  Integrating quantum materials with fibre optics adds advanced functionalities
to a variety of applications, and introduces fibre-based quantum devices such
as remote sensors capable of probing multiple physical parameters. However,
achieving optimal integration between quantum materials and fibres is
challenging, particularly due to difficulties in fabrication of quantum
elements with suitable dimensions and an efficient photonic interface to a
commercial optical fibre. Here we demonstrate a new modality for a
fibre-integrated van der Waals quantum sensor. We design and fabricate a
hole-based circular Bragg grating cavity from hexagonal boron nitride (hBN),
engineer optically active spin defects within the cavity, and integrate the
cavity with an optical fibre using a deterministic pattern transfer technique.
The fibre-integrated hBN cavity enables efficient excitation and collection of
optical signals from spin defects in hBN, thereby enabling all-fibre integrated
quantum sensors. Moreover, we demonstrate remote sensing of a ferromagnetic
material and of arbitrary magnetic fields. All in all, the hybrid fibre-based
quantum sensing platform may pave the way to a new generation of robust,
remote, multi-functional quantum sensors.
