It is fair to say that the words ‘quantum spectroscopy’ are not exactly common currency. Very few people understand them, and even if you Google the term, you might not find much that’s understandable to a non-expert. Actually, it’s probably fair to say that either word on its own would be enough to put most people off reading further—but stay with me, because quantum spectroscopy is turning out to offer huge potential in life sciences and medicine.
Let’s try to unpick the terms a bit and explain what we mean.
– A blog post by Stefan Kröll, Professor at Atomic Physics, Lund University
Getting started in the quantum world
My interest in the quantum world started with materials that could retain what is known as quantum superposition states over a long period.
Superposition is a term to describe a system that has two possible states, and it can exist in both at the same time. Electrons, for example, can either spin up or spin down. But they can also exist in a complex state of both—until you measure them, at which point they are by definition in one or other state. Normally, quantum superpositions only exist for nanoseconds at a time. However, in some materials, this state can persist for milliseconds or longer. That may not sound like much, but it’s a huge difference: orders of magnitude.
We were looking at light-matter interactions in these materials, and about 10 or 15 years ago, we realised that they could also slow down the speed of light. We were working with a group in the US, and together we realised that this might be the answer to eliminate the detrimental effect that tissue scattering has on optical imaging in the body.
The next question, of course, is what is tissue scattering? When you pass light through human tissue—which is basically what is needed for optical imaging—it is scattered differently by our cells and our various types of tissue, because they all affect the light slightly differently. This means that it is almost impossible to use optical imaging in the human body because you can’t get a clear image. We can use ultrasound or other parts of the electromagnetic spectrum like X-rays instead. However, anyone who has ever looked at either an X-ray or an ultrasound will be aware that they have their limitations. X-rays, for example, mostly see bone structures, so you can’t see clearly what’s happening at a muscular level.
A possible step is to combine light and ultrasound. Optical contrast will in many cases provide more information than simply using ultrasound. Light passes through the tissue much faster than ultrasound waves do. The light passing through the ultrasound focus can then be shifted in frequency by the ultrasound. If you can slow down the frequency-shifted light only —for example, by passing it through a filter made with one of these materials that hold its quantum superposition for longer—then you can eliminate the interference with the rest of the light, and then you more or less have the power of optical imaging within tissue with ultrasound resolution.
Moving the work forwards
We are by no means yet there on quantum spectroscopy. However, we have made considerable progress. For example, we have found some materials that work in infrared light, and particularly, two that allow us to see a clear difference between non-oxygenated and oxygenated blood. This has obvious applications in the heart, brain and circulatory system, but we still need to work out exactly how we might use it.
We are doing some work on how we can use this technology for breast cancer screening, and to remove blockages that occur during strokes or other conditions where something blocks the blood flow. This may be particularly helpful with unconscious patients because it will allow us to see immediately if our actions have had an impact on blood flow. In stroke, in particular, acting quickly is vital.
Unpicking the quantum problem
The precise applications are perhaps somewhat beyond my expertise. I am a physicist by background. My focus is on finding the right materials to make filters that do what is needed to permit imaging at the level required and deliver what doctors need. Does it matter to patients or doctors that the physics behind these filters is based on quantum technology? No. It’s just a very powerful new technology that looks likely to give us a significant step up in imaging quality—and therefore improve both diagnosis and treatment of many conditions.