Atomic clocks have been heavily studied by physicists for decades. The way these clocks work is by having atoms, such as rubidium or cesium, that are "ticking" (that is, oscillating) between two quantum states. As such, atomic clocks are extremely precise, but can be fragile to shaking or other perturbations, like temperature fluctuations. Additionally, these clocks need a special laser to probe the clock. Both factors can make atomic clocks imprecise, difficult to study, and expensive to make.
A team of physicists are proposing a new type of laser that could change the future path of atomic clocks. In this team, JILA Fellow Murray Holland and Research Associate Simon Jäger theorized a new type of laser system in a paper recently published in Physical Review Letters. The designed laser system utilizes a dense beam of excited atoms that cooperatively lose their energetic excitations by collective emission into an optical cavity. The design put forth by Holland and Jäger relies on excitation of the atoms outside of the laser cavity, making the equipment more robust and less sensitive to cavity shaking. In studying their new laser design, the Holland group collaborated with the Nicholson Laboratory out of the Center for Quantum Technologies in Singapore. The team hoped to use this stronger and more robust laser to develop an active atomic clock. Holland explained that "an active atomic clock produces its own stable light. And that is something of a Holy Grail for us theoretical physicists. This would combine light and matter into a single device, making an integrated coherent source, which is something that could eventually replace conventional atomic clocks." The impact of making better atomic clocks is of widespread benefit to society as it may lead to new discoveries in quantum physics, and new applications of quantum information science for industry and technology.
This new, more robust laser system would also be able to be implemented in other fields. In their paper, the team suggested that this system could be used to improve space technology, geodesy, and astrophysical measurements. With this more robust laser system, more accurate measurements could be made of some of our Earth’s properties, such as its composition and magnetic fields, as well as properties of other planets in our solar system.
A New Method for Quantum Entanglement
In designing this new laser system, the researchers found that their laser system could have two regimes, one of superradiance and one of subradiance. "Superradiant light is made by basically taking a lot of atoms, and putting them inside an optical cavity. And inside the cavity they talk to each other," Holland said. "And these atoms can synchronize together and generate this extremely pure light. The other side of the coin is subradiance. It's the same set up consisting of atoms placed inside an optical cavity, but instead of all emitting light in phase together, they sort of conspire against each other. And that destructive lack of reinforcement causes very little light to come out, but it also causes the atoms to become correlated in a quantum entangled way." Jäger and Holland explained that this laser system has a critical threshold for subradiance, and that close to this threshold, their laser actually produces a huge amount of entangled atoms. Their research on subradiance was published in a second paper in Physical Review Letters.
Pushing further, the team emphasized that quantum entanglement emerges in this laser system due to the interplay between driving, dissipation, and long-range interactions. As the laser system contained many different controls and parameters, finding the right ones were vital for their research. “One of these parameters is the beam flux—how many atoms there are inside the laser system,” commented Holland. “Lasers have a sort of critical threshold. So, if you crank up the parameters, eventually there’s a sort of phase transition to producing laser light, and it happens quite suddenly.” By varying the parameters, the team was excited to find a controllable boundary that separated the production of ultrastable light and the generation of vast quantities of entangled atoms.
Finding a new method for generating quantum entanglement would not only be valuable to processes like quantum communications, but could help with further study of condensed matter and quantum information science. The research team, including Holland and Jäger, were extremely excited to see experimental groups being interested in both the superradiant and subradiant regimes of this cavity setup. "There are multiple groups around the world that are very actively putting the numbers together and trying to think about whether a real engineering model could be developed from this theory," Holland stated. Jäger, for his part, added: “It is, I think, a very nice feeling when people are actually so interested in a theory that one creates."
Written by Kenna Castleberry, JILA Science Communicator