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

Quantum correlations -- without entanglement

Few people doubt the "quantumness" of entanglement. Quantifying the quantum correlation of entanglement is something that is relatively regular right now. However, things change a bit when it comes to quantum correlations other than entanglement. However, there is a growing interest in the use of non-entanglement quantum correlations in a number of possible future applications.
“A few years ago, scientists proposed quantum discord as a quantum correlation measure that goes beyond the entanglement paradigm,” Roberto Serra tells PhysOrg.com. “Quantum discord may be present, even in separate, non-entangled states. However, some doubt was being shed on the quantum qualities of non-entangled states because of the difficulty in quantifying the correlations.”
In order to remedy the difficulty in “seeing” the correlations in a laboratory setting, Serra, a scientist at the Federal University of ABC in São Paulo, Brazil, worked with a group to create a technique that makes it possible to recognize nonclassical correlations in quantum discord. Serra worked with a team from different Brazilian institutions of higher learning, including the Brazilian Agricultural Research Corporation's and the Brazilian Center for Physics Research both in Rio de Janeiro, and the Physics Institute of São Carlos, in São Paulo State. The results of the work can be seen in Physical Review Letters: Experimentally Witnessing the Quantumness of Correlations.”
“Nuclear Magnetic Resonance systems at room temperature were used to test principles of quantum computation with a good level of success,” Serra explains. “The quantum nature of these demonstrations was questioned because there is no entanglement in such a system. In our experiment we reveled directly the quantum nature of this system at room temperature. We used a sample of chloroform molecules, since it’s the simplest two-qubit system. We folded a qubit in the carbon nucleus and another one in the hydrogen nucleus.”
Next the Brazilian scientists were able to manipulate the system. Even though they used hot quantum bits, the system actually works as a quantum mechanical one. “We displaced the system from the thermal equilibrium by a very tiny deviation, and the phase coherence present there could encode quantum correlations as the measured by the quantum discord,” Serra says.
“Our methods can be applied to another system, such as an optical system. This can enable us to say if a given system is purely classical in nature, or if it has truly quantum correlations,” he continues. Serra thinks that using this test, which is relatively simple to perform in a laboratory setting, could help lay to rest the debate over whether or not these other types of correlations are truly quantum.
“We test the quantumness of discord at room temperature, and this very robust quantumness can be used to get an advantage in quantum protocols,” Serra insists. He believes that this method can already be used for metrology. “We are involved now in a test of principles in quantum metrology using this type of system, and exploiting this very tiny nonclassical correlation. We are testing those right now, to see about advantages over classical protocols, and we hope to have new results in the next few months.”
“We hope to develop future applications, and advance our comprehension about the rule played by this kind of quantumness in tasks as, for example, quantum communications,” Serra continues. “We are building collaborations between theoretical and experimental researchers, and we hope that we can do more to show the usefulness of other quantum correlations beyond entanglement.”

13 year old researcher finds tree inspired solar collection more efficient

Aidan Dwyer, a 13 year old Junior High School student from New York state, noticed that the phyllotaxy of the leaves on trees he was observing while hiking through the Catskill Mountains, did so in the form of a Fibonacci sequence. Wondering if there was a reason for it, he deduced that it might be because such an arrangement provides the most efficient means of solar power collection for the trees. To find out if this was the case, he built a small solar tree from PVC pipe and small solar panels, then built another in a normal flat panel array, attached voltage readers to both, and lo and behold, discovered the tree model array was indeed more efficient, at least during times of low or indirect sunlight. Dwyer won a Young Naturist Award for his efforts after writing and submitting his essay, The Secret of the Fibonacci Sequence in Trees.
The Young Naturist Awards are given (by the American Museum of Natural History) to two students from each grade, K-12, every year for young scientists who have investigated questions they have in the areas of biology, Earth science, ecology, and astronomy. Dwyer’s entry, took the known, that tree leaves grow in a Fibonacci sequence (where each number is the sum of the previous two) and applied it in a novel way that advanced the study of solar energy.

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The spiral on trees showing the Fibonacci Sequence

With trees, The Fibonacci pattern shows up in the way that limbs spiral around the trunk, specifically in the fraction that arises when computing the number of limbs it takes to complete a run all the way around the tree. Dwyer gives the example of the Oak tree which takes five branches to spiral two times around the trunk, giving the fraction 2/5. This is important because it was the basis of the model he built to replicate the tree structure. Interestingly, to find this fraction, and those of other types of trees, he fashioned his own measuring device out of a clear plastic tube with circle protractors on it. Branch angles were measured by inserting them into the tube.

Next, he built a small model tree (mimicking the Oak’s Fibonacci series as closely as possible) out of various sizes of PVC pipe to which he affixed small solar panels. After that, he put together a traditional flat panel solar array comprised of the same size solar panels. Then, after hooking up both to a data logger connected to a voltage meter, he then let them sit.

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The two models collecting sunlight

After analyzing his data, he found that the tree design appeared to be far more efficient than the traditional flat-panel structure during so-called off peak times, such as when the sun was low, and that the model appeared to be close to 50% more efficient overall during the winter. Not bad for someone who’s still a kid.

Dwyer theorizes that tree branches have evolved such an arrangement because it’s likely the most efficient pattern available, i.e. the one that best takes into account the shading created by branches or leaves hiding one another from direct sunlight.

Quantum memory works at room temperature

Quantum memory works at room temperature

How to store a photon at room temperature


A quantum memory for photons that works at room temperature has been created by physicists in the UK. The breakthrough could help researchers to develop a quantum repeater device that allow quantum information to be transmitted over long distances.

Quantum bits (or qubits) of information can be transmitted using photons and put to use in a number of applications, including cryptography. These schemes rely on the fact that photons can travel relatively long distances without interacting with their environment. This means that photon qubits are able, for example, to remain in entangled states with other qubits – something that is crucial for many quantum-information schemes.

However, the quantum state of a photon will be gradually changed (or degraded) due to scattering as it travels hundreds of kilometres in a medium such as air or an optical fibre. As a result, researchers are keen on developing quantum repeaters, which take in the degraded signal, store it briefly, and then re-emit a fresh signal. This way, says Ian Walmsley of the University of Oxford, "you can build up entanglement over much longer distances".
Difficult to repair

A quantum memory, which stores and re-emits photons, is the critical component of a quantum repeater. Those made so far in laboratories must be maintained at extremely cold temperatures or under vacuum conditions. They also only tend to work over very narrow wavelength ranges of light and store the qubit for very short periods of time. Walmsley and his colleagues argue that it isn't feasible to use such finicky systems in intercontinental quantum communication – these links will need to cross oceans and other remote areas, where it's difficult to send a repair person to fix a broken cryogenic or vacuum system.

Moreover, they should also absorb a broad range of frequencies of light and store data for periods much longer than the length of a signal pulse. Walmsley calls this combination a "key enabling step for building big networks". The broad range of frequencies means the memory can handle larger volumes of data, while a long storage time makes it easier to accumulate multiple photons with desired quantum states.

Working towards this goal, Walmsley and his team made a cloud of caesium atoms into a quantum memory that operates at an easy-to-achieve temperature of about 62 °C. Unlike previous quantum memories, the photons stored and re-emitted do not have to be tuned to a frequency that caesium electrons would like to absorb. Instead, a pulse from an infrared control laser converts the photon into a "spin wave", encoding it in the spins of the caesium electrons and nuclei.
Paint it black

Walmsley compares the cloud of caesium atoms to a pane of glass – transparent, so it allows the light through. The first laser paints the glass black in a sense, allowing it to absorb all the light that reaches it. However, instead of becoming dissipating as heat and as it would in the darkened glass, the light that passed into the caesium cloud is stored in the spin wave.

Up to 4 µs later, a second laser pulse converts the spin wave back into a photon and makes the caesium transparent to light again. The researchers say that the caesium's 30% efficiency in absorbing and re-emitting photons could increase with more energetic pulses from the control laser, while the storage time could be improved with better shielding from stray magnetic fields, which disturb the spins in the caesium atoms.

Even at 30% efficiency, Ben Buchler of the Australian National University in Canberra calls the device "a big deal" because it absorbs a wide band of photon frequencies. Due to Heisenberg's uncertainty principle, the ultra-short single-photon pulses from today's sources don't have well defined energies, so an immediately useful quantum memory must be able to absorb a wide range of frequencies – which Buchler says high-efficiency memories can't yet do.
Noise not a problem

Background noise, or extra photons generated in the caesium clouds that are unrelated to the signal photons, was a major concern for room-temperature memories. "People thought that if you started using room-temperature gases in storage mode, you'd just have a lot of noise," says Walmsley.

Temperatures near absolute zero suppress these extra photons other memories. But because the control and signal pulses in the Oxford team's set-up are far from caesium's favoured frequencies, the cloud was less susceptible to photon-producing excitations and the noise level remained small even at room temperature.

Hugues de Riedmatten of the Institute of Photonic Sciences in Barcelona, Spain, says that the researchers showed that the remaining noise is fundamental to the system, not caused by their set-up. If improvements cannot further reduce the noise, it will be challenging to maintain the integrity of the signal across a large, complex network, he explains.

Nevertheless, he says, "This approach is potentially very interesting because it may lead to a quantum memory for single photon qubits at room temperature, which would be a great achievement for quantum-information science."

The work is described in a paper to be published in Physical Review Letters and a preprint on arXiv.

The 'quantum magnet': Physicists expand prospects for engineering unusual materials

The physicists prepared a chain of single-atom magnets (red spheres with black arrows indicating north-south orientation) that repel one another, and aligned them (back row) with an external field. By reducing the field, they were able to observe reorientation (front row) caused by the magnetic repulsion (yellow helix) and minute quantum fluctuations. The background shows an image of the individual magnets, each comprising a single atom, as observed in the experiment.

  Harvard physicists have expanded the possibilities for quantum engineering of novel materials such as high-temperature superconductors by coaxing ultracold atoms trapped in an optical lattice -- a light crystal -- to self-organize into a magnet, using only the minute disturbances resulting from quantum mechanics. The research, published in the journal Nature, is the first demonstration of such a “quantum magnet” in an optical lattice.

As modern technology depends more and more on materials with exotic quantum mechanical properties, researchers are coming up against a natural barrier.
“The problem is that what makes these materials useful often makes them extremely difficult to design,”said senior author Markus Greiner, an associate professor in Harvard’s Department of Physics. “They can become entangled, existing in multiple configurations at the same time. This hallmark of quantum mechanics is difficult for normal computers to represent, so we had to take another approach.”
That approach is using a so-called “quantum simulator” — the properties of a quantum material are simulated with an artificial quantum system that can behave similarly, but that is easier to manipulate and observe.
The physicists found that when they applied a force to a crystal formed by ultracold atoms trapped in an optical lattice, a Mott insulator, the atoms behaved like a chain of little magnets that repelled one another, in the presence of an external magnetic field that sought to align them.
“When the external magnetic field was strong, all of the magnets aligned to it, forming a paramagnet,” said co-author Jonathan Simon, a postdoctoral fellow in physics. “When we reduced the magnetic field, the magnets spontaneously anti-aligned to their neighbors, producing an antiferromagnet.”
While such self-organization is common in everyday materials, it typically depends on temperature to jostle the system into the new order, like shaking a Boggle game to help the dice settle, the researchers say. “But the temperature was so low that thermal fluctuations were absent,” explained Simon. “Our fluctuations arose from quantum mechanics.”
When quantum mechanics takes over, things get bizarre. “Quantum fluctuations can make the magnets point in multiple directions simultaneously,” Greiner said. “This ‘quantum weirdness’ gives rise to many of the fascinating properties of quantum magnets.”
Greiner and his colleagues used a quantum gas microscope to observe individual magnets at temperatures of one billionth of a degree above absolute zero (-273 Celsius). They were able to watch as quantum fluctuations flipped the magnets around, turning a paramagnet into an antiferromagnet and back again.
“Observing quantum magnetism in a cold gas is a crucial first step toward quantum simulation of real magnetic materials,” Greiner said. “There remain many exciting questions to answer, and we have only just scratched the surface. By studying the bizarre and wonderful ways that quantum mechanics works, we open new perspectives not only for developing novel high-tech materials, but also for quantum information processing and computation.”

A team of physicists experimentally produces quantum-superpositions, simply using a mirror.

Standing in front of a mirror, we can easily tell apart ourselves from our mirror image. The mirror does not affect our motion in any way. For quantum particles, this is much more complicated. In a spectacular experiment in the labs of the Heidelberg University, a group of physicists from Heidelberg Unversity, together with colleagues at TU Munich and TU Vienna extended a gedankenexperiment by Einstein and managed to blur the distinction between a particle and its mirror image. The results of this experiment have now been published in the journal Nature Physics.
Emitted Light, Recoiling Atom
When an atom emits light (i.e. a photon) into a particular direction, it recoils in the opposite direction. If the photon is measured, the motion of the atom is known too. The scientists placed atoms very closely to a mirror. In this case, there are two possible paths for any photon travelling to the observer: it could have been emitted directly into the direction of the observer, or it could have travelled into the opposite direction and then been reflected in the mirror. If there is no way of distinguishing between these two scenarios, the motion of the atom is not determined, the atom moves in a superposition of both paths.
“If the distance between the atom and the mirror is very small, it is physically impossible to distinguish between these two paths,” Jiri Tomkovic, PhD student at Heidelberg explains. The particle and its mirror image cannot be clearly separated any more. The atom moves towards the mirror and away from the mirror at the same time. This may sound paradoxical and it is certainly impossible in classical phyiscs for macroscopic objects, but in quantum physics, such superpositions are a well-known phenomenon. “This uncertainty about the state of the atom does not mean that the measurement lacks precision”, Jörg Schmiedmayer (TU Vienna) emphasizes. “It is a fundamental property of quantum physics: The particle is in both of the two possible states simultaneousely, it is in a superposition.” In the experiment the two motional states of the atom – one moving towards the mirror and the other moving away from the mirror – are then combined using Bragg diffraction from a grating made of laser light. Observing interference it can be directly shown that the atom has indeed been traveling both paths at once.