Air quality directly impacts our health and the health of the planet, yet it remains a matter of global and individual concern. Today, 91% of the world’s population live where atmospheric pollution reaches harmful levels and the average global temperature is on the rise, changing the climate in yet unknown ways. Cities are at the centre of the issue as they accommodate 55% of global population and account for 70% of the total carbon dioxide emission, the main contributor to global warming.

To act efficiently, governments and regulatory bodies must quantify the problem with live, precise and continuous atmospheric measurements that capture entire cities. However, there is no sensor currently capable of undertaking this task. Here, we propose to develop a Single photon sensor for Mid-Infrared Lidar (SMIL), the first step towards realising an atmospheric imager that covers a whole city with a single device in real-time.

A ground-based light detection and ranging (lidar) presents a unique opportunity to fulfil this task as it offers live 3D imaging and capabilities of full atmospheric monitoring, including arbitrary gas molecules, dust particles (aerosols) and wind and temperature sensing. Despite being versatile, the performance of lidar is bound to the performance of its photodetector. Today the challenge of imaging the composition of kilometre-scale urban atmosphere remains outstanding, due to a lack of high-performance photodetection systems operating at mid-infrared (mid-IR) wavelengths (i.e. 2 – 10 μm). This spectral range is crucial to gas sensing because it contains fundamental vibrational absorption bands of most molecules.

This project will unlock the mid-IR spectral range to efficient and low noise detection at the single-photon level for the first time by employing superconducting technology optimised for 2 μm radiation. We will develop a high-performance superconducting nanowire single photon detector (SNSPD) with over 80% detection efficiency, low noise levels below 100 c/s, high detection rates of 20 Mc/s, and high temporal resolution better than 100 ps. This new generation of superconducting detectors will challenge the leading position of semiconducting HgCdTe avalanche photodiodes, currently dominating high performance mid-IR detection. The specifications of 2 μm SNSPD imply at least 3 orders of magnitude improvement in signal-to-noise detection ratio and overall sensitivity allowing for further, faster and more precise lidar measurements. We will build a table-top CO2 gas sensing lidar and conduct tests in controlled laboratory environment. We aim to achieve sensing performances equivalent to CO2 concentration sensitivity of 1 part per million (ppm), 5 km maximum ranging distance, 10 m depth resolution, and 1 min acquisition time.

The combination between mid-IR SNSPDs and lidar technologies has never been exploited in gas sensing applications. This synergy offers a unique opportunity for advancing current capabilities of remote sensing by realising the first city-wide live atmospheric imager and has a great potential to pave the way for new applications in environmental science, chemistry, industry and medicine.