  Long-duration quantum memories for photonic qubits are essential components
for achieving long-distance quantum networks and repeaters. The mapping of
optical states onto coherent spin-waves in rare earth ensembles is a
particularly promising approach to quantum storage. However, it remains
challenging to achieve long-duration storage at the quantum level due to
read-out noise caused by the required spin-wave manipulation. In this work, we
apply dynamical decoupling techniques and a small magnetic field to achieve the
storage of six temporal modes for 20, 50 and 100 ms in a
$^{151}$Eu$^{3+}$:Y$_2$SiO$_5$ crystal, based on an atomic frequency comb
memory, where each temporal mode contains around one photon on average. The
quantum coherence of the memory is verified by storing two time-bin qubits for
20 ms, with an average memory output fidelity of $F=(85\pm 2)\%$ for an average
number of photons per qubit of $\mu_\text{in}$ = 0.92$\pm$0.04. The qubit
analysis is done at the read-out of the memory, using a type of composite
adiabatic read-out pulse we developed.
