An apparatus includes a light source, an optical fiber, having a tapered region, coupled to the light source, a stretchable microresonator in contact with the tapered region of the fiber and in contact with a magnetostrictive material, and a polarization controller that controls polarization of light in the tapered region.

A sensor (1, 2, 3, 4, 5, 6, 7, 8) comprising a first diamond substrate (9) with at least one colour centre (15), the sensor (1, 2, 3, 4, 5, 6, 7, 8) further comprising a first piezomagnetic (10) or piezoelectric primary element (11), which primary element (10, 11) is arranged to interact with the colour centre(s) (15) of the first diamond substrate (9).