The hydroxyl radical (the OH molecule) carries both electric and magnetic dipole moments and, as a diatomic molecule, admits a comparatively simple and accurate model. This makes it a natural quantum probe for the joint estimation of electric and magnetic fields. Here we study simultaneous estimation of both fields using the tools of multiparameter quantum estimation theory, explicitly accounting for the performance loss caused by measurement incompatibility. We analyze and optimize both stationary and dynamical estimation strategies. In the stationary regime we consider ground and thermal states of the Stark-Zeeman Hamiltonian and identify optimal operating points. For thermal probes we find a nontrivial multiparameter effect: Increasing the temperature can reduce the overall estimation error by weakening parameter correlations. In the dynamical regime we study both pure and thermal initial states, illustrating nontrivial manifestations of incompatibility for mixed probes. Finally, we show that an optimal sequential control protocol can overcome limitations due to noncommutativity, and we assess its robustness in the multiparameter setting.

Collisions between electrons and radiating atoms broaden spectral absorption and emission lines in dense plasmas. High densities also introduce screening and pressure ionization effects that distort the wave functions of both bound and free electrons. In order to study how dense plasma effects influence the electron broadening of spectral lines, this paper incorporates electron wave functions from an average-atom (AA) model to calculate the linewidth of the B iii $2p\text{−}2s$ transition at $T=10\mathrm{eV}$ for mass densities ranging from $\rho ={10}^{-4}\text{to}0.4\mathrm{g}/\mathrm{cc}$. The calculation method uses the impact approximation, allowing the linewidth to be written in terms of electron-collision cross sections and an interference term. Compared to an otherwise identical calculation that uses Coulomb free wave functions, the AA method is found to modify both the cross sections and the resulting linewidth at sufficiently high density by introducing screening and pressure-ionized bound states. Screening lowers the cross sections at low energies and near electron excitation thresholds, while pressure-ionized bound states introduce resonances into the continuum. Thus, as the density increases, the relative linewidth between the AA and Coulomb calculations follows a general decrease because of screening, with sharp increases at various intervals due to pressure ionization. The AA results are also compared with a common approach to introduce screening through the interaction potential and reduced models that use the Bethe formula for the inelastic electron-collision cross sections.

We present a rapid evaporative cooling scheme for a strongly interacting ${}^6\mathrm{Li}$ Fermi gas in an optical dipole trap. The method uses a magnetic-field-gradient-induced tilt of the trapping potential to accelerate cooling in the unitarity-limited regime. In evaporation based only on lowering the optical trap depth, the unitarity-limited scattering cross section can support runaway cooling; however, the cooling rate slows around $T/T_F\simeq 0.5$, and the runaway behavior is no longer maintained. We improve on this approach by applying a magnetic-field gradient when the gas temperature reaches about half the Fermi temperature. The induced tilt opens an escape channel for energetic atoms while keeping the trap frequencies nearly unchanged. This modification increases the cooling speed and cools the gas below the superfluid transition temperature, reducing $T/T_F$ from 0.28 to 0.16 in $\sim 25\mathrm{ms}$; by contrast, optimized ODT lowering reaches only $T/T_F\approx 0.20$ after approximately $100\mathrm{ms}$. Our results provide a simple and robust route for rapidly cooling a strongly interacting Fermi gas into the superfluid regime, facilitating studies of the physics of unitary Fermi superfluids.

Topological boundary states are a hallmark of topological phases, yet their direct detection remains challenging when they hybridize with bulk states or fail to form isolated in-gap modes. Here, we introduce a dynamical approach for directly distilling boundary-localized states in topological lattices via non-Hermitian masking, achieved by imposing loss on the bulk region of the lattice without targeting specific eigenmodes. Under full-field excitation, bulk modes are dynamically suppressed, while boundary-localized states survive as the longest-lived modes, enabling their direct visualization. We experimentally demonstrate this method in tailored photonic honeycomb lattices using defocused incoherent input, demonstrating the dynamical distillation of both static and Floquet-induced corner states. Notably, the proposed approach does not rely on mode-selective excitation or prior knowledge of the spatial distributions of boundary states, and enables their simultaneous visualization even in the presence of spatial extension or hybridization. Our results establish a general and experimentally accessible strategy for isolating and visualizing boundary-localized states in topological systems.

The light deflection under a strong gravitational field, referred to as strong gravitational lensing, provides a powerful probe of spacetime geometry. Besides, laboratory analog models are employed to study the effects of curved spacetime and explore the design of optical devices. Here, applying the framework of analog gravity, we investigate pulse echoes from a pointlike source on two curved surfaces of revolution derived from Schwarzschild spacetime, the Flamm paraboloid, and the surface of the two-dimensional Fermat metric, and reveal the interplay between the photon sphere and the pulse. We calculate the echo response using analytical geodesic solutions and the Huygens-Fresnel principle. Particularly, when the spatial scale of pulse duration is comparable to the photon sphere, continuous echo tails appear along bright interference fringes in temporal response. Analysis in both the temporal and frequency domains reveals that these echo tails are a signature of resonance between the incoming pulse and the photon sphere. This work provides a wave-optics perspective on pulse interference and echo formation in curved geometries, offering a tabletop platform for exploring the simulation of strong gravitational lensing phenomena.

Vacuum magnetic birefringence (VMB) is an effect predicted by quantum electrodynamics in which the vacuum behaves as a nonlinear optical medium and exhibits a birefringence in the presence of a magnetic field. In this work, an interferometric scheme is introduced to measure this effect by sensing the changes in the frequencies of fields stabilized to the resonances of an optical cavity. Results are presented from a prototype setup with this sensing technique implemented on a 19-m test cavity without a magnetic field. We propose using this scheme for a measurement of VMB with a 245-m-long optical cavity and a string of 24 superconducting magnets, arranged for the ALPS II experiment. In this manuscript we examine potential sources of noise in the prototype and project these results in terms of the sensitivity of the full-scale experiment.

Reference-frame-independent measurement-device-independent quantum key distribution (RFI-MDI-QKD) offers a promising approach for building star-type quantum secure networks, removing the need for reference frame alignment and eliminating all detector side channel attacks. While prior experiments confirmed the practicality of RFI-MDI-QKD, they largely assume symmetric channels, an idealization rarely met in real networks. Here, we present an experimental demonstration of RFI-MDI-QKD under asymmetric channel loss. By combining an optimized seven-intensity decoy-state method with a joint-study method, we achieve a secret key rate of 46.47 bit/s over 100 km with an asymmetric channel (Alice at 40 km and Bob at 60 km), surpassing the key rate of conventional added-attenuation methods by more than tenfold. These results pave the way for high-rate, ultrasecure quantum networks.

Highly nonclassical states of light—such as the approximate Gottesman-Kitaev-Preskill states, states exhibiting cubic nonlinear squeezing, or catlike states—can be generated from experimentally accessible Gaussian states via photon-counting measurements on selected modes, conditioned on specific outcomes of these heralding events. A simple yet important example of this approach involves performing photon number measurements on one mode of a two-mode entangled Gaussian state. The heralding probability of this scheme is a key figure of merit, as it determines the generation rate of the target nonclassical state. In this work, we show that the maximum heralding probability for the two-mode setting can be calculated analytically, and we investigate its dependence on the number of detected photons $n$. Our results show that the number of required experimental trials scales only polynomially with $n$. Generation of highly complex optical quantum states with high stellar rank is thus possible in principle in this setting, given access to sufficiently strong squeezing.

We report on a simple and easy to implement method of feedback cooling trapped ions to temperatures below those achievable using only Doppler cooling. Additionally, the feedback cooling results in significantly shorter cooling times. For selected parameters, we demonstrate cooling to temperatures below $\hslash \mathrm{\Gamma }/2k_{\mathrm{B}}$. The motion of a single ion is monitored in real time, allowing for the generation of a feedback signal that is applied to an auxiliary trap electrode. Motion detection is implemented by imaging the fluorescence photons emitted by the ion onto a knife edge and detecting the transmitted light, a method used so far to cool trapped nanoparticles. The intensity modulation of the fluorescence resulting from the ion motion is used to generate and apply the feedback signal and also to determine the ion temperature. While the method benefits from a high rate of detected scattered photons, which can be a challenge, and which we address by using a parabolic mirror for collecting the fluorescence, we expect the method to also be applicable when using lenses with moderate numerical apertures.

Quantum annealing (QA) is an efficient method for finding the ground-state energy of the problem Hamiltonian. However, in practical implementation, the system suffers from decoherence. Recently, “localized virtual purification” (LVP) was proposed to suppress decoherence in the context of noisy intermediate-scale quantum devices. We consider observables with spatially local support in the lattice. In this case, the LVP scheme requires evaluating the expectation value using the reduced density matrix of the subsystem where the observable acts and its surroundings. In this work, we propose a method to mitigate decoherence errors in QA using LVP. The key idea is to use the so-called classical shadow (CS) method to construct the reduced density matrix. Thanks to the use of the CS method, unlike the previous schemes to mitigate decoherence errors in QA, we do not need either two-qubit gates or midcircuit measurements, which means that our method is hardware efficient.

The vibrational excitation of molecular hydrogen $\left({\mathrm{H}}_2\right)$ by low-energy electrons is one of the most fundamental reactions in atomic and molecular physics, playing a key role in the chemistry of the interstellar medium and studies of fusion plasmas. Crossed-beam experiments and electron swarm measurements have long been used to determine scattering cross sections for vibrational excitation, but the two methods produce conflicting results, a disagreement that has persisted for more than five decades and remains unsolved. Theoretical treatments have historically relied on approximations tailored to specific processes, such as separating the direct and resonant pathways or employing model polarization potentials, with no single method accurately describing all vibrational transitions. The absence of benchmark-quality calculations, combined with decades-long disagreements between experimental methods, means that even for this simple problem, there are no definitive cross-section data, leaving the reliability of theoretical approaches applied to more complex molecules uncertain and without quantified error bounds. Here we present ab initio calculations of low-energy vibrational excitation in ${\mathrm{H}}_2$ that treat electronic and vibrational motion on equal footing, providing a unified description of direct and resonant scattering across all vibrational transitions. The results are in stark contrast with previous theoretical estimates for many vibrational transitions, revealing systematic differences in both magnitude and energy dependence. At the same time, they provide strong confirmation of the swarm-derived cross sections, unlike previously accepted calculations that favored the beam data. This reversal of the long-standing situation shifts attention away from the swarm data and back onto the interpretation of the crossed-beam measurements. By providing accurate and self-consistent cross sections for vibrational excitation of ${\mathrm{H}}_2$, the present calculations will enable more reliable modeling of low-temperature plasmas and interstellar clouds.

We present ab initio calculations using the $R$-matrix with time dependence method for high-order harmonic generation (HHG) in argon in a short, intense pulse regime. The calculations employ a six-cycle ${sin}^2$ pulse at 850 nm with peak intensity $2.3\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ and, for comparison with the experiment by Guo et al. [J. Phys. B: At. Mol. Opt. Phys. 51, 034006 (2018)], a Gaussian pulse with the same frequency and peak intensity. Both pulse shapes yield the expected harmonic structure in the region above the ionization threshold (approximately 15.82 eV in $LS$ coupling). The spectra exhibit strong carrier-envelope-phase sensitivity. The energy region leading up to the ionization threshold contains spectral features arising from residual coherent dipole oscillations (free-induction decay) that strongly depend on spectral windowing and the post-pulse propagation time. We show that the HHG spectrum, particularly below the ionization threshold, is a defined quantity that depends on analysis choices rather than being a uniquely determined observable. Comparison between theoretical predictions and experimental observations in this energy regime, therefore, requires explicit specification of these parameters.

We investigate the thermodynamic properties of lattice Bose gases in optical cavities in the Mott-insulator limit. We find that the system assumes anomalous thermodynamic behavior that can be traced back to the breaking of fundamental additivity by its infinite-long-range interaction. Specifically, the system shows striking ensemble inequivalence between the canonical ensemble and the grand canonical one, sharply manifesting in the distinct anomalous structure of the thermodynamic phase diagram in the canonical ensemble. In particular, in the temperature regime at around half of the on-site energy, the system manifests negative compressibility and anomalous reentrant phase transitions where the ordered charge density wave phase revives from the homogenous phase upon increasing the temperature. Direct experimental observation of the anomalous behavior can be realized in the current experiments with well-controlled total particle-number fluctuations.

We recently showed that macroscopic nuclear spin ensembles prepared in coherent spin states can dramatically enhance the interaction rates of weakly interacting cosmic relics, such as dark matter and the cosmic neutrino background, through collective quantum effects analogous to Dicke superradiance, where the deexcitation and excitation rates scale as the square of the number of spins $N^2$. We thus coined these processes superradiant interactions. In this paper, we propose a protocol to realize this enhancement and boost the discovery potential for such relics. We show how concepts from quantum optics can be adapted to nuclear spins coupled to superconducting circuits, enabling high-sensitivity systems. The spins are first initialized into a coherent spin state via a $\pi /2$ Rabi pulse from the ground state. When the circuit is sufficiently detuned from resonance, the spin-circuit interaction implements a squeezing Hamiltonian. Because squeezing must outpace spin relaxation and dephasing, the protocol favors macroscopic ensembles and high-quality superconducting circuits. During this squeezing phase, the standard quantum variance is reduced by up to 4.8 orders of magnitude, equivalent to 48 dB of squeezing, for circuits with quality factors $Q\sim {10}^8\text{–}{10}^9$. The signal imprinted on the spins during the squeezing protocol can be magnified by further utilizing the squeezing interactions, easing the requirement for shot-noise-limited readout. This protocol has the potential to significantly accelerate axion and dark photon dark matter searches and extend the reach of existing axion experiments to probe QCD axion-nuclear spin couplings. More broadly, it paves the way for detecting coherent inelastic interactions from other cosmic relics, most notably the cosmic neutrino background, and establishes nuclear-spin-based systems as a class of quantum, ultra-low-threshold detectors.

We investigate a nonequilibrium quantum thermometry protocol in which an ensemble of qubits, acting as temperature probes, is weakly coupled to a macroscopic thermal bath. The temperature of the bath, the parameter of interest, is encoded in the dissipator of a Markovian thermalization process. For some relevant initial states, we observe a peak in the Quantum Fisher Information (QFI) during the transient of the thermalization, indicating enhanced sensitivity in early-time dynamics. This effect becomes more pronounced at higher bath temperatures and is further enhanced when the initial reduced state of the qubits has a large ground-state population and/or it is highly coherent. We also focus on the role of initial quantum correlations in the thermometric performance, which emerge as a central feature of this work. We find strong numerical evidence that, given same single-qubit reduced states, the inclusion of quantum correlations among the qubits of the ensemble always yields an enhanced QFI. Moreover, even if none of the considered states outperform the (pure, separable) ground state, maximally entangled states display QFIs values remarkably close to the standard quantum limit when probing extremely hot thermal baths. Finally, although the Markovian dynamics does not permit superlinear scaling of the QFI with the number of probes, we identify the most effective initial states for designing high-precision quantum thermometers within this setting. We also provide concrete guidelines for experimental implementations.

We study single-shot distinguishability of locally implementable multipartite unitaries under Local Operations and Classical Communication (LOCC) and global operations. As unitary discrimination depends on both the choice of probing states and the measurements on the evolved states, we classify LOCC and global distinguishability into two categories: adaptive strategies, where probing states are chosen based on measurement outcomes from other subsystems, and restricted strategies, where probing states remain fixed. Our findings uncover three surprising features in the bipartite setting and establish new structural limits for unitary discrimination: (i) Certain pairs of unitaries are globally distinguishable with restricted strategies but indistinguishable under LOCC, even with adaptive strategies. (ii) There exist sets of four unitaries that are distinguishable via LOCC, yet remain globally indistinguishable with restricted strategies. (iii) Some sets of unitaries are globally indistinguishable when probed with separable states, but become distinguishable via LOCC.

Heisenberg's uncertainty relation is at the origin of understanding minimum uncertainty states and squeezed states of light. In the recent past, the sum uncertainty relation was formulated by Maccone and Pati [L. Maccone and A. K. Pati, Phys. Rev. Lett. 113, 260401 (2014)] and is claimed to be stronger than the existing Heisenberg-Robertson product uncertainty relation. We analyze the minimum uncertainty states for the sum uncertainty relation using the variational approach. We claim that the minimum uncertainty states for the sum uncertainty relation are always the minimum uncertainty states for the traditional product uncertainty relation, using the example of position-momentum pair as well as angular momentum operators. We show that the coherent and squeezed states of radiation remain completely unaffected by the sum uncertainty relation.

Spontaneous diffusion (i.e., nonconservation of energy) is a prominent, testable prediction of collapse and hybrid classical-quantum gravitational models. Without smearing of the mass density operator, the associated heating (or energy increase) rate diverges, yet the smearing distribution is arbitrary and, on scales much larger than the smearing length $r_C$, much of the phenomenology is expected to be insensitive to this choice. We propose to resolve this arbitrariness as follows: for a fixed $r_C$, select the distribution that minimizes the heating rate. Conceptually, this should identify the minimal deviation from standard quantum mechanics and provide models that, once experimentally refuted, would strongly disfavor all variants with different distributions. We apply this approach to the most investigated collapse models: GRW (for Ghirardi-Rimini-Weber), CSL (for continuous spontaneous localization), and DP (for Diósi-Penrose). Notably, the Gaussian is optimal only for the GRW case. Finally, we apply it to the Tilloy-Diósi hybrid classical-quantum model of Newtonian gravity, leading to its minimally deviating variant. This version of the model is entirely determined by only one free parameter $r_C$, and, if experimentally refuted, would strongly disfavor any other version of it.

Cat states are an important resource for fault-tolerant quantum computing, where they serve as building blocks for a variety of fault-tolerant primitives. Consequently, the ability to prepare high-quality cat states at large fault distances is essential. While optimizations for low fault distances or small numbers of qubits exist, higher fault distances can be achieved via generalized constructions with potentially suboptimal circuit sizes. In this work, we propose a cat state preparation scheme based on preparing two cat states with low-depth circuits, followed by a transversal cnot and measurement of one of the states. This scheme prepares $w$-qubit cat states fault tolerantly up to fault distances of 9 using $\lceil {log}_{2}w\rceil +1$ depth and at most $3w-2$ cnots and $2w$ qubits. We discuss that the combinatorially challenging aspect of this construction is the precise wiring of the transversal cnot and propose three methods for finding these: two based on Satisfiability Modulo Theory solving and one heuristic search based on a local repair strategy. Numerical evaluations show that our circuits achieve a high fault distance while requiring fewer resources than generalized constructions.