Light has been slowed to the speed of a bicycle

In a cold cloud of sodium atoms, a light pulse crawled at 17 metres a second. That is about sixty kilometres an hour. In vacuum it still does 300,000 kilometres a second.

Lene Hau and colleagues cooled a gas of sodium until a large fraction of the atoms sat in one quantum state, then dressed that gas with a control laser so that a second pulse could enter it without being absorbed. Electromagnetically induced transparency is the name. The price of the transparency is a brutal slope in the refractive index, and the group velocity — the speed of the pulse’s envelope — falls with that slope. In the condensate they measured 17 metres a second. The vacuum speed had not changed. The pulse was spending its time as a shared excitation of the atoms and the field, and the atoms are slow.

Seventeen metres a second is a sprint on a bicycle, not a walk, and not a room you could cross while watching a sunbeam. The cloud was a fraction of a millimetre, the pulse was microseconds, and outside the cloud the light was ordinary light again. Later experiments stored the excitation in the atoms and let it go as light a moment afterward. That is not a photon parked on a shelf. It is the information of the pulse, written into the gas and read back. The astonishment is that the writing is possible at all.