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11 anni fa
sabato 29 settembre 2012
martedì 28 febbraio 2012
Quantum Microphone Captures Extremely Weak Sound.
A "quantum microphone" based on a Single Electron Transistor (SET) detects sound waves on a chip surface, so called Surface Acoustic Waves (SAW). The waves make the charge of the atoms underneath the quantum microphon oscillate. Since the quantum microphone is an extremely sensitive charge detector, very low sound levels can be detected. (The size of the waves are exaggerated in the picture). (Credit: Philip Krantz, Chalmers)
Source: Science Daily
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ScienceDaily (Feb. 27, 2012) — Scientists from Chalmers University of Technology have demonstrated a new kind of detector for sound at the level of quietness of quantum mechanics. The result offers prospects of a new class of quantum hybrid circuits that mix acoustic elements with electrical ones, and may help illuminate new phenomena of quantum physics.
The results have been published in Nature Physics.The "quantum microphone" is based on a single electron transistor, that is, a transistor where the current passes one electron at a time. The acoustic waves studied by the research team propagate over the surface of a crystalline microchip, and resemble the ripples formed on a pond when a pebble is thrown into it. The wavelength of the sound is a mere 3 micrometers, but the detector is even smaller, and capable of rapidly sensing the acoustic waves as they pass by.
On the chip surface, the researchers have fabricated a three-millimeter-long echo chamber, and even though the speed of sound on the crystal is ten times higher than in air, the detector shows how sound pulses reflect back and forth between the walls of the chamber, thereby verifying the acoustic nature of the wave.
The detector is sensitive to waves with peak heights of a few percent of a proton diameter, levels so quiet that sound can be governed by quantum law rather than classical mechanics, much in the same way as light.
"The experiment is done on classical acoustic waves, but it shows that we have everything in place to begin studies of proper quantum-acoustics, and nobody has attempted that before," says Martin Gustafsson, PhD student and first author of the article.
Apart from the extreme quietness, the pitch of the waves is too high for us to hear: The frequency of almost 1 gigahertz is 21 octaves above one-lined A. The new detector is the most sensitive in the world for such high-frequency sound.
lunedì 27 febbraio 2012
Replacing Electricity With Light: First Physical 'Metatronic' Circuit Created.
Figure A. When the plane of the electric field is in line with the nanorods the circuit is wired in parallel. Figure B. When the plane of the electric field crosses both the nanorods and the gaps the circuit is wired in series. (Credit: Image courtesy of University of Pennsylvania).
Source: Science Daily
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ScienceDaily (Feb. 23, 2012) — The technological world of the 21st century owes a tremendous amount to advances in electrical engineering, specifically, the ability to finely control the flow of electrical charges using increasingly small and complicated circuits. And while those electrical advances continue to race ahead, researchers at the University of Pennsylvania are pushing circuitry forward in a different way, by replacing electricity with light.
Different arrangements and combinations of electronic circuits have different functions, ranging from simple light switches to complex supercomputers. These circuits are in turn built of different arrangements of circuit elements, like resistors, inductors and capacitors, which manipulate the flow of electrons in a circuit in mathematically precise ways. And because both electric circuits and optics follow Maxwell's equations -- the fundamental formulas that describe the behavior of electromagnetic fields -- Engheta's dream of building circuits with light wasn't just the stuff of imagination. In 2005, he and his students published a theoretical paper outlining how optical circuit elements could work.
Now, he and his group at Penn have made this dream a reality, creating the first physical demonstration of "lumped" optical circuit elements. This represents a milestone in a nascent field of science and engineering Engheta has dubbed "metatronics."
Engheta's research, which was conducted with members of his group in the electrical and systems engineering department, Yong Sun, Brian Edwards and Andrea Alù, was published in the journal Nature Materials.
In electronics, the "lumped" designation refers to elements that can be treated as a black box, something that turns a given input to a perfectly predictable output without an engineer having to worry about what exactly is going on inside the element every time he or she is designing a circuit.
"Optics has always had its own analogs of elements, things like lenses, waveguides and gratings," Engheta said, "but they were never lumped. Those elements are all much larger than the wavelength of light because that's all that could be easily built in the old days. For electronics, the lumped circuit elements were always much smaller than the wavelength of operation, which is in the radio or microwave frequency range."
Nanotechnology has now opened that possibility for lumped optical circuit elements, allowing construction of structures that have dimensions measured in nanometers. In this experiment's case, the structure was comb-like arrays of rectangular nanorods made of silicon nitrite.
The "meta" in "metatronics" refers to metamaterials, the relatively new field of research where nanoscale patterns and structures embedded in materials allow them to manipulate waves in ways that were previously impossible. Here, the cross-sections of the nanorods and the gaps between them form a pattern that replicates the function of resistors, inductors and capacitors, three of the most basic circuit elements, but in optical wavelengths.
"If we have the optical version of those lumped elements in our repertoire, we can actually make designs similar to what we do in electronics but now for operation with light," Engheta said. "We can build a circuit with light."
In their experiment, the researchers illuminated the nanorods with an optical signal, a wave of light in the mid-infrared range. They then used spectroscopy to measure the wave as it passed through the comb. Repeating the experiment using nanorods with nine different combinations of widths and heights, the researchers showed that the optical "current" and optical "voltage" were altered by the optical resistors, inductors and capacitors with parameters corresponding to those differences in size.
"A section of the nanorod acts as both an inductor and resistor, and the air gap acts as a capacitor," Engheta said.
Beyond changing the dimensions and the material the nanorods are made of, the function of these optical circuits can be altered by changing the orientation of the light, giving metatronic circuits access to configurations that would be impossible in traditional electronics.
This is because a light wave has polarizations; the electric field that oscillates in the wave has a definable orientation in space. In metatronics, it is that electric field that interacts and is changed by elements, so changing the field's orientation can be like rewiring an electric circuit.
When the plane of the field is in line with the nanorods, as in Figure A, the circuit is wired in parallel and the current passes through the elements simultaneously. When the plane of the electric field crosses both the nanorods and the gaps, as in Figure B, the circuit is wired in series and the current passes through the elements sequentially.
"The orientation gives us two different circuits, which is why we call this 'stereo-circuitry,'" Engheta said. "We could even have the wave hit the rods obliquely and get something we don't have in regular electronics: a circuit that's neither in series or in parallel but a mixture of the two."
This principle could be taken to an even higher level of complexity by building nanorod arrays in three dimensions. An optical signal hitting such a structure's top would encounter a different circuit than a signal hitting its side. Building off their success with basic optical elements, Engheta and his group are laying the foundation for this kind of complex metatronics.
"Another reason for success in electronics has to do with its modularity," he said. "We can make an infinite number of circuits depending on how we arrange different circuit elements, just like we can arrange the alphabet into different words, sentences and paragraphs.
"We're now working on designs for more complicated optical elements," Engheta said. "We're on a quest to build these new letters one by one."
This work was supported in part by the U.S. Air Force Office of Scientific Research.
Andrea Alù is now an assistant professor at the University of Texas at Austin.
Scientists Score New Victory Over Quantum Uncertainty.
Michael Chapman, a professor in the School of Physics at Georgia Tech, poses with optical equipment in his laboratory. Chapman’s research team is exploring squeezed states using atoms of Bose-Einstein condensates. (Click image for high-resolution version. Credit: Gary Meek) (Credit: Image courtesy of Georgia Institute of Technology, Research Communications)
Source: Science Daily
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ScienceDaily (Feb. 26, 2012) — Most people attempt to reduce the little uncertainties of life by carrying umbrellas on cloudy days, purchasing automobile insurance or hiring inspectors to evaluate homes they might consider purchasing. For scientists, reducing uncertainty is a no less important goal, though in the weird realm of quantum physics, the term has a more specific meaning.
For scientists working in quantum physics, the Heisenberg Uncertainty Principle says that measurements of properties such as the momentum of an object and its exact position cannot be simultaneously specified with arbitrary accuracy. As a result, there must be some uncertainty in either the exact position of the object, or its exact momentum. The amount of uncertainty can be determined, and is often represented graphically by a circle showing the area within which the measurement actually lies.Over the past few decades, scientists have learned to cheat a bit on the Uncertainty Principle through a process called "squeezing," which has the effect of changing how the uncertainty is shown graphically. Changing the circle to an ellipse and ultimately to almost a line allows one component of the complementary measurements -- the momentum or the position, in the case of an object -- to be specified more precisely than would otherwise be possible. The actual area of uncertainty remains unchanged, but is represented by a different shape that serves to improve accuracy in measuring one property.
This squeezing has been done in measuring properties of photons and atoms, and can be important to certain high-precision measurements needed by atomic clocks and the magnetometers used to create magnetic resonance imaging views of structures deep inside the body. For the military, squeezing more accuracy could improve the detection of enemy submarines attempting to hide underwater or improve the accuracy of atom-based inertial guidance instruments.
Now physicists at the Georgia Institute of Technology have added another measurement to the list of those that can be squeezed. In a paper appearing online February 26 in the journal Nature Physics, they report squeezing a property called the nematic tensor, which is used to describe the rubidium atoms in Bose-Einstein condensates, a unique form of matter in which all atoms have the same quantum state. The research was sponsored by the National Science Foundation (NSF).
"What is new about our work is that we have probably achieved the highest level of atom squeezing reported so far, and the more squeezing you get, the better," said Michael Chapman, a professor in Georgia Tech's School of Physics. "We are also squeezing something other than what people have squeezed before."
Scientists have been squeezing the spin states of atoms for 15 years, but only for atoms that have just two relevant quantum states -- known as spin ½ systems. In collections of those atoms, the spin states of the individual atoms can be added together to get a collective angular momentum that describes the entire system of atoms.
In the Bose-Einstein condensate atoms being studied by Chapman's group, the atoms have three quantum states, and their collective spin totals zero -- not very helpful for describing systems. So Chapman and graduate students Chris Hamley, Corey Gerving, Thai Hoang and Eva Bookjans learned to squeeze a more complex measure that describes their system of spin 1 atoms: nematic tensor, also known as quadrupole.
Nematicity is a measure of alignment that is important in describing liquid crystals, exotic magnetic materials and some high temperature superconductors.
"We don't have a spin vector pointing in a particular direction, but there is still some residual information in where this collection of atoms is pointing," Chapman explained. "That next higher-order description is the quadrupole, or nematic tensor. Squeezing this actually works quite well, and we get a large degree of improvement, so we think it is relatively promising."
Experimentally, the squeezing is created by entangling some of the atoms, which takes away their independence. Chapman's group accomplishes this by colliding atoms in their ensemble of some 40,000 rubidium atoms.
"After they collide, the state of one atom is connected to that of the other atom, so they have been entangled in that way," he said. "This entanglement creates the squeezing."
Reducing uncertainty in measuring atoms could have important implications for precise magnetic measurements. The next step will be to determine experimentally if the technique can improve the measurement of magnetic field, which could have important applications.
"In principle, this should be a straightforward experiment, but it turns out that the biggest challenge is that magnetic fields in the laboratory fluctuate due to environmental factors such as the effects of devices such as computer monitors," Chapman said. "If we had a noiseless laboratory, we could measure the magnetic field both with and without squeezed states to demonstrate the enhanced precision. But in our current lab environment, our measurements would be affected by outside noise, not the limitations of the atomic sensors we are using."
The new squeezed property could also have application to quantum information systems, which can store information in the spin of atoms and their nematic tensor.
"There are a lot of things you can do with quantum entanglement, and improving the accuracy of measurements is one of them," Chapman added. "We still have to obey Heisenberg's Uncertainty Principle, but we do have the ability to manipulate it."
domenica 16 maggio 2010
Quantum Dynamics of Matter Waves Reveal Exotic Multibody Collisions.
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ScienceDaily (May 16, 2010) — At extremely low temperatures atoms can aggregate into so-called Bose Einstein condensates forming coherent laser-like matter waves. Due to interactions between the atoms fundamental quantum dynamics emerge and give rise to periodic collapses and revivals of the matter wave field.
A group of scientists led by Professor Immanuel Bloch (Chair of Experimental Physics at the Ludwig-Maximilians-Universität München (LMU) and Director of the Quantum Many Body Systems Division at the Max Planck Institute of Quantum Optics in Garching) has now succeeded to take a glance 'behind the scenes' of atomic interactions revealing the complex structure of these quantum dynamics. By generating thousands of miniature BECs ordered in an optical lattice the researchers were able to observe a large number of collapse and revival cycles over long periods of time.
The research is published in the journal Nature.
The experimental results imply that the atoms do not only interact pairwise -- as typically assumed -- but also perform exotic collisions involving three, four or more atoms at the same time. On the one hand, these results have fundamental importance for the understanding of quantum many-body systems. On the other hand, they pave the way for the generation of new exotic states of matter, based on such multi-body interactions.
The experiment starts by cooling a dilute cloud of hundreds of thousands of atoms to temperatures close to absolute zero, approximately -273 degrees Celsius. At these temperatures the atoms form a so-called Bose-Einstein condensate (BEC), a quantum phase in which all particles occupy the same quantum state. Now an optical lattice is superimposed on the BEC: This is a kind of artificial crystal made of light with periodically arranged bright and dark areas, generated by the superposition of standing laser light waves from different directions. This lattice can be viewed as an 'egg carton' on which the atoms are distributed. Whereas in a real egg carton each site is either occupied by a single egg or no egg, the number of atoms sitting at each lattice site is determined by the laws of quantum mechanics: Depending on the lattice height (i.e. the intensity of the laser beam) the single lattice sites can be occupied by zero, one, two, three and more atoms at the same time.
The use of those "atom number superposition states" is the key to the novel measurement principle developed by the researchers. The dynamics of an atom number state can be compared to the dynamics of a swinging pendulum. As pendulums of different lengths are characterized by different oscillation frequencies, the same applies to the states of different atom numbers. "However, these frequencies are modified by inter-atomic collisions. If only pairwise interactions between atoms were present, the pendulums representing the individual atom number states would swing synchronously and their oscillation frequencies would be exact multiples of the pendulum frequency for two interacting atoms," Sebastian Will, graduate student at the experiment, explains.
Using a tricky experimental set-up the physicists were able to track the evolution of the different superimposed oscillations over time. Periodically interference patterns became visible and disappeared, again and again. From their intensity and periodicity the physicists found unambiguous evidence that the frequencies are actually not simple multiples of the two-body case. "This really caught us by surprise. We became aware that a more complex mechanism must be at work," Sebastian Will recalls. "Due to their ultralow temperature the atoms occupy the energetically lowest possible quantum state at each lattice site. Nevertheless, Heisenberg's uncertainty principle allows them to make -- so to speak -- a virtual detour via energetically higher lying quantum states during their collision. Practically, this mechanism gives rise to exotic collisions, which involve three, four or more atoms at the same time."
The results reported in this work provide an improved understanding of interactions between microscopic particles. This may not only be of fundamental scientific interest, but find a direct application in the context of ultracold atoms in optical lattices. Owing to exceptional experimental controllability, ultracold atoms in optical lattices can form a "quantum simulator" to model condensed matter systems. Such a quantum simulator is expected to help understand the physics behind superconductivity or quantum magnetism. Furthermore, as each lattice site represents a miniature laboratory for the generation of exotic quantum states, experimental set-ups using optical lattices may turn out to be the most sensitive probes for observing atomic collisions.
The research is published in the journal Nature.
The experimental results imply that the atoms do not only interact pairwise -- as typically assumed -- but also perform exotic collisions involving three, four or more atoms at the same time. On the one hand, these results have fundamental importance for the understanding of quantum many-body systems. On the other hand, they pave the way for the generation of new exotic states of matter, based on such multi-body interactions.
The experiment starts by cooling a dilute cloud of hundreds of thousands of atoms to temperatures close to absolute zero, approximately -273 degrees Celsius. At these temperatures the atoms form a so-called Bose-Einstein condensate (BEC), a quantum phase in which all particles occupy the same quantum state. Now an optical lattice is superimposed on the BEC: This is a kind of artificial crystal made of light with periodically arranged bright and dark areas, generated by the superposition of standing laser light waves from different directions. This lattice can be viewed as an 'egg carton' on which the atoms are distributed. Whereas in a real egg carton each site is either occupied by a single egg or no egg, the number of atoms sitting at each lattice site is determined by the laws of quantum mechanics: Depending on the lattice height (i.e. the intensity of the laser beam) the single lattice sites can be occupied by zero, one, two, three and more atoms at the same time.
The use of those "atom number superposition states" is the key to the novel measurement principle developed by the researchers. The dynamics of an atom number state can be compared to the dynamics of a swinging pendulum. As pendulums of different lengths are characterized by different oscillation frequencies, the same applies to the states of different atom numbers. "However, these frequencies are modified by inter-atomic collisions. If only pairwise interactions between atoms were present, the pendulums representing the individual atom number states would swing synchronously and their oscillation frequencies would be exact multiples of the pendulum frequency for two interacting atoms," Sebastian Will, graduate student at the experiment, explains.
Using a tricky experimental set-up the physicists were able to track the evolution of the different superimposed oscillations over time. Periodically interference patterns became visible and disappeared, again and again. From their intensity and periodicity the physicists found unambiguous evidence that the frequencies are actually not simple multiples of the two-body case. "This really caught us by surprise. We became aware that a more complex mechanism must be at work," Sebastian Will recalls. "Due to their ultralow temperature the atoms occupy the energetically lowest possible quantum state at each lattice site. Nevertheless, Heisenberg's uncertainty principle allows them to make -- so to speak -- a virtual detour via energetically higher lying quantum states during their collision. Practically, this mechanism gives rise to exotic collisions, which involve three, four or more atoms at the same time."
The results reported in this work provide an improved understanding of interactions between microscopic particles. This may not only be of fundamental scientific interest, but find a direct application in the context of ultracold atoms in optical lattices. Owing to exceptional experimental controllability, ultracold atoms in optical lattices can form a "quantum simulator" to model condensed matter systems. Such a quantum simulator is expected to help understand the physics behind superconductivity or quantum magnetism. Furthermore, as each lattice site represents a miniature laboratory for the generation of exotic quantum states, experimental set-ups using optical lattices may turn out to be the most sensitive probes for observing atomic collisions.
Story Source:
Adapted from materials provided by Ludwig-Maximilians-Universität München.
Journal Reference:
Sebastian Will, Thorsten Best, Ulrich Schneider, Lucia Hackermüller, Dirk-Sören Lühmann, Immanuel Bloch. Time-resolved observation of coherent multi-body interactions in quantum phase revivals. Nature, 2010; 465 (7295): 197 DOI: 10.1038/nature09036
Adapted from materials provided by Ludwig-Maximilians-Universität München.
Journal Reference:
Sebastian Will, Thorsten Best, Ulrich Schneider, Lucia Hackermüller, Dirk-Sören Lühmann, Immanuel Bloch. Time-resolved observation of coherent multi-body interactions in quantum phase revivals. Nature, 2010; 465 (7295): 197 DOI: 10.1038/nature09036
domenica 9 maggio 2010
Anton Zeilinger vs. Daniel Salart about "Spooky action at a distance".
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Comment on: Testing the speed of ‘spooky action at a distance’.
(Johannes Kofler, Rupert Ursin, Časlav Brukner, Anton Zeilinger)
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In a recent experiment, Salart et al. addressed the important issues of the speed of hypothetical communication and of reference frames in Bell-type experiments. The authors report that they "performed a Bell experiment using entangled photons" and conclude from their experimental results that "to maintain an explanation based on spooky action at a distance we would have to assume that the spooky action propagates at speeds even greater than the bounds obtained in our experiment", exceeding the speed of light by orders of magnitude. Here we show that, analyzing the experimental procedure, explanations with subluminal or even no communication at all exist for the experiment.
In order to explain the violation of Bell inequalities within the view where, to use the author‟s wording, "correlated events have some common causes in their shared history", one needs to assume hypothetical communication between the observer stations. This communication must be faster than light if the outcome at one station is space-like separated from all relevant events at the other station.
In the experiment pairs of time-bin entangled photons were sent over 17.5 km optical fibers to two receiving stations, located in Jussy and Satigny, both equipped with a Franson-type interferometer and detectors. The out-comes were observed space-like separated from each other. The phase in the interferometer, i.e. the setting, in Jussy was continuously scanned, while the setting at Satigny was kept stable.
However, if the setting at one side remains unchanged, the results at both observer stations can be described by a "common-cause" without having to invoke any communication at all, let alone superluminal spooky action at a distance. This is signified, e.g., by the fact that no formulation of a bipartite Bell type inequality exists which does not use at least two settings at each side. Therefore, contrary to the claim in the paper, no Bell test was performed.
Furthermore, had the experiment been repeated with a second stable setting at Satigny, a "common-cause" explanation would still be possible. This is because in order to exclude subluminal communication, it is crucial that the outcome event on each side is space-like separated from the setting choice on the other side – which was not done in Ref. [1]. Thus, such experimental data – even if they were taken with two measurement settings at Satigny and even granting the fair-sampling assumption – could be explained by a "common-cause" model. In other words, the experiment tests the superluminal speed of hypothetical influences between outcome events under the assumption of no, not even subluminal, hypothetical influences between setting choices and outcome events.
We also remark that in a Franson-type experiment like the one reported in Ref. [1] the considered Clauser-Horne-Shimony-Holt Bell inequality is not applicable even with perfect detectors because of the inherent postse-lection.2 One would (i) have to use a chained Bell inequality2, (ii) achieve fast switching with a rate depending on the geometry of the interferometer, and (iii) reach a better visibility than the one reported in Ref. [1]. None of these three issues is covered by the experiment.
We would like to stress that this comment should not be seen as a defence of local realism. And neither do we demand that Ref. [1] must present a loophole-free Bell test. However, it is the purpose of our comment to point out "common-cause" explanations of an experiment which aims at putting "stringent experimental bounds on the speed of all such hypothetical influences".
In a recent experiment, Salart et al. addressed the important issues of the speed of hypothetical communication and of reference frames in Bell-type experiments. The authors report that they "performed a Bell experiment using entangled photons" and conclude from their experimental results that "to maintain an explanation based on spooky action at a distance we would have to assume that the spooky action propagates at speeds even greater than the bounds obtained in our experiment", exceeding the speed of light by orders of magnitude. Here we show that, analyzing the experimental procedure, explanations with subluminal or even no communication at all exist for the experiment.
In order to explain the violation of Bell inequalities within the view where, to use the author‟s wording, "correlated events have some common causes in their shared history", one needs to assume hypothetical communication between the observer stations. This communication must be faster than light if the outcome at one station is space-like separated from all relevant events at the other station.
In the experiment pairs of time-bin entangled photons were sent over 17.5 km optical fibers to two receiving stations, located in Jussy and Satigny, both equipped with a Franson-type interferometer and detectors. The out-comes were observed space-like separated from each other. The phase in the interferometer, i.e. the setting, in Jussy was continuously scanned, while the setting at Satigny was kept stable.
However, if the setting at one side remains unchanged, the results at both observer stations can be described by a "common-cause" without having to invoke any communication at all, let alone superluminal spooky action at a distance. This is signified, e.g., by the fact that no formulation of a bipartite Bell type inequality exists which does not use at least two settings at each side. Therefore, contrary to the claim in the paper, no Bell test was performed.
Furthermore, had the experiment been repeated with a second stable setting at Satigny, a "common-cause" explanation would still be possible. This is because in order to exclude subluminal communication, it is crucial that the outcome event on each side is space-like separated from the setting choice on the other side – which was not done in Ref. [1]. Thus, such experimental data – even if they were taken with two measurement settings at Satigny and even granting the fair-sampling assumption – could be explained by a "common-cause" model. In other words, the experiment tests the superluminal speed of hypothetical influences between outcome events under the assumption of no, not even subluminal, hypothetical influences between setting choices and outcome events.
We also remark that in a Franson-type experiment like the one reported in Ref. [1] the considered Clauser-Horne-Shimony-Holt Bell inequality is not applicable even with perfect detectors because of the inherent postse-lection.2 One would (i) have to use a chained Bell inequality2, (ii) achieve fast switching with a rate depending on the geometry of the interferometer, and (iii) reach a better visibility than the one reported in Ref. [1]. None of these three issues is covered by the experiment.
We would like to stress that this comment should not be seen as a defence of local realism. And neither do we demand that Ref. [1] must present a loophole-free Bell test. However, it is the purpose of our comment to point out "common-cause" explanations of an experiment which aims at putting "stringent experimental bounds on the speed of all such hypothetical influences".
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Reply to the: "Comment on: Testing the speed of `spooky action at a distance' "
(D. Salart, A. Baas, C. Branciard, N. Gisin, and H. Zbinden)
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Quantum correlations cannot be described by local common causes. This prediction of quantum theory, surprising as it might appear, has been widely con rmed by numerous experiments. In our Nature Letter [1] we considered this point as established and addressed another issue: the alternative assumption that quantum correlations are due to supra-luminal influences of a first event onto a second event. For this purpose we believe that it suffices to observe 2-photon interferences with a visibility high enough to potentially violate Bell's inequality, as we reported (over 2 x 17.5 km). Simultaneously closing other loopholes, like the locality loophole as desired by Koer and colleagues, would certainly be an interesting addition, as would be any Bell tests that simultaneously address several of the loopholes.
Indeed, to rigorously exclude any common cause explanation of the observed quantum correlation one should, ideally, simultaneously close the locality and the detection loophole (and assume the existence of independent randomness and that quantum measurements are nished when detectors re or at least when a mesoscopic mass has sufficiently moved as insured in our experiment, see our recent article [2]). This is a formidable task and any progress towards achieving it is most welcome. So far, however, all experiments have addressed at most one of these loopholes; ours is no exception.
Concerning the comment on the use of a Franson interferometer for testing quantum nonlocality, we stress that this is not a fundamental issue. In principle it suffices to replace the entrance beam splitters of each interferometer by a fast switch. In this way the non-interfering lateral peaks observed in the 2-photon interferogram would disappear. However, in practice such switches suffer due to losses of around 3 dB. Hence, with today's technology it is much more
convenient to replace the ideal switch by a passive coupler, as we did in our experiment in a way very similar to [3].
Quantum correlations cannot be described by local common causes. This prediction of quantum theory, surprising as it might appear, has been widely con rmed by numerous experiments. In our Nature Letter [1] we considered this point as established and addressed another issue: the alternative assumption that quantum correlations are due to supra-luminal influences of a first event onto a second event. For this purpose we believe that it suffices to observe 2-photon interferences with a visibility high enough to potentially violate Bell's inequality, as we reported (over 2 x 17.5 km). Simultaneously closing other loopholes, like the locality loophole as desired by Koer and colleagues, would certainly be an interesting addition, as would be any Bell tests that simultaneously address several of the loopholes.
Indeed, to rigorously exclude any common cause explanation of the observed quantum correlation one should, ideally, simultaneously close the locality and the detection loophole (and assume the existence of independent randomness and that quantum measurements are nished when detectors re or at least when a mesoscopic mass has sufficiently moved as insured in our experiment, see our recent article [2]). This is a formidable task and any progress towards achieving it is most welcome. So far, however, all experiments have addressed at most one of these loopholes; ours is no exception.
Concerning the comment on the use of a Franson interferometer for testing quantum nonlocality, we stress that this is not a fundamental issue. In principle it suffices to replace the entrance beam splitters of each interferometer by a fast switch. In this way the non-interfering lateral peaks observed in the 2-photon interferogram would disappear. However, in practice such switches suffer due to losses of around 3 dB. Hence, with today's technology it is much more
convenient to replace the ideal switch by a passive coupler, as we did in our experiment in a way very similar to [3].
lunedì 25 gennaio 2010
A new computer algorithm allows scientists to view nuclear fission in much finer detail than ever before.
An elevation plot of the highest energy neutron flux distributions from an axial slice of the reactor is shown superimposed over the same slice of the underlying geometry. This figure shows the rapid spatial variation in the high energy neutron distribution between within each plate along with the more slowly varying, global distribution. The figure is significant since UNIC allows researchers to capture both of these effects simultaneously. (Credit: Image courtesy of Argonne National Laboratory)
Source: ScienceDaily
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ScienceDaily (Jan. 25, 2010) — Ever wanted to see a nuclear reactor core in action? A new computer algorithm developed by researchers at the U.S. Department of Energy's (DOE) Argonne National Laboratory allows scientists to view nuclear fission in much finer detail than ever before.
A team of nuclear engineers and computer scientists at Argonne National Laboratory are developing the neutron transport code UNIC, which enables researchers for the first time to obtain a highly detailed description of a nuclear reactor core.
The code could prove crucial in the development of nuclear reactors that are safe, affordable and environmentally friendly. To model the complex geometry of a reactor core requires billions of spatial elements, hundreds of angles and thousands of energy groups -- all of which lead to problem sizes with quadrillions of possible solutions.
Such calculations exhaust computer memory of the largest machines, and therefore reactor modeling codes typically rely on various approximations. But approximations limit the predictive capability of computer simulations and leave considerable uncertainty in crucial reactor design and operational parameters.
"The UNIC code is intended to reduce the uncertainties and biases in reactor design calculations by progressively replacing existing multilevel averaging techniques with more direct solution methods based on explicit reactor geometries," said Andrew Siegel, a computational scientist at Argonne and leader of Argonne's reactor simulation group.
UNIC has run successfully at DOE leadership computing facilities, home to some of the world's fastest supercomputers, including the energy-efficient IBM Blue Gene/P at Argonne and the Cray XT5 at Oak Ridge National Laboratory. Although still under development, the code has already produced new scientific results.
In particular, the Argonne team has carried out highly detailed simulations of the Zero Power Reactor experiments on up to 163,840 processor cores of the Blue Gene/P and 222,912 processor cores of the Cray XT5, as well as on 294,912 processor cores of a Blue Gene/P at the Jülich Supercomputing Center in Germany. With UNIC, the researchers have successfully represented the details of the full reactor geometry for the first time and have been able to compare the results directly with the experimental data.
Argonne's UNIC code provides a powerful new tool for designers of safe, environmentally friendly nuclear reactors -- a key component of our nation's current and future energy needs. By integrating innovative design features with state-of-the-art numerical solvers, UNIC allows researchers not only to better understand the behavior of existing reactor systems but also to predict the behavior of many of the newly proposed systems having untested design characteristics.
Development of the UNIC code is funded principally by DOE's Office of Nuclear Energy through the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program. The Argonne UNIC project is a key part of the NEAMS efforts to replace the traditional "test-based" approach to nuclear systems design with a new "science-based" approach in which advanced modeling and simulation play a dominant role.
The code could prove crucial in the development of nuclear reactors that are safe, affordable and environmentally friendly. To model the complex geometry of a reactor core requires billions of spatial elements, hundreds of angles and thousands of energy groups -- all of which lead to problem sizes with quadrillions of possible solutions.
Such calculations exhaust computer memory of the largest machines, and therefore reactor modeling codes typically rely on various approximations. But approximations limit the predictive capability of computer simulations and leave considerable uncertainty in crucial reactor design and operational parameters.
"The UNIC code is intended to reduce the uncertainties and biases in reactor design calculations by progressively replacing existing multilevel averaging techniques with more direct solution methods based on explicit reactor geometries," said Andrew Siegel, a computational scientist at Argonne and leader of Argonne's reactor simulation group.
UNIC has run successfully at DOE leadership computing facilities, home to some of the world's fastest supercomputers, including the energy-efficient IBM Blue Gene/P at Argonne and the Cray XT5 at Oak Ridge National Laboratory. Although still under development, the code has already produced new scientific results.
In particular, the Argonne team has carried out highly detailed simulations of the Zero Power Reactor experiments on up to 163,840 processor cores of the Blue Gene/P and 222,912 processor cores of the Cray XT5, as well as on 294,912 processor cores of a Blue Gene/P at the Jülich Supercomputing Center in Germany. With UNIC, the researchers have successfully represented the details of the full reactor geometry for the first time and have been able to compare the results directly with the experimental data.
Argonne's UNIC code provides a powerful new tool for designers of safe, environmentally friendly nuclear reactors -- a key component of our nation's current and future energy needs. By integrating innovative design features with state-of-the-art numerical solvers, UNIC allows researchers not only to better understand the behavior of existing reactor systems but also to predict the behavior of many of the newly proposed systems having untested design characteristics.
Development of the UNIC code is funded principally by DOE's Office of Nuclear Energy through the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program. The Argonne UNIC project is a key part of the NEAMS efforts to replace the traditional "test-based" approach to nuclear systems design with a new "science-based" approach in which advanced modeling and simulation play a dominant role.
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Adapted from materials provided by DOE/Argonne National Laboratory.
Adapted from materials provided by DOE/Argonne National Laboratory.
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