Breakthrough for Quantum Measurement

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."

Comments

Heisenberg Uncertainty Principle?

[SRA8]

Wouldnt this violate the Heisenberg Uncertainty Principle?

Re:Heisenberg Uncertainty Principle?

[Anonymous Coward]

I'm not sure

Re:Heisenberg Uncertainty Principle?

[Centurix]

Maybe. Maybe not, who can tell?

Re:Heisenberg Uncertainty Principle?

[arrrrg]

> Wouldnt this violate the Heisenberg Uncertainty Principle?

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?

[maxzilla]

Not neccisarily, by one quantum bit changing can be measured because it influences a number of other bits in a uniform way. I think you could imagine one gear spinning that drives a set of other gears, as the quantum bit changes it changes the whole setup of the capacitor so it is evident which way the bit was moving without changing its spin by measuring it. you change the spin of one of the bits in the capacitor, not all of the bits or the original bits

Re:Heisenberg Uncertainty Principle?

[BlkItlStl]

I don't think so. I'm pretty sure that has to do with limits on measuring momentum and location. http://en.wikipedia.org/wiki/Uncertainty_principle

Re:Heisenberg Uncertainty Principle?

[Obvius]

Heisenburg's Uncertainty Principle - DxDp>=hbar/2 Or as my old prof used to say "When you've got energy, you don't have the time. And when you've got the time, you don't have the energy."

Re:Heisenberg Uncertainty Principle?

[arrrrg]

I should clarify: reading the qubit will destroy all quantum effects (superposition as well as entanglement), effectively making the qubits look like ordinary bits (when you open the box, the cat's either dead or alive, not both). However, quantum computers are designed with this in mind; reading the output destroys any quantum properties it may have, but a computation can be repeated many times to get an idea of what uncertainty was present in the output.

Re:Heisenberg Uncertainty Principle?

[Anonymous Coward]

The uncertainty principle just states that you cannot know both
the position and momentum of a particle at the same time ... in
other words you can know both but not precisely, if you know one
precisely you do not know the other ... because you have interacted
with it.

If you know that the particle is in a certain band, you do not need
to know its location ... that IS its location ... or it is essentially
trapped .... you do not care.

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. ... best I can do.

Re:Heisenberg Uncertainty Principle?

[Haydn Fenton]

Netcraft confirms it, Schrödingers cat is dead.

Re:Heisenberg Uncertainty Principle?

[mumrah]

The Heisenberg Uncertainty Principle states that the more precisly you measure momentum, the less precisly you can measure position. This is a specific case of a more general uncertainty principle that states if two observables (momentum, position, speed, energy) do not commute, then they cannot be measured to close percision.

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*d/dt. Long story short, x and p do not commute because of the time derivative in the p operator. This physically means that these two particular observables cannot both be measured with high precision, and as i turns out the product of the uncertainty in x with the uncertainty in p must be less than or equal to hbar/2 (which is the Heisenberg Uncertainty Principle). Not sure how relavent all of this is.

Shroedinger's cat?

[mcrbids]

Does this mean that we can find out if the cat is dead without opening the box? Sure sounds like it.

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?

[Anonymous Coward]

Does this mean that we can find out if the cat is dead without opening the box? Sure sounds like it.

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?

[tendays]

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 an ant that has been seeing a cat alive all along

Oh, and finally, there are many indirect but observable consequences of these superposition states, and they are precisely what quantum computing takes advantage of.

by the way, sorry I know this is off topic, but is anyone else having problems logging in in slashdot.org subdomains? That problem started for me a couple of weeks ago : I'm logged in slashdot.org but not in science.slashdot.org or it.slashdot.org etc so I can't mod in these domains unless I comment first

Re:Shroedinger's cat?

[slavemowgli]

I don't think you understand. It's not about someone being *aware* of whether the metaphorical cat is dead or alive - it's about the quantum state being disturbed. Measuring it can disturb it; whether the measurement is presented to a human mind afterwards or whether it's thrown away, for example, is irrelevant. So, yes, if an ant crawls into the box, the cat will either be dead or alive.

As for QM not making any testable hypotheses - that's also not true. Quite the opposite, in fact; QM works exceptionally well so far, and our findings continue to match its predictions. Do read up on it a bit - it's a very fascinating topic.

Re:Shroedinger's cat?

[Y2]

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 an ant that has been seeing a cat alive all along

Why do you think the human observer is more special than the ant? Why don't you believe that when you open the box you become correlated with both the ant and the cat, and enter a state which is a mixture of you seeing a dead cat and seeing a live cat - with your own two states each, as you say, "unaware of the other?"

That's what the many-worlds interpretation is all about, not some sci-fi multiple universes schtick we always run across.

Re:Shroedinger's cat?

[tomhudson]

I always had a problem with that experiment - it implies that human intellegence has some link with the state of the universe.

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 change much, the thermometer does, and registers the change.

Now, substitute a drop of water for the bucket. Doesn't matter how hot that water is, sticking it on the cold thermometer is going to change its temperature by a significant amount.

Now go to the sub-atomic level. There is no way that ANY measurement can't help but affect the thing being measured. Its like the drop of water - you tried to measure its temperature, and in doing so, changed its temperature.

Objects affect each other when they interact - or, as the saying goes, "shit happens"

Is quantum computing useful beyond decryption?

[jimmyhat3939]

The problem with quantum computing, as I understand it, is there are very very few applications.

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

Re:Is quantum computing useful beyond decryption?

[smeek]

Quantum computing is also good for solving problems in quantum mechanics. No, really.

Re:Is quantum computing useful beyond decryption?

[Jace of Fuse!]

In particular, something like rendering an environment in real-time won't be helped because there's an unpredictable input (the human).

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...

Re:Is quantum computing useful beyond decryption?

[mattpalmer1086]

I shall consider myself lightened-up :) But seriously, it's not obvious to me that it would be useful for calculating results in quantum physics, just because it is based on that theory. Sounds likely, but IANAQP. "Obvious" and "quantum physics" aren't words I normally encounter close to one another ;)

Quantum Cryptography??

[laggist]

juz wondering.. would this result mean anything to the already available systems whereby quantum properties are used to securely send data from point to point??

no fair

[FidelCatsro]

You changed the state of the quantum bits by measuring them

Implications in reverse order

[Flying pig]

Talk about looking for grant funding! Problem is, scientific illiterates in Government etc. think they understand what a quantum computer can do (application a long way in the future if at all) but not what you can do with very low noise parametric amplifiers (which might be relatively near term applications.) In terms of exciting progress in studies of brain function, small scale biochemistry, remote sensing and signal processing, very low noise amplifiers are critical components, whereas quantum computers don't yet exist, and by the time they do conventional computers should be adequate to deal with a lot of the data processing.

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

[insignificant1]

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)

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, gets funding from NSA, QuIST/DARPA, etc.

I'm sure it's a windfall for the physicists to do fundamental research, though, so "Hoorah Quantum Computers!!! (and cut me that check...)"

At least one other group (http://qdev.boulder.nist.gov/) is working on research similar to that published on PhysicsWeb, specifically using Josephson junctions in creating quantum bits & logic. That group was just recently setting up an honest phase measurement system, though, so are probably a bit behind in the research mentioned.

The reason that group plays with Josephson junction devices (how they justify it under the NIST banner)? Voltage standards. The reason they tell other funding agencies? Quantum computing, communication, and good-old-Alice-and-Bob.

Re:Implications in reverse order

[Kagura]

So, for example, whereas factoring a 64-bit prime number might take a fairly hefty digital computer a decent chunk of time

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

[ericcantona]

presumably, given entanglement, measurement of qbit state allows potentially for instant communication ? (which would be really spooky!).

Re:spooky action at a distance

[gauge boson]

presumably, given entanglement [wikipedia.org], measurement of qbit state allows potentially for instant communication ?

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 (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 - 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 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, since nobody knows if they exist.

Unchanged State

[squoozer]

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

[quoll]

The article seems misleading in its wording. It says "read the value of a qubit without changing its value." This can't mean that it doesn't change the original quantum value, as this makes no sense. Reading a quantum value (a qubit) collapses the probability to the value read, by definition. This means that the value is no longer quantum. The original probability function cannot be read (though it can be calculated).

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

[anomalousman]

They can do what they say, but it's a lot more trivial than measuring the entire quantum state of the system, which is, as others have suggested above, impossible.

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".

Credit where it's due.

[kimmop]

The article isn't totally clear about it but the Finnish university in question is the Helsinki University of Technology (in the city of Espoo) and not the University of Helsinki. These are the largest two universities in Finland and both have Physics departments so the distinction is important.

Re:Credit where it's due.

[grimJester]

I just realized I've studied at both but graduated from neither. How did you collapse those wavefunctions again?

Crap!

[werewolf1031]

I changed the article by reading it! Someone tell me what it says now...?

Re:Crap!

[$RANDOMLUSER]

"I changed the article by reading it" is a terrific joke.

"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.

mis-statement (I think)

[sdedeo]

It's been awhile; I do GR now, not QM (much simpler.) But any measurement will change the state; this is the famous "collapse of the wavefunction" (in the Copenhagen interpretation.) What they mean is that the measurement will collapse the wavefunction as usual, but that it will not then alter the system being measured so that the state changes. i.e., if the amplitude is 0.1 A and 0.9 B, and the measurement collapses the wf to B 90% of the time as it will, then when the measurement is done the system will be B 90% of the time as expected, and it will be B "the right" times.

Quantum cryptographic links?

[evilviper]

Sounds to me like the security of quantum fiber-optic links are now in question. This isn't directly applicable to taping one, but it's a start.

(Not a quantum physicist, but I can play one on slashdot can't I?)

Re:Quantum cryptographic links?

[Anonymous Coward]

Sounds to me like the security of quantum fiber-optic links are now in question. This isn't directly applicable to taping one, but it's a start.

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 of QKD. The only way it becomes insecure is if we learn that quantum mechanics is an incorrect theory.

(Not a quantum physicist, but I can play one on slashdot can't I?)

Sure. Just try to play a better one next time! ;)

Some corrections

[Catullus]

Quantum computers are not known to be very good at solving NP-complete problems, and in fact it is considered very unlikely that they will be able to solve such problems efficiently. Grover's algorithm provides a square-root speed-up in solving any problem in NP; however, this is not enough to make an unfeasible problem feasible, and for any given NP-complete problem, there is likely to be a classical algorithm that outperforms this "brute force" approach.

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...

Helsinki U. of Tech., not Helsinki U.

[msmikkol]

Just a minor correction to the linked article: Mika Sillanpää worked at the Helsinki University of Technology, not at the Helsinki University when he wrote the paper in question.

Because it isn't an insulator, of course

[Flying pig]

The description of the Josephson Junction is aimed at all the non-physicists out there. The "insulating layer" is a bandgap layer. The point is that cooper paired electrons can tunnel through it, i.e. it acts as a superconductor itself. It is an insulator for ordinary electrons only. And the definition of capacitor is nothing at all to do with physical conductors or insulators. It is a region of space where a potential gradient can be created, and the capacitance is the measure of how much energy has to be pumped into the region in order to create a given potential gradient. "Empty space" requires the lowest energy and has the lowest capacitance per unit volume, while certain ceramics with relatively mobile but limited electrons have very high values. If you cannot create a potential difference across your region of space, you have no capacitance - and at first sight, if that region is superconducting you cannot have a potential difference.

Re:So what does Linus have to say about it.

[kimmop]

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!

And how is that different from debugging C pointers? ;)

Shor's Algorithm

[stelmach]

I'm trying to grasp what the implications are of this. Let's take Shor's algorithm as an example. It is my understanding that the Quantum Fourier Transform (QFT) is applied to the result of the algorithm to peak the probability amplitudes, which will help the result to collapse into the correct state when measured. So does this mean that the QFT will not need to be applied, and the result of Shor's algorithm can be read with 100% accuracy?

A Golden Age is Coming

[TheZorch]

Quantum Computers will usher in a golden age in computing. There are all sorts of applications that they could be used for. For a time they'll serve a role that most super-computers today serve and that's for engineering computations and scientific experiments that require massive number crunching.

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 to any kind of critical issue 100x faster. Please, no "2001/Hal" comments. I'm being serious.

Also, quantum computers could be used for gene sequencing that can be done in minutes rather than hours, months, or years for the creation of new drugs or gene therapy. A single quantum computer could be used to replace dozens of computers in a corporation's server room. Just one machine could do the work of 20 or more so you don't need a separate database server, email server, web server, web proxy server, or any other kind of server a large company would need. These computers would benefit businesses like Ford, GM, and all the other car makers allowing them to make better engineered cars.

I can also imagine the graphics industry would benefit. Imagine if Pixar had one of these machines. Imagine being able to render a movie at final-draft production quality in "realtime". We'd also finally have a computer that could make Virtual Reality better than it has been in the past. The applications for this technology aren't as limited as you might think.

thank goodness

[revery]

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.

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?

[necro81]

From RTFA, I am wondering if this new discovery will actually be of much use to anyone. The apparatus involves cooling down to a few millikelvin. I am guessing that this is so that the thermal noise in the circuit is greatly reduced, and also because the superconducting threshold of whatever their Johnson capacitor is made from might in fact be that cold. Pure copper becomes a superconductor, but not until several degrees Kelvin, I believe.

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?

They're so silly

[jav1231]

Hey, it could be worse. Quantum Theorists could be using their imagination to sit around and dream about getting laid.

Probabilistic computing?

[Urusai]

Or aleatory computing? I realize there are certain problems that are deterministically intractable but with feasible probabilistic "solutions", but statistics based computing is just...dirty. I don't think a lot of people understand that quantum computing doesn't actually provide hard answers, that you have to run the same "algorithm" a lot of times to get an approximation.

Re:Wrong

[tomhudson]

Um, no. You're wrong. The 2-slit experiments I'm referring to neither require nor use a mirror. You're referring to something else.

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 is no need to resort to a superposition of states (which has always been a bit of a brainfuck imho) when there are better models. Superposition is only required if the time scale is unidirectional and can't be "rotated out" of the question and replaced by another vector, which has never been shown to be the case. The copenhagen gang lacked sufficient imagination to see the obvious.

But that's just my opinion, and this is slashdot, and its not Tuesday :-)