August 13, 2026 -- At the X-ray free-electron laser SwissFEL, scientists are now able to capture the strange quantum states that only emerge at extremely low temperatures. In this interview, Bill Pedrini and Simon Gerber from the Center for Photon Science at the Paul Scherrer Institute PSI offer two complementary perspectives: one on how a globally unique experimental station is built, and the other on how it opens a window onto a seldom-seen quantum world.

How cold can you go?

Simon Gerber: Recently, we measured at 35 millikelvin – just 35 thousandths of a degree above absolute zero. That’s as low as you can go in our experimental setup at this point and, to our knowledge, the coldest at which measurements have ever been made with X-ray scattering. 

Why so cold?

Simon Gerber: There are many quantum states that only exist at very low temperatures. Our vision is to use the SwissFEL to image these quantum states and their dynamics as close as possible to absolute zero (temperature). No one has done that before.

Can you give us an idea how you do that?

Simon Gerber: The inspiration comes from X-ray experiments for structural biology – a principle known as diffraction before destruction. When we do experiments with X-rays, one thing we know for sure is that the sample will heat up and suffer from radiation damage. But X-ray free-electron laser (FEL) measurements are incredibly fast – the pulses are ultra-intense, but they only last for tens of femtoseconds, or even less. So, we can take a snapshot before “destruction” happens. We thought: if this works so well for biology, why can’t we transfer the concept to quantum states?

Bill Pedrini: An FEL acts like a super-fast camera on the atomic world. Our idea is that the shutter of SwissFEL is closed, the sample is as cold as can be and then… ‘pschup!’: one intense light pulse and we take a snapshot of the quantum state before the X-rays suppress it by heating it up.

Is it then possible to make a ‘movie’ of a quantum state in action by taking multiple snapshots?

Simon Gerber: Not in this way exactly, because once you measure with an ultra-intense X-ray pulse, then the quantum state is gone. But we can study the dynamics by coherently driving these quantum states with electrical or optical pulses and seeing how they evolve. In this way, we control the state of the quantum system and then read it out with the FEL, for example when states are switched or quantum information protocols are performed. This is one of the ultimate aims of our efforts.

The experiments take place at the SwissFEL Cristallina experimental station. I understand the setup here is quite special.

Simon Gerber: Indeed. The cryostat, which we use to cool samples to such low temperatures while we study them with the FEL, is unique. We started thinking about performing these types of experiments as early as 2016, from the first days of SwissFEL. Figuring out how we could actually realise this took some effort. Ultimately, it’s all teamwork and only possible when many persons with complementary expertise work towards the same goal.

Bill Pedrini: The difficult part was how to design the cryostat, and incorporate windows to allow the X-rays in. The other part – building the beamline and experimental infrastructure – was complicated, but we could benefit tremendously from established technology and expertise concentrated here at PSI.

Can you tell us more about your first measurement?

Simon Gerber: Yes. We studied a material with an unusual type of magnetism – a quantum antiferromagnet. This had already been investigated several years ago at PSI’s Swiss Spallation Neutron Source SINQ and so it was an ideal proof-of-concept experiment, built on a foundation established with neutron scattering. 

Bill Pedrini: In this material, the antiferromagnetic state emerges only below a couple of hundred millikelvin. This is very cold. With our cryostat, we have a base temperature of 35 millikelvin, so it didn’t give us much headroom at all. 

Simon Gerber: When you build something new, the first step is benchmarking it. This was a challenging but realistic benchmark. Now we know that we can access this ultra-cold regime with SwissFEL. It’s an exciting time.

Aside from curiosity, why is it important for us to access this regime?

Bill Pedrini: These quantum properties have potential applications in future technologies. We’ve designed the experimental station in such a way that we can in principle measure actual quantum devices.

Now that the first experiments are a success, what’s next?

Bill Pedrini: We’ve already had our first couple of external user groups and we’re looking forward to what future ones will bring us. It’s all about quantum states that emerge at very low temperatures and are not measurable with other techniques. This can be because they’re very fragile and prone to disappear when attempting to measure them, or because only very small crystals of the quantum material exist, or because one is interested in the dynamics as explained before.

What motivates you personally to do these types of experiments?

Simon Gerber: These quantum states are so intangible. What I find amazing is that here we have a camera that takes snapshots at this record cold 35 millikelvin. The quantum state is there and – snap – we take a photo of it. We see it in our space, on our detectors. That’s incredible.

Bill Pedrini: This is the boundary of technology. Most of the equipment that we use here at Cristallina and SwissFEL can’t be bought off the shelf. But we have a need for it and so we develop it. The challenge of conceiving new solutions, and further developing techniques and equipment is what motivates me. 

Simon Gerber: And this pushes technology that is useful for way beyond quantum physics… 

Could you say more about what you’re referring to?

Simon Gerber: We’re investigating fundamental physics, but we need to invent technical solutions to realise our goals. Take the microwaves that we use to drive quantum states, for example: this is related to synchronisation, and we had to invent new ways of doing that. Synchronising signals have broad applications, not only at SwissFEL.

Our frontier experiments are a catalyst for technology development. If we weren’t pushing boundaries then these technologies would never be developed.

What about spillover into local industry?

Simon Gerber: A practical example is the lightweight, stiff carbon stand for the cryostat. This was developed with a local mechanical engineering company. To meet our specifications, they needed to figure out how to very accurately glue carbon fibre and aluminium. This know-how will have benefits for other endeavours, where high precision, as well as sturdy and lightweight designs are needed.

Bill Pedrini: Or simply have a look at our floor. The Cristallina experimental station is designed to be flexible: as well as the quantum experiments, it’s used for studying proteins. This flexibility requires instrumental setups weighing several tonnes to be moved around precisely and without shocks. Our solution is to have an ultra-flat, extremely smooth granite floor on which the instruments float on air pads. The level of precision this demanded from local industry is useful in many other contexts – within and outside Switzerland.