There have always been certain "Rules of the Road" when it comes to our Universe, and one of them is the Second Law of Thermodynamics.
This law states that when energy changes from one form to another, or when matter moves freely, entropy (disorder) in a closed system always increases. Entropy is a measure of the spread of matter and energy to everywhere in the Universe to which it has access.
The best way to understand entropy is to consider my kitchen: Every day there are dirty dishes in the sink and the countertops are covered with various substances. Every day, I wash the dishes and clean the countertops, thus decreasing their entropy, but the very next day, the sink is again full of dirty dishes and the counters are covered.
This was the fate of everything in our Universe until July 29, 2021. That's when researchers posted a preprint on the arXiv website entitled, "Observation of Time-Crystalline Eigenstate Order on a Quantum Processor," which announced the discovery of "time crystals". A preprint is an academic paper that has yet to be peer-reviewed or formally published. A preprint's findings can be challenged or even overturned entirely.
If you're thinking, "What's the big deal, I've heard of time crystals before," it may be because they have been a staple in the BBC's long-running sci-fi series, Dr. Who. Indeed, the fan site DWLegacy Wiki states, "The surest way to obtain time crystals is to buy them ... They are sold in bundles of 1, 6, 13, 27, and 70 ..." In the real world, however, time crystals are a little harder to come by.
What's a "time crystal"?
A time crystal is an object that is comprised of a novel phase of matter and that moves in a regular, repeating cycle continuously and without using any energy. One of the discoverers of time crystals, Roderich Moessner who is director of the Max Planck Institute for the Physics of Complex Systems in Dresden, Germany recently told Quanta Magazine, that with time crystals, "You evade the second law of thermodynamics." That's a pretty big deal!
Time crystals are the first objects ever created that spontaneously break "time-translation symmetry." A translation is just a movement from one place to another, whether spatially or in time. Up until now, a stable object moving through time should remain the same, but time crystals don't. They are both stable and ever-changing at the same time.
Time crystals are actually a new phase of matter. Up until now, the four phases of matter, which all rely upon temperature, are: Solid, liquid, gas, and plasma.
In the solid phase, an object's molecules are closely bound together, and the volume of a solid is determined by its shape.
In the liquid phase, the molecular forces are weaker than in the solid phase, but a liquid still has a fixed volume, and it takes the shape of its container.
In the gas phase, the molecular forces are very weak, and a gas takes both the shape and the volume of its container.
The plasma phase only occurs during very high temperatures and pressures, when electrons are stripped from their orbits around an atom's nucleus and leave behind a positively charged ion. What results is a mixture of neutrally charged atoms, negatively charged free electrons, and positively charged ions. This mixture, or plasma, both respond to and generate electromagnetic forces.
In the four phases, atoms are locked into the lowest energy state that is permitted by the ambient temperature, and they don't change over time. A time crystal is the first "out-of-equilibrium" phase of matter that while stable, is also in an evolving state.
Dr. Who or Dr. Wilczek?
Time crystals were first envisioned in 2012 by the Nobel Prize-winning MIT professor Frank Wilczek who was then teaching a class on regular crystals. In regular crystals, individual atoms prefer to be at a specific point in space, and examples include the salt in your salt shaker and snowflakes.
Wilczek began considering how crystals might behave over time, with some of them undergoing periodic motion then returning to their initial configuration. The most interesting notion in Wilczek's time crystal idea was that; despite moving, time crystals would require no input of energy, and their motion would continue indefinitely, just like a perpetual motion machine.
By 2014, Wilczek's time crystal concept had been shot down by Haruki Watanabe and Masaki Oshikawa in a paper entitled, "Absence of Quantum Time Crystals." The authors stated, "In analogy with crystalline solids around us, Wilczek recently proposed the idea of 'time crystals' as phases that spontaneously break the continuous time translation into a discrete subgroup . . . here we first present a definition of time crystals based on the time-dependent correlation functions of the order parameter. We then prove a no-go theorem that rules out the possibility of time crystals defined as such, in the ground state or in the canonical ensemble of a general Hamiltonian, which consists of not-too-long-range interactions." All you need to take away from that statement is the "no-go" part.
Then, in 2015, a graduate physicist student at Princeton named Vedika Khemani, along with Moessner, Shivaji Sondhi who was Khemani's Ph.D. advisor, and Achilleas Lazarides of Loughborough University in the UK published a paper which described possible methods for creating time crystals, and it set off a five year race between Khemani's group and another group of physicists to create the first time crystal. However, none of their efforts was successful, and that's when Khemani and company got the idea to approach Google.
In 2019, Google's Sycamore quantum computer completed a task in 200 seconds that would have taken an ordinary computer 10,000 years to solve. After that, scientists were able to make the regular computer smarter so that it wouldn't take it that long to solve the same problem. It was good that Sycamore solved that problem because it hadn't been too successful up until that point. Sycamore was still too error-prone to solve cryptographic or algorithmic problems, so when Google Scholar Kostya Kechedzhi was approached by Khemani and her team, he was thrilled to put his machine to work as a scientific tool to study new physics or chemistry.
In the fall of 2014, Khemani had joined Sondhi, who was on a sabbatical at the Max Planck Institute in Dresden, Germany. There, Khemani, and Sondhi began looking at the work of Nobel Prize-winning physicist Philip Anderson, specifically at an area of quantum mechanics called Anderson localization. This involves an electron, which normally can be pictured as a wave which spreads out over time. The higher the amplitude of the wave in certain places, the greater is the probability of finding the electron at that place.
Anderson had discovered that a random defect within a crystal lattice can cause an electron's wave to break up, allowing it to interfere with itself. This meant that the wave was canceled out everywhere except for at a very small location. Then, a team at Princeton and Columbia universities found that several particles could get stuck in a similar fixed state as well.
Also at the Max Planck Institute, along with Khemani and Sondhi, were Moessner and Lazarides, who were at work "tickling" crystals with a laser. The four quickly realized that if they tickled a localized chain of particles in a certain way, the particles' spins would flip back and forth between two states continuously, and without absorbing any energy from the laser.
The group published their findings, calling the new entity a "pi spin-glass phase," but it took a reviewer of the article to realize that what had actually been discovered were time crystals. In March 2016, the other group of physicists chasing time crystals, which was comprised of Chetan Nayak, now of Microsoft Station Q and the University of California, Santa Barbara, Dominic Else, and Bela Bauer, published a paper describing Floquet time crystals.
Last month, that group reported in the journal Science that they'd created "prethermal" time crystals, which would not run forever and would instead come into equilibrium. That's when Moessner and company realized that Sycamore had everything they needed to create a time crystal that would change state continually and not require any energy input.
Like all quantum computers, Sycamore is comprised of "qubits," which are quantum particles that can be in two possible states simultaneously. Sycamore's qubits are superconducting aluminum strips which the Google engineers programmed to simulate a particle's spin — pointing either up or down.
By tuning the strength of the interactions between the qubits, destructive interference was introduced, which was the same as that produced by random defects within a crystal, and the 20 qubits locked into a set orientation. The researchers then tickled the system with microwaves, which caused the system to flip back and forth between two "many-body localized states;" the spins neither absorbed nor dissipated energy from the microwaves, which left the disorder, or entropy, of the system unchanged.
In much the same way that calculus was simultaneously and independently discovered by Isaac Newton and Gottfried Wilhelm Leibniz, on July 5, 2021 — mind you, the two had a rough rivalry going on — a third team of researchers at Delft University of Technology in the Netherlands reported that they too had built a time crystal, this time in a diamond.
A use for time crystals
While a time crystal is the first out-of-equilibrium phase of matter, many more might be possible. Even though Albert Einstein made headway in unifying space and time into what we now call "space-time", the fact remains that time is still fundamentally different, in that it only flows in one direction.
As for the potential uses for time crystals, the qubits within quantum computers exhibit superposition, that is, they can exist in multiple states at one time. That means that data input into a quantum computer, stored within it, or output from it will change depending on when it is observed. By generating time crystals in quantum bits, it would make that data consistently viewable. In other words, it would allow users to view the exact data they need, with a large probability that it is correct.
Personally, I think the best use of time crystals would be in my kitchen, where in one state there would be dirty dishes and messy counters, and in the other state they would both be clean. And, best of all, I wouldn't have to do a thing.