the drone is power-supplied by only two lithium batteries. Such a high-efficiency drone-based SNSPD system is promising for applications of three-dimensional high-resolution imaging and sensing or avionic quantum communications. Moreover, such lightweight and low-power systems are even able to be installed on spaceborne platforms, further facilitating deep-space optical communications and space explorations.

Biographies of the authors are not available.