A quantum computer uses not bits but qubits, which can be a blend of 0 and 1 – a quantum superposition of the two states. Therefore, in quantum RAM, the address qubits would not identify a single memory cell but a certain superposition of all possible memory cells.
However, if a quantum-computer designer were to copy how classical RAM is accessed, they would hit a problem, according to a paper posted online by Vittorio Giovannetti of the Scuola Normale Superiore in Pisa, Italy, and colleagues.
This is because an address qubit would control a lot of switches simultaneously at each level of the RAM memory tree. With so many quantum systems linked together, or “entangled”, they would become highly susceptible to interference from the environment. Their delicate quantum states would get scrambled, and the information would be lost before it can be retrieved.
A quantum computer built this way would come up blank every time it tried to retrieve something from its memory.
Giovannetti’s idea is to send the address down the branching tree of connections in such a way that it only affects one switch at a time.
The first address qubit sets a switch at the first branching point to go one way or the other; the second qubit is sent that way and sets the switch at the next branching point, and so on. The total number of entangled quantum systems is smaller, and they are not so susceptible to interference, allowing information to be retrieved from memory intact.