We report the production of an intense and controlled beam of electronically excited sulfur atoms in the ${}^{1}D_2$ state using a multistage Zeeman decelerator. Sulfur atoms, generated via photolysis of ${\mathrm{CS}}_2$, are produced in both the ground ${}^{3}P_J$ and excited ${}^{1}D_2$ states. We demonstrate that both can be manipulated using the decelerator, and that temporal separation between them can be achieved by operating in deceleration mode. This enables the generation of sulfur atom beams with a well-defined velocity, narrow velocity spreads, and an enhanced quantum-state purity. To assess the suitability of the beam for scattering studies, we performed a proof-of-principle elastic collision experiment with $\mathrm{S}\left({}^{1}D_2\right)$ and argon atoms. The observed velocity-map-imaging signal confirms that the $\mathrm{S}\left({}^{1}D_2\right)$ beam density is sufficient for detailed scattering studies. These results form the foundation for future studies of reactive and quenching processes involving $\mathrm{S}\left({}^{1}D_2\right)$ atoms at tunable and well-defined collision energies.

Entangled photon pairs are key elements in quantum communication and quantum cryptography. State-of-the-art sources of entangled photons are mainly based on parametric down-conversion from nonlinear crystals, which is probabilistic in nature, and on cascade emission from biexciton quantum dots, which finds difficulties in generating entangled photons in the visible regime. Here, we use the Wigner-Weisskopf theory to provide a demonstration that polarization-entangled photon pairs can be emitted from two interacting quantum emitters with two-level-system behavior and perpendicular transition dipole moments. These emitters can represent a large variety of systems (e.g., organic molecules, quantum dots, and diamond color centers) offering a large technological versatility, for example, in the spectral regime of the emission. We show that a highly entangled photon pair can be postselected from this system by including optical filters. Additionally, we verify that the photon entanglement is not significantly affected by small changes in the detection directions and in the orientation between the dipole moments.

The polarization of incident light may be tuned with the Brewster effect that is widely used in photography and optical coatings. The Brewster angle demonstrating zero reflection is usually observed for transverse-magnetic polarized light, but recent studies have discovered the generalized Brewster effect in transverse-electric polarization using nanostructures. In this work, we discover the way of the Brewster angle and perfect absorption control in transverse-electric polarization with two-dimensional plasmonic materials and nanostructures. We derive the analytical model of the reflectionless metasurface described by the surface conductivity within and beyond the total internal reflection state leading to the perfect absorption and generalized Brewster's angle, respectively. Then, we support our analytical results by the full-wave numerical simulation using the effective two-dimensional sheet obeying the Drude-Lorentz dispersion model and the real design of plasmonic anisotropic metasurface based on gold nanodisks. The results obtained open new opportunities for the planar polarization optics and related devices including polarization transformers, optical isolators, antireflection coatings, and biodetectors.

We propose and implement a protocol to measure the state-dependent motion of Rydberg atoms induced by dipole-dipole interactions. Our setup enables simultaneous readout of both the atomic internal state and position on a one-dimensional array of optical tweezers. We benchmark the protocol using two atoms in the same Rydberg state, which experience van der Waals repulsion, and measure velocities in agreement with theoretical predictions. When preparing the atoms in a different pair state, we observe an oscillatory dynamics that we attribute to the proximity of a macrodimer bound state. Finally, we perform a Stern-Gerlach-like experiment in which a superposition of the two previous pair states results in the separation of the atomic wave packet into two macroscopically distinct trajectories, thereby demonstrating spin-motion coupling mediated by the interactions.

We study the temporal dynamics of light interacting with a one-dimensional lattice of cold atoms. In such a system, a photonic band gap opens up, yielding an efficient Bragg reflection for an incident field incoming with the right angle and detuning. Here, we report two new effects appearing in the Bragg reflection. First, for some detunings, there is a “flash,” i.e., a transient increase of the reflected intensity when the incident field is switched off. Second, the subsequent extinction of the reflected field is clearly superradiant, with decay rates up to eight times the natural decay rate of the atomic excited state. Numerical simulations are in qualitative agreement with the observations, which can be explained by a classical photonic model. Our results are a step towards exploiting this photonic band gap in atomic systems for quantum-optical applications.

Information obtained from noise characterization of a quantum device can be used in classical decoding algorithms to improve the performance of quantum error-correcting codes. Focusing on the surface code under local (i.e., single-qubit) noise, we present a versatile, numerical method to quantify the performance improvement gained by adapting an optimal surface-code decoder to specific, user-defined features of the physical noise model. Our method is based on a tensor-network decoding algorithm, which uses the syndrome information as well as a process matrix description of the noise to compute a near-optimal correction. By selectively mischaracterizing the noise model input to the decoder and measuring the resulting loss in fidelity of the logical qubit, we can determine the relative importance of individual noise parameters for decoding. We apply this method to several physically relevant uncorrelated noise models with features such as coherence, spatial inhomogeneity, and bias. While noise generally requires many parameters to describe completely, we find that to achieve near-optimal decoding it appears only necessary to adapt the decoder to a small number of critical parameters.

We investigate a quantum metrological protocol operating in a non-Markovian environment by employing the quantum Brownian motion model, in which the system is linearly coupled to a reservoir of harmonic oscillators. Specifically, we use a position-momentum (PM) correlated Gaussian state as a probe to examine how memory effects influence the evolution of the system's covariance matrix in the weak-coupling regime under both high- and low-temperature conditions. To confirm the presence of non-Markovian behavior, we apply two well-established non-Markovianity quantifiers. Furthermore, we estimate both the channel's sample temperature and its non-Markovianity witness parameter. Our results demonstrate that non-Markovianity and PM correlations can jointly be valuable resources to enhance metrological performance.

We present a quantum theory of a magneto-optical trap (MOT) from first principles based on the quantum kinetic equation for the atomic density matrix, taking into account the recoil effects caused by the interaction of atoms with the laser field. An efficient method for solving the quantum kinetic equation is proposed. It is shown that the steady-state solution describing the atoms in the MOT has a significantly nonequilibrium nature and can be described within the framework of a two-temperature distribution. The momentum distribution of cold atoms in the MOT depends on the magnetic field gradient and, in general, significantly differs from the momentum distribution of atoms in the optical molasses, which is usually used as an approximation to describe the MOT. We have also shown that with an increase in the magnetic field gradient, a spatial two-component distribution of atoms in the trap is formed even for a single-particle approximation when interatomic interactions are neglected.

We investigate strong-field-induced electronic coherences in argon and molecular nitrogen ions created by high-intensity, few-cycle infrared laser pulses. This is a step toward the long-sought goal of strong-field coherent control in molecular chemistry. We employ high-intensity, few-cycle infrared laser pulses in a pump-probe setup to investigate a recent prediction that electronic coherences in nitrogen molecules change the ion yields versus pump-probe delay. [Yuen et al., Phys. Rev. A 109, L011101 (2024)]. The predicted coherence signals in molecular nitrogen could not be resolved above the optical interference of the pump and probe pulses; a simultaneous measurement clearly resolved the induced cation fine-structure coherence in strong-field-ionized argon. The results of our comparison with simulations suggest that optical interference effects manifest differently in each ionic species and must be carefully accounted for when interpreting experimental data. We find that nonsequential double ionization in the low-intensity region of the focal volume can reduce the visibility of coherence generated by two-pulse sequential ionization, and we quantify the importance of pulse shape and spectral characteristics for isolating the desired coherence signals.

We theoretically investigate the role of confinement-induced resonances (CIRs) in low-dimensional ultracold atomic mixtures in the formation of weakly bound dimers. To this end, we examine the scattering properties of a binary atomic mixture confined by a quasi-one-dimensional (quasi-1D) potential. In this regime, the interspecies two-body interaction is modeled as an effective 1D zero-range pseudopotential, with a coupling strength $g_1\mathrm{D}$ derived as a function of the three-dimensional scattering length $a$. This framework enables the study of CIRs in harmonically confined systems, with particular attention paid to the case of mismatched transverse trapping frequencies of the two atomic species. Finally, we consider the Bose-Fermi mixture of ${}^{87}\mathrm{Rb}$ and ${}^{87}\mathrm{Sr}$ and identify values of the experimentally accessible parameters for which CIRs can be exploited to create weakly bound molecules.

We study classical and quantum echoes in a Kerr oscillator driven by a frequency-controlling pulsed perturbation. We consider the dynamical response to the perturbation for a single coherent state and for Schrödinger cat states constructed as both balanced and imbalanced superpositions of two coherent states. For individual coherent states, we demonstrate that a weak parametric drive yields a long-lived sequence of classical echoes. Cat states are found to exhibit distinct quantum echoes that are sensitive to the initial relative phase and weights of the coherent states in superposition. We examine the effect of dissipation on quantum echoes and quantum revivals of cat states. We demonstrate that, even when dissipation suppresses quantum revivals, quantum echoes can be recovered by properly tuning the timing and strength of the perturbation. These results may be useful for characterizing and mitigating errors of cat qubits.

In this work, we consider a three-level ladder-type atom driven by a coherent field, inspired by the experimental work of Gasparinetti et al. [Phys. Rev. A 100, 033802 (2019)]. When driven on two-photon resonance, the atom is excited into its highest energy state $|f\rangle$ by absorbing two photons simultaneously. The atom then de-excites via a cascaded decay $|f\rangle \to |e\rangle \to |g\rangle$. Here we present a theoretical study of the atomic fluorescence spectrum where, upon strong coherent driving, the spectrum exhibits seven distinct frequencies corresponding to transitions among the atomic dressed states. We characterize the quantum statistics of the emitted photons by investigating the second-order correlation functions of the emitted field. We do so by considering the total field emitted by the atom and focusing on each of the dressed-state components, taking in particular a secular-approximation and deriving straightforward, transparent analytic expressions for the second-order auto- and cross-correlations.

Systems in contact with an environment provide a ubiquitous challenge in quantum dynamics. Many fascinating phenomena can arise if the coupling is strong, leading to non-Markovian dynamics of the system, or collective, where the environment can become highly excited. We introduce a significant improvement of the hierarchy of pure states (HOPS) approach, which is able to efficiently deal with such highly excited, non-Markovian environments in a nearly unitary way. As our method relies on quantum trajectories, we can obtain dynamics efficiently, also for large system sizes, by (1) avoiding the quadratic scaling of a density matrix and (2) exploiting the localization properties of the trajectories with an adaptive basis. We provide the derivation of the nuHOPS (nearly unitary hierarchy of pure states) method, compare it to the original HOPS, and discuss numerical subtleties based on an illustrative dephasing model. Finally, we show its true power using the Dicke model as the paradigmatic example of many emitters decaying superradiantly inside a cavity. We reach numerically exact solutions for up to 1000 emitters. We apply our method to study emerging higher-order correlations in the emitter system or the cavity mode environment and their scaling with the number of emitters.

Remarkable breakthroughs in quantum science and technology are demanding more efficient methods in analyzing quantum many-body states. A significant challenge in this field is to verify whether a quantum state prepared by quantum devices in the laboratory accurately matches the desired target pure state. Recent advancements in randomized measurement techniques have provided fresh insights in this area. Specifically, protocols such as classical shadow tomography and shadow overlap have been proposed. Building on these developments, we investigate the fundamental properties of schemes that certify pure quantum states through random local Haar measurements. We derive bounds for sample fluctuations that are applicable regardless of the specific estimator construction. These bounds depend on the operator size distribution of either the observable used to estimate fidelity or the valid variation of the reduced density matrix for arbitrary observables. Our results unveil the intrinsic interplay between operator complexity and the efficiency of quantum algorithms, serving as an obstacle to local certification of pure states with long-range entanglement.

With four electrons, ${\mathrm{He}}_2$ is one of very few molecules for which first-principles quantum-chemical calculations that include the treatment of nonadiabatic, relativistic, and quantum-electrodynamics corrections are possible. Precise spectroscopic measurements are needed as references to test these calculations. We report here on a spectroscopic measurement of the $c{{}^3{\mathrm{\Sigma }}_g}^{+}\leftarrow a{{}^3{\mathrm{\Sigma }}_u}^{+}$ electronic transition of ${}^4\mathrm{He}_2$ at a precision ($\mathrm{\Delta }\nu /\nu$) of $2.5\times {10}^{-10}$ and with full resolution of the rotational, spin-spin, and spin-rotational fine structures. The focus of the investigation is placed on the measurement and analysis of transitions to the rotational levels of the $c{}^3{\mathrm{\Sigma }}_g^{+}(v=4)$ state, which are located energetically above the $\mathrm{He}\left(1{}^1\mathrm{S}_0\right)+\mathrm{He}\left(2{}^3\mathrm{S}_1\right)$ dissociation limit and can decay by tunneling predissociation through a barrier in the potential of the $c$ state. The new data include a full set of the energy levels of the $a{}^3{\mathrm{\Sigma }}_u^{+}(v=0)$ and $c{}^3{\mathrm{\Sigma }}_g^{+}(v=4)$ states with rotational quantum numbers $N$ up to 9 and 10, respectively, and full sets of fine-structure intervals in these levels for comparison with the results of first-principles calculations carried out in a parallel investigation [Rácsai, Jeszenszki, Margócsy, and Mátyus, arXiv:2506.23879v1 [physics.chem-ph]. The new data were combined with data from earlier measurements of the spectrum of the $c\text{−}a$ band system of ${}^4\mathrm{He}_2$ to derive full sets of molecular constants for the $a{}^3{\mathrm{\Sigma }}_u^{+}(v=0)$ and $c{}^3{\mathrm{\Sigma }}_g^{+}(v=4)$ states with much improved precision over previous experimental results. A pronounced broadening of the linewidths of the transitions to the $c{}^3{\mathrm{\Sigma }}_g^{+}(v=4,N=10)$ fine-structure levels is attributed to tunneling predissociation through a barrier in the potential of the $c$ state and is quantitatively accounted for by calculations of the predissociation widths.

Ultrafast science studies the dynamics of electrons in matter with extreme temporal precision, typically in the attosecond and femtosecond time domain. Recent experimental and theoretical progress has put metal-insulator-metal (MIM) tunneling nanojunctions in the spotlight of ultrafast science. Waveform-controlled laser fields can induce ultrafast currents in these junctions, opening the door to petahertz electronic operation and attosecond-scale scanning tunneling microscopy. Inspired by our strong-field model for attosecond tunneling microscopy [B. Ma and M. Krüger, Phys. Rev. Lett. 133, 236901 (2024)], here we generalize our model to MIM nanojunctions. We introduce several refinements and corrections, accounting for the image potential inside the gap and boundary effects. Moreover, we also find that the Keldysh parameter, which is a hallmark parameter for ultrafast light-matter interactions, alone is insufficient for describing the physics in thin MIM nanojunctions. We introduce a new parameter $\zeta$ that accounts for the effects of the limited size of the junction and provide several interpretations of the parameter that shed new light on the complex physics in the light-driven junction. Most strikingly, we find that for $\zeta >1$ photon-assisted tunneling is dominating the ultrafast electron transport across the junction, regardless of the value of the Keldysh parameter. Here one or more photons are absorbed and the electron undergoes static tunneling through the barrier. $\zeta <1$ is required for a regime in which the laser field and the Keldysh parameter dominate the transport. We also discuss the three-step model of electron transport across the nanojunction in the adiabatic regime, the energy cutoff of the transported electrons, and the carrier-envelope phase control of the net current. Our theory model provides a rich toolbox for understanding and predicting the physics of ultrafast MIM nanojunctions.

In the tunneling regime and at sufficiently low field amplitudes, the shape of orientation-dependent molecular ionization rate curves usually resembles the shape of the ionized orbital. As the ionizing field strength increases, the shape of the ionization rate can deviate from this pattern. The oft-cited explanation is that the increasing contribution of excited states relative to the ground state modifies the distribution. In this paper we show that orbital distortion and parabolic channel effects, which are independent of excited-state effects, can also significantly modify the angular dependence of the yields of widely studied molecules where excited state effects are negligible. For example, as the ionizing field increases we find that (1) in CO, orbital distortion is responsible for the switch of the position of the global maximum in the orientation-dependent ionization rate to the opposite orientation, and (2) in ${\mathrm{CH}}_3\mathrm{Br}$, the interplay between orbital distortion and parabolic channel effects transforms a local minimum to a local maximum. To simulate orbital distortion and parabolic channel effects, we use the one-electron weak-field asymptotic theory including the first-order correction [OE-WFAT(1)] in the integral representation. Since OE-WFAT(1) incurs expensive computations when the number of orientation angles is large, we also reformulate the original OE-WFAT(1) algorithm into a partial-wave expansion form, which greatly enhances the efficiency of the method.

Because of its simple structure, the hydrogen atom is often used as a testbed for quantum electrodynamics. Spectroscopy of trapped atomic samples can greatly improve the accuracy of these tests. Trapping atomic hydrogen in an optical dipole trap or an optical lattice has never been achieved. Only trapping in magnetic fields that lead to large Zeeman shifts has been demonstrated. Standard techniques of atomic physics are difficult to apply to atomic hydrogen. The small mass of the atom and the large photon energy of the 1S-2P cooling transition significantly complicate Doppler cooling. This proposal introduces a photon recoil-assisted loading scheme that uses these properties to our advantage to load atomic hydrogen into an optical dipole trap without laser cooling. The magic wavelength (515 nm) for the 1S-2S clock transition (1.3-Hz natural linewidth) is easily accessible with current laser technology. Since the 1S-2S clock transition can be driven Doppler free, we do not require a very low temperature. Besides improving spectroscopy for fundamental science, such a system can also be used as a “computable” atomic clock that may one day justify the redefinition of the SI second in terms of the Rydberg constant.

We report observation of self-oscillating supersonic flows in a two-dimensional atomic superfluid. By imposing a local particle sink with strong loss, we induce a convergent radial flow with a spatially bounded supersonic region, forming an acoustic analog of a black-hole horizon and an inner horizon around the sink. The observed superflow appears to be modulated by quasiperiodic bursts of superluminal signals. We measure their frequencies and find agreement with numerical simulations of the frequencies of ring soliton oscillations within the black-hole horizon. The solitons seen in the simulations are emitted from the region between the two horizons in a process that we attribute to the Landau instability. The presented experiment demonstrates a new method for creating supersonic flows in atomic superfluids, which may find applications in quantum simulations of curved spacetime, supersonic turbulence, and self-oscillating dynamics in dissipative many-body systems.

The Bacon-Shor code is a quantum error correcting subsystem code composed of weight-2 check operators that admits a single logical qubit, and has distance $d$ on a $d\times d$ square lattice. We show that when viewed as a Floquet code, by choosing an appropriate measurement schedule of the check operators, it can additionally host several dynamical logical qubits. Specifically, we identify a period-4 measurement schedule of the check operators that preserves logical information between the instantaneous stabilizer groups. Such a schedule not only measures the usual stabilizers of the Bacon-Shor code, but also measures and promotes gauge operators of the parent subsystem code to additional temporary stabilizers that protect the dynamical logical qubits against errors. We show that the code distance of these Floquet-Bacon-Shor codes scales as $\mathrm{\Theta }(d/\sqrt{k})$ on an $n=d\times d$ lattice with $k$ dynamical logical qubits, along with the logical qubit of the parent subsystem code. Unlike the usual Bacon-Shor code, the Floquet-Bacon-Shor code family introduced here can therefore saturate the subsystem bound $kd=O\left(n\right)$. Moreover, several errors are shown to be self-corrected purely by the measurement schedule itself. This work provides insights into the design space for dynamical codes and expands the known approaches for constructing Floquet codes.