Squeezed.
The amplitude of an ordinary light wave (top) is equally uncertain everywhere, whereas phase-squeezed light (middle) is more uncertain in some places than in others. In a squeezed vacuum (bottom), only

G. Breitenbach et al., Nature 387 (1997)

Physicists Successfully Store and Retrieve Nothing

It sounds like a headline from the spoof newspaper The Onion, but for physicists, this is actually an achievement: Two teams have stored nothing in a puff of gas and then retrieved it a split second later. Storing a strange form of vacuum builds on previous efforts in which researchers stopped light in its tracks (ScienceNOW, 22 January 2001) and may mark a significant step toward new quantum information and telecommunication technologies.

To stop light, researchers first shine an intense and continuous beam of laser light into a gas of atoms. That "control beam" tickles the atoms to allow a pulse of laser light of another wavelength to enter the gas. To trap the pulse, researchers turn off the control beam, which causes the pulse to imprint itself on the atoms. To release it again, they turn on the control laser.

So storing a vacuum might sound ridiculously simple: Follow the same procedure but leave out the pulse, and you store nothing. However, Alexander Lvovsky of the University of Calgary in Canada and his colleagues and Mikio Kozuma of the Tokyo Institute of Technology in Japan and his group have stored a very peculiar type of nothingness called a "squeezed vacuum."

To see what this is, begin with a normal light wave. Classically, this is a smooth wave of electromagnetic fields with equally spaced peaks and dips. But throw in quantum mechanics and things get more complicated. The precise height of the wave becomes uncertain, so the wave gets fuzzy (see figure). Physicists have learned how to manipulate that inevitable uncertainty--for example, making it smaller at the peaks and larger in between. That makes "phase-squeezed light." Now imagine turning down the intensity of the phase-squeezed light to zero. The wave itself goes away, but the waxing and waning uncertainty remains, creating a squeezed vacuum.

This is what Lvovsky and Kozuma stored. To make a pulse of squeezed vacuum, both used a device called an optical parametric amplifier, the heart of which is a crystal whose optical properties can be controlled by laser light. Kozuma and colleagues stored pulses of squeezed vacuum for up to 3 microseconds in rubidium atoms chilled to near absolute zero, they report in a paper to be published in Physical Review Letters. Lvovsky and colleagues stored their pulses for 1 microsecond in warm rubidium gas and say they reconstructed the squeezed vacuum in greater detail. Their results will appear in the same journal.

Proving that the squeezed vacuum survived its confinement is tricky, as it's hard to measure nothing. To probe the retrieved vacuum, researchers "mixed" it with the same ordinary laser light that was used to excite the optical parametric amplifier and make the squeezed vacuum. They then observed the telltale up and down in the uncertainty in that light beam, which was effectively transferred from the resurrected vacuum.

"I'm very impressed," says physicist Alexander Kuzmich of the Georgia Institute of Technology in Atlanta. "It's a real technical achievement." The ability to store squeezed states could help pave the way to new types of quantum networks that would carry uncrackable coded messages, says Kuzmich, who in 2006 stored and retrieved a single photon. More conceptually, such experiments might help spell out the boundary between the quantum and classical realms, he says. "There is something we still don't understand about that transition."

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