When you cool devices to low temperatures then the motions of the electrons and the atoms are greatly reduced and then this simplifies the structure and makes it easier to understand the physics of the device.Dilution refrigerators are very applicable for this application, so this is a really exciting field at the moment.
People often ask me how cold a dilution refrigerator can actually achieve. If you think of your domestic refrigerator at home in your kitchen then our machines can get something like 20,000 times colder.
My recent work on cryo-free systems has been enormously successful. The original concept of designing cryo-free systems was to basically eliminate liquid helium. It’s extremely expensive and it requires a lot of expertise to handle it.
There’s a number of challenges in designing ultra-low equipment. We need to first of all have a really good understanding of the physics and the requirements of the machine. There’s hundreds of joints, welds and seals, all of which need to be leak tight to atomic scale. We’re running at 0.1 of a degree above absolute zero.
Tiny heat inputs at these temperatures, something like one ten millionth of the heat load from an electric lightbulb would be too much for the machine to accommodate and would cause it to warm up.
In a nutshell, how a dilution refrigerator works is that it uses a mixture of two isotopes of helium. It uses helium-3 which is extremely rare and the more abundant helium-4. If you cool down a mixture of helium-3 and helium-4 it will phase separate with the lighter atoms floating on the top of the denser helium-4 atoms. So this is very similar to oil and water. So if you can think of the oil floating on the top, then you’ve got a distinct boundary between the oil and the water.
If we can force atoms to cross the phase boundary from that concentrated phase into that dilute phase, this provides a cooling process.It can operate for weeks and months below ten millikelvin.
I’ve worked on a number of extremely exciting projects actually over the years. The machines that Professor von Klitzing, Bob Richardson and Doug Osheroff received their Nobel prize for, they were all on machines I designed.The fascination of working at ultra-low temperatures for me and the reason why I really enjoy it is the challenge of achieving the performance that the customer requires, it’s using my physics education and background to good practical use, coming up with well-engineered solutions and then physically seeing the equipment being built from the concept that you’ve designed and it ultimately being installed in a customer’s lab is extremely rewarding.I’ve been working in the field since 1985 and I still find it very enjoyable.