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FQXI ARTICLE

February 6, 2023

The Quantum Engineer: Q&A with Alexia Auffèves

Experiments seek to use quantum observations as fuel to power mini motors.

FQXi Awardees: Alexia Auffèves

September 22, 2021

Alexia Auffèves

CNRS

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.

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.

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.

In quantum physics,

looking at a system

can transfer energy

to it, just like a hot

source.

looking at a system

can transfer energy

to it, just like a hot

source.

- Alexia Auffèves

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.

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.

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