The Quantum Engineer: Q&A with Alexia Auffèves

The quantum world is notoriously difficult to understand. One of the greatest mysteries is how a physical system can exist in many different states at the same time, and only ’pick’ one when we observe it. Physicists and philosophers are still debating the nature of this mysterious measurement postulate. Alexia Auffèves, a senior researcher at CNRS in France, who heads Quantum Engineering Grenoble, and her colleagues, received an FQXi grant of over $1,200,000 to explore whether the act of observation could be used as fuel—allowing us to extract energy from quantum systems. Their series of groundbreaking experiments will not only have philosophical implications, but could actually help us improve the energetic efficiency of quantum communication and computation.

You started your career as a PhD student with Serge Haroche, who was later awarded the Nobel Prize in Physics for devising methods to study the quantum behaviour of individual particles of light, or ‘photons.’ What was that like?

Inspiring. He and his brilliant team of permanent professors would come up with ideas, that would then percolate in the group of PhD students—and we would do the experiments. The results would then go back to the masters, and they would write the stuff in a way that would make the physics so crystal clear. Everything was supposed to be simple. The name of the group was the Electrodynamics of Simple Systems, and I’ve kept this love of "simple systems" and at the same time very deep concepts.

One deep concept that you are currently investigating relates to the physical nature of information. How can you exploit the act of observation to create fuel to power a tiny engine?

We will not to use a hot source as a fuel for the engine, but we are going to use the fact that in quantum physics when you measure a system, you not only extract information on it, but you also perturb it, that is, you change its state, and also its energy. Therefore in quantum physics, looking at a system can transfer energy to it, just like a hot source would do. This is what we called quantum heat, and we have just experimentally evidenced the reality of this concept. The next step is to convert this energy input, this "measurement fuel" into work, just as in regular engines.

How will you create the mini engine in the lab? And how will you demonstrate that it produces work, or useful energy, rather than just heat?

For the engine that we have proposed to build, we use a quantum system called a superconducting qubit—an element in a superconducting circuit. This qubit has two identifiable energy levels, analogous to the one and zero in a normal computer bit. We then excite this qubit with light, and measure it in such a way that there is more light exiting the qubit than inputing the qubit. This is work, here extracted as photons.

What kind of applications could come out of this?

The measurement-driven engine is a proof of concept. It shows that there is an energetic footprint as a result of looking at a quantum system. Now if I want to think in terms of application, what I am actively working on is the other way around, which is estimating the energy cost of performing a quantum computation or communication. And in this case, the fluctuation that is induced by the quantum noise is not a resource anymore, it is a perturbation that I have to fight against. So in one situation, I am using the noise as a resource. In the other, which is much more practical, I am estimating the energy cost of fighting that noise. So it could be used to make quantum technologies more energy efficient.

What can this teach us about the nature and role of measurement, which is different depending on which of the various interpretations of quantum mechanics you subscribe to?

I have become aware that the way we build concepts in quantum thermodynamics depends on our favorite interpretation of quantum mechanics, and especially of the measurement postulate. Is it something that is disgustingly practical and, if we developed a better theory, we could get rid of it? Or, on the other hand, is it something that is really fundamental and is the soil for all quantum concepts? I believe that the measurement postulate is at the root of everything.

And actually, if we have this dichotomy in mind, then we understand that when people are building thermodynamic concepts around the measurement postulate, what they prefer to call either ’heat’ or ’work’ is nothing but a matter of interpretation.