Watched quantum pot boils slower

By Eric Smalley, Technology Research News

The Greek philosopher Zeno argued that motion does not exist because at any given point in an arrow's flight, it is not moving, and because it is still at all points of flight it never actually moves.

Though mathematicians solved this paradox long ago by introducing the concept of real numbers to describe quantities of duration and distance, the notion of freezing motion by observing it turns out to be a very real effect in the strange world of quantum physics.

Researchers at the University of Texas at Austin have demonstrated the Quantum Zeno Effect and Anti-Zeno Effect in sodium atoms trapped in a pair of laser beams.

The trapped sodium atoms escape over time, a process physicists call decay. By observing the atoms repeatedly at very short time intervals the researchers were able to either reduce or increase the probability that the atoms would escape after a certain amount of time.

The Quantum Zeno Effect holds the tantalizing possibility of bolstering the notoriously fragile quantum systems used in today's rudimentary quantum computing experiments. Likewise, the related Anti-Zeno Effect could hinder certain quantum computing schemes because the frequent measurements could hasten the demise of a quantum computer's tenuous bits.

Quantum bits, or qubits, can represent the ones and zeros of computing using distinct states of atoms or subatomic particles like magnetic orientation. If large-scale quantum computers can be built, they would be phenomenally fast for certain applications like cracking codes and searching large databases.

The researchers induced the Quantum Zeno Effect by observing their quantum system every microsecond, or millionth of a second. They brought about the Anti-Zeno Effect by observing the system every five microseconds.

"Just by looking at the system periodically one can modify decay," said Martin C. Fischer, who was a graduate student at the University of Texas at Austin when the research was conducted. "If measurements are performed at very short time intervals, the decay can be strongly suppressed, whereas observations at intervals slightly larger can enhance the decay."

The Quantum Zeno Effect doesn't produce a specific slower rate of decay but rather increases the probability that the atoms will be in the trap at a given point in time. "We achieved about 40 percent larger probability of survival at a certain time of decay, but that, of course, is not really a fixed number," said Fischer.

Delaying the escape of trapped sodium atoms isn't itself useful for quantum computing, but a better understanding of the Quantum Zeno Effect and Anti-Zeno Effect could play a role in quantum computing, said Fischer.

Although it's early to speculate about applications in quantum computing, the experiment does show that reduction of decay by measurement is possible, said Fischer. "Quantum computing requires more elements, but this experiment might provide an important ingredient," he said.

Preliminary investigations into using the Quantum Zeno Effect to make sturdier quantum computers are not all that promising, said Paul Kwiat, a physics professor at the University of Illinois at Urbana-Champaign.

"But there are new paradigms all the time," he added. "I wouldn't be surprised" if in five years the effect could be used in quantum computing, said Kwiat. "I also would not be surprised if it's not useful."

On the other hand, the Anti-Zeno Effect could be cause for concern, said Kwiat.

The equivalent of decay in a quantum computer, decoherence, occurs when energy from the environment knocks qubits out of their quantum states. Qubits are made from atoms or subatomic particles that are in the quantum state of superposition, which is a mixture of the two states that are used to represent the ones and zeros of computing. Controlling this mixture allows quantum computers to examine all possible answers to a problem at the same time.

Because it is challenging to stave off decoherence even for small numbers of qubits, researchers are looking to use error correction schemes. But quantum error correction schemes require frequent observations and some researchers fear they could induce the Anti-Zeno Effect, said Kwiat.

Whether or not the Quantum Zeno Effect can be harnessed to improve quantum computers, many researchers say it will be at least two decades before practical quantum computers can be developed.

"I don't think I will see a functional quantum computer on a level that could compete with conventional PCs in the next couple of decades. But you never know," said Fischer.

Fischer's research colleagues were Braulio Gutiérrez-Medina and Mark G. Raizen of the University of Texas at Austin. Their research has been accepted for publication in the journal Physical Review Letters. The research was funded by the National Science Foundation (NSF), the R. A. Welch Foundation and the Sid W. Richardson Foundation.

Timeline:   >20 years
Funding:   Government; Private
TRN Categories:   Quantum Computing
Story Type:   News
Related Elements:  Technical paper, "Observation of the Quantum Zeno and Anti-Zeno effects in an unstable system," Physical Review Letters, accepted for publication


September 5, 2001

Page One

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Quantum current closer to computing

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Software spins tales into animations

Watched quantum pot boils slower


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