Imagine extremely small and comfortably installed systems in an environment where almost nothing is going to interact with them: it’s a bit like they were trapped due to a disease that would affect small particles. Under these conditions, these systems can start behaving strangely: they can, for example, both work and chat with a friend. When their state is distributed in this way, they are said to be in a state of superposition.
If two Atoms are together in this partially distracted state, and despite their confinement, one of the two talkative Atoms receives a call from their boss, then they suddenly have to stop chatting. For the abandoned atom, there are two possibilities: either he gets a signal from his friend who has decided to get back to work more seriously or tell him all about his conversation, in which case he will do the same; either his friend says nothing to him anymore and he can with a certain probability choose to come back to work alone or to go to the coffee machine to find others to talk to. In any case, after the interaction between one of the two atoms with its hierarchy, the two atoms will no longer be in their original state of “distracted” superposition.
So what ? Well then, in the case where the boss has managed to remotivate the first atom and the second, by receiving this information, has also focused again, the overall motivation of these two workers has been enhanced by their interaction with this external element . and immutable, that is their responsibility.
The fascinating thing is that in quantum physics, instead of this sudden increase in motivation, we can see a sudden increase in energy. Note that the energy does not come from a dark reservoir. Like the boss, who sometimes starts his own motivation by intervening with his employees, the energy is also transferred from one system to another.
By exploiting this phenomenon – transferring energy to a quantum system by measuring it, we direct our research towards new “quantum motors”, which, for example, would be able to amplify the light from a laser.
What is the energy of a quantum system?
Energy can take various forms: chemical as for cells and batteries, thermal as when taking a very hot bath, or for example kinetic when throwing a petanque ball. Like two magnets that need to be energized to loosen them, it is also possible to store energy through the interaction between two systems.
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For quantum systems, it is not that different. Moreover, this is normal because a quantum system is nothing more than a system like the others … but so small that its behavior reveals its different energy levels, we then talk about a fundamental level when the energy is minimal. Of course, quantum physics, how counterintuitive it is, respects the principle of energy conservation well. If an atom goes from a high energy to a lower energy, it emits a photon, which compensates for this difference in energy.
Quantum measurement is indiscreet
So far, we understand that our talkative atoms can certainly have more or less energy, and that they are sensitive to interference, but one question remains: how is it possible to transfer energy to a quantum system by measuring it?
To understand this, we must remember it all action requires interaction. Even when measuring the temperature in your home, it required molecules in the ambient air to smash into your thermometer to cause the liquid in it to expand, and then reflected photons from this unit of measurement back to your eye so you could see , what it points to. Thus, if one can read the result of a measurement on a microscopic scale, it is because this measurement is encoded on a sufficiently large system that it complies with the rules of classical physics.
Quantum systems are no exception to this rule.
The difference, however, is that in quantum physics the measuring apparatus is larger, not smaller, than the object being measured. Thus, the interaction between the measuring device and the system is no longer insignificant. Everything happens as if the boss of our two atoms could not observe the state of motivation of his employees without them being aware of it. The privacy of the atoms is therefore well preserved.
How to transfer energy by measuring?
Measurement in quantum physics, in other words, acts as a disturbing element. It can therefore sometimes increase the energy in our duo of atoms. But after the intervention of their boss, it happens that the motivation of the atoms (or their energy) decreases. Thus, this disruption, if it becomes daily, will, on average, generate neither gain nor loss of motivation.
Not all interactions have the same effects, and there are disorders that are, on average, much more useful. Imagine, for example, that the boss finds that one of her employees works faster than the other, but has difficulty motivating herself. So even though the interaction between them makes them work less time, as the best employee is more motivated, the amount of work done is preserved during this interaction (just as energy is conserved when two atoms interact). Thus, once the fast worker is motivated, there is only left to cut in the interaction between the employees to make them work longer and increase the total amount of work.
By choosing an appropriate type of measurement (a measurement that says nothing about their energy, or in the form of specialists who do not commute with Hamiltonian of the nuclear system), it is also possible in quantum physics to transfer l energy to measured systems, even in average.
To better understand this phenomenon, let’s take two magnets together. The energy associated with their interaction is negative because energy had to be used to separate them. Since each magnet has weight, they both have potential energy. To measure the weight of the second magnet, attach a dynamometer to it (our measuring instrument, a kind of weight on which you attach the object whose weight you want to know) and pull the spring until the specified value no longer varies. Under these conditions, the magnets are so far apart that the magnetic interaction between them becomes insignificant. It is then possible to rotate the first magnet without changing its energy. By then depositing the second magnet close to the first, their identical poles will then come face to face. The magnets will therefore repel each other and it is now possible to extract energy from the system.
In quantum physics there is no reason to make the interaction between atoms insignificant, the measurement itself can modify the state of the atoms, a bit as if it allowed the first magnet to rotate directly.
Measurement as an energy source or “quantum motors”
Using this type of mechanism, we therefore work at Institut Néel in Grenoble to imagine “quantum motors”, where there is no need for “thermal baths”, ie reservoirs of hot and cold energy, but simply to measure the state of our system to then be able to extract more energy from it.
Be careful, these motors should not be seen as the technology of the future for our electric cars and mobile phones. It is simply a transfer of energy from our measuring device to a quantum system: the measuring device is so large that the energy it loses is very small in relation to its total energy.
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The interest in these “quantum engines” is twofold. On the one hand, it lies in their ability to modify the energy of quantum systems, for example, amplifying the light from a laser. On the other hand, they allow the study of the process of quantum measurement, which is still one of the major gray areas for physicists, because by measuring a quantum system one chooses a particular state, but there is no one, there is currently no consensus on the equation that makes it possible to describe this development. A better understanding of these basic phenomena is essential to set up and optimize the energy consumption of quantum processors, the essential building blocks of quantum computers.