  Phononic quantum networks feature distinct advantages over photonic networks
for on-chip quantum communications, providing a promising platform for
developing quantum computers with robust solid-state spin qubits. Large
mechanical networks including one-dimensional chains of trapped ions, however,
have inherent and well-known scaling problems. In addition, chiral phononic
processes, which are necessary for conventional phononic quantum networks, are
difficult to implement in a solid-state system. To overcome these seemingly
unsolvable obstacles, we have developed a new network architecture that breaks
a large mechanical network into small and closed mechanical subsystems. This
architecture is implemented in a diamond phononic nanostructure featuring
alternating phononic crystal waveguides with specially-designed bandgaps. The
implementation also includes nanomechanical resonators coupled to color centers
through phonon-assisted transitions as well as quantum state transfer protocols
that can be robust against the thermal environment.
