STED microscopy reveals crystal colour centres with nanometric resolution

Because they have spin states that can be optically polarized and detected, fluorescent nitrogen vacancies in diamond(1-3) have considerable potential for applications in quantum cryptography(4,5) and computation(6-8), as well as for nanoscale magnetic imaging(9,10) and biolabelling(11,12). However, their optical detection and control are hampered by the diffraction resolution barrier of far-field optics. Here, we show that stimulated emission depletion (STED) microscopy(13,14) is capable of imaging nitrogen-vacancy centres with nanoscale resolution and Angstrom precision using focused light. The far-field optical control of the population of their excited state at the nanoscale expands the versatility of these centres and demonstrates the suitability of STED microscopy to image dense colour centres in crystals. Nitrogen-vacancy defects show great potential as tags for far-field optical nanoscopy(15) because they exhibit nearly ideal STED without bleaching. Measured point-spread functions of 5.8 nm in width demonstrate an all-physics-based far-field optical resolving power exceeding the wavelength of light by two orders of magnitude.