BLOCH STATES: NOT FOR ELECTRONS ONLY It is often essential to consider an electron traveling through a solid as being a wave that spreads out through the whole of the solid. The quantum description of this spread-out electron was formulated by Felix Bloch in the 1920s. Physicists have since sought to extend this idea of a "Bloch state" to guest atoms in a crystal, but an atom's mass is so large (and its equivalent wavelength so small) that a Bloch state for an atom has been difficult to observe. Now, physicists from Japan (Ryosuke Kadono, KEK, ryosuke.kadono@kek.jp) have seen clear signs of a Bloch state for a muonium "atom", in effect a light isotope of hydrogen whose proton is replaced by a positively charged muon particle having 1/9 of the proton's mass. Performing experiments at the Rutherford Appleton lab in England, the researchers studied spin-polarized muonium (Mu) atoms in a KCl crystal cooled down to 10 mK. Measuring how long it took the atoms to lose their initial polarization in the presence of an external magnetic field provided information on their energy state and matched the predictions of a Bloch model. Further studies may offer new insights into the energy bands of atoms in crystals. (Kadono et al., Physical Review Letters, 2 August 1999.)