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Finding the looking glass: researcher uncovers barrier between quantum and classical world
19 February 2001

By Catherine Zandonella, Media Relations

San Francisco - Like Alice stepping through the looking glass, the journey into the quantum realm of subatomic particles is fraught with strange phenomena.

Now, new research indicates that the looking glass is more than a helpful analogy - there is indeed a perceptible barrier between the large-scale world we know and understand, and the wacky quantum world of half-dead, half-alive cats and particles that can be in two places at once.

Rather than a gradual transition between the two realms as was previously thought, a computer scientist working at the University of California, Berkeley, has proven the existence of an actual turning point where classical behavior stops and quantum behavior starts.

Just as water freezes to become ice, the classical world shifts to quantum behavior at a specific energy threshold, at least for a certain type of quantum system. Controlling the transition could lead to the construction of robust quantum computers, said Dorit Aharonov, who completed her work on this project while a post-doctoral researcher at UC Berkeley. Now a faculty member in the computer science and engineering department at Hebrew University in Jerusalem, Aharonov will present this work on Monday, Feb. 19 at the annual meeting of the American Association for the Advancement of Science in San Francisco.

The research emerged from work Aharonov conducted with Michael Ben-Or of Hebrew University on a theoretical model of a quantum computer that could operate in the presence of noise from the classical world.

Noise wreaks havoc with quantum computers by interfering with the delicate quantum bits, or qubits, that perform the calculations. Unlike conventional computer bits (the 0s and 1s that make up the data in your computer), qubits are spinning subatomic particles that can assume the value of 0 or 1 simultaneously. This superposition of states, called entanglement, greatly expands the potential for super-fast parallel computing. But noise from the outside world knocks the qubits off their spin, causing them to decouple into ordinary classical particles, a process known as decoherence.

An active area of research, quantum computing can be used for data encryption and calculating quantum effects. While extremely powerful, quantum computers are not well-suited to replace their classical counterparts in the areas of word processing and arithmetic calculations.

In recent years, scientists have created error-correcting codes that compensate for noise - as long as the noise is below a certain value. These codes, in effect, hide the quantum calculations from the outside world. The success of these codes implies the existence of a specific level of noise that quantum computers can tolerate, leading Aharonov to calculate a distinct noise value at which the system goes classical.

Until now, most physicists have believed a continuum exists between the quantum and classical worlds. Instead, Aharonov's work shows that, rather than undergoing a gradual transition from the quantum to the classical realm, the computers undergo a rapid transition that happens at a specific noise level, analogous to the phase transition that water undergoes when it turns to ice at 0 degrees Celsius.

What is the value of this "melting point" for entanglement? To compute that value for general quantum systems, said Aharonov, would require calculating the entanglement for hundreds of qubits. "The current formula for entanglement is so complicated that it is simply impossible to compute such a value for large systems," she said.

An easier way to find the value would be to measure it directly, said Aharonov, but scientists have yet to invent a device capable of such measurements. "We need a quantum thermometer for measuring entanglement," she said.

Although its value remains unknown, the fact that the phase transition exists is quite significant for the field of quantum computing, said Aharonov, because it shows that fault-tolerant quantum computers can operate indefinitely, as long as the noise level can be kept down.

As yet, Aharonov has proven that the phase transition exists only for fault-tolerant quantum computers. If other types of quantum systems can co-exist with noise, might we start seeing cats that are simultaneously alive and dead, teleportation, and other more bizarre aspects of the quantum realm? Aharonov refused to speculate.

This research was published in Phys. Rev. A 62, 062311 (November, 2000).

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