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(1-3), and electron-spin qubits have been demonstrated in quantum dots(4-6). The phase coherence time (tau(2)) and the single qubit figure of merit (Q(M)) of superconducting and electron-spin qubits are similar-at tau(2)similar to ms and Q(M)similar to 10-1,000 below 100 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 7,8), but these systems cannot be scaled up, although some proposals for qubits based on two-dimensional nanostructures should be scalable(9-11). Here we report that a new family of spin qubits based on rare-earth ions demonstrates values of tau(2) (similar to 50 ms) and Q(M) (similar to 1,400) at 2.5 K, which suggests that rare-earth qubits may, in principle, be suitable for scalable quantum information processing at He-4 temperatures.