  Er$^{3+}$:Y$_2$SiO$_5$ crystal is a promising candidate with a great variety
of its potential applications in quantum information processing and quantum
communications ranging from optical/microwave quantum memories to circuit QED
and microwave-to-optics frequency converters. Some of the above listed
applications require ultra-low temperature environment, i.e., temperatures
$T\lesssim0.1~$K. Most of the experiments with erbium doped crystals have been
so far carried out at temperatures above 1.5~K. Therefore, only little
information is known about Er$^{3+}$:Y$_2$SiO$_5$ coherence properties at
millikelvins. Here, we investigate optical decoherence of
$^{167}$Er:Y$_2$SiO$_5$ crystal by performing 2- and 3-pulse echo experiments
at millikelvin temperature range and at weak and moderate magnetic fields. We
show that the deep freezing of the crystal results in an increase of optical
coherence time by one order of magnitude compared to temperature of 1.5 Kelvin
and magnetic field of $\sim$0.2~T, taken as a reference point. We further
describe the detailed investigation of the decoherence mechanisms in this
regime.
