A time crystal, first proposed by physicist Frank Wilczek in 2012, is a phase of matter which repeats in time, similar to how a regular crystal's structure repeats in space. What that means is that the particles in the crystal perpetually switch between two states without requiring the input of more energy and without losing any energy.
These crystals are the first objects to break what is known as "time-translation symmetry," a rule in physics that says that a stable object will remain unchanged throughout time. Time crystals avoid this rule, being both stable and ever-changing.
So, for example, ice when stable will remain ice and will only change when temperature or another factor makes it unstable. A time crystal would change even when in its ground state, acting differently than all other phases of matter.
But scientists still needed to figure out how to create this phase of matter. They turned to a phenomenon called many-body localization.
Today @Nature published our quantum computing research on time crystals. Excited to see more experiments like this not only in physics, but future quantum applications in other fields too #GoogleAI https://t.co/d54qdMtlW4 https://t.co/0dWPR4wTOz— Sundar Pichai (@sundarpichai) November 30, 2021
Many-body-localization is when a one-dimensional chain of quantum particles gets stuck in a fixed state. Each particle in the chain has a magnetic orientation (known as "spin") that points up, down, or a certain probability of both directions, according to Quanta Magazine.
For example, imagine the particles are set up so that the first one points up, the next one points down, the one after that points down and the one after that points up. Usually, the spins would quantum mechanically fluctuate and align, if possible. But with random interference between the particles confining their activity, the row of particles can get stuck in a particular configuration, unable to rearrange or settle into thermal equilibrium. They'll point in that configuration indefinitely.
In 2014, Vedika Khemani, a condensed matter physicist who is now at Stanford, and Shivaji Sondhi who was her doctoral advisor at Princeton at the time, found that many-body localized systems could exhibit a special kind of order: if all of the spins in the system were flipped, it would be another stable, many-body localized state, according to Quanta Magazine.
What this means is that if the system were to be prodded with a laser, it would forever cycle between the two states without absorbing or releasing energy from the laser.
Khemani and Sondhi, together with Achilleas Lazarides and Roderich Moessner at the Max Planck Institute for Physics of Complex Systems, were able to find such a system where the spins of the particles flipped between patterns that repeated forever, at a period twice that of the period of the laser.
This system is unique because it is a system of millions of things that oscillate between two states, only completing a cycle when prodded twice by the laser, and doing so without absorbing or releasing energy.
An article on Stanford's website stressed that while this may sound like a "perpetual motion machine," which would break the laws of physics by allowing perpetual motion without any external energy source, this is not the case.
Entropy - a measure of the disorder in the system - remains stationary, not increasing, but not decreasing either, which means it still fits into the second law of thermodynamics which rules that disorder cannot decrease, but allows for the disorder to remain at a constant level as long as the process is reversible.
An example of a reversible process is a gas flowing through a pipe that is tight in the middle. As the flow moves through the constricted part of the pipe, its pressure, temperature and velocity change, but these values return to their original conditions after they enter the widened part of the pipe, meaning that the change in entropy is zero.
While other attempts have gotten close to making a time crystal, the crystal described in the study published in Nature is the first to meet all the requirements needed to make a truly infinitely stable time crystal.
The access to Google’s Sycamore quantum computing hardware was what allowed the researchers to make their breakthrough.
Although the hardware is still imperfect, meaning that the experiment was still limited in size and duration, the researchers created a number of protocols that allowed them to assess the stability of the time crystal, including running the simulation forward and backward in time and scaling the size.
“We managed to use the versatility of the quantum computer to help us analyze its own limitations,” said Roderich Moessner, co-author of the paper and director at the Max Planck Institute for Physics of Complex Systems. “It essentially told us how to correct for its own errors, so that the fingerprint of ideal time-crystalline behavior could be ascertained from finite time observations.”
The researchers used a chip with 20 qubits - controllable quantum particles which maintain two possible states, 0 and 1, at the same time - made out of superconducting aluminum strips, according to Quanta Magazine. The states were programmed to represent up or down spins.
The programmers were able to randomize the interaction strengths of the qubits, creating the interference needed to lock the particles into a set pattern of spins instead of letting them align.
The researchers tested a large number of initial configurations to see if they could all be locked into an eternally oscillating pattern of spins which would oscillate at twice the period of the prodding, probing over a million states in just a single run of the machine.
They were also able to extrapolate trends from the relatively small systems that could be created on the Sycamore hardware to much large systems. The researchers were also able to show that, except for decoherence in the processor itself, there was no increasing entropy in the simulated system itself.
All of these findings together substantially bolstered the case for the existence of time crystals more so than past experiments were able to.
“I am optimistic that with more and better qubits, our approach can become the main method in studying non-equilibrium dynamics,” said Pedram Roushan, a researcher at Google and senior author of the paper.
“We think that the most exciting use for quantum computers right now is as platforms for fundamental quantum physics,” said Matteo Ippoliti, a postdoctoral scholar at Stanford and co-lead author of the work. “With the unique capabilities of these systems, there’s hope that you might discover some new phenomenon that you hadn’t predicted.”
On Saturday, another team at QuTech, a collaboration between the Delft University of Technology and the Netherlands Organisation for Applied Scientific Research (TNO), published their findings on a time crystal they had created with a quantum processor which lasted about eight seconds.
“While a perfectly isolated time crystal can, in principle, live forever, any real experimental implementation will decay due to interactions with the environment,” said Joe Randall on QuTech's website. “Further extending the lifetime is the next frontier.”
The QuTech team used nine quantum bits and manipulated them to lock their spins into a periodically inverting pattern which formed from a variety of different initial states.
The team referred to the research conducted on the Google Sycamore quantum computer, with Tim Taminiau, lead investigator at QuTech, saying that "It is extremely exciting that multiple experimental breakthroughs are happening simultaneously."
"All these different platforms complement each other. The Google experiment uses two times more qubits, our time crystal lives about ten times longer," said Taminiau.