[jdrov]

This is a good question. The "directions" you mention would, in our system, typically be considered to depend on the nature of the drive. For instance, if you repeatedly rotate the magnetization by 180 degrees, you can imagine the magnetization going up-down-up-down-... repeatedly, whereas if you instead used rotations of 181 degrees, it would take a long time for the state to come back around to pointing along its exact original orientation.

The proposed signature of a "discrete time crystal" was to observe the magnetization point up-down-up-down-... even when you used e.g. 181 degree rotations, if you allow dipole-dipole interactions to act for long enough between rotations. This is what we observe: "wrapped" magnetization when we use imperfect rotations with short nuclear spin interaction times, then locked up-down-up-down-... magnetization when we use imperfect rotations with longer nuclear spin interaction times.

A last subtelty when comparing to traditional oscillating systems is that the response is not at the same frequency as the drive, but will have a period determined by both the drive period T and the symmetry of the dipole interactions. Our system's interactions have 2 symmetric states, so the response period is at 2T. Other systems have other symmetries; for instance, the research team at Harvard showed oscillations at 3T using a spin system with different interaction symmetries.

[brandmeyer]

(HN doesn't do private messages, or this would be sent privately)

Thanks for coming out here and fielding our totally ignorant questions. Its an amazing and beautiful world out there, thank you for sharing your discoveries about it.

[jdrov]

Thanks for the kind words. I've focused a good deal in the past few years on teaching/communication (see my profile for a link to some of my basic-physics lectures for student taking the MCAT), and I'm very grateful for the opportunity to discuss our work with this community. Thanks for your interest and great questions!

[downer68]

Actually, HN does do /almost/ private messages, if you hellban your account, and the recipient of your reply comment has “show dead” turned on.

(of course, anybody can activate the “show dead” option, but, in reality, there’s no such thing as privacy on a web server, since there’s always a system administrator noticing unhashed passwords scroll through the log stream)