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Classical Chaos Occurs In The Quantum World, Scientists Find

Classical Chaos Occurs In The Quantum World, Scientists Find

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aaaaEven tiny, easily overlooked events can completely change the behavior of a complex system, to the point where there is no apparent order to most natural systems we deal with in everyday life.

The weather is one familiar case, but other well-studied examples can be found in chemical reactions, population dynamics, neural networks and even the stock market. Scientists who study “chaos” — which they define as extreme sensitivity to infinitesimally small tweaks in the initial conditions — have observed this kind of behavior only in the deterministic world described by classical physics.

Until now, no one has produced experimental evidence that chaos occurs in the quantum world, the world of photons, atoms, molecules and their building blocks.

This is a world ruled by uncertainty: An atom is both a particle and a wave, and it’s impossible to determine its position and velocity simultaneously. And that presents a major problem. If the starting point for a quantum particle cannot be precisely known, then there is no way to construct a theory that is sensitive to initial conditions in the way of classical chaos. Yet quantum mechanics is the most complete theory of the physical world, and therefore should be able to account for all naturally occurring phenomena.

jessenThe problem is that people don’t see [classical] chaos in quantum systems,” said Professor Poul Jessen of the University of Arizona. “And we believe quantum mechanics is the fundamental theory, the theory that describes everything, and that we should be able to understand how classical physics follows as a limiting case of quantum physics.”

Experiments Reveal Classical Chaos In Quantum World

Now, however, Jessen and his group in UA’s College of Optical Sciences have performed a series of experiments that show just how classical chaos spills over into the quantum world. The scientists report their research in the Oct. 8 issue of the journal Nature in an article titled, “Quantum signatures of chaos in a kicked top.” Their experiments show clear fingerprints of classical-world chaos in a quantum system designed to mimic a textbook example of chaos known as the “kicked top.”

The quantum version of the top is the “spin” of individual laser-cooled cesium atoms that Jessen’s team manipulate with magnetic fields and laser light, using tools and techniques developed over a decade of painstaking laboratory work.

“Think of an atom as a microscopic top that spins on its axis at a constant rate of speed,” Jessen said. He and his students repeatedly changed the direction of the axis of spin, in a series of cycles that each consisted of a “kick” and a “twist”.

bbbbBecause spinning atoms are tiny magnets, the “kicks” were delivered by a pulsed magnetic field. The “twists” were more challenging, and were achieved by subjecting the atom to an optical-frequency electric field in a precisely tuned laser beam. They imaged the quantum mechanical state of the atomic spin at the end of each kick-and-twist cycle with a tomographic technique that is conceptually similar to the methods used in medical ultrasound and CAT scans. The end results were pictures and stop-motion movies of the evolving quantum state, showing that it behaves like the equivalent classical system in some significant ways.

One of the most dramatic quantum signatures the team saw in their experiments was directly visible in their images: They saw that the quantum spinning top observes the same boundaries between stability and chaos that characterize the motion of the classical spinning top. That is, both quantum and classical systems were dynamically stable in the same areas, and dynamically erratic outside those areas.

A New Signature Of Chaos Called ‘Entanglement’

Jessen’s experiment revealed a new signature of chaos for the first time. It is related to the uniquely quantum mechanical property known as “entanglement.”

Entanglement is best known from a famous thought experiment proposed by Albert Einstein, in which two light particles, or photons, are emitted with polarizations that are fundamentally undefined but nevertheless perfectly correlated. Later, when the photons have traveled far apart in space, their polarizations are both measured at the same instant in time and found to be completely random but always at right angles to each other.

“It’s as though one photon instantly knows the result for the other and adjusts its own polarization accordingly,” Jessen said.

By itself, Einstein’s thought experiment is not directly related to quantum chaos, but the idea of entanglement has proven useful, Jessen added.

“Entanglement is an important phenomenon of the quantum world that has no classical counterpart. It can occur in any quantum system that consists of at least two independent parts,” he said.

Theorists have speculated that the onset of chaos will greatly increase the degree to which different parts of a quantum system become entangled. Jessen took advantage of atomic physics to test this hypothesis in his laboratory experiments. The total spin of a cesium atom is the sum of the spin of its valence electron and the spin of its nucleus, and those spins can become quantum correlated exactly as the photon polarizations in Einstein’s example.

In Jessen’s experiment, the electron and nuclear spins remained unentangled as a result of stable quantum dynamics, but rapidly became entangled if the dynamics were chaotic. Entanglement is a buzzword in the science community because it is the foundation for quantum cryptography and quantum computing.

“Our work is not directly related to quantum computing and communications,” Jessen said. “It just shows that this concept of entanglement has tendrils in all sorts of areas of quantum physics because entanglement is actually common as soon as the system gets complicated enough.”

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Science has seen the future…and it’s invisible!

In the early 1990s, George Bush Senior led the U.S. into war with Iraq’s Saddam Hussein. “Operation Desert Storm” became the first war to be televised “live.” Amid the images of explosions and soldiers and tanks covered in desert camouflage, the war also shed light on the Stealth jet fighter. Though it had been in use by the military since the early 1970s, for the first time it registered in the popular consciousness that this sleek jet fighter was virtually invisible to radar. At the time, being invisible to radar was a concept that seemed to come straight from the movies, rather than an evening news report.

Fast forward to 2008, with American forces embedded in a much different Iraq and the talk about invisibility circulating at the Pentagon has gone beyond radar, and into the realm of sight. Or out of sight, quite literally. Invisibility, once thought to be scientifically impossible and an outlandish concept promoted only in science fiction, is back, so to speak, on the radar.

In fact, one of the world’s foremost physicists, Michio Kaku, has put his academic mind to some of science fiction’s other concepts, such as teleportation and force fields, and is convinced that they, too, can become reality. At Duke University, Kaku explains, researchers funded by the military were able in 2006 to render a microscopic object invisible to microwave radiation. Then, a few months ago, researchers at Cal Tech and in Germany achieved the same result with visible light.

“They were able to achieve invisibility to red and green light. Single colours of light can be bent in a way consistent with invisibility on a microscopic scale using nanotechnology,” Kaku says. This has huge potential on the battlefield. Imagine a tank being invisible to enemy forces. No wonder the Pentagon is bankrolling research in this field. “The next step is to do a large object at one light colour,” Kaku says. “Within 10 years, we may be able to make an object completely invisible to one colour of light.”


And that is only one of the seemingly outrageous accomplishments in the works that Kaku discusses in his new book, Physics of the Impossible.

While the chances of someone being teleported – as in the recent hit movie Jumper – is highly unlikely, Kaku points out that teleportation of an inorganic molecule has already been achieved. And how about the fact that, while time travel poses philosophical questions that can twist your mind like trying to squeeze water out of a soaking wet towel, on principle it does not violate the known laws of physics. In the introduction of the book, he warns against ruling out great possibilities because “in my own short lifetime, I have seen the seemingly impossible become established fact over and over again.” Commenting on his book, which was published in March, Kaku says: “We are taking ideas that are usually the property of science fiction and we are looking at them with a very serious analysis with the most recent advances in physics. Science is doubling every 10 years – it’s almost too much information to print. As a result, the public is really quite unaware of the breakthroughs that we are looking forward to over the next few decades.”

How is it possible to make something invisible? Kaku believes that by using metamaterials, a substance with optical properties not found in nature, scientists will be able to eventually render subjects invisible. Another seemingly impossible idea that Kaku deals with is travel outside of our solar system. “The idea of warping in space comes from Einstein not Star Trek, and the invention the atomic bomb was predicted almost to the date in an H.G Wells novel.”

While the concept of bringing a mega-size starship with hundreds of people aboard to another star is not likely, he says NASA is making advances toward sending billions of self-reproducing nanosized exploration vessels throughout the galaxy. Some of the changes that excite Kaku are the possibilities of computers carrying information through light instead of electricity, or computers functioning on DNA molecules. Another reality that may change our view of the possible is the question of extraterrestrial life.

“It’s almost a certainty; microbial life for sure,” Kaku explains. “The odds are that there are civilizations much more advanced than us. We can count 100 billion stars in our galaxy and 100 billion galaxies in the visible universe. That’s 100 billion squared for the number of stars in the visible universe. The probability that one of these stars has a planet that will have life more intelligent than us, I think, is 100 per cent.” This marriage of science fact and science fiction, while exciting, Michio concedes, is nothing new. Instead, Michio points out that they are interrelated traditions.

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