Gapped 2D Dirac materials, in which inversion symmetry is broken by a gap-opening perturbation, feature a unique valley transport regime. The system ground state hosts dissipationless persistent valley currents existing even when topologically protected edge modes are absent or when they are localized due to edge roughness. Topological valley currents in such materials are dominated by bulk currents produced by electronic states just beneath the gap rather than by edge modes. Dissipationless currents induced by an external bias are characterized by a quantized half-integer valley Hall conductivity. The under-gap currents dominate magnetization and the charge Hall effect in a light-induced valley-polarized state.

We outline a designer approach to endow widely available plain materials with topological properties by stacking them atop other nontopological materials. The approach is illustrated with a model system comprising graphene stacked atop hexagonal boron nitride. In this case, the Berry curvature of the electron Bloch bands is highly sensitive to the stacking configuration. As a result, electron topology can be controlled by crystal axes alignment, granting a practical route to designer topological materials. Berry curvature manifests itself in transport via the valley Hall effect and long-range chargeless valley currents. The nonlocal electrical response mediated by such currents provides diagnostics for band topology.topological bands | graphene | van der Waals heterostructure E lectronic states in topological materials possess unique properties including a Hall effect without an applied magnetic field (1-3) and topologically protected edge states (4, 5). Accessing nontrivial electron topology depends on identifying materials in which symmetry and interactions produce topological Bloch bands. Such bands can only arise when multiple requirements, such as a multiband structure with a Berry phase and suitable symmetry, are fulfilled. As a result, topological bands are found in only a handful of exotic materials in which good transport properties are often lacking. Formulating practical methods for transforming widely available materials with a reasonably high carrier mobility (such as silicon or graphene) into a topological phase remains a grand challenge.Here, we lay out an approach for engineering designer topological materials out of stacks of generic materials-"Chernburgers." Our scheme naturally produces (i) topological bands with different Chern invariant values, and (ii) tunable topological transitions.As an illustration, we analyze graphene on hexagonal boron-nitride heterostructures (G/hBN), where broken inversion symmetry is expected to generate Berry curvature (6, 7), a key ingredient of topological materials. Indeed, recently valley currents have been demonstrated in a G/hBN system (8) signaling the presence of Berry curvature (6). As we will show, Berry curvature in G/hBN can be molded by stacking configuration, leading to a large variability in properties. Transitions between different topological states can be induced by a slight change in stacking angle.Topological bands in G/hBN arise separately for valley K and valley K′. Graphene bandstructure reconstruction due to the coupling to hBN produces superlattice minibands (9-14), with Berry curvature ΩðkÞ developing near avoided crossings. The minibands for each valley possess a valley Chern numberwhere the integral is taken over the entire superlattice Brillouin zone (SBZ) in one valley (K or K′). As discussed below, for commensurate stackings (Fig. 1A) C v = ±1 for the lowest minibands. In contrast, for incommensurate moiré superlattice structures (Fig. 1B), the invariant [1] vanishes in these minibands, C v = 0. The difference in the behavior for the...

We demonstrate photon-mediated interactions between two individually trapped atoms coupled to a nanophotonic cavity. Specifically, we observe superradiant line broadening when the atoms are resonant with the cavity, and level repulsion when the cavity is coupled to the atoms in the dispersive regime. Our approach makes use of individual control over the internal states of the atoms, their position with respect to the cavity mode, as well as the light shifts to tune atomic transitions individually, allowing us to directly observe the anti-crossing of the superradiant and subradiant two-atom states. These observations open the door for realizing quantum networks and studying quantum many-body physics based on atom arrays coupled to nanophotonic devices. arXiv:1909.09108v1 [quant-ph]

The realization of an efficient quantum optical interface for multi-qubit systems is an outstanding challenge in science and engineering. Using two atoms in individually-controlled optical tweezers coupled to a nanofabricated photonic crystal cavity, we demonstrate entanglement generation, fast non-destructive readout, and full quantum control of atomic qubits. The entangled state is verified in free space after being transported away from the cavity by encoding the qubits into long-lived states and using dynamical decoupling. Our approach bridges quantum operations at an optical link and in free space by a coherent one-way transport, potentially enabling an integrated optical interface for atomic quantum processors.

We present a simple method for narrowing the intrinsic Lorentzian linewidth of a commercial ultraviolet grating extended-cavity diode laser (TOPTICA DL Pro) using weak optical feedback from a long external cavity. We achieve a suppression in frequency noise spectral density of 20 dB measured at frequencies around 1 MHz, corresponding to the narrowing of the intrinsic Lorentzian linewidth from 200 kHz to 2 kHz. Provided additional active low-frequency noise suppression and long-term drift compensation, the system is suitable for experiments requiring a tunable ultraviolet laser with narrow linewidth and low high-frequency noise, such as precision spectroscopy, optical clocks, and quantum information science experiments.