In the past few decades, driven by the development of quantum photonics science and technology, detecting single photons with high efficiency and high accuracy in time has become the essential requirement for many game-changing applications
Since 2001, a new type of single-photon detector called superconducting nanowire single-photon detectors (SNSPDs) emerged as the leading technology in single-photon detection due to its unprecedented high efficiency and timing accuracy.
Detecting longer wavelength photons (for example the 2-4 μm mid-infrared range) is highly important for applications including infrared fluorescence and spectroscopy, planetary soil studies, remote light detection and ranging, as well as mid-infrared two-photon entanglement and interference, thus detecting mid-infrared single-photons are highly important to the SNSPD community. At the same time, long-wave photons have lower energy and are therefore more difficult to detect.
Previously, -Low energy gap superconductors, e.g. WSi was used for mid-infrared single-photon detection The price to this, is that such devices need to be operated at almost absolute zero degree temperature (millikelvin range)! Such fridges are complex and usually very expensive. Also, WSi based SNSPDs usually show worse timing performance.
As a comparison, more economic Gifford-McMahon (GM) Cryocoolers are several times less costly, but their base temperature is around 3 Kelvin. How to achieve mid-infrared detection in a GM coolers using conventional superconductors with high critical temperature has become an urgent challenge for researchers.
Recently, researcher from Delft University of Technology (the Netherlands), the Dutch commercial company Single Quantum B.V, as well as researcher from KTH (Sweden) use optimized NbTiN to achieve mid-infrared detection in a GM coolers. The research results are published in Photonics Research, Volume 10, No. 4, 2022 (Jin Chang, Johannes W. N. Los, Ronan Gourgues, et al, "Efficient mid-infrared single-photon detection using superconducting NbTiN nanowires with high time resolution in a Gifford-McMahon cryocooler," Photon. Res. 10, 1063-1070 (2022)).
This work employed optimized polycrystalline NbTiN superconducting film with reduced thickness and better film quality, even at ~2.5 K in a GM cooler, the detectors from such superconducting films showed >70% system detection efficiency at 2000 nm and >80% internal efficiency at 4000 nm. The researcher also systematically studied the dark count (false clicks) origins in such detectors and offered the best available mid-infrared SNSPDs in economic GM coolers.
"Until recently the area of mid-infrared SNSPDs was limited to low-gap superconductors. I am very happy that we could show excellent performance from NbTiN detectors in a conventional GM cryostat" says Dr. Esmaeil Zadeh, assistant professor in TU Delft, who supervised the project; "the main question now, is where is the limit to this? How far can we still push this? We know the fundamental limit is 2-times the bandgap of our superconductor but that is still a factor of ~100 further than where we stand today; it will be very exciting to see if that limit can be achieved both from a fundamental point of view but also for the numerous applications which are out there."
Dr. Jin Chang from TUDelft mentioned: "Currently, superconducting single-photon detection technology is becoming more and more mature, and opening the window of mid-infrared single-photon detection will enable a series of revolutionary quantum optics applications. The exploration of the limit of long-wavelength single-photon detection will help us understand the working principle of superconducting nanowires from the underlying physical level.
In the future, we hope to use superconducting nanowires to achieve the detection of single photons at long-wave infrared and even terahertz frequencies. In these bands, high-quality single-photon detectors will bring a great improvement in cosmic astronomical observations, and we will obtain information such as the origin of the universe, but this will require more dedicated research and exploration in the future."
Prof. Val Zwiller from KTH, Sweden comments on this work as follows:
"Opening the infrared and even the mid-infrared to single-photon detection with high efficiency and signal-to-noise ratio will enable a wide range of experiments and applications. In KTH Stockholm, we will use these detectors in quantum microscopes to image deep in tissue as well as in environmental monitoring to generate live 3D maps of atmosphere pollution over a whole city. The future holds exciting innovations: we will open the mid-infrared range for quantum optics and will explore the ultimate limit for single-photon detection in wavelength."
Figure.1 State-of-the-art mid-infrared SNSPD system: (a) close-cycle GM cooler, (b) 2.4K stage inside cryostat, (c) an SNSPD mounted on PCB, (d) single SNSPD after fabrication on Si wafer, (e) SEM image of meandered nanowire and (f) system efficiency over 70% at 2 μm wavelength.