Quantum physics mimics spooky action into the past Author: University of Vienna

[xchrom]

http://atomiumculture.eu/content/quantum-physics-mimics-spooky-action-past

Physicists of the group of Prof. Anton Zeilinger at the Institute for Quantum Optics and Quantum Information (IQOQI), the University of Vienna, and the Vienna Center for Quantum Science and Technology (VCQ) have, for the first time, demonstrated in an experiment that the decision whether two particles were in an entangled or in a separable quantum state can be made even after these particles have been measured and may no longer exist. Their results will be published this week in the journal "Nature Physics".

Entangled States
According to the Austrian physicist Erwin Schrödinger, entanglement is the characteristic trait of quantum mechanics. In addition to its crucial role for the foundations of physics, entanglement is also a key resource for upcoming quantum information technologies such as quantum cryptography and quantum computation. Entangled particles exhibit correlations which are stronger and more intricate than those allowed by the laws of classical physics. If two particles are in an entangled quantum state, they have perfectly defined joint properties at the expense of losing their individual properties. This is like having two dice which have no orientation until they are subject to measurement, upon which they certainly show the same (random) side up. In contrast, so-called separable quantum states allow for a classical description, because every particle has well-defined properties on its own. Two dice, each one of them with its own well-defined orientation, are in a separable state. Now, one would think that at least the nature of the quantum state must be an objective fact of reality. Either the dice are entangled or not. Zeilinger's team has now demonstrated in an experiment that this is not always the case.

Exciting realization of a "Gedankenexperiment"
The authors experimentally realized a "Gedankenexperiment" called "delayed-choice entanglement swapping", formulated by Asher Peres in the year 2000. Two pairs of entangled photons are produced, and one photon from each pair is sent to a party called Victor. Of the two remaining photons, one photon is sent to the party Alice and one is sent to the party Bob. Victor can now choose between two kinds of measurements. If he decides to measure his two photons in a way such that they are forced to be in an entangled state, then also Alice's and Bob's photon pair becomes entangled. If Victor chooses to measure his particles individually, Alice's and Bob's photon pair ends up in a separable state. Modern quantum optics technology allowed the team to delay Victor's choice and measurement with respect to the measurements which Alice and Bob perform on their photons. "We found that whether Alice's and Bob's photons are entangled and show quantum correlations or are separable and show classical correlations can be decided after they have been measured", explains Xiao-song Ma, lead author of the study.

According to the famous words of Albert Einstein, the effects of quantum entanglement appear as "spooky action at a distance". The recent experiment has gone one remarkable step further. "Within a naïve classical word view, quantum mechanics can even mimic an influence of future actions on past events", says Anton Zeilinger.


*** that's the whole article.

[bananas]

1. The universe uses lazy evaluation

http://en.wikipedia.org/wiki/Lazy_evaluation

[Jim__]

2. Do I understand this correctly?

Two different pairs of entangled photons are created: a and a' - entangled with each other, and b and b' entangled with each other but not a and a'. A pair of photons a and b are sent to Victor. Then a' is sent to Alice and b' is sent to Bob.

Alice and Bob make measurements against their photons a' and b'. After Alice and Bob make their measurement, Victor makes his measurement; and whether or not Alice and Bob got measurements indicating that the photons are entangled depends upon how Bob will measures his pair of photons in the future.

This would seem to indicate that if a pair of photons are ever to be entangled, then even if we measure them before they have undergone the entangling event, they will measure as if entangled. Is that what this experiment indicates?

[xchrom]

3. I know next to nothing... But that's how I understood it. Nt

[Jim__]

5. Thanks. Strange stuff.

[bananas]

4. Ok, that's more like predictive branching

Kind of cool that quantum computers will have all this stuff built in automatically.
http://en.wikipedia.org/wiki/Branch_predictor

[Jim__]

6. That sounds like statistical prediction.

I get the impression that what they are talking about here - although the article isn't clear on the details - is more than statistical prediction.

[bananas]

7. I meant the rollback and do-over when the wrong prediction is made. nt

[DireStrike]

8. To rail against fate

What if: You repeat the experiment until a' and b' show up as unentangled. After that, you entangle a and b.

If the entanglement is ignoring time, it should be impossible to measure a' and b' as unentangled in the first place. That would also imply that every a and b would have to have been, or become entangled at some point in time, based solely on the intentions of the experimenters!

Either that, or it would become impossible to entangle a and b after a' and b' had been measured as not entangled.

Then, there's this: ""Within a naïve classical word view, quantum mechanics can even mimic an influence of future actions on past events", says Anton Zeilinger."

I suspect that I don't understand the experiment after all.

[Jim__]

9. The article says that this is an experiment proposed by Asher Peres.

So, I googled to see if I could find any more information on it. The one thing that may be pertinent is that Peres said in quantum mechanics interpretation is unnecessary. That doesn't tell us how to interpret this; but it seems to be telling us not to try.

from wikipedia:

...

As classical physics and non-mathematical language cannot match the precision of quantum mechanics mathematics, anything said outside the mathematical formulation is necessarily limited in accuracy.[citation needed]

Also, the precise ontological status of each interpretation remains a matter of philosophical argument. In other words, if we interpret the formal structure X of quantum mechanics by means of a structure Y (via a mathematical equivalence of the two structures), what is the status of Y? This is the old question of saving the phenomena, in a new guise.

Some physicists, for example Asher Peres and Chris Fuchs, argue that an interpretation is nothing more than a formal equivalence between sets of rules for operating on experimental data, thereby implying that the whole exercise of interpretation is unnecessary.

...

Several theories have been proposed which modify the equations of quantum mechanics to be symmetric with respect to time reversal.[32][33][34] This creates retrocausality: events in the future can affect ones in the past, exactly as events in the past can affect ones in the future. In these theories, a single measurement cannot fully determine the state of a system (making them a type of hidden variables theory), but given two measurements performed at different times, it is possible to calculate the exact state of the system at all intermediate times. The collapse of the wavefunction is therefore not a physical change to the system, just a change in our knowledge of it due to the second measurement. Similarly, they explain entanglement as not being a true physical state but just an illusion created by ignoring retrocausality. The point where two particles appear to "become entangled" is simply a point where each particle is being influenced by events that occur to the other particle in the future.

...

[bananas]

10. Are you familiar with the delayed choice quantum eraser?

http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser