Several Quantum Calculations Combined At NIST

Al writes "Researchers at the National Institute of Standards and Technology (NIST) have demonstrated a crucial step toward building a practical quantum computer: multiple computing operations on quantum bits. The NIST team performed five quantum logic operations and 10 transport operations (meaning they moved the qubit from one part of the system to another) in series, while reliably maintaining the states of their ions — a tricky task because the ions can easily be knocked out of their prepared state. The researchers used beryllium ions stored within so-called ion traps and added magnesium ions to keep the beryllium ones cool and prevent them from losing their quantum state." In related news, another reader links to an Australian study indicating that quantum computers "can continue to work perfectly even if half their components, or qubits, are missing."

Comments

This may be slightly off-topic, but

[Anonymous Coward]

Seriously WTF is Quantum Computing? I've looked at the wiki articles and googled things, and I'm still lost. I did read that unless you have an education in this area you just won't get it, but help me out here.

Re:This may be slightly off-topic, but

[mrsteveman1]

They are computers that leap from datacenter to datacenter, solving previously unsolvable problems, and hoping each time that the next leap will be the leap home.

Re:This may be slightly off-topic, but

[jpmorgan]

To truly understand a quantum computer you need a fairly strong understanding of linear algebra, although knowing quantum mechanics isn't actually necessary. I'll repost an explanation I wrote for another site:

Not 100% accurate, but here's a rough way to understand a quantum computer: If you've ever heard of the concept that whenever there's some chance, the universe 'splits' and both events occur, that's what's going on. When the quantum computer makes a qubit 1 and 0 at the same time, it basically uses a truly random event to determine which value the bit will be. The universe 'splits' and down one path there is a 1, and down the other there is a 0. Except the quantum computer 'splits' the universe in such a way that the two universes can interact with each other. It is even possible to have the quantum computer compute something on every input at once and then search through all the different universes to find an answer; this is known as Gover's algorithm.

The critical part is coherence: making sure that the only difference between the different universes is inside the quantum computer itself. So long as coherence is maintained, the universes can merge back together and all you're left with is the right answer (99.99999% of the time). If coherence isn't maintained then the universes can't remerge, and you don't get a correct answer. Decoherence is actually extremely hard to deal with, and the biggest engineering challenge in designing a quantum computer.

Re:This may be slightly off-topic, but

[xerent_sweden]

So basically it's quantum physics applied on computer science. Computers of today are based on semiconductors and diodes, which allows us to build electric circuits with memory. In this case, it's voltage applied or voltage off - one or zero. Quantum computing is a whole new world of computing; because it's based on the principles of quantum physics. This means that a quantum computer does not resemble the computers of today at all. In a quantum computer, information is stored in "qubits", which is 0, 1 or "undetermined / both". This is a direct application of the wave/particle duality of matter (wiki: De Broglie-wavelength). Working out how a quantum computer - which behaves totally differently from anything we have today - and constructing such a device is really hard. Theoretically, such devices would be more efficient than our computers - and that's an understatement. This story means that we've taken yet another small step towards practical quantum computers, but also that it'll be reposted at least 100 times before working quantum computers are reality. (Off the top of my head, please correct me if I wrote something in error. Thanks! :)

Re:This may be slightly off-topic, but

[MichaelSmith]

The critical part is coherence: making sure that the only difference between the different universes is inside the quantum computer itself. So long as coherence is maintained, the universes can merge back together and all you're left with is the right answer (99.99999% of the time).

How does the observer in the universe with the right answer know their answer is right?

Re:This may be slightly off-topic, but

[jpmorgan]

Typically with these searches you know the answer you want, and you're interested in which input gives you that answer (the inverse problem). An important caveat about Grover's algorithm is that, while it's significantly faster than classical unordered search, it's still non-polynomial.

Re:This may be slightly off-topic, but

[FooAtWFU]

You might check it with a classical-computing algorithm. For NP problems, verification of the answer is often substantially faster than computing the answer itself.

Re:This may be slightly off-topic, but

[Pseudonym]

Usually the answer is one that's difficult to compute but easy to check, such as any problem in NP. Checking that you have a factor of a number is much easier than producing a factor, and checking that a proof is correct is much easier than producing a correct proof.

The other option is to simply run it more than once. If you have an algorithm which is wrong 1% of the time (and that 1% is uncorrelated to the "input"), then if you run it ten times, the chance that all of them are wrong is extremely small.

Having said that, the "many universes" model is, according to most quantum mechanics, not an accurate picture. It's better to think of quantum algorithms as being probabilistic algorithms that works with quantum probability theory rather than classical probability theory.

Re:This may be slightly off-topic, but

[blincoln]

f you've ever heard of the concept that whenever there's some chance, the universe 'splits' and both events occur, that's what's going on. When the quantum computer makes a qubit 1 and 0 at the same time, it basically uses a truly random event to determine which value the bit will be. The universe 'splits' and down one path there is a 1, and down the other there is a 0. Except the quantum computer 'splits' the universe in such a way that the two universes can interact with each other. It is even possible to have the quantum computer compute something on every input at once and then search through all the different universes to find an answer; this is known as Gover's algorithm.

As big a fan as I am of the Many Worlds theory, and as much as I think it's the one that makes the most sense in terms of explaining quantum phenomena, my understanding is that it's far from accepted as fact, with the Copenhagen interpretation being in the lead.

So to keep things on an even keel, is there a similarly straightforward explanation that can be given which doesn't depend on Many Worlds?

Re:This may be slightly off-topic, but

[iris-n]

You just couldn't resist using Everett's interpretation, could you?

I don't think it is a good idea using it to explain something to laymen. They usually end up thinking that quantum mechanics is some kind of inaccessible black magic.

Just to be clear here, it is possible (and it is what's done most of the times) to describe quantum mechanics without ever talking about splitting universes.

Let's see: the qubit can hold some combination of 0 and 1 (NOT 0 and 1). By the same reason (superposition), the quantum computer can perform multiples paths of computation at the same time, which can be used to accelerate the computation of some algorithms.

Quantum computers are quite sensible to noise; it causes decoherence, which can be understood as a loss of quantumness. In other words, a qubit that suffered too much decoherence can't hold a superposition of 0 and 1 anymore.

See? It wasn't that difficult.

Re:This may be slightly off-topic, but

[sanman2]

Ever seen that Superman comicbook issue where he changes back and forth between Superman and Clark Kent so fast that he appears as 2 people in front of the media, and fools them into thinking that Clark and Superman are 2 different guys?

Well, at the tiny smallscale - aka, the quantum level - small particles are being buffeted between different states so quickly, that to us it can look like they're in 2 states at once (like being in 2 different places at the same time - like that Superman comic)

If you're Superman able to use his superspeed to fool people into thinking you're in 2 places at the same time, then you could lead 2 different lives, or even have a conversation with yourself on camera.

If you're a qubit able to be in 2 different states at once, then you could be used to perform twice as many state operations as a regular bit. And if there are 2 qubits, then they can do 4 times as many operations, 3 qubits can do 8 times as many, etc, etc.

So the advantages pile up rather quickly.

Re:This may be slightly off-topic, but

[ajs]

Quantum computing is a whole new world of computing; because it's based on the principles of quantum physics. This means that a quantum computer does not resemble the computers of today at all. In a quantum computer, information is stored in "qubits", which is 0, 1 or "undetermined / both".

Yeah, I'm still not buying it. Quantum computing has yet to do anything that lives up to even its basic promise. If it's possible to harness these states, we should have been able to demonstrate, at the very least, the capacity for computation on the scale of, say, the average pocket calculator. And yet, we've still gotten nowhere. Why? Well, there are many possible reasons, but I tend to favor the simplest: quantum mechanics is a field that is built on some very nice math and as many intersections with reality as we've been able to test, but fundamentally we don't really understand what's going on at the level of the atom, much less at the level of its constituent parts. As we gain that understanding, our math is likely to be revised and refined several times.

Put simply: we're like Aristarchus of Samos measuring the relative sizes and distances of the sun and moon. He was more accurate than any before him, and his understanding drove countless others' discoveries, but if he'd tried to put a man on the moon using that math, he would have failed.

We're doing roughly that: trying to put a man on the moon at quantum scale, and while the discoveries of Plank and Dirac and all of their successors to the current day have enlightened us as to the nature of the quantum world, we're still not so much farther along than Aristarchus. I expect there to be at least one more wave of truly physics-shattering discoveries on par with the uncertainty principle before we even start to be able to perform real computation at the quantum scale. Even then, it's entirely possible that those discoveries will invalidate the entire idea of using superposition for computation.

Then again, I could be completely wrong, and quantum computing could be workable tomorrow. Just don't go betting the farm on it.

Re:This may be slightly off-topic, but

[SeekerDarksteel]

Wow, a decent summary of quantum computing on the internet. It's so weird not having to pull out the baseball bat and perform some facial readjustment in a qc thread. Just a little added information. When we refer to qubits as being "both" 0 and 1 at the same time, it's not necessarily a 50/50 split. It is in the form (a+bi)|0> + (c + di)|1>, where |0> refers to the 0 state and |1> to the 1 state. |a+bi| = sqrt(a*a + b*b) is the probability that, if measured in the 0/1 basis, it will result in 0, and |c + di| the probability it will result in 1.

The presence of i (the imaginary number, in case that wasn't clear), is important. Also, you can measure a qubit in any basis, not just 0/1, which is actually vital to the way some quantum algorithms work. (Notably quantum key exchange, which relies on the fact that a potential eavesdropper doesn't know what basis he should be measuring the qubit in.) A good way to imagine a single qubit is a bloch sphere. Imagine a sphere, where straight up is 0, and straight down is 1. Anything on the equator is a 50/50 superposition of 0 and 1.
Also, to say that quantum computers are more "efficient" than classical computers isn't quite precise enough for my tastes. It's not that they're capable of doing the same things as a classical computer can, just faster. It's that they're able to do things classical computers simply cannot do due to the way superposition works. And those things allow it to solve a number of problems more efficiently.

Re:This may be slightly off-topic, but

[smallfries]

Ziggy computes a 98.3% chance that is the correct definition for Quantum Computing.

Re:This may be slightly off-topic, but

[maxwell demon]

I consider that a very bad explanation of quantum computers (and yes, I work in the field of quantum information, so I know quite well about it). Nothing against many-worlds, but using it to explain a quantum computer is IMHO misplaced. The working of quantum computers is "interpretation-invariant" and adding many-worlds here only muddles the waters.

Even the usual statement that a qubit is "at the same time 0 and 1", while in some sense true, isn't really helpful. Indeed, a single qubit can be modeled by direction in three-dimensional space (for most implementations it's an abstract space, but if you use the spin of spin-1/2-particles to represent the qubit, then those directions are literally the directions of real space). Operations on a single qubit (other than measurement, which is special) are just rotations in that space. Where the power of quantum computing comes from is entanglement: If you have several quantum systems, the states they can assume are more than just "the first qubit is in state X, the second qubit is in state Y". There are "entangled" states where the single qubits are in no single state, but the whole system is still in a well defined space.

The whole point is that for classical systems (even classical analog systems), the state for the whole system is the direct sum of the states of the separate systems, while for quantum system, it's the direct product. That is, if a classical system has an n-dimensional state space, then k copies of it have an n*k-dimensional state space. That is, the size of the state space grows linearly (of course the number of states grows exponentially because every dimension gives a new factor). OTOH, for quantum systems, k copies of a system with an n-dimensional state space have an n^k-dimensional state space. That is, the dimension of the state space grows exponentially rather than linearly. Therefore it's not surprising that for some problems, you get an exponential speedup: By adding a linear amount of physical resources (qubits) you add an exponential amount of computational resources (states space dimensions).

Now those extra dimensions can only be used in very limited ways, therefore not every problem gets an exponential speedup. At that point it should be stressed that those are state space dimensions, not real space dimensions; e.g. two classical particles in three-dimensional space have together 12 state space dimensions (for each particle 3 dimensions for position and 3 dimensions for momentum; note that the classical physics state space is usually called phase space).

Re:Begs the Question

[jpmorgan]

That's a horribly misleading summary. Quantum computation is plagued with error... the same thing occurs in classical scenarios but we have error correction schemes to deal with that (for example, error correcting codes). Analagously there's quantum error correction which lets you recover your quantum information after corruption, however previously it was fairly limited in capability. The new research is a way to improve quantum error correction, so that the original information is recoverable after much more substantial corruption than was possible before.

Take cover!

[SEWilco]

Release the cat jokes!

Re:Begs the Question

[Eternauta3k]

Even better:

• Remove half of the components
• You still have a working computer
• Repeat

</misinterpreting the summary for fun and profit>

Re:works with half their components missing

[dmartine40]

Hmph! It'd just be a bunch of Schrodinger's cats taped together!

Oblig. Bad Car Analogy

[PPH]

...quantum computers "can continue to work perfectly even if half their components, or qubits, are missing."

Based on the number of spare parts I end up with after every time I tinker with it, so can my car.

Re:Entangled Backups

[PieSquared]

No, no, no. That's an off-site *mirror*, not a backup. Just... hyperdimensional RAID 1.

Re:Entangled Backups

[Clever7Devil]

"Hyperdimensional RAID 1" Sounds like a Dalek Universal Domination plan. "EXTerminate!"