The essence of quantum computing is simple to state: If you could entangle N two-level qubits, you'd have access not just to 2N combinations of binary states but also to their myriad quantum superpositions. Running on just 50 or so qubits, a quantum computer could crack integer factorization and other problems beyond the reach of the most powerful classical computer.

Physicists have already made single qubits from a range of ingredients, including Josephson junctions, quantum dots, and trapped ions. Other, more exotic recipes, like those based on topological states of fractional quantum Hall systems and superconducting strontium ruthenate, await their qubit debut.

As in juggling, the more objects you have to coordinate, the more precise and delicate your control has to be. Curiously, the record for juggling balls, 10, is about the same for entangling qubits. Three years ago, Rainer Blatt of the University of Innsbruck in Austria and...

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