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February 13, 2007

Lene Vestergaard Hau — Master of Light


Hau, a professor of physics at Harvard, is the early 21st century wizard of the photon.

She and her colleagues have succeeded in stopping light in its tracks — and then recreating it in a different location to proceed on its merry quantum way.

Watch a movie from her lab showing "light storage and revival in distant clouds."

Kenneth Chang wrote about the work of her group, just published in a cover article in Nature magazine (top), in a story that appeared in the February 8, 2007 New York Times; it follows.

    Wizardry at Harvard: Physicists Move Light

    It’s like three-card monte. Now you see it. Now you don’t. Then you see it — over there.

    In a quantum mechanical sleight of hand, Harvard physicists have shown that they can not only bring a pulse of light, the fleetest of nature’s particles, to a complete halt, but also resuscitate the light at a different location and let it continue on its way.

    That ability to catch, store, move and release light could be used in future computers to process information encoded in the light pulses.

    “It’s been a wonderful problem to try to wrap your brain around,” said Lene Vestergaard Hau, a professor of physics at Harvard and senior author of a paper describing the experiment that appears today in the journal Nature. “There are so many doors that open up.”

    In 1999, Dr. Hau headed a team of scientists that slowed light, which travels a brisk 186,282 miles a second when unimpeded, to a leisurely 38 miles an hour by shining it into an exotic, ultracooled cloud of sodium atoms. At temperatures a fraction of a degree above absolute zero, the atoms coalesce into a single quantum mechanical entity known as a Bose-Einstein condensate. Shining a laser on the cloud tunes its optical properties so that it becomes molasses when a second light pulse enters it.

    Then, in 2001, Dr. Hau and a second team of physicists, this one from the Harvard-Smithsonian Center for Astrophysics, brought light to a complete halt by slowly turning off the laser. The Bose-Einstein cloud turned opaque, trapping the light pulse inside. When the laser was turned back on, the trapped light pulse flew out.

    The latest results add an additional twist: transporting the pulse to a second Bose-Einstein cloud and regenerating the light there. “That’s the sort of stuff we find really sexy in this business,” said Eric A. Cornell, a senior scientist at the National Institute of Standards and Technology.

    In the new Harvard experiment, when the initial pulse slammed into the first Bose-Einstein cloud, the collision caused 50,000 to 100,000 of the sodium atoms to start spinning, almost like small tops, and pushed this small clump forward at less than a mile an hour.

    Dr. Hau described the clump of atoms as a “metacopy” of the light pulse. Although it consisted of sodium atoms instead of particles of light, it exactly captured the characteristics of the light pulse.

    The clump floated out from the rest of the cloud, traveled about two-tenths of a millimeter and burrowed into a second Bose-Einstein cloud. When a laser was shined on the second cloud, the atom clump transformed into a new pulse of light identical to the original pulse.

    It was refinements to the 2001 experimental technique that extended the time the particles maintain quantum collective behavior. This allowed the clump to reach the second cloud.

    Transforming a light signal into a clump of atoms could be a way of storing information. (“You could put it on the shelf for a while,” Dr. Hau said.) It could also enable a way of performing calculations in future optical computers that employ quantum algorithms to speed through certain types of calculations.

    But one hurdle to building a computer that calculates with light is that it is difficult to grab onto and manipulate a quick-moving light pulse. Performing calculations with atomic clumps would be much easier with the result changed back into light and then sped to the next step.

    “That has been a missing link,” Dr. Hau said.

    The advance could also find applications in quantum cryptography, which can hide messages in codes that cannot be broken.

    Dr. Hau said the current apparatus was just a proof of the concept and far from anything that could be used practically for any applications.

    But that has not stopped other physicists from starting to ponder what the applications might be, just as her earlier experiments have spurred physicists and engineers in a new active field of research, looking for ways to harness slow light for use in optical networks.

    Currently, optical signals need to be changed into electronic ones for processing and then changed back into light. All-optical devices could save on costs and power use.


Now we're ready to up the degree of difficulty, with your brain all warmed up and buzzing around this idea.

Here's the first paragraph of the Nature paper.

    Coherent control of optical information with matter wave dynamics

    In recent years, significant progress has been achieved in manipulating matter with light, and light with matter. Resonant laser fields interacting with cold, dense atom clouds provide a particularly rich system. Such light fields interact strongly with the internal electrons of the atoms, and couple directly to external atomic motion through recoil momenta imparted when photons are absorbed and emitted. Ultraslow light propagation in Bose–Einstein condensates represents an extreme example of resonant light manipulation using cold atoms. Here we demonstrate that a slow light pulse can be stopped and stored in one Bose–Einstein condensate and subsequently revived from a totally different condensate, 160 mum away; information is transferred through conversion of the optical pulse into a travelling matter wave. In the presence of an optical coupling field, a probe laser pulse is first injected into one of the condensates where it is spatially compressed to a length much shorter than the coherent extent of the condensate. The coupling field is then turned off, leaving the atoms in the first condensate in quantum superposition states that comprise a stationary component and a recoiling component in a different internal state. The amplitude and phase of the spatially localized light pulse are imprinted on the recoiling part of the wavefunction, which moves towards the second condensate. When this 'messenger' atom pulse is embedded in the second condensate, the system is re-illuminated with the coupling laser. The probe light is driven back on and the messenger pulse is coherently added to the matter field of the second condensate by way of slow-light-mediated atomic matter-wave amplification. The revived light pulse records the relative amplitude and phase between the recoiling atomic imprint and the revival condensate. Our results provide a dramatic demonstration of coherent optical information processing with matter wave dynamics. Such quantum control may find application in quantum information processing and wavefunction sculpting.


Here's the Nature Editor's Summary of this remarkable, groundbreaking work.

    A trick of the light

    The cover graphic represents a remarkable experiment. A light pulse stopped and extinguished in one box is revived from a completely different box in a separate location and sent back on its way. In the actual experiment, a slow light pulse was stopped and stored in one Bose–Einstein condensate (the first 'box'), then revived from a different condensate, 160 mum away. Information was transferred by converting the optical pulse into a travelling matter wave more amenable to manipulation than light. The experiment (video streams of which can be seen online) shows that the interaction of resonant laser fields with Bose-condensed atom clouds is a powerful way of manipulating light with matter, and vice versa. Such quantum control may find application in quantum information processing and the controlled sculpting of atomic wavefunctions.


Here's a link to Harvard's press release about the findings.

Look for a Nobel Prize in Physics for this work circa 2015.

February 13, 2007 at 02:01 PM | Permalink


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I've listened to both the NPR report and read the description above. One person in the NPR report asked Dr. Hau about being able to not only transfer light to matter and back to light again, but asked if you could also convert matter to light, shoot the light beam to another location, recapture it, and recreate the object again at the other side, a la Star Trek transporter. She did acknowledge that the process works both ways indistinguishably, but seemed to sidestep the question at the same time. I also don't see any discussion about this concept in any of the reports I have read.

Why isn't anybody talking about this concept? Even if converting light to matter and back to light is easier to do and provides a path to a more immediately practical application, I would think the concept would at least be mentioned as something to pursue in the future.

This is wonderful stuff! Thank you!


Posted by: Thomas Perry | Feb 27, 2007 8:27:05 PM

Well hell, I coulda told you all that.

Posted by: Flautist | Feb 13, 2007 8:13:58 PM

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