Many researchers are vying to exploit the quantum mechanical properties of superposition and entanglement to achieve massive parallel processing. They hope that a quantum computer can be far faster than today’s supercomputers and tackle problems that elude even the biggest ones.

The basic building block of a quantum computer is a two-level quantum system known as a quantum bit, or qubit. In its ground state, the qubit represents a “0” and in its excited state, a “1.” Because a qubit may be in a superposition of those states, a quantum computer with n qubits can be in an arbitrary superposition of 2 n different states simultaneously.

Physics presents no dearth of possible two-level systems as candidates for the qubits. Researchers have extensive experience with single-particle candidates such as atoms, ions, and nuclei. But they can also make qubits from solid-state systems, such as Josephson junctions or quantum dots, which exploit...

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