We all know crystals as an arrangement of atoms in a three-dimensional lattice with examples being sugar, salt and diamonds. The arrangements we know are all periodic.
A new form of matter, called a time crystal by the Harvard physicists that created it, could offer a new perspective into the enigmatic behavior of quantum systems. The crystals take that idea of periodically arranged atoms and add another dimension. The team suggests that the atoms of certain materials can exhibit a periodic structure across time under certain conditions.
A team of post-doctoral fellows led by Professors of Physics Mikhail Lukin and Eugene Demler used a small piece of diamond to build a quantum system. Millions of atomic scale impurities known as nitrogen vacancy (NV) centers were embedded in the diamond and microwave pulses were used to “kick” the system out of equilibrium. This resulted in the NV center’s spins flipping at precise time intervals, one of the key characteristics of a time crystal.
The creation of a time crystal is important because it proves that materials that were previously only theoretical can in fact exist. The time crystals also offer physicists an alluring window into the behavior of systems that are out of equilibrium.
Lukin notes that understanding the physics of non-equilibrium quantum systems is an area that is of interest for many quantum technologies. The subject is currently being studied on a broad, ongoing basis. A quantum computer is if fact a quantum system that is very much out of equilibrium and Lukin added that their work is only scratching the surface.
Although understanding such systems could help guide researchers in the study of quantum computing, the technology behind time crystals also has applications that could be useful in the short term.
Lukin explains that the original inspiration for this work was to improve precision measurement and the team believes their work might be useful in this specific area. He cited the example of building a magnetic field sensor where it would be possible to use NV-center spins, making the non-equilibrium states of matter that they create beneficial.
Initially it seemed that it would be impossible to build such systems. Various researchers did in fact prove that it would not be possible to create a time crystal in a quantum system that was at equilibrium. Lukin explained that most things are normally in equilibrium. If you for example have a cold body and a hot body and you bring them together, their temperature will equalize. He added however that not all systems are like that.
A diamond is a common example of a material that is out of equilibrium. Diamonds are crystallized forms of carbon that form under immense pressure and heat. A diamond is meta-stable and once it adopts the crystal formation, it will stay that way, even when the pressure and heat are no longer present.
Researchers only recently discovered that non-equilibrium systems could exhibit the characteristics of a time crystal. This is especially true for those known as “driven” systems that can be prodded with periodic energy pulses. One of the features that makes the substance behave as a time crystal is that the crystal’s response across time will remain strong with respect to perturbations.
If you push on a solid crystal, the distance between atoms may change slightly, but the crystal itself survives. The concept behind a time crystal is to have that same type of order in a time domain, but it must be tough.
Another important element is that if you keep pushing a system away from equilibrium it will start to heat up. There is however a class of system where this does not happen. It became apparent the time crystal effect is strongly linked to the idea that when a system is excited, it does not absorb energy.
When Lukin and colleagues started building their system, they began with a small piece of diamond that was embedded with so many NV centers it looked black.
The diamond was then exposed to microwave pulses. This changed the orientation of the spins of the NV centers. All the spins that were pointed up were turned down, and a next pulse turned them back up. The timing of the pulses was changed to determine if the material would continue to respond like a time crystal, i.e. the researchers checked if the system was robust.
Lukin explained that if all the spins are not oriented fully up or down each time and very rapidly, the system would be completely random. In this case, however, the system responded in a periodic, time crystalline manner, as the interactions between the NV centers stabilized the response.
Such systems demonstrate that two critical components, a very high density of quantum bits and long quantum memory times, are not mutually exclusive. This is critical in the development of useful quantum sensors and quantum computers. Although you need both of those for many applications, it is well known that the two requirements are usually contradictory.
Although much work still needs to be done, the research shows that the desired combination can be achieved. The team believes these effects will enable a new generation of quantum sensors, and could possibly have other applications, like atomic clocks, in the future.