Quantum Computing: On the Not-So-Far Horizon?
Dean
Quoted:
Read the rest here.Delsing and colleagues at Chalmers University began by embedding their Cooper-pair transistor in a resonant circuit. Next, they cooled the device down to millikelvin temperatures and measured how the phase of a radio-frequency signal changed when it was reflected from the circuit. Based on these measurements, the team was able to show that the device behaved like a quantum capacitor. Hakonen and co-workers in Helsinki and Moscow group employed a similar technique. Both teams found that the devices behaved as predicted by theory.
The effect could be used to read out quantum bits (qubits) in a reliable way because the quantum capacitance of the excited state of the qubit has the opposite sign to the ground state. These states could be used as the "1s" and "0s" in a quantum computer. Indeed Hakonen and colleagues have already used this approach to read the value of a qubit without changing its value -- which is almost always a problem when measuring the quantum state of any system.
Quantum computers wont be inside Playstations anytime soon. But this makes them a LOT nearer.
Using a resonant read-out scheme to measure a qubit has also been done before (and is not being reported here anyway). A friend of mine does something similar with a superconducting SQUID, using its variable inductance in a resonant circuit to do the read out (here, I'm mentioned in the acknowledgements). However, this is not the same as measuring the qubit without decohering its state--it does decohere, just slower. A means to measure it without decohering it would be a violation of the no-cloning theorem. And besides, you don't actually need that to make a quantum computer useful. The whole point of the algorithms which we have now is to reduce the superposition down to a single result, which you read out while collapsing the state. That shouldn't matter, as the collapsed state gives you the answer you're looking for. In fact, a read-out that doesn't collapse the state may give you a wrong answer. The only requirement is not to decohere it before you reach the final state. That's why I always argued that these slow, resonant measurements aren't necessary. You can easily do an RSFQ readout on picosecond timescales (for certain meanings of the word easily), capturing the state as it currently stands well before your decoherence time runs out.