IS A QUANTUM STANDING WAVE COMPOSED OF 2 TRAVELING WAVES

We compare the scattering of an atom by two different quantized standing-wave configurations. The first one is established in a cavity by a pair of fixed mirrors. The other consists of two independent counterpropagating traveling waves, as could occur in a ring configuration. We show that in the quantum regime (of small photon numbers) atoms are scattered differently by a true standing wave than by a superposition of two counterpropagating waves of equal amplitudes and opposite directions. This behavior is a manifestation of momentum conservation. In the case of traveling waves each wave depletes its momentum independently, whereas the standing wave that is fixed in space acts as a potentially infinite sink or source for momentum.