Quantum Random Access Memory (QRAM)
The hypothetical device that would let a quantum computer query a classical database in superposition. Many famous quantum algorithms quietly assume one exists.
QRAM, quantum random access memory, is the hypothetical device that would let a quantum computer query a classical database in superposition: ask for 'all the addresses at once' and get the contents entangled with the addresses. Many celebrated quantum algorithms, especially in machine learning, quietly assume a QRAM in their opening paragraph.
Building one is brutally hard. The leading 'bucket-brigade' architecture routes queries through a tree of switches that must all stay coherent; errors anywhere poison the result. Experimental demonstrations exist, including recent superconducting implementations of small bucket-brigade QRAMs, but with fidelities and sizes far from useful, and some complexity arguments suggest the energetics may never favor it at scale.
The millikelvin's rule of thumb: when a quantum machine-learning claim sounds too good, check whether a QRAM is hiding in the assumptions.