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A full-scale quantum computer could produce reliable results even if its components performed no better than today’s best first-generation prototypes, according to a paper by a scientist at the National Institute of Standards and Technology (NIST) in the March 3 issue of the journal Nature*.

The new NIST architecture for quantum computing relies on several levels of error checking to ensure the accuracy of quantum bits (qubits). The purple spheres represent qubits that are either used in error detection or in actual computations. The yellow spheres are qubits that are measured to detect or correct errors but are not used in final computations.

In theory, such a quantum computer could be used to BREAK commonly used encryption codes, to improve optimization of complex systems such as airline schedules, and to simulate other complex quantum systems.

A key issue for the reliability of future quantum computers—which would rely on the unusual properties of nature’s smallest particles to store and process data—is the fragility of quantum states. Today’s computers use millions of transistors that are switched on or off to reliably represent values of 1 or 0. Quantum computers would use atoms, for example, as quantum bits (qubits), whose magnetic and other properties would be manipulated to represent 1 or 0 or even both at the same time. These states are so delicate that qubit values would be unusually susceptible to errors caused by the slightest electronic "noise."

To get around this problem, NIST scientist Emanuel Knill suggests using a pyramid-style hierarchy of qubits made of smaller and simpler building blocks than envisioned previously, with teleportation of data at key intervals to continuously double-check the accuracy of qubit values. Teleportation was demonstrated last year by NIST physicists, who transferred key properties of one atom to another atom without using a physical link.

"There has been a tremendous gap between theory and experiment in quantum computing,” Knill says. "It is as if we were designing today's supercomputers in the era of vacuum tube computing, before the invention of transistors. This work reduces the gap, showing that building quantum computers may be easier than we thought. However, it will still take a lot of work to build a useful quantum computer."
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