Superconducting single-photon detectors get sub-millikelvin temperature resolution

In the science fiction movie ‘Predator’, the alien uses an advanced infrared thermal camera to get super night vision and to locate Dutch's hiding area accurately. Any object whose absolute temperature is not zero will produce infrared radiation. There is an infrared radiation difference between two objects with different temperatures, which is detected and resolved by thermal detection and imaging technology. Nowadays, high-resolution infrared thermal detection and imaging technologies have important research and market value in many fields, such as forest fire warning, industrial non-destructive testing, public security, and military defense. Some extreme domains look forward to high-resolution infrared thermal detection for remote sensation and accurate location as the development of time and technology. For example, satellite ocean remote sense conducts wide-area monitoring of ocean temperature changes (such as small temperature differences on the sea surface caused by underwater targets), which is an important part of supporting marine environmental information assurance and retrieving global climate models. The temperature resolution of thermal detectors must be on the order of 1 mK if temperature changes in the ocean will be resolved accurately [2]. In these frontier fields, the targets usually have extreme characteristics, including long distance or small size. For example, a thermal radiation power received is calculated on the order of pW according to Planck's law if the infrared system with an optical aperture of 0.2 m is used to detect a room temperature target with a size of 1 m and a distance of 100 km. It is urgent to develop a new thermal detection technology with high sensitivity and low noise in such ultra-low-light scenarios.

A highly sensitive infrared detector is one of the basics for detection with high temperature resolution. In the existing commercial infrared detectors, cooled detectors can obtain higher temperature resolution than uncooled detectors, but the temperature resolution of these detectors is still distributed in 20-40 mK due to noise and low sensitivity. Single-photon is the basic unit of optical energy, and the realization of single-photon detection implies the highest sensitivity of a photodetector. Superconductors have the intrinsic advantages of higher sensitivity and lower noise compared with semiconductors, opening up new avenues for limited detection in extreme scenarios. Superconducting nanowire single-photon detectors (SNSPDs) have a low superconducting energy gap (< 5 meV), and their detection spectra can range from X-ray to long-wave infrared, providing a theoretical basis for high temperature resolution detection.

The research group proposed a new thermal detection technology with high temperature resolution based on the optimized design and fabrication of high-sensitivity and low-noise infrared SNSPD. The SNSPD obtained sub-mK temperature resolution in spite of pW absorbed radiation power and provided technical support for high-resolution thermal detection and imaging in ultra-low-light scenarios.

The research group analyzed the temperature-resolving model based on photon counting. The noise equivalent temperature difference (NETD) is a key technical parameter to estimate the temperature resolution of an infrared detector, which defines the minimum temperature difference to be resolved. The smaller the NETD, the higher the temperature resolution. Conventional semiconductor infrared detectors (such as mercury cadmium telluride, quantum dots, and vanadium oxide bolometers, etc.) output analog signals (such as photovoltage or photocurrent). In contrast, SNSPD is a photon counting detector, and its detection mechanism can be described as follows: superconducting nanowires with biased current will undergo phase transition after absorbing a photon, jumping from a superconducting zero-resistance state to a normal high-resistance state. The sudden change of the resistance will eventually result in a detectable electrical pulse (Figure 1). When the target temperature changes ΔT, the change of photon counting rate C_R of the SNSPD is defined as ΔC_R. The noise of SNSPD is shot noise, which mainly includes the background counts caused by the photon noise and the intrinsic dark counts. There is a relationship between the root-mean-square noise dn and the integration time τ: δn ∝ 1/τ^0.5. Thus, an integral time-dependent temperature-resolving model is obtained: NETD = (1/τ)^0.5ΔT / (ΔC_R / δn)

The research group measured the temperature resolution of SNSPD to blackbody sources with different temperatures (the temperature T_b distributed between 400 and 1000 K) (Figure 2). It was found that the intrinsic NETD of SNSPD can be lower than 0.1 mK based on the low intrinsic dark count rate, demonstrating the high temperature resolution of the superconducting single photon detection schemes. In addition, the total noise of SNSPD is dominated by the background count caused by photon noise in the system measurement, which has good temporal stability and follows Poisson distribution. When T_b = 400, 800, and 1000 K were set, the corresponding NETDs are 1.93 mK, 1.76 mK, and 1.02 mK at 10 s integration time, respectively. Research shows that the NETD can continue to break through to the sub-mK level after further system optimization. For example, NETD = 0.65 mK was obtained after system optimization and T_b = 600 K, which was shot noise-limited. The continuous increase of τ will further reduce the NETD. For example, when τ is increased by 4 times, the NETD can be reduced by 2 times. This has advantages in high temperature resolution detection of static targets. In addition, the research group also found that the SNSPD was less affected by the circuit and maintain a stable minimum NETD in a wide current-bias band, indicating the high anti-interference ability. Notably, the radiation power absorbed by the SNSPD was distributed between 0.1 and 1 pW when detecting the blackbody with adjustable temperatures. This work focused on the verification of high temperature resolution of the SNSPD and has not yet conducted in-depth and comprehensive research on the complex system and environmental noise faced in practical applications. These will be addressed in the future works.

In summary, the research group proposed a new thermal detection technology based on the SNSPD with high temperature resolution. The NETD could still reach the shot noise limit at pW absorbed radiation power and breaks through to less than 1 mK. This work is expected to solve the core detector problem of achieving high-temperature-resolved detection in frontier extreme scenarios.

Qi Chen, Fei Zhou, Chen Wei, and Yue Dai contributed equally to this work. Both the professor Labao Zhang of Nanjing University and the researcher Haiyong Gan of the National Institute of Metrology, China, are co-corresponding authors. Academician Peiheng Wu of Nanjing University gave in-depth guidance to this work. This work was supported by the National Natural Science Foundation of China, the Innovation Program for Quantum Science and Technology, and the Natural Science Foundation of Jiangsu Province.

 

Reference:

[1] Chen et al. Sub-millikelvin-resolved superconducting nanowire singlephoton detector operates with sub-pW infrared radiation power. Nat. Sci. Rev. (accepted).

[2] Fulton et al. Low Coseismic Friction on the Tohoku-Oki Fault Determined from Temperature Measurements. Science, 342, 1214-1217 (2013).

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