Faster Than Slow Light ?
Faster Than Slow Light ? |
Electromagnetically Induced Transparency alters a normally opaque body so that light can propagate through it. When the induced transparency is shut off, light is trapped inside the opaque body. Yet more amazingly, when the transparency is again induced, the light that was trapped within the body is released.
As the 20th century dawned, Physics was turned on its head by a young clerk in a government office. In the great experiments of the early 1900s, great minds struggling to explain why the speed of light appears the same to all observers in all inertial frames of reference. Our young hero proposed that since the laws of physics were unaltered in moving frames of reference, and thus the speed of light must be such an unalterable law. And, he was able to work out equations showing how it was possible for the speed of light to be unaltered, if we are willing to accept some astonishing ideas about the warping of space and time. Over the next 50 years Albert Einstein became a household word as his theories passed test after test.
Now at the dawn of the 21st century, physicists hhave learned astonishing new tricks. It has long been known that the speed of light is slower in matter than in free space, this is how we make lenses ouut of glass. Light freely propagates at 300,000,000 meters/sec, or nearly 8 times around the world in one second. In glass it may travel half this speed. But at Harvard and other research facilities light has been slowed again and again, down to as fast as a horse can run, and eventually stopped completely.
"Two years ago we slowed it down to 38 miles an hour; now we've been able to park it then bring it back up to full speed." said Lene Hau, professor of physics, at Harvard in Boston. Hau and her team did it by passing a beam of light through a small cloud of atoms cooled to temperatures a billion times colder than those in the spaces between stars. The atom cloud was suspended magnetically in a chamber pumped down to a vacuum 100 trillion times lower than the pressure of air in the room where you are reading this.
"It's nifty to look into the chamber and see a clump of ultracold atoms floating there," Hau says. "In this odd state, light takes on a more human dimension; you can almost touch it."
She and her team continued to tweak their system until they finally brought light to a complete stop. The light dims as it slows down, so you think that it's being turned out. Then Hau shoots a yellow-orange laser beam into the cloud of atoms, and the light emerges at full speed and intensity. Hau's team stopped light for one-thousandth of a second. Atomically speaking, "this is an amazingly long time," Hau notes. "But we think it can be stopped for much longer."
Harvard Gazette interview with Lene Hau, professor of physics, Rowland Institute for Science at Harvard in Boston, 24 January 2001.
The key is to cool them to within a billionth of a degree of "absolute zero," at which atoms have the least possible energy, and they all but cease to move around. The atoms enter a new phase called a "Bose-Einstein Condensate" (BEC) where the wave functions of individual atoms merge. Hau and her group figured out a way to trap atoms of Sodium vapor and condense them into a BEC that entangles light and slows it down. Two laser beams glow yellow-orange like sodium streetlights, and the cigar-shaped cloud of BEC atoms is about eight-thousandths of an inch long and about a third as wide.
Researchers at the Harvard-Smithsonian Center for Astrophysics (CfA) used an easier method. They shot laser beams through a dense cloud of rubidium and helium gas. The light bounced from atom to atom, gradually slowing down until it stopped. No BEC was needed, supervacuum or ultra-cold not required. In fact, the chamber where the light stopped was at a temperature of 176 degrees F. In their convenient setup, however, only half of the incoming light was stored, then recovered, and the storage time was much shorter.
Philip R. Hemmer of Hanscom Air Force Base has slowed light to 45 m/s in an exotic type of crystal. At Texas A&M researchers have stopped the light and even made it drift backwards.
This is all possible due to Electromagnetically Induced Transparency (EIT). In a Bose-Einstein Condensate and certain other states of matter can produce quantum entanglement of atoms, and a laser beam can be used to manipulate the quantum states of the atoms and switch the matter between transparent and opaque states for a certain frequency of light. A "coupling" laser at right angles to the test beam is turned on, causing the entanglement to become transparent. Light from a "probe" laser entering the atomic entanglement transfers its energy to the atoms. Light energy raises the atoms to higher energy levels in ways that depend on the frequency and intensity of the light. When the coupling laser illuminating the cloud at right angles to the incoming beam is turned off, it stops the beam from the probe laser inside the cloud. When the coupling laser is turned on again, the atoms transfer their energy back to the light, and the light resumes its journey, leaving the cloud at full speed and intensity.
Aside from possible practical applications in computing and storage, this is a fascinating concept. For instance suppose a large state of such matter contains a car and driver circling on a track. When the transparency is shut off, the speed of light is essentially zero, at least at one wavelength. Does the car continue to move? Even if painted exactly the color of the probe beam? Is the car then going faster than light?
What would young Albert think if he were here today!
http://www.sciencenews.org/articles/20010127/fob1.asp
http://www.spie.org/app/Publications/magazines/oerarchive/may/may99/cover2.html
http://www.lns.cornell.edu/spr/1999-02/msg0015137.html
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