In a new research paper, Google scientists claim to have used quantum processors for a useful scientific application: observing real time crystals.
If “time crystals” sound sci-fi, that’s because they are. Time crystals, as the researchers put it, are nothing less than a new “phase of matter,” which has been theorized over the years as a new state that might join the ranks of solids, liquids, gases, crystals, and more. The paper is still in preprint and still needs peer review.
Time crystals are also hard to find. But the Google scientists are now rather excited to say that their results establish a “scalable approach” to studying time crystals on current quantum processors.
Understanding why time crystals are interesting requires some background in physics—specifically knowledge of the second law of thermodynamics, which states that systems naturally tend to be in a state called “maximum entropy.”
Case in point: if you pour some milk into a coffee cup, the milk will end up dissolving throughout the coffee rather than staying on top, bringing the whole system into equilibrium. This is because there are far more ways to have the coffee spread randomly throughout the coffee than to sit on top of the cup in a more orderly fashion.
As described by the second law of thermodynamics, this irresistible drive to thermal equilibrium reflects the fact that everything tends to move toward less useful random states. Over time, systems inevitably degenerate into chaos and disorder—aka entropy.
Time crystals, on the other hand, cannot reach thermal equilibrium. Rather than slowly degenerating into randomness, they get stuck in two high-energy configurations, switching between them—a back-and-forth process that can go on forever.
To better explain this, Curt von Keyserlingk, a lecturer at the University of Birmingham’s School of Physics and Astronomy, who was not involved in Google’s latest experiment, pulled some slides from an introductory talk to prospective undergraduates. “They usually pretend to understand, so it might be useful,” warns von Keyserlingk.
It started with a thought experiment: Take a box in a closed system isolated from the rest of the universe, put in a few dozen coins and shake it a million times. As the coins flip, tumble, and bounce around each other, they move randomly and become increasingly chaotic. When opening the box, expect to see about half of the coins come up heads and half face down.
It doesn’t matter whether the experiment starts with more coins up or down: the system forgets what the initial configuration was, and becomes increasingly random and chaotic as it shakes.
This closed system, when translated into the quantum domain, is the perfect setting for trying to find time crystals, and the only one known so far. “The only stable time crystals we envision in closed systems are quantum mechanical,” von Keyserlingk said.
Enter Google’s quantum processor Sycamore, which is known for achieving quantum supremacy and is now looking for some kind of useful quantum computing application.
By definition, quantum processors are perfect tools for replicating quantum mechanical systems. In this scenario, Google’s team represented coins in a box with qubits spinning up and down in a closed system; instead of shaking the box, they applied a specific set of quantum operations that changed the state of the qubits , they repeated many times.
This is where time crystals defy all expectations. Looking at the system after a certain number of manipulations or shaking reveals that the configuration of the qubits is not random, but rather looks very similar to the original setup.
“The first element that makes up a time crystal is that it remembers what it was doing in the first place. It doesn’t forget,” von Keyserlingk said. “The boxed coin system will forget, but the time crystal system will not.”
It doesn’t stop there. Shake the system an even number of times and you’ll get a similar configuration to the original – but shake it an odd number of times and you’ll get another setup where the tail has been flipped front and back – the opposite.
And no matter how many operations are performed on the system, it always flips, switching back and forth between these two states periodically.
Scientists call this a breakthrough in time symmetry – which is why time crystals are called time crystals. This is because the action performed to stimulate the system is always the same, and the response is just every other shock.
“In Google’s experiment, they do a set of operations on this spin chain, and then they do the exact same thing over and over again. They do the same thing at step 100, if they go this far,” von Keyserlingk Say.
“So they subject the system to a set of conditions that has a symmetry, but the way the system responds breaks that symmetry. It’s the same every two cycles instead of every cycle. That’s what makes it really time. crystals.”
From a scientific point of view, the behavior of time crystals is fascinating: contrary to all other known systems, they do not tend towards disorder and chaos. Unlike coins in a box, which mix and settle with roughly half heads and half tails, they violate the laws of entropy by trapping in a special state of time crystals.
In other words, they violate the second law of thermodynamics, which essentially defines the direction of all natural events.
Such special systems are not easy to observe. Time crystals have been a topic of interest since Frank Wilczek, a MIT professor who won the Nobel Prize in 2012, started thinking about them. Since then, the theory has been refuted, debated, and refuted many times.
To date, several attempts have been made to create and observe time crystals, with varying degrees of success. Just last month, a team at Delft University of Technology in the Netherlands published a preprint showing that they had built a time crystal in a diamond processor, albeit a smaller system than Google claims.
The search giant’s researchers used a chip with 20 qubits as a time crystal — far more than what has been achieved so far and beyond what can be achieved with conventional computers, according to von Keyserlingk.
About 10 qubits can be easily simulated using a laptop, von Keyserlingk explained. Add more, and you quickly hit the limits of current hardware: every additional qubit requires exponential amounts of memory.
The scientist did not say the new experiment was a demonstration of quantum supremacy. “For me, they are not far enough away to be implemented with classical computers, because there might be a clever way to put it on classical computers that I hadn’t thought of,” von Keyserlingk said.
“But I think this is the most convincing experimental demonstration of time crystals to date.”
The scope and control of Google’s experiments means that time crystals can be viewed for longer periods of time, detailed sets of measurements can be made, the size of the system can be changed, and more. In other words, it’s a useful demonstration that can really advance science — so it could be the key to showing that quantum simulators play a central role in advancing discoveries in physics.
Of course, there are some caveats. Like all quantum computers, Google’s processors still suffer from decoherence, which causes the quantum state of the qubits to decay, meaning the oscillations of the time crystal inevitably die out as the environment interferes with the system.
However, the preprint argues that this problem can be alleviated as processors become more effectively isolated.
One thing’s for sure: Time crystals won’t be in our lives anytime soon, because scientists haven’t yet found a clear useful application for them. Therefore, Google’s experiment is unlikely to explore the commercial value of time crystals. Instead, it showcased what could be another early application of quantum computing, and yet another demonstration of the company’s technological prowess in a new and competitive field.
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