Entanglement allows particles to have a much closer relationship than is possible in classical physics: if two particles are entangled, we can know the state of one particle by measuring the state of the other. For example, two particles can be entangled such that the polarization of one particle is always "horizontal" when the spin of the other is "vertical", and vice versa; or that the spin of one particle is "up" when the other is "down", and vice versa. An additional feature of quantum mechanics is that the particle can exist in a superposition of both these states at the same time. By taking advantage of such quantum phenomena, a quantum computer could, in principle, outperform a classical computer for certain tasks.

Although physicists can now routinely entangle photons and send them over long distances down optical fibres, these particles are difficult to store for long periods and so are not ideal as qubits for real quantum information systems. In contrast, qubits based on ground-state atoms have long lifetimes and so can be stored. Kuzmich and colleagues have now succeeded in remotely entangling two such atomic qubits using a photon (Phys. Rev. Lett. 96 030405).

The Georgia Tech team made each long-lived qubit using "collective" spin states of a cold cloud of about 100,000 rubidium-85 atoms. Only a single spin is "flipped" in these collective states but the flip is distributed over all of the atoms involved in the qubit. The physicists began by preparing an entangled state of one of these atomic qubits and a single photon in a magneto-optical trap in their laboratory.

Next, the scientists transmitted the photon down an optical fibre to a magneto-optical trap in another lab located 5.5 metres away. Finally, they converted the photon into another atomic qubit, also consisting of rubidium-85 atoms. The team then measured the resulting entanglement of the two atomic qubits by "transferring" their quantum states onto photons and then measuring the polarization correlations of the photons.

"It should now be possible to teleport quantum states of matter over long distances," says Kuzmich. "The breakthrough also indicates that atoms and photons can be used for larger quantum networks -- though further work on practical issues is still necessary."

Meanwhile, in a separate experiment, Weinfurter and colleagues have entangled a single trapped atom with a single photon at a wavelength of 0.78 microns, which is suitable for low-loss communication over long distances, using similar experimental techniques to the Georgia Tech group (Phys. Rev. Lett. 96 030404). The entanglement is between the polarization of the photon and the internal site of a rubidium-87 atom stored in an optical trap. Kuzmich and colleagues have also demonstrated atom-photon entanglement at "telecommunications" wavelengths of 1.5 microns (quantum-physics/0601055).