  Quantum random access memory (QRAM) is a common architecture resource for
algorithms with many proposed applications, including quantum chemistry,
windowed quantum arithmetic, unstructured search, machine learning, and quantum
cryptography. Here we propose two bucket-brigade QRAM architectures based on
high-coherence superconducting resonators, which differ in their realizations
of the conditional-routing operations. In the first, we directly construct
controlled-$\mathsf{SWAP}$ ($\textsf{CSWAP}$) operations, while in the second
we utilize the properties of giant-unidirectional emitters (GUEs). For both
architectures we analyze single-rail and dual-rail implementations of a bosonic
qubit. In the single-rail encoding we can detect first-order ancilla errors,
while the dual-rail encoding additionally allows for the detection of photon
losses. For parameter regimes of interest the post-selected infidelity of a
QRAM query in a dual-rail architecture is nearly an order of magnitude below
that of a corresponding query in a single-rail architecture. These findings
suggest that dual-rail encodings are particularly attractive as architectures
for QRAM devices in the era before fault tolerance.
