  A single qubit provides sufficient computational capabilities to construct a
universal quantum classifier when assisted with a classical subroutine. This
fact may be surprising since a single qubit only offers a simple superposition
of two states and single-qubit gates only make a rotation in the Bloch sphere.
The key ingredient to circumvent these limitations is to allow for multiple
data re-uploading. A quantum circuit can then be organized as a series of data
re-uploading and single-qubit processing units. Furthermore, both data
re-uploading and measurements can accommodate multiple dimensions in the input
and several categories in the output, to conform to a universal quantum
classifier. The extension of this idea to several qubits enhances the
efficiency of the strategy as entanglement expands the superpositions carried
along with the classification. Extensive benchmarking on different examples of
the single- and multi-qubit quantum classifier validates its ability to
describe and classify complex data.
