Understanding destruction of nth-order quantum coherence in terms of   multi-path interference
  The classic example of the destruction of interference fringes in a ``which-way'' experiment, caused by an environmental interaction, may be viewed as the destruction of first-order coherence as defined by Glauber many years ago (Glauber). However, the influence of an environment can also destroy the $n$th-order quantum coherence in a quantum system, where this high order coherence is captured. We refer to this phenomenon as the $n$% th-order decoherence. In this paper we show that, just as the first-order coherence can be understood as the interference of the amplitudes for two distinct paths, the higher order coherence may be understood as the interference of multiple amplitudes corresponding to multiple paths. To see this, we introduce the concept of $n$th-order ``multi-particle wave amplitude''. It turns out that the $n$th-order correlation function can be expressed as the square norm of some ``multi-particle wave amplitude'' for the closed system or as the sum of such square norms for the open system. We also examine, as a specific example, how an environment can destroy the second order coherence by eliminating the interference between various multiple paths. 
D.L. Zhou, P. Zhang, and C.P. Sun