“Last month, a team of physicists from UC Berkeley said they’d created a blueprint for a new phase of matter called a time crystal.”
“A time crystal isn’t something you can hold in your hands, and it isn’t something you can grow in your kitchen with some table salt and a glass of water.”
“For a long time, the time crystal concept existed only on paper as a mathematical oddity. It’s only now that time crystals have been realized in a lab in (quantum) physical form.”
“First let’s ignore that extra dimension—time—and consider your run-of-the-mill 3D crystal. A crystal is basically just a number of atoms arranged in a periodic, or repeating, pattern in space.”
“Before a liquid crystallizes, the space it occupies is homogeneous.”
“space exhibits symmetry.”
“Yet when the water crystallizes, the atoms form rigid, set arrangements. The space occupied by the crystal has become periodic. The crystal has broken spatial symmetry because it exhibits repeating patterns in some directions—anyone who’s grown salt water into sodium chloride crystals has seen them push up— rather than being the same in all directions.”
“In 2012, the Nobel laureate Frank Wilczek predicted that the periodicity of crystals could be extended into the fourth dimension: time. Wilczek imagined a system in its lowest possible energy state, which would effectively render it frozen in space like a normal crystal.”
“just like how in the previous example the water was the same throughout the space it occupied (spatial symmetry), objects exist through time in a similar way, which means that just like the atoms in a spatial crystal occur at regular intervals in space, the movement of the Wilczek’s 4D crystal occurs at regular intervals in time (aka periods)”
“when the time crystal breaks time-translation symmetry that means that it is making a particular period in time special”
“Just like physics allows for the spontaneous formation of crystals, whose periodicity breaks the symmetry of space, so too should it allow for the spontaneous formation of time crystals, whose periodicity break the symmetry of time.”
“Wilczek’s idea was visionary in its originality and elegant in its simplicity, but ultimately he got the details wrong. One of the most glaring problems was that his time crystal approached something that looked suspiciously like perpetual motion—after all, where did the system in its lowest possible energy state (meaning energy cannot be extracted from it) get the energy to produce the periodic motion in the first place?”
“It wasn’t until 2016 that a group of physicists working at Station Q, a Microsoft research facility at UC Santa Barbara, figured out a way to correct the theoretical problems with Wilczek’s time crystals and provided the stepping stone to actually make them.”
“The group, led by physicist Chetan Nayak, built on prior research from Princeton University, which found that time crystals can spontaneously break a fundamental symmetry called time-translation symmetry to exhibit periodicity over time.”
“a time crystal should occur in a type of quantum system known as a Floquet many body localized driven system. This is basically just a fancy way of describing a system that is intrinsically out of thermal equilibrium”
“In other words, these systems never heat up, and cannot be characterized by any temperature, since the very idea of temperature supposes equilibrium.”
“non-equilibrium Floquet systems are able to host new states of matter that wouldn’t be possible in equilibrium systems, like the glass of water that turns to ice crystals”
“Whereas, say, equilibrium systems like liquids and gasses can spontaneously break natural spatial symmetries, by considering a non-equilibrium system, the Microsoft and UCSB researchers were able to predict spontaneously broken time-translation symmetry, aka a time crystal.”
“When a time crystal is driven, or pushed, at a certain period or frequency, it doesn’t respond at the same frequency that it was driven—in other words, if a laser is pulsed (the driving mechanism) at a chain of ions (the medium of the time crystal) every ten seconds, those ions will exhibit a period not of ten seconds, but twenty, thirty or some other multiple of the original period.”
“To help explain why this was so remarkable, consider this analogy.”
“Imagine three people playing jump rope: Bob and Rob hold the end of the rope and Alice jumps in the middle. Every three seconds, Bob and Rob’s arms make one full rotation, the rope goes around once and their arms return to their original position, which establishes the time-translation symmetry where the period is three seconds.”
“Now, to make a time crystal in this analogy, you have to break this time-translation symmetry by having the system respond at a different frequency. What that would mean is that Bob and Rob’s arms make multiple full rotations, but the rope only makes one full rotation. Put differently, Bob and Rob’s arms might make four full rotations, but Alice only has to jump over the rope once—which is pretty damn weird.”