Public Release: 

Creating time crystals

Physicists create new form of matter that may hold the key to developing quantum machines

Harvard University





Other co-authors of the study are Junichi Isoya, Shinobu Onoda,and Hitoshi Sumiya from University of Tsukuba, Takasaki Advanced Research Institute and Sumitomo, Fedor Jelezko from University of Ulm, Curt von Keyserlingk from Princeton University and Norman Y. Yao from UC Berkeley.




"One specific area where we think this might be useful, and this was one of our original motivations for this work, is in precision measurement," Lukin said. "It turns out, if you are trying to build...for example, a magnetic field sensor, you can use NV-center spins," he said. "So it's possible these non-equilibrium states of matter which we create may turn out to be useful."

The notion that such systems could be built at all, however, initially seemed unlikely. In fact several researchers (names are Patrick Bruno, Haruki Watanabe, Masaki Oshikawa) went so far as to prove that it would be impossible to create a time crystal in a quantum system that was at equilibrium.

"Most things around us are in equilibrium," Lukin explained. "That means if you have a hot body and a cold body, if you bring them together, their temperature will equalize. But not all systems are like that."

One of the most common examples of a material that is out of equilibrium, he said, is something many people wear on a daily basis - diamond.

A crystallized form of carbon that forms under intense heat and pressure, diamond is unusual because it is meta-stable, meaning that once it adopts that crystal formation, it will stay that way, even after the heat and pressure are removed.

It is only very recently, Lukin said, that researchers began to realize that non-equilibrium systems - particularly those known as "driven" systems, which researchers can "kick" with periodic energy pulses, can exhibit the characteristics of a time crystal.

One of those characteristics, he said, is that the crystal's response across time will remain robust with respect to perturbations.

"A solid crystal is if you push on it, maybe the distance between atoms changes a little, but the crystal itself survives," he said. "The idea of a time crystal is to have that type of order in a time domain, but it must be robust."

One other important ingredient is typically if you keep pushing a system away from equilibrium it starts heating up, but it turns out there is a class of systems which are resistant to this heating," Lukin added. "It turns out the time crystal effect is strongly related to this idea that a system is excited, but it not absorbing energy."

To build such a system, Lukin and colleagues began with a small piece of diamond which was embedded with so many NV centers it appeared black.

"We subject that diamond to microwave pulses, which change the orientation of the spins of the NV centers," Lukin explained. "That basically takes all the spins that are pointed up and turns them down, and a next pulse turns them back up."

To test the robustness of the system, Lukin and colleagues varied the timing of the pulses to see whether the material would continue to respond like a time crystal.

"If you don't orient all the spins fully up or down each time, then very rapidly, you will end up with a completely random system ," Lukin said. "But the interactions between the NV centers stabilize the response: they force the system to respond in a periodic, time crystalline manner."

Such systems could ultimately be critical in the development of useful quantum computers and quantum sensors, Lukin said, because they demonstrate that two critical components - long quantum memory times and a very high density of quantum bits - arent' mutually exclusive.

"For many applications you want both of those," Lukin said. "But these two requirements are usually contradictory....this is a well-known problem. The present work shows that we can achieve the desired combination. There is still a lot of work to be done, but we believe these effects might enable us to create a new generation of quantum sensors, and could possibly in the long run have other applications to things like atomic clocks."

