Light impresses atoms
By Eric Smalley, Technology Research NewsBringing a pulse of light to a full stop is easy. Just hold a piece of black paper in its path. But bringing a pulse of light to a full stop, storing its information in a group of atoms and later reconstituting the beam is another matter.
Two teams of researchers have separately achieved this feat, in effect developing techniques for storing all the information contained in a pulse of light.
Though the information can be stored for only fractions of a second, the techniques could be used to transport information about the quantum states of particles like atoms and photons. One of the biggest hurdles to building quantum computers is figuring out how to move quantum information in and among them.
"We're able to... take a pulse of light, store the information describing that pulse of light in a vapor cell of rubidium atoms, and then at a later time... reconstruct the light pulse from the information that's been stored in the atoms and have the light pulse reemitted from the cell," said David F. Phillips, a physicist at the Harvard-Smithsonian Center for Astrophysics.
The technique allowed the atoms to store an impression of the light wave that made up the pulse. The impression contained all of the information about the light wave, including its polarity and coherence, said Phillips. "What we're storing is information, and what we're not storing is the energy," he said.
Both teams' experiments involved shooting a pair of laser beams into a cloud of atoms. Phillips' team used atoms that were heated. The other team, led by Harvard University physics professor Lene Vestergaard Hau, used atoms that were cooled to just above absolute zero. The key to both experiments is the Electromagnetically Induced Transparency process.
Light doesn't travel through opaque matter, like rubidium gas, because its atoms absorb photons. To produce Electromagnetically Induced Transparency, the researchers fired a strong laser beam into the rubidium atoms, essentially overloading them, said Phillips. "So now [the] beam just propagates on through the cell as though the atoms weren't there," he said.
At the same time, the researchers sent a weaker pulse of light of a different frequency into the rubidium atoms. The interaction between the light pulse and the rubidium atoms introduced drag, which slowed and compressed the light pulse.
It also altered the atoms' spins. An atom can be thought of as a tiny spinning top with a magnetic field aligned along its axis. Atoms are ordinarily oriented up or down, but they can be forced into orientations in between.
The researchers gradually dimmed the laser beam, which increased the interaction between the light pulse and the atoms. The greater the interaction, the slower the light pulse and the more the atoms' spins were altered.
When the laser was turned off completely, the light pulse also disappeared. However, at that point all of the information about the light pulse was imprinted in the collective spins of the atoms. The researchers then turned the laser beam back on, which reconstituted the light pulse from the information stored in the atoms.
The researchers were able to keep the laser beam off for as long as 500 microseconds, Phillips said. "We're limited by how long the atoms stay in the volume of the laser beam as it goes through the cell," he said. There are one million microseconds in one second.
The information about the pulse stored in the atoms' spins was classical, meaning the information described the light as a wave rather than a collection of individual photons. The researchers' next step is to use the technique to convey quantum information about the atoms.
"In the future, we'd like to... turn our experiment inside out and start with some quantum state in our atoms, apply the right control fields... have it emit a pulse that contains the quantum information [from] the atoms, transport that pulse to a different set of atoms and write it into the new atoms and thus transport the quantum state," said Phillips.
The researchers described this as an alternative to the more difficult work of conveying quantum information between single atoms, which is the focus of most current quantum computing efforts.
"They're converting photon pulses to excitations of many atoms," which could lead to exchanging collective quantum information between groups of atoms, said John Preskill, professor of theoretical physics and director of the Institute for Quantum Information at the California Institute of Technology. "This is something that is technically easier [than exchanging information between single atoms] but on the other hand it's not clear if it's useful from a quantum information processing point of view."
Researchers have pretty clear ideas about how to compute using information encoded in single atoms, but computing using information encoded collectively in groups of atoms will likely prove more difficult, said Preskill. "Even so, I think it's a nice experiment," he said.
A third team of researchers is preparing an experiment aimed at not only storing the information from a pulse of light, but halting the actual pulse. "We [will] have a pulse of light in our cell which stays stationary," said Olga Kocharovskaya, an associate professor of physics at Texas A&M University.
This will allow the light to interact with the atoms for an extended period of time, which could provide an efficient means of producing entangled photons and other nonlinear optical effects that are useful for quantum computing, she said. Freezing an actual light pulse could pave the way for combining the storage and transportation of quantum information with quantum processing, she said.
Using the light storage techniques to transport quantum information in practical applications will require practical quantum computers, said Phillips. "To my eyes it looks like 50 years. It's not going to be less than 10 years," he said.
Phillips' research colleagues were Annet Fleischhauer, Ronald L. Walsworth and Mikhail D. Lukin of the Harvard-Smithsonian Center for Astrophysics. Their research is scheduled to be published in the January 29, 2001 issue of Physical Review Letters. The research was funded by the National Science Foundation and the Office of Naval Research.
Hau's research colleagues were Chien Liu, Zachary Dutton and Cyrus H. Behroozi of the Rowland Institute for Science and Harvard University. Their research is scheduled to be published in the January 25, 2001 issue of Nature. The research was funded by the Defense Advanced Research Projects Agency (DARPA), the U.S. Air Force Office of Scientific Research and the U.S. Army Research Office.
Kocharovskaya's research colleagues were Yuri Rostovtsev and Marlan O. Scully of Texas A&M University. They published their work in the January 22, 2001 issue of Physical Review Letters. The research was funded by the Office of Naval Research, the National Science Foundation, the Welch Foundation and the Texas Advanced Technology Program.
Timeline: 2 years; 10-50 years
Funding: Government; Private
TRN Categories: Quantum Computing
Story Type: News
Related Elements: Technical paper, "Storage of light in atomic vapor," scheduled to appear in Physical Review Letters, January 29, 2001; Technical paper, "Observation of coherent optical information storage in an atomic medium using halted light pulses," scheduled to appear in Nature, January 25, 2001; Technical paper, "Stopping Light via Hot Atoms," Physical Review Letters, January 22, 2001
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January 24, 2001
Page One
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