Discrepancies from in-beam- and in-bottle-type experiments measuring the neutron lifetime are on the $4\sigma$ standard deviation level. In a recent publication Fornal and Grinstein proposed that the puzzle could be solved if the neutron would decay on the one percent level via a dark decay mode, one possible branch being $n\to \chi +e^{+}{e}^{-}$. With data from the Perkeo II experiment we set limits on the branching fraction and exclude a one percent contribution for 95% of the allowed mass range for the dark matter particle.

The unstable radium nucleus is appealing for probing new physics due to its high mass, octupole deformation, and energy level structure. Ion traps, with long hold times and low particle numbers, are excellent for work with radioactive species, such as radium and radium-based molecular ions, where low activity, and hence low total numbers, is desirable. We address the challenges associated with the lack of stable isotopes in a tabletop experiment with a low-activity ($\sim 10\mu \mathrm{Ci}$) source where we laser-cool trapped radium ions. With a laser-cooled radium ion we measured the $7p{}^2{P}_{1/2}^o$ state’s branching fractions to the ground state, $7s{}^2{S}_{1/2}$, and a metastable excited state, $6d{}^2{D}_{3/2}$, to be $p=0.9104(7)$ and 0.0896(7), respectively. With a nearby tellurium reference line we measured the $7s{}^2{S}_{1/2}\to 7p{}^2{P}_{1/2}^o$ transition frequency, 640.096 63(6) THz.

We consider the effect of phase backaction on the correlator $⟨I(t)I(t+\tau )⟩$ for the output signal $I(t)$ from continuous measurement of a qubit. We demonstrate that the interplay between informational and phase backactions in the presence of Rabi oscillations can lead to the correlator becoming larger than 1, even though $|⟨I⟩|\le 1$. The correlators can be calculated using the generalized “collapse recipe,” which we validate using the quantum Bayesian formalism. The recipe can be further generalized to the case of multitime correlators and arbitrary number of detectors, measuring non-commuting qubit observables. The theory agrees well with experimental results for continuous measurement of a transmon qubit. The experimental correlator exceeds the bound of 1 for a sufficiently large angle between the amplified and informational quadratures, causing the phase backaction. The demonstrated effect can be used to calibrate the quadrature misalignment.

Chirality describes not only the structural property of three-dimensional objects, but also an intrinsic feature of electromagnetic fields. Here we report a strategy to realize a Bessel beam superchiral “needle” by focusing a twisted radially polarized beam on a planar dielectric interface. By tailoring the light spatial distribution in the pupil plane of a high numerical aperture lens, the chirality of the local field at the focus can be enhanced by 11.9-fold than that of a circular polarized beam. Through a combined interaction of chiral and achiral transitions, the dimension of the region with enhanced chiral sensitivity can be shrunk down to $\lambda /25$. This theoretical work paves the way towards a completely new label-free imaging technique using the enhanced circular dichroism for sparse subdiffraction chiral objects (e.g., individual molecules).

We experimentally demonstrate the phenomenon of electroosmotic dipole flow that occurs around a localized surface charge region under the application of an external electric field in a Hele-Shaw cell. We use localized deposition of polyelectrolytes to create well-controlled surface charge variations, and show that, for a disk-shaped spot, the internal pressure distribution that arises results in uniform flow within the spot and dipole flow around it. We further demonstrate the superposition of surface charge spots to create complex flow patterns, without the use of physical walls.

The relativistic magnetron is the most compact and efficient high-power microwave source. In order to further increase the total efficiency of microwave generation in a relativistic magnetron with diffraction output with a low-energy state of electrons that is formed between two virtual cathodes (VCs) in the interaction space, the second VC at the downstream end of the interaction space is replaced with a magnetic mirror that reflects all leakage electrons back into the interaction space. Previously these leakage electrons were deposited on the anode surface without interacting with the microwaves. Particle-in-cell simulations show that this configuration of relativistic magnetron increases the electronic efficiency from 86% to 92%, and increases the total efficiency up to the maximal electronic efficiency. This is a record high efficiency of microwave generation in a relativistic magnetron or any other gigawatt-class, gigahertz-frequency HPM source.

We report on an experimental observation of two phases of hydrogen atoms in solid ${\mathrm{H}}_2$ films at temperatures of 0.1–0.8 K, characterized by a large enhancement of the nuclear spin polarization compared to that given by Boltzmann statistics ($p=0.15$ at $T=0.15\mathrm{K}$). The first phase with $p=0.35(5)$ is formed spontaneously during sample storage in a high magnetic field ($B=4.6\mathrm{T}$). The second phase with an even higher nuclear polarization, $p=0.75(7)$, can be achieved at $T\le 0.55\mathrm{K}$ by repeating sequences of dynamic nuclear polarization followed by a system relaxation. Upon warming through the range 0.55–0.65 K, the highly nuclear-polarized phase undergoes a phase transition to the spontaneously polarized phase which breaks down at $T\simeq 0.8\mathrm{K}$, and the nuclear polarization gradually converges to the Boltzmann distribution. We discuss possible scenarios for explaining the nature of the observed phenomena.

The accurate calculation of excited state properties of interacting electrons in the condensed phase is an immense challenge in computational physics. Here, we use state-of-the-art equation-of-motion coupled-cluster theory with single and double excitations (EOM-CCSD) to calculate the dynamic structure factor, which can be experimentally measured by inelastic x-ray and electron scattering. Our calculations are performed on the uniform electron gas at densities corresponding to Wigner-Seitz radii of $r_s=5$, 4, and 3 corresponding to the valence electron densities of common metals. We compare our results to those obtained using the random-phase approximation (RPA), which is known to provide a reasonable description of the collective plasmon excitation and which resums only a small subset of the polarizability diagrams included in EOM-CCSD. We find that EOM-CCSD, instead of providing a perturbative improvement on the RPA plasmon, predicts a many-state plasmon resonance, where each contributing state has a double-excitation character of 80% or more. This finding amounts to an ab initio treatment of the plasmon linewidth, which is in good quantitative agreement with previous diagrammatic calculations, and highlights the strongly correlated nature of lifetime effects in condensed-phase electronic structure theory.

The Lifshitz transition, a change in Fermi surface topology, is likely to greatly influence exotic correlated phenomena in solids, such as high-temperature superconductivity and complex magnetism. However, since the observation of Fermi surfaces is generally difficult in the strongly correlated systems, a direct link between the Lifshitz transition and quantum phenomena has been elusive so far. Here, we report a marked impact of the pressure-induced Lifshitz transition on thermoelectric performance for SnSe, a promising thermoelectric material without a strong electron correlation. By applying pressure up to 1.6 GPa, we have observed a large enhancement of the thermoelectric power factor by more than 100% over a wide temperature range (10–300 K). Furthermore, the high carrier mobility enables the detection of quantum oscillations of resistivity, revealing the emergence of new Fermi pockets at $\sim 0.86\mathrm{GPa}$. The observed thermoelectric properties linked to the multivalley band structure are quantitatively reproduced by first-principles calculations, providing novel insight into designing the SnSe-related materials for potential valleytronic as well as thermoelectric applications.

We propose the idea of a spin-lattice liquid, in which spin and lattice degrees of freedom are strongly coupled and remain disordered and fluctuating down to low temperatures. We show that such a state arises naturally from a microscopic analysis of a class of molybdate pyrochlore compounds, and is driven by a giant magnetoelastic effect. Finally, we argue that this could explain some of the experimental features of ${\mathrm{Y}}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$.

Entangled photon sources are crucial for quantum optics, quantum sensing, and quantum communication. Semiconductor quantum dots generate on-demand entangled photon pairs via the biexciton-exciton cascade. However, the pair of photons are emitted isotropically in all directions, thus limiting the collection efficiency to a fraction of a percent. Moreover, strain and structural asymmetry in quantum dots lift the degeneracy of the intermediate exciton states in the cascade, thus degrading the measured entanglement fidelity. Here, we propose an approach for generating a pair of entangled photons from a semiconductor quantum dot by application of a quadrupole electrostatic potential. We show that the quadrupole electric field corrects for the spatial asymmetry of the excitonic wave function for any quantum dot dipole orientation and fully erases the fine-structure splitting without compromising the spatial overlap between electrons and holes. Our approach is compatible with nanophotonic structures such as microcavities and nanowires, thus paving the way towards a deterministic source of entangled photons with high fidelity and collection efficiency.

We identify the quantum metric dipole as the geometric origin of the nonreciprocal directional dichroism which describes the change in the refractive index upon reversing the light propagation direction. Specifically, we find that the static limit of the nonreciprocal directional dichroism corresponds to a quadrupolar transport current from the quantum metric dipole, in response to a quadrupolar electric field. Moreover, at a finite frequency, we demonstrate that the steepest slope of the averaged quantum metric dipole gives rise to a peak in the differential refractive index between counterpropagating lights. Finally, we illustrate both features in a low-energy model.

We investigate the effect of an anisotropic substrate on the turbulent dynamics of a simulated two-dimensional active nematic. This is introduced as an anisotropic friction and an effective anisotropic viscosity, with the orientation of the anisotropy being defined by the substrate. In this system, we observe the emergence of global nematic order of topological defects that is controlled by the degree of anisotropy in the viscosity and the magnitude of the active stress. No global defect alignment is seen in passive liquid crystals with anisotropic viscosity or friction confirming that ordering is driven by the active stress. We then closely examine the active flow generated by a single defect to show that the net kinetic energy of the flow is dependent on the orientation of the defect relative to the substrate, resulting in a torque on the defect to align it with the anisotropy in the substrate.

Directing individual motions of many constituents to a coherent dynamical state is a fundamental challenge in multiple fields. Here, based on the spherical crystal model, we show that topological defects in particle arrays can be a crucial element in regulating collective dynamics. Specifically, we highlight the defect-driven synchronized breathing modes around disclinations and collective oscillations with strong connection to disruption of crystalline order. This work opens the promising possibility of an organizational principle based on topological defects, and may inspire new strategies for harnessing intriguing collective dynamics in extensive nonequilibrium systems.

We study the phase diagram and quantum critical region of one of the fundamental models for electronic correlations: the periodic Anderson model. Employing the recently developed dynamical vertex approximation, we find a phase transition between a zero-temperature antiferromagnetic insulator and a Kondo insulator. In the quantum critical region, we determine a critical exponent $\gamma =2$ for the antiferromagnetic susceptibility. At higher temperatures, we have free spins with $\gamma =1$ instead, whereas at lower temperatures, there is an even stronger increase and suppression of the susceptibility below and above the quantum critical point, respectively.

Spin squeezing can improve atomic precision measurements beyond the standard quantum limit (SQL), and unitary spin squeezing is essential for improving atomic clocks. We report substantial and nearly unitary spin squeezing in ${}^{171}\mathrm{Yb}$, an optical lattice clock atom. The collective nuclear spin of $\sim {10}^3$ atoms is squeezed by cavity feedback, using light detuned from the system’s resonances to attain unitarity. The observed precision gain over the SQL is limited by state readout to 6.5(4) dB, while the generated states offer a gain of 12.9(6) dB, limited by the curvature of the Bloch sphere. Using a squeezed state within 30% of unitarity, we demonstrate an interferometer that improves the averaging time over the SQL by a factor of 3.7(2). In the future, the squeezing can be simply transferred onto the optical-clock transition of ${}^{171}\mathrm{Yb}$.