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On May 27 2014 20:37 urboss wrote:Thanks for your answers, it shows me that there is a lot of stuff one has to consider. Show nested quote +All the randomness that people associate with quantum mechanics stems from uncertainty about outcomes in highly isolated systems. Once systems come in contact with other systems, they tend to get disturbed. This disturbance often equates to a "measurement" which collapses all the involved wave functions - and thus there is no longer any randomness left in the systems.
This stuff confuses me. Is "measurement" something tangible that can itself be represented by a wave function? Also, you say that "measurement" collapses all the wave functions in real systems. Does that mean that quantum physics only applies to a highly isolated system? Or in other words, does it mean that quantum physics cannot be used to explain the real world?
First I want to address quantum decoherence. I don't imagine it's easy to investigate as it's based very heavily on the mathematical framework used to study quantum mechanics. It's a combination of ideas from that framework, more or less, and that makes it a fairly abstract kind of concept. Essentially, though, quantum decoherence is the concept I'm referring to when I'm saying that a whole of lot of quantum systems all next to each other and all interacting will quickly start to look like a classical system.
Your questions about measurements are very on point. The fact is: we don't know what exactly constitutes a measurement. In fact, we've kind of defined it in the reverse way in that a measurement is basically "whatever collapses the wave function." 'Measurement' is a nice word for scientists to use because we need to measure things to get our bearings and to make sense of everything. It turns out that doing this (recovering information from a system) always seems to collapse the wave function. So observing a system (getting a look at its insides) always collapses it. But that verb "observe" is very mischievous and gets a lot of journalists and laymen into trouble. A rock can be an observer in that it can disturb and collapse a quantum system - it just can't relay back to us any useful information.
The rough picture we have at the moment is that anything that reacts in a tangible sort of way with a quantum system will collapse that system. Things bumping into it. Things exerting strong magnetic/electric potentials on it. But specifically it's hard to say.
Consider a single atom (a collection of protons, neutrons, and electrons). The usual picture we have of atoms is that the protons and neutrons are more or less in the "middle" where they are, and the electrons exist in a superposition where their locations and paths aren't fixed until they're "measured." In a chemically and electrically neutral environment, there are (in general) not any strong external influences on atoms. Put this atom inside the brain. It's kind of "next to" a whole bunch of other atoms. But if they're chemically neutral (i.e. none of them desperately want another electron or are trying to rid themselves of an extra) and the environment is electrically neutral "enough" then maybe their quantum-like traits are preserved for some time until the system is sufficiently disturbed.
But it's difficult to actually know for sure one way or the other. My personal instinct (i.e. I couldn't convince anyone it's true) is that atoms in our brain often have their electrons in a quantum state, if only for very small amounts of time. But the fact that we don't know exactly what it is that constitutes a measurement makes it difficult to say either way. What kind of interactions and what strength of interactions can disturb a quantum system so that it collapses the wave function? That's a question for which we'd all very much like a definitive answer.
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On May 27 2014 22:16 urboss wrote:Thanks, this makes sense now. To come back to the initial question, it seems that the problem with randomness was something that was addressed in 1980 by Quantum Decoherence. I'm just pasting from Wikipedia: Show nested quote +When the Copenhagen interpretation was first expressed, Niels Bohr postulated wave function collapse to cut the quantum world from the classical. This tactical move allowed quantum theory to develop without distractions from interpretational worries. Nevertheless it was debated, for if collapse were a fundamental physical phenomenon, rather than just the epiphenomenon of some other process, it would mean nature were fundamentally stochastic, i.e. nondeterministic, an undesirable property for a theory. This issue remained until quantum decoherence entered mainstream opinion after its reformulation in the 1980s.
Do you by chance know how Quantum Decoherence solves the problem with randomness?
You're asking terribly deep questions, I like it. 
A few comments into this thread airen asked if I subscribed to any particular "interpretation" of quantum mechanics. I replied with Copenhagen, which basically believes in the wave function and the collapse of it when we take a measurement.
Interpretations are just ways of making sense out of everything, or at least trying to. There are tons of different interpretations, and decoherence is another way to view things.
How it solves the problem with randomness, in very simple terms, is something like this:
Take the photon example again, except this time we don't want to think about "collapsing the wave function". The wave function is what it is, but then something (read: our polarizer) interacts with it. As a result of this interaction, the wave functions of the photon and polarizer become entangled with each other. Our nice, coherent wave function that described our photon has interacted with something, and therefore has lost its coherence, it de-cohered and became entangled with the environment.
Are you familiar with the concept of interference? You add two sine waves together and if they're both at values greater than 0 at one point, they experience constructive interference- the total wave is bigger than the individual waves. If one is positive and one is negative, the destructively interfere and the total wave can be smaller or even zero at that point. Well a wave function is, as expected, a bunch of waves. We get probabilities of things happening by (sort of) squaring the wave function, and when we do this we get interference terms...
The role of interference is key in the concept of decoherence, but that requires math that's probably a little too sophisticated to be useful for me to explain... so take these statements as you will. In classical physics, which is not random, you don't get these "interference" terms in expressions for the probability of something happening. In quantum, you do. Decoherence, mathematically, actually removes these interference terms from the probability expressions, leaving just the classical result.
Instead of seeing a "superposition" of lots of possibilities (the wave function), we have a classical statistics.
Maybeeee that answered your question? I think this is still a pretty active area of research. People are always arguing about what quantum means and what something is doing before you measure it and stuff like that.
No one knows what the correct way to think about everything is... its nice to be familiar with the different explanations though. Most of the popular ones agree with what we know to be true about physics. Which interpretation is "right" or "wrong" is, in my opinion, a debate that probably won't ever end or be supported with definitive proofs.
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I just want to say this thread has been fun to watch, keep it up.
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Argh, only just found this blog series and will read and reread, so please keep on keeping on. =)
What makes this stuff so fascinating is that it deals with the concept of Reality itself and how we think about it.
All we have is math and logic and interpretations, and we don't even know what constitutes an observer? AAAH!
I remember waay back when I first read about the light particle/wave duality and how that boggled my mind leading
to all sorts of questions that made my head hurt at the time. Well gosh darn it, THIS time maybe I have the brain cells, I'll follow the links to other links and figure this all out and return with the Truth of Everything.
Well, probably not. But I will read with interest, and ponder with the rest of you.
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I don't have much to say on the subject, but I found this article about a recent study where a team managed to "teleport" information across a distance of 10 feet using quantum entanglement.
Here's the article: http://phys.org/news/2014-05-team-accurately-teleported-quantum-ten.html
Some really interesting stuff! It shows how quantum theory can be used practically. Perhaps one day we will have super fast quantum computers
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On June 03 2014 21:27 sorrowptoss wrote:I don't have much to say on the subject, but I found this article about a recent study where a team managed to "teleport" information across a distance of 10 feet using quantum entanglement. Here's the article: http://phys.org/news/2014-05-team-accurately-teleported-quantum-ten.htmlSome really interesting stuff! It shows how quantum theory can be used practically. Perhaps one day we will have super fast quantum computers
Does anyone knowledgeable have something to say about this? I'm very skeptical of nonlocal information transfer, and everything I've read ends up explaining these events as a misunderstanding about the nature of the observation -- where the intervention really occurred that caused the measurement to happen. But I'm no expert and I'm curious what others think about this.
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I read through the Science paper a couple of times today, so here is a quick summary in very simplified words(since not everyone has a university account that gives them access to all those journals... or has patience to decode):
So Alice has a qubit (bit of quantum information) she wants to teleport, q1. Alice also has q2 which is entangled with Bob's q3. Alice has to measure q1 and q2. These measurements yield information that tell Bob how to "decode" the measurement he got. (Oversimplified, but think of Alice measuring the polarization of a photon and defines north as "up". If her polarization is "down", she has to tell Bob the definition of her axes so he knows that north means up. Make sense?) The catch: Alice has to call Bob on the phone to tell him her results; this still requires the classical distribution of information.
This is a complicated experiment because it requires Alice to get a lot of information out of one single measurement, and it requires that Bob's q3 retains coherence (it isn't interfered with before he deliberately measures it). This sort of thing has been done before but never over this large of a distance. They had more success with this experiment because they used matter qubits--nuclear spins have long coherence times, nitrogen vacancy centers allow for longer entanglement duration. Compare to photons which are difficult to measure and retain these properties for much shorter time scales.
Why is this called teleportation? Well, the information that was located with Alice appeared on Bob's end. However, he needed a key to know what he was looking at.
If anyone else read the paper and got something different from it, please please chime in. This is not an area of research I'm involved with, so I could very easily be misinterpreting something.
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On June 04 2014 07:18 Ideal26 wrote:
I read through the Science paper a couple of times today, so here is a quick summary in very simplified words(since not everyone has a university account that gives them access to all those journals... or has patience to decode):
So Alice has a qubit (bit of quantum information) she wants to teleport, q1. Alice also has q2 which is entangled with Bob's q3. Alice has to measure q1 and q2. These measurements yield information that tell Bob how to "decode" the measurement he got. (Oversimplified, but think of Alice measuring the polarization of a photon and defines north as "up". If her polarization is "down", she has to tell Bob the definition of her axes so he knows that north means up. Make sense?) The catch: Alice has to call Bob on the phone to tell him her results; this still requires the classical distribution of information.
This is a complicated experiment because it requires Alice to get a lot of information out of one single measurement, and it requires that Bob's q3 retains coherence (it isn't interfered with before he deliberately measures it). This sort of thing has been done before but never over this large of a distance. They had more success with this experiment because they used matter qubits--nuclear spins have long coherence times, nitrogen vacancy centers allow for longer entanglement duration. Compare to photons which are difficult to measure and retain these properties for much shorter time scales.
Why is this called teleportation? Well, the information that was located with Alice appeared on Bob's end. However, he needed a key to know what he was looking at.
If anyone else read the paper and got something different from it, please please chime in. This is not an area of research I'm involved with, so I could very easily be misinterpreting something.
Thanks very much, all the cogent details that I was looking for!
It would seem that this is good progress for reasons of stability and reproducibility, without having done any groundbreaking miracle ansible stuff, which the headlines always make it out to be. But of course one wonders if there's a clever way of arranging your qubits so a modicum of classical information will decode them all.
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EPT
