— In the fundamentally fuzzy world of quantum mechanics, it can be difficult to keep clear memories, and that could be a problem for future quantum computers.
Now three physicists in Italy and the US have proposed a method for retrieving quantum information from memory that should make total quantum recall more reliable.
Quantum computers have the potential to do some kinds of calculation with unprecedented speed, as small-scale demonstrations have confirmed. However, to perform most of these calculations effectively these machines will eventually need to access something resembling random access memory (RAM) a large store of quantum information that can be selectively accessed.
Ordinary RAM contains a large array of memory cells, each holding one bit of information a binary 0 or 1. To check the contents of particular cell, a computer accesses it using its address a string of bits that identifies the cell's location.
How this works physically is that all the cells are connected up to a branching tree of connections, with switches on each branch. The address bits open and close these switches in a way that leaves just one path along the tree open, connecting to the target memory cell.
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.
"It's a good idea," says quantum computing expert Charles Bennett, of IBM's Watson Research Center in Yorktown Heights, New York, US, although he has reservations.
"I think the advantage of the proposed scheme over conventional addressing is less clear-cut than the authors suggest," Bennett told New Scientist. This is partly because some conventional addressing schemes do not require all switches to be flipped at once, and also because even inactive switches might contribute to interference, he says.