Universal Quantum Random Access Memory: A Data-Independent Unitary Construction

Abstract: We present a construction for Quantum Random Access Memory (QRAM) that achieves a single, data-independent unitary operator. Unlike routing-based approaches or circuit methods that yield data-dependent unitaries, our Universal QRAM encodes data in memory qubits that act as quantum control signals within a block-diagonal permutation structure. The key insight is that memory qubits serve as control signals, enabling coherent lookup when addresses are in superposition. For N addresses with K-bit data words, the construction requires $\log_2 N + K + NK$ qubits and decomposes into exactly $NK$ multi-controlled gates. We verify the construction for $N \in {2, 4, 8, 16}$ and $K \in {1, 2, 3, 4}$, confirming that the resulting unitary is a pure permutation matrix with zero error across all data configurations. This approach simplifies QRAM implementation by separating fixed circuit structure from variable data encoding.