Physicist proves impossibility of quantum time crystals

Is it possible that a moving object could have zero energy? The common sense answer is no, since motion itself is kinetic energy, but this answer has been challenged recently by the concept of quantum time crystals. First proposed in 2012 by the Nobel Laureate Frank Wilczek at MIT, quantum time crystals are theoretical systems that exhibit periodic oscillations in their ground state, i.e., their state of lowest possible energy.

Since then, researchers Tongcang Li et al., at the University of California, Berkeley, have proposed an experimental set-up of a time crystal based on charged particles (ions) in a ring-shaped ion trap. They argue that under a weak applied magnetic field, the ions should begin to rotate around the ion trap, and that, because the ions are in their ground state, their rotation theoretically would persist indefinitely.

But not everyone is embracing the concept of quantum time crystals. Physicist Patrick Bruno at the European Synchrotron Radiation Facility in Grenoble, France, has identified some holes in the concept and has proven a "no-go theorem" that rules out the possibility of spontaneous ground-state rotation for a broad class of systems that might be categorized as quantum time crystals.

Bruno's argument, which is published in a recent issue of Physical Review Letters, expands upon his Comment on Wilczek's original paper, both of which were published in Physical Review Letters in March.