“This is a new phase of matter, period, but it is also really cool because it is one of the first examples of non-equilibrium matter.”
Total energy of a body is a sum of its potential and kinetic energy. When a body isn’t moving (i.e. K.E=0), the total energy of the body is equal to its potential energy, and this energy is the least possible energy at that state and the body is said to be in equilibrium. Thus, lower the energy, the more stable is the system. Thus, when a body is in motion it isn’t stable, right? Wrong.
A new discovery argues our above idea of stable objects. These restless entities are being called as ‘time crystals’. Now, normally when we talk of crystal we image a solid, that is made up of well structured, repetitive blocks of atomic arrangements, the repetition going on in space. For example, a diamond, or a sugar cube. Now even though the laws of physics are the same in every direction, these crystals form their associated repeating chain of blocks only in a particular direction, and it seems that the laws of physics worked differently. This breaking of symmetry is called, as one would guess, ‘symmetry Breaking’ and is a thing that is still being studied and new properties of this effect are being discovered. Now, here comes the crazy part. In 2012, nobel laureate Frank Wilczek proposed that in the four dimensional space-time(i.e. the three space coordinates and time), it should be possible that similarly to Spacial Symetry Breaking (like in solid crystals) their exists a phenomena of ‘temporal/time symetry breaking’, and crystals formed by the above process were termed as time crystals. An elusive idea indeed. So, in simple sense, time crystals have a structure that repeats itself in time. Try imagining a ring that is rotating on an axis, now after some time T the ring makes a complete rotation, and returns to its original state, and the cyclic motion continues, and so it can be said to have a repititive structure in time. The only difference between a time crystal and the above rotating ring is that a time crystal is stable whereas the ring will stop rotating, unless force is applied, and reach the state of stable or no motion.
“Observation of the discrete time crystal… confirms that symmetry breaking can occur in essentially all natural realms, and clears the way to several new avenues of research.”
This idea of time crystals attracted a lot of criticism until 2015, when two independent groups, one from Princeton University and other from UC Barabara’s Station Q, proved that the idea was theoretically possible. On the same path, Norman Yao made a blue-print to create the strange time-crystals. What he suggested was to create a quantum system, such as a group of ions arranged in a ring, and cool them until they are in their lowest energy state. In these circumstances, the laws of physics would suggest that the ring should be perfectly stationary. But if time symmetry was to be broken the ring had to vary periodically in time. No energy could be, though, manifested from this motion, because of the low (near 0K) temperature, and so such a crystal points towards the violation of the second law of thermodynamics.
Things are but not so simple. The main problem is that the quantum world is not governed by time-dependent variables, so time symmetry cannot be broken on this scale. So in ordinary circumstances, cooling a ring of ions to their lowest energy state would leave them stationary. Here comes another property of a quantum system, that though it is not governed by time dependent variables, a quantum system evolves in time, and a the same idea of quantum system was used to create them.
Using Yao’s blueprint, two independent groups, one from University of Maryland and the other from UC Berkeley Harvard University, confirmed the creation of time crystals, despite using two completely different quantum systems. The team from University of Maryland published their findings in Physics Review Letters on January 2017. Yao was a co-author in both the studies.
The team from University of Maryland ,headed by Chris Monroe, used a quantum system of 10 Ytterbium ions. They used an idea of Anderson localization and many body localisation. To explain the above terms, one should know that quantum mechanics is based on the principle of probability, and thus particles like electrons are not confined to a well-defined position but rather smeared in space. Sometimes, it may happen that an electron interfered with itself and in a closed system has a well defined position in space and this is called Anderson localisation, similarly it was found that groups of quantum particles interact with each other in a way that causes them all to become localized. This so-called many body localization is a delicate state that maintains the quantum particles in an out-of-equilibrium state. In other words, it forces them to be localized. And that’s exactly how this chain of ytterbium ions behaves. Another important property of these ions is spin. The team used two lasers, one was used to create a magnetic field and the other was used to ‘flip’ the spin of the ions. The lasers were applied periodically, with a time period T, and as the spin of the first ion was flipped, it caused the other ion to slip too, and this change travelled along the chain of ions, and this process of applying the laser turning the spin, and the change of spin travelling down the chain continued. Now, the interesting part was that intuitively one would expect the chain of ions to change the spins with the same period as that of the application of the lasers, but on measuring it was found that the quantum system vibrated with twice the time period, and changing the time period of the lasers did not affect the time period of the ions. Thus, the chain of ions was independently flipping with a time period that was its intrinsic property, and thus, a time crystal was formed. A crystal whose stable state was a state of motion. Yao commented on the findings saying this was a greater discovery than what it seems, this opens the doors to a greater field of non-equilibrium stable phases of matter. The team then measured some of the properties of the time crystal. The continued vibration of ions on removing the pulses represents the rigidity of the ions. It was found that if the perturbations applied were macroscopic, the crystal melted. Yao also described how the time crystal would change phase, like an ice-cube melting, under different magnetic fields and laser pulsing.
“Wouldn’t it be super weird if you jiggled the Jell-O and found that somehow it responded at a different period?” Yao said. “But that is the essence of the time crystal. You have some periodic driver that has a period ‘T’, but the system somehow synchronizes so that you observe the system oscillating with a period that is larger than ‘T’.”
The Harvard team headed by Mikhail Lukin used densely packed nitrogen vacancy centres in diamonds to set up their time crystals. The team employed microwave radiation to alternately flip the spins of the impurities and to generate their spin-spin interactions, all in the presence of random disorder occurring natively within the diamond bulk. Despite using a completely different quantum system to that of Monroe’s group, the researchers observed the same significant features of a time-crystal state: oscillations at integer multiples of the drive period T (both 2T and 3T in this work) and robustness to perturbations in the drive parameters. Furthermore, both groups have performed initial measurements of the location of the time-crystal phase boundary predicted by Yao et al., outside of which the time crystal “melts” into a symmetry-unbroken state. Such similar results achieved in two wildly disparate systems underscore that time crystals are a broad new phase of matter, not simply a curiosity relegated to small or narrowly specific systems.
“Such similar results achieved in two wildly disparate systems underscore that time crystals are a broad new phase of matter, not simply a curiosity relegated to small or narrowly specific systems,”
-Phil Richerme, of Indiana University
Time Crystals did not mark the end of a discovery but the beginning of a much bigger mystery. What, if any, are the other possible states of out-of-equilibrium but stable matter? How does the time crystal continue to be in motion despite its low energy state? Is the second law of thermodynamics finally broken? The notion,as was expected earlier that laws of physics are symmetrical in time and cannot vary as time passes, was also challenged. Time-Symmetry has finally been broken. Yao suggested that time crystals can be used as permanent memory in quantum communication in place of qubits. Phsysicists are engaged in solving these mysteries. symmetry breaking, time crystals and related phenomena are still unknown to a great extent and new information is coming out constantly. More exciting findings are on their way, lets stay buckled up!
“For the last half-century, we have been exploring equilibrium matter, like metals and insulators. We are just now starting to explore a whole new landscape of non-equilibrium matter.”
-The Cosmogasmic Person
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