  Here we present a model for a small system combined with an explicit entropy
bath that is comparably small. The dynamics of the model is defined by a simple
matrix, M. Each row of M corresponds to a macrostate of the system, e.g. net
alignment, while the elements in the row represent microstates. The constant
number of elements in each row ensures constant entropy, which allows
reversible fluctuations, similar to information theory where a constant number
of bits allows reversible computations. Many elements in M come from the
microstates of the system, but many others come from the bath. Bypassing the
bath states yields fluctuations that exhibit standard white noise; whereas with
bath states the power spectral density varies as S(f)~1/f over a wide range of
frequencies, f. Thus, the explicit entropy bath is the mechanism of 1/f noise
in this model. Both forms of the model match Crooks' fluctuation theorem
exactly, indicating that the theorem applies not only to infinite reservoirs,
but also to finite-sized baths. The model is used to analyze measurements of
1/f-like noise from a sub-micron tunnel junction.
