  A long-time quantum memory capable of storing and measuring quantum
information at the single-qubit level is an essential ingredient for practical
quantum computation and com-munication. Recently, there have been remarkable
progresses of increasing coherence time for ensemble-based quantum memories of
trapped ions, nuclear spins of ionized donors or nuclear spins in a solid.
Until now, however, the record of coherence time of a single qubit is on the
order of a few tens of seconds demonstrated in trapped ion systems. The qubit
coherence time in a trapped ion is mainly limited by the increasing magnetic
field fluctuation and the decreasing state-detection efficiency associated with
the motional heating of the ion without laser cooling. Here we report the
coherence time of a single qubit over $10$ minutes in the hyperfine states of a
\Yb ion sympathetically cooled by a \Ba ion in the same Paul trap, which
eliminates the heating of the qubit ion even at room temperature. To reach such
coherence time, we apply a few thousands of dynamical decoupling pulses to
suppress the field fluctuation noise. A long-time quantum memory demonstrated
in this experiment makes an important step for construction of the memory zone
in scalable quantum computer architectures or for ion-trap-based quantum
networks. With further improvement of the coherence time by techniques such as
magnetic field shielding and increase of the number of qubits in the quantum
memory, our demonstration also makes a basis for other applications including
quantum money.
