Quantum decoherence measured

17 Jun 2002

Quantum decoherence, the collapse of a quantum superposition (a uniquely quantum-mechanical phenomenon in which an atom, photon, or other quantum system acts as if it simultaneously exists in more than one state) into a single definite state, has been quantitatively measured for the first time. Serge Haroche and his co-workers at the Ecole Normale Superieure in Paris (haroche@physique.ens.fr) send individual rubidium atoms-- each of which is in a superposition of two states--through a cavity containing a microwave field.

Each of the two quantum states shifts the phase of the microwave field by a different amount--so the field also falls into a superposition of two states. However, as the cavity field exchanges energy with its surroundings, the superposition can collapse into a single definite state. The researchers measured this decoherence by measuring correlations between the energy levels of pairs of atoms sent through the cavity with various time delays between the atoms.

The ENS team discovered that decoherence (as measured by the disappearance of evidence of quantum-mechanical interference between the two cavity states) proceeds at a faster rate with time, and also when the differences between the two phase shifts are increased and therefore made more distinguishable from one another. tTe results will be published in the upcoming Physical Review Letters.