In theory, 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 full-scale quantum computer could produce reliable results even if its components performed no better than today’s best first-generation prototypes, according to researchers at the National Institute of Standards and Technology (NIST).

The image illustrates how qubits are grouped in blocks to form the levels. To implement the architecture with three levels, a series of operations is performed on 36 qubits (bottom row) — each one representing a 1, a 0, or both at once. The operations on the nine sets of qubits produce two reliably accurate qubits (top row). 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.

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, and teleportation of data at key intervals to continuously double-check the accuracy of qubit values. NIST physicists, who transferred key properties of one atom to another atom without using a physical link, demonstrated teleportation last year.

Use of this architecture could lead to reliable computing even if individual logic operations made errors as often as 3% of the time — performance levels already achieved in NIST laboratories with qubits based on ions (charged atoms). The proposed architecture could tolerate several hundred times more errors than scientists had generally thought acceptable.

The findings are based on several months of calculations and simulations on large, conventional computer workstations. The new architecture, which has yet to be validated by mathematical proofs or tested in the laboratory, relies on a series of simple procedures for repeatedly checking the accuracy of blocks of qubits. This process creates a hierarchy of qubits at various levels of validation.

Find out more at: http://math.nist.gov/mcsd/highlights/knill-qc-noisydevices.html