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It Takes Two to Entangle

21 February 2003 (All day)
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Practical quantum computers may be a step closer now that physicists have figured out how to meld the quantum states of pairs of electrified dabs of metal, which might easily be assembled into circuits.

The quantum computer of tomorrow will crunch many numbers simultaneously by encoding them in quantum bits that can be set to 0, 1, or 0-and-1 at the same time. Those "qubits" might consist of hard-to-handle individual atoms, nuclei, electrons, or photons, all of which can be forced into two different quantum states at once. Or they might be manufactured of more tractable bits of metal. Since 1999, researchers have fashioned such "solid-state" qubits by coaxing a tiny patch of superconductor, called a Cooper pair box, into a condition in which it simultaneously does and does not contain one extra pair of electrons. But in a quantum computer, the qubits must also intertwine so that reading one affects the states of the others. For example, two qubits can be entangled so that if the first reads 0, the other will automatically read 1, and vice versa. This strange linkage is called entanglement. Even though researchers have entangled atoms, nuclei, and photons, no one had managed to entangle two solid-state qubits.

Now, a team of physicists has done the job by placing two Cooper pair boxes within nanometers of a sliver of metal. The metal conveys electric forces between the boxes, so that when both are placed in state with and without an extra electron pair, the two quantum states meld, report Jaw-Shen Tsai of the NEC Fundamental Research Laboratories in Tsukuba and the Institute of Physical and Chemical Research in Wako, Japan, and colleagues in the 20 February issue of Nature. To see the entanglement, the researchers monitored the tiny current leaking out of each Cooper pair box, which oscillated in a telltale way.

"This is a breakthrough for solid-state qubits," says Siyuan Han, a physicist at the University of Kansas in Lawrence, who is working on a different scheme for making qubits from tiny rings of superconductor. The researchers still have to show that the entangled states will last long enough to perform basic logical calculations, he says. But if they succeed it should be relatively easy to construct circuits from the pairs of bits, Han says--especially when the alternative is wiring up individual atoms, nuclei, or photons.

Related sites
Jaw-Shen Tsai's home page
How Stuff Works' explanation of quantum computing

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