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.

We investigate the out-of-equilibrium dynamics of quantum information in one-dimensional systems undergoing a quantum quench using a local perspective based on the information lattice. This framework provides a scale- and space-resolved decomposition of quantum correlations, enabling a hydrodynamic description of the information flow through well-defined local densities—termed local information—and currents. We apply this framework to three local quenches in noninteracting fermionic chains: (i) the release of a single particle into an empty tight-binding chain, (ii) the connection of two critical chains via the removal of a central barrier, and (iii) the coupling of a topological Kitaev chain to a critical chain. In each case, the information lattice reveals the local structure of correlation buildup and information interface effects, going beyond global measures such as the von Neumann entropy. In particular, through the information lattice, we uncover the signatures in the local information flow associated with topological edge modes and analytically explain the fractional von Neumann entropy values observed in Majorana quench protocols. Our approach is general and applicable to interacting, disordered, and open systems, providing a powerful tool for characterizing quantum information dynamics.

We investigated two deprotonated paracoumaric acid model chromophores of the photoactive yellow protein, ${\mathrm{pCE}}^{-}$ and ${\mathrm{pCK}}^{-}$, using action-absorption spectroscopy. Employing cryogenic cooling, we determined the ${\mathrm{S}}_0\text{−}{\mathrm{S}}_1$ band origins and associated vibrational resonances, which were found to reside within the electronic continuum, rendering them susceptible to autodetachment. Resonances observed below the electron-detachment threshold were attributed to dipole-bound states of the neutral radical core, probed via resonant two-photon excitation and detachment. Above the electron-detachment limit, we identified dipole-resonance states susceptible to autodetachment. The experimental approach, combining 6 K helium-buffer gas cooling with an ion-storage ring, proved effective in resolving vibrational modes of both the neutral radical core and the valence-excited ${\mathrm{S}}_1$ state, revealing soft-mode resonances down to 50–100 ${\mathrm{cm}}^{-1}$. Time-resolved action detection indicates that internal conversion competes with autodetachment.

We consider the problem of reproducing one quantum measurement given the ability to perform another. We give a general framework and specific protocols for this problem. For example, we show how to use available “imperfect” devices a small number of times to implement a target measurement with average error that drops off exponentially with the number of imperfect measurements used. We hope that could be useful in near-term applications as a type of lightweight error mitigation of the measuring devices. As well as the view to practical applications, we consider the question from a general theoretical perspective in the most general setting where both the available and target measurements are arbitrary generalized quantum measurements. We show that this general problem in fact reduces to the ability to reproduce the statistics of (complete) von Neumann measurements, and that in the asymptotic limit of infinitely many uses of the available measurement, a simple protocol based upon “classical cloning” can perfectly achieve this task. We show that asymptotically any nontrivial quantum measurement can reproduce any other. We also study optimal protocols for a fixed number of uses of the available measurement. This includes, but is not limited to, improving both noisy and lossy quantum measurements. Furthermore, we show that, in a setting where we perform multiple measurements in parallel, we can achieve finite-rate measurement reproduction, by using block-coding techniques from classical information theory. Finally, we show that advantages can also be gained by making use of probabilistic protocols.

We propose that a nuclear clock based on the Th v ion can surpass the accuracy of clocks built with other thorium ions. The ${\mathrm{Th}}^{4+}$ ion has a rigid closed-shell core with zero total electron angular momentum, suppressing frequency shifts from black-body radiation and stray external fields that act mainly on electrons. We calculate the energy shift of the nuclear clock transition frequency in ${}^{229}\mathrm{Th}$ due to the Coulomb field of atomic electrons and find a relative frequency difference of $2.8\times {10}^{-7}$ between Th iv and Th v, 12 orders of magnitude larger than the projected ${10}^{-19}$ fractional uncertainty of a nuclear clock. We also perform calculations for Th v energy levels, ionization potential, static polarizability, quadratic Zeeman shift, and the black-body radiation shift of the nuclear line. Additionally, we determine the nuclear transition frequencies in two thorium ions and a neutral atom: ${\nu }_N=2,020,406.964\left(70\right)\mathrm{GHz}$ in Th iii, ${\nu }_N=2,020,408.264\left(100\right)\mathrm{GHz}$ in Th ii, and ${\nu }_N=2,020,408.364\left(100\right)\mathrm{GHz}$ in Th i.

Dissipative phase transitions (DPTs) are defined by sudden changes in the physical properties of nonequilibrium open quantum systems and they present characteristics that have no analog in closed and thermal systems. Several methods to detect and characterize DPTs have been suggested in the literature, the most famous of which, the Liouvillian gap, can be derived from a spectral analysis of the Liouvillian superoperator that governs the complex interplay between coherent and dissipative dynamics. Here we consider the output current, defined as the average total quantum jumps per unit time between the open quantum system and the environment. We propose that output current fluctuations and in particular their dynamical correlations, their power spectrum, and their characteristic timescale can provide valuable information about DPTs, confirming a dramatic change of behavior at the critical point. We validate our proposal using the dissipative $XYZ$ model and the nonlinear driven-dissipative Kerr model, showing good agreement with previous estimates of the location of the critical point. Compared to previous approaches, our proposal could already be experimentally tested in optical systems, providing a practical method to detect criticality in quantum open systems.

Nondemolition readout and high-fidelity unconditional reset of qubits are key requirements for practical quantum computation. For superconducting qubits, readout and reset are typically realized using resonators based on dispersive approximation. However, violations of the approximation often lead to detrimental effects on the qubit. In this work, we demonstrate nondemolition fluorescence readout and high-fidelity unconditional reset of a fluxonium qubit via dissipation engineering without employing a resonator. We design and implement a planar filter to protect the qubit from energy relaxation while enhancing the relaxation of the readout transition. By appropriately selecting the readout transition, we achieve fluorescence readout with enhanced quantum nondemolitionness. We also realize fast, high-fidelity, and all-microwave unconditional reset of the fluxonium qubit. These results highlight the potential of superconducting quantum computing architectures without relying on dispersive interaction between qubits and resonators.

We present a general theory of gain-modified emission dynamics between two quantum emitters (two level systems) in a linear gain medium, demonstrating how gain modifies the usual radiative decay rates and interemitter coupling and decay rates, from purely lossy systems. We derive a Born-Markov master equation that shows explicitly how gain modifies the usual rates appearing for two emitters in a loss-only medium and introduces new gain terms. We then present Bloch equations in the bare state basis as well as a dressed-state basis (using superradiant and subradiant states). As an application of the theory, we show calculations for a fully three-dimensional metal dimer in a gain-compensated medium containing two quantum emitters and show explicit solutions in terms of the quasinormal modes, which easily allows one to obtain all the relevant decay rates. We study the modified decay dynamics, gain excited pumping, as well as the entanglement entropy and spectral emission. We also demonstrate how a breakdown of the weak excitation approximation in the presence of gain requires a careful treatment beyond a linear response, which yields nontrivial incoherent coupling terms that typically are not included in dipole-dipole coupling models and heuristic gain pumping models.

We perform narrowline cooling of single dysprosium atoms trapped in a one-dimensional optical tweezer array, employing the narrow single-photon transition at 741 nm. At the trapping wavelength of 532 nm, the excited state is less trapped than the ground state. To obtain efficient cooling performances, we chirp the frequency of the cooling beam to subsequently address the red sidebands of different motional states. We demonstrate the effectiveness of the cooling protocol through Raman thermometry, which we characterize for our experimental conditions. We obtain an array of 75 atoms close to the motional ground state in the radial direction of the tweezers. Our results demonstrate the possibility to manipulate the motional degree of freedom of dysprosium in optical tweezer arrays, a key ingredient to exploit the potential of lanthanide-based tweezer platforms for quantum science.

Through left- or right-handed twisting, we investigate the impact of mirror-asymmetry (chirality) of the conducting boundary conditions of an equilaterial triangular cross section electromagnetic resonator. We observe the generation of eigenmodes with nonzero electromagnetic helicity as a result of the coupling of near degenerate ${\mathrm{TE}}_{11(p+1)}$ and ${\mathrm{TM}}_{11p}$ modes. This can be interpreted as an emergence of magnetoelectric coupling, which in turn produces a measurable shift in resonant mode frequency as a function of twist angle. We show that this coupling mechanism is equivalent to introducing a nonzero chirality material parameter ${\kappa }_{\text{eff}}$ or axion field ${\theta }_{\text{eff}}$ to the medium. Our findings demonstrate the potential for real-time, macroscopic manipulation of electromagnetic helicity.