Obtaining ultra-slow light
(a) A cloud of sodium atoms, cooled to nanokelvin temperatures, is trapped by an electromagnet and freely suspended in the centre of a vacuum chamber held at room temperature. The cloud is then illuminated from the side by a "coupling" laser, which creates a new optical medium from entangled states between the cold atoms and the laser field. The optical properties of this coupled system can be controlled dynamically by varying the intensity of the coupling laser. "Probe" laser pulses are then fired through the cloud in the z direction. To determine the speed of these pulses, we use a photomultiplier tube (PMT) to measure the time it takes them to pass through the system. A third, vertical laser beam the "imaging beam" is used to measure the size of the cloud; its shadow (top right) is recorded by a charge-coupled device camera (CCD2). Another camera (CCD1) images the cloud along the z direction (top left).
(b) The three internal quantum states of the sodium atoms, 1, 2 and 3, that are used to slow down light. The coupling laser is tuned to the transition between states 2 and 3, while the probe laser is tuned to the transition between 1 and 3.