Breakthrough for Quantum Measurement

said_captain_said_wo writes to tell us that PhysicsWeb is reporting that two teams of physicists have developed a new method for measuring the state of quantum bits in a quantum computer without disturbing the state. From the article: "In the future, the Josephson capacitance could be used for operations in a large-scale quantum computer," says Mika Sillanpaa of Helsinki University. "The Josephson inductance and Josephson capacitance together would also allow us to build new types of quantum 'band engineered' electronic devices, such as low-noise parametric amplifiers."

Comments

Implications in reverse order

[Flying pig]

Talk about looking for grant funding! Problem is, scientific illiterates in Government etc. think they understand what a quantum computer can do (application a long way in the future if at all) but not what you can do with very low noise parametric amplifiers (which might be relatively near term applications.) In terms of exciting progress in studies of brain function, small scale biochemistry, remote sensing and signal processing, very low noise amplifiers are critical components, whereas quantum computers don't yet exist, and by the time they do conventional computers should be adequate to deal with a lot of the data processing.

Not to knock the discovery, which is very interesting, but it's a pity quantum computers have to be dragged into everything to justify research. I doubt that Tom's Hardware will be reviewing millikelvin coolers for your qubit box any time in the next 20 years (though I'd like to be proved wrong)

Physicist in the House

[anomalousman]

They can do what they say, but it's a lot more trivial than measuring the entire quantum state of the system, which is, as others have suggested above, impossible.

The Heisenberg Unccertainty principle implies that measuring a quantity must add noise in the conjugate quantity. For example, measuring the momentum of an object spreads out the wavefunction. Another example, measuring the state of a qubit (whether it is a zero or a one) destroys the relative phase between the zero and the one.

So the "non-destructive" measurement they are talking about means that they aren't changing it from a zero to a one or vice-versa. But they are (and must) destroy the information about the phase of the qubit state during the measurement. For a more in-depth discussion, look up "quantum nondemolition measurements".