One of the weirdest bits of science to have come out recently – time crystals – are apparently forming the center of a new program at DARPA, the US military’s research institute. What interest the army has in this odd state of matter is, naturally, classified.
Time crystals are not easy to get one's head around. In effect, in the same way that atoms are arranged periodically in space to form a crystal, they can also be arranged periodically in time to form a time crystal. These oscillate in a stable repetitive pattern, meaning that they could be used as an incredibly precise clock, or may find use in the ever-elusive quantum computer.
Up until very recently though, they only existed on paper, a theoretical state of matter that many a clever physicist couldn’t figure out how to get from complex mathematics to the laboratory.
That was until 2016. A group working out of UC Santa Barbara managed to correct the theoretical problems in the original workings and came up with a way in which to make the time crystals, although it wasn’t exactly detailed. In doing so, however, they revealed another oddity about the system – when a time crystal is pushed at a certain frequency, it does not respond at the same frequency. So if a laser was pulsed at the crystal every 10 seconds, it would oscillate every 20 or 30 seconds.
The best way to describe this is to imagine two people holding each end of a jump rope, with a third person hopping up and down at the center. Usually, for every time the two rotate their arms, the rope rotates once and the person in the middle jumps once. But with time crystals, it would be as if the two people turning the rope rotated their arms three times, but the rope only turned once, and so the person in the middle only had to jump once. See, it’s confusing.
Following the recipe created at UC Santa Barbara, however, scientists at UC Berkeley managed to actually make time crystals, stringing together a chain of 10 ytterbium ions whose electron spins were entangled. They then pulsed two lasers at the chain and caused the spins of the ions to flip in sequence. Importantly, this repetitive pattern of flipping was twice the period of the laser pulse that initiated it.
They had created time crystals, a new form of matter. What is more, by altering the period of laser pulses, they could change the duration of the pattern (or phase of the time crystals), basically akin to turning a liquid into a gas.
How and why DARPA are interested in this is still not known, and physicists themselves don’t actually know the full implications or applications of time crystals, which is one of the reasons why this frontier is so exciting.