  An N-bit quantum state requires a vector of length $2^N$, leading to an
exponential increase in the required memory with N in conventional
statevector-based quantum simulators. A proposed solution to this issue is the
decision diagram-based quantum simulator, which can significantly decrease the
necessary memory and is expected to operate faster for specific quantum
circuits. However, decision diagram-based quantum simulators are not easily
parallelizable because data must be manipulated dynamically, and most
implementations run on one thread. This paper introduces ring
communication-based optimal parallelization and automatic swap insertion
techniques for multi-node implementation of decision diagram-based quantum
simulators. The ring communication approach is designed so that each node
communicates with its neighboring nodes, which can facilitate faster and more
parallel communication than broadcasting where one node needs to communicate
with all nodes simultaneously. The automatic swap insertion method, an approach
to minimize inter-node communication, has been employed in existing multi-node
state vector-based simulators, but this paper proposes two methods specifically
designed for decision diagram-based quantum simulators. These techniques were
implemented and evaluated using the Shor algorithm and random circuits with up
to 38 qubits using a maximum of 256 nodes. The experimental results have
revealed that multi-node implementation can reduce run-time by up to 26 times.
For example, Shor circuits that need 38 qubits can finish simulation in 147
seconds. Additionally, it was shown that ring communication has a higher
speed-up effect than broadcast communication, and the importance of selecting
the appropriate automatic swap insertion method was revealed.
