Researchers Demonstrate Macroscopic Entanglement

Hank Hogan

With one experiment, researchers have heartened Star Trek fans and quantum computing enthusiasts. A team at the University of Aarhus' Quantum Optics Center in Denmark has demonstrated quantum entanglement involving trillions of atoms over a distance of millimeters. Previous-ly, entanglement had been done with only four or so atoms and over very small separations. Because the work uses laser pulses, teleportation based on this type of entanglement can be achieved, in theory, over long distances.

According to Eugene S. Polzik, professor of physics at the university and a researcher on the project, a carefully constructed experimental chamber was key to the mass entanglement of quantum spins. "We put the spins in a quiet environment with magnetic shielding and reduced collisional depolarization," he explained.

Quantum entanglement describes two physically separated bodies that are connected through a common quantum state, so that knowing the status of one automatically reveals that of the other. Teleportation involves transporting the state of one to the other.

In the experiment, the researchers placed two cells, separated by a few millimeters, of approximately 10¹² cesium atoms in a 0.9-G magnetic field in the experimental chamber. They used a Ti:sapphire laser with a power level of a few milliwatts to circularly polarize the atoms in the cells. The cesium's resonance transition determined the precise wavelength used.

A 0.45-ms pulse from the Ti:sapphire entangled the atoms, marrying their spins. And another, blue-detuned pulse probed one cell to verify that entanglement still existed after up to 0.5 ms.

Quantum computing applications

The phenomena of teleportation and entanglement are crucial to the development of quantum computing. Because of its inherently parallel nature, a quantum computer could solve problems that today's machines cannot.

Polzik and his co-workers hope this work will enable such computers. The team plans to investigate the application of the technique to the teleportation of atomic states and to quantum memory using atomic spin.