JILA physicists have demonstrated a novel laser design based on synchronized听emissions of light from the same type of atoms used in advanced atomic clocks. The laser could be听stable enough to improve atomic clock performance a hundredfold and even serve as a clock itself,听while also advancing other scientific quests such as making accurate 鈥渞ulers鈥� for measuring听astronomical distances.
, the 鈥渟uperradiant鈥澨齦aser鈥檚 output of red light is expected听to be about 10,000 times less听sensitive than conventional lasers听to pervasive mechanical vibrations,听or noise. As a result, the new laser听can lock onto an exact frequency, or听color, more tightly, making it 100 times听sharper as a precision tool.
The work was done at JILA, a partnership听of the National Institute of听Standards and Technology (NIST) and听the University of Colorado Boulder.听NIST has long been a world leader听in developing ultra-stable lasers, and听the new work provides a qualitatively听new approach for advancing the field听further.
The same JILA group demonstrated the basic principle for a superradiant laser in 2012. Now the scientists听have built the laser using the same type of atoms used in JILA鈥檚 world-leading strontium lattice听clock. In fact, the new laser might be used as an atomic clock all by itself.听
Strontium atoms were chosen because they have an excellent 鈥渕emory鈥� of their exact color or frequency.听They can potentially store this information for 2.5 minutes, compared to the mere 100 billionths of听a second of typical atoms. This allows the superradiant laser to store and protect most of the laser鈥檚听color information inside the atoms. In contrast, ordinary lasers store this information in light bouncing听between two mirrors, and any mirror vibrations scramble it. The ability to maintain a precise frequency听is crucial for applications like atomic clocks, which rely on lasers to make atoms 鈥渢ick鈥� from one energy听state to another.
鈥淏ut here is the rub: The very long memory of the atoms is awesome, but it also makes it extremely听difficult to get the atoms to emit any light, which provides the information for us to use,鈥� said JILA/NIST听scientist James Thompson. 鈥淏ut in this superradiant laser, for the first time, we have coaxed these听atoms to emit their light 10,000 times faster than they would normally like to emit it.鈥�
JILA鈥檚 superradiant laser uses 200,000 strontium atoms stacked in layers of 5,000 and trapped in a听hollow enclosure鈥攁 cavity鈥攂etween two mirrors (these mirrors do vibrate, but the frequency information听is stored in the atoms). The atoms are chilled to temperatures near absolute zero and levitated in a听vacuum by an optical lattice, a 鈥渃rystal of light鈥� created by intersecting external laser beams.
The experiment begins by briefly shining light on the atoms to prepare them in their long-lived excited,听or high-energy, state. An environmental signal鈥攓uantum noise of empty space鈥攑rompts the strontium听atoms to spontaneously start ticking as their outer electrons begin to bounce back and forth from听one side of the atom to the other. The oscillation is like a miniature antenna that radiates a very small听amount of light into the cavity. This very weak light, consisting of only a few light particles, or photons,听bouncing back and forth inside the cavity, allows the atoms to communicate and synchronize with each听other. This synchronization phenomenon is also evident in pendulum clocks placed near each other,听and even in the flashing of fireflies.
As the synchronization spreads and strengthens, more and more light is emitted, until eventually all the听atoms have moved from an excited (high-energy) to a calm (low-energy) state. Light bounces back and听forth between the mirrors nearly 30,000 times before leaking out through the mirrors. All of the energy听initially stored inside of the atoms has been converted into a pulse of laser light lasting 50 hundredths of听a second.
When synchronized, the collection of small antennas act like a single 鈥渟uper antenna鈥� that broadcasts听power into the cavity at a much higher than normal rate鈥攁 process called superradiance because the听collective emission is 1,000 times more intense than independently radiating atoms. The emission rate听increases proportionally to the number of atoms squared, making the laser much brighter than is possible听without synchronization.
Future studies will investigate use of the pulsed superradiant laser light as an absolute frequency听reference for such applications as atomic clocks. In addition, researchers hope to create a continuous听superradiant laser beam by constantly returning atoms to the excited state.
鈥淭he superradiant laser light is still billions of times weaker than typical lasers, but the key point is that听the color or frequency of the light should be very stable,鈥� Thompson said.
Such a laser might be just as stable as the atoms used in the most advanced clocks. Today鈥檚 best听atomic clocks are limited in part by laser noise. Because a superradiant laser essentially uses an atomic听clock as its energy source, the laser light both reads out the ticking of the atoms and is immune to听cavity mirror vibrations. Better lasers may also have applications in space science, perhaps as rulers of听light that could reach across distances as vast as from the Earth to the Sun, potentially enabling the听detection of gravity waves in space, for example.
The research was funded by the Defense Advanced Research Projects Agency, Army Research Office,听National Science Foundation and NIST.
JILA鈥檚 superradiant laser is expected to be more stable than ordinary lasers,听making it a sharper tool for improving the performance of atomic clocks. The听laser is based on the same atom听used in strontium lattice atomic clocks and听might even serve as a clock itself. Image听credit: NIST
