Optical-frequency combs enable measurement precision at the 20 th digit, and accuracy entirely commensurate with their reference oscillator. A new direction in experiments is the creation of ultracompact frequency combs by way of nonlinear parametric optics in microresonators. We refer to these as microcombs, and here we report a silicon-chip-based microcomb optical clock that phase-coherently converts an optical-frequency reference to a microwave signal. A low-noise comb spectrum with 25 THz span is generated with a 2 mm diameter silica disk and broadening in nonlinear fiber. This spectrum is stabilized to rubidium frequency references separated by 3.5 THz by controlling two teeth 108 modes apart. The optical clock's output is the electronically countable 33 GHz microcomb line spacing, which features an absolute stability better than the rubidium transitions by the expected factor of 108. Our work demonstrates the comprehensive set of tools needed for interfacing microcombs to state-of-the-art optical clocks.Optical clocks leverage the narrow, unvarying transitions of atoms to realize exceptionally stable laser frequencies measured at below the 10 -17 level (1, 2). Optical-frequency combs facilitate the measurement and use of these atomic references by providing a dense set of clock-referenced lines that span more than an octave (3). By way of their extraordinary measurement precision, frequency combs have enabled advances in diverse fields from spectroscopy of atoms and molecules (4, 5) to astronomy (6). Some of the most exciting, but yet unrealized, applications call for operation in environments that are unsuitable for existing comb technology.A new class of frequency combs has emerged based on optical microresonators (7,8). Here the comb generation relies on nonlinear parametric oscillation and cascaded four-wave mixing driven by a single CW laser. Such microcombs offer revolutionary advantages including chip-based photonic integration, uniquely large comb-mode spacings in the 10's of GHz range, and monolithic construction with small size and power consumption. Progress in microcomb development has included frequency control of their spectra (9-12), characterization of their noise properties (13-15), a Rb-stabilized microcomb oscillator (16), and demonstration of phase-locked (13,17,18) and modelocked states (19,20). However, the milestone of all-optical frequency control of a microcomb to an

Important features in the spectral and temporal photoluminescence excitation of single nitrogen-vacancy (NV) centers in diamond are reported at conditions relevant for quantum applications. Bidirectional switching occurs between the neutral (NV(0)) and negatively charged (NV(-)) states. Luminescence of NV(-) is most efficiently triggered at a wavelength of 575 nm which ensures optimum excitation and recharging of NV(0). The dark state of NV(-) is identified as NV(0). A narrow resonance is observed in the excitation spectra at 521 nm, which mediates efficient conversion to NV(0).

However, in contrast to a mode-locked laser comb there is little consensus on possible modelocking mechanisms (e.g. a saturable absorber) in microcombs, which could act to enforce equidistance of the modes and align the phases to generate ultrashort optical pulses. This is the

No abstract

No abstract