We experimentally demonstrate cooling of trapped ions by collisions with cotrapped, higher-mass neutral atoms. It is shown that the lighter ${{}^{39}\mathrm{K}}^{+}$ ions, created by ionizing ${}^{39}\mathrm{K}$ atoms in a magneto-optical trap (MOT), when trapped in an ion trap and subsequently allowed to cool by collisions with ultracold, heavier ${}^{85}\mathrm{R}\mathrm{b}$ atoms in a MOT, exhibit a longer trap lifetime than without the localized ${}^{85}\mathrm{R}\mathrm{b}$ MOT atoms. A similar cooling of trapped ${}^{85}\mathrm{R}{\mathrm{b}}^{+}$ ions by ultracold ${}^{133}\mathrm{C}\mathrm{s}$ atoms in a MOT is also demonstrated in a different experimental configuration to validate this mechanism of ion cooling by localized and centered ultracold neutral atoms. Our results suggest that the cooling of ions by localized cold atoms holds for any mass ratio, thereby enabling studies on a wider class of atom-ion systems irrespective of their masses.

The maximum speed at which a liquid can wet a solid is limited by the need to displace gas lubrication films in front of the moving contact line. The characteristic height of these films is often comparable to the mean free path in the gas so that hydrodynamic models do not adequately describe the flow physics. This Letter develops a model which incorporates kinetic effects in the gas, via the Boltzmann equation, and can predict experimentally observed increases in the maximum speed of wetting when (a) the liquid’s viscosity is varied, (b) the ambient gas pressure is reduced, or (c) the meniscus is confined.

A major challenge in optical quantum processing is implementing large, stable interferometers. We offer a novel approach: virtual, measurement-based interferometers that are programed on the fly solely by the choice of homodyne measurement angles. The effects of finite squeezing are captured as uniform amplitude damping. We compare our proposal to existing (physical) interferometers and consider its performance for BosonSampling, which could demonstrate postclassical computational power in the near future. We prove its efficiency in time and squeezing (energy) in this setting.

We introduce a tensor renormalization group scheme for coarse graining a two-dimensional tensor network that can be successfully applied to both classical and quantum systems on and off criticality. The key innovation in our scheme is to deform a 2D tensor network into small loops and then optimize the tensors on each loop. In this way, we remove short-range entanglement at each iteration step and significantly improve the accuracy and stability of the renormalization flow. We demonstrate our algorithm in the classical Ising model and a frustrated 2D quantum model.

The source-free Maxwell action is invariant under electric-magnetic duality rotations in arbitrary spacetimes. This leads to a conserved classical Noether charge. We show that this conservation law is broken at the quantum level in the presence of a background classical gravitational field with a nontrivial Chern-Pontryagin invariant, in parallel with the chiral anomaly for massless Dirac fermions. Among the physical consequences, the net polarization of the quantum electromagnetic field is not conserved.

We propose new searches for axionlike particles (ALPs) produced in flavor-changing neutral current (FCNC) processes. This proposal exploits the often-overlooked coupling of ALPs to $W^{\pm }$ bosons, leading to FCNC production of ALPs even in the absence of a direct coupling to fermions. Our proposed searches for resonant ALP production in decays such as $B\to K^{(*)}a$, $a\to \gamma \gamma$, and $K\to \pi a$, $a\to \gamma \gamma$ could greatly improve upon the current sensitivity to ALP couplings to standard model particles. We also determine analogous constraints and discovery prospects for invisibly decaying ALPs.

Using proton-proton collision data corresponding to an integrated luminosity of $3.0{\mathrm{fb}}^{-1}$, recorded by the LHCb detector at center-of-mass energies of 7 and 8 TeV, the $B_c^{+}\to D^0{K}^{+}$ decay is observed with a statistical significance of 5.1 standard deviations. By normalizing to $B^{+}\to {\overline{D}}^0{\pi }^{+}$ decays, a measurement of the branching fraction multiplied by the production rates for $B_c^{+}$ relative to $B^{+}$ mesons in the LHCb acceptance is obtained, $R_{D^{0}K}=(f_c/f_u)\times \mathcal{B}(B_c^{+}\to D^0{K}^{+})=(9.3_{-2.5}^{+2.8}\pm 0.6)\times {10}^{-7}$, where the first uncertainty is statistical and the second is systematic. This decay is expected to proceed predominantly through weak annihilation and penguin amplitudes, and is the first $B_c^{+}$ decay of this nature to be observed.

We report the selective population of Rb or Cs $np$ ${{}^{2}P}_{\frac{3}{2}}$ ($n=5$, 6; $F=4$, 5) hyperfine states by the photodissociation of a transient, alkali-rare gas diatomic molecule. Circularly polarized (${\sigma }^{-}$), amplified spontaneous emission (ASE) on the $D_2$ line of Rb or Cs (780.0 and 852.1 nm, respectively) is generated when Rb-Xe or Cs-Xe ground state collision pairs are photoexcited by a ${\sigma }^{+}$-polarized optical field having a wavelength within the $D_2$ blue satellite continuum, associated with the $B{{}^2\mathrm{\Sigma }}_{\frac{1}{2}}^{+}\leftarrow X{{}^2\mathrm{\Sigma }}_{\frac{1}{2}}^{+}$ ($\text{free}\leftarrow \text{free}$) transition of the diatomic molecule. The degree of spin polarization of Cs ($6p$ ${}^2{P}_{\frac{3}{2}}$), specifically, is found to be dependent on the interatomic distance ($R$) at which the excited complex is born, a result attributed to the structure of the $B{{}^2\mathrm{\Sigma }}_{\frac{1}{2}}^{+}$ state. For Cs-Xe atomic pairs, tuning the wavelength of the optical field from 843 to 848 nm varies the degree of circular polarization of the ASE from 63% to almost unity because of the perturbation, in the $5\le R\le 6\AA$ interval, of the ${{}^2\mathrm{\Sigma }}_{\frac{1}{2}}^{+}$ potential by a $d\sigma$ molecular orbital associated with a higher ${}^2\mathrm{\Lambda }$ electronic state. Monitoring only the Cs $6p$ ${}^2{P}_{\frac{3}{2}}$ spin polarization reveals a previously unobserved interaction of CsXe ($B{{}^2\mathrm{\Sigma }}_{\frac{1}{2}}^{+}$) with the lowest vibrational levels of a ${}^2\mathrm{\Lambda }$ state derived from Cs $(5d)+\mathrm{Xe}$. By inserting a molecular intermediate into the alkali atom excitation mechanism, these experiments realize electronic spin polarization through populating no more than two $np$ ${{}^{2}P}_{\frac{3}{2}}$ hyperfine states, and demonstrate a sensitive spectroscopic probe of $R$-dependent state-state interactions and their impact on interatomic potentials.

Parametric envelope and sum envelope resonances are analyzed to revisit the validity of an assumed stop band and design limit of high intensity linear accelerators at a longitudinal phase advance of 90° per focusing lattice period. While the 90° limit is unquestioned in the transverse plane, we show here that it can be dropped as longitudinal limit for lattices with two or more rf gaps per focusing period. A new limit arises, however, from a novel transverse-longitudinal parametric sum envelope instability. The resulting sum instability rule allows the phase advance to exceed 90° longitudinally provided that transversely it remains correspondingly under 90°. We suggest that the additional design freedom opens the possibility for larger accelerating gradients and stronger longitudinal focusing with potential length and cost saving in the design of advanced superconducting linear accelerator concepts—as long as technological cavity limits are not reached.

Cooling atomic gases into strongly correlated quantum phases requires estimates of the entropy to perform thermometry and establish viability. We construct an ansatz partition function for models of Laughlin states of atomic gases by combining high temperature series expansions with exact diagonalization. Using the ansatz we find that entropies required to observe Laughlin correlations with bosonic gases are within reach of current cooling capabilities.

We study the response of the one-dimensional charge density wave in ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_3$ to different types of excitation with femtosecond optical pulses. We compare direct excitation of the lattice at midinfrared frequencies with injection of quasiparticles across the low energy charge density wave gap and with charge transfer excitation in the near infrared. For all three cases, we observe a fluence threshold above which the amplitude-mode oscillation frequency is softened and the mode becomes increasingly damped. We show that all the data can be collapsed onto a universal curve in which the melting of the charge density wave occurs abruptly at a critical lattice excursion. These data highlight the existence of a universal stability limit for a charge density wave, reminiscent of the Lindemann criterion for the melting of a crystal lattice.

In quantum many-body systems with a U(1) symmetry, such as particle number conservation and axial spin conservation, there are two distinct types of excitations: charge-neutral excitations and charged excitations. The energy gaps of these excitations may be independent from each other in strongly correlated systems. The static susceptibility of the U(1) charge vanishes when the charged excitations are all gapped, but its relation to the neutral excitations is not obvious. Here we show that a finite excitation gap of the neutral excitations is, in fact, sufficient to prove that the charge susceptibility vanishes (i.e., the system is incompressible). This result gives a partial explanation for why the celebrated quantization condition $n(S-m_z)\in \mathbb{Z}$ at magnetization plateaus works even in spatial dimensions greater than one.

We computationally study the micromechanics of shear-induced size segregation and propose distinct migration mechanisms for individual large and small particles. While small particles percolate through voids without enduring contacts, large particles climb under shear through their crowded neighborhoods with anisotropic contact network. Particle rotation associated with shear is necessary for the upward migration of large particles. Segregation of large particles can be suppressed with inadequate friction, or with no rotation; increasing interparticle friction promotes the migration of large particles, but has little effect on the percolation of small particles.

When two populations of “particles” move in opposite directions, like oppositely charged colloids under an electric field or intersecting flows of pedestrians, they can move collectively, forming lanes along their direction of motion. The nature of this “laning transition” is still being debated and, in particular, the pair correlation functions, which are the key observables to quantify this phenomenon, have not been characterized yet. Here, we determine the correlations using an analytical approach based on a linearization of the stochastic equations for the density fields, which is valid for dense systems of soft particles. We find that the correlations decay algebraically along the direction of motion, and have a self-similar exponential profile in the transverse direction. Brownian dynamics simulations confirm our theoretical predictions and show that they also hold beyond the validity range of our analytical approach, pointing to a universal behavior.

High-dimensional entanglement offers promising perspectives in quantum information science. In practice, however, the main challenge is to devise efficient methods to characterize high-dimensional entanglement, based on the available experimental data which is usually rather limited. Here we report the characterization and certification of high-dimensional entanglement in photon pairs, encoded in temporal modes. Building upon recently developed theoretical methods, we certify an entanglement of formation of 2.09(7) ebits in a time-bin implementation, and 4.1(1) ebits in an energy-time implementation. These results are based on very limited sets of local measurements, which illustrates the practical relevance of these methods.

We report the first direct measurement of decays of the ${\mathrm{\Lambda }}_c^{+}$ baryon involving the neutron. The analysis is performed using $567{\mathrm{pb}}^{-1}$ of $e^{+}{e}^{-}$ collision data collected at $\sqrt{s}=4.599\mathrm{GeV}$ with the BESIII detector at the BEPCII collider. We observe the decay ${\mathrm{\Lambda }}_c^{+}\to n{K}_S^0{\pi }^{+}$ and measure the absolute branching fraction to be $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to n{K}_S^0{\pi }^{+})=[1.82\pm 0.23(\text{stat})\pm 0.11(\text{syst})]\%$. A comparison to $\mathcal{B}[{\mathrm{\Lambda }}_c^{+}\to p(\overline{K}\pi {)}^0]$ provides an important test of isospin symmetry and final state interactions.