  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],
whereas electron-spin qubits have been demonstrated in quantum dots [4-6]. The
phase coherence time (Tau2) and the single qubit figure of merit (QM) of
superconducting and electron-spin qubits are similar -- Tau2 ~ microseconds and
QM ~10-1000 below 100mK -- 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 tau2 (up to tens of ms) [7, 8], but these systems cannot be
scaled up, although some proposals for qubits based on 2D nanostructures should
be scalable [9-11]. Here we report that a new family of spin qubits based on
rare-earth ions demonstrates values of Tau2 (~ 50microseconds) and QM (~1400)
at 2.5 K, which suggests that rare-earth qubits may, in principle, be suitable
for scalable quantum information processing at 4He temperatures.
