Time crystals. That might sound like something pulled straight out of the pages of classic science fiction, but it is a real system which has recently been observed in the lab independently by two groups at the University of Maryland and Harvard University. The theory predicting time crystals was not proposed by Spock, but rather Nobel laureate Frank Wilczek from MIT.

Wilczek outlined the premise – a material which could break translational time symmetry by oscillating indefinitely in a type of perpetual motion. Translational symmetry just means it looks the same whether you run it forwards or backwards – and is fundamental to the laws of Physics.

The basic analogy is as follows – a regular crystal lattice such as diamond breaks the isotropic symmetry of space because atoms exist in thermodynamic equilibrium at specific locations (ie it doesn't look the same from every direction). Now imagine a type of lattice where the state of a particle is fixed in a repeating arrangement in time, even in its lowest energy state. Wilczek proposed that this effect could be physically observed by applying a static magnetic field to a ring of Quantum particles.

Time Crystal Methodology

As it turns out, Wilczek’s original idea was slightly flawed because calculations revealed the system he described would not occupy the ground state energy.

The solution was to introduce the ‘discrete time crystal’ (Floquet crystal) which breaks discrete translational time symmetry, not continuous symmetry. To observe this phase, one must apply a periodic driving force to a set of spins, as set out in 2016 by Norman Yao from the University of California, Berkley, in his 'Physical Review Letters' paper.

In January 2017 Christopher Monroe (Maryland) and Mikhail Lukin (Harvard) published the experimental observation of time crystals in the journal Nature, both confirming Yao’s predications.

Periodic microwave pulse

The practical approach of the two groups differed although the underlying principle was the same. Monroe et al. trapped an array of strongly interacting ytterbium ions and exposed them to a periodic laser pulse.

They identified spin oscillations (by simply measuring the spin polarisation with time) and crucially found that the measured period was independent of the rate of the laser pulse. Lukin et al. instead took advantage of nitrogen vacancy centres (NVCs) which are naturally occurring defects in diamond and provided a source of interacting spins for the experiment. They applied a periodic microwave pulse to the sample and observed spin oscillations at an integer multiple to the pulse frequency of microwaves. Once again, the spin oscillations were found to be completely independent of the driving force.

It appears that the long-range interactions between are critical for the stability/ rigidity of the time crystal to be maintained.

As Yao puts it: “It’s an emergent phenomenon… It requires many particles and many spins to talk to each other and collectively synchronise.”

This work opens the door to further research into non-equilibrium quantum systems. The reliable invariance of the time crystal’s ‘tick’ could prove useful for future applications in quantum computing.