  Trapped ions constitute one of the most promising systems for implementing
quantum computing and networking. For large-scale ion-trap-based quantum
computers and networks, it is critical to have two types of qubits, one for
computation and storage, while the other for auxiliary operations like runtime
qubit detection, sympathetic cooling, and repetitive entanglement generation
through photon links. Dual-type qubits have previously been realized in hybrid
systems using two ion species, which, however, introduces significant
experimental challenges for laser setup, gate operations as well as the control
of the fraction and positioning of each qubit type within an ion crystal. Here
we solve these problems by implementing two coherently-convertible qubit types
using the same ion species. We encode the qubits into two pairs of clock states
of the 171Yb+ ions, and achieve fast and high-fidelity conversion between the
two types using narrow-band lasers. We further demonstrate that operations on
one qubit type, including sympathetic laser cooling, gates and qubit detection,
have crosstalk errors less than 0.03% on the other type, well below the error
threshold for fault-tolerant quantum computing. Our work showcases the
feasibility and advantages of using coherently convertible dual-type qubits
with the same ion species for future large-scale quantum computing and
networking.
