Quantum Random Access Memory Architectures Using 3D Superconducting Cavities

Abstract

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-SWAP (CSWAP) 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. Published by the American Physical Society 2024