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Quantum entanglement, whereby two or more objects are linked by an unseen connection, has some famously spooky effects. As quantum researcher Anton Zeilinger has said, entanglement can be thought of as a pair of dice that always land on the same number.
One of the most intriguing applications of this entanglement is quantum teleportation, in which the quantum state of a particle or atom is transferred to its entangled partner, even if they are separated physically. Such relaying of quantum information could form the backbone of long-distance quantum communication channels, but such a network remains far on the horizon.
A group of researchers, however, report today in Science that they've made headway in quantum teleportation, and thus communication. The team, led by physics graduate student Steven Olmschenk at the University of Maryland, College Park, succeeded in teleporting quantum information between ytterbium ions (charged atoms) three feet (one meter) apart.
Quantum teleportation has been demonstrated over macroscopic distances—hundreds of meters in at least one case—for photons, the fundamental particles of electromagnetic radiation, but ions are better candidates for quantum memory because they can store information for relatively long periods of time. (Christopher Monroe, a study co-author and the leader of the trapped-ion research group to which Olmschenk and several other co-authors belong, wrote about the potential for ions to serve as quantum bits, or qubits, in Scientific American last year.) The fundamental advantage of quantum information systems is that whereas a conventional digital bit can be 0 or 1, a qubit can be in a so-called superposition of 0 and 1 simultaneously.
Information is teleported from one ion to another by encoding quantum information onto the first ion. Once the ion is entangled with another, the state of each ion is indefinite until the first one is measured—an action that projects the other ion into one of two states. Conventional (nonquantum) communication channels relay information, gleaned from the first ion's measurement, as to which of those two states is correct, and a pulse of microwave energy sets the second ion into the state representing the information encoded on the first.
"We write information to the first ion, we perform this teleportation protocol, and it transfers the information over to the second ion," Olmschenk explains. He notes that this is the first teleportation experiment between two matter qubits that were a long distance apart.
Paul Kwiat, a physics professor at the University of Illinois at Urbana–Champaign, says that distant teleportation between potential qubits of quantum memory is a definite milestone. "The whole point of teleportation is getting the information far away," Kwiat says, noting that in prior micron-scale teleportation demonstrations with matter qubits, the researchers might have been better off simply moving the qubits physically from point to point. (A micron is one millionth of a meter, or about one twenty-five-thousandth of an inch.)
The ability to transmit information between bits of quantum memory could form the basis of so-called quantum repeaters, point-to-point networks that relay data down the line. "The idea is to in some sense boost the information along the way—to send it a short distance and then have it in some sense be amplified and sent on again and again to complete a transfer over a long distance," Olmschenk says.
Kwiat also sees this work finding applications in quantum communication as a link between quantum processors. But he would like to see the system boosted to higher operating speeds—in the current incarnation it takes an average of 12 minutes, or about 30 million attempts, to secure entanglement between a pair of ions.
Olmschenk agrees. "If you want to use this for real quantum communication purposes," he says, "we'd like it to go much faster." Toward that end, he says that small improvements in collecting and detecting the photons emitted by the ions, which are used to establish ion-to-ion entanglement, could provide a major boost in teleportation efficiency.