Can thermal light be represented by a mixture of single pulses? It cannot; only a modified mixture can yield the correct first-order correlation function at equal space-points. Still, this fails to reproduce higher orders.
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.
The relationship between thermal light and coherent pulses is of fundamental interest, and is also central to relating coherent optical experiments on photophysical processes to the natural occurrence of those processes in sunlight. We now know that thermal light cannot be represented as a statistical mixture of single pulses. In this paper we ask whether or not thermal light can be represented as a statistical mixture of sets of pulses. We consider thermal light in a one-dimensional waveguide, and find a convex decomposition into products of orthonormal coherent states of localized, nonmonochromatic modes.
We investigate the connection between thermal light and coherent pulses, constructing mixtures of single pulses that yield the same first-order, equal-space-point correlation function as thermal light. We present mixtures involving (i) pulses with a Gaussian line shape and narrow bandwidths and (ii) pulses with a coherence time that matches that of thermal light. We characterize the properties of the mixtures and pulses. Our results introduce an alternative description of thermal light in terms of multifrequency coherent states, valid for the description of broadband linear light-matter interactions. We anticipate our results will be relevant to time-resolved measurements that aim to probe the dynamics of systems as if they were excited by natural thermal light.