We uncover the fine structure of a silicon vacancy in isotopically purified silicon carbide (4H-28 SiC) and reveal not yet considered terms in the spin Hamiltonian, originated from the trigonal pyramidal symmetry of this spin-3/2 color center. These terms give rise to additional spin transitions, which would be otherwise forbidden, and lead to a level anticrossing in an external magnetic field. We observe a sharp variation of the photoluminescence intensity in the vicinity of this level anticrossing, which can be used for a purely all-optical sensing of the magnetic field. We achieve dc magnetic field sensitivity better than 100 nT/ √ Hz within a volume of 3 × 10 −7 mm 3 at room temperature and demonstrate that this contactless method is robust at high temperatures up to at least 500 K. As our approach does not require application of radiofrequency fields, it is scalable to much larger volumes. For an optimized light-trapping waveguide of 3 mm 3 the projection noise limit is below 100 fT/ √ Hz.

One of the challenges in the field of quantum sensing and information processing is to selectively address and coherently manipulate highly homogeneous qubits subject to external perturbations. Here, we present room-temperature coherent control of high-dimensional quantum bits, the so-called qudits, associated with vacancy-related spins in silicon carbide enriched with nuclear spin-free isotopes. In addition to the excitation of a spectrally narrow qudit mode at the pump frequency, several other modes are excited in the electron spin resonance spectra whose relative positions depend on the external magnetic field. We develop a theory of multipole spin dynamics and demonstrate selective quantum control of homogeneous spin packets with sub-MHz spectral resolution. Furthermore, we perform two-frequency Ramsey interferometry to demonstrate absolute dc magnetometry, which is immune to thermal noise and strain inhomogeneity.

We present a theory of topological edge states in one-dimensional resonant photonic crystals with a compound unit cell. Contrary to the traditional electronic topological states the states under consideration are radiative, i.e., they decay in time due to the light escape through the structure boundaries. We demonstrate that the states survive despite their radiative decay and can be detected both in time-and frequency-dependent light reflection.PACS numbers: 78.67. Pt,78.47.jg Introduction. Topological insulator is an electronic material that has a band gap in its interior like an ordinary insulator but possesses conducting states on its edge or surface. The surface states of topological insulators have been extensively studied both in two-and threedimensional materials [1]. Recently an untrivial link has been revealed between such seemingly distinct systems as topological insulators, one-dimensional (1D) quasicrystals, and periodic 1D crystals with compound unit cell [2][3][4]. Particularly, it has been demonstrated that the 1D Aubry-André-Harper (AAH) model, or a "bichromatic" system (both incommensurate and commensurate), exhibits topological properties similar to those attributed to systems of a higher dimension [2][3][4]. This model allows states at sharp boundaries between two distinct topological systems. The system is described by a 1D tightbinding Hamiltonian with nearest-neighbor hopping and an on-site potential [5]. In the generalized AAH model both the hopping terms and the on-site potential are cosine modulated. It is the modulation phase that adds the second degree of freedom and permits one to relate the descendent 1D model with a 2D "ancestor" system which has a 2D band structure and quantized Chern numbers. In this Letter, instead of quasiparticles which tunnel from one site to another, we consider a 1D sequence of sites with resonant excitations long-range coupled through an electromagnetic field [6]. Such system is open, its eigenfrequencies are complex and its eigenstates are quasistationary due to the radiative decay. Hence, the resonant optical lattice stands out of the standard classification of topological insulators, developed for conservative and Hermitian electronic problems [7]. Nevertheless, we show here that this 1D bichromatic resonant photonic crystal demonstrates the topological properties in spite of being open and formulate general condition for the edge state existence. We also demonstrate how the radiative character of the problem opens new pathways to optical detection of the edge states. This provides an important insight into the rapidly expanding field of the electromagnetic topological states in photonic crystals [8,9], coupled cavities [10], waveguide arrays [11][12][13], and metamaterials [14].

We develop a rigorous theoretical approach for analyzing inelastic scattering of photon pairs in arrays of two-level qubits embedded in a waveguide. Our analysis reveals strong enhancement of the scattering when the energy of incoming photons resonates with the double-excited subradiant states. We identify the role of different double-excited states in the scattering such as superradiant, subradiant, and twilight states, being a product of single-excitation bright and subradiant states. Importantly, the N -excitation subradiant states can be engineered only if the number of qubits exceeds 2N . Both the subradiant and twilight states can generate long-lived photon-photon correlations, paving the way to a storage and processing of quantum information.

Polariton-mediated light-sound interaction is investigated through resonant Brillouin experiments in GaAsOptimizing optomechanical coupling in GaAs nanocavities thus does not rely only on increasing the confinement of optical and acoustic fields at the same location in the device, in other words in nanofabricating resonators with efficient acoustic-optic overlap and high-