  Quantum sensing techniques have been successful in pushing the sensitivity
limits in numerous fields, and hold great promise for scanning probes that
study nano-scale devices and novel materials. However, forming a nano-scale
qubit that is simple and robust enough to be placed on a scanning tip, and
sensitive enough to detect various physical observables, is still a great
challenge. Here we demonstrate a conceptually new qubit implementation in a
carbon nanotube that achieves these requirements. In contrast to the prevailing
semiconducting qubits that use electronic states in double quantum dots, our
qubit utilizes the natural electronic wavefunctions in a single quantum dot.
Using an ultraclean nanotube we construct a qubit from two wavefunctions with
significantly different magnetic moments and spatial charge distributions,
making it sensitive to both magnetic and electric fields. We use an array of
gates to directly image these wavefunctions and demonstrate their localized
moments. Owing to their different spatial structure, these wavefunctions also
show radically different transport properties, giving us a simple
transport-based qubit readout mechanism. Due to its narrow coherence-limited
transition, the qubit demonstrates significantly better electric field
detection sensitivity than a single electron transistor. Moreover, with the
same qubit we demonstrate simultaneous probing of magnetic fields with DC
sensitivity comparable to that of NV centers. Our technique has minimal
requirements for device complexity, which can be implemented using a number of
straightforward fabrication methods. These features make this atomic-like qubit
a powerful new tool that enables a variety of new nanoscale imaging
experiments.
