The border between science and science fiction is sometimes almost imperceptible. And we owe it, of course, to our increasingly precise understanding of the world in which we live. That macroscopic world that we can see with our eyes and in which the processes seem to run in a single direction in time: from the present to the future.
We are so intimately used to observing this phenomenon that it is very difficult for us to accept the possibility of reversing a process in time. To recover it as it was before having undergone any change that we could consider permanent. But it’s not impossible. Quantum physics has just shown us that it is feasible both theoretically and practically.
Quantum physics and our intuition are, once again, about to collide
Our intuition invites us to conclude that the irreversibility of processes is a fundamental law. And the second law of thermodynamics proves us right. It can be formulated in many different ways, but all of them, if correct, invite us to conclude that physical phenomena are irreversible.
If we put a container of very hot water on our kitchen counter and do nothing with it, the water will cool down. And if we drop a glass and it explodes when it hits the ground, it won’t come back together on its own. Heat exchange and entropy are precisely two properties closely linked to the second law of thermodynamics.
Entropy is usually defined as the magnitude that measures the degree of disorder in a physical system. It is perhaps an excessive simplification, but it can help us understand what we are talking about without being forced to resort to complex concepts. In any case, this thermodynamic principle is of a statistical nature, and, furthermore, classical physics is deterministic.
This means that it is possible to predict the evolution of a physical system over time if we know its initial state and the differential equations that describe its behavior. However, in the domain of quantum physics, in the world of the very small, of particles, the reversibility of physical processes is possible. It has been so from a theoretical point of view for a long time, and now it is also so in practice.
Quantum physics allows it: a photon has gone back in time
Physicists have flirted with the possibility of reversing processes in time for many years. In fact, some theorists are working on very peculiar tools that quantum mechanics has placed in their hands: the universal rewind or rewind protocols. We do not need to know in detail how these mechanisms work, but it is helpful to know that they are used to reverse the changes that a quantum system has undergone without knowing its initial state. And without knowing what those changes consisted of either.
Universal reversion protocols are used to reverse the changes that a quantum system has undergone without knowing what its initial state was.
It almost looks like magic, but it’s not. It’s science. And, precisely, the Spanish theoretical physicist Miguel Navascués leads a research team at the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences who is an expert in this discipline. Miguel and his collaborators have designed an innovative theoretical reversal protocol that proposes, in broad strokes, what procedure can be used to get a quantum system to recover its initial state without knowing what changes it has undergone.
Putting something like this into practice is not easy, which has meant that experimental physicists working in this area have not been very successful. Fortunately, the landscape has changed. And it is that the team of experimental physicists from the University of Vienna led by Philip Walther has managed to successfully implement the universal reversion protocol designed by Miguel Navascués and his team.
The heart of their experiment is sophisticated optical equipment made up of several interferometers and fiber optic links that together behave like a quantum switch. Knowing in detail how this device works is outside the purpose of this article because, as we can guess, its complexity is extraordinary. Even so, anyone who is not easily intimidated and is curious can consult the article published by Navascués, Walther and their teams in the magazine Optica. It is very worth it.
The heart of his experiment is sophisticated optical equipment made up of several interferometers and fiber optic links that together behave like a quantum switch.
A note before we go any further: an interferometer is an optical device that uses a light source (usually a laser) to measure very precisely changes introduced into a physical system. Described in this way it seems very complicated, and yes, it is complicated, but we can use an example close in time to illustrate what we are talking about.
The LIGO experiments, in the United States, and Virgo, in Italy, used to identify and analyze gravitational waves are interferometers. And, as we have just seen, both incorporate sophisticated optical equipment and a laser that allows them to measure the gravitational disturbances generated by massive objects in the cosmos that are subjected to a certain acceleration. These disturbances propagate through the space-time continuum at the speed of light in the form of waves, and are picked up by interferometers.
In a way, the quantum switch that the Navascués and Walther teams have built is similar to LIGO or Virgo, but on an infinitely smaller scale because its purpose is to identify and measure the changes introduced into a quantum system. What they have achieved is astounding: they have successfully reversed the evolution in time of a photon without previously knowing its initial state or what changes it had undergone. In practice it is the same as traveling back in time.
This schematic describes the ingenious optical equipment designed by researchers from the University of Vienna and the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences.
It seems reasonable to think that achieving this with a single particle, with a photon, is not very interesting, but nothing is further from the truth. The result that these researchers have obtained, which has already been peer-reviewed, is extraordinary because it opens wide the doors that will probably allow us to understand much better the rules that underlie the world in which we live. The rules, in short, of quantum mechanics.
What allows this experiment to stand out from previous ones that also tried to demonstrate the possibility of reversing the state of a quantum system is that the universal reversion protocol of Navascués and Walther has managed to do it without having to no prior information about the state of the quantum system. We can see it as if they had managed to perfectly recompose a porcelain vase without knowing the number of fragments they initially had, their shape, and much less that they belonged to a vase and were made of porcelain.
In the conclusions of their article, these researchers insist on something very important: the results they have obtained are not only valid in quantum systems of a photonic nature, which are those that work with light; they are coherent with other quantum systems. For this reason, the applications of this technology can be very numerous, especially in the field of quantum computing.
And it is that universal reversion protocols can in theory be used to solve one of the biggest challenges currently posed by quantum computers: bug fixing. In fact, this is probably the highest wall that quantum computing researchers will have to break down to make quantum computers capable of solving the kinds of complex problems in which they are theoretically vastly superior to classical supercomputers.
Top image: Giallo
More information: Optics
