Samstag, 24. Januar 2009

Quantum information teleported between distant atoms


New technique can move fragile quantum data
between atoms without destroying it.

A qubit walks into a bar, unsure of whether to order drink A or drink B. If the bartender asks the qubit what it wants, the qubit will collapse and be destroyed. But now researchers can instantly teleport the original, intact qubit to another “bar” far away.

In the Jan. 23 Science, a team is reporting what is the first successful transfer of a qubit — an undecided bit of quantum information — between two widely separated, charged atoms. Because the quantum information instantly hops from one atom to the other without ever crossing the space between the two, scientists call the transfer “teleportation.”


Being able to teleport such information between atoms could aid the development of ultrafast quantum computers and extremely secure quantum communication, the researchers point out.

“The catch with quantum information is that you can’t read it without destroying it,” says study coauthor Steven Olmschenk, a physicist at the University of Maryland in College Park. “Somehow you have to send it from one point to another without ever having read it.

To read the quantum information contained in an atom or a photon, scientists musmeasure some property of that particle. But in the quantum world, the act of measuring a particle alters it. Until it’s measured, an atom or photon can remain in an ambiguous state of all possible values simultaneously. Whenever a particle is measured, though, this range of possibilities “collapses” into a single, distinct value. The original, uncommitted state is lost, and it’s this ability to hold multiple values at once that gives qubits such potential for high-performance computing.

Scientists have previously teleported unmolested qubits between photons of light, and between photons and clouds of atoms. But researchers have long sought to teleport qubits between distant atoms. Light’s high speed of travel makes photons good transporters of information, but for storing quantum information, atoms are a much better choice because they’re easier to hold on to.

“This is a big deal,” comments Myungshik Kim, a quantum physicist at Queen’s University Belfast in the United Kingdom. “To store information as it is in quantum form, you have to have a teleportation scheme available between two stationary qubits. Then you can store them and manipulate them later on.”

To teleport the qubit, Olmschenk’s team first linked the fates of two charged atoms of ytterbium, which were suspended in a vacuum chamber by electric fields. Zapping one of the atoms with a microwave pulse excited an electron in that atom, thus putting that electron into a mixture of two possible states. Researchers then zapped each atom with an ultrafast laser that caused each atom to emit a single photon of light. The wavelengths, or colors, of these photons depended on which states the electrons were in. Crossing these photons in a beamsplitter sometimes entwined the states of those electrons, a bizarre quantum phenomenon called entanglement.

When two particles become entangled, their separate quantum identities get blended so that a single equation represents both. So entangling the two electrons caused the original qubit — the unknown, unresolved mixture of two possible states — to become essentially shared between the two atoms.

The researchers then measured the first atom, thus destroying the delicate quantum information it contained, and also destroying the entanglement. That left the original qubit intact in only the second, recipient atom, completing the teleportation.

While the work marks a fundamental achievement in manipulating quantum information, Eugene Polzik, a physicist at the Niels Bohr Institute in Copenhagen, notes that the efficiency of the procedure is still too low to be useful. Currently, only about one out of every 100 million attempts results in a successful entanglement, though Olmschenk says this rate could be significantly improved.

“This very low efficiency is partly due to technical reasons,” such as a small lens for capturing photons released by the atoms and low detection efficiency for those photons, Polzik comments. “It is nonetheless a spectacular achievement.”


By Patrick Barry for Science News (outside link)

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24. Jan. 2009
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