No-photon laser: Physicists demonstrate 'superradiant' laser design

JILA's superradiant laser traps 1 million rubidium atoms in a space of about 2 centimeters between two mirrors. The atoms synchronize their internal oscillations to emit laser light. Credit: Burrus/NIST Physicists at JILA have demonstrated a novel "superradiant" laser design, which has the potential to be 100 to 1,000 times more stable than the best conventional visible lasers. This type of laser could boost the performance of the most advanced atomic clocks and related technologies, such as communications and navigation systems as well as space-based astronomical instruments. print this article email this article 7 text-to-speech save as pdf send feedback share to facebook share to twitter share to linkedin share to google share Described in the April 5, 2012, issue of Nature, the JILA laser prototype relies on a million rubidium atoms doing a sort of synchronized line dance to produce a dim beam of deep red laser light. JILA is a joint institute of the National Institute of Standards and Technology and the University of Colorado Boulder (CU). JILA/NIST physicist James Thompson says the new laser is based on a powerful engineering technique called "phased arrays" in which electromagnetic waves from a large group of identical antennas are carefully synchronized to build a combined wave with special useful features that are not possible otherwise. "It's like what happens in the classical world but with quantum objects," Thompson explains. "If you line up lots of radio antennas that each emit an oscillating electric field, you can get all their electric fields to add up to make a really good directional antenna. In the same way, the individual atoms spontaneously form something like a phased array of antennas to give you a very directional laser beam." Physicists at JILA have demonstrated a novel "superradiant" laser design with the potential to be 100 to 1,000 times more stable than the best conventional visible lasers. This type of laser could boost the performance of the most advanced atomic clocks and related technologies, such as communications and navigation systems as well as space-based astronomical instruments. An ordinary laser relies on millions of particles of light (photons) ricocheting back and forth between two mirrors, striking atoms in the lasing material and generating copies of themselves to build up intense light. Photons with synchronized wave patterns leak out of the mirrored cavity to form a laser beam. The laser frequency, or color, wobbles slightly because the mirrors are vibrating due to either the motion of atoms in the mirrors or environmental disturbances—which can be as subtle as people walking past the room or cars driving near the building. That doesn't happen in the new JILA laser simply because the photons don't hang around long enough. The atoms are constantly energizing and emitting synchronized photons, but on the average, very few—less than one photon, in fact —stick around between the mirrors. This average, which scientists calculate indirectly based on the laser beam's output power, is just enough to maintain an oscillating electric field to sustain the atoms' synchronized behavior. Nearly all photons escape before they have a chance to become scrambled by the mirrors and disrupt the synchronized atoms—thus averting the very effect that causes laser frequency to wobble in a normal laser. Thompson engineered a system that first traps the atoms in laser light between two mirrors and then uses other low-power lasers to tune the rate at which the atoms switch back and forth between two energy levels. The atoms emit photons each time their energy level drops. The atoms ordinarily would emit just one photon per second, but their correlated action boosts that rate 10,000-fold—making the light superradiant, Thompson says. This "stimulated emission" meets the definition of a laser (Light Amplification by the Stimulated Emission of Radiation). "This superradiant laser is really, really dim—about a million times weaker than a laser pointer," Thompson says. "But it is much brighter than one would expect from the ordinary uncoordinated emissions from individual atoms." Thompson's measurements show that the stability of the laser beam frequency is less than 1/10,000th as sensitive to mirror motion as in a normal optical laser. This result suggests the new approach might be used in the future to improve the best lasers developed at NIST as much as 1,000-fold. Just as important, such lasers might be moved out of the vibration-controlled laboratory environment to be used in real-world applications. Despite its dim light, the extraordinary stability of the superradiant laser can be transferred by using it as part of a feedback system to "lock" a normal laser's output. The bright laser, potentially 100 to 1,000 times more stable than today's best lasers, could then be used in the most advanced atomic clocks to induce the atomic oscillations that are the pendulum ticks of super-accurate clocks. The added stability allows for a better match to the atoms' exact frequency, significantly boosting the precision of the clock. The improvement would extend to atomic clock-based technologies such as GPS, optical communications, advanced geodetic surveys and astronomy. Thompson's work confirms predictions made several years ago by JILA/NIST Fellow Jun Ye and JILA/CU theorist Murray Holland, who is also a co-author of the new Nature paper. Thompson stresses that for the new laser design to achieve its highest potential stability and be of practical use, it will need to be re-created using different atoms, such as strontium, which are better suited for use in advanced atomic clocks. As a nonregulatory agency of the U.S. Department of Commerce, NIST promotes U.S. innovation and industrial competitiveness by advancing measurement science, standards and technology in ways that enhance economic security and improve our quality of life. More information: J.G. Bohnet, Z. Chen, J.M. Weiner, D. Meiser, M.J. Holland and J.K. Thompson. A steady state superradiant laser with fewer than one intracavity photon. Nature. Apr. 5, 2012. http://www.nature. … re10920.html

Speed limit on the quantum highway

January 26, 2012 by Olivia Meyer-Streng

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Propagation of quantum correlations in an optical lattice. Left: artist’s view (Graphic by woogie works animation studio). Right: a) In the initial state, each lattice site is filled by exactly one atom. The height of the barrier between the sites is then abruptly lowered, bringing the system out of equilibrium. b) After the barrier has been lowered, an entangled doublon-holon pair is formed. The correlated doublons and holons move across the system with opposite momenta. (Graphic: MPQ)

(PhysOrg.com) -- Physicists at the Max Planck Institute of Quantum Optics have measured the propagation velocity of quantum signals in a many-body system.

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A quantum computer based on quantum particles instead of classical bits, can in principle outperform any classical computer. However, it still remains an open question, how fast and how efficient quantum computers really may be able to work. A critical limitation will be given by the velocity with which a quantum signal can spread within a processing unit. For the first time, a group of physicists from the Quantum Many-Body Systems division at the Max-Planck Institute of Quantum Optics (Garching near Munich) in close collaboration with theoretical physicists from the University of Geneva (Switzerland) has succeeded in observing such a process in a solid-state like system. The physicists generated a perfectly ordered lattice of rubidium atoms and then induced a quantum excitation – an “entangled” pair of a doubly occupied lattice site next to a hole. With the aid of a microscope they observed how this signal moved from lattice site to lattice site. “This measurement gives insight into very elementary processes involved in the communication and processing of quantum information”, Professor Immanuel Bloch points out.

The communication and processing of information in a quantum computer is based on concepts that are inherently different from those used in classical computers. This is due to the fundamental differences between quantum particles and classical objects. Whereas the latter are, for example, either black or white, quantum particles can take on both colours at the same time. It is only at the process of measurement that the particles decide on one of the two possible properties. As a consequence of this peculiar behaviour, two quantum objects can form one entangled state in which their properties are strictly connected, i.e. quantum correlated. At present there is no general model for predicting how fast a quantum correlation can travel after it is generated.

Now physicists from the Quantum Many-Body Systems division have been able to directly observe such a process. They start the experiment by generating an extremely cold gas of rubidium atoms. The ensemble is then kept in a light field which divides it into several parallel one-dimensional tubes. Now the tubes are superimposed with yet another light field, a standing laser light wave. By the periodic sequence of dark and bright areas, the atoms are forced to form a lattice structure: exactly one atom is trapped in each bright spot, and is separated from the neighbouring atom by a dark area which acts as a barrier.

Changing the intensity of the laser light controls the height of this barrier. At the beginning of the experiments, it is set to a value that prevents the atoms from moving to a neighbouring site. Then, in a very short time, the height of the barrier is lowered such that the system gets out of equilibrium and local excitations arise: Under the new conditions one or the other atom is allowed to “tunnel” through the barrier and reach its neighbouring site. If this happens, entangled pairs are generated, each consisting of a doubly occupied site, a so-called doublon, and a hole, named holon. According to a model developed by theoretical physicists from the University of Geneva around Professor Corinna Kollath, both doublon and holon move through the system – in opposite directions – as if they were real particles (see figure). “Regarding one entangled pair, it is not defined whether the doublon sits on the right or on the left side of the holon. Both constellations are present at the same time”, Dr. Marc Cheneau, a scientist in the Quantum Many-Body Systems division, explains. “However, once I observe a doubly occupied or an empty site, I exactly know where to find its counterpart. This is the correlation we are talking about.”

Now the scientists observe how the correlations are carried through the system. Using a new microscopic technique, they can directly image the single atoms on their lattice sites. In simplified terms, they make a series of snapshots each showing the position of the doublons and the holons at that very moment. The propagation velocity of this correlation can be deduced from the distance the two partners have moved apart in a certain period of time. The experimental results are in very good agreement with the predictions of the model mentioned above.

“As long as quantum information is communicated with light quanta, we know, that this is done with the speed of light,” Dr. Cheneau points out. “If, however, quantum bits or quantum registers are based on solid-state structures, things are different. Here the quantum correlation has to be passed on directly from bit to bit. Once we know how fast this process can happen, we have the key to understand, what will limit the velocity of future quantum computers.”

More information: Marc Cheneau, Peter Barmettler, Dario Poletti, Manuel Endres, Peter Schauß, Takeshi Fukuhara, Christian Gross, Immanuel Bloch, Corinna Kollath and Stefan Kuhr, Light-cone-like spreading of correlations in a quantum many-body system, Nature, DOI:10.1038/nature10748