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مكان : آرامگاه بوعلي سينا

زمان : جمعه 31 فروردين ماه    از ساعت 10 صبح تا 22 شب

اهم برنامه ها :

٭ پخش تصاوير و اسلايدهاي نجومي

٭ آشنايي با صور فلكي

٭ اجراي نمايش هاي نجومي

٭ مسابقه نقاشي ، ساخت ابزار نجومي (ويزه كودكان)

٭ پرواز بادباكهاي نجومي

٭ مسابقه نجومي و اهداي جوايز به بهترين عكس گرفته شده از اين روز

٭ رصـد اسـمان شب

Using a laser-cooling technique that could one day allow scientists to observe quantum behavior in large objects, MIT researchers have cooled a coin-sized object to within one degree of absolute zero.

This study marks the coldest temperature ever reached by laser-cooling of an object of that size, and the technique holds promise that it will experimentally confirm, for the first time, that large objects obey the laws of quantum mechanics just as atoms do.

Although the research team has not yet achieved temperatures low enough to observe quantum effects, 'the most important thing is that we have found a technique that could allow us to get (large objects) to ultimately show their quantum behavior for the first time,' said MIT Assistant Professor of Physics Nergis Mavalvala, leader of the team.

The MIT researchers and colleagues at Caltech and the Albert Einstein Institute in Germany will report their findings in an upcoming issue of Physical Review Letters.

Quantum theory was developed in the early 20th century to account for unexpected atomic behavior that could not be explained by classical mechanics. But at larger scales, objects' heat and motion blur out quantum effects, and interactions are ruled by classical mechanics, including gravitational forces and electromagnetism.

MIT researchers have developed a technique to cool this

dime-sized mirror (small circle suspended in the center o

f large metal ring) to within one degree of absolute zero

'You always learn in high school physics that large objects don't behave according to quantum mechanics because they're just too hot, and the thermal energy obscures their quantum behavior,' said Thomas Corbitt, an MIT graduate student in physics and lead author of the paper. 'Nobody's demonstrated quantum mechanics at that kind of (macroscopic) scale.'

To see quantum effects in large objects, they must be cooled to near absolute zero. Such low temperatures can only be reached by keeping objects as motionless as possible. At absolute zero (0 degrees Kelvin, -237 degrees Celsius or -460 degrees Fahrenheit), atoms lose all thermal energy and have only their quantum motion.

In their upcoming paper, the researchers report that they lowered the temperature of a dime-sized mirror to 0.8 degrees Kelvin. At that temperature, the 1 gram mirror moves so slowly that it would take 13 billion years (the age of the universe) to circle the Earth, said Mavalvala, whose group is part of MIT's LIGO (Laser Interferometer Gravitational-wave Observatory) Laboratory.

The team continues to refine the technique and has subsequently achieved much lower temperatures. But in order to observe quantum behavior in an object of that size, the researchers need to attain a temperature that is still many orders of magnitude colder, Mavalvala said.

To reach such extreme temperatures, the researchers are combining two previously demonstrated techniques-optical trapping and optical damping. Two laser beams strike the suspended mirror, one to trap the mirror in place, as a spring would (by restoring the object to its equilibrium position when it moves), and one to slow (or damp) the object and take away its thermal energy.

Combined, the two lasers generate a powerful force--stronger than a diamond rod of the same shape and size as the laser beams--that reduces the motion of the object to near nothing.

Using light to hold the mirror in place avoids the problems raised by confining it with another object, such as a spring, Mavalvala said. Mechanical springs are made of atoms that have their own thermal energy and thus would interfere with cooling.

As the researchers get closer and closer to reaching the cold temperature they need to see quantum behavior, it will get more difficult to reach the final goal, Mavalvala predicted. Several technical issues still stand in the way, such as interference from fluctuations in the laser frequency.

'That last factor of 100 will be heroic,' she said.

Once the objects get cold enough, quantum effects such as squeezed state generation, quantum information storage and quantum entanglement between the light and the mirror should be observable, Mavalvala said.

Other authors on the paper are Christopher Wipf, MIT graduate student in physics; David Ottaway, research scientist at MIT LIGO; Edith Innerhofer (formerly a postdoctoral fellow at MIT); Yanbei Chen, leader of the Max Planck (Albert Einstein Institute) group; Helge Muller-Ebhardt and Henning Rehbein, graduate students at the Albert Einstein Institute; and research scientists Daniel Sigg of LIGO Hanford Observatory and Stanley Whitcomb of Caltech.

The research was funded by the National Science Foundation and the German Federal Ministry of Education and Research.

Ever since 1887, when Norwegian mathematician Sophus Lie discovered the mathematical group called E8, researchers have been trying to understand the extraordinarily complex object described by a numerical matrix of more than 400,000 rows and columns.

Now, an international team of experts using powerful computers and programming techniques has mapped E8--a feat numerically akin to the mapping of the human genome--allowing for breakthroughs in a wide range of problems in geometry, number theory and the physics of string theory.

'Although mapping the human genome was of fundamental importance in biology, it doesn't instantly give you a miracle drug or a cure for cancer' said mathematician Jeffrey Adams, project leader and mathematics professor at the University of Maryland. 'This research is similar: it is critical basic research, but its implications may not become known for many years.'

Team member David Vogan, a professor of mathematics at the Massachusetts Institute of Technology (MIT), presented the findings today at MIT.

The effort to map E8 is part of a larger project to map out all of the Lie groups--mathematical descriptions of symmetry for continuous objects like cones, spheres and their higher-dimensional counterparts. Many of the groups are well understood; E8 is the most complex.

The project is funded by the National Science Foundation (NSF) through the American Institute of Mathematics.

It is fairly easy to understand the symmetry of a square, for example. The group has only two components, the mirror images across the diagonals and the mirror images that result when the square is cut in half midway through any of its sides. The symmetries form a group with only those 2 degrees of freedom, or dimensions, as members.

A continuous symmetrical object like a sphere is 2-dimensional on its surface, for it takes only two coordinates (latitude and longitude on the Earth) to define a location. But in space, it can be rotated about three axes (an x-axis, y-axis and z-axis), so the symmetry group has three dimensions.

In that context, E8 strains the imagination. The symmetries represent a 57-dimensional solid (it would take 57 coordinates to define a location), and the group of symmetries has a whopping 248 dimensions.

Because of its size and complexity, the E8 calculation ultimately took about 77 hours on the supercomputer Sage and created a file 60 gigabytes in size. For comparison, the human genome is less than a gigabyte in size. In fact, if written out on paper in a small font, the E8 answer would cover an area the size of Manhattan.

While even consumer hard drives can store that much data, the computer had to have continuous access to tens of gigabytes of data in its random access memory (the RAM in a personal computer), something far beyond that of home computers and unavailable in any computer until recently.

The computation was sophisticated and demanded experts with a range of experiences who could develop both new mathematical techniques and new programming methods. Yet despite numerous computer crashes, both for hardware and software problems, at 9 a.m. on Jan. 8, 2007, the calculation of E8 was complete.

The E8 root system consists of 240 vectors in an 8-dimensional space. Those vectors are the vertices (corners) of an 8-dimensional object called the Gosset polytope 421. In the 1960s, Peter McMullen drew by hand a 2-dimensional representation of the Gosset polytope 421. This image was computer generated by John Stembridge, based on McMullen's drawing.

Every time you switch on a light bulb, 10 to the power of 15 (a million times a billion) visible photons, the elementary particles of light, are illuminating the room in every second. If that is too many for you, light a candle. If that is still too many, and say, you just want one and not more than one photon every time you press the button, you will have to work a little harder. A team of physicists in the group of Professor Gerhard Rempe at the Max Planck Institute of Quantum Optics in Garching near Munich, Germany, have now built a single-photon server based on a single trapped neutral atom. The high quality of the single photons and their ready availability are important for future quantum information processing experiments with single photons. In the relatively new field of quantum information processing the goal is to make use of quantum mechanics to compute certain tasks much more efficiently than with a classical computer. (Nature Physics online, March 11th, 2007)
 

A single atom trapped in a cavity generates a

single photon after being triggered by a laser pulse.

After the source is characterised, the subsequent

photons can be distributed to a user

A single atom, by its nature, can only emit one photon at a time. A single photon can be generated at will by applying a laser pulse to a trapped atom. By putting a single atom between two highly reflective mirrors, a so called cavity, all of these photons are sent in the same direction. Compared with other methods of single-photon generation the photons are of a very high quality, i.e. their energy varies very little, and the properties of the photons can be controlled. They can for instance be made indistinguishable, a property necessary for quantum computation. On the other hand, up to now, it was not possible to trap a neutral atom in a cavity and at the same time generate single photons for a sufficiently long time to make practical usage of the photons.

In 2005 the team around Prof. Rempe was able to increase the trapping times of single atoms in a cavity significantly by using three dimensional cavity cooling. In the present article they report on results where they have been able to combine this cavity cooling with the generation of single photons in a way that a single atom can generate up to 300,000 photons. In their current system the time the atom is available is much longer than the time needed to cool and trap the atom. Because the system can therefore run with a large duty cycle, distribution of the photons to a user has become possible: The system operates as a single-photon server.

The experiment uses a magneto-optical trap to prepare ultracold Rubidium atoms inside a vacuum chamber. These atoms are then trapped inside the cavity in the dipole potential of a focused laser beam. By applying a sequence of laser pulses from the side, a stream of single photons is emitted from the cavity. Between each emission of a single photon the atom is cooled, preventing it from leaving the trap. To show that not more than one photon was produced per pulse, the photon stream was directed onto a beam splitter, which directed 50% of the photons to a detector, and the other 50% to a second detector. A single photon will be detected either by detector 1 or by detector 2. If detections of both detectors coincide, more than one photon must have been present in the pulse. It is thus the absence of these coincidences that proves that one and not more than one photon is produced at the same time, which is demonstrated convincingly in the work presented.

With the progress reported now, quantum information processing with photons has come one step closer. With the single-photon server operating, Gerhard Rempe and his team are now ready to take on the next challenges such as deterministic atom-photon and atom-atom entanglement experiments.

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