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."

Comments

In case of Slashdotting...

[Anonymous Coward]

FOR evidence of the power of simplicity, you need look no further than a computer. Everything it does is based on the manipulation of binary digits, or bits--units of information that can be either 0 or 1. Using logical operations to combine those 0s and 1s allows computers to add, multiply and divide, and from there go on to achieve all the feats of the digital age. But at each step of the complex operations involved, each bit has a definite value.

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-

Re:You don't need to understand grammer..

[tm2b]

Are you serious? You started a grammar flame:

• on Slashdot,
• with an ellipse at the beginning,
• and mispelled grammar

in the process? Are you trolling for "funny" points or something?

Anyway, since you put "Quantum" at the beginning of your sentence, where it would have been capitalized anyway, it wasn't at all clear that it was capitalization that you were talking about. Sorry.

Eigenvalues

[manganese4]

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