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Quantum Experiment Shows Effect Before Cause
steveb3210 writes "Physicists have demonstrated that making a decision about whether or not to entangle two photons can be made after you've already measured the states of the photons."
Here's the article's description of the experiment: 'Two independent sources (labeled I and II) produce pairs of 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. 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. 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. ... Ma et al. found to a high degree of confidence that when Victor selected entanglement, Alice and Bob found correlated photon polarizations. This didn't happen when Victor left the photons alone.'
Nevermind -- why bother telling you if you already know :-(
http://en.wikipedia.org/wiki/Thiotimoline
"To those who are overly cautious, everything is impossible. "
*Looks at physics degree.*
*Tosses it in the trash.*
Victor should decide not to entangle the photons whenever Alice and Bob's polarizations are correlated. That'll rip physics a new one...
AIUI, the notion that information can't be transferred faster than the speed of light is based on the fact that it would violate causality. I have wondered whether causality is an assumption rather than an actual property of the universe.
If it is (I'm not qualified to interpret this experiment), we'll have a lot of new physics coming down the pike over the next few decades.
Sheesh, evil *and* a jerk. -- Jade
The summary doesn't say what the time delay is between when Alice and bob measure their polarization and when victor makes his choice.
FTFA:
Due to the 104-meter fiber-optic cable, Victor's measurements occurred at least 14 billionths of a second after those of Alice and Bob
Victor is Bipolarized making him erratic and unpredictable. Might want to try adding lithium atoms into the mix and see if the results stabilize.
1. PROFIT!!!
2. ???
3. Collide some photons!
The argument goes like this:
`I refuse to prove that I exist,' says God, `for proof denies faith, and without faith I am nothing.'
`But,' says Man, `The Babel fish is a dead giveaway, isn't it? It could not have evolved by chance. It proves you exist, and so therefore, by your own arguments, you don't. QED.'
`Oh dear,' says God, `I hadn't thought of that,' and promptly disappears in a puff of logic.
`Oh, that was easy,' says Man, and for an encore goes on to prove that black is white and gets himself killed on the next zebra crossing.
I really which quantum people would stop acting like they know what they are talking about.
This is just a really shitty description/way of looking at a series of events and is more or less wrong in the same way that saying your traveling back in time by looking at old stars in the sky from far off distances.
The only thing out of order here is the observers note taking and logic. Due various other quantum flux it may appear to happen in a certain order even though it didn't and its just a matter of appearance due to propagation effects.
Its a bad observation and bad description of that observation, not a causality violation.
Or at least try...
So a key part of the experiment was that the pair of photons sent to "Victor" went through a 104 meter cable to ensure that whatever Victor did, Alice and Bob measured their polarizations first.
Presumably, one could extend this cable to increase the amount of time between Alice and Bob's measurement and Victor's decision to entangle or not.
Presumably long enough for Alice and Bob to send the result of their measurement to Victor.
And then instead of an RNG, Victor chooses to entangle based on whatever would contradict Alice and Bob's measurement.
Come on, we have to try...
P.S. the paper says they aren't violating causality, and it only looks like they are if you're looking at it wrong.
The enemies of Democracy are
The experiment in the article is ... awesome. Though if history is any indication, hoards of raving Slashdoters will try their damnedest to force this into a classical mechanistic world-view.
So here's a fun experiment you can do at home! (Craftsmanship is important for good results.)
1) Start by setting up up a classic double-slit experiment. A laser pointer and some household junk is all you need.
* Observe the interference pattern.
2) Stop denying that you went to see "Avatar" 36 times and grab a couple pairs of 3D movie glasses.
2a) Alternately, you can just buy a polarizing filter sheet. (this is the better way)
3) Being careful to note orientation of the filter, place the filters in front of the slits with one oriented 90 degree to the other. (This is only tricky because the distance between the two slits is so small.)
* What happened to the interference pattern? You "tagged" the individual photons so that you could, in principle, know which slit they passed through, so instead of going through both, they went through just one.
4) Place a third sheet of polaroid between the slits and the detector screen, oriented half-way between the two other filters (if one sheet is vertically oriented and the other horizontally, this sheet will be oriented at 45 degrees)
* The interference pattern is back? WTF? You took the tag away, so that you couldn't know which slit a photon passed through. You "erased" the which-path information so each photon went through both slits, instead of just one of them.
Do the experiment. Accept that the physical world is weird as shit. Shut-up and calculate.
Required reading for internet skeptics
FTFA:
They probably hired the cable guy that got fired from CERN a few months ago.
What this article is saying, is that victor's decision to entangle his photons has a direct effect on the results that alice and bob get from their double blind measurements.
So, either there is retrograde communication on time's axis, or....
The decision that victor makes is predetermined, by the act of measurement undergone by alice and bob. (Meaning victor doesn't really have as much free will as he thinks he does.)
Proposed followup experiment:
Alice and bob examine their photons, tell each other, but not victor. Victor decides to entangle or not entangle. Examine new correlation.
This will test "does a correlation between alice and bob indicate that victor will entangle?".
If it does, you have a reasonably strong test case for many worlds.
First of all, quantum effects like this don't allow the passage of information (no quantum entanglement effect does, it would violate relativity). Alice and Bob don't know if their photons are entangled simply by examining them. As a rule, quantum effects are worthless for transmitting information of any kind: both parties know what the other's state is if they know the photon's were entangled, but that is insufficient to transmit any kind of information (it is very useful for encrypting information, but not transmitting it), so you cannot build a useful transistor system using this.
Secondly, the Ars article rightly points out that concluding that effect proceeded cause should be rejected without much much better evidence. I can't explain the results, but throwing out causality so rapidly would be foolish.
One thought I had was that the detectors might actually be in a quantum state (basically, entangled with the photon they observe) after making their observation, which isn't collapsed into an entangled (or not) state with the other photon until Victor makes his decision. In other words, these results might not show up if you increase the timescale, because the quantum state of the detectors after they sense the photons (which, if it lasts long enough, can be affected by Victor after they detect the photon polarization without violating causality) might collapse before he decides to entangle the photons or not. I am, of course, not a quantum physicist, so that might not be possible.
"None can love freedom heartily, but good men; the rest love not freedom, but license." --John Milton
They want sub-millisecond latency on high-frequency transactions? We'll give them negative latency! Let's see what they do then!
Article says! It's on the order of 14billionths of a second.
When you say it like that, it sounds small, but if I did my math right, 14billionths of a second is the same amount of time as 28 clock cycles on a 2GHz processor.
I have a pretty bad grasp/understanding of this stuff, but if two atoms are entangled, changing the state in one affects the other, right?
No. All that happens is that when the particles are entangled they will have a correlated state when measured. e.g. if one has positive spin the other will have negative. Measuring -- or changing -- the state breaks the entanglement, so you can't simply use it like an FTL telegraph.
Besides, they are working on this now, so it hardly seems futile?
They are not working on FTL communication. The "quantum communication" they are talking about is like the GP said, in a sense a new form of encryption. You can't use entanglement to communicate FTL. However you can use it to determine if your communications have been intercepted -- due to the property that measuring the entangled particles breaks the entanglement. This is awesome because it means you could transmit a shared encryption key, and detect if anyone snooped it, and either send a new one if it was, or use the shared key if it wasn't.
The enemies of Democracy are
Not exactly. Let me explain: when you observe a property of one of an entangled pair of objects, you automatically know the state of the other. This isn't exactly a problem, until you add Heisenbergs uncertainty principle, which states that the more you know about one property of an object, the less you can know about another (position and velocity of an electron being the classic example, but for entangled objects a better example is spin and velocity).
If observing the spin of one entangled electron lets you know the spin of both (but changes the speed only of the first, since you only observed that electron), then you logically should be able to observe the speed of the other entangled electron (which would alter it's spin... but you already know that) and know both spin and speed of both electrons precisely. This violates the uncertainty principle, so instead what happens is observing the spin of the first electron causes both electrons to change in speed, but they do so randomly: in other words, you can change one of an entangled pair by observing the other, but you cannot do so in a controlled fashion. Again, to do otherwise would be to allow one to know both spin and speed of the electron, which is impossible.
Similar logic holds true for entangled photons: observing one changes the other, but not in a controlled fashion. However, both parties can know the polarity of the other's photon (if they are entangled) just fine, which allows them to share certain secret information, which is why quantum networks are theoretically 100% secure. Anyone trying to eavesdrop will actually change the state of the photons by doing so, which can be detected. The details are, obviously, somewhat complex.
"None can love freedom heartily, but good men; the rest love not freedom, but license." --John Milton
You'll end up with two subprime mortgage contracts before you even have a house to lose.
Assuming you know enough information to determine that a particle has been disentangled (and I think that this is the case), then you have faster-than-light transmission of information.
Nope. The only way you'd know that the particles had been disentangled is when the person on Mars sent you, via normal communication channels, the information they had measured and you saw that it was not correlated with what you had measured.
That's what was going on in this experiment -- Alice and Bob could not tell just by looking at their individual particles whether or not they were entangled. Even comparing their measurements doesn't tell them, since they could have gotten the same results as they would have in the case of entanglement through chance alone. Only when Victor told them which particles were entangled could they sort their data sets into entangled and non- and see that in fact the entangled set showed the expected correlation.
BTW, this is at a high level how Quantum Encryption works -- along with regular data, you send information about your entangled particle. If the information was snooped, then the entanglement is broken, and what you measure will have no correlation with the measurements you were sent. That's the only way to tell. You can't just look at the particle and say "yep, it's entangled".
The enemies of Democracy are
No problem, I had to have it explained to me once too. They say newborns have an intuitive understanding of some basic physics, but nobody is born understanding quantum mechanics.
Frankly I don't think anyone dies understanding quantum mechanics. :)
The enemies of Democracy are
So, are they working on something that makes light travel a long distance and/or go slower before making that "decision", thus achieving a substantial delay that could actually be used for "time travelling information"?
Under the simple interpretation, nothing "goes back in time." It's essentially two Schrodinger's cats (A & B) being in a superimposed state for several nano-seconds. Then V adds a constraint, and eventually the A, B, and V information bubbles interact and collapse into an observed state that the scientists record.
The meta-computer that runs our universe probably printed a log message like 'ATOMIC MERGE-OP unexpected long delay on eval: d=7m, t=23ns.' If scientists persist in this sort of research, the person running this universe will probably just ^C the app.