Slashdot Mirror
Can Quantum Entanglement Create Faster-Than-Light Communication? (mit.edu)
Slashdot reader StartsWithABang writes: If you were to send a space probe to a distant star system, gather information about it and send it back to Earth, you'd have to wait years for the information to arrive. But if you have an entangled quantum system -- say, two photons, one with spin +1 and one with spin -1 -- you could know the spin of the distant one instantly by measuring the spin of the one in your possession.
This "incredible idea to exploit quantum weirdness" for communication was the subject of a recent Forbes article [which blocks ad-blockers] as well as a NASA mission directorate. ("Entanglement-assisted Communication System for NASA's Deep-Space Missions: Feasibility Test and Conceptual Design".) And Friday MIT News reported a research team is now making progress toward capturing paired electron halves for quantum computing on gold film. "Our first goal is to look for the Majorana fermions, unambiguously detect them, and show this is it. "
This week even 85-year-old Star Trek actor William Shatner cited quantum entanglement in a discussion of Star Trek's transporter technology, arguing that "Although a lot of the concepts in science fiction are absurd to our Newtonian minds, anything is possible because of the new language of quantum physics."
This "incredible idea to exploit quantum weirdness" for communication was the subject of a recent Forbes article [which blocks ad-blockers] as well as a NASA mission directorate. ("Entanglement-assisted Communication System for NASA's Deep-Space Missions: Feasibility Test and Conceptual Design".) And Friday MIT News reported a research team is now making progress toward capturing paired electron halves for quantum computing on gold film. "Our first goal is to look for the Majorana fermions, unambiguously detect them, and show this is it. "
This week even 85-year-old Star Trek actor William Shatner cited quantum entanglement in a discussion of Star Trek's transporter technology, arguing that "Although a lot of the concepts in science fiction are absurd to our Newtonian minds, anything is possible because of the new language of quantum physics."
TLDR: No.
Next story please.
I thought Slashdot had a policy of rejecting Forbes links. Fuck StartsWithABang. Fire EditorDavid. And no, faster than light communication isn't possible through quantum entanglement. The no-communication theorem shows that it's not possible through quantum entanglement. This is a shit article from a shit poster posted by a shit editor.
They already have tested it, and it can't go faster than light. Apparently some people haven't gotten the message.
I used to think this was an option too, but the more I read about it, the more it became obvious that it wouldn't work. This is because, while you would easily and immediately have an influence on the paired quantumdot at Earth, even if you were 10 lightyears away, there is no way to direct or guide to any particular state in front. Meaning, the moment to interact with your entangled electron or photon, it would 'set' its state, but in a random way.
So the information encoded in entanglement is only extractable when you look at correlations between measurements on both the entangled systems. So to access that correlation information, you would need communication anyway, and that communication could not be FTL. If you only look at either system, but not the other, then you need no such communication, but you also can extract no information from the entanglement. This is actually a good thing, because much of science is done by ignoring entanglements, and the reason we get away with that is the information we are ignoring cannot interfere with our interpretation of the results of our experiment.
Suppose we split up two qubits in an entangled |00+|11state, where we've established that Alice is going to measure two overlapping bell curves with their double-slit experiment.Suppose Bob likes wavy interference patterns. The rules of quantum mechanics allow Bob to do, on his qubit, any unitary transformation like |0|112|0+12|112|012|1.
This takes our state to:
14|00+14|01+14|1014|11
Now supposing that Bob measures his qubit as 0 or 1, then Alice must measure either the wavy interference patterns 12|f0(x)+f1(x)|2 or 12|f0(x)f1(x)|2.
Bob can thereby instantaneously change, from a quantum perspective, what the outcomes of Alice's measurement are going to be.
Alice's wavefunction must change instantaneously and might even change retroactively: she may have already measured her qubit before Bob does this unitary transformation and measurement: nevertheless, to satisfy the predictions of quantum mechanics, her measurements must be consistent with Bob's manipulations. But that can't send messages. Because this thing that Bob has done is not directly visible to Alice. That's for a couple of reasons, the first being that this only generates one photon of results on the double-slit screen, which isn't enough to see the pattern! But suppose we measure lots and lots of these qubits to try and see the pattern: then the problem is that Alice doesn't know which ones Bob measured as 0 or which ones Bob measured as 1. Since there was a 50/50 chance of Bob getting either, what Alice sees is therefore:
14|f0(x)+f1(x)|2+14|f0(x)f1(x)|2=12|f0(x)|2+12|f1(x)|2.
Alice therefore still measures two overlapping bell curves, overall!
Where are the interference patterns?! That is very simple: when Bob and Alice compare their measurements in the first case, Bob's 0-measurement can be used to "filter" Alice's patterns into 12|f0(x)|2,
the bell curve of photons which passed through only the first slit, and his 1-measurement filters the results to give 12|f1(x)|2,
Bob's transformation then changes how he can filter Alice's patterns: Alice's overlapping bell curves are now made up of the ones he measured 0
for, which describe one wavy pattern, and the ones he measured 1 for, which describe the other wavy pattern, and they add up into the non-wavy pattern.
--- "To pee or not to pee, that is the question." ---
seriously, please reject all stories with links to forbes from now on.
Anons need not reply. Questions end with a question mark.
You can't tell if the state has changed without measuring it. The first of the entangled pair of particles (one at home, one on your spaceship) to be measured will mean the other will be measured (when it is) in a complementary state. That's all that happens. We're not talking about some particle giving off a photon of light when its partner is measured or anything like that. Also measuring breaks the entanglement. Purposefully changing the state of one of them also breaks the entanglement. So you can't have a bunch of them that you keep on measuring, waiting for one of them to change state. It just doesn't work that way.
Generally, those aren't his entire articles. They're summaries, with links back to Forbes. It's a holding spot so he can double dip on ad revenue.
"Freedom in the USA is not the ability to do what you want. It is the ability to stop others from doing what THEY want"
Someone more able to work in the detail of QM should comment.
Off the top of my head: I guess this is saying that if when they measure in this manner the result comes out in a certain way they know the photon still has an un-collapsed wave function? Presumably if it had a definite state it would be either vertically or horizontally polarised ? I'm still not sure that the wave function of the 'second' particle will actually show locally as having collapsed just because the 'first' particle was measured. It's just that when you perform a full measurement you'll get the complementary value.
I don't think it actually claims it can measure *what* determined state it is in (what kind of polarisation has taken place). Because in that case, you can't but have a collapsed wavefront, since you actually determined the exact state of the particle itself.
What they're saying is that they can determine *whether* or not a qubit has an undetermined or determined state (without saying anything more about the determined state, thus).
They say they can measure whether it's 'set', or whether it's 'not set', without collapsing it. And, following logic, that alone would be enough to impart some information, indeed. Because if one had 100 individual qubits that are supposed to be in an undetermined state, yet when you measure it at Alpha Centauri (without the wave-function being collapsed), and it would turn out some of them were undetermined, but some aren't anymore (because of polarisation of the entangled qubits deliberately done on Earth), you could create a pattern that sends information. Even if there were random fluctuations individually, you could still filter it out statistically.
IF true, it should be possible, in principle, to have FTL communication. Since that is no small matter, and would earn you a nobel-prize, it's strange to see no paper dealing with this, even after 3 years since its publication...
--- "To pee or not to pee, that is the question." ---