Image courtesy of Robert Strasser, Kees Scherer, and Michael Büker on Flickr.
In grade school, many of us learned that there are three basic states of matter in the universe: solid, liquid, and gas. However, in recent years, scientists have created several more, including one state that seems to bend the laws of physics: a “time crystal.” While it may sound more like an Avenger’s oddity than a scientific reality, time crystals have unique properties that can be used for extremely precise timekeeping. Previously, they have been very difficult to create and maintain. Now, researchers from the United States and Poland have innovated a new way of creating time crystals that could allow them to leave the confines of sophisticated laboratories and be used for everyday applications.
Most people are familiar with normal, garden-variety crystals: quartz, diamonds, snowflakes. These crystals consist of repeating patterns of atoms layered on top of each other to form a 3D structure. In 2012, theoretical physicist and Nobel laureate Frank Wilczek wondered if a similar phenomenon could exist where a crystal’s pattern would repeat in time rather than in 3D space. The crystal’s default state would be to switch back and forth between two different structures. What makes time crystals so strange and unique is that they spontaneously break time-translation symmetry, which says that a stable object will act the same throughout time.
We have an intuitive sense of time-translation symmetry for objects in our daily lives. For example, imagine that you’re holding a tray with a rubber ball on it. You tilt the tray from side to side, making the ball roll across the tray repeatedly. You expect the ball to make one trip across the tray each time it is tilted—however, if the ball suddenly decided to rocket back and forth across the tray fifty times faster than the speed of your tilting, you would certainly be very surprised. This change breaks time-translation symmetry, just like time crystals do. Time crystals are naturally not in static equilibrium, making them a new state of matter.
Scientists first created a time crystal in 2016, and several groups have devised different versions of time crystals since then. However, they all have one unfortunate characteristic in common: they can’t ever be taken out of the lab due to their complicated and precise configurations. In fact, after a certain amount of time, even the crystals in the lab will devolve and stop their periodic motion. These factors severely limit the lifetime of time crystals and prevent them from being used for everyday applications outside the lab. However, a few groups of scientists have proposed the creation of a time crystal out of light that wouldn’t be bound by these limitations, and one recently succeeded in creating one of these time crystals.
In 2018, Hossein Taheri was a recently graduated electrical and computer engineering Ph.D. who had just started his lab at the University of California at Riverside. While visiting a friend for lunch at the California Institute of Technology and chatting about physics projects, his friend mentioned a curious concept he had never heard about before: time crystals. When he returned home that day, he found and read a review article that discussed time crystals, and it struck a chord with him. “Within a few days, I was just thinking that, well, we can create something with these properties!” Taheri said.
He shared his thoughts with Andrey Matsko, a group lead and collaborator at the NASA Jet Propulsion Laboratory working with quantum and nonlinear optics. Matsko agreed that their work might relate to time crystals and greenlighted delving further into this line of research. In their first couple of papers, they wrote very cautiously about the concept. “But little by little, we realized that, well, we are on the right track. Why not shoot higher?” Taheri said. Taheri then decided to contact and collaborate with Krzysztof Sacha, a physics professor at Jagiellonian University and one of the first researchers to study time crystals.
Their team’s approach is simpler and less expensive than those previously used to create time crystals. They shine two lasers into a tiny crystal cavity roughly two millimeters across, and the beams bounce around the walls of the cavity. The beams are stabilized using a method called self-injection locking, which was developed by a team at OEwaves Inc. led by president and CEO Lute Maleki. If the beams are tuned to the correct power and frequency, the interacting light eventually spontaneously resonates at a frequency entirely different from the properties of either input laser beam, creating a time crystal in the cavity. Previous studies had only ever used solid-state physics to create time crystals–theirs was the first to use light and optics.
Their light-based time crystal is revolutionary because it isn’t subject to the limitations of solid-state physics. Previous solid-state time crystals required extremely cold cryogenic environments and complicated, expensive equipment. In contrast, optics work just fine at room temperature and can also create time crystals with a much longer lifetime. “What we are offering with this work is that if you have a resonator and two lasers, and you spend probably a few thousand dollars on your setup, you in principle can generate a time crystal,” Taheri said. This pioneering innovation makes the study of time crystals vastly more accessible and could pave the way for using time crystals in everyday applications.
The main application of light-based time crystals comes from their remarkably accurate timekeeping abilities. While they are some orders of magnitude less precise than state-of-the-art atomic clocks, they aren’t picky about their environmental conditions and can thus provide highly accurate timing while being rugged enough to load onto a plane or car. This tradeoff in precision is necessary since the time crystal generated by the team cannot be more stable than the two lasers that created it. In order to have more stable lasers, the environment around the crystal would need to be tightly controlled, or the system setup would have to be more complicated.
Light-based time crystals could also enable entirely new avenues of physics research through the creation of bigger time crystals. The “size” of a time crystal refers to the ratio between two different time periods: the time it takes for one complete back-and-forth structure change of the time crystal and the period of the laser’s light waves. A “bigger” time crystal has a larger ratio. Most previous time crystals only had a size of two or three, while this research team’s light-based system could create crystals with a size of twenty or even fifty. These bigger time crystals would essentially provide scientists with a larger “laboratory space,” allowing them to do more complicated experiments. In particular, larger time crystals can be used to mimic condensed matter time crystal experiments that are otherwise difficult or even impossible to conduct in smaller time crystals. But beyond this known application, the study of the crystals themselves is still young and could result in fascinating new science that researchers can’t yet imagine. “This, for me, is the most interesting application,” Matsko said.
It’s extraordinary that just ten years after Frank Wilczek’s musings about the existence of time crystals, we are now close to seeing them outside the laboratory. But this transition from a dream to reality isn’t a novel process–even the first cell phone was inspired by Star Trek’s sci-fi communicator devices. Scientists who dare to adventure have the incredible power to forge these far-fetched ideas into our world’s reality.