19 November 2010

Quantum world more ordered than thought

Cosmos Online
Two of the cornerstones of quantum theory are unexpectedly and fundamentally linked, which explains why quantum theory "isn't as weird as it could be", scientists said.
Werner Heisenberg

Werner Heisenberg, whose Uncertainty Principle says it is impossible to know about certain pairs of properties to high precision.

SYDNEY: Two of the cornerstones of quantum theory are unexpectedly and fundamentally linked, which explains why quantum theory “isn’t as weird as it could be”, scientists said.

Quantum mechanics is a hugely successful physical theory that describes our world at the micro-scale. Although aspects of the theory are deeply counterintuitive, repeated experiments have shown that it does accurately predict the results of real-world events.

The new research, published this week in the journal Science, deals with two of the theory’s most important and mind-bending properties: Heisenberg’s Uncertainty Principle and nonlocality. Originally thought to be distinct concepts, two physicists have shown that the two are fundamentally linked, and that the link must hold for all physical theories.

Einstein: “Spooky action at a distance”

Heisenberg’s Uncertainty Principle says that it is impossible to know certain things about a quantum particle – such as its momentum and position – simultaneously. Knowledge of one of these properties affects the accuracy with which you can learn the other.

Non-locality, which is related to the phenomenon of entanglement, means that when two quantum particles are entangled, they can influence each other instantaneously. Albert Einstein famously referred to non-locality as “spooky action at distance”, because it defies classical physics, in which two objects separated in space cannot interact with something mediating the interaction.

Until now, researchers had treated these two phenomena as separate entities. In fact, 75 years ago Albert Einstein and colleagues suggested that quantum mechanics must be incomplete because in their opinion the two phenomena were potentially contradictory.

Nonlocality depends on two factors

Stephanie Wehner of Singapore’s Centre for Quantum Technologies and the National University of Singapore, and Jonathan Oppenheim of the United Kingdom’s University of Cambridge, set out to try and understand the limits of nonlocality in quantum mechanics.

They found an equation that shows how the degree of non-locality is determined by the uncertainty principle. “More specifically, the degree of nonlocality of any theory is determined by two factors: the strength of the uncertainty principle and the strength of a property called “steering,” which determines which states can be prepared at one location given a measurement at another,” the researchers wrote in their paper.

“Quantum theory is pretty weird, but it isn’t as weird as it could be. We really have to ask ourselves, why is quantum mechanics this limited? Why doesn’t nature allow even stronger non-locality?” Oppenheim said in a statement.

The breakthrough came because they approached the problem using the techniques of computer science, Wehner told Cosmos Online. “When adopting the viewpoint that actually both of these concepts are about encoding and retrieving information, it is actually very natural to imagine that these two concepts are linked.”

Rephrasing the phenomena of quantum physics in terms that might be familiar to a computer hacker, the researchers treat non-locality as the result of as one entity, which they dub Alice, creating and encoding information, and another entity, called Bob, retrieving information from this encoding.

How well Alice and Bob can encode and retrieve information is determined by the uncertainty relations, the researchers found.

Cannot get rid of link now

“Our discovery is really an example of how concepts from different branches of science, physics and computer science, can come together to shed light on very fundamental questions about nature,” Wehner said.

The theoretical framework that Wehner and Oppenheim have discovered creates a fundamental link between the two concepts.

“In fact, this link is so fundamental that we could not get rid of it even if we were one day to replace quantum mechanics with some other physical theory,” Wehner said. “There’s a lot known about these concepts individually and it would be great to see whether we can know transfer knowledge of one, to gain a deeper understanding of the other.”

Still searching for understanding…

Australian researchers said the results could open new vistas in our understanding of quantum mechanics. “It’s a fascinating connection that people really hadn’t appreciated before,” said Andrew Doherty from the University of Sydney’s Quantum Control Laboratory.

“Understanding how these two fundamental aspects of quantum mechanics are connected will most likely lead to further advances in this field,” added Alessandro Fedrizzi, a researcher at the Centre for Quantum Computer Technology at the University of Queensland in Brisbane. “However, as always with fundamental research at this level, it is hard to predict what they will be.”

But Professor Howard Wiseman, director of the Centre for Quantum Dynamics at Griffith University on the Gold Coast expressed some caution. “We’re still searching for an explanation for why quantum correlations are the way they are that can be explained to a lay person,” he said.


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