Debunking Time Crystals
Recently Google made headlines for creating a ‘Time Crystal’ in its quantum computer. But what is this Time Crystal?
These days, it is all abuzz in the physics circles. Professors are giving lectures about it; penners are writing lengthy articles about its possible applications; people (like me) are going bonkers over it without clearly understanding it, and science fiction writers are smirking under their cloaked pseudonyms, yes, it is this thing called as a ‘Time Crystal’.
Recently Google made headlines (yet again) for creating the first of its kind ‘Time Crystal’ in its quantum computer, being described as a breakthrough in the field of condensed matter physics. But what is this Time Crystal and why is it such a headline these days?
A Starting Definition
Jumping right into the topic, a time crystal is a system of quantum particles that is in kind of a perpetual oscillation between two positions, without spending energy.
Let’s boil it down part by part. The definition follows the following features:
- Quantum System — Well, this should be self-explanatory to the reader.
- Perpetual oscillations — This is an interesting point. What this means is that the motion of the constituent particles of the crystal should continue as long as the particle is not disturbed once created. Disturbance refers to any kind of thermal or mechanical disturbance. We’ll cover this aspect in more detail later.
- Without spending Energy — Clearly, the most interesting point of the three, this concept requires some decent level of understanding, as time crystals seem to evade a “fundamental” law of nature, an aspect which we’ll cover later. But this means that this kind of “perpetual motion” in these crystals happens without the expense of energy.
What it is
Starting with a simple analogical picture, imagine a simple pendulum swinging back and forth in space with a specific period. We know, from daily experiences and the Second Law of Thermodynamics, that this pendulum will eventually come to rest, as the First Law of Thermodynamics necessitates that it loses a part of its energy in the process. Thus, these two fundamental laws of physics completely reject the idea of any sort of perpetual motion.
Now here’s when things get crazy. A time crystal is a configuration of particles that does the same things by the pendulum, except the fact that it continues forever if allowed.
What this means is that the constituent particles of the crystal continuously jump between two configurations without losing energy, similar to what happens in resonating structures like Benzene. In terms of condensed matter physics, it means that the crystal lattice of a time crystal regains itself after regular intervals of time, again, without the loss of energy.
A thing which I would like to highlight here is that while jumping between the two lattices, the system does not undergo “real” motion unlike in a pendulum, as motion requires that the object undergoing the motion possesses kinetic energy, which is converted from the object’s internal energy (in the absence of an external force) and leads to a reduction in its internal energy.
So, we can also conclude that a time crystal is in its lowest possible energy state. Also, the time crystal seems to violate the Second Law of Thermodynamics, a *fundamental* law of the Universe (and a thing known as the Time-Translation Symmetry), which is what makes it so interesting a topic of research.
But how and where does a Quantum Computer come into this story?
What Makes A Time Crystal
As told previously, a time crystal involves a crystal oscillating between two stable configurations of low-energy. As a result, a two-level system would be our most perfect physical starting point of a time crystal. And as the reader might know, such a system is achieved with the help of ‘particle spins’, in modern applications… and Quantum Computers.
Thus, we use the property of particle spins or ‘spinon’ quasiparticles as our time particles. So, for example, if we have a system of electrons, with about half of the electrons spin down, and the others spin up, that would be our apt starting point for a time crystal.
Next, a microwave pulse is targetted onto the system, which causes them to change their spin orientations, without changing their internal energies.
Repeating this microwave pulse eventually results in the system of spinons continuously oscillating between the two configurations at fixed intervals of time, without gaining or losing energy which… is a time crystal.
Well, quantum computers are two-level systems, have near-perfect microwave pulses (measured by their gate fidelity), and they can model qubit spins, so why shouldn’t they be used for these experiments?
I would here like to highlight one specific caveat. As previously told, time crystals behave as they should only in the absence of any disturbances. But, in practice, such conditions are non-existent, at least till now. In modern quantum computers too, there’s always some “noise”, mostly thermal or other radiations, which is what makes a time crystal highly susceptible, and difficult to physically realize.
Time Crystals continue to be an interesting topic of research, especially for their implications on the Second Law of Thermodynamics, which is a part of the greater conquest to understand the very nature of time itself. And again, this research highlights another fascinating application of quantum computers.
Here I am adding a few additional resources for those who want to dig deeper and the mathematically inclined readers:
- Eternal Change for No Energy: A Time Crystal Finally Made Real | Quanta Magazine — One of my primary sources of information and an excellent article
- Observation of Time-Crystalline Eigenstate Order on a Quantum Processor | arXiv — Preprint of the paper explaining the creation of Time Crystals on Sycamore
- Observation of a many-body-localized discrete time crystal with a programmable spin-based quantum simulator | arXiv — Another experiment showing the observation of a time crystal from the Netherlands
- Phys. Rev. Lett. 109, 160401 — The original paper by Nobel-laureate Frank Wilczek stating the theoretical physics behind a time crystal
- Time crystals enter the real world of condensed matter | physicsworld
- What the heck is a time crystal, and why are physicists obsessed with them? | Popular Science