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.

分析传统分叉结构和 “桶式队列” 两种随机存取器（RAM）架构，并提出了一些原型实现，显示出在每个 qRAM 调用期间只需要进行 O（n）个二量子比特物理互动，从而减少所需资源，可以构建大型 qRAMs，无需广泛的量子纠错。

量子随机访问存储器的架构设计

Architectures for a quantum random access memory

A random access memory (RAM) uses n bits to randomly address N=2^n distinct
memory cells. A quantum random access memory (qRAM) uses n qubits to address
any quantum superposition of N memory cells. We present an architecture that
exponentially reduces the requirements for a memory call: O(log N) switches
need be thrown instead of the N used in conventional (classical or quantum) RAM
designs. This yields a more robust qRAM algorithm, as it in general requires
entanglement among exponentially less gates, and leads to an exponential
decrease in the power needed for addressing. A quantum optical implementation
is presented.

本文介绍了一种使用 n 个量子位对 N 个内存单元进行随机访问的量子随机访问内存（qRAM）体系结构，其中只需 O（log N）个切换开关即可进行内存调用，因此可以大大减少量子门之间的量子纠缠和降低量子访问所需的功率。