Researchers Control the Flip of Electron Spin

karvind writes "According to PhysOrg, physicists in Europe, California and at Ohio University now have found a way to manipulate the spin of an electron with a jolt of voltage from a battery. In this experiment voltage was applied to Indium Arsenide based quantum dot which flipped the spin of electron inside it and emitted a photon. The scientists were able to manipulate how long it would take for the electron to flip its spin and emit a photon - from one to 20 nanoseconds. This may have possible applications in optoelectronics and quantum cryptography. Results were published in the latest issue of Physics Review Letters"

Not exactly ...

[maxwell demon]

The title of the linked-to article in Physical Review Letters is:
"Voltage Control of the Spin Dynamics of an Exciton in a Semiconductor Quantum Dot"
(Emphasis by be)
Now an exciton [wikipedia.org] is something quite different from an electron [wikipedia.org].

Re:Not exactly ...

[Anonymous Coward]

Technically yes. But in semiconductors you cannot get isolated electron. You excite an electron from the valence band to conduction band and leave behind a hole (sorry for the technical details). The work manipulates this excited electron and flips the spin. PhysOrg is not a site for hardcore physcists.

quantum crypto

[cryptoz]

Cryptography is in a desperate state right now. Virtually every product that needs to include it has in implemented in such a way that it's basically useless. And so quantum crypto is rolling in more and more these days with newer and better discoveries (like the one here) coming out periodically. However, yeah, it's great, w00t, applications for quantum crypto, etc, but that doesn't really mean much. We already have messages that are unbreakable through brute-force. All that needs improvement through our crypto is more secure ways of entering information into computers and sending it without screwing up by keeping the message in RAM, storing it in a temp file, etc.

So sure, the ability to flip the electron spin is great for quantum cryptography itself, but...how does that help anyone, really?

Re:Quantum computing?

[heelios]

Not exactly I am afraid. There are still huge issues to quantum computing. Namely isolation and data retrieval.

A quantum computer (or at least it's processor) needs to be totally shielded to the outside world while it operates as any interraction or mesurement from the outside world will break the theory. Also, at this moment, you cannot retrieve the processed data without interfering, right? So as soon as you get the data from one of the virtual processors working in 'other worlds', the thing breaks and you can't get anything anymore from it. So it's in fact pretty useless I'm afraid.

I don't think we're going to see a quantum computer in the years to come, and much less under our desks. Even if they were invented I believe our governments will keep them away from us as they could be quite mean to encryption.

Re:Quantum computing?

[jfern]

This advance deals with manipulating the spin of a single electron (a single qubit) The hard part of quantum computing is reliably maniuplating two qubits. With single qubit operations and measurements, and a two qubit CNOT, one can perform arbitrary quantum computation.

Re:what happens when the elecron is "entangled" ..

[maxwell demon]

My question now is: are they still connected afterwards!

No. The measurement destroys the entanglement.

Re:very interesting

[Anonymous Coward]

I don't think Einstein wasted his life on arguing against QM. If it was not him, the subtleness of QM wouldn't have been exposed. And the issue raised by him isn't resolved completely. You can refer to J Bell's paper to understand why it is not a trivial problem. Currently QM is accepted because it works and there had been various non-intutive way to explain them (hidden variables, parallel universe etc). And again if you think nothing useful came out of GUT, you are only reading popular science articles and not Phy Rev papers.

Re:As Usual

[whimsy]

PCR [wikipedia.org] is 20 years old and ubiquitous in industry.

Entanglement doesn't work that way

[Asparfame]

By "Pair off two electrons", I presume you mean put them in an entangled state where the spins of the two electrons are correlated? (For example, in the state |up, down> + |down, up>).

In that case, your system won't work. Putting one of the electroncs in this spin-flipping device would destroy the fragile entanglement. In other words, flipping the spin of one would do nothing to the other.

This is how it always is with entanglement -- entangled particles only remain entangled as long as you leave their entangled properties alone. Once you measure or modify the properties of one, the entanglement is ruined.

Re:Entanglement doesn't work that way

[Asparfame]

Proviso: When I said that modifying the properties of one member of the pair ruins the entanglement, that was not completely correct. If you managed to come up with a scheme to flip the spin of one without measuring the spin, then entanglement would be maintained. However, this would still not flip the spin of the other electron -- the entanglement would not have a different character.

Example: You start with the electrons having opposite (but indeterminate) spins, in the entangled state

|down, up> + |up, down>

(normalization constant ignored)

Now you flip the spin of the first electron. This puts you in the entangled state

|up, up> + |down, down>

Entanglement is preserved, however, you have not "flipped the spin" of the second electron. You have changed the sense of the correleation though. But you still haven't transmitted any information. The spin of each individual electron was indeterminate before you meddled, and was after you meddled.

When I said the measuring the relevant property of one of the pairs ruins the entanglement, well, that was still correct. And try as you might, there is no way to transmit classical information without performing a measurment.

Re:but what does "indeterminate" mean?

[maxwell demon]

It means the spin does not yet have a determined value. And this can indeed be checked. There are probability inequalities (the so-called Bell inequalities, named after Bell who found them) which must hold if the result of measurement should be pre-determined for each particle. The laws of quantum mechanics violate those inequalities, and experiments by Aspect have shown that nature obeys quantum mechanics also in this respect (the violation of the Bell inequalities has been measured).

If those measured correlations mean interaction between those systems or not depends on which interpretation of quantum mechanics you prefer. Since there are interpretations where you don't need such an interaction, it's clear that you cannot use it to instantaneously transmit information with this effect (otherwise such interpretations couldn't possibly exist).