Quantum entanglement is a counterintuitive scenario in which a pair of systems cannot be described as two separate systems, no matter how far apart they are. If two particles are entangled, for example, then it is possible to produce exactly the same change in the joint system by interacting with either particle. Entanglement allows quantum objects to be manipulated in ways that are wholly unavailable to everyday, macroscopic, objects, and this can be exploited to gain immense advantages in terms of efficiency in certain computations. It is therefore of major interest to be able to confine and manipulate quantum objects so that we can harness this fascinating quantum-mechanical effect for computing and communication purposes.
Take, for example, a pair of boots. It is true that by looking at one boot you can probably tell what colour the other one is, or whether it is for the left or right foot. But the boots are nevertheless distinct objects because they interact distinctly with other things. For instance, turning the left boot around produces a different effect from turning the right boot around. If the boots were entangled particles, however, these two operations would be indistinguishable.
Now two teams in Germany and the US have performed contrasting experiments that exploit light–matter interactions at the level of single atoms and photons. Peter Maunz and colleagues in Gerhard Rempe’s group at the Max Planck Institute for Quantum Optics in Garching caught a single rubidium atom in a laser trap, and then demonstrated a new kind of laser cooling that is more efficient than existing techniques (Nature 428 50).
Meanwhile, Boris Blinov and colleagues in Chris Monroe’s group at the University of Michigan have detected quantum entanglement between the angular-momentum state of a single trapped cadmium atom and the polarization state of a photon that it had emitted. This is the first time that atom– photon entanglement has been directly observed. Moreover, the atom does not change position, which means that it can act as memory (Nature 428 153).
In the May issue of Physics World Andrew Steane at the Centre for Quantum Computation at the University of Oxford in the UK describes this work in more detail.
