Teleport lifts quantum computing

By Eric Smalley, Technology Research News

A leading candidate architecture for quantum computing -- trapped ions -- has received a significant boost with a pair of teleportation experiments that show is possible to reliably and readily shuttle information within a quantum computer.

The experiments, from the University of Innsbruck in Austria and the National Institute of Standards and Technology (NIST), have also advanced the practice of teleportation.

Each team transported the states of charged atoms and showed that it is possible to do so on demand. Previous teleportation experiments used photons and were probabilistic, meaning each attempt had only a certain probability of succeeding.

The method allows quantum bits, or qubits, to be stored before and after teleportation so that quantum computers can process information, move it to a different part of the computer and then process it further.

Teleportation is based on the strange phenomenon of entanglement, which links the traits of particles like atoms and photons regardless of the distance between the particles. When one of a pair of entangled particles is observed, it assumes a definite state, and at the same instant, the other particle assumes the opposite state.

Teleporting the state of a particle is akin to faxing a document and in the process destroying the original. Teleportation requires three particles. Two of the particles are entangled and one is held by the sender and the other sent to the receiver.

The third particle is teleported when the sender brings it into contact with her half of the entangled pair and measures both particles, which transfers the quantum information of the third particle to the entangled particle and in the process destroys the original. The sender then gives the receiver via an ordinary communication line information about how she measured the particles. The receiver performs the same measurement on his half of the entangled pair, which converts it into a copy of the third particle.

The University of Innsbruck researchers teleported the quantum states of calcium ions trapped in a radio-frequency electric field. Electrodes shaped the field in a way that forced the ions into a row. The NIST researchers used a similar setup to teleport the quantum states of beryllium ions.

Qubits formed from ions last much longer than qubits formed from photons, which exist for a small fraction of a second and are difficult to store. "In related experiments using our beryllium ion qubits, we have observed [quantum] states to be preserved for longer than 10 minutes," said David Wineland, group leader for ion storage at NIST. This is more than enough time to use the information in computing operations.

The ion teleportation schemes provide a means for moving information within a quantum computer without having to move the ions themselves. In most large-scale quantum processor schemes, moving information by moving qubits directly takes a relatively long time, said Wineland. In contrast, teleportation speed is determined by the time it takes to transmit classical signals, which is limited only by the speed of light, he said.

Ion-based quantum computer architectures generally require that quantum information be moved frequently. Trapped-ion quantum information processors are likely to need several small quantum registers, or sets of qubits, to store, process and error-check information, said Rainer Blatt, a professor of physics at the University of Innsbruck. Information could be readily transmitted between registers using teleportation, he said.

The experiments are significant because the quantum states of ions, which are relatively massive particles, have been teleported, said Steven van Enk, a researcher at Lucent Technologies' Bell Labs. "The advantage is that the state of an ion can be stored for a long time," he said. Teleportation has only previously been achieved using photons, which are massless particles of light, and whole light beams.

In addition, the process is reliable, and the state that has been teleported is available for another round of quantum information processing, said van Enk. "The significance is that the experiments were done in setups that are very promising candidates for quantum computing." The two teams used different techniques, both of which can ultimately be used in quantum computers, he added.

There are two big advantages to using teleportation to transfer information within quantum computers, said van Enk. It makes it possible to perform a two-ion operation on ions that are not close to one another, and it increases the computer's error threshold, which means the computers would be easier to make, he said. Without teleportation, the error threshold, or acceptable error rate, is around one in 10,000, or 1 percent of 1 percent; teleportation increases the error threshold to about 1 percent, making the computer more resilient to errors, he said.

Practical applications for full quantum computers that have several hundred thousand qubits are one or two decades away, but specialized applications that require only a few qubits are two to five years away, and applications that require 10 to 20 qubits are five to ten years away, said Blatt. "One of the not-so-distant applications might be a quantum repeater, that is a small quantum computer node for long distance quantum communication," he said.

Useful simulations of other quantum systems and practical applications in quantum measurement be possible within the next decade, said Wineland. A useful factoring machine is likely to take much longer, he added.

Blatt's research colleagues were Mark Riebe, Hartmut Häffner, Christian F. Roos, Wolfgang Hänsel, Jan Benhelm, Gavin P. T. Lancaster, Timo W. Körber, Christoph Becher and Ferdinand Schmidt-Kaler of the University of Innsbruck, and Daniel F. V. James of the Los Alamos National Laboratory. The work appeared in the June 17, 2004 issue of Nature. The research was funded by the Austrian Science Fund (FWF), the Austrian Ministry for Education, Science and Culture (bm:bwk), the Institute for Quantum Information (IQI), the Advanced Research and Development Activity (ARDA), the European Union (EU), and the Austrian Academy of Sciences.

Wineland's research colleagues were Murray Barrett, John Chiaverini, Tobias Schaetz, Joe Britton, Wayne Itano, John Jost, Emanuel Knill, Chris Langer, Dietrich Leibfried and Roee Ozeri. The work appeared in the June 17, 2004 issue of Nature. The research was funded by the Advanced Research and Development Activity (ARDA), the National Security Agency (NIST), and the National Institute of Standards and Technology (NIST).

Timeline:   1-2 decades
Funding:   Government, Private
TRN Categories:  Quantum Computing and Communications
Story Type:   News
Related Elements:  Technical paper, "Deterministic Quantum Teleportation with Atoms," Nature, June 17, 2004; Technical paper, "Deterministic Quantum Teleportation of Atomic Qubits", Nature, June 17, 2004.


July 14/21, 2004

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