## Quantum information: Building a quantum computer

发布时间：2019-03-13 03:20:03来源：未知点击：

By Vlatko Vedral (Image: The New York Times/Redux/eyevine) Read more: “Instant Expert 33: Quantum information“ There are many ways of making the “qubits” for a quantum computer to crunch, from polarising light to cooling atoms to taming the collective motions of electrons. But any qubit must fulfil some stringent criteria, particularly in proving robust, or “coherent”, in the face of buffeting from its surrounding classical environment. No single sort of qubit has yet ticked all the boxes In 1997, David DiVincenzo of IBM wrote down some desirable conditions that remain a rough, though not exhaustive, checklist for what any practical quantum computer must achieve. Scalability To out-gun a classical computer, a quantum computer must entangle and manipulate hundreds of qubits. Quantum computers built so far have just a handful. Scaling up is a big hurdle: the larger the system, the more prone it is to “decohere” in the face of environmental noise, losing its essential quantumness. “To out-gun a classical computer, we must entangle hundreds of qubits. So far we have managed a handful” Initialisation We must be able to reliably set all the qubits to the same state (to zero, say) at the beginning of a computation. Coherence The time before decoherence kicks in must be a lot longer than the time to switch a quantum logic gate – preferably,