Abstract

Quantum bits (qubits) are the basic building blocks of any quantum computer. Superconducting qubits have been created with a top-down approach that integrates superconducting devices into macroscopic electrical circuits, and electron-spin qubits have been demonstrated in quantum dots. The phase coherence time (|[tau]|2) and the single qubit figure of merit (QM) of superconducting and electron-spin qubits are similar — at |[tau]|2|[nbsp]||[sim]||[nbsp]||[micro]|s and QM|[nbsp]||[sim]||[nbsp]|10–1,000 below 100|[nbsp]|mK — and it should be possible to scale up these systems, which is essential for the development of any useful quantum computer. Bottom-up approaches based on dilute ensembles of spins have achieved much larger values of |[tau]|2 (up to tens of milliseconds; refs|[nbsp]|7,8), but these systems cannot be scaled up, although some proposals for qubits based on two-dimensional nanostructures should be scalable. Here we report that a new family of spin qubits based on rare-earth ions demonstrates values of |[tau]|2 (|[sim]|50|[nbsp]||[micro]|s) and QM (|[sim]|1,400) at 2.5|[nbsp]|K, which suggests that rare-earth qubits may, in principle, be suitable for scalable quantum information processing at 4He temperatures.