  Bilayer graphene is a promising platform for electrically controllable qubits
in a two-dimensional material. Of particular interest is the ability to encode
quantum information in the so-called valley degree of freedom, a two-fold
orbital degeneracy that arises from the symmetry of the hexagonal crystal
structure. The use of valleys could be advantageous, as known spin- and
orbital-mixing mechanisms are unlikely to be at work for valleys, promising
more robust qubits. The Berry curvature associated with valley states allows
for electrical control of their energies, suggesting routes for coherent qubit
manipulation. However, the relaxation time of valley states -- which ultimately
limits these qubits' coherence properties and therefore their suitability as
practical qubits -- is not yet known. Here, we measure the characteristic
relaxation times of these spin and valley states in gate-defined bilayer
graphene quantum dot devices. Different valley states can be distinguished from
each other with a fidelity of over 99%. The relaxation time between valley
triplets and singlets exceeds 500ms, and is more than one order of magnitude
longer than for spin states. This work facilitates future measurements on
valley-qubit coherence, demonstrating bilayer graphene as a practical platform
hosting electrically controlled long-lived valley qubits.
