Thermal light cannot be represented as a statistical mixture of single   pulses
  We ask whether or not thermal light can be represented as a mixture of single broadband coherent pulses. We find that it cannot. Such a mixture is simply not rich enough to mimic thermal light; indeed, it cannot even reproduce the first-order correlation function. We show that it is possible to construct a modified mixture of single coherent pulses that does yield the correct first-order correlation function at equal space points. However, as we then demonstrate, such a mixture cannot reproduce the second-order correlation function. 
Aur\'elia Chenu, Agata M. Bra\'nczyk, Gregory D. Scholes and J. E.   Sipe
quant-ph