We experimentally study the fundamental torsional mechanical mode of the nanofiber waist of a tapered optical fiber. We find that this oscillator with a resonance frequency of $f=161.7\mathrm{kHz}$ features a quality factor of up to $Q={10}^7$ and a $Q$-frequency product of $Qf=1\mathrm{THz}$. The polarization fluctuations of a transmitted laser field serve as a probe for the torsional motion. We damp the oscillator's thermal motion from room temperature to 28(7) mK by means of active feedback. Our results might enable new types of fiber-based sensors and lay the foundation for a promising hybrid quantum optomechanical platform.

According to the Schiff theorem, an external electric field vanishes at atomic nucleus in a neutral atom in a stationary state, i.e., it is completely shielded by electrons. This makes a nuclear electric dipole moment (EDM) unobservable. We show that if atom or molecule is not in a stationary state (e.g., in a superposition of two stationary states), electric field on the nucleus is not zero and interaction with nuclear EDM does not vanish. In molecules this effect is enhanced by the ratio of nuclear mass to electron mass, $M_n/m_e$, since nuclei in a molecule are slow (compare to electrons) and do not provide efficient screening in a nonstationary environment. Electric field on the nucleus may also affect nuclear reactions.

We propose a high-quality deterministic single-photon source which can not only works in the ultrastrong light-matter coupling regime, but also emits two single photons with an arbitrary time separation, in one excitation process. We find that the special solutions of the two-qubit quantum Rabi and Jaynes-Cummings models which have at most one photon and constant eigenenergies can be used to implement this proposal through two consecutive adiabatic evolutions, and the system goes back to the initial state of the next period automatically after photon emission. Due to their peculiarities and reach of the ultrastrong coupling, the adiabatic evolution can be quite fast and further accelerated with the Stark shift. This Letter paves the way to fast quantum information protocols which take advantage of the ultrastrong coupling and avoid its dynamical complexity simultaneously.

Universality is a fundamental concept in physics that allows for the description of properties of systems that are independent of microscopic details. In this work, we demonstrate that the propagation of slow light in the form of a dark-state polariton can encode universal aspects of an impurity strongly coupled to a Bose-Einstein condensate. This unveils a nondemolition scheme to probe impurity physics beyond the quasiparticle properties. Based on a field theory that includes the two-body correlations at the exact level we demonstrate that under appropriate conditions, the damping rate of slow-light propagation reveals the high-energy universal tail in the polaron spectrum.

Maximally entangled single-particle states (MESPS) are opening new possibilities in quantum technology, as they have the potential to encode more information and are robust to decoherence compared to their nonlocal two-particle counterparts. We find that a single coin can generate MESPS at recurrent time steps (periodically) via discrete-time quantum walks on both 4- and 8-site cyclic graphs. This scheme is resource saving, with possibly the most straightforward experimental realization since the same coin is applied at each time step. We also show that recurrent MESPS can be generated on any arbitrary $k$-site cyclic graph, $k\in \{3,4,5,8\}$ via effective-single (identity and arbitrary coin) or two-coin evolution sequences. Beyond their use in fundamental research, we propose an application of the generated MESPS in quantum cryptography protocols. MESPS as cryptographic keys can strengthen quantum-secure communication.

We study the coexistence of energy-time entanglement with fiber-optical communication in the telecom $C$ band. The property of noise from wavelength-multiplexed classical channels is characterized with different wavelength settings. With the largest noise, i.e., the worst case, we measure the entanglement property of the distributed energy-time entangled photon pairs at different data rates of fiber-optical communication. After being distributed over 40-km spooled fiber coexisting with a bidirectional data rate of 20 Gbps, a visibility of $82.01\pm 1.10\%$ is obtained by measuring the Franson interference. With the Bennett-Brassard-Mermin 1992 (BBM92) protocol, a secret key rate of 343 bits per second is reached over the 40-km spooled fiber coexisting with a bidirectional data rate of 10 Gbps. Our result paves the way for developing a cost-effective quantum entanglement network compatible with fiber communication systems.

We study the energetics of chemical reactions between ultracold ground-state alkaline-earth-metal diatomic molecules. We show that the atom-exchange reactions forming homonuclear dimers are energetically allowed for all heteronuclear alkaline-earth-metal combinations. We perform high-level electronic structure calculations on the potential energy surfaces of all possible homo- and heteronuclear alkaline-earth-metal trimers and show that trimer formation is also energetically possible in collisions of all considered dimers. Interactions between alkaline-earth-metal diatomic molecules lead to the formation of deeply bound reaction complexes stabilized by large nonadditive interactions. We check that there are no barriers to the studied chemical reactions. This means that all alkaline-earth-metal diatomic molecules are chemically unstable at ultralow temperature, and optical lattice or shielding schemes may be necessary to segregate the molecules and suppress losses.

The radiative excitation of the 8.3 eV isomeric state of ${}^{229}\mathrm{Th}$ is an long-standing challenge due to the lack of tunable far-ultraviolet (FUV) sources. In this work, we propose an efficient two-photon pumping scheme for ${}^{229}\mathrm{Th}$ using the optonuclear quadrupolar effect, which only requires a 300 nm $\mathrm{UV}\text{−}\mathrm{B}$ pumping laser. We further demonstrate that population inversion between the nuclear isomeric and ground states can be achieved at room temperature using a two-step pumping process. The nuclear laser, which has been pursued unsuccessfully for decades, may be realized using a watt-level UV-B pumping laser and ultrawide band gap thorium compounds (e.g., $\mathrm{Th}{\mathrm{F}}_4$, ${\mathrm{Na}}_2\mathrm{Th}{\mathrm{F}}_6$, or ${\mathrm{K}}_2\mathrm{Th}{\mathrm{F}}_6$) as the gain medium.

We report on a nonlinear scattering effect that challenges the notion of topological protection for wave packets propagating in chiral edge modes. Specifically, in a Floquet topological system close to resonant driving and with a nonlinear potential, we demonstrate how a wave packet propagating in a chiral edge mode may be irreversibly deflected by scattering off a localized wave packet, or pass the collision region virtually unaffected in an approximately linear fashion. An experimentally accessible knob to tune between those two scenarios is provided by the relative phase between the involved wave packets. This genuinely nonlinear interference phenomenon is in stark contrast to linear scattering off a static impurity, which cannot destroy a topological edge state. Besides corroborating our findings with numerically exact simulations, we propose two physical platforms where our predictions may be verified with state-of-the-art experimental techniques: first, a coupled waveguide setting where nonlinearity has been engineered via an intensity-dependent optical index, and second, a Bose-Einstein condensate of cold atoms in an optical honeycomb lattice governed by a nonlinear Gross-Pitaevskii equation that effectively accounts for many-body interactions.

We report the engineering of molecular potentials at large interatomic distances. The molecular states are generated by off-resonant optical coupling to a highly excited, long-range Rydberg molecular potential. The coupling produces a potential well in the low-lying molecular potential, which supports a bound state. The depth of the potential well, and thus the binding energy of the molecule, can be tuned by the coupling parameters. We characterize these molecules and find good agreement with a theoretical model based on the coupling of the two involved adiabatic potential energy curves. Our results open numerous possibilities to create long-range molecules between ultracold ground-state atoms and to use them for ultracold chemistry and applications such as Feshbach resonances, Efimov physics, or the study of halo molecules.

Hybrid quantum-classical algorithms have been suggested to control the quantum entanglement of many-body systems in noisy intermediate-scale quantum technology, and yet their applicability is limited by the numbers of qubits and quantum operations. Here, we propose a mean-operator theory which overcomes limitations by combining the advantages of hybrid algorithms and standard mean-field theory. We demonstrate that an introduction of a mean operator prepares an entangled target many-body state with a significantly reduced number of quantum operations. We also show that a class of mean operators is expressed as time-evolution operators, which indicates that our theory is directly applicable to quantum simulations with Rydberg atoms and trapped ions.

Quantum acoustic systems, which integrate surface or bulk phonons with superconducting qubits, offer a unique opportunity to investigate phononic interference and scattering processes in the quantum regime. In particular, the interaction between a superconducting qubit and a phononic oscillator allows the qubit to sense the oscillator's excitation spectrum and underlying interference effects. Here, we present measurements revealing the Fano interference of a resonantly trapped piezoelectric surface acoustic wave (SAW) mode with a broad continuum of surface phonons in a system consisting of a SAW resonator coupled to a superconducting qubit. The experiments highlight the existence of additional weakly coupled mechanical modes and their influence on the qubit-phonon interaction and underscore the importance of phononic interference in quantum acoustic architectures that have been proposed for quantum information processing applications.

We study the transport of interacting bosons through an Aharonov-Bohm cage—a building block of flat-band networks—with coherent pump and sink leads. In the absence of interactions the cage is insulating due to destructive interference. We find that the cage stays insulating up to a critical value of the pump strength in the presence of mean-field interactions, while the quantum regime induces particle pair transport and weak conductance below the critical pump strength. A swift crossover from the quantum into the classical regime upon further pump strength increase is observed. We solve the time-dependent master equations for the density matrix of the many-body problem in the classical, pure quantum, and pseudoclassical regimes. We start with an empty cage and switch on driving. We characterize the transient dynamics, and the complexity of the resulting steady states and attractors. Our results can be readily realized using experimental platforms involving interacting ultracold atoms, superconducting circuits, and photons on fine-tuned optical lattices.

A confined electron laser is a “wiggled” electron beam confined by the ponderomotive forces inside an electromagnetic cavity. The ponderomotive forces counteract the beam expansion due to space-charge forces, thereby enabling lasing at low energies. In this Letter we show the possibility of lasing at a wavelength of approximately 4 nm in the soft-x-ray regime. We simulate various cavity parameters and show that electron-beam confinement and consequently the initiation of the lasing process are independent of the cavity resonance wavelength. We also show that ponderomotive confinement can compensate for the beam divergence due to nonzero emittance, thereby enabling lasing with large emittance values for which the free-electron lasing condition is violated. Our results support the feasibility of a few femtosecond, compact, and coherent soft-x-ray source.

We develop an analytical model for calculation of optical spectra for metal nanostructures of arbitrary shape supporting localized surface plasmons (LSPs). For plasmonic systems with characteristic size below the diffraction limit, we obtain an explicit expression for optical polarizability that describes the lineshape of optical spectra solely in terms of the metal dielectric function and LSP frequency. The amplitude of the LSP spectral band is determined by the effective system volume that, for long-wavelength LSPs, can significantly exceed the physical volume of metal nanostructure. Within the quasistatic approach, we derive the exact LSP Green's function and establish general spectral properties of LSPs, including the distribution and oscillator strength of the LSP states. These results can be used to model or interpret the experimental spectra of plasmonic nanostructures and to tune their optical properties for various applications.

We study the impact of finite-range physics on the zero-range-model analysis of three-body recombination in ultracold atoms. We find that temperature dependence of the zero-range parameters can vary from one set of measurements to another as it may be driven by the distribution of error bars in the experiment, and not by the underlying three-body physics. To study finite-temperature effects in three-body recombination beyond the zero-range physics, we introduce and examine a finite-range model based upon a hyperspherical formalism. The systematic error discussed in this Letter may provide a significant contribution to the error bars of measured three-body parameters.

We report on the observation of the $4f^{14}6s^2{}^{1}S_0-4f^{13}5d6s^2(J=2)$ transition at 431 nm in ${}^{171}\mathrm{Yb}$ by depleting atoms in a magneto-optical trap formed by the $6s^2{}^{1}S_0-6s6p{}^{3}P_1$ intercombination transition. The absolute frequency of the transition to the $F=3/2$ state is determined to be $695171054858.1\left(8.2\right)$ kHz against physical realization of Coordinated Universal Time maintained by the National Metrology Institute of Japan with a frequency comb. The $g$ factor of the transition to the $F=3/2$ state and the A hyperfine constant are measured to be $g_J=1.54\left(13\right)$ and 1123.273(13) MHz, respectively. More precise spectroscopy of this transition can lead to searches for time variation of the fine-structure constant and searches for new physics with isotope shift measurements.

We study multiqubit quantum logical gates between distant quantum modules in a network, which are appealing in quantum computation by offering advantages including faster execution and higher fidelity. We provide a protocol to implement the fan-out multiqubit controlled-not gate $C^N$ with one control qubit and $N-1$ target qubits at a distance, which obviates the need for a direct coherent quantum interaction. We present a proof-of-principle experimental demonstration of the $C^3$ gate in a photonic quantum network with a fidelity of 0.86 and exploit this quantum gate to map representative qubit states to generalized three-qubit Greenberger-Horne-Zeilinger states with an average fidelity of 0.83, demonstrating a crucial step to implement the three-qubit quantum error correction codes in distributed quantum computation. We expect that the nonlocal multiqubit quantum gates may contribute critically to the development of distributed quantum computation.

Stokes drift is a classical fluid effect in which traveling waves transfer momentum to tracers of the fluid, resulting in a nonzero drift velocity in the direction of the incoming wave; this effect is the driving mechanism allowing particles, i.e., impurities, to be transported by the flow. In a classical (viscous) fluid this happens usually due to the presence of viscous drag forces; because of the eventual absence of viscosity in quantum fluids, impurities are driven by inertial effects and pressure gradients only. We present theoretical predictions of a Stokes drift analogous in quantum fluids finding that, at the leading order, the drift direction and amplitude depend on the initial impurity position with respect to the wave phase, and at the second order, our theoretical model recovers the classical Stokes drift but with a coefficient that depends on the relative particle-fluid density ratio. Our theoretical predictions are obtained for classical impurities using multitime analytical asymptotic expansions. Numerical simulations of a two-dimensional Gross-Pitaevskii equation coupled with a classical impurity corroborate our findings. Our findings are experimentally testable, for instance, using fluids of light obtained in photorefractive crystals.

Jahn-Teller (JT) polaritons emerge when the two perpendicular, normal-incidence electromagnetic modes of a Fabry-Perot cavity interact with JT active systems. These JT polaritons are characterized by a mixed $(+/-)$–circular electromagnetic polarization mediated by the molecular JT vibronic coupling of the material subsystem. Consequently, the exchange of photonic and vibronic angular momenta can be very efficient. Due to the photonic-vibronic coupling, the magnitude and direction of the cavity polarization varies for different eigenstates of the cavity-molecule system. This type of light-matter coupling results in polarization inverted states in the polaritonic system—states that can be reached resonantly with either right or left circularly polarized light but which are characterized by a cavity-photon polarization opposite to the external fields used to excite the system.