Quantum Random Access Memory Architectures Using 3D Superconducting Cavities

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 cavity-controlled controlled-<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:mrow><a:mstyle mathsize="0.85em"><a:mi>SWAP</a:mi></a:mstyle></a:mrow></a:math> (<e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><e:mrow><e:mstyle mathsize="0.85em"><e:mi>CSWAP</e:mi></e:mstyle></e:mrow></e:math>) operations, while in the second, we utilize the properties of giant-unidirectional emitters (GUEs). For both architectures, we analyze single- 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 postselected 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.