Proposal would marry atom and photon

By Eric Smalley, Technology Research News

Neither atoms nor photons are ideal for building quantum computers. Atoms are easy to store and manipulate but difficult to transport. Photons, on the other hand, are hard to manipulate and harder still to store. But they're made to move.

Some researchers are trying to figure out how to use the best of both. After all, conventional computers use both electricity and magnetism to handle bits: electricity for manipulating and moving them and magnetism for long-term storage. The goal is to build a quantum computer that uses both atoms and photons. The key is linking an atom to a photon in the quantum mechanical state of entanglement.

"Having one of each entangled means that a quantum device could readily store and manipulate the atom while sending the photon off to a distant receiver," said Michael G. Moore, a postdoctoral fellow at the Institute for Theoretical Atomic and Molecular Physics at the Harvard-Smithsonian Center for Astrophysics.

But while converting an electric bit to a magnetic bit is relatively straightforward, transferring information between a photon and an atom in an orderly fashion is a major challenge. One scheme, proposed by Moore and a colleague at the University of Arizona, calls for entangling atom-photon pairs by firing a laser into a Bose Einstein condensate.

A Bose Einstein condensate is an exotic form of matter formed by chilling atoms to near absolute zero. The atoms in a Bose Einstein condensate share the same wave function, meaning they are in the same state and orientation. This is analogous to the photons in a laser beam.

"By using a Bose Einstein condensate it should be possible to... create entangled atom-photon pairs in a highly controlled manner," said Moore.

Two particles can become entangled, or linked, when they are in the quantum mechanical condition of superposition, which is a mixture of all possible states. When one of the entangled particles is measured, it collapses out of superposition into a random state and the other particle immediately collapses into the same state, regardless of the physical distance between them.

When a photon of the right wavelength hits an atom, it bounces off rather than being absorbed. Sometimes when a photon bounces off an atom the two become entangled. However, if a second photon hits the atom it knocks the atom out of its quantum mechanical state and breaks the entanglement with the first photon.

The advantage of using a Bose Einstein condensate is that it contains large numbers of atoms -- typically about one million -- relative to the number of photons, said Moore. This makes it more likely that only one photon will hit each atom.

Quantum computers hold the promise of solving problems that ordinary computers cannot, such as searching massive databases and cracking powerful encryption codes, but quantum computers are likely decades away. Entangled atom-photon pairs could find use sooner, however.

The entangled pairs could be used for quantum cryptography and quantum teleportation, said Moore. Quantum cryptography and quantum teleportation have already been demonstrated using entangled photons.

Using quantum cryptography, a sender can transmit a series of individual photons to a receiver. Anyone eavesdropping on the communications would necessarily alter the state of the photons, revealing the security breach.

In quantum teleportation, the quantum state of a particle can be reproduced in another location by using a pair of particles that are entangled but separated in space as a sort of quantum fax machine.

It should be possible to demonstrate quantum cryptography or quantum teleportation with atom-photon pairs in five to ten years, said Moore.

Moore's research colleague was Pierre Meystre, a professor of physics at the University of Arizona. They published the research in the December 11, 2000 issue of Physical Review Letters. The research was funded by the Office of Naval Research, the National Science Foundation, the Army Research Office and the Joint Services Optics Program.

Timeline:   5-10 years
Funding:   Government
TRN Categories:   Quantum Computing
Story Type:   News
Related Elements:  Technical paper, "Generating Entangled Atom-Photon Pairs from Bose-Einstein Condensates," Physical Review Letters, December 11, 2000


February 7, 2001

Page One

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DNA induced to superconductivity

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Proposal would marry atom and photon


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