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

Re:Heisenberg Uncertainty Principle?

[arrrrg]

> Wouldnt this violate the Heisenberg Uncertainty Principle?

Reading a qubit doesn't violate the Uncertainty Principle by itself; if qubits couldn't be read or written, they'd be worthless. The issue you are probably thinking of is that entanglements between qubits will be destroyed by the reading process (and there is no way to "read" such entanglements without destroying the individual qubit values).

Re:Heisenberg Uncertainty Principle?

[arrrrg]

I should clarify: reading the qubit will destroy all quantum effects (superposition as well as entanglement), effectively making the qubits look like ordinary bits (when you open the box, the cat's either dead or alive, not both). However, quantum computers are designed with this in mind; reading the output destroys any quantum properties it may have, but a computation can be repeated many times to get an idea of what uncertainty was present in the output.

Credit where it's due.

[kimmop]

The article isn't totally clear about it but the Finnish university in question is the Helsinki University of Technology (in the city of Espoo) and not the University of Helsinki. These are the largest two universities in Finland and both have Physics departments so the distinction is important.

Re:Heisenberg Uncertainty Principle?

[Anonymous Coward]

The uncertainty principle just states that you cannot know both
the position and momentum of a particle at the same time ... in
other words you can know both but not precisely, if you know one
precisely you do not know the other ... because you have interacted
with it.

If you know that the particle is in a certain band, you do not need
to know its location ... that IS its location ... or it is essentially
trapped .... you do not care.

If you have your cow in a pasture, you do not care where it is, as
long as it is eating grass or hay in the pasture and how not escaped. ... best I can do.

Re:Unchanged State

[quoll]

The article seems misleading in its wording. It says "read the value of a qubit without changing its value." This can't mean that it doesn't change the original quantum value, as this makes no sense. Reading a quantum value (a qubit) collapses the probability to the value read, by definition. This means that the value is no longer quantum. The original probability function cannot be read (though it can be calculated).

The statement without changing its value must refer to reading the value reliably. When reading the state of an individual subatomic particle it is extremely easy to have the result perturbed by noise. Given that there is a probability of reading an alternative value, then it is not normally possible to tell when the wrong value was read. It appears that this makes the process much more reliable.

IAAQP (I Am A Quantum Physicist). Though I could still learn a thing or two about subatomic physics.

Re:spooky action at a distance

[gauge boson]

presumably, given entanglement [wikipedia.org], measurement of qbit state allows potentially for instant communication ?

No, it doesn't. The closest you can come is instant synchronization of states, but you don't get to choose what state that is. For example, you can have two particles entangled to have the same (or opposite, as in the EPR thought experiment) spin orientation, but you can't send a signal from one to the other by choosing the orientation. Instead, it's random whether each one is spin up or spin down - the only guarantee is the relationship between the measurements. This would be great for things like cryptographic key exchange, since you can't have a man-in-the-middle attack if there is no middle, but it's useless for sending information. See: The No-Communication Theorem (warning: requires crazy math skills to avoid the MEGO effect)

nothing can travel faster than light.

I call bullshit. Relativity prohibits* local superluminal motion; non-locally, it's fair game. See, for example, the Alcubierre Warp Drive - the only question of whether it's possible or not (aside from new physics) rests on whether there's any local superluminal energy propagation at the edge of the spacetime bubble. Plus, QM allows for lots more in the way of non-local effects (even if you assume hidden variables, since Bell's Theorem rules out local hidden variables based on current experimental results), though, as I noted above, it still doesn't allow for superluminal communication (or teleportation, for that matter).

* Minor caveat: this is not counting tachyons, since nobody knows if they exist.

Helsinki U. of Tech., not Helsinki U.

[msmikkol]

Just a minor correction to the linked article: Mika Sillanpää worked at the Helsinki University of Technology, not at the Helsinki University when he wrote the paper in question.

Because it isn't an insulator, of course

[Flying pig]

The description of the Josephson Junction is aimed at all the non-physicists out there. The "insulating layer" is a bandgap layer. The point is that cooper paired electrons can tunnel through it, i.e. it acts as a superconductor itself. It is an insulator for ordinary electrons only. And the definition of capacitor is nothing at all to do with physical conductors or insulators. It is a region of space where a potential gradient can be created, and the capacitance is the measure of how much energy has to be pumped into the region in order to create a given potential gradient. "Empty space" requires the lowest energy and has the lowest capacitance per unit volume, while certain ceramics with relatively mobile but limited electrons have very high values. If you cannot create a potential difference across your region of space, you have no capacitance - and at first sight, if that region is superconducting you cannot have a potential difference.