Ben Lang

The quantum physics of superconducting circuits.

Electricity and light interact in many ways. One specific interaction that is particularly open to control and measurement is the interaction of superconducting circuits with light in the microwave part of the spectrum. This level of control means that I (as a theorist) can explore bizarre "what ifs" with some hope they may be experimentally feasible. For example, what if the light photons could only be born in groups of six? Strange and unexplained quantum behaviour lurks in these exotic situations, and if we see anything technologically useful then the circuits can actually be made to exploit it.

I loved quantum physics and became interested in all its strangeness. I worked on electricity-light interactions in my PhD, but moving to superconductors was a chance to do something that was in some ways the same and in others very different. It was the "right distance" from what I already knew. Plus, it was there. Nobody knows of every research topic in existence, and even if they did they could never pick a favourite to commit to. So, post-hoc rationalisations aside, their is an element of the pinball machine to the topics everyone works in. I got some lucky bounces.

I debug simulation code, re-arrange equations on paper and stare at graphs while wondering "why is it doing that?". I also chat with colleagues, speculating about things, read stuff and then edit text and figures to try and explain what I have found.

I love getting weirded-out by the strange things quantum physics can do, the headspin is great fun. I also enjoy the sharing of these strange ideas. When in the right moods I draw a lot of fun and purpose from diving into some simulation code or doing a load of algebra for an equation.

The computer simulations are very good at getting the answer, but that isn't really what we care about. What we are looking for is an understanding of how and why something happens, and a simulation of it happening doesn't necessarily give you that. Often the answer in these cases is to find (or devise) a new way of plotting the relevant simulation data, this can make the "why" much more transparent. When that fails to answer every question the next step is to do maths.

I found an electric circuit that, the more damped it is, the more photons it holds in equilibrium. To take an analogy, its a bucket with holes in it by which water escapes. In a quantum setting I found that, with more loss (more holes) you actually end up with more water in the bucket. The big question is: Why? How can more friction result in the circuit retaining more energy? Does it happen in other branches of quantum physics? I also want to understand to what extent quantum theory can be thought of a "probability theory, with negatives".

In the ideal case I would hope to uncover general principles, concepts and methods for understanding quantum physics. The theory is quite old now, but I think many of the more basic issues of what it means remain unsolved. It also continues to throw up oddities that get us by surprise (the friction example above) and a deeper understanding of that effect and others would be wonderful. I have no idea what (if any) applications might follow, but sturdier conceptual frameworks and knowledge of these unexpected effects might lead to places we didn't know we wanted to find.

I do not currently teach.

Its sounds weirdly specific, but decomposing whatever it is you are trying to understand to find its eigenspectrum is (I think) basically always the right way of approaching physics problems. If you are choosing some specific initial conditions and then advancing time in a simulation you will probably learn very little, "what happens if the marble starts here and is left to roll" is not an interesting question, and the answer tells you very little about what you really want to know "how do marbles behave?".