Portland, Ore. — Scientists at the National Institute of Standards and Technology predict a sixfold improvement in the speed of synchronizing atomic clocks, and more than a doubling of magnetic-sensor sensitivity, by applying the lessons learned from NIST's demonstration of the world's largest quantum computer.

NIST showed that six qubits could be entangled in synchronized "Schrödinger's cat" states of superposition — simultaneously spinning "up" and "down" — thereby enabling both ones and zeros to be superimposed. The previous world's record was an IBM Corp. five-qubit-device quantum computer using flourine atoms (www.eetimes.com/story/technology/OEG20000822S0007) instead of the beryllium atoms used here by NIST.

"There is no limit to how many qubits you can entangle, so we have to find the limits experimentally," said NIST physicist Dietrich Leibfried. "We have a friendly competition with a group of researchers at the University of Innsbruck [Austria] who have actually entangled eight ions, but their entangled state is not as sensitive as our Schrödinger cat state — where every bit simultaneously affects every other bit.

"Our goal is to do quantum information processing — build a so-called quantum computer," Leibfried said. "As a milestone along the way, we have created this demonstration of how much control we already have over quantum states."

The IBM five-qubit quantum computer and Innsbruck's eight-qubit design use different quantum effects than the more-sensitive Schrödinger's cat state. "We are entangling the ions in a very controlled way," said Leibfried.

In 1935, Austrian physicist Erwin Schrödinger proposed an experiment that took until the 21st century to test — namely, that a single atom could affect the state

of macroscopic systems. In Schrödinger's experiment, the state of a single atom is linked with whether a cat in a box is alive or dead. Theoretically, the cat is both alive and dead until the state of the atom is measured, thereby sealing its fate.

"In our experiment we measure the state of one atom, and the others are like Schrödinger's cats. That is, when we measure the state of any one atom, depending on what we measure, all the others collapse to that same state," said Leibfried.

The point of Schrödinger's experiment is that there must be some limit to the number of atoms that can exhibit quantum effects. In a quantum computer, a qubit, or quantum bit, takes on the property of being both a 1 and a 0 simultaneously — a phenomenon called superposition — thereby speeding calculations by allowing operations to be performed on superimposed ones and zeros at the same time. Likewise, in the NIST experiment

all six qubits simultaneously represented both a 1 and a 0 but also were linked to one another with absolute synchronization, enabling sixfold speedups of applications such as synchronizing atomic clocks.

"Schrödinger's point was that we do not experience quantum effects in the macroscopic world, so there must be some number of atoms between our six atoms and the 1026 atoms in a cat where quantum effects disappear," said Leibfried. "At NIST, we would like to know where this boundary is between individual atoms, where we observe these quantum effects, and the macroscopic world, where these superpositions don't happen."

In applications, Schrödinger cat states could make it possible to set atomic clocks six times faster than today, since it would be six times easier to synchronize their frequencies. Likewise, entangled qubits could enable fault tolerance in quantum computers by providing sixfold easier verification that a quantum calculation had been performed without disturbances.

NIST also proposes using the six-synchronized states to build more-sensitive sensors. Higher sensitivity to disturbances could enable quantum encryption algorithms that would foil undetected eavesdropping.

The NIST experiment held the six atoms stationary in an electromagnetic trap. It used ultraviolet lasers to cool them almost to absolute zero and then synchronize their states by entangling them. The Schrödinger cat states lasted about 50 microseconds and could be repeated every millisecond.