English: Animation of a Hahn echo decay. This file does not animate in Wikipedia if it is resized (as of May 2011), due to a bug. Examples of the Hahn echo include the spin echo and the photon echo. The red arrows can be thought of as spins. Applying the first pulse rotates the spins by 90 degrees, producing an equal superposition of spin up and spin down. The spins then "spread out" because each is in a slightly different environment. This spreading out looks like decoherence, but it can be refocused by a second pulse which rotates the spins by 180 degrees. Several simplifications are used in this animation: no decoherence is included until the blue graph appears, when only T2 decoherence is included. Also, each spin experiences perfect pulses during which the environment provides no spreading. Animation made by Gavin W Morley with POV-Ray 3.6 and Photoshop; some rights reserved.