玻色-爱因斯坦凝聚.ppt

Quantum Statistics Predicted 1924……Created 1995 The Nobel Prize in Physics 2001 What is an atom laser? An atom laser is analogous to an optical laser, but it emits matter waves instead of electromagnetic waves. Its output is a coherent matter wave, a beam of atoms which can be focused to a pinpoint or can be collimated to travel large distances without spreading. The beam is coherent, which means, for instance, that atom laser beams can interfere with each other. Compared to an ordinary beam of atoms, the beam of an atom laser is extremely bright. One can describe laser-like atoms as atoms marching in lockstep. Although there is no rigorous definition for the atom laser (or, for that matter, an optical laser), all people agree that brightness and coherence are the essential features. Atomic trap ATOMIC TRAP cools by means of two different mechanisms. First, six laser beams (red) cool atoms, initially at room temperature, while corralling them toward the cen-ter of an evacuated glass box. Next, the laser beams are turned off, and the magnetic coils (copper) are ener-gized. Current flowing through the coils generates a magnetic field that further confines most of the atoms while allowing the energetic ones to escape. Thus, the average energy of the remaining atoms decreases, mak-ing the sample colder and even more closely confined to the center of the trap. Ultimately, many of the atoms attain the lowest possible energy state allowed by quan-tum mechanics and become a single entity known as a Bose-Einstein condensate. The parts of an atom laser A laser requires a cavity (resonator), an active medium, and an output coupler. In the MIT atom laser, the resonator is a magnetic trap in which the atoms are confined by magnetic mirrors. The active medium is a thermal cloud of ultracold atoms, and the output coupler is an rf pulse which controls the reflectivity of the magnetic mirrors. The gain process in an atom laser The analogy to spontaneous emission in the o