The ability to engineer photon emission and photon scattering is at the heart of modern photonics applications ranging from light harvesting, through novel compact light sources, to quantuminformation processing based on single photons. Nanophotonic waveguides are particularly well suited for such applications since they confine photon propagation to a 1D geometry thereby increasing the interaction between light and matter. Adding chiral functionalities to nanophotonic waveguides lead to new opportunities enabling integrated and robust quantum-photonic devices or the observation of novel topological photonic states. In a regular waveguide, a quantum emitter radiates photons in either of two directions, and photon emission and absorption are reverse processes. This symmetry is violated in nanophotonic structures where a non-transversal local electric field implies that both photon emission [1,2] and scattering [3] may become directional. Here we experimentally demonstrate that the internal state of a quantum emitter determines the chirality of single-photon emission in a specially engineered photonic-crystal waveguide. Single-photon emission into the waveguide with a directionality of more than 90% is observed under conditions where practically all emitted photons are coupled to the waveguide. Such deterministic and highly directional photon emission enables on-chip optical diodes, circulators operating at the single-photon level, and deterministic quantum gates. Based on our experimental demonstration, we propose an experimentally achievable and fully scalable deterministic photon-photon CNOT gate, which so far has been missing in photonic quantum-information processing where most gates are probabilistic [4]. Chiral photonic circuits will enable dissipative preparation of entangled states of multiple emitters [5], may lead to novel topological photon states [6,7], or can be applied in a classical regime to obtain highly directional photon scattering [8][9][10].Truly 1D photon-emitter interfaces are desirable for a range of applications in photonic quantum-information processing [11]. To this end, photonic-crystal waveguides constitute an ideal platform featuring on-chip integration with the ability to engineer the light-matter coupling. Recent experiments have achieved a coupling efficiency for a single quantum dot (QD) to a photoniccrystal waveguide in excess of 98%, thus constituting a deterministic 1D photon-emitter interface [12]. Standard photonic-crystal waveguides are mirror symmetric around the center of the waveguide and as a consequence the mode polarization is predominantly linear at the positions where light intensity is high. By designing a photonic-crystal waveguide that breaks this symmetry, modes that are circularly polarized at the field maxima can be engineered. We refer to this novel type of waveguide as a glide-plane waveguide (GPW), cf. Supplementary Material for further descriptions of the structural parameters. In a GPW, a QD with a circularly polarized transition dipole emits preferential...

Magnetometers based on ensembles of nitrogen-vacancy centres are a promising platform for continuously sensing static and low-frequency magnetic fields. Their combination with phase-sensitive (lock-in) detection creates a highly versatile sensor with a sensitivity that is proportional to the derivative of the optical magnetic resonance lock-in spectrum, which is in turn dependant on the lock-in modulation parameters. Here we study the dependence of the lock-in spectral slope on the modulation of the spin-driving microwave field. Given the presence of the intrinsic nitrogen hyperfine spin transitions, we experimentally show that when the ratio between the hyperfine linewidth and their separation is ≳ 1/4, square-wave based frequency modulation generates the steepest slope at modulation depths exceeding the separation of the hyperfine lines, compared to sine-wave based modulation. We formulate a model for calculating lock-in spectra which shows excellent agreement with our experiments, and which shows that an optimum slope is achieved when the linewidth/separation ratio is ≲ 1/4 and the modulation depth is less then the resonance linewidth, irrespective of the modulation function used.

Ensembles of nitrogen-vacancy centers in diamond are a highly promising platform for high-sensitivity magnetometry, whose efficacy is often based on efficiently generating and monitoring magneticfield-dependent infrared fluorescence. Here, we report on an increased sensing efficiency with the use of a 532-nm resonant confocal cavity and a microwave resonator antenna for measuring the local magnetic noise density using the intrinsic nitrogen-vacancy concentration of a chemical-vapor deposited singlecrystal diamond. We measure a near-shot-noise-limited magnetic noise floor of 200 pT= ffiffiffiffiffiffi Hz p spanning a bandwidth up to 159 Hz, and an extracted sensitivity of approximately 3 nT= ffiffiffiffiffiffi Hz p , with further enhancement limited by the noise floor of the lock-in amplifier and the laser damage threshold of the optical components. Exploration of the microwave and optical pump-rate parameter space demonstrates a linewidth-narrowing regime reached by virtue of using the optical cavity, allowing an enhanced sensitivity to be achieved, despite an unoptimized collection efficiency of < 2%, and a low nitrogen-vacancy concentration of about 0.2 ppb.

We demonstrate magnetic-field sensing using an ensemble of nitrogen-vacancy centers by recording the variation in the pump-light absorption due to the spin-polarization dependence of the total ground-state population. Using a 532 nm pump laser, we measure the absorption of native nitrogen-vacancy centers in a chemical-vapor-deposited diamond placed in a resonant optical cavity. For a laser pump power of 0.4 W and a cavity finesse of 45, we obtain a noise floor of ∼100 nT/ √ Hz spanning a bandwidth up to 125 Hz. We project a photon shot-noise-limited sensitivity of ∼1 pT/ √ Hz by optimizing the nitrogen-vacancy concentration and the detection method.

Time-integrated and time-resolved microphotoluminescence studies have been performed on Inx Ga1−xN quantum disks at the tips of GaN nanocolumns. The results are analyzed in the context of current theories regarding an inhomogeneous strain distribution in the disk which is theorized to generate lateral charge separation in the disks by strain induced band bending, an inhomogeneous polarization field distribution, and Fermi surface pinning. It is concluded that no lateral separation of carriers occurs in the quantum disks under investigation. Internal field screening by an increased carrier density in the QDisks at higher excitation densities is observed via a blue-shift of the emission and a dynamically changing decay time. Other possible explanations for these effects are discussed and discounted. Cathodoluminescence studies have also been carried out on the nanocolumns to provide insight into the physical origin of the luminescence.