Argentine physicists created “time crystals”

Argentine physicists created “time crystals”
Argentine physicists created “time crystals”

With few exceptions, almost all the solid matter that is part of our daily life has a crystalline structure: ice, salt, sugar, concrete, metals, precious stones and most minerals; among them, some of those used in the technologies that define the modern world, such as quartz in watches or the semiconductors in our many electronic devices.

One of the defining characteristics of crystals is that their atoms and molecules are arranged in space in an orderly, regular and periodic manner. About a dozen years ago, Nobel Prize winner Frank Wilczek, awarded for his work in high energy physics, launched a bold hypothesis: he proposed that solid matter could not only be periodic in space, but also in time. That is, given certain conditions, a material could oscillate according to a pattern over time without the need for external disturbances.

As sometimes happens in science, later work showed that this approach, although tempting, was incorrect. However, based on the questions that arose from these ideas, several groups remained interested in the topic. Now, Argentine researchers have just achieved it: they developed a system that, induced by the disturbance of a continuous laser, presents a sustained periodic oscillation, “time crystals”. The discovery has just been published in none other than Science.

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Alex Fainstein (second from left) with part of his team. (PHOTO: Ramiro Sáenz Valenzuela, IB Press)

“We are happy, because it is a Science completely made here – Alex Fainstein, a Conicet researcher at the Bariloche Atomic Center of the National Atomic Energy Commission, a graduate and teacher at the Balseiro Institute, is proud. Except for the device, manufactured in Germany (we do not have that capacity for now), the idea, the theoretical and experimental work is local.”

The space is homogeneous; That is, if you put a group of atoms in a cubicle, they can be anywhere. However, What happens in most materials, if they are cooled and brought to their most fundamental state, is that they form a crystal, their atoms are located in space at the same distance from each other..

“Although in space one point is the same as the next, in crystals, the atoms are located as if there were a box of eggs below that indicates their position,” Fainstein illustrates, “they are arranged periodically. Since physicists see in the equations many similarities between spatial and temporal coordinates, Wilczek said: ‘If in fundamental states there is a symmetry breaking and the atoms are arranged as if there were a box of eggs underneath, couldn’t the same thing happen? over time? I mean, He asked whether the fundamental state of a material will not be periodic in both space and time.”.

Ignacio Carraro-Haddad, Dimitri Chafatinos, Alexander Kusnetsov and Ignacio Papuccio-Fernández (Photo courtesy of IB Press)

That a material suddenly acquires a periodic temporal structure could be imagined with a lake that without any external disturbance, instead of being mirrored, begins to oscillate. Wilczek found a system that in its most stable state, which typically occurs when it is isolated and at low temperatures, instead of being still, oscillated. It was what he called “crystals,” but instead of being space crystals, like the ones we know, he called them “time crystals” or “crystals in time.”

He published his paperbut less than a year later there was another that showed that it was incompatible with the equations of quantum mechanics”says Fainstein.

However, the same people who questioned it began to think that the idea was interesting and that it would be good to look for conditions in which a system undergoes a transition towards something that varies periodically in time without being induced, as is done when pushing a hammock.

What the group led by Fainstein, who has been working in optomechanics for many years, did was use a laser, an optical table and a single “nanocavity” that functions as a “tiny mirror trap” to create it.

“Basically, we work with these devices called ‘optical microcavities’ that can be thought of as two mirrors that are very close, and that have the ability to confine or capture light –says Ignacio Carraro-Haddad, first author of the paper and doctoral candidate in Physics at the Balseiro Institute. One shoots a laser at it and can confine that light emission within the cavity. But the interesting thing is that these cavities not only contain light, but also vibrations. The light is bouncing and so are the mechanical vibrations.”

The work team at the Photonics and Optomechanics Laboratory of the Bariloche Atomic Center (Artistic photo by Ramiro Sáenza Valenzuela, Prensa IB)

“We shoot light that is confined and interacts with electrons forming millions of particles that are a mixture of light and electrons –Details Fainstein–. As our laser is continuous (not pulsed), the number of photons we are sending is the same all the time. We excite the system and then we find out what is happening to it because it emits different light than what it receives, of other colors.”

What the scientists were able to see is that when they excited the system with the laser, at first light of a single color, of a single frequency, came out. But when they increased the power, two different colors automatically appeared, a very strong suggestion that the system was oscillating. And when they continued to increase the power, The system formed from the interaction between light, electrons and sound generated its own dynamics where everything oscillated in unison, like the “ticking” of a clock.. Then, the theoretical physicists in the group developed a model to explain what these particles are like, how they interact, and whether they can give rise to the phenomena they were observing.

Carraro-Haddad, from Salta and only 24 years old, explains it like this: “The laser excites electrons from the semiconductor material, and these electrons couple with the light that is confined in the cavity, combining in a quantum superposition of electron and light that has the name of a quasiparticle resulting from the coupling between light and matter, the ‘polariton’. We trap the polaritons, we fix them in a place in space, and then, because they have matter and they have light, they can interact with the vibrations of the cavity. It occurs as a coordinated dance between light, electrons and mechanical vibrations. The oscillation that light has is the ‘crystal of time’, because it is ordered periodically. The interesting thing is that mechanical vibrations stabilize the frequency of the time crystal, giving it a well-defined rhythm. One can imagine the vibrations as a metronome that dictates the rhythm to the time crystal. And that metronome activates itself: you simply hit it with a continuous laser, the metronome turns on and the polaritons begin to dance spontaneously. And when one increases the power further, this dance of the electrons doubles its period, that is, it takes twice as long to make that oscillatory movement; “It would be like going from a quarter note to a white note.”

Gonzalo Usaj, professor at the Balseiro Institute and co-author of the work (IB Press)

This is a remarkable work that causes me healthy envy – comments Juan Pablo Paz, full professor at the UBA, senior researcher at Conicet and former Vice Minister of Science and Technology between 2019 and 2023, who did not participate in the experiment.. On the one hand, it is the result of several years of studies by the group led by Alex Fainstein at the Bariloche Atomic Center. They had already published in very high impact journals, but now with the publication in Science, crown a line entirely conceived and promoted from Argentina. The theoretical analysis work and, more importantly, the original idea, were coined in Bariloche. There are few occasions in which it is possible to publish a contribution of these characteristics that is part of a scientific and technological trend that is booming worldwide: quantum science and technology, which promise to generate applications that revolutionize metrology (allowing super measurements). precise measurements of different physical magnitudes), and the transmission and processing of information. They built a system using novel ‘quantum’ materials in which something remarkable happens: beyond a certain threshold, they begin to oscillate with a certain frequency that characterizes the temporal crystal. The material with which this crystal is formed arises from the interaction of light (photons) with which the system is excited and the vibrations (excitons) of the solid substrate that composes it. Although I am not an expert on the subject, I think it is spectacular. My congratulations to the team, who are not in vain at the head of one of the ‘High Impact Federal Networks’ that the now defunct Ministry of Science, Technology and Innovation financed last year seeking to promote excellence and focus Argentine scientific studies on topics of national and international relevance and interest.”

Although they clarify that they are a basic science group (motivated by curiosity, which tries to find and describe novel behaviors in the matter), the researchers do not miss that any discovery of this type inspires possible applications. “I would say there are two interesting things,” Fainstein suggests. On the one hand, today The information spreads from one place to another by light, by optical fibers, with lasers, and then it must be transformed into an electronic circuit so that a computer can process it. All of this involves a translation between two things that cost energy and time. So the more processing you can do directly with light, without needing to convert it into electrical currents, the better. That’s called integrated photonics. Finding new materials that are capable of doing different processing at very high frequencies is part of what is being sought. What we do has to do with integrated photonics, because precisely what we are looking for is to couple light with vibrations in the order of gigahertz. Our ‘time crystal’ oscillates at tens of gigahertz. They are also trying to couple light with microwaves, because communication between cell phones is by microwave, so there is always an instance in which one would like to transmit information that comes via fiber optics through microwaves. But if you have to convert to electricity, you lose energy and time. If one could make a direct translation from light to microwaves it would be greateither. “We feed our system with light and it oscillates at microwave frequencies, so the translation would be direct.”

And he concludes: “The second possibility, a little more fanciful, but which I definitely want to try before I retire, has to do with the world of quantum computing. Today, the most promising quantum computers, like the ones Google has, are based on superconducting circuits that operate and manage all their information through microwaves. If I wanted to communicate one computer with another (which is how the world of information processing works today) the problem is how to connect them.. Since all information is handled with microwaves, one computer has to ‘talk’ to another by sending it microwaves. And that can’t be done. In quantum, when the microwaves are removed from the cryostat, which is at millikelvin [apenas por encima del cero absoluto], the external world has infinite noise and information is immediately lost. So what they have to do is convert it into light, which are much higher frequencies, where there is no noise in the environment, because the noise comes from the temperature and the temperature does not influence the light. So, you have to convert microwave information into light, but in the quantum limit: a microwave photon into a light photon. Since our system is so efficient that it oscillates on its own, you don’t even have to force it to oscillate in such a way that the light connects with the microwaves. Our idea is to try to make that conversion from microwaves to light in the limit of a single photon.”

Dimitri Chafatinos, Alexander Kuznetsov and Ignacio Papuccio-FernándezA are also authors of this work. A. Reynoso, A. Bruchhausen, K. Biermann, PV Santos and Gonzalo Usaj.

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