Atomic Crowds Tied by Quantum Thread.

Scientists have been predicting that the strangeness of quantum mechanics will lead to computing and communications devices of unprecedented power. In pursuit of those trophies, researchers have struggled to control the frail, fleeting quantum states of minuscule particles. Now, a relatively simple and robust way of manipulating quantum states may be at hand.

For years, physicists have been exploiting a quantum phenomenon, known as entanglement, to intertwine the quantum states of charged atoms, or ions. They've been able to entangle as many as four ions so far (SN: 4/15/00, p. 255).

Eugene S. Polzik and his colleagues at the University of Aarhus in Denmark have now entangled two gas clouds, each of about a trillion cesium atoms. That huge leap reflects the team's development of a type of entanglement of atoms that's different from that established in earlier work, the researchers say.

Entangled entities have a coordinated quantum state. For example, if one particle in an entangled pair has an upwardly oriented magnetic field, or so-called spin, its partner's spin points down. Which partner has which spin remains hidden until the spin of one of them is measured.

In the smaller-scale experiments, each ion was entangled with every other. However, in the Sept. 27 NATURE, the Aarhus team describes a technique that treats large atomic ensembles as single quantum entities.

In their experiment, Polzik and his colleagues first injected cesium gas into a pair of 3-centimeter-long glass capsules. Next, the scientists shot laser pulses through each cell to impart a different collective spin state onto each cloud. Then, the researchers fired another laser pulse of a carefully chosen wavelength through both clouds.

The orientation, or polarization, of that pulse's electromagnetic field is itself a quantum state. Once the pulse entered the capsules, it became entangled with the clouds' spins, creating the first photon-atom entanglements, Polzik says. Finally, with the act of measuring polarization of the emerging pulse, the researchers forced the spins of the clouds to entangle.

"This is a spectacular experiment and a significant advance in experimental capabilities in quantum [systems]," comments H. Jeff Kimble of the California Institute of Technology in Pasadena.

The new work departs sharply from earlier research in several ways. To become entangled, individual ions must be close together and chilled to nearly absolute zero. But the atom clouds became entangled at room temperature and while centimeters apart--a vast distance from an atomic perspective. Also, the clouds stayed linked for half a millisecond, a duration that looks promising for developing practical quantum devices.

While the technical ease of entangling atom clouds by this method has great appeal, using ensembles of particles also has drawbacks, remarks Wolfgang Tittel of the University of Geneva. For instance, data stored as quantum states could become corrupted more readily in such clouds than in individual particles.

Several years ago, Kimble, Polzik, and their colleagues entangled beams of laser light containing countless photons. They also teleported--or transferred via entanglement--a quantum state of one beam to the other (SN: 4/3/99, p. 220).

Although such transfers could be important in future quantum-communication technology, photons aren't matter. Scientists expect both quantum communications and information processing to require matter, such as atoms, to be the data bits in calculations and memory.

While researchers have yet to pull off teleportation between separated clumps of matter, it looks like the stage is now set, Tittel says.