Quantum bits, or qubits, form the heart of quantum-information processing schemes. Because of the quantum parallelism and entanglement that arise from the superposition of states in two-level qubit systems, researchers expect eventual quantum computers to tackle tasks, such as factoring large numbers and simulating large quantum systems, that no ordinary computers can do in a practical time frame.

Quantum computing involves preparing, manipulating, and reading out the quantum states of a many-qubit system. So it is desirable to have qubits that can be individually controlled. Moreover, they should be scalable; that is, simply adding more qubits should create a larger circuit capable of more complex calculations. Solid-state qubits satisfy these requirements.

Fortunately, very small solid-state devices can behave quantum mechanically. As the size of a bulk conductor becomes increasingly smaller, its quasi-continuous electron conduction band turns into discrete energy levels. An example is a quantum dot, in which electrons are...

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