Official "Science News" Thread

[cheesehead]

Quantum mechanics just got stranger (again):

"Quantum decision affects results of measurements taken earlier in time"

http://arstechnica.com/science/news/...beforehand.ars

Quantum entanglement is a state where two particles have correlated properties: when you make a measurement on one, it constrains the outcome of the measurement on the second, even if the two particles are widely separated. It's also possible to entangle more than two particles, and even to spread out the entanglements over time, so that a system that was only partly entangled at the start is made fully entangled later on.

This sequential process goes under the clunky name of "delayed-choice entanglement swapping." And, as described in a Nature Physics article by Xiao-song Ma et al., it has a rather counterintuitive consequence. You can take a measurement before the final entanglement takes place, but the measurement's results depend on whether or not you subsequently perform the entanglement.

. . .

The picture certainly looks like future events influence the past, a view any right-minded physicist would reject. The authors conclude with some strong statements about the nature of physical reality that I'm not willing to delve into (the nature of physical reality is a bit above my pay grade).

As always with entanglement, it's important to note that no information is passing between Alice, Bob, and Victor: the settings on the detectors and the BiSA are set independently, and there's no way to communicate faster than the speed of light. Nevertheless, this experiment provides a realization of one of the fundamental paradoxes of quantum mechanics: that measurements taken at different points in space and time appear to affect each other, even though there is no mechanism that allows information to travel between them.

It seems to me that this reverse-causality result (measurement results depend on a decision later in time) was previously known in some different form. Was it?

Can some arrangement of the two-slit experiment provide a way to demonstrate a measurement result that depends on a decision later in time, without communication?

[Dubslow]

Originally Posted by cheesehead

Quantum mechanics just got stranger (again):

"Quantum decision affects results of measurements taken earlier in time"

http://arstechnica.com/science/news/...beforehand.ars

It seems to me that this reverse-causality result (measurement results depend on a decision later in time) was previously known in some different form. Was it?

Can some arrangement of the two-slit experiment provide a way to demonstrate a measurement result that depends on a decision later in time, without communication?

I have a PDF copy of the paper, if anyone's interested.

[only_human]

Originally Posted by cheesehead

It seems to me that this reverse-causality result (measurement results depend on a decision later in time) was previously known in some different form. Was it?

The Wikipedia page on reverse causality seems to be a bit disappointing:
http://en.wikipedia.org/wiki/Retrocausality

I'm sure I have heard of some double experiments involving changing the decision to measure until after the photon has passed through a slit affecting the interference pattern but I didn't find anything briefly looking right now.

Physics World gave a double slit experiment top honors as Breakthrough of the Year in 2011:
Physics World reveals its top 10 breakthroughs for 2011 The secret lives of photons revealed

The experiment reveals, for example, that a photon detected on the right-hand side of the diffraction pattern is more likely to have emerged from the optical fibre on the right than from the optical fibre on the left. While this knowledge is not forbidden by quantum mechanics, Steinberg says that physicists have been taught that "asking where a photon is before it is detected is somehow immoral".
"Little by little, people are asking forbidden questions," says Steinberg, who adds that his team's experiment will "push [physicists] to change how they think about things".

Feynman diagrams have always been able to show things like an an electron emitting a photon causing the electron to recoil in any direction (including backward in time) and then absorb the same photon.

I feel that if any quantum behavior can occur like this there is no reason to exclude entanglement.

[Jeff Gilchrist]

Researchers use GPS data to speed up tsunami warnings
http://www.cbc.ca/news/technology/st...-research.html

[ewmayer]

Originally Posted by cheesehead

Quantum mechanics just got stranger (again):

"Quantum decision affects results of measurements taken earlier in time"

Unless I greatly misunderstand the thrust of the paper, the apparent conundrum is strictly a result of how one defines "measurement". To me "measurement" implies the act of observation - as long as the observer is not aware of the state of the system, no measurement has been made, hence no paradox.

Too lazy/busy to read the whole thing, so flame away if I missed the point.

[cheesehead]

Originally Posted by ewmayer

Unless I greatly misunderstand the thrust of the paper, the apparent conundrum is strictly a result of how one defines "measurement". To me "measurement" implies the act of observation

... and that is also how the paper uses it, according to my non-flaming understanding.

Originally Posted by http://arstechnica.com/science/2012/04/decision-to-entangle-effects-results-of-measurements-taken-beforehand/

. . .

Delayed-choice entanglement swapping consists of the following steps. (I use the same names for the fictional experimenters as in the paper for convenience, but note that they represent acts of measurement, not literal people.)

1. Two independent sources (labeled I and II) produce pairs photons such that their polarization states are entangled. One photon from I goes to Alice, while one photon from II is sent to Bob. The second photon from each source goes to Victor. (I'm not sure why the third party is named "Victor".)
2. Alice and Bob independently perform polarization measurements; no communication passes between them during the experiment—they set the orientation of their polarization filters without knowing what the other is doing.
3. At some time after Alice and Bob perform their measurements, Victor makes a choice (the "delayed choice" in the name). He either allows his two photons from I and II to travel on without doing anything, or he combines them so that their polarization states are entangled. A final measurement determines the polarization state of those two photons.

The results of all four measurements are then compared. If Victor did not entangle his two photons, the photons received by Alice and Bob are uncorrelated with each other: the outcome of their measurements are consistent with random chance. (This is the "entanglement swapping" portion of the name.) If Victor entangled the photons, then Alice and Bob's photons have correlated polarizations—even though they were not part of the same system and never interacted.

. . .

Alice and Bob do make observations. They each know what their own measurement is (and it doesn't change later), but they just don't communicate their measurements to each other or to Victor until after Victor has also made both the delayed choice and the final measurement (which doesn't change later).

(This paragraph is all from my own non-flaming understanding.) There could be quibbles about what "after" means, but all three observers can easily have low enough relative velocity to each other so that all communication can be arranged to exclude unintended overlap. In part 3, Alice and Bob can each send a message to Victor announcing that they've completed their measurements, but not what the measurement was. Victor waits to make the delayed choice until after receiving these notifications. After the final measurement, Victor notifies Alice and Bob that that measurement has been made, but not what the measurement is. Then all three send their measurements to each other and everyone else.

I write all this while still experiencing a slightly unreal feeling of non-flaming amazement about the reverse-causality conclusion.

[ewmayer]

Originally Posted by cheesehead

I write all this while still experiencing a slightly unreal feeling of non-flaming amazement about the reverse-causality conclusion.

I should have been clearer - in this case the causality issue relates to the individual observers exchanging their data - no information exchange constitutes "no definitive measurement". So causality is not violated, but indeed, it is amazing how far entanglement requires one to bend the normal macroscopic rules of causality in order to accommodate the result.

[retina]

Originally Posted by cheesehead

In part 3, Alice and Bob can each send a message to Victor announcing that they've completed their measurements, but not what the measurement was.

Perhaps better if Alice, Bob and Victor all send randomly salted hashes of their results to an independent dropbox client. Then when all three results are in they all send the salts along with the plaintext result and it can all be checked to ensure that no one cheated.

[cheesehead]

Originally Posted by retina

Perhaps better if Alice, Bob and Victor all send randomly salted hashes of their results to an independent dropbox client. Then when all three results are in they all send the salts along with the plaintext result and it can all be checked to ensure that no one cheated.

Good idea, but that would be in addition to what I meant: semaphores for the purpose of notifying each other that the measurement had been made, for timing/synchronizing purposes.

I.e., Alice and Bob tell Victor that they've each made their measurements, so that Victor can know that his choice and measurement both occur after Alice's and Bob's measurements in all of their reference frames.

Or is there a flaw in my attempt to ensure that Victor's actions occur after Alice's and Bob's?

[cheesehead]

Originally Posted by ewmayer

I should have been clearer - in this case the causality issue relates to the individual observers exchanging their data - no information exchange constitutes "no definitive measurement".

Even if retina's suggestion is implemented?

So causality is not violated,

I can see that if there's no information exchange, no one except the observer who made a measurement can know what the measurement will be revealed to be, but need more explanation about whether an independent observer (of messages, not making measurements herself) can/can't eliminate at least some possibilities.

[retina]

Originally Posted by cheesehead

Good idea, but that would be in addition to what I meant: semaphores for the purpose of notifying each other that the measurement had been made, for timing/synchronizing purposes.

I.e., Alice and Bob tell Victor that they've each made their measurements, so that Victor can know that his choice and measurement both occur after Alice's and Bob's measurements in all of their reference frames.

Yes, there would still be the need for synchronisation. But I think the synchronisation would need to be controlled by the dropbox server. We need to ensure that there is no direct path from either Alice or Bob to Victor or each other.