**It’s not yet possible to build a general-purpose quantum computer with enough quantum bits (qubits) to do anything useful, but many researchers hope eventually to achieve that goal. They’re mostly using one of two strategies to store and manipulate qubits: superconducting loops or trapped ions. Both of these approaches are prone to error, though, making it necessary to build in much redundancy so that the needed error correction can be done. It might ultimately require thousands of physical qubits working together to make up a single error-corrected logical qubit. And that could make it impossible to construct a general-purpose quantum computer.**

**That’s why some researchers (in particular, some working at Microsoft) have long considered a third strategy: building a “topological” quantum computer, which, if it were ever to come to pass, would have qubits that were much less prone to error. Constructing a topological quantum computer would, however, be much harder than what’s being explored now with superconducting loops and trapped ions. Indeed, it’s very hard even to describe what a topological supercomputer uses for qubits.**

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