[ww520]

Crystal has repeatable structure over space. Its lattice is the same shape over and over again.

Time crystal is a kind of crystal that has repeatable structure over time. It just means it changes its "shape" in a repeated fashion over time. Usually an external force is required to force a crystal to change its shape over time such as with the piezoelectric effect. Time crystal is supposed to just keep changing its "shape" over time by itself. In the Yale case, the changing of "shape" is the flipping of the nuclear-spin magnetization periodically.

[iandanforth]

Kind of like this?

https://www.youtube.com/watch?v=1n-HMSCDYtM

[ww520]

That's a great illustration of repeated structural change over time; admittedly that requires an external power to spin it initially.

[FeepingCreature]

Kind of like this?

https://www.youtube.com/watch?v=OrCTmfQWCmQ

[olfactory]

So could a time crystal be viewed as a source for time information in a system?

[ww520]

The original idea of time crystal is a material that has periodic structures in time, at the lowest energy state [1]. That means it can have repeated structural change over time by itself at rest. In that case, the crystal itself has an intrinsic periodicity in motion, and can be used as a source of time information.

However, now it seems to be changed to that an external energy source is driving the frequency of the structural change of the crystal over time. Like in the Yale experiment, an external pulsing light shines at the system at certain pulsing frequency and they found the system reverses its nuclear spin at twice as slow as the light's pulsing frequency. There's no intrinsic timing property shown in the crystal, so it cannot be used as a source of time information. I've asked the same question. [2]

[1] https://physics.aps.org/articles/v5/116

[2] https://news.ycombinator.com/reply?id=16997756