Breakthrough for Quantum Measurement 201
said_captain_said_wo writes to tell us that PhysicsWeb is reporting that two teams of physicists have developed a new method for measuring the state of quantum bits in a quantum computer without disturbing the state. From the article: "In the future, the Josephson capacitance could be used for operations in a large-scale quantum computer," says Mika Sillanpaa of Helsinki University. "The Josephson inductance and Josephson capacitance together would also allow us to build new types of quantum 'band engineered' electronic devices, such as low-noise parametric amplifiers."
Heisenberg Uncertainty Principle? (Score:4, Insightful)
Re:Heisenberg Uncertainty Principle? (Score:5, Funny)
Re:Heisenberg Uncertainty Principle? (Score:4, Funny)
Re:Heisenberg Uncertainty Principle? (Score:5, Funny)
Schrödingers cat (Score:2)
Re:Heisenberg Uncertainty Principle? (Score:5, Informative)
Reading a qubit doesn't violate the Uncertainty Principle by itself; if qubits couldn't be read or written, they'd be worthless. The issue you are probably thinking of is that entanglements between qubits will be destroyed by the reading process (and there is no way to "read" such entanglements without destroying the individual qubit values).
Re:Heisenberg Uncertainty Principle? (Score:5, Informative)
Re:Heisenberg Uncertainty Principle? (Score:2)
Probabilistic computing? (Score:3, Informative)
Re:Heisenberg Uncertainty Principle? (Score:2)
They are still uncertain, until they are measured. Does measuring one make the other certain?
Quantum cryptography? Wassup with that now?
o.0
Re:Heisenberg Uncertainty Principle? (Score:3, Informative)
Re:Heisenberg Uncertainty Principle? (Score:2)
If I understod it right, you just described quantum entanglement. So, looking for that single gear will change the spin of all the other gears.
The GP had a very good point, because what the submiter says is known to be impossible. That is also why I'm reading the comments, to discover what the researches really did. But it seems that I'll need to RTFA.
Re:Heisenberg Uncertainty Principle? (Score:3, Informative)
Re:Heisenberg Uncertainty Principle? (Score:2)
For instance, in condensed matter physics, the wave functions that have to do with superfluidity have an uncertainty relationship between the number of particles participating in the condensate and the quantum phase of the condensate.
Re:Heisenberg Uncertainty Principle? (Score:2)
It should be noted that the uncertainity principle also applies to Wikipedia itself, insofar that you can never be entirely sure if the information posted there is legit
Re:Heisenberg Uncertainty Principle? (Score:4, Funny)
Re:Heisenberg Uncertainty Principle? (Score:5, Informative)
the position and momentum of a particle at the same time
other words you can know both but not precisely, if you know one
precisely you do not know the other
with it.
If you know that the particle is in a certain band, you do not need
to know its location
trapped
If you have your cow in a pasture, you do not care where it is, as
long as it is eating grass or hay in the pasture and how not escaped.
Re:Heisenberg Uncertainty Principle? (Score:2, Informative)
The reason that x (position) and p (momentum) do not commute comes from their operators. In x-space (what you're used to), the operator for x is just x, the operator for p is -i*hbar
Re:Heisenberg Uncertainty Principle? (Score:2)
Re:Heisenberg Uncertainty Principle? (Score:2)
"You keep using that word. I do not think it means what you think it means."
There are certain pairs of quantities ("conjugate variables") which cannot both be measured to arbitrary precision. If you measure, for example, the position of a particle along the x-axis (in your choice of coordinate system) very precisely, you cannot measure the momentum along that same axis very well. The product of the two imprecisions (or uncertainties) is a sma
Re:Heisenberg Uncertainty Principle? (Score:2)
Re:Heisenberg Uncertainty Principle? (Score:2)
The only confusi
Shroedinger's cat? (Score:5, Funny)
IANASPP (I Am Not A Sub-atomic Particle Physicist) but this seems to be quite a breakthrough that might save millions of subatomic cats from untimely deaths...
Anybody with some actual knowledge care to elucidate?
Re:Shroedinger's cat? (Score:3, Interesting)
That's exatcly what it means, the way the headline presents it, which would mean that QP is disproved, which again leads me to beleive that the poster has misunderstood something.
Re:Shroedinger's cat? (Score:2)
I guess what they managed is to do a measurement which behaves like an ideal quantum measurement: After you measured the system to be in state x, it really is in state x (unlike e.g. a typical photon measurement where after measuring the state of the photon, it isn't actually in any state because it doesn't exist any more).
Which means that if the system is already in an eigenstate of the measurem
Re:Shroedinger's cat? (Score:2)
Re:Shroedinger's cat? (Score:2)
Re:Shroedinger's cat? (Score:2)
Re:Shroedinger's cat? (Score:2)
I always had a problem with that experiment - it implies that human intellegence has some link with the state of the universe. I don't buy that for one minute. What happens if an ant crawls into the box for example? Because it's not 'really' an observer the cat is still half alive?? Plus since it's by definition unobservable it's also unprovable - so the whole experiment relies on philosophy rather th
Re:Shroedinger's cat? (Score:2, Interesting)
What happens if an ant crawls into the box for example? Because it's not 'really' an observer the cat is still half alive??
Yes the cat is still half alive, and the ant is half seeing a dead cat and half seeing a cat alive. What happens when the ant walks in the box is that its state gets correlated to that of the cat.
Note that the either state of the ant is unaware of the other.
When you open the box you will either see a dead cat and an ant that has been seeing a dead cat all along, or a living cat and
Re:Shroedinger's cat? (Score:3, Insightful)
Re:Shroedinger's cat? (Score:2)
And that room could be inside a much larger house, and that house inside one of those pretentious "gated communities", which is
Re:Shroedinger's cat? (Score:3, Informative)
As for QM not making any testable hypotheses - that's also not true. Quite the opposite, in fact; QM works exceptionally
Don't confuse measurement with knowing (Score:2)
Likewise if we use a machine to measure the position of a particle, the superposition collapses whether or not a human intelligence ever reads the results. In QM measurement is an act with consequences, just like swinging an axe at a tree. But it doesn't require a human intelligence.
Re:Shroedinger's cat? (Score:2)
I think this is a common misunderstanding. The whole buisiness with the "Cat" is a rather over-stretched metaphor. When you read "observation" think "interaction". In this sense anything else is an observer if it interacts with the "Cat" (probes its state). This gets more complex of course when one realises that "everything is connected to everything else" [*]
I realise the following is probably more detailed than necessary, however it's wort
Re:Shroedinger's cat? (Score:2)
The cat is a thought experiment that's useful for getting across the general idea. As far as I know, it's never been argued as something that would actually be performed, because cats are macroscopic.
You can use the concept on the quantum scale to predict lab-testable results like the two-slit experiment [wikipedia.org]. If you put detectors (e.g. observers) in the slits, the effect disappears.
Re:Shroedinger's cat? (Score:3, Informative)
Sounds weird, but its already been proven to be the case - look for two-slit diffraction experiments if yo really want to warp your brain. And no, it doesn't mean that humans are special - its just one case where takeing a measurement alters the state of an object.
Think on a macro scale. You take a cold thermometer and put it in a big bucket - the bucket's temperature doesn't c
Wrong (Score:2)
Actually, it has been found not to be the case. The experiment essentially uses a mirror instead of a cat, and either breaks or doesn't break the mirror. A mirror is used because (using an other aspect of QM) you can distinguish a mirror that is in a superposition of states (broken | unbroken) from one that is definite
Re:Wrong (Score:3, Interesting)
As for the temperature, there is no way that a thermometer of any size can't affect the sample its measuring unless it already is at the same temperature as the sample.
Every measurement, even in the macro universe, affects the thing measured. That this would NOT be the case at the quantum level, while counterintuitive at first, would upon reflection be surprising.
Also, there
Re:Wrong (Score:2)
The 2-slit experiments I'm referring to neither require nor use a mirror. You're referring to something else.
The two-slit experiments you are referring to do not put (or claim to put) macroscopic objects in a superposition of states (which is what Schroedinger's cat is all about). So they don't prove (as you claimed) that this actually happened. The only experiments to date that I am aware of to actually test the issue did use mirrors (actually, a metal grid that served as a mirror at the wavelengths
Re:Wrong (Score:2)
an accurate measurement of anything IS impossible. WE already know that.
actually, you're making an assumption that strict causality exists. It has never been proven, and is an unworkable assumption because it then requires artificial constructs such as superpositions of states.
BTW, you misquote me when you say:
Re:Wrong (Score:2)
A few things we seem to agree on
Now back to our regularly scheduled debate
You are misquoting me right after quoting me verbatim. My
Interesting discussion (Score:2)
According to some theories, that actually happens. We can already see that here on earth, where the number of receivers can affect the load of a proximate transmitter, due to coupling. Think of the 2 coils of a transformer. Now separate the coils by a foot ... now a yard
Yea, its possible (Score:2)
Sure, just ask the cat.
Re:Shroedinger's cat? (Score:2)
Re:Shroedinger's cat? (Score:2)
Probably just as well. If you were, you would be too small to use a keyboard or to set up your experiments.
Sorry. Well, half-sorry. I just couldn't resist.
Re:Shroedinger's cat? (Score:2)
No, it means that you can find out that the cat is alive without killing it in the process.
Most quantum measurements require you to kill the cat in order to determine that it was alive when the measurement started. For instance, drop a large weight on the box and listen for screams coming out. Think of measuring photons: the usual way to measure a photon is to allow it to be absorbed by something such as a photomultiplier tub
Is quantum computing useful beyond decryption? (Score:3, Interesting)
Essentially, it's only useful in a situation where you need to repeatedly run the same computation over and over again with different input values to see which of those values produces a valid output.
I have a friend who has suggested repeatedly that eventually computers will contain some sort of quantum processor that helps with such tasks as gaming. I don't think this is realistic because of the serialness of the tasks that quantum computing tackles. In particular, something like rendering an environment in real-time won't be helped because there's an unpredictable input (the human).
--
Free 411! 1-800-411-SAVE [1800411save.com]
Re:Is quantum computing useful beyond decryption? (Score:5, Funny)
Re:Is quantum computing useful beyond decryption? (Score:2, Insightful)
Re:Is quantum computing useful beyond decryption? (Score:5, Insightful)
Durring the 1/60th (or less) of a second that your system is rendering a single frame in that game, the state of the scene and all objects (as well as light positions, textures, and overlays) is very static. It just doesn't seem like it to you, because you are very slow compared to your computer.
There could be hundreds of applications of a Quantum Co-Processor in a game, from testing for occlusion in a 3D scene, to making AI decisions in computer controlled characters.
Quantum Computing may very well not be immediately useful in many traditional computation tasks ("While this value is true then do that") but it will open up whole new ways of tackling processes that are time consuming with today's methods ("do any of these things give us this, that, or something in between?").
Just thinking about it gives me that Fuzzy Logic Feeling...
QC gaming accelerators... (Score:2)
attached to an MMORG server...
Full disclosure: I work for them...
Paul B.
Some corrections (Score:4, Informative)
Grover's algorithm is only the provably best implementation in a "black box" setting, which is unrealistic for many problems.
Finally, quantum computers are not known to be able to do anything useful for protein folding - this would be an application of an efficient quantum algorithm for the graph isomorphism problem, which nobody has come up with yet...
Quantum Cryptography?? (Score:2, Insightful)
no fair (Score:4, Funny)
Implications in reverse order (Score:5, Interesting)
Not to knock the discovery, which is very interesting, but it's a pity quantum computers have to be dragged into everything to justify research. I doubt that Tom's Hardware will be reviewing millikelvin coolers for your qubit box any time in the next 20 years (though I'd like to be proved wrong)
Re:Implications in reverse order (Score:2, Informative)
I concur. NIST Boulder, as an example that I am familiar with, is developing certain techniques that can be used for quantum computing. ( http://tf.nist.gov/ion/index.htm [nist.gov])
But the reason why the Time and Frequency division at NIST cares is because these techniques may yield better clocks in the future. (In fact, many breakthroughs in fundamental theoretical/experimental physics are applicable to clocks.) Meantime, however, the project gets mainstream-media publicity for quantum computing implications, ge
Re:Implications in reverse order (Score:2)
As a mathematician in training, my biggest worry is that all the interesting cryptography jobs will have been obsoleted by the time I get that far.
Re:Implications in reverse order (Score:2, Funny)
I can factor even the largest Mersenne primes in under two seconds in my head. Maybe I can help these scientists out a little bit with factoring primes...
spooky action at a distance (Score:2, Interesting)
Re:spooky action at a distance (Score:5, Informative)
No, it doesn't. The closest you can come is instant synchronization of states, but you don't get to choose what state that is. For example, you can have two particles entangled to have the same (or opposite, as in the EPR thought experiment) spin orientation, but you can't send a signal from one to the other by choosing the orientation. Instead, it's random whether each one is spin up or spin down - the only guarantee is the relationship between the measurements. This would be great for things like cryptographic key exchange, since you can't have a man-in-the-middle attack if there is no middle, but it's useless for sending information. See: The No-Communication Theorem [wikipedia.org] (warning: requires crazy math skills to avoid the MEGO effect)
nothing can travel faster than light.
I call bullshit. Relativity prohibits* local superluminal motion; non-locally, it's fair game. See, for example, the Alcubierre Warp Drive [arxiv.org] - the only question of whether it's possible or not (aside from new physics) rests on whether there's any local superluminal energy propagation at the edge of the spacetime bubble. Plus, QM allows for lots more in the way of non-local effects (even if you assume hidden variables, since Bell's Theorem [wikipedia.org] rules out local hidden variables based on current experimental results), though, as I noted above, it still doesn't allow for superluminal communication (or teleportation, for that matter).
* Minor caveat: this is not counting tachyons [wikipedia.org], since nobody knows if they exist.
Re:spooky action at a distance (Score:2)
Didn't the IBM guys already demonstrate information teleportation? IIRC they figured out some way to get additional particles to synchronize with the Bell pair, then read the state of the additional particles, leaving the Bell pair un-read.
Re:spooky action at a distance (Score:2)
Re:spooky action at a distance (Score:2)
If Alice and Bob share an entangled pair, and both do a measurement, they both get a completely random result. It's just that both get the same result. But there's no way for Alice to influence the result which Bob gets. Think of it as a one-time pad which materializes only on observation.
Now, with quantum teleportation, Alice doesn't just measure her pair particle, but a combination of
Danger of thought experiments (Score:2)
Unchanged State (Score:3, Interesting)
I thought the state had to be changed to measure it or am I confusing a technique used in quantum crytography with this technique in quantum computing. As an ex-chemist my understanding of things quantum was never that good anyway but I seem to remember someone saying that in order to measure something you had to change it. Any physicists in the house?
Re:Unchanged State (Score:5, Informative)
The statement without changing its value must refer to reading the value reliably. When reading the state of an individual subatomic particle it is extremely easy to have the result perturbed by noise. Given that there is a probability of reading an alternative value, then it is not normally possible to tell when the wrong value was read. It appears that this makes the process much more reliable.
IAAQP (I Am A Quantum Physicist). Though I could still learn a thing or two about subatomic physics.
Physicist in the House (Score:5, Interesting)
The Heisenberg Unccertainty principle implies that measuring a quantity must add noise in the conjugate quantity. For example, measuring the momentum of an object spreads out the wavefunction. Another example, measuring the state of a qubit (whether it is a zero or a one) destroys the relative phase between the zero and the one.
So the "non-destructive" measurement they are talking about means that they aren't changing it from a zero to a one or vice-versa. But they are (and must) destroy the information about the phase of the qubit state during the measurement. For a more in-depth discussion, look up "quantum nondemolition measurements".
Re:Physicist in the House (Score:2)
Re:Physicist in the House (Score:2)
Re:Physicist in the House (Score:2)
Can you elaborate a bit? NEWB WARNING (Score:2)
Is it our processes for trying to detect and measure states that create a change or do we not know why?
Credit where it's due. (Score:5, Informative)
Re:Credit where it's due. (Score:2, Informative)
Re:So what does Linus have to say about it. (Score:2)
However I imagine debugging on a quantum computer will be no fun: After all, quantum programs will behave different when you look at them with a debugger!
Re:So what does Linus have to say about it. (Score:2, Funny)
Crap! (Score:5, Funny)
Re:Crap! (Score:5, Insightful)
"The author changed the article by writing it" may be the best analogy to quantum computing I've run into. At the moment he finished the article, the author caused the article to collapse from all the articles it might have been the the article it actually was. As he was writing it, it simultaneously passed through all the possible articles (states) it might have been, to become the final article (state). Becoming is infinite, being is finite.
Re:Crap! (Score:2)
I'm honored. :-)
Re:Crap! (Score:2)
mis-statement (I think) (Score:3, Interesting)
Re:mis-statement (I think) (Score:2)
One more pedantic point though - I've never liked the use of the word 'collapse' in the Copenhagen interpretation. It has a
Re:mis-statement (I think) (Score:2)
Not quite. If the system is originally in an eigenstate of the operator being measured, then it should be possible to perform a measurement without changing the state. For instance, if a photon is in a pure momentum eigenstate, then it should be possible to measure the photon's momentum without changing its state.
The problem is that most quantum measurements destroy the state, even in such a case. For instance, with the photon, we usually absorb the photon in order to
Quantum cryptographic links? (Score:3, Interesting)
(Not a quantum physicist, but I can play one on slashdot can't I?)
Re:Quantum cryptographic links? (Score:2, Informative)
The security of quantum key distribution (QKD) does not depend on the technology of the eavesdropper: it is assumed she can do any attack allowed by quantum mechanics. The security only depends on Alice and Bob's (the legitimate users) ability to actually produce and measure the quantum states used by the protocol. Finally, there are *proofs* of the security o
But is it a flux capacitor? (Score:2)
Because it isn't an insulator, of course (Score:5, Informative)
Re:Because it isn't an insulator, of course (Score:2)
Re:But is it a flux capacitor? (Score:2)
Helsinki U. of Tech., not Helsinki U. (Score:4, Informative)
Holographs vs. qbits (Score:2)
Holographic storage or quantum computing?
Shor's Algorithm (Score:2, Interesting)
A Golden Age is Coming (Score:2, Interesting)
For long term space travel like the proposed mission to Mars a quantum computer would be invaluable. It would be able to monitor the crew and spacecraft faster than today's computers and will be able to react t
thank goodness (Score:3, Funny)
I'm very glad, as I have a current-model parametric amplifier and man is it LOUD....
I should have figured as much, seeing as how it goes up to 11.
Useful? (Score:3, Interesting)
In any event, cooling down to such temperatures implies a couple of things: lots and lots of very expensive equipment to cool down a tiny tiny volume of space. Even the first transistors didn't require such great lengths.
The article also makes reference to the capacitance of the Johnson capacitor changing signs depending on the state of the qubit, which is part of how the whole thing works. Does this mean that someone has discovered negative capacitance? Whoa! What would that mean?
Re:Useful? (Score:2)
"... [T]he superconducting threshold of whatever their Johnson capacitor is made from might in fact be that cold."
As a Canadian, I can vouch for this. In extremely cold temperatures, my Johnson shrinks significantly and becomes much easier to measure.
... But what exactly is a 'Johnson capacitor'? Is that like Viagra or something?
They're so silly (Score:3, Funny)
Brian Josephson (Score:2)
Look at his web page [cam.ac.uk]! He's pushing cold fusion, ESP and other paranormal powers, and all kinds of bizarre theories. He's gotten into fights with the highly respected archiv.org [archiv.org] physics publication site over their habit of removing crackpot papers. In short Josephson is an embarrassment to the scientific community, someone w
Re:Brian Josephson (Score:2)
Besides, you never know, he might even be right about one of those things. If you throw enough darts you might just hit something.
Re:Inifinite storage. (Score:2)
It could store something which classical disks cannot, which is quantum states. Which would make a quantum hard disk quite interesting, but not for storing more classical information, but for storing quantum information which you simply couldn't store on a classical disk at all.