  The intrinsic valley degree of freedom makes bilayer graphene a unique
platform for emerging types of semiconducting qubits. The single-carrier
quantum dot ground state exhibits a two-fold degeneracy where the two states
have opposite spin and valley quantum numbers. By breaking the time-reversal
symmetry of this ground state with an out-of-plane magnetic field, a novel type
of qubit (Kramers qubit), encoded in the two-dimensional spin-valley subspace,
becomes accessible. The Kramers qubit is robust against known spin- and
valley-mixing mechanisms, as it requires a simultaneous change of both quantum
numbers, potentially resulting in long relaxation and coherence times. We
measure the relaxation time of a single carrier in the excited states of a
bilayer graphene quantum dot at small ($\sim \mathrm{mT}$) and zero magnetic
fields. We demonstrate ultra-long spin-valley relaxation times of the Kramers
qubit exceeding $30~\mathrm{s}$, which is about two orders of magnitude longer
than the spin relaxation time of $400~\mathrm{ms}$. The demonstrated
high-fidelity single-shot readout and long relaxation times are the foundation
for novel, long-lived semiconductor qubits.
