David Hume (B.S. ’02) attended graduate school at the University of Colorado Boulder (CU) and conducted his Ph.D. research in (future Nobel laureate) David Wineland’s lab at the National Institute of Standards and Technology (NIST) in Boulder. After a post-doc appointment at the University of Heidelberg, he returned to NIST to continue his research on high precision measurements of trapped ions, in particular the use of aluminum ions as atomic clocks.
Physics seniors and Society of Physics Student officers Anthony Kelly and Gabija Ziemyte had the opportunity to ask Dr. David Hume a few questions when he visited the University of Kentucky to give a departmental colloquium in November of 2022. Below are edited excerpts from Anthony and Gabija’s conversation with Dr. Hume.
Anthony: I’ll start off with a fun question I've been asking a lot of professors recently – what is your favorite equation?
David: My favorite equation. Well, I have a lot I suppose, but one that sticks out is the equation for the size of the wave function in a quantum mechanical harmonic oscillator – square root of ħ over 2m omega. That is one I use a lot in the research that I do.
Gabija: That is a great segue into telling us more about your research.
David: I work in the Ion Storage Group at the National Institute of Science and Technology in Boulder, and the thing that ties all research together is that it's based on trapped laser cooled ions. We trap individual ions, cool them close to absolute zero, and use them to do precision measurements.
A large part of the group is devoted to quantum information processing, so you can use the ions as the carrier of quantum information. If you isolate two states in the atom, you can label those as zero and one. Of course, the system can be in a superposition of both zero and one, and that's the basic building block of a quantum computer. We can make those interact with each other to make quantum gates.
My research is in the precision measurement side of the group. The original goal of many of these experiments was to try to make more and more accurate atomic clocks. You can base a clock on an internal resonant frequency, and the name of the game is trying to control all the environmental effects that might perturb that resonance frequency.
Gabija: How did you get involved in that specific subject? Did you start out in your career knowing that's where you wanted to go?
David: No, I didn't. When I started at UK, I didn't even know exactly what I wanted to study. I was interested in science and math and technical things like that, but I spent some time in the engineering program. I then had the feeling that I wanted to try something different. In my first undergraduate year, I started looking around a little bit and thought I should sit in on some classes in the Physics Department. I sat in on one of Dr. Straley's classes and it was like a switch flipped immediately. There was a big difference between that class and what I had experienced up to that point – in that the professor seemed so interested in what he was teaching, and the students were really interested in what was going on. It clicked with me, so it was a pretty easy decision to then join the physics department.
The way I made my way to this particular career path was, as in a lot of cases, a bunch of coincidences. I did a summer research REU at Los Alamos National Laboratory, and there I was exposed to quantum information. I was working on theory that summer, but my advisor told me about a group at University of Colorado at Boulder (CU) that was working on experimental quantum information and doing very interesting stuff. I followed up on that and was able to get a position in the group.
Anthony: Which physics class did you sit in on?
David: I remember taking classical mechanics with Dr. Straley, but I don't think that was the class I sat in on. The first day of a lot of those core classes really stuck in my mind. I took introduction to quantum mechanics with Dr. Susan Gardner, and her enthusiasm was infectious from the very beginning. I took electricity and magnetism from Dr. Nicholas Martin, and I remember an entertaining introduction that he had. Dr. Wolfgang Korsch taught a lab class. The first day of a lot of these classes really made impressions on me.
Anthony: It doesn’t sound like you really changed your research focus. Did you go into graduate school wanting to do quantum computing or did your interest fluctuate at all?
David: Not really. I was fascinated by quantum information, basically the concept that you could represent information with totally different mathematics and that there was a way to harness that to do something that's potentially more powerful than what we can do with normal computers. I was really fascinated by that concept, and I started working in the Wineland group at NIST when I was a graduate student at CU. There were a couple of projects going on in the lab at the same time; we actually shared the lasers between two different projects. One project was doing quantum computing where they would load several ions into the trap and do gates between them. That's what I started on, but there was an opening to work on the other experiment, which was an optical clock based on the aluminum ion using a lot of the same lasers. An interesting thing about that is that we couldn't do the measurements that we needed directly on the aluminum ion because it doesn't have the right atomic structure. To get information out of the atomic system, you need to scatter a lot of photons, but aluminum doesn't have the right structure for that. Therefore, the trick they played, which I still find interesting, is that you can perform a quantum gate between an ion in the trap and the one that you measure. In that way you can transfer information from one ion to the other and then scatter a bunch of photons, so the atomic clock research uses an application of the quantum gates that had been developed on the other side of the group. So, I went away from quantum computing, but I’m still using some of those basic building blocks.
Gabija: What's one of the coolest things that you've discovered through your research so far?
David: I really like the idea of trying to answer questions experimentally that you have no idea what the answer is going to be – exploratory experiments. In fact, I don't get to do a whole lot of that. For the most part, we can calculate exactly what is going to happen if we apply the right laser pulses to the ions – it can be done with just a pen and paper for the most part. That's the power in the system.
However, I would like to learn something new about the universe. We can make frequency measurements with our optical clocks to 18 digits of accuracy, but we cannot calculate the actual frequency of the e optical clock. While it cannot be calculated, it is determined by fundamental constants, like the fine structure constant. If the clock frequency were to change, it would mean that the fundamental constants of the Standard Model have changed. So, our measurements are a unique and sensitive probe of whether we need extensions to the Standard Model.
Gabija: When you were an undergrad, what was your favorite class not limited to physics?
David: My physics classes were by far my favorite classes. It's hard to pick a favorite. I really like the modern physics classes because they expose you to mind-bending ideas – fascinating things like quantum mechanics and relativity. A great experience in my life has been going from having been exposed to those far out, remote, and counterintuitive ideas to now working in the lab where I get to see them daily. I see quantum jump effects every day – that's the signal that we get out of the ion trap experiment – and I see the effects of relativistic time dilation in the shifting frequency of the clock.
Anthony: Do you have any advice for current undergraduates who want to pursue graduate school?
David: Find something that you like and stick with it. I think I've benefited from focusing on something and making sure to follow my interests. It's been important for me to also surround myself with good people. Graduate school can get difficult, and if you're not in a great working environment, it makes it more challenging. If you find really good people with whom to work, that could be a big benefit.
Gabija: How did your application process to graduate school go? Did you apply to a lot of schools, or did you already know?
David: I applied to, I think, four schools. For me there was a big draw to the University of Colorado Boulder because I'm also interested in rock climbing. I've been rock climbing for a long time, and I went there at least 50% for the physics and about 50% for the climbing – it's just so accessible there. What I found was that in graduate school, I ended up spending 95% of my time in the lab and only 5% out climbing, but at least it was convenient when I did.
Gabija: This is more of a fun question: what's your favorite particle?
David: My favorite particle is the aluminum ion. I work a lot with the aluminum ion, so I'm biased. If it must be a fundamental particle – I'm not a particle physicist, so I like the normal things – I find the electron to be so beautiful. The simpler the better, and the electron seems to be about as simple as it gets.
Anthony: What are your thoughts on the different interpretations of quantum mechanics?
David: I find it very interesting. Early in graduate school, I became fascinated with the Many-Worlds Interpretation of quantum mechanics. It seemed elegant to me – you are taking the superposition principle seriously and not having to introduce a measurement postulate where things collapse to a particular state in the superposition. It’s frustrating that we can't distinguish interpretations from experiment so far. I like thinking about it.
Gabija: What fields other than physics – math, philosophy, etc. – do you think are useful to know as a physicist?
David: It depends on what you end up doing. For me, I'm an experimental physicist, so statistics and data analysis have been the most useful.
Anthony: How and when did you find out you were more interested in experimental physics versus theoretical?
David: I didn't really know, and it was what ended up working out in terms of the research position as much as anything. I had a good tip from my advisor at Los Alamos that the Dave Wineland group at CU was a good one to work in. I hadn't done much experimental physics until then. I had worked a little bit in an engineering lab in the material science department, and I had worked in a chemistry lab when I was a high school student. I had done a little bit of theoretical work with quantum information, but I still didn't know exactly what the best fit was. I became interested in how the experiments ran. It was a great experience just learning how to make things work. You go in the morning to turn everything on and align all your knobs and the different mirror mounts to get the lasers on. It was the experience of seeing it all work and making it happen.
Anthony: Is that your favorite part of being in the lab?
David: I still do prefer working in the lab to working at a desk. If I had my way, I think I'd do more of that and less work at the computer.
Gabija: Did you attend colloquia or lectures when you were here at UK? Were there any ones that stood out?
David: I remember attending the physics colloquia here. One that sticks out to me was by Professor MacAdam, an atomic physics professor. He invented this thing that he called the Stark ball based on a Stark cylinder, which was a way that one could apply a well-controlled electric field by applying the right voltages to a set of electrodes that could be rotated a full 180 degrees. His Stark ball was a spherical version of that, and he figured out the optimal points at which to put the electrodes. By applying the right voltages, he could apply an electric field in any direction. He described that invention in some detail, and he had one in his hand that he could show us. It's actually not so different from the kinds of things that I have to think about now with the ion traps.
Anthony: What are your future career plans?
David: I don't have any changes that I am planning to make in my career. I'm in charge of a small group of people working on optical clocks, and I'm interested in seeing how far we can push that. These clocks continue to improve in terms of their accuracy and their stability. What's interesting to me is that when you see a graph of how the performance is over time, it's this Moore’s law kind of thing, where it's a factor of 10 per decade. It almost looks boring, like it's predetermined that you are going to get a factor of 10 improvement per decade, but if you go back and see how those factors of 10 were made, it's a lot of completely new techniques. New things were invented, so there's a lot of new physics going into every factor of 10.
Gabija: If you had to go into any other field, what would you be your first choice?
David: I'm fascinated by observational astrophysics. There's just so much out there, and there is greater and greater capability of resolving and understanding. I was really excited about the first observation of gravitational waves in 2015. I had heard about that research over the years, and I knew that they were pushing down the precision to be better and better for many decades – it was one of the long quests in science. The signal that the researchers showed just slapped you in the face – it was so obvious, and they had such a great fit to the signal based on what we understood about colliding black holes. That was really inspiring – that a significant long-term scientific effort paid off.
