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.

Figure 1

Bifurcation graph of the RAM addressing.

Figure 2

Bifurcation graph of the bucket-brigade architecture. Here the third memory cell is addressed (address register 010).

Figure 3

Basic level structure of the atoms in a possible bucket-brigade implementation. Some extra energy levels needed to implement Raman transitions are not shown.