April 23, 2024

Researchers from Stanford and Google, along with others, have created and observed a new phase in the matter. This is commonly known as the time crystal.

A considerable effort is underway to create a computer that can harness the power of quantum Physics to perform complex computations. Although many technological hurdles still prevent the creation of a quantum computer, early prototypes of such computers can perform remarkable feats.

Creating a new phase in matter is called a “time crystal.” This is similar to a crystal’s structure that repeats in space. But, unlike a crystal, a time crystal does not require additional energy and can run indefinitely. It works in the same way as a clock without batteries. This phase of matter was a long-standing challenge, which has finally been solved through experimentation and theory.

A team of scientists from Stanford University, Google Quantum AI, and the Max Planck Institute for Physics of Complex Systems (Oxford University) published their research on November 30, 2021, in the journal Nature. They describe how they created a time crystal with Google’s Sycamore quantum computing hardware.

“The overall picture is that we are taking devices that are meant to be quantum computers of tomorrow and considering them complex quantum systems in themselves,” stated Matteo Ippoliti (postdoctoral scholar at Stanford) and co-lead author. “Instead of computation, we are using the computer as an experimental platform to detect and realize new phases of matter.”

The team’s excitement lies in the creation of a new phase in the matter and the possibility of exploring new regimes within their field of condensed material physics. This study studies new phenomena and properties caused by the interaction of many objects in a system. These interactions can produce properties that are far more valuable than those of individual things.

Vedika Khemani (assistant professor of Physics at Stanford) said that time crystals were a striking example of a new non-equilibrium type of quantum phase of matter. She was also a senior author of the paper. While most of our knowledge of condensed matter is based upon equilibrium systems, these quantum devices offer us fascinating insight into non-equilibrium regimes within many-body Physics.

How time-clear is it, and what isn’t?

These are the essential ingredients for making this time crystal: A physics equivalent to a fruit fly and something to give it some energy. The Ising model is the fruit fly of Physics. It is an old tool for understanding physical phenomena such as phase transitions, magnetism, and magneticity. It consists of a lattice in which each site is occupied with a particle that can exist in two states. These are represented by spin up or spin down.

Khemani’s graduate school years were spent at Princeton University with Shivaji Sondhi (then at Princeton University), Achilleas Lazarides, and Roderich Moessner from the Max Planck Institute for Physics of Complex Systems. They unintentionally discovered the recipe for creating time crystals. They looked at non-equilibrium in many-body localized systems, systems in which particles remain in the same state when they began and cannot be reset to equilibrium. They wanted to explore the possible phases that could develop in these systems when they were periodically “kicked.” They found a stable non-equilibrium degree and discovered one in which the spins of particles alternated between patterns that repeated in time forever at twice the speed of the laser. This made them a time crystal.

The laser’s periodic kick establishes a rhythm to the dynamics. The “dance” of spins should follow this rhythm. However, in a time crystal, it doesn’t. Instead, the reels alternate between two states and complete a cycle after being kicked twice by the laser. The system’s “time-translation symmetry” has been broken. Symmetries are fundamental in physics and can often be broken. This explains the origins of regular crystals and magnets. However, time translation symmetry is unique because it cannot be broken in equilibrium, unlike other balances. The periodic kick is a loophole that makes it possible to make time crystals.

Although it is not common, doubling oscillation periods is uncommon. Stable oscillations are ubiquitous in the quantum dynamics of a few particle systems. A time crystal is a collection of millions of objects exhibiting this type of coordinated behavior without releasing any energy.

Sondhi, a professor of physics at Oxford, co-authored the paper. “It is a completely robust Phase of Matter, where you are not fine-tuning states or parameters, but your system remains quantum.” It doesn’t consume energy or feed it. It’s endless and involves many highly interacting particles.

This may seem like a “perpetual motion machine,” but a closer inspection reveals that time crystals do not break any laws. Entropy, a measure that measures the disorder in the system, remains stationary over time and marginally fulfills the second law.

Different research teams conducted many experiments to achieve various quasi-time-crystal milestones between the time-crystal development plan and the quantum computer experiment. It was a formidable challenge to provide all the ingredients for “many-body localization, ” which is the phenomenon that allows an infinitely stable time-crystal.

Khemani and her colleagues found the final step in achieving time crystal success by working with a Google Quantum AI team. This group combined Google’s Sycamore quantum computing hardware with 20 “spins,” using the quantum version, or qubits, of a classic computer’s bits.

Another time crystal, published in Science this week, shows the intense interest in time crystals. Researchers at Delft University of Technology, Netherlands, created the crystal by combining qubits with a diamond.

Quantum opportunities

Thanks to the unique capabilities of quantum computers, the researchers could confirm their claim that there was a true time-crystal. The finite size and imperfect coherence time (of the quantum device) meant their experiment could not be extended. Therefore, oscillations of time crystals could only last a few hundred cycles. However, the researchers developed several protocols to assess the stability of their creation. They also ran the simulation forward and reverse in time and scaled it.

Moessner was co-author of this paper and director of the Max Planck Institute for Physics of Complex Systems. “It taught us how to correct its errors so that we could determine the fingerprint of ideal time crystal behavior from finite-time observations.”

An ideal time crystal shows indefinite oscillations between all states. This is a vital sign. This robustness in choosing conditions was a major experimental challenge. The researchers developed a protocol that allowed them to examine over a million of these states in a single machine run. It took only milliseconds to run the protocol. This is similar to looking at a physical crystal from multiple angles to confirm its repetitive structure.

“A distinctive feature of our quantum processor’s ability to create highly complicated quantum states,” stated Xiao Mi (a researcher at Google) and co-lead author. These states enable the adequate verification of the phase structures in the matter without the need to explore the entire computational space, which is a difficult task otherwise.

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