New Scientist: Quantum Quiet

From: Ziana Astralos (
Date: Tue Jan 09 2001 - 20:16:14 MST

Daily News

Quantum quiet

1900 GMT, 4 January 2001

Adrian Cho

Delicate quantum states can be shielded from destructive noise, say physicists, bringing quantum computers a step closer

Information encoded in peculiar quantum states is immune to a common type of noise, physicists report. Such protective states could help give the quantum computers of tomorrow enough time to think clearly.

In the strange world of quantum mechanics, tiny objects can exist in two different states at the same time. For example, a magnetic ion can point both up and down at the same time.

This makes ions good candidates for "q-bits," the data bits in a quantum computer that can represent 0 and 1 at the same time. Unfortunately, the up-and-down state ordinarily lasts only a fraction of a second before noise from the surrounding environment destroys it.

However the information will survive 3.6 times longer if it is encoded in certain more complex quantum states involving a pair of ions, report physicists David Kielpinski, David Wineland and their colleagues at the National Institute of Standards and Technology in Boulder, Colorado.

The results should cheer researchers struggling to increase the lifetimes of q-bits, says Paul Kwiat, a physicist at Los Alamos National Laboratory in New Mexico. He says: "It's a crucial first step because you don't get to a factor of 10 without getting to a factor of three first."

Getting phased

A single ion in an up-and-down state gets muddled because any stray magnetic field causes the up and down components of the state to have slightly different energies. This causes the two components to collect different amounts of phase, a quantity similar to time measured on a stopwatch. The stopwatch for the higher energy component runs faster.

Random magnetic noise quickly obliterates the initial phase between the up and down components. But that information is vital for any quantum computation.

The team shut out such noise by encoding and storing the initial phase of the single ion in particular states of a pair of ions. In these states, the first ion points up and the second points down.

Such states collect phase at precisely the same rate because both have one ion pointing along and one ion pointing against the magnetic field, Kielpinski says: "The noise can't distinguish which q-bit is which. Up-down is the same as down-up to the noise."

The special states form a so-called decoherence-free subspace, and such states can be implemented in other q-bit systems, such as nuclear spins in large organic molecules or tiny rings of superconductors.

"Maybe someone will find a q-bit that is extremely robust by itself without using decoherence-free subspaces," Wineland says. "But most people think this is the way to go."

The research is published in Science.

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