Time Crystals!

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Matt O'Dowd (1:00)

This episode is supported by The Great Courses Plus. What exactly are time crystals? Are they the bling inside your time turner? The flux in your flux capacitor? Are they the heart of the TARDIS? In today's edition of Space Time Journal Club, we find out.

In Space Time Journal Club, we review new scientific papers that are making waves. We pick them apart to turn the technobabble into simple English. At least as much as is possible. Then we fight about it in the comments.

This week, we're going to take a look at a recent publication by Norman Yao et al. in Physical Review Letters, entitled, Discrete Time Crystals, Rigidity, Criticality and Realizations. This paper proposed an approach to making these bizarre objects, a recipe that two other research teams have now followed and have actually synthesized these things. But first up, what on earth are time crystals? The idea was first proposed in 2012 by Nobel Laureate Frank Wilczek of MIT.

He suggested a type of matter that exhibits a sort of fundamental oscillation over time. So some property of the material goes through a repeating cycle.

But how does that make it a time crystal? The analogy is that regular crystals have a periodic cycle through space. Molecular patterns repeat again and again along their lattices. Time crystals repeat some internal state with constant separations in time. The name time crystal is somewhat out there. But Wilczek wasn't the first to use it in reference to a regularly repeating system. That may have been Arthur Winfrey in his The Geometry of Biological Time, where it's used to describe periodic biological systems. But Wilczek was clever to apply it here, because the name made the internet go completely bonkers. And so here we are. Wilczek came up with a simple model in which charged particles in a superconducting ring break what we call continuous time translational symmetry. That's a fancy way of saying that the system looks different on a global level from one instant to the next. Normal matter, that is in what we call thermal equilibrium, only has random internal motion. In solid matter that would be the vibrational buzz of its constituent atoms. But from one instant to the next that buzz stays random. In regular matter in equilibrium, statistical properties stay the same over time. Wilczek's imaginary system broke this time translational symmetry because there are global statistical differences in the state of the matter. Non-random patterns that change over time. Big deal. Lots of things change over time. Cups of coffee cool down. Planets orbit the sun. The universe expands.

But cups of coffee in the universe are not in equilibrium. And the planets are macroscopic moving objects. Wilczek proposed an actual substance that was in perpetual motion while in equilibrium. More, he imagined a substance for which oscillations were the most fundamental lowest energy or ground state. This would break time translational symmetry, which makes most physicists nervous.

Well, physicists can chill.

In 2015, Haruki Watanabe of UC Berkeley and Masaki Oshikawa of the University of Tokyo showed from theoretical arguments that time translational symmetry can't be broken by a quantum system in equilibrium. That sounds bad for time crystals. But that's where this new paper by Yao et al comes in. Their answer is to throw away this equilibrium thing. Thermal equilibrium means a closed system. No energy in, no energy out.

Norman Yao, also at UC Berkeley, and his team proposed a way to make time crystals by using some sort of external input of energy to force the oscillating states. The idea goes like this. Set up a chain of ions, so electrically charged atoms. These atoms have spin values, quantum mechanical angular momenta from their electrons. Spins in nearby atoms like to line up with each other due to interacting magnetic fields, either direct alignment or opposite alignment are both a lower energy state than random alignment. This is the same effect that results in magnetic materials. So you prepare a string of ions with aligned spins. Now cause those spins to flip back and forth using a laser. A laser is just a very well ordered electromagnetic wave with a known period or frequency.