This is neat. Scattering effects are eliminated allowing
measurement of the particle by itself.
This is a beautiful result in quantum physics.
Some possibilities toward
computation is mentioned but that appears to be long shot.
In the meantime this is pretty
good confirmation of theory well beyond expectations in terms of precision.
Near-Perfect Particle Measurement Achieved
by Clara Moskowitz, LiveScience Senior Writer
Date: 13 July 2011 Time: 01:01 PM ET
An atom consists of a nucleus of protons and neutrons, with electrons
orbiting around.
The mind-bending laws of quantum mechanics say we can't observe the
smallest particles without affecting them. Physicists have now caused the
smallest-ever disturbance while making a quantum measurement — in fact,
almost the minimum thought to be possible.
This disturbance is called back-action, and it is one of the hallmarks
of quantum
mechanics, which governs the actions of the very small. It arises from the
supposition that before a measurement is made, particles exist in a sort of
limbo state, being neither here nor there while retaining the possibility of
either.
Once an observer intervenes, the particle is forced to
"choose" a state ? to settle on one possibility, eliminating the
other options. Thus, the state of the particle is altered by the act of
measuring it.
"The atom changes because you are looking," explained physicist
Peter Maunz of Duke
University .
[The
Coolest Little Particles in Nature]
Usually the small difference caused by this back-action is dwarfed by
the interference to particles caused by laboratory imperfections. But for
the first time, scientists have achieved a quantum measurement with virtually
no additional disturbance beyond what quantum mechanics deems unavoidable.
The researchers, led by Jurgen Volz of the Université Pierre et Marie
Curie in Paris
"I think it was a significant step forward," said Maunz, who
did not participate in the new research but wrote an accompanying essay in the
same issue of Nature. [Twisted
Physics: 7 Mind-Blowing Findings]
In the new experiment, Volz and colleagues trapped a single atom of
rubidium in a cavity between two mirrors. They then shined laser light on the trapped
atom. What happened next depended on which of two energy states the atom was
in. In one state, the atom would "ignore" the light, which would
bounce back and forth between the mirrors and eventually leak to a detector
beyond the mirrors.
In the second state, the atom would absorb and re-emit the light
photons in a process called scattering. Scattering changes the energy of the
atom, and the researchers wanted to prevent that effect; the only disturbance
they wanted was from the effect of their observation.
So they set the mirrors at a precise distance where the presence of an
atom in the second state would prevent the light from bouncing back and forth
between the mirrors. Instead, all the light would reflect off the first mirror,
leaving the cavity dark. The light would hit a detector in front of the first
mirror.
In either case, the state of the atom could be determined without
causing the scattering effect.
"Experiments done before used atoms in free space and shined a
laser beam on them," Maunz told LiveScience. "They could tell which
of the two states the atoms were in, but they scattered a lot of photons. In
this experiment they managed to determine the state of the atom without
scattering photons."
While the researchers were able to limit this disturbance, there will
always be a certain amount of back-action caused by any measurement.
Ultimately, Maunz said, the experiment could help point the way toward quantum
computers, which would use particles as bits to run complex calculations
quickly.
"At the end of computation you have to read out which state
[the particle] is in," Maunz said. "If you can read it out without
disturbing the system, that's an advantage there."
You can follow LiveScience.com senior writer Clara Moskowitz on Twitter
@ClaraMoskowitz. Follow
LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.
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