One-atom maser: Non-separable atom pairs
  When a one-atom maser is operated in the standard way -- excited, resonant two-level atoms traverse the resonator at random times -- the emerging atoms are entangled with the cavity field. As a consequence, the results of measurements on different atoms tend to be correlated. We show that truly non-classical correlations can be found between two successive atoms by studying the properties of the reduced state of such an atom pair. In particular, we calculate its degree of separability and find parameter ranges in which it is markedly less than unity. 
Berthold-Georg Englert, Pavel Lougovski, Enrique Solano, Herbert   Walther