Several Quantum Calculations Combined At NIST 91
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."
Re:This may be slightly off-topic, but (Score:4, Informative)
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 (Score:5, Informative)
Re:Begs the Question (Score:5, Informative)
Re:This may be slightly off-topic, but (Score:5, Informative)
Re:This may be slightly off-topic, but (Score:3, Informative)
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 (Score:3, Informative)
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 (Score:3, Informative)
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.
Quick Quantum Computing Explanation (Score:1, Informative)
In the quantum universe, you can take a fundamental property like "position" and put a particle into a superposition state. A particle can be at position a with some probability and position b with some probability. Amazing, huh?
Now, the second component is that you can use quantum entanglement to create superposition states of multiple particles. Einstein had this great idea where measuring the state of one particle tells you what the state of another entangled particle is. This is fundamentally what allows for the improvement over classical algorithms. You can know the states of two things with one measurement.
One more example of quantum powers: When you put a particle in a superposition state, you can store a lot of information this way (sort of). You can give it a probability to be in place a of: 0.1234567891 and probability to be at place b of 1 - 0.123456789. However, you have to make repeated measurements to access this information.
Anyway, it's a fascinating subject. Caltech has a good page online somewhere. I hope that people everywhere can get an appreciation of this brilliant new field!
Re:This may be slightly off-topic, but (Score:2, Informative)
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).