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Latest Research on Quantum Computing
zeristor writes "The The Economist is running a story
about the latest progress in Quantum computing. It seems that what has been glossed over in Physics as a minor detail, the decoherence of the superposition of states, is actually quite fundamental to Quantum computing.
The decoherence can be measured by something called the Loschmidt echo (is this esoteric or am I just thick? This sounds like a bad episode of Star Trek.)
Also goes on to explain how entanglement can be prolonged. All in all very interesting developments."
Come on trolls, you're getting slow. It's the second article down the page. I've never seen a quantum computing article without somebody making the obligatory "It will even run Duke Nukem Forever!" joke.
We won't know until somebody reads the article and actually understands what it means.
I found it interesting that something that sounds quite fundamental to quantum physics has been passed over for so long with a 'And then something happens'.
The same cannot be said of many properties in quantum physics, such as the spin of an atomic nucleus (loosely speaking, which way it is pointing) or the position of an electron orbiting such a nucleus. At a small scale, such properties can have more than one value at once. In 1994, Peter Shor, a mathematician then at AT&T's Bell Laboratories in New Jersey, realised that a computer that used such quantum properties to represent information could factorise large numbers extremely quickly. This is an important problem, because much of modern cryptography is based on the difficulty of factorising large numbers--so being able to do so quickly would render many modern codes easily breakable. Then, in 1996, a colleague of Dr Shor's at Bell Labs, Lov Grover, showed that such a quantum computer would be able to search through an unsorted database much faster than an ordinary computer--another important application.
Computer technology
With these insights, quantum computing, which had first been thought of as a possibility in the early 1980s, became a hot topic of research. It was clear to many physicists that using "qubits"--which, unlike ordinary bits, can exist in a "superposition" of the values 0 and 1 simultaneously--might yield an exponential improvement in computing power. This is because a pair of qubits could be in four different states at once, three qubits in eight, and so forth. What Dr Shor and Dr Grover showed was that the improvement, if the technological hurdles could be overcome, would be not hypothetical, but real, and useful for important problems.
The technology necessary to manipulate qubits, in their various incarnations, is challenging. So far, nobody has managed to get a quantum computer to perform anything other than the most basic operations. But the field has been gathering pace, and was the topic of much discussion among the scientists gathered in Montreal for the annual March meeting of the American Physical Society, the largest physics conference in the world.
There are currently several different approaches to quantum computing, all of which rely on fundamentally different technologies, including ultra-cold ions that are cooled by lasers, pulses of laser light, nuclear-magnetic resonance and solid-state devices such as superconducting junctions or quantum dots (which are confined clouds of electrons). What all these technologies have in common is that they can be used to invoke and exploit the bizarre phenomenon of superposition.
Superposition is not simple. Though a qubit may, for a while, be in a state of superposition between 0 and 1, it must eventually choose between the two. And in even the best quantum computers, that choice, or "decoherence", happens in a fraction of a millisecond. Just how the choice is made, and how to prolong the preceding period of "coherence" that allows quantum computations to be made, constitute a long-unexplained gap at the heart of modern physics. For nearly 80 years, since the inception of quantum theory in the 1920s, most physicists were content to gloss over the process. What is perhaps surprising is that the technological challenge of quantum computing is now a driving force behind efforts to understand the most abstract and philosophical underpinnings of quantum mechanics.
Echoes of the future
Until a qubit interacts with the macroscopic world, which follows the classical laws of physics, it behaves according to the laws of quantum mechanics, which are well understood, at least by physicists. However, the interaction with the classical world--decoherence-
Ok, lets consider two dice to be our collections of qubits. They can each hold the superposition of the numbers 1 - 6. Shaking and throwing the dice cause the superposition of the values and decoherence happens when they come to rest.
The question is, what do we use as 'bang-bang' pulses in order to keep the dice from decohering until we can coerce them into making our point?
1. ???
2. Go shoot craps
3. Profit!
Why don't you send me a quantum echo from the future when this is all running nicely...
....to understand quantum mechanics, apparently. "Quantum" is not a proper noun, not even in the context of quantum mechanics!
-psy
Doesn't it make you feel good to know that our freedoms are protected by politicans, lawyers and journalists.
A number of threads have mentioned eigenvalues/eigenstates and how a system is represented by them.
Eigenvalues and eigenstates have meaning when in terms of an operator which represents the perturbation to or observation of the system.
Every operator has a characteristic set of eigenvectors. Every quantum system is described by a wavefunction and prior to a pertubation/observation this wavefunction can be described as a linear combonation of the eigenvectors of the operator.
Following a perpurbation/observation decribed by the operator, the quantum system will be described by a one and only one eigenvector of the operator.
Of course, the probability of a particular eigenvector being chosen is represented by the square of the eigenvalue of the eigenvector.
for more see wikipedia
I make my face look like this and concerned words come out.
IANAQP (I am not a quantum physist) but I think this decoherence is the same thing as the observer or observation of the state of a quantum variable in the Copenhagen interpretation of quantum physics.
recall schroedingers cat in the box - in a state of aliveness and deadness until you open the box at which point the wave equation collapses and it assumes one or the other.
The question is what makes this happen -- the answer seems to be -- we don't know.
To make quantum computers work we probably need to answer the question....then again maybe I should RTA.
The most rational explanation I've come up with so far is that we are actually existing inside of a simulated reality. If you were going to simulate a universe but only populate it with a couple of billion people, you would do much better to only pay attention to what each individual sees. That way, you could ignore the virtually infinite processing and memory requirements, and just have a processor per person, and interconnections between people in order to make the experience consistent. So Quantum Physics is just that the descisions about what happened are postponed until needed, and the logic glosses over some of the details.
Ask a metaphysical question, get a metaphysical answer...
while (sig==sig) sig=!sig;
while (sig==sig) sig=!sig; ::cringe::
:)
I just thought of an actual use for that code. Assume sig points to an audio port. The output tone would then indicate the load on the system. Owwwwww, owwww, pain, pain, lol.
As for Quantum weirdness, it kinda makes sense to me. It's hard to describe what I picture, but I'll take a stab at it. Our entire perceived reality would just be a single point or surface in a much larger system or reality. When it seems a quantum value has more than one value, it really has both values in neighboring areas of that higher dimention system. It would be a wave or ripple. Those two areas can interact or interfere to produce the mixed result here. There would be other "here"'s that see other results. What we think of as "quantum randomness" would actually be non-random effects caused by our contact/interaction with the surrounding unperceived reality and ripples.
Picture a ripple spreading out in all directions, relecting back, and interfering with another part of itself. We would only see a single point of that entire process. From our point of view that "ripple" was a particle, and to us it seems that particle took two or more paths at the same time and then interfered with itself. Exactly what quantum mechanics describes.
Or not
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- - You can't take something off the Internet! That's like trying to take pee out of a swimming pool.
Looking around on the web I found this rather good site on Quantum Computing.
This Loschmidt echo thing seems to be buried quite deep, I have not found it referenced in my Quantum Mechanics books.