Architectures for a quantum random access memory

TL;DR

This paper analyzes two quantum RAM architectures and proposes implementations that significantly reduce the active quantum interactions per memory call, potentially enabling scalable large-scale quantum memory devices.

Contribution

It introduces proof-of-principle implementations of qRAM architectures that require only O(n) active interactions per call, reducing resource demands.

Findings

Only O(n) two-qubit interactions are needed per qRAM call.

Large-scale qRAMs could operate with minimal quantum error correction.

The proposed architectures are feasible with current quantum technology.

Abstract

A random access memory, or RAM, is a device that, when interrogated, returns the content of a memory location in a memory array. A quantum RAM, or qRAM, allows one to access superpositions of memory sites, which may contain either quantum or classical information. RAMs and qRAMs with n-bit addresses can access 2^n memory sites. Any design for a RAM or qRAM then requires O(2^n) two-bit logic gates. At first sight this requirement might seem to make large scale quantum versions of such devices impractical, due to the difficulty of constructing and operating coherent devices with large numbers of quantum logic gates. Here we analyze two different RAM architectures (the conventional fanout and the "bucket brigade") and propose some proof-of-principle implementations which show that in principle only O(n) two-qubit physical interactions need take place during each qRAM call. That is, although a qRAM needs O(2^n) quantum logic gates, only O(n) need to be activated during a memory call. The resulting decrease in resources could give rise to the construction of large qRAMs that could operate without the need for extensive quantum error correction.