We report on the local control of the transition frequency of a spin $1/2$ encoded in two Rydberg levels of an individual atom by applying a state-selective light shift using an addressing beam. With this tool, we first study the spectrum of an elementary system of two spins, tuning it from a nonresonant to a resonant regime, where “bright” (super-radiant) and “dark” (subradiant) states emerge. We observe the collective enhancement of the microwave coupling to the bright state. We then show that after preparing an initial single spin excitation and letting it hop due to the spin-exchange interaction, we can freeze the dynamics at will with the addressing laser, while preserving the coherence of the system. In the context of quantum simulation, this scheme opens exciting prospects for engineering inhomogeneous $XY$ spin Hamiltonians or preparing spin-imbalanced initial states.

Coulomb interaction between two closely spaced parallel layers of conductors can generate the frictional drag effect by interlayer Coulomb scattering. Employing graphene double layers separated by few‐layer hexagonal boron nitride, we investigate density tunable magneto- and Hall drag under strong magnetic fields. The observed large magnetodrag and Hall-drag signals can be related with Laudau level filling status of the drive and drag layers. We find that the sign and magnitude of the drag resistivity tensor can be quantitatively correlated to the variation of magnetoresistivity tensors in the drive and drag layers, confirming a theoretical formula for magnetodrag in the quantum Hall regime. The observed weak temperature dependence and $\sim B^2$ dependence of the magnetodrag are qualitatively explained by Coulomb scattering phase-space argument.

Kitaev’s honeycomb lattice spin model is a remarkable exactly solvable model, which has a particular type of spin liquid (Kitaev spin liquid) as the ground state. Although its possible realization in iridates and $\alpha \text{−}{\mathrm{RuCl}}_3$ has been vigorously discussed recently, these materials have substantial non-Kitaev direct exchange interactions and do not have a spin liquid ground state. We propose metal-organic frameworks (MOFs) with ${\mathrm{Ru}}^{3+}$ (or ${\mathrm{Os}}^{3+}$), forming the honeycomb lattice as promising candidates for a more ideal realization of Kitaev-type spin models, where the direct exchange interaction is strongly suppressed. The great flexibility of MOFs allows generalization to other three-dimensional lattices for the potential realization of a variety of spin liquids, such as a Weyl spin liquid.

In this Letter, we study the motion and wake patterns of freely rising and falling cylinders in quiescent fluid. We show that the amplitude of oscillation and the overall system dynamics are intricately linked to two parameters: the particle’s mass density relative to the fluid $m^{*}\equiv {\rho }_p/{\rho }_f$ and its relative moment of inertia $I^{*}\equiv I_p/I_f$. This supersedes the current understanding that a critical mass density ($m^{*}\approx 0.54$) alone triggers the sudden onset of vigorous vibrations. Using over 144 combinations of $m^{*}$ and $I^{*}$, we comprehensively map out the parameter space covering very heavy ($m^{*}>10$) to very buoyant ($m^{*}<0.1$) particles. The entire data collapse into two scaling regimes demarcated by a transitional Strouhal number ${\text{St}}_t\approx 0.17$. ${\text{St}}_t$ separates a mass-dominated regime from a regime dominated by the particle’s moment of inertia. A shift from one regime to the other also marks a gradual transition in the wake-shedding pattern: from the classical two-single ($2S$) vortex mode to a two-pair ($2P$) vortex mode. Thus, autorotation can have a significant influence on the trajectories and wakes of freely rising isotropic bodies.

We study the electronic structure of ${\mathrm{HgBa}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{8+\delta }$ (Hg1223; $T_c=134\mathrm{K}$) using photoemission spectroscopy (PES) and $x$-ray absorption spectroscopy (XAS). Resonant valence band PES across the O $K$ edge and Cu $L$ edge identifies correlation satellites originating in O $2p$ and Cu $3d$ two-hole final states, respectively. Analyses using the experimental O $2p$ and Cu $3d$ partial density of states show quantitatively different on-site Coulomb energy for the Cu site ($U_{dd}=6.5\pm 0.5\mathrm{eV}$) and O site ($U_{pp}=1.0\pm 0.5\mathrm{eV}$). ${\mathrm{Cu}}_2{\mathrm{O}}_7$-cluster calculations with nonlocal screening explain the Cu $2p$ core level PES and Cu $L$-edge XAS spectra, confirm the $U_{dd}$ and $U_{pp}$ values, and provide evidence for the Zhang-Rice singlet state in Hg1223. In contrast to other hole-doped cuprates and $3d$-transition metal oxides, the present results indicate weakly correlated oxygen holes in Hg1223.

We have investigated concentrated suspensions of polydisperse hard spheres and have determined the dynamics and sizes of individual particles using confocal microscopy. With increasing concentration, the dynamics of the small and large particles start to differ. The large particles exhibit slower dynamics and stronger localization. Moreover, as the particle size increases, the local volume fraction ${\varphi }_{\mathrm{loc}}$ also increases. In the glass state, the localization length significantly decreases beyond ${\varphi }_{\mathrm{loc}}\approx 0.67$. This suggests a link between local crowding and dynamical heterogeneities. However dynamical arrest of subpopulations seems not directly linked to a large value of ${\varphi }_{\mathrm{loc}}$, indicating the importance of collective effects.

Finding the optimal encoding strategies can be challenging for communication using quantum channels, as classical and quantum capacities may be superadditive. Entanglement assistance can often simplify this task, as the entanglement-assisted classical capacity for any channel is additive, making entanglement across channel uses unnecessary. If the entanglement assistance is limited, the picture is much more unclear. Suppose the classical capacity is superadditive, then the classical capacity with limited entanglement assistance could retain superadditivity by continuity arguments. If the classical capacity is additive, it is unknown if superadditivity can still be developed with limited entanglement assistance. We show this is possible, by providing an example. We construct a channel for which the classical capacity is additive, but that with limited entanglement assistance can be superadditive. This shows entanglement plays a weird role in communication, and we still understand very little about it.

We demonstrate experimentally the resonant excitation of plasma waves by trains of laser pulses. We also take an important first step to achieving an energy recovery plasma accelerator by showing that a plasma wave can be damped by an out-of-resonance trailing laser pulse. The measured laser wakefields are found to be in excellent agreement with analytical and numerical models of wakefield excitation in the linear regime. Our results indicate a promising direction for achieving highly controlled, GeV-scale laser-plasma accelerators operating at multikilohertz repetition rates.

The optical selection rules for interband transitions in ${\mathrm{WSe}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{MoSe}}_2$ transition metal dichalcogenide monolayers are investigated by polarization-resolved photoluminescence experiments with a signal collection from the sample edge. These measurements reveal a strong polarization dependence of the emission lines. We see clear signatures of the emitted light with the electric field oriented perpendicular to the monolayer plane, corresponding to an interband optical transition forbidden at normal incidence used in standard optical spectroscopy measurements. The experimental results are in agreement with the optical selection rules deduced from group theory analysis, highlighting the key role played by the different symmetries of the conduction and valence bands split by the spin-orbit interaction. These studies yield a direct determination of the bright-dark exciton splitting, for which we measure $40\pm 1\mathrm{meV}$ and $55\pm 2\mathrm{meV}$ in ${\mathrm{WSe}}_2$ and ${\mathrm{WS}}_2$ monolayer, respectively.

We demonstrate a new technique for detecting the amplitude of arbitrarily chosen components of radio-frequency waveforms based on stroboscopic backaction evading measurements. We combine quantum nondemolition measurements and stroboscopic probing to detect waveform components with magnetic sensitivity beyond the standard quantum limit. Using an ensemble of $1.5\times 10^6$ cold rubidium atoms, we demonstrate entanglement-enhanced sensing of sinusoidal and linearly chirped waveforms, with 1.0(2) and 0.8(3) dB metrologically relevant noise reduction, respectively. We achieve volume-adjusted sensitivity of $\delta B\sqrt{V}\approx 3.96\mathrm{fT}\sqrt{{\mathrm{cm}}^3/\mathrm{Hz}}$, comparable to the best rf magnetometers.

We present experimental evidence for the different mechanisms driving the fluctuations of the local density of states (LDOS) in disordered photonic systems. We establish a clear link between the microscopic structure of the material and the frequency correlation function of LDOS accessed by a near-field hyperspectral imaging technique. We show, in particular, that short- and long-range frequency correlations of LDOS are controlled by different physical processes (multiple or single scattering processes, respectively) that can be—to some extent—manipulated independently. We also demonstrate that the single scattering contribution to LDOS fluctuations is sensitive to subwavelength features of the material and, in particular, to the correlation length of its dielectric function. Our work paves a way towards complete control of statistical properties of disordered photonic systems, allowing for designing materials with predefined correlations of LDOS.

We present a simple trick that allows us to consider the sum of all connected Feynman diagrams at fixed position of interaction vertices for general fermionic models, such that the thermodynamic limit can be taken analytically. With our approach one can achieve superior performance compared to conventional diagrammatic Monte Carlo algorithm, while rendering the algorithmic part dramatically simpler. By considering the sum of all connected diagrams at once, we allow for massive cancellations between different diagrams, greatly reducing the sign problem. In the end, the computational effort increases only exponentially with the order of the expansion, which should be contrasted with the factorial growth of the standard diagrammatic technique. We illustrate the efficiency of the technique for the two-dimensional Fermi-Hubbard model.

The interfacial thermal resistance determines condensation-evaporation processes and thermal transport across material-fluid interfaces. Despite its importance in transport processes, the interfacial structure responsible for the thermal resistance is still unknown. By combining nonequilibrium molecular dynamics simulations and interfacial analyses that remove the interfacial thermal fluctuations we show that the thermal resistance of liquid-vapor interfaces is connected to a low density fluid layer that is adsorbed at the liquid surface. This thermal resistance layer (TRL) defines the boundary where the thermal transport mechanism changes from that of gases (ballistic) to that characteristic of dense liquids, dominated by frequent particle collisions involving very short mean free paths. We show that the thermal conductance is proportional to the number of atoms adsorbed in the TRL, and hence we explain the structural origin of the thermal resistance in liquid-vapor interfaces.

We study the growth and saturation of the superradiant instability of a complex, massive vector (Proca) field as it extracts energy and angular momentum from a spinning black hole, using numerical solutions of the full Einstein-Proca equations. We concentrate on a rapidly spinning black hole ($a=0.99$) and the dominant $m=1$ azimuthal mode of the Proca field, with real and imaginary components of the field chosen to yield an axisymmetric stress-energy tensor and, hence, spacetime. We find that in excess of 9% of the black hole’s mass can be transferred into the field. In all cases studied, the superradiant instability smoothly saturates when the black hole’s horizon frequency decreases to match the frequency of the Proca cloud that spontaneously forms around the black hole.

We report the results of a first experimental search for lepton number violation by four units in the neutrinoless quadruple-$\beta$ decay of ${}^{150}\mathrm{Nd}$ using a total exposure of 0.19 kg yr recorded with the NEMO-3 detector at the Modane Underground Laboratory. We find no evidence of this decay and set lower limits on the half-life in the range $T_{1/2}>(1.1–3.2)\times 10^{21}\mathrm{yr}$ at the 90% C.L., depending on the model used for the kinematic distributions of the emitted electrons.

We calculate the Casimir force and its gradient between a spherical and a planar gold surface. Significant numerical improvements allow us to extend the range of accessible parameters into the experimental regime. We compare our numerically exact results with those obtained within the proximity force approximation (PFA) employed in the analysis of all Casimir force experiments reported in the literature so far. Special attention is paid to the difference between the Drude model and the dissipationless plasma model at zero frequency. It is found that the correction to PFA is too small to explain the discrepancy between the experimental data and the PFA result based on the Drude model. However, it turns out that for the plasma model, the corrections to PFA lie well outside the experimental bound obtained by probing the variation of the force gradient with the sphere radius [D. E. Krause et al., Phys. Rev. Lett. 98, 050403 (2007)]. The corresponding corrections based on the Drude model are significantly smaller but still in violation of the experimental bound for small distances between plane and sphere.

We present a scanning probe microscopy technique for spatially resolving transport in cold atomic gases, in close analogy with scanning gate microscopy in semiconductor physics. The conductance of a quantum point contact connected to two atomic reservoirs is measured in the presence of a tightly focused laser beam acting as a local perturbation that can be precisely positioned in space. By scanning its position and recording the subsequent variations of conductance, we retrieve a high-resolution map of transport through a quantum point contact. We demonstrate a spatial resolution comparable to the extent of the transverse wave function of the atoms inside the channel and a position sensitivity below 10 nm. Our measurements agree well with an analytical model and ab initio numerical simulations, allowing us to identify a regime in transport where tunneling dominates over thermal effects. Our technique opens new perspectives for the high-resolution observation and manipulation of cold atomic gases.

We report attosecond-scale probing of the laser-induced dynamics in molecules. We apply the method of high-harmonic spectroscopy, where laser-driven recolliding electrons on various trajectories record the motion of their parent ion. Based on the transient phase-matching mechanism of high-order harmonic generation, short and long trajectories contributing to the same harmonic order are distinguishable in both the spatial and frequency domains, giving rise to a one-to-one map between time and photon energy for each trajectory. The short and long trajectories in ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ are used simultaneously to retrieve the nuclear dynamics on the attosecond and ångström scale. Compared to using only short trajectories, this extends the temporal range of the measurement to one optical cycle. The experiment is also applied to methane and ammonia molecules.

Tip streaming generates micron- and submicron-sized droplets when a thin thread pulled from the pointy end of a drop disintegrates. Here, we report streaming from the equator of a drop placed in a uniform electric field. The instability generates concentric fluid rings encircling the drop, which break up to form an array of microdroplets in the equatorial plane. We show that the streaming results from an interfacial instability at the stagnation line of the electrohydrodynamic flow, which creates a sharp edge. The flow draws from the equator a thin sheet which destabilizes and sheds fluid cylinders. This streaming phenomenon provides a new route for generating monodisperse microemulsions.

Space is represented in the mammalian brain by the activity of hippocampal place cells, as well as in their spike-timing correlations. Here, we propose a theory for how this temporal code is transformed to spatial firing rate patterns via spike-timing-dependent synaptic plasticity. The resulting dynamics of synaptic weights resembles well-known pattern formation models in which a lateral inhibition mechanism gives rise to a Turing instability. We identify parameter regimes in which hexagonal firing patterns develop as they have been found in medial entorhinal cortex.