[Image: E. Edwards/JQI]
Two groups of researchers have published full details of their creation of “time crystals”—a state of matter that displays persistent periodicities in time, as a crystalline solid does in space (Nature, doi: 10.1038/nature21413; 10.1038/nature21426). Quite apart from its significant theoretical interest, the new state of matter could, in the view of the research groups and others, have potential applications in the quantum information domain, particularly in quantum memory and quantum metrology.
Symmetry-breaking—in time
Although natural mineral crystals, from quartz to snowflakes, may appear highly symmetrical, from the point of view of physics they are actually classic examples of symmetry breaking. As the material transitions from a liquid to a crystalline solid phase, atoms begin to prefer specific locations in a periodic order, breaking the translational symmetry of empty space, in which nothing distinguishes any one point from any other.
In 2012, in series of papers in Physical Review Letters, the Nobel laureate Frank Wilczek and others explored the possibility of a material that analogously broke the translational symmetry of time, showing time periodicities in its lowest-energy state in the same way a crystalline solid shows periodicities in space. Wilczek dubbed such a state of matter, which he suggested could occur in both the classical and quantum realms, a time crystal. While perhaps uncomfortably reminiscent of a perpetual-motion machine, a time crystal would not break the second law of thermodynamics, as no useful work could be extracted from the system.
A time crystal recipe
Subsequent theoretical analysis showed that Wilczek’s original vision—a time crystal in the thermal-equilibrium state—couldn’t actually exist. But a number of studies in 2016 suggested a potential workaround: A so-called Floquet time crystal. In this scheme, the system is not in thermal equilibrium, and it would require a periodic driving force to nudge the system to continue its time-crystalline behavior. But the material would still acquire a long-lived, periodic pulse that was out of sync with pulse of the driving force—and, thus, that would break time symmetry.
A subsequent PRL paper by Norman Yao of the University of California, Berkeley (USA), and colleagues—published in January 2017, but posted on the arXiv preprint server in August 2016—even proposed a recipe for creating time crystals, by subjecting the quantum spin property of a chain of cold, trapped ions to a periodic alternating drive that would flip the spins. The Yao team also framed a criterion for knowing when you had a time crystal: The oscillations of the spin magnetizations within the crystal would occur not at the period of the driving force, but at an integer multiple of that period—and would persist even if the drive parameters were changed.
Two roads to a time crystal
In the wake of the Yao group’s observations, two research teams—one led by Chris Monroe of the University of Maryland, USA, and one led by OSA Fellow Mikhail Lukin of Harvard University, USA—jumped on the problem, and devised alternative paths to demonstrating a Flouquet time crystal in action. Monroe’s group subjected an array of ten trapped ytterbium atoms to alternating laser pulses, to flip the atoms’ spins and allow them to interact. As the Yao group had predicted, the spins began oscillating on their own, at twice the period of the laser driving force—and the oscillations remained locked at that period even when the frequency of the driving laser pulses was changed slightly.
Lukin’s group, meanwhile, used a diamond sample with around one million nitrogen-vacancy (NV) defects, a common platform for quantum-computing investigations, and bombarded it with an oscillating microwave pulse to achieve the requisite spin flips. They found that the NV-riddled diamond also started showing the behavior of a time crystal, with long-lived spin oscillations at two and three times the period of the driving microwave oscillations—again consistent with the Yao group’s predictions.
Quantum memory platform?
Now that we have time crystals, what do we do with them? The papers from both the Maryland and Harvard groups stress the intrinsic interest of having isolated these new nonequilibrium phases, and of their potential usefulness as launch pads for further studies in the area. But the ability to create and maintain these independently oscillating crystals, according to the Monroe team, opens up interesting possibilities in using them for new forms of quantum memory. And the Lukin group suggests these first steps could ultimately contribute to the ability “to create and stabilize coherent quantum superposition states for applications such as quantum metrology.”