Time crystal in a quantum computer

Time crystal in a quantum computer

There is a huge worldwide effort to design a computer that can harness the power of quantum physics to perform calculations of unprecedented complexity. While formidable technological hurdles still stand in the way of creating such a quantum computer, today’s early prototypes are still capable of remarkable feats.

The Google Sycamore chip used in the creation of a time crystal. (Image credit: Google Quantum AI)

For example, the creation of a new phase of matter called “time crystal”. Just as the structure of a crystal repeats itself in space, a time crystal repeats itself in time, and more importantly, does so ad infinitum and without any further input of energy – like a clock that runs indefinitely without any batteries. . The quest to realize this phase of matter has been a long-standing challenge in theory and experiment – a challenge that has finally come to fruition.

in research published November 30 in Naturea team of scientists from Stanford University, Google Quantum AI, the Max Planck Institute for Complex Systems Physics and the University of Oxford detail their creation of a time crystal using Sycamore quantum computing hardware from Google.

“The big picture is that we’re taking the devices that are supposed to be the quantum computers of the future and thinking of them as complex quantum systems in their own right,” said Matteo Ippoliti, Stanford postdoctoral fellow and co-lead author. . work. “Instead of computation, we put the computer to work as a new experimental platform to realize and detect new phases of matter.”

For the team, the excitement of their achievement lies not just in creating a new phase of matter, but in opening up opportunities to explore new regimes in their field of matter physics. condensed, which studies the new phenomena and properties caused by the collective interactions of several objects in a system. (Such interactions can be much richer than the properties of individual objects.)

“Time crystals are a striking example of a new kind of non-equilibrium quantum phase of matter,” said Vedika Khemani, assistant professor of physics at Stanford and senior author of the paper. “While much of our understanding of condensed matter physics is based on equilibrium systems, these new quantum devices give us a fascinating window into new non-equilibrium regimes in many-body physics. “

What time crystal is and is not

The basic ingredients to craft this time crystal are: The physical equivalent of a fruit fly and something to kick it. The fruit fly of physics is the Ising model, a long-standing tool for understanding various physical phenomena – including phase transitions and magnetism – which consists of a lattice where each site is occupied by a particle that can be in two states, represented by a spin up. or down.

During his graduate years, Khemani, his doctoral supervisor Shivaji Sondhi, then at Princeton University, and Achilleas Lazarides and Roderich Moessner at the Max Planck Institute for the Physics of Complex Systems came across this recipe. to unwittingly craft Time Crystals. They were studying non-equilibrium localized many-body systems – systems where particles remain “stuck” in the state they started out in and can never relax back to an equilibrium state. They were interested in exploring the phases that might develop in such systems when periodically “kicked” by a laser. Not only did they manage to find stable out-of-equilibrium phases, but they found one where the spins of the particles flipped between patterns that repeated in time forever, at a period twice the period of driving the laser, creating a time crystal.

The periodic kick of the laser establishes a rhythm specific to the dynamic. Normally, the “dance” of the towers should synchronize with this rhythm, but this is not the case in a time crystal. Instead, the spins switch between two states, only completing a cycle after being hit by the laser twice. This means that the “temporal translation symmetry” of the system is broken. Symmetries play a fundamental role in physics, and they are often broken – explaining the origins of regular crystals, magnets and many other phenomena; however, time translation symmetry is distinctive because unlike other symmetries, it cannot be broken in equilibrium. The periodic kick is a loophole that makes time crystals possible.

The doubling of the period of oscillation is unusual, but not unprecedented. And long-lived oscillations are also very common in the quantum dynamics of few-particle systems. What makes a time crystal unique is that it is a system of millions of things that exhibit this kind of concerted behavior without any energy entering. Where flee.

“It’s a completely robust phase of matter, where you don’t adjust parameters or states, but your system is still quantum,” said Sondhi, professor of physics at Oxford and co-author of the paper. “There is no energy supply, there is no energy leakage, and it goes on forever and it involves many strongly interacting particles.”

Although it may sound suspiciously close to a “perpetual motion machine”, closer examination reveals that Time Crystals do not violate any laws of physics. Entropy – a measure of disorder in the system – remains stationary over time, satisfying the second law of thermodynamics at the margin by not decreasing.

Between the development of this blueprint for a time crystal and the quantum computer experiment that made it happen, many experiments by many different teams of researchers have gone through various near-crystalline milestones. However, providing all the ingredients for the recipe for “many-body localization” (the phenomenon that makes for an infinitely stable time crystal) remained a daunting challenge.

For Khemani and his collaborators, the final step to success with the Time Crystal was working with a Google Quantum AI team. Together, this group used Google’s Sycamore quantum computing hardware to program 20 “spins” using the quantum version of a classical computer’s bits of information, called qubits.

Revealing how intense the interest in time crystals is currently, another time crystal has been published in Science this month. This crystal was created using qubits in a diamond by researchers at the Delft University of Technology in the Netherlands.

Quantum Opportunities

The researchers were able to confirm their claim of a real time crystal thanks to the special abilities of the quantum computer. Although the finite size and coherence time of the (imperfect) quantum device meant that their experiment was limited in size and duration – so that time crystal oscillations could only be observed for a few hundred cycles rather than indefinitely – the researchers devised various protocols to assess the stability of their creation. These included running the simulation forward and backward in time and scaling its size.

A view of Google’s dilution refrigerator, which houses the Sycamore chip. (Image credit: Google Quantum AI)

“We managed to use the versatility of the quantum computer to help us analyze its own limitations,” said Moessner, co-author of the paper and director of the Max Planck Institute for the Physics of Complex Systems. “He basically told us how to correct his own mistakes, so that the fingerprint of ideal temporal crystal behavior could be determined from finite-time observations.”

A key signature of an ideal time crystal is that it exhibits indefinite oscillations of all States. Verifying this robustness to choice of states was a key experimental challenge, and the researchers devised a protocol to probe more than a million states of their time crystal in a single run of the machine, requiring only a few milliseconds of time. ‘execution. It’s like looking at a physical crystal from multiple angles to check its repeating structure.

“A unique feature of our quantum processor is its ability to create very complex quantum states,” said Xiao Mi, researcher at Google and co-lead author of the paper. “These states allow efficient verification of the phase structures of matter without the need to survey the entire computational space – an otherwise intractable task.”

Creating a new phase of matter is undeniably exciting on a fundamental level. Moreover, the fact that these researchers were able to do so indicates the growing usefulness of quantum computers for applications other than computing. “I am optimistic that with more and better qubits, our approach can become a primary method for studying non-equilibrium dynamics,” said Pedram Roushan, Google researcher and lead author of the paper.

“We think the most exciting use of quantum computers right now is as platforms for fundamental quantum physics,” Ippoliti said. “With the unique capabilities of these systems, there is hope that you will discover a new phenomenon that you did not anticipate.”

This work was led by Stanford University, Google Quantum AI, the Max Planck Institute for Complex Systems Physics, and the University of Oxford. The complete list of authors is available in the Nature paper.

This research was funded by the Defense Advanced Research Projects Agency (DARPA), a Google Research Award, the Sloan Foundation, the Gordon and Betty Moore Foundation, and Deutsche Forschungsgemeinschaft.

To read all of Stanford’s science stories, subscribe to the bi-weekly Stanford Scientific Summary.