The two physics stories that dominated the news in 2011 were questions rather than solid scientific results, namely "Do neutrinos travel faster than light?" and "Has the Higgs boson been found?". However, there have also been some fantastic bona fide research discoveries over the last 12 months, which made it difficult to decide on the Physics World 2011 Breakthrough of the Year. But after much debate among the Physics World editorial team, this year's honour goes to Aephraim Steinberg and colleagues from the University of Toronto in Canada for their experimental work on the fundamentals of quantum mechanics. Using an emerging technique called "weak measurement", the team is the first to track the average paths of single photons passing through a Young's double-slit experiment – something that Steinberg says physicists had been "brainwashed" into thinking is impossible. We have also awarded nine runners-up (see below). The choice between first and second place was particularly close this year because the number-two finding also involves weak measurement – this time to map the wavefunction of a bunch of photons. But we felt that Steinberg's finding edged it. Other breakthroughs in the list include the first "space–time" cloak, a laser made from a living cell and a new way to measure cosmic distances.
1st place: Shifting the morals of quantum measurement
Steinberg's work stood out because it challenges the widely held notion that quantum mechanics forbids us any knowledge of the paths taken by individual photons as they travel through two closely spaced slits to create an interference pattern. This interference is exactly what one would expect if we think of light as an electromagnetic wave. But quantum mechanics also allows us to think of the light as photons – although with the weird consequence that if we determine which slit individual photons travel through, then the interference pattern vanishes. By using weak measurements Steinberg and his team have been able to gain some information about the paths taken by the photons without destroying the pattern. In the experiment, the double slit is replaced by a beamsplitter and a pair of optical fibres. A single photon strikes the beamsplitter and travels along either the right or the left fibre. After emerging from the closely spaced ends of the parallel fibres, it creates an interference pattern on a detector screen. The weak measurement is performed by passing the emerging photons through a piece of calcite, which imparts a tiny rotation in the polarization of the photon. The amount of rotation depends on the direction of travel of the photon – in other words, its momentum. The photons are then "post-selected" according to where they strike the screen, which allows the researchers to determine the average direction of travel of photons that arrive there. The experiment reveals, for example, that a photon detected on the right-hand side of the diffraction pattern is more likely to have emerged from the optical fibre on the right than from the optical fibre on the left. While this knowledge is not forbidden by quantum mechanics, Steinberg says that physicists have been taught that "asking where a photon is before it is detected is somehow immoral". "Little by little, people are asking forbidden questions," says Steinberg, who adds that his team's experiment will "push [physicists] to change how they think about things".
2nd place: Measuring the wavefunction
Second place goes to another group that has asked a "forbidden question". Led by Jeff Lundeen at the National Research Council of Canada in Ottawa – a former colleague of Steinberg – a team has used weak measurement to map out the wavefunction of an ensemble of identical photons without actually destroying any of them. Quantum tomography, in contrast, maps out the wavefunction at the expense of destroying the state. As well as boosting our understanding of the fundamentals of quantum mechanics, the technique could prove useful in cases where tomography cannot be used.
3rd place: Cloaking in space and time
Coming in at third place are two teams – one at Cornell University in the US led by Alexander Gaeta, and the other at Imperial College London headed by Martin McCall. In early 2011 McCall's team published a theoretical analysis of how an event in space and time could be cloaked, which he later described in a special Physics World feature. A few months later, Gaeta and colleagues built a device that uses two "split time lenses" to do just that. As well as changing our ideas about what can and cannot be cloaked, space–time cloaking could also be used in the perfect bank heist – at least in theory.
4th place: Measuring the universe using black holes
Fourth spot on the list goes to Darach Watson and colleagues at the University of Copenhagen, Denmark, and the University of Queensland, Australia, who have worked out a way of using supermassive black holes – which power active galactic nuclei (AGNs) – as "standard candles" for making accurate measurements of cosmic distances. The work is important because AGNs can be found just about everywhere in the universe, and unlike the supernovae currently used as standard candles, the light from AGNs endures for long periods of time.
5th place: Turning darkness into light
Christopher Wilson and colleagues of Chalmers University of Technology in Sweden together with physicists in Japan, Australia and the US have bagged fifth place because they are the first to see the dynamical Casimir effect in the lab. The effect arises when a mirror is moving so quickly through a vacuum that pairs of virtual photons – which are always appearing and then annihilating – are pulled apart to create real photons that can then be detected. As well as shedding new light on the Casimir effect, the team's use of a superconducting quantum interference device (SQUID) as the mirror make this an extremely clever experiment.
6th place: Taking the temperature of the early universe
Just after the Big Bang, the universe was a complicated soup of free quarks and gluons that eventually condensed to form the protons and neutrons we see today. Sixth place in our top 10 goes to a team of physicists in the US, India and China that has made the best calculation yet of this condensation temperature: two trillion degrees Kelvin. As well as providing important insights into the early universe, the work also advances our understanding of quantum chromodynamics, which describes the properties of neutrons, protons and other hadrons.
7th place: Catching the flavour of a neutrino oscillation
Seventh place is awarded to the international team of physicists working on the Tokai-to-Kamioka (T2K) experiment in Japan. The researchers fired a beam of muon neutrinos 300 km underground to a detector, where they found that six neutrinos had changed, or "oscillated", into electron neutrinos. While the measurement is not good enough to claim the discovery of the muon-to-electron neutrino oscillation, it is the best evidence yet that one "flavour" of neutrino can oscillate into another.
8th place: Living laser brought to life
In a fascinating bit of biophysics, Malte Gather and Seok Hyun Yun at Harvard Medical School in the US share eighth place for being the first to make a laser from a living biological cell. By shining intense blue light onto green fluorescent protein molecules inside an embryonic kidney cell, the molecules generate light that is intense, monochromatic and directional. The cells survive the ordeal and this amazing phenomenon could potentially be used to distinguish cancerous cells from healthy ones.
9th place: Complete quantum computer made on a single chip
Ninth place goes to Matteo Mariantoni and colleagues at the University of California, Santa Barbara for being the first to implement a quantum version of the "Von Neumann" architecture found in PCs. Based on superconducting circuits and integrated on a single chip, the new device has been used to perform two important quantum-computing algorithms. Its development moves us closer to the creation of practical quantum computers that solve real-life problems.
10th place: Seeing pure relics from the Big Bang
Michele Fumagalli and Xavier Prochaska of the University of California, Santa Cruz and John O'Meara of Saint Michael's College in Vermont take 10th spot for being the first to catch sight of clouds of gas that are pure relics of the Big Bang. Unlike other clouds in the distant universe – which appear to contain elements created by stars – these clouds contain just the hydrogen, helium and lithium created by the Big Bang. As well as confirming predictions of the Big Bang theory, the clouds provide a unique insight into the materials from which the first stars and galaxies were born.
Source:physicsworld
Shabir. Barzanjeh
Quantum optics is a field of research in physics, dealing with the application of quantum mechanics to phenomena involving light and its interactions with matter.
Friday, February 3, 2012
Wednesday, February 1, 2012
Another Step Back for Wave-Particle Duality
Published December 2, 2011 | Physics 4, 102 (2011) | DOI: 10.1103/Physics.4.102
A new thought experiment makes it clearer than ever that photons aren’t simply particles or waves.
Quantum physics tells us that a photon isn’t strictly a particle or strictly a wave. And yet most of us will revert back—whenever we can—to familiar concepts of billiard balls or vibrating strings when picturing photons in our heads. A new thought experiment, proposed in Physical Review Letters, hopes to break us of these old habits. The authors imagine a type of quantum switch that controls whether a simple optical measurement tests for particlelike or wavelike behavior in a single photon. This slight reworking of a famous experiment demonstrates with logical precision the futility of trying to label the photon as a particle or a wave.
The wave-particle duality is often illustrated by splitting a light beam so that it travels along two separate paths that later merge to form an interference pattern from the combined beams. For a dim beam delivering photons one-at-a-time, this interference suggests that each photon is a wave that travels down both paths simultaneously. But if the paths are observed individually, then the photon will behave like a particle, traveling down only one path or the other and generating no interference. The fact that no experiment can measure both the wave and the particle behaviors simultaneously is called the principle of complementarity.
Someone unhappy with this indeterminacy could imagine that the photon somehow knows what measuring devices lie ahead and thereby decides in advance to go down both paths or one path, accordingly. To deny us this conceit, John Wheeler, at the University of Texas, proposed a thought experiment 30 years ago in which a “wave detector” suddenly switches to a “particle detector” during the photon’s travels, or vice versa.
This so-called delayed-choice experiment was performed in 2007 using an interferometer [1]. In the normal setup, a beam splitter creates two separate light beams that later recombine in a second beam splitter. Detectors placed at the two outputs of this beam splitter both register an interference pattern. However, this wave detector can be turned into a particle detector by removing the second beam splitter, so that the two paths no longer interfere. In the experiment, the choice to add or remove the second beam splitter was made after an individual photon had already passed through the first beam splitter. The data showed that particle and wave behavior were unaffected by the delayed choice, as expected from standard quantum mechanics.
Radu Ionicioiu, now at the Institute for Quantum Computing in Waterloo, Canada, and Daniel Terno of Macquarie University in Sydney, Australia, wanted to see what happens in the thought experiment if the delayed choice is made through quantum means. They imagined that the interferometer contains a quantum device—perhaps an atom in a cavity or a micro-mirror placed on a cantilever—that can exist in two possible states. One state selects the particle experiment, and the other selects the wave experiment. This quantum control element can be placed in a combination, or superposition, of its two states, making the whole experiment participate in the wave-particle duality.
“We show you can do both wave and particle experiments at once,” Ionicioiu says. This means the choice of wave vs particle can be delayed indefinitely. The photon can be observed at one of the detectors and still not “know” if it is supposed to be a wave or a particle. It’s only when the observer decides to measure the state of the quantum control that the photon’s behavior can be identified as wavelike or particlelike.
The idea of having measurements depend on some secondary quantum effect was explored previously in the so-called quantum eraser [2]. But this new, simpler thought experiment allows a direct analysis of the information flows between experimental elements, Ionicioiu says. Within this simplified framework, the theorists evaluated certain alternative theories to standard quantum mechanics known as “hidden variable” theories and showed that they would have to be absurdly complicated to reproduce the results of their quantum delayed-choice experiment.
This proof doesn’t come as a surprise. “We know very well that quantum phenomena cannot be described by hidden-variable theories, unless they have utterly implausible and unacceptable properties,” says Berthold-Georg Englert of the National University of Singapore. Marco Genovese of the Italian Metrological Institute (INRIM) in Turin agrees that the basic concepts here are not new. But he still very much likes this simple thought experiment for the way it points out the paradoxes that emerge from treating the photon as a classical particle or wave.
-Shabir Barzanjeh.
References
V. Jacques, E Wu, F. Grosshans, F. Treussart, P. Grangier, A. Aspect, and J.-F. Roch, “Experimental Realization of Wheeler’s Delayed-Choice Gedanken Experiment,” Science 315, 966 (2007).
Y-H. Kim, R. Yu, S. P. Kulik, Y. Shih, and M. O. Scully, “Delayed ‘Choice’ Quantum Eraser,” Phys. Rev. Lett. 84, 1 (2000).
A new thought experiment makes it clearer than ever that photons aren’t simply particles or waves.
Quantum physics tells us that a photon isn’t strictly a particle or strictly a wave. And yet most of us will revert back—whenever we can—to familiar concepts of billiard balls or vibrating strings when picturing photons in our heads. A new thought experiment, proposed in Physical Review Letters, hopes to break us of these old habits. The authors imagine a type of quantum switch that controls whether a simple optical measurement tests for particlelike or wavelike behavior in a single photon. This slight reworking of a famous experiment demonstrates with logical precision the futility of trying to label the photon as a particle or a wave.
The wave-particle duality is often illustrated by splitting a light beam so that it travels along two separate paths that later merge to form an interference pattern from the combined beams. For a dim beam delivering photons one-at-a-time, this interference suggests that each photon is a wave that travels down both paths simultaneously. But if the paths are observed individually, then the photon will behave like a particle, traveling down only one path or the other and generating no interference. The fact that no experiment can measure both the wave and the particle behaviors simultaneously is called the principle of complementarity.
Someone unhappy with this indeterminacy could imagine that the photon somehow knows what measuring devices lie ahead and thereby decides in advance to go down both paths or one path, accordingly. To deny us this conceit, John Wheeler, at the University of Texas, proposed a thought experiment 30 years ago in which a “wave detector” suddenly switches to a “particle detector” during the photon’s travels, or vice versa.
This so-called delayed-choice experiment was performed in 2007 using an interferometer [1]. In the normal setup, a beam splitter creates two separate light beams that later recombine in a second beam splitter. Detectors placed at the two outputs of this beam splitter both register an interference pattern. However, this wave detector can be turned into a particle detector by removing the second beam splitter, so that the two paths no longer interfere. In the experiment, the choice to add or remove the second beam splitter was made after an individual photon had already passed through the first beam splitter. The data showed that particle and wave behavior were unaffected by the delayed choice, as expected from standard quantum mechanics.
Radu Ionicioiu, now at the Institute for Quantum Computing in Waterloo, Canada, and Daniel Terno of Macquarie University in Sydney, Australia, wanted to see what happens in the thought experiment if the delayed choice is made through quantum means. They imagined that the interferometer contains a quantum device—perhaps an atom in a cavity or a micro-mirror placed on a cantilever—that can exist in two possible states. One state selects the particle experiment, and the other selects the wave experiment. This quantum control element can be placed in a combination, or superposition, of its two states, making the whole experiment participate in the wave-particle duality.
“We show you can do both wave and particle experiments at once,” Ionicioiu says. This means the choice of wave vs particle can be delayed indefinitely. The photon can be observed at one of the detectors and still not “know” if it is supposed to be a wave or a particle. It’s only when the observer decides to measure the state of the quantum control that the photon’s behavior can be identified as wavelike or particlelike.
The idea of having measurements depend on some secondary quantum effect was explored previously in the so-called quantum eraser [2]. But this new, simpler thought experiment allows a direct analysis of the information flows between experimental elements, Ionicioiu says. Within this simplified framework, the theorists evaluated certain alternative theories to standard quantum mechanics known as “hidden variable” theories and showed that they would have to be absurdly complicated to reproduce the results of their quantum delayed-choice experiment.
This proof doesn’t come as a surprise. “We know very well that quantum phenomena cannot be described by hidden-variable theories, unless they have utterly implausible and unacceptable properties,” says Berthold-Georg Englert of the National University of Singapore. Marco Genovese of the Italian Metrological Institute (INRIM) in Turin agrees that the basic concepts here are not new. But he still very much likes this simple thought experiment for the way it points out the paradoxes that emerge from treating the photon as a classical particle or wave.
-Shabir Barzanjeh.
References
V. Jacques, E Wu, F. Grosshans, F. Treussart, P. Grangier, A. Aspect, and J.-F. Roch, “Experimental Realization of Wheeler’s Delayed-Choice Gedanken Experiment,” Science 315, 966 (2007).
Y-H. Kim, R. Yu, S. P. Kulik, Y. Shih, and M. O. Scully, “Delayed ‘Choice’ Quantum Eraser,” Phys. Rev. Lett. 84, 1 (2000).
Monday, November 28, 2011
Coherent Schrödinger's cat still confounds
The famous paradox of Schrödinger's cat starts from principles of quantum physics and ends with the bizarre conclusion that a cat can be simultaneously in two physical states – one in which the cat is alive and the other in which it is dead. In real life, however, large objects such as cats clearly don't exist in a superposition of two or more states and this paradox is usually resolved in terms of quantum decoherence. But now physicists in Canada and Switzerland argue that even if decoherence could be prevented, the difficulty of making perfect measurements would stop us from confirming the cat's superposition.
Erwin Schrödinger, one of the fathers of quantum theory, formulated his paradox in 1935 to highlight the apparent absurdity of the quantum principle of superposition – that an unobserved quantum object is simultaneously in multiple states. He envisaged a black box containing a radioactive nucleus, a Geiger counter, a vial of poison gas and a cat. The Geiger counter is primed to release the poison gas, killing the cat, if it detects any radiation from a nuclear decay. The grisly game is played out according to the rules of quantum mechanics because nuclear decay is a quantum process.
If the apparatus is left for a period of time and then observed, you may find either that the nucleus has decayed or that it has not decayed, and therefore that the poison has or has not been released, and that the cat has or has not been killed. However, quantum mechanics tells us that, before the observation has been made, the system is in a superposition of both states – the nucleus has both decayed and not decayed, the poison has both been released and not been released, and the cat is both alive and dead.
Mixing micro and macro
Schrödinger's cat is an example of "micro-macro entanglement", whereby quantum mechanics allows (in principle) a microscopic object such as an atomic nucleus and a macroscopic object such as a cat to have a much closer relationship than permitted by classical physics. However, it is clear to any observer that microscopic objects obey quantum physics, while macroscopic things obey the classical physics rules that we experience in our everyday lives. But if the two are entangled it is impossible that each can be governed by different physical rules.
The most common way to avoid this problem is to appeal to quantum decoherence, whereby multiple interactions between an object and its surroundings destroy the coherence of superposition and entanglement. The result is that the object appears to obey classical physics, even though it is actually following the rules of quantum mechanics. It is impossible for a large system such as a cat to remain completely isolated from its surroundings, and therefore we do not perceive it as a quantum object.
While not disputing this explanation, Christoph Simon and a colleague at the University of Calgary, and another at the University of Geneva, have asked what would happen if decoherence did not affect the cat. In a thought experiment backed up by computer simulations, the physicists consider pairs of photons (A and B) generated from the same source with equal and opposite polarizations, travelling in opposite directions. For each pair, photon A is sent directly to a detector, but photon B is duplicated many times by an amplifier to make a macroscopic light beam that stands in for the cat. The polarizations of the photons in this light beam are then measured.
Two types of amplifier
They consider two different types of amplifier. The first measures the state of photon B, which has the effect of destroying the entanglement with A, before producing more photons with whatever polarization it measures photon B to have. This is rather like the purely classical process of observing the Geiger counter to see whether it has detected any radiation, and then using the information to decide whether or not to kill the cat. The second amplifier copies photon B without measuring its state, thus preserving the entanglement with A.
The researchers ask how the measured polarizations of the photons in the light beam will differ depending on which amplifier is used. They find that, if perfect resolution can be achieved, the results look quite different. However, with currently available experimental techniques, the differences cannot be seen. "If you have a big system and you want to see quantum features like entanglement in it, you have to make sure that your precision is extremely good," explains Simon. "You have to be able to distinguish a million photons from a million plus one photons, and there is no current technology that would allow you to do that."
Quantum-information theorist Renato Renner of ETH Zurich is impressed: "Even if there was no decoherence, this paper would explain why we do not see quantum effects and why the world appears classical to us, which is a very fundamental question of course." But, he cautions, "The paper raises a very fundamental question and gives us an answer in an interesting special case, but whether it is general remains to be seen."
The research will be published in Physical Review Letters.
Friday, October 14, 2011
New twist on Brownian motion seen for the first time
Remembering motion
In the mid-20th century, however, physicists began to realize that when the densities of the particle and fluid are similar, the kicks are not completely random. Instead, "persistent correlations" are predicted between the motions of the fluid and the particle. These arise because particles moving through a fluid will cause the surrounding fluid to move, which in turn will affect the motion of the particle and so on. For example, a person swimming at a constant speed will pull some of the surrounding water with them. But if they stop suddenly, they will feel a push forward from the moving water. Researchers refer to this as "hydrodynamic memory", but its observation has remained elusive for the tiny single particles that undergo Brownian motion. Now, Sylvia Jeney at EPFL in Switzerland and colleagues in Switzerland and Germany claim to have seen clear evidence for this effect in the Brownian motions of particles. Their measurements are based on the idea that this hydrodynamic "memory" gives rise to the power spectrum of the particle being described by "coloured noise", rather than white noise. In the context of Brownian motion, white noise means that the particle fluctuates with the same magnitude (or power) regardless of the frequency of the fluctuation. Jeney's experiments, however, show that higher frequencies actually have higher magnitudes of fluctuation – which means that the noise is no longer white but is coloured.
Specialized trap
Jeney's group made the measurement by trapping a single micrometre-sized melamine sphere in optical tweezers created by a tightly focused laser beam. Although similar to a commercial set-up already used by biophysicists, the researchers spent several years optimizing their apparatus. In particular, they improved the time resolution of the system by a factor of 1000 and boosted its spatial resolution so it can measure distances of less than a nanometre. The experiments involved single particles trapped by the tweezers and immersed in liquid. The parameters of the experiment were chosen so that time it takes for the fluid to diffuse over the diameter of the particle is about one-sixth of the time it takes for the sphere to reach its equilibrium position in the tweezers. This diffusion time is the timescale on which the hydrodynamic memory is expected to occur and therefore the set-up allowed the researchers to study the correlated behaviour. "Currently, there are two maybe three labs in the world that have similar high-precision set-ups," explains Jeney. She says that the team wants to establish the optical-trapping technique as an advanced biophysical tool.
Source: IOP/Physics
Gathered by: Sh.Barzanjeh(shabirbarzanjeh@gmail.com)
Tuesday, September 13, 2011
Fine-tuning lasers to find waves in gravity
SYDNEY: 'Squeezing' laser light could significantly improve the accuracy of detectors searching for Einstein's elusive gravitational waves.
Gravitational waves were predicted by Einstein but have long remained undetected. To look for them, scientists use devices called laser interferometers, which measure the time it takes a split beam of laser light to travel between suspended mirrors. The waves are expected to distort the laser's travel time - but so far scientists can't measure this accurately enough.
The accuracy of these devices is limited by a quantum phenomenon of light called 'shot noise' - a type of electronic interference.
Using a new quality of laser light, which radiates much more calmly than a conventional laser, researchers reported yesterday in Nature Physics that they have curbed this interference and improved measuring accuracy in their detectors by roughly 50%.
"Squeezed light is a completely new approach," said physicist and lead author Roman Schnabel, from the Max Planck Institute for Gravitational Physics in Germany.
"One can say that for the first time a `technology' is based on one of the distinct features of quantum physics itself. We were able to leave the stage of laboratory experiments and realize a real application."
Travel time weakens waves
The findings are an an exciting step forward for the Laser Interferometry Gravitational-Wave Observatory (LIGO) project in its quest to observe gravitational waves using Earth-based detectors.
Albert Einstein first predicted the existence of gravitational waves in 1916 in his theory of general relativity. The presence of large amounts of mass or energy can distort the space-time fabric causing it to curve, and when they move suddenly, this curvature ripples outward - like the ripples in a pond after a fish jumps.
Violent astronomical events such as black hole collisions and supernovae can cause gravitational waves. In the immediate vicinity of these objects, gravitational waves would be immensely strong, said Schnabel.
However, after travelling billions of light years to reach the Earth they are significantly weakened, making them incredibly difficult to detect. So far they have eluded scientists.
Theoretical predictions based on Einstein's theory indicate current detectors must be improved by another factor of about three to 10 to reach a high probability of successful detection, said Schnabel.
"Squeezed light is a new technology, which has now proven to significantly contribute to realising this last factor," Schnabel said. "[And] the improvement factor of 1.5 is just the beginning. A factor of three due to squeezed light is possible with today's technology."
Lasers superimposed
In a laser interferometer, a laser is split into two beams and shot down long, perpendicular vacuum tubes before reflecting off mirrors back to where they started.
If the distance measured by the light is exactly the same, all the light will be directed back to the original source, but if there is any difference in the distance, some light will be redirected to a photodetector for further analysis.
The idea is that space-time ripples caused by gravitational waves will cause the distance measured by the light beam to change, and the amount of light falling on the photodector to vary.
"We now feed the squeezed light into the interferometer, in addition to our normal laser light," explained Schnabel. "If the two light fields then superimpose, the resulting laser beam has a much more uniform intensity, compared to the original signal beam.
"We thus smooth out the irregularities caused by quantum physical effects in the detector signal," he added.
New view on the universe
"This is the first time this technology has been used outside of a test laboratory anywhere in the world," said David McClelland, a physicist at the Australian National University in Canberra and a key investigator for LIGO-Australia.
"The detection of gravitational waves would open a new window for astronomy and create a completely new way of sensing the Universe, akin to being able to hear for the very first time," he added.
The LIGO collaboration is in the process of testing a squeezed light source built at the ANU on the 4 km long LIGO interferometer in Washington State in the U.S. According to Schnabel, testing on this observatory and Europe's envisaged 10km Einstein Telescope, could further improve detection capabilities.
source:Cosmos Online
Gathered by: Sh.Barzanjeh(shabirbarzanjeh@gmail.com)
Gravitational waves were predicted by Einstein but have long remained undetected. To look for them, scientists use devices called laser interferometers, which measure the time it takes a split beam of laser light to travel between suspended mirrors. The waves are expected to distort the laser's travel time - but so far scientists can't measure this accurately enough.
The accuracy of these devices is limited by a quantum phenomenon of light called 'shot noise' - a type of electronic interference.
Using a new quality of laser light, which radiates much more calmly than a conventional laser, researchers reported yesterday in Nature Physics that they have curbed this interference and improved measuring accuracy in their detectors by roughly 50%.
"Squeezed light is a completely new approach," said physicist and lead author Roman Schnabel, from the Max Planck Institute for Gravitational Physics in Germany.
"One can say that for the first time a `technology' is based on one of the distinct features of quantum physics itself. We were able to leave the stage of laboratory experiments and realize a real application."
Travel time weakens waves
The findings are an an exciting step forward for the Laser Interferometry Gravitational-Wave Observatory (LIGO) project in its quest to observe gravitational waves using Earth-based detectors.
Albert Einstein first predicted the existence of gravitational waves in 1916 in his theory of general relativity. The presence of large amounts of mass or energy can distort the space-time fabric causing it to curve, and when they move suddenly, this curvature ripples outward - like the ripples in a pond after a fish jumps.
Violent astronomical events such as black hole collisions and supernovae can cause gravitational waves. In the immediate vicinity of these objects, gravitational waves would be immensely strong, said Schnabel.
However, after travelling billions of light years to reach the Earth they are significantly weakened, making them incredibly difficult to detect. So far they have eluded scientists.
Theoretical predictions based on Einstein's theory indicate current detectors must be improved by another factor of about three to 10 to reach a high probability of successful detection, said Schnabel.
"Squeezed light is a new technology, which has now proven to significantly contribute to realising this last factor," Schnabel said. "[And] the improvement factor of 1.5 is just the beginning. A factor of three due to squeezed light is possible with today's technology."
Lasers superimposed
In a laser interferometer, a laser is split into two beams and shot down long, perpendicular vacuum tubes before reflecting off mirrors back to where they started.
If the distance measured by the light is exactly the same, all the light will be directed back to the original source, but if there is any difference in the distance, some light will be redirected to a photodetector for further analysis.
The idea is that space-time ripples caused by gravitational waves will cause the distance measured by the light beam to change, and the amount of light falling on the photodector to vary.
"We now feed the squeezed light into the interferometer, in addition to our normal laser light," explained Schnabel. "If the two light fields then superimpose, the resulting laser beam has a much more uniform intensity, compared to the original signal beam.
"We thus smooth out the irregularities caused by quantum physical effects in the detector signal," he added.
New view on the universe
"This is the first time this technology has been used outside of a test laboratory anywhere in the world," said David McClelland, a physicist at the Australian National University in Canberra and a key investigator for LIGO-Australia.
"The detection of gravitational waves would open a new window for astronomy and create a completely new way of sensing the Universe, akin to being able to hear for the very first time," he added.
The LIGO collaboration is in the process of testing a squeezed light source built at the ANU on the 4 km long LIGO interferometer in Washington State in the U.S. According to Schnabel, testing on this observatory and Europe's envisaged 10km Einstein Telescope, could further improve detection capabilities.
source:Cosmos Online
Gathered by: Sh.Barzanjeh(shabirbarzanjeh@gmail.com)
Sunday, August 21, 2011
Quantum mechanics rule 'bent' in classic experiment
Researchers have bent one of the most basic rules of quantum mechanics, a counterintuitive branch of physics that deals with atomic-scale interactions.
Its "complementarity" rule asserts that it is impossible to observe light behaving as both a wave and a particle, though it is strictly both.
In an experiment reported in Science, researchers have now done exactly that.
Light can interfere with itself just as water ripples can add to or cancel one another
They say the feat "pulls back the veil" on quantum reality in a way that was thought to be prohibited by theory.
Quantum mechanics has spawned and continues to fuel spirited debates about the nature of what we can see and measure, and what nature keeps hidden - debates that often straddle the divide between the physical and the philosophical.
For instance, a well-known rule called the Heisenberg uncertainty principle maintains that for some pairs of measurements, high precision in one necessarily reduces the precision that can be achieved in the other.
One embodiment of this idea lies in a "two-slit interferometer", in which light can pass through one of two slits and is viewed on a screen.
Let a number of the units of light called photons through the slits, and an interference pattern develops, like waves overlapping in a pond. However, keeping a close eye on which photons went through which slits - what may be termed a "strong measurement" - destroys the pattern.
Continue reading the main story
Young's two-slit experiment
A central idea in quantum mechanics is that light and matter can behave as both particle and wave
However, the idea of "complementarity" prevents observation of both behaviours simultaneously
In the two-slit experiment, light is passed through two tiny holes and is then viewed on a screen
The two beams interfere with each other, forming a rippled "diffraction pattern" - as if the light were made of a number of waves adding or cancelling
However, if one of the holes is blocked, the light can be seen as a single beam on the screen - as if light were made of particles
The new work, for the first time, observes both kinds of behaviour at the same time
Now, Aephraim Steinberg of the University of Toronto and his colleagues have sidestepped this limitation by undertaking "weak measurements" of the photons' momentum.
The team allowed the photons to pass through a thin sliver of the mineral calcite which gave each photon a tiny nudge in its path, with the amount of deviation dependent on which slit it passed through.
By averaging over a great many photons passing through the apparatus, and only measuring the light patterns on a camera, the team was able to infer what paths the photons had taken.
While they were able to easily observe the interference pattern indicative of the wave nature of light, they were able also to see from which slits the photons had come, a sure sign of their particle nature.
The trajectories of the photons within the experiment - forbidden in a sense by the laws of physics - have been laid bare.
On one level, the experiment appears to violate a central rule of quantum mechanics, but Professor Steinberg said this was not the case.
He explained to BBC News that "while the uncertainty principle does indeed forbid one from knowing the position and momentum of a particle exactly at the same time, it turns out that it is possible to ask 'what was the average momentum of the particles which reached this position?'" .
"You can't know the exact value for any single particle, but you can talk about the average."
Philosophical beginnings
Marlan Scully of Texas A&M University, a quantum physicist who has published on the idea of sneaking around this quantum limit before, said: "It's a beautiful series of measurements by an excellent group, the likes of which I've not seen before.
"This paper is probably the first that has really put this weak measurement idea into a real experimental realisation, and it also gave us the trajectories."
He said that the work would - inevitably - raise philosophical issues as well.
"The exact way to think about what they're doing will be researched for some time, and the weak measurement concept itself will be a matter of controversy - but now we have a very pretty experiment with these weak measurements," he added.
For his part, Professor Steinberg believes that the result reduces a limitation not on quantum physics but on physicists themselves.
"I feel like we're starting to pull back a veil on what nature really is," he said.
"The trouble with quantum mechanics is that while we've learned to calculate the outcomes of all sorts of experiments, we've lost much of our ability to describe what is really happening in any natural language.
"I think that this has really hampered our ability to make progress, to come up with new ideas and see intuitively how new systems ought to behave."
Gathered by: Sh.Barzanjeh(shabirbarzanjeh@gmail.com)
Its "complementarity" rule asserts that it is impossible to observe light behaving as both a wave and a particle, though it is strictly both.
In an experiment reported in Science, researchers have now done exactly that.
They say the feat "pulls back the veil" on quantum reality in a way that was thought to be prohibited by theory.
Quantum mechanics has spawned and continues to fuel spirited debates about the nature of what we can see and measure, and what nature keeps hidden - debates that often straddle the divide between the physical and the philosophical.
For instance, a well-known rule called the Heisenberg uncertainty principle maintains that for some pairs of measurements, high precision in one necessarily reduces the precision that can be achieved in the other.
One embodiment of this idea lies in a "two-slit interferometer", in which light can pass through one of two slits and is viewed on a screen.
Let a number of the units of light called photons through the slits, and an interference pattern develops, like waves overlapping in a pond. However, keeping a close eye on which photons went through which slits - what may be termed a "strong measurement" - destroys the pattern.
Continue reading the main story
Young's two-slit experiment
A central idea in quantum mechanics is that light and matter can behave as both particle and wave
However, the idea of "complementarity" prevents observation of both behaviours simultaneously
In the two-slit experiment, light is passed through two tiny holes and is then viewed on a screen
The two beams interfere with each other, forming a rippled "diffraction pattern" - as if the light were made of a number of waves adding or cancelling
However, if one of the holes is blocked, the light can be seen as a single beam on the screen - as if light were made of particles
The new work, for the first time, observes both kinds of behaviour at the same time
Now, Aephraim Steinberg of the University of Toronto and his colleagues have sidestepped this limitation by undertaking "weak measurements" of the photons' momentum.
The team allowed the photons to pass through a thin sliver of the mineral calcite which gave each photon a tiny nudge in its path, with the amount of deviation dependent on which slit it passed through.
By averaging over a great many photons passing through the apparatus, and only measuring the light patterns on a camera, the team was able to infer what paths the photons had taken.
While they were able to easily observe the interference pattern indicative of the wave nature of light, they were able also to see from which slits the photons had come, a sure sign of their particle nature.
The trajectories of the photons within the experiment - forbidden in a sense by the laws of physics - have been laid bare.
On one level, the experiment appears to violate a central rule of quantum mechanics, but Professor Steinberg said this was not the case.
He explained to BBC News that "while the uncertainty principle does indeed forbid one from knowing the position and momentum of a particle exactly at the same time, it turns out that it is possible to ask 'what was the average momentum of the particles which reached this position?'" .
"You can't know the exact value for any single particle, but you can talk about the average."
Philosophical beginnings
Marlan Scully of Texas A&M University, a quantum physicist who has published on the idea of sneaking around this quantum limit before, said: "It's a beautiful series of measurements by an excellent group, the likes of which I've not seen before.
"This paper is probably the first that has really put this weak measurement idea into a real experimental realisation, and it also gave us the trajectories."
He said that the work would - inevitably - raise philosophical issues as well.
"The exact way to think about what they're doing will be researched for some time, and the weak measurement concept itself will be a matter of controversy - but now we have a very pretty experiment with these weak measurements," he added.
For his part, Professor Steinberg believes that the result reduces a limitation not on quantum physics but on physicists themselves.
"I feel like we're starting to pull back a veil on what nature really is," he said.
"The trouble with quantum mechanics is that while we've learned to calculate the outcomes of all sorts of experiments, we've lost much of our ability to describe what is really happening in any natural language.
"I think that this has really hampered our ability to make progress, to come up with new ideas and see intuitively how new systems ought to behave."
Gathered by: Sh.Barzanjeh(shabirbarzanjeh@gmail.com)
Thursday, July 28, 2011
Generation of motional nonlinear coherent states and their superpositions via an intensity-dependent coupling of a cavity field to a micromechanical membrane
This paper is available in J. Phys. B: At. Mol. Opt. Phys. 44 (2011) 105504 (14pp)
In this paper, we have introduced a physical scheme that allows
one to generate and control the nonclassical properties of
motional nonlinear coherent states and their superpositions
for an undamped vibrating micromechanical membrane inside
an optical cavity. We have shown that if the cavity field
is initially prepared in a Fock state, the motional state of
the membrane may evolve to a family of nonlinear coherent
states. We have been interested in analysing the nonclassical
properties of the generated state of the membrane, including
the quadrature squeezing and the sub-Poissonian statistics. In
particular, we have found that the Lamb–Dicke parameter and
the membrane’s reflectivity lead to an enhancement of the
nonclassical properties. As we have seen, with increasing
the Lamb–Dicke parameter and the membrane’s reflectivity,
the sub-Poissonian behaviour and quadrature squeezing of the
motional state of the membrane are considerably strengthened.
In addition, the scheme offers the possibility of generating
various types of the so-called nonlinear multicomponent
Schr¨odinger cat states of the membrane. We have shown
that the separation between nonlinear coherent components is
increased by increasing the parameters η and rc.
We have also extended our treatment to a more realistic situation in
which the photon leakage from the cavity as a relevant source
of decoherence is included and examined its influence on the
nonclassical characteristics of the generated motional states of
the membrane. We have shown that it is possible to control the
effect of the cavity field damping on the nonclassical behaviour
of the motional state of the membrane via the Lambe–Dicke
parameter and the membrane’s reflectivity. In particular,
we have found that the generated motional NLSCSs of the
membrane can be more robust against decoherence than the
usual Schr¨odinger cat states.
Gathered by: Sh.Barzanjeh(shabirbarzanjeh@gmail.com)
In this paper, we have introduced a physical scheme that allows
one to generate and control the nonclassical properties of
motional nonlinear coherent states and their superpositions
for an undamped vibrating micromechanical membrane inside
an optical cavity. We have shown that if the cavity field
is initially prepared in a Fock state, the motional state of
the membrane may evolve to a family of nonlinear coherent
states. We have been interested in analysing the nonclassical
properties of the generated state of the membrane, including
the quadrature squeezing and the sub-Poissonian statistics. In
particular, we have found that the Lamb–Dicke parameter and
the membrane’s reflectivity lead to an enhancement of the
nonclassical properties. As we have seen, with increasing
the Lamb–Dicke parameter and the membrane’s reflectivity,
the sub-Poissonian behaviour and quadrature squeezing of the
motional state of the membrane are considerably strengthened.
In addition, the scheme offers the possibility of generating
various types of the so-called nonlinear multicomponent
Schr¨odinger cat states of the membrane. We have shown
that the separation between nonlinear coherent components is
increased by increasing the parameters η and rc.
We have also extended our treatment to a more realistic situation in
which the photon leakage from the cavity as a relevant source
of decoherence is included and examined its influence on the
nonclassical characteristics of the generated motional states of
the membrane. We have shown that it is possible to control the
effect of the cavity field damping on the nonclassical behaviour
of the motional state of the membrane via the Lambe–Dicke
parameter and the membrane’s reflectivity. In particular,
we have found that the generated motional NLSCSs of the
membrane can be more robust against decoherence than the
usual Schr¨odinger cat states.
Gathered by: Sh.Barzanjeh(shabirbarzanjeh@gmail.com)
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