Scalable Passive QRAM

Abstract

We present a blueprint for implementing the physical QRAM operation in a scalable, passive manner, proving its feasibility in principle. Our construction is passive because for an $n$-qubit address over $N=2^n$ memory bits the energy cost per QRAM query is $O(\log N)$, and to our knowledge it is the first construction to use $\mathrm{polylog}N$ energy. The runtime for each query is $O({\log }^{2}N)$. It is achieved by evolving a pre-manufactured time-independent 4-local Hamiltonian with $O(N)$ terms where every qubit participates in at most 4 terms.