Interview: Greg Dmochowski, Experimental Quantum Optics

I’m psyched to present a new Pop Physics feature: Interview With A Physicist! We’ll be speaking to people working in all kinds of physics-related fields to get a better sense of what it’s actually like to spend your days probing reality’s deeper mysteries.

Greg Dmochowski and The Lasers
Dmochowski in front of his group's experimental setup.

Our first interviewee is Greg Dmochowski, who’s half-way through a PhD in Physics at the University of Toronto, working in the impressive-sounding field of Experimental Quantum Optics. (The post gets a little long, but I couldn’t bear to cut it down any more than this. Too interesting.)


Pop Physics: What are you working on right now?

We are trying to make two beams of light interact with each other. “Interact” really just means that we want some noticeable effect to appear on one beam as a result of the other beam. Normally, light doesn’t interact with other light; two beams just pass through each other completely unaffected. But light talks a great deal with atoms. By looking at light after it’s passed through an atom, you can infer information about what state the atom was in. Equally importantly, the state of the atom can in turn be affected by the interaction with light – the interaction goes both ways.

So the basic idea is this: you have two light beams (lasers) and some atoms. You shine one of the lasers at the atoms and then measure this light after the interaction with these atoms. This first laser beam will be called the ‘probe’ beam. When you measure the probe beam, you will be able to infer something about the state of the atoms – how many atoms there were in the ground state, for example.

Now, while leaving this probe beam on and continuously measuring it, you then shine the second laser beam (the ‘pump’ beam) through the atoms. This pump laser will affect the atoms – transfer some of them into the excited state, for example. As a result, the original probe laser that you are measuring will feel this change in the number of excited atoms, and your probe measurement will indicate this. In effect, the pump beam has interacted with the probe beam, using the atoms as a mediator of the interaction. Strictly speaking, the two laser beams haven’t interacted with each other. But they have both interacted with the atoms in such a way as to be able to infer the presence of the pump beam just by measuring the probe beam.

And that is what we are after – we want to be able to infer the presence of a pump beam by looking at the effect on the probe beam.

Laser Play
An unrelated yet cool laser photo.

So why are you doing this?

In a word, computing. Specifically, all-optical quantum computing.

In your computer there are billions (trillions?) of tiny devices called transistors. These transistors are really just switches. They are either on or off – when they are on, they allow a current to pass through. These transistors make up billions of “bits” in your computer, which store all of your information. All computer data is stored as a (long) series of zeros and ones, representing off or on. Importantly, the state of a bit can be changed by the use of a second current. That is, multiple bits talk to each other, manipulating the information that is stored, and it is this “processing” that allows you to do computing.

Over-simplistically, we want to create such a device but using light in place of currents – we want one light beam to ‘switch’ a second light beam ‘on’ or ‘off’. Moreover, if we use a single photon as our pump beam (a photon is a particle of light) then we can make use of quantum mechanical effects that would provide a huge improvement in terms of computing power. Instead of a single bit being either 1 or 0 (on or off), a quantum bit (qubit) can also be a combination of 0 and 1.

Cubit arm
Not to be confused with an arm.

As a result, quantum computing would be more powerful. It would allow us to simulate much larger, much more complicated systems – systems which are just too large for our regular computers to handle in any reasonable time (the age of the universe, for example). And light is a great candidate for quantum computing. It is very effective for storage of quantum information – light hardly interacts with its environment (meaning that the information storage is very robust), and it travels very, very fast, which means communication speeds will be as high as we can expect them to be.

Trouble is, in order to do a computation, it isn’t enough to have a bunch of bits stored in memory. You need the ability to manipulate these bits and do operations on them – have the bits talk to each other. This is where the lack of interaction of light is suddenly a curse – we need to be able to interact these qubits with each other.

So basically, we’ve been trying to establish a proof-of-principle quantum ‘transistor’. Then we need thousands of these to make something useful of it all.

Have you made much progress so far?

Yes! We are still rather far from seeing the effects at the single photon level, which is the holy grail, but we have been able to get two beams of light interacting with each other via atoms.

Do you mean that you don’t consider the pump beam to be ‘singular’, because it’s made up of many photons?

Exactly. The pump beam is sometimes called the ‘signal’ beam, because it would be the single photon in which your quantum information would be stored, often in the form of polarization. However, we have only been able to see the effect of many thousands of these pump photons on the probe beam. The effect of a single photon is just too small for the amount of noise we have in our measurement system.


What first got you interested in physics?

I don’t know how I would have formulated an answer to this question as a kid. Probably something along the lines of, “because it explains what I see.” To me, physics explained why things were the way they were. I was always a curious kid; I was curious about the world around me. Questions I specifically remember puzzling over include: Why do clouds move? What are clouds, for that matter? Why does the bath tub water form a spiral (vortex) when I pull out the plug? And the more I asked and sought out answers to such questions, the more familiar I became with the approach of physics. As far as the observable world around us goes, physics is the best source for answers to questions dealing with what we observe.


What do you love about your work today? What do you hate about it?

I love that everyday I get to discuss terribly interesting things with fellow curious people. I can share with colleagues the marvel of the physical world. I really like the experimental aspect of it – the fact that I am actually making something, building something, measuring something, making it work the way we would like it to work. Or at the very least, debugging and figuring out why it doesn’t work the way we’ve designed it to.

I hate the low wage and the long hours.  Around $20,000 a year and you end up working considerably more than 40 hours a week if you want to accomplish something. The hourly rate is s*** and Toronto is a fairly expensive place to live.  I also dislike the politics involved in research. There is too much salesmanship involved – making people think your research is sexier and more useful than it really is. It’s a necessary evil most of the time. For example, I have lost a passion for quantum computing. I think humans could do without quantum computers these days. Yet, I find light-matter interaction interesting and want to learn about it, expand our understanding of it. This means that I have to try to sell quantum computing in hopes of getting published and securing funding.


What does a normal day of work look like for you?

A normal day in the lab for me is hard to describe because it depends on what stage of the project we’re in. Sometimes I’m sitting at a desk working on a calculation, either on paper or using computer software. Before you go and measure something in the lab, you have to know what you’re looking for – how you’re going to measure it, how big of an effect you expect to see, is the noise in your measurement small enough to see the effect, etc.

Other times, once we’ve worked out enough of those details, we spend our time setting up the necessary optics, electronics and atoms to do these measurements. It’s really fun doing this stuff: manipulating laser beams – changing their frequency, polarization, beam waists, powers, etc – to exert forces on atoms, to trap atoms and cool them down to millions of times colder than room temperature. Imagine that: we’re tightly focusing intense laser beams on these atoms and the net effect is to keep the atoms stationary and very cold.

Unrelated military laser experiment
The supercool glasses are mandatory. (This photo is also totally unrelated to Dmochowski's work.)
Other times, once we’ve set things up correctly, we’re in the stage of ‘taking data’. In some experiments, you have to repeat measurements many thousands, even millions, of times in order to see the effect that you are trying to see. Often, the things we measure are probabilistic (due to quantum effects or because noise dominates the signal), so we can’t tell on a single run of the experiment what will happen. So you spend some time programming, writing code, to automate the running of your experiment. In the end, you make all the equipment, the electronics and lasers and computers, work together and repeat some measurement for hours and hours, sometimes even days and days.

Still other times, once you have this data, you have to present it to the community. So you write a paper about it and try to get it published. Here enters a lot of silliness.

These days, we’re at the exciting stage where we have worked out a lot, but not all, of the questions we need to know and we’ve set up a lot of the necessary stuff, and have seen some preliminary results. We spend our days discussing how to fine tune the experiment, working out the finer details of the physics involved, improving areas of our experimental set-up and seeing real progress towards our goal. The beginning of an experiment can be hard because the goal seems so far away and so many details are yet to be worked out. It all gets disheartening sometimes. But these days it’s gravy.


What do you plan to do next?

Work harder so that I can finish my PhD. Then I don’t know. A post-doc would be a fun excuse to travel to a new place and continue playing with physics research. But maybe I will prefer to be sedentary and stay in Toronto, in which case I would probably look for an R&D job for some company. I still feel as though something would be missing from my life if I didn’t engage in physics. It is an excellent way to gain perspective on why we’re here and all that big picture stuff while shedding light on everyday, mundane things.