From: scerir (scerir@libero.it)
Date: Thu Jan 25 2001 - 07:36:33 MST
And now you can freely download that paper
http://www.nature.com/nature/fow/010125.html
Damien Broderick wrote:
> Here's a better summary of the halted-light experiment:
> ===================================================
> PHYSICS NEWS UPDATE
> The American Institute of Physics Bulletin of Physics News
> Number 521 January 18, 2001 by Phillip F. Schewe, James
> Riordon, and Ben Stein
>
> AT LAST! LIGHT BROUGHT TO A HALT. For the first time,
> physicists in two separate laboratories have effectively brought a
> light pulse to a stop. In the process, physicists have accomplished
> another first: the non-destructive and reversible conversion of the
> information carried by light into a coherent atomic form. Sending a
> light pulse into specially prepared rubidium (Rb) vapor, a group at
> the Harvard-Smithsonian Center for Astrophysics led by Ron
> Walsworth (617-495-7274) and Mikhail Lukin (617-496-7611) has
> (1) slowed the pulse's "group velocity" to zero and (2) stored its
> information in the form of an atomic "spin wave," a collective
> excitation in the Rb atoms. (A spin wave can be visualized as a
> collective pattern in the orientation of the atoms, which spin like
> tops and hence act like tiny bar magnets. "Spin" is merely the name
> for the tiny magnetic vector in each of the atoms.) The atomic spin
> wave is coherent and long-lived, which enables the researchers to
> store the light pulse's information and then convert it back into a
> light pulse with the same properties as the original pulse. This new
> accomplishment in a simple system increases the promise for
> quantum communication, which may someday be used to connect
> potentially ultrafast quantum computers in a large network
> analogous to the Internet.
> Usually photons (the quanta of light) are absorbed by atoms,
> destroying the information carried by the light. With the present
> method, in principle, no information in the light pulse is lost.
> Previous efforts to slow light (such as Hau et al., Nature, 18
> February 1999) have reduced the signal speed to about 1 mph
> (Update 472) by using a process called electromagnetically
> induced transparency (EIT; see Updates 37, 344 and Stephen
> Harris's article in Physics Today, July 1997). Walsworth, Lukin
> and colleagues have gone the rest of the way to a zero light-pulse
> speed by using a novel technique which was recently proposed
> theoretically (Lukin, Yelin and Fleischhauer, Phys. Rev. Lett. 1
> May 2000; Fleischhauer and Lukin, Phys. Rev. Lett. 29 May
> 2000).
> The light storage experiment begins with the Harvard-
> Smithsonian scientists shining a "control" laser beam into a glass
> cell filled with rubidium vapor (about 70-90 degrees Celsius),
> which puts the atoms into a conventional EIT state in which they
> cannot absorb light in the traditional sense. The scientists then
> send in a "signal" pulse of light which contains the information
> they want to store. As the pulse enters the rubidium cell its
> propagation speed is reduced to about 2,000 mph. Since the front
> edge of the signal pulse enters the cell (and hence is decelerated)
> first, the pulse experiences dramatic spatial compression: from
> several kilometers in free-space to a few centimeters inside the
> rubidium vapor. The light in the vapor cell interacts with the
> atoms (see figure at http://www.aip.org/physnews/graphics),
> changing the atoms' spin states coherently and creating a joint
> atom-photon system known as a polariton. (For a nice descriptions
> of polaritons see Phys Rev Focus, 26 April 2000:
> http://focus.aps.org/v5/st19.html)
> The light-atom interaction causes the polaritons to act as if they
> have an effective mass; so one way to understand the signal pulse's
> reduced speed is that the mixture with atoms, in the form of a
> polariton, effectively weighs down the otherwise massless photons.
> Next, the Harvard-Smithsonian scientists stop the signal pulse of
> light by gradually turning off the control beam, which causes more
> atoms to be mixed with fewer photons, thereby increasing the
> polariton mass and further reducing the signal pulse's speed. When
> the control beam is completely off the polariton is purely atomic,
> the light pulse is effectively halted, and no signal pulse emerges
> from the glass cell during the storage period.
> At this point there are no photons remaining in the cell. The
> light does not go into warming of the atoms, as is the usual case.
> Instead the photons are expended in the creation of the atomic spin
> wave. Thus, the information that the light pulse carried (all that
> one can know about the photons) is stored in the atomic spin wave,
> waiting to be released as a light pulse that is in principle identical
> to the incident pulse.
> An alternative way to understand the slowing of light is to think
> of the signal pulse as a wave made of many different components,
> each with a different frequency. The Rb atoms bend or "refract"
> the individual components of the light by different amounts
> depending on each component's frequency. The vapor cell's
> frequency-dependent index of refraction causes the component
> waves to add together in such a way that the group velocity, the
> velocity of the composite pulse, slows appreciably. The dimming
> of the control beam makes the vapor's index of refraction more
> sharply dependent on frequency, and this serves to reduce the
> group velocity further. The dimming causes the atoms to become
> transparent to a narrower range of frequencies. But
> simultaneously, the light wave (or more precisely, the combination
> of light wave and atomic spin wave) is continually slowing down,
> maintaining its shape but narrowing its range of component
> frequencies so that the atoms are still unable to absorb it. After a
> relatively long delay the control beam can be turned back on,
> reverting the polariton to being a light wave by coaxing the atoms
> to emit the exact signal light pulse that entered the medium.
> In brief: (1) the length of a light pulse is compressed from
> kilometers to centimeters in a properly-prepared rubidium vapor;
> (2) the information carried by the light pulse is then imprinted
> upon the ensemble of rubidium atoms in the form of long-lived
> spin waves; and (3) the light pulse can later be read out on demand.
> This new light storage method is robust because information is
> maintained in collective atomic spin states, which are much less
> sensitive to dissipation, losses, and quantum-computer-crashing
> decoherence effects than are excited electronic states in atoms.
> Scientists believe that the light storage method is quite general
> and that the simplicity of its implementation is a big advantage.
> They even speculate that the technique may be utilized in certain
> solid-state materials. The Harvard-Smithsonian demonstration
> experiment is exciting news for scientists worried about preserving
> the coherence of quantum information transfer. With further work,
> this technique should allow for the storage and transmission of
> photon quantum states useful for quantum communication and
> computation. (Phillips et al., Physical Review Letters, 29 January
> 2001.)
> Walsworth and Lukin say that a very similar result has been
> recently obtained by Lene Hau's group (Harvard/Rowland Institute
> of Science) in an ultra-cold atomic gas. In addition, an upcoming
> theory paper (Kocharavskaya et al., Phys Rev. Lett., 22 January)
> discusses a novel technique for making a light beam not only stop
> in its tracks but reverse its direction; this effect could be useful for
> non-linear optics applications.
>
This archive was generated by hypermail 2.1.5 : Sat Nov 02 2002 - 08:05:19 MST