Counting photons [for Potter]
- From: Sam Wormley <swormley1@xxxxxxxxx>
- Date: Wed, 29 Aug 2007 22:12:14 GMT
Crueltyfree: Counting photons without killing them
http://www.sciencenews.org/articles/20070825/fob6.asp
Davide Castelvecchi
Disclaimer: No particles were harmed in the making of this
experiment. Physicists have found a way to count photons as they zip
along, without destroying them. The researchers say that the
technique will enable scientists to probe quantum effects that so far
have been the subject only of speculation.
In physics labs, detecting light has long been synonymous with
absorbing photons. Typically, the photons cease to exist and the
light's energy transforms into an electrical signal. Physicists can
count single photons--but they haven't been able to count them
and keep them.
"Up to now, when you measure light, it's a destructive process," says
Serge Haroche of the École Normale Supérieure of Paris. Now, Haroche
and his colleagues have shown how to count photons nondestructively
while they bounce back and forth between two mirrors.
Haroche's team began by introducing small numbers of photons into the
space between two niobium-coated screens. Kept at less than 1 kelvin,
the niobium became superconducting, which made the screens into
virtually perfect mirrors. The photons could bounce back and forth up
to a billion times, lingering inside their hall of mirrors for more
than a tenth of a second.
The team then shot rubidium atoms one by one across the photons'
path. The atoms were in a highly excited state in which their
electrons were especially sensitive to the photons' electric fields.
The electrons responded with a shift in the timing of their orbits,
essentially acting as the hands of microscopic clocks. The amount of
shift was proportional to the number of photons between the two
mirrors.
Quantum uncertainty dictates that the number of photons could not be
well defined at the start of the experiment. Measuring the influence
of the photons on a single rubidium atom yielded only incomplete
information about the number of photons. But after the researchers
had shot about 100 atoms through the chamber--gaining information
and reducing uncertainty at each step--the number of photons
converged to a definite value. Subsequent measurements confirmed that
count. So far, the team has managed to count up to seven photons,
Haroche says.
While the photons didn't die, their lives would never be the same. In
any experiment, measuring one physical quantity with increasing
precision leads to increased fuzziness in a related quantity. In this
case, obtaining a precise count of the photons came at the expense of
losing knowledge about the relative timing, or phase, of the photons'
wavelike fluctuations. The findings appear in the Aug. 23 Nature.
David Hume of the National Institute of Standards and Technology in
Boulder, Colo., says that the results are "an elegant demonstration
of the measurement process in quantum mechanics."
The experiment highlights a little-known aspect of quantum physics:
When quantities go from a fuzzy state to one with a precise value,
the transition can take place in small increments. In that way,
measurements can extract partial information (SN: 5/12/07, p. 292).
Haroche says that his team's setup could be a means for testing new
quantum phenomena in which photons occupy multiple states
simultaneously. "Quantum physics textbooks are illustrated by thought
experiments," Haroche says. "Now we are doing those experiments."
References:
Guerline, C. . . . and S. Haroche 2007. Progressive field-state
collapse and quantum non-demolition photon counting. Nature 448(Aug.
23):889-893. Abstract available at
http://dx.doi.org/10.1038/nature06057.
Further Readings:
Castelvecchi, D. 2007. Degrees of quantumness: Shades of gray in
particle-wave duality. Science News 171(May 12):292. Available at
http://www.sciencenews.org/articles/20070512/fob3.asp.
Hume, D.B., T. Rosenband, and D.J. Wineland. Preprint. High-fidelity,
adaptive qubit measurements through repetitive information transfer.
Abstract and preprint available at http://arxiv.org/abs/0705.1870.
Sources:
Serge Haroche
Laboratoire Kastler Brossel
École Normale Supérieure
CNRS
Université Pierre et Marie Curie
24 rue Lhomond
75231 Paris Cedex 05
France
David B. Hume
National Institute of Standards and Technology
Time and Frequency Division 847
325 Broadway
Boulder, CO 80305
Luis Orozco
Joint Quantum Institute
Department of Physics
University of Maryland, College Park
College Park, MD 20742
David J. Wineland
National Institute of Standards and Technology
Time and Frequency Division 847
325 Broadway
Boulder, CO 80305-3328
From Science News, Vol. 172, No. 8, Aug. 25, 2007, p. 117.
.
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