Time Crystals — A space-time odyssey
Crystals — These beautiful structures continue to amaze scientists till date. These are interesting formations of atoms that repeat their arrangements in space. Table Salt (Sodium Chloride), sugar, quartz, and diamond are some well-known examples of such crystals, but what if I tell you there are crystals that repeat themselves in time?
This article will present to you, a crystal-clear introduction to Time Crystals. It won’t take you much time to read this, so give this a shot!
Disclaimer: Nature loves symmetry. Our job here is to provoke mother nature by smashing what it loves. This is not for the people who fear her
Time crystals are 4-dimensional structures that not only repeat their atomic patterns in space, but also repeat in time. To get a fine understanding of this, we need to acquaint ourselves with the concept of symmetry breaking. Imagine a bottle of pure water and the orientation of its constituent molecules (assume you have a powerful microscope to view the molecules with ease, although buying one is simply out of our scope). As it is in liquid form, the molecules, proving that they are bad at hide ’n’ seek, move about and can be found anywhere and as a result, water is going to look the same from every viewpoint. Now, pitch the bottle into a freezer and wait for the water to turn into ice (By the way, water once turned into ice, will be rude because it is cold by nature) Look at the molecular configuration now, and you will see regular patterns that repeat in space. However, you will notice that the arrangement is not as symmetric as that when in liquid form i.e there is a loss of symmetry (To people who have OCD, my apologies). This is symmetry-breaking, the primordial concept involved in the formation of crystals.
Let us now apply this concept in the temporal domain. Consider the molecular arrangement of ice. As time passes, the configuration will stay the same (assume you have exams and staring at the bottle of ice in the freezer is a cool time killer). We say this is temporally symmetric as the system is the same throughout time. What if the next second, the configuration changes to some other regular pattern, and back to the original pattern the next second? This is temporal symmetry-breaking, the core concept behind time crystals, and the original idea of MIT professor and Nobel Laureate Frank Wilczek, who proposed it in 2012. To be frank, his proposal was mind-blowing.
These crystals may initially appear to defy physics with style, and might continue to do so because they happen to be non-equilibrium phases. We know that a whopping majority of matter prefers being in equilibrium but time crystals aren’t part of the mainstream. They cannot exist in thermal equilibrium. The atoms in the crystals can never get themselves to settle down in motionless equilibrium, like a lot of spatial crystals do. Also, these crystals exist in the lowest possible energy state i.e the ground state. Although it is theoretically impossible for particles to move while in the ground state because they would have to expend energy for it (Just like when your mom grounds you because you were a mischievous kid and you can’t move out of your bedroom), Wilczek proposed that the crystals can possess movement in, such a state (also called the zero-point energy of a system, but not like there’s no point about it or anything) by periodically switching the atomic alignment back and forth, so that the net energy of the system continues to be zero. This seemed straight out of hardcore science fiction for people back then (some of them hated sci-fi, really). A whole flock of scientists all over the world got triggered and argued saying “Your proposal is trash, these crystals can never be realized. Go get a life, Frankie!”. However, a small plot twist crept in, one small tweak that led to the synthesis of time crystals in laboratories today! The tweak is pretty simple — “To drive the crystals to switch states periodically, you need to give it a nudge”. That’s right, drive the crystal system with some initial excitation (with a fixed time period) and soon, the crystal particles will oscillate through multiple states in its own frequency. This is what scientist Christopher Munroe and his colleagues observed while claiming to have successfully created time crystals in their laboratories at the University of Maryland in October 2016 (However, their labs may remind you of Disneyland). They achieved this by initially trapping a chain of ytterbium atoms within radio frequency electromagnetic fields called Paul Traps (It’s a trap!), then proceeding to fire a pair of lasers to pick from the two possible quantum spin states of the atoms. The lasers are pulsed at the desired frequency with the help of an acousto-optic modulator (that uses sound waves to diffract electromagnetic waves and shift their frequencies, but don’t try screaming at the tubelight, hoping to change its color). The driving frequency kickstarts the oscillation of the atoms. After some time, the atomic system was observed to oscillate at a different sub-harmonic frequency. At this point, applying external perturbation would not disrupt the oscillation of the system, thus demonstrating the property of rigidity and the fact that these crystals practised Zen Buddhism. However, as the maxim goes; “Too many cooks spoil the broth”, excess external perturbation will collapse the system and bring it down to the initial driving frequency i.e melting the time crystal.
If you think that’s enough, there’s more. Later that year, scientist Mikhail Lukin synthesized time crystals using a different method. His team doped diamond crystals with a high concentration of impurities called nitrogen-vacancy centers. They kept adding impurities until the diamonds started Lukin dirty. This system exhibited a strong dipole-dipole coupling and a lasting spin coherence. Microwave fields drove the configuration and laser pulses were fired to read out the ensemble spin states. Just as observed in the previous experiment, the system picked up its own oscillation frequency (the spin polarization rate turned out to be half that of the driving lasers), a subharmonic of the driving frequency. Human beings, who are the most intelligent life forms in Earth find it hard to pick up in college academics, while these inanimate crystals could pick up easily!
So we have seen how time crystals work and a quick tour through how they were synthesized, blazing through the barriers of classical physics. This scientific breakthrough is of prime significance for two reasons:
- This opens new avenues for the research of non-equilibrium systems. Till now, all of us are well-acquainted with systems that remain in equilibrium (No wonder why you would never get up in the morning despite a chain of annoying alarms). Interface a hot body with a cold one and both bodies will have their temperatures equalized, but now this may not be the case. In fact, out-of-equilibrium systems turn out to be quite useful. Quantum computers, for example, are essentially quantum systems that hate being in equilibrium, and the next point mentions the importance of these sweet crystals in those powerful computing machines
- Time crystals have a long memory retaining time because of the property of rigidity as discussed above, and are hence useful as possible near-perfect quantum memory devices. Think of these as the storage drives for a PC
Right now, time crystals are new, exotic and have recently secured their admission into the existing states of matter, and it will take some time for the physicists to exploit their full potential, but I believe the day is not far, when we might witness a proven and tested application of them. Before we close, let me disclose: writing this article was a time-consuming process.
Written and researched by Tushar Sairam.
