FEATURE QUANTUM SENSING
Greenhouse gases feel the squeeze
Combining quantum mechanics and photonics could greatly enhance sensing systems being used in environmental monitoring. The University of Bristol’s Dr Alex Clark explains a novel such sensing method to Abigail Williams
A
research team at the University of Bristol is working on a novel quantum sensing
method with the potential to both significantly improve the detection of greenhouse gases in the atmosphere and provide environmental scientists with highly accurate data. The new method builds on – and overcomes the limitations of – an earlier frequency comb technique, which was developed by 2005 Nobel laureates in physics John Hall and Theodor Hänsch, by exploiting a quantum state with reduced noise called ‘squeezed light’. Electro Optics talked to Dr Alex Clark, Senior Lecturer and Royal Society University Research Fellow in the Quantum Engineering Technology Laboratories at the University of Bristol, to find out
more about the new sensing method and the photonics- based spectroscopy technology it uses, as well as about its potential role in improving environmental monitoring and research about the sources and scale of greenhouse gases.
Can you outline the development of your proposed new quantum sensing method for greenhouse gas detection? Here at the University of Bristol in the Quantum Engineering Technology Laboratories, or ‘QET Labs’ for short, we are working on combining the principles of quantum mechanics with optics and photonics to enhance sensing and imaging systems. We can use pulses of light and fast single-photon detection to measure the distance to plumes of gases; we can generate beams of light with correlated intensity for more sensitive measurements, and we can use interferometric techniques to even detect mid-infrared gas absorption while only ever detecting visible light.
Dr Alex Clark 16 Electro Optics May 2024
What photonics-based spectroscopy technology does – or would – the method use? Photonic technology is at the heart of many of our experiments. We develop enhanced light sources based on nonlinear optical interactions such as spontaneous parametric down-conversion or spontaneous four-wave mixing. In both of these processes, photons from a bright laser
An integrated silicon photonic chip for methane sensing
pulse can be converted into correlated pairs of photons – single particles of light. As these are always generated in pairs, the intensities of the resulting beams of light are correlated, even though they can have very different optical wavelengths compared to the laser, and compared to one another! These processes are seeded by vacuum fluctuations, which is why they are often termed quantum. By passing one of the correlated beams through a gaseous medium and then detecting it, while directly detecting the other, we are able to use one to know the amount of incident light and the other, that experienced some absorption, to tell us about whether a gas is present and at what concentration. There are extensions to this, such
as nonlinear interferometry, where the beams are not all detected. In this case, the correlated beams, after one has interacted with a gas, are passed back through the generation process again and, due to their phase relationship with one another, the presence and concentration of the gas is mapped onto a change in the properties of both beams. If we are careful about how we set our experiment up, we can generate correlated beams of very different wavelengths – one visible and one infrared. Gases often have strong absorption in the infrared as that is where molecular bonds have vibrational resonances, but infrared light is hard to detect. In our system, we only ever have to detect the visible light, which can be done with
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Picture courtesy of Dr Arthur Cardoso, QET Labs, University of Bristol
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