Superradiant laser may boost atomic clocks, create ‘rulers’ for space
JILA physicists have demonstrated a novel laser design based on synchronizedemissions of light from the same type of atoms used in advanced atomic clocks. The laser could bestable enough to improve atomic clock performance a hundredfold and even serve as a clock itself,while also advancing other scientific quests such as making accurate “rulers” for measuringastronomical distances.
, the “superradiant”laser’s output of red light is expectedto be about 10,000 times lesssensitive than conventional lasersto pervasive mechanical vibrations,or noise. As a result, the new lasercan lock onto an exact frequency, orcolor, more tightly, making it 100 timessharper as a precision tool.
The work was done at JILA, a partnershipof the National Institute ofStandards and Technology (NIST) andthe ǸԹ.NIST has long been a world leaderin developing ultra-stable lasers, andthe new work provides a qualitativelynew approach for advancing the fieldfurther.
The same JILA group demonstrated the basic principle for a superradiant laser in 2012. Now the scientistshave built the laser using the same type of atoms used in JILA’s world-leading strontium latticeclock. In fact, the new laser might be used as an atomic clock all by itself.
Strontium atoms were chosen because they have an excellent “memory” of their exact color or frequency.They can potentially store this information for 2.5 minutes, compared to the mere 100 billionths ofa second of typical atoms. This allows the superradiant laser to store and protect most of the laser’scolor information inside the atoms. In contrast, ordinary lasers store this information in light bouncingbetween two mirrors, and any mirror vibrations scramble it. The ability to maintain a precise frequencyis crucial for applications like atomic clocks, which rely on lasers to make atoms “tick” from one energystate to another.
“But here is the rub: The very long memory of the atoms is awesome, but it also makes it extremelydifficult to get the atoms to emit any light, which provides the information for us to use,” said JILA/NISTscientist James Thompson. “But in this superradiant laser, for the first time, we have coaxed theseatoms to emit their light 10,000 times faster than they would normally like to emit it.”
JILA’s superradiant laser uses 200,000 strontium atoms stacked in layers of 5,000 and trapped in ahollow enclosure—a cavity—between two mirrors (these mirrors do vibrate, but the frequency informationis stored in the atoms). The atoms are chilled to temperatures near absolute zero and levitated in avacuum by an optical lattice, a “crystal of light” created by intersecting external laser beams.
The experiment begins by briefly shining light on the atoms to prepare them in their long-lived excited,or high-energy, state. An environmental signal—quantum noise of empty space—prompts the strontiumatoms to spontaneously start ticking as their outer electrons begin to bounce back and forth fromone side of the atom to the other. The oscillation is like a miniature antenna that radiates a very smallamount of light into the cavity. This very weak light, consisting of only a few light particles, or photons,bouncing back and forth inside the cavity, allows the atoms to communicate and synchronize with eachother. This synchronization phenomenon is also evident in pendulum clocks placed near each other,and even in the flashing of fireflies.
As the synchronization spreads and strengthens, more and more light is emitted, until eventually all theatoms have moved from an excited (high-energy) to a calm (low-energy) state. Light bounces back andforth between the mirrors nearly 30,000 times before leaking out through the mirrors. All of the energyinitially stored inside of the atoms has been converted into a pulse of laser light lasting 50 hundredths ofa second.
When synchronized, the collection of small antennas act like a single “super antenna” that broadcastspower into the cavity at a much higher than normal rate—a process called superradiance because thecollective emission is 1,000 times more intense than independently radiating atoms. The emission rateincreases proportionally to the number of atoms squared, making the laser much brighter than is possiblewithout synchronization.
Future studies will investigate use of the pulsed superradiant laser light as an absolute frequencyreference for such applications as atomic clocks. In addition, researchers hope to create a continuoussuperradiant laser beam by constantly returning atoms to the excited state.
“The superradiant laser light is still billions of times weaker than typical lasers, but the key point is thatthe color or frequency of the light should be very stable,” Thompson said.
Such a laser might be just as stable as the atoms used in the most advanced clocks. Today’s bestatomic clocks are limited in part by laser noise. Because a superradiant laser essentially uses an atomicclock as its energy source, the laser light both reads out the ticking of the atoms and is immune tocavity mirror vibrations. Better lasers may also have applications in space science, perhaps as rulers oflight that could reach across distances as vast as from the Earth to the Sun, potentially enabling thedetection of gravity waves in space, for example.
The research was funded by the Defense Advanced Research Projects Agency, Army Research Office,National Science Foundation and NIST.