We propose networking superconducting quantum circuits by transducing their excitations (typically 4–8 GHz) to 200–500 MHz photons for transmission via superconducting cryogenic coaxial cables. This frequency down-conversion reduces transmission losses and, counterintuitively, reduces noise. We introduce a multioctave asymmetrically threaded superconducting quantum interference device circuit capable of the required efficient, high-rate transduction. For a 100-meter cable with $Q_i={10}^5$ at 10 mK, we theoretically show that our approach achieves single-photon fidelities of 0.962 at 200 MHz versus 0.772 at 8 GHz, and triples the lower bound on quantum channel capacity. This method enables highly efficient networking of superconducting qubits while maintaining high fidelities, combining improved performance with the practical advantages of flexible, compact coaxial cables.

Phononic resonators are becoming increasingly important in quantum information science, both for applications in quantum computing, communication, and sensing as well as in experiments investigating fundamental physics. Here, we study the decoherence of phonons confined in a surface acoustic wave (SAW) resonator strongly coupled $(g/2\pi \approx 9\mathrm{MHz})$ to a superconducting transmon qubit. By comparing experimental data with numerical solutions to the Markovian master equation, we report a surface phononic energy decay rate of ${\kappa }_1/2\pi =480\mathrm{kHz}$ and a pure dephasing rate of ${\kappa }_{\varphi }/2\pi =180\mathrm{kHz}$. These rates are in good agreement with the level of decoherence we extract from qubit-assisted spectroscopic measurements of the SAW resonator. We additionally find that the timescales over which coherent driven dynamics and decoherence occur are comparable, highlighting the need to model the composite device as an open quantum system. We discuss possible sources of decoherence in SAW-based quantum acoustic devices and the application of these devices in future quantum acoustic dissipation engineering experiments. The decoherence characterization techniques we employ are broadly applicable for investigating and benchmarking the effects of loss and dephasing on mechanical resonators in the quantum regime.

The photoionization (PI) spectrum of the CH molecule is evaluated theoretically and compared with available experimental data for photon energies from the photoionization threshold up to 12 eV. The vibrational motion and the vibronic coupling between the lowest vibrational and electronic states of CH and ${\mathrm{CH}}^{+}$ are accounted for in the present study. The multichannel quantum-defect theory with the vibrational frame transformation is employed in modeling the PI cross section of CH. The PI cross section is calculated for several initial vibrational levels of CH. A good agreement with the experiment by Gans et al. [J. Chem. Phys. 144, 204307 (2016)] for positions of observed resonances and a qualitative agreement for the intensities of the resonances are found.

We theoretically investigate the product-state distribution of weakly bound diatomic van der Waals molecules via ultracold three-body recombination of bosonic alkali-metal atoms. We find a two-level hierarchy of spin propensity rules at zero magnetic field. The primary propensity rule states that nearly all molecular products conserve the total hyperfine spin of a reactant atomic pair, while molecular products not conserving the total spin are highly suppressed. For the dominant molecular products, there is a secondary propensity to conserve certain spin components of the reactant pair such as the atomic hyperfine spins, or the total electronic or nuclear spins. The second propensity varies across species and depends fundamentally on the interplay between effective electronic spin exchange and hyperfine interactions. The spin sensitivity of product-state distribution can potentially open up new avenues for controlling state-to-state reaction rates in ultracold three-body recombination.

A framework for computing the superfluid gap in an effective field theory (EFT) of fermions interacting via momentum-independent contact forces is developed. The leading universal corrections in the EFT are one-loop in-medium effects at the Fermi surface, and reproduce the well-known Gor'kov-Melik-Barkhudarov result. The complete subleading universal corrections are presented here, and include one-loop effects away from the Fermi surface, two-loop in-medium effects, as well as modifications to the fermion propagator. Together, these effects are found to reduce the gap at low densities. Applications to neutron superfluidity in neutron stars are also discussed.

We predict the existence of a period-4 temporal dynamics in the nonlinear stage of modulation instability of light inside a passive ring fiber Kerr cavity. We investigate analytically and numerically the stability of period-doubled continuous-wave solutions of the Ikeda map describing waves oscillating between two well-defined power levels at consecutive round trips. We prove that conditions exist for which spectral sidebands can be amplified leading to the formation of a temporal modulation pattern which repeats itself identically every four cavity round trips. We show that this dynamics exists for realistic parameters of a driven fiber cavity, which makes it potentially accessible to experimental observation.

This paper presents a comprehensive theoretical framework for nonlinear optical trapping, verified by numerical simulations. The roles of thermal nonlinearity, two-photon absorption (TPA), and Kerr nonlinearity are thoroughly examined in the formation of split traps. We demonstrate the formation of an attractive scattering force generated by TPA, which is distinct from previously reported attractive scattering forces J. Chen et al. [Nat. Photon. 5, 531 (2011); O. Brzobohatỳ et al., Nat. Photon. 7, 123 (2013)]. We also introduce efficient methods for accurately calculating trap stiffness by modeling nonlinear trap potentials as nonlinear harmonic oscillators. Furthermore, we offer an alternative perspective on nonlinear traps by considering them as hybrid traps. This work enhances the understanding of the nonlinear optical trapping mechanism and provides a robust framework for future experimental studies in quantum mechanics and high-precision optical manipulation.

The rise of aluminum gallium arsenide on-insulator (AlGaAsOI) and lithium niobate on-insulator (LNOI) platforms presents new opportunities for advancing on-chip and ultracompact nonlinear photonic devices. In this study, we investigate theoretically and via numerical simulations second-harmonic generation (SHG) in AlGaAsOI and LNOI waveguides across a wide range of pulse widths, from the subpicosecond to the continuous-wave regime, and peak power levels spanning from milliwatts to hundreds of watts. Our research aims to identify optimal time and power regimes for achieving high SHG conversion efficiency, exploring the detrimental impact of linear and nonlinear losses as well as dispersion effects. Additionally, we assess the performance of SHG against manufacturing errors and surface roughness, comparing our findings with state-of-the-art solutions. These results offer insights into the robustness and potential applications of SHG in these advanced photonic platforms.

We investigate a generic tripartite quantum system featuring a single qubit interacting concurrently with two quantized harmonic oscillators via nonlinear multiphoton Jaynes-Cummings (MPJC) interactions. Assuming the qubit is initially prepared in a superposition state and the two oscillators are in arbitrary Fock states, we analytically trace the temporal evolution of this tripartite pure initial state. We identify four broad cases, each further divided into two subcases, and derive exact analytical solutions for most cases. Notably, we obtain perfect transfer of excitations between the oscillators by carefully selecting system parameters. In addition, we extensively examine the manner in which the nonclassicalities of various initial oscillator Fock states, quantified by the volume of negative regions in the associated Wigner functions, evolve under the MPJC Hamiltonian, considering diverse system parameters including environmental effects. Besides producing substantial enhancements in the initial value for higher-photon-number states, our analysis reveals that driven solely by the initial qubit energy, with both oscillators initialized in the vacuum state, the nonlinear MPJC interaction yields a significant amount of nontrivial Wigner negativity in the oscillators. The additional nonlinearity introduced by the multiphoton process plays a pivotal role in surpassing the initial nonclassicalities of the photon-number states.

Recently, Jentschura [Phys. Rev. A 109, 012802 (2024)] rederived the multipole corrections to the dipole part of the atom-wall interaction described by the Lifshitz theory using the concept of volume dielectric permittivity. These corrections were computed for the hydrogen and positronium atoms in close proximity to the $\alpha$-quartz wall and claimed to be numerically significant within the short-range regime. Here, it is shown that the application areas of the obtained expressions in both the short- and long-range asymptotic regimes are indicated incorrectly, in contradiction with those dictated by the Lifshitz theory. As a result, within the valid application areas, all multipole corrections to the Casimir-Polder dipole part of the atom-wall interaction turned out to be negligibly small.

It is shown that the criticism formulated in the preceding Comment fails to appreciate recently obtained results for the upper limit of the short-range expansion of atom-surface interactions, and that the application of our results to physisorption is based on a valid extension of Lifshitz theory to the physisorption range, which can be accomplished by refining the concept of the atom-surface distance with the help of a reference-plane that takes the response function of the solid into account. Some details on the calculation of the reference-plane are recalled from the literature.

A general entanglement generation protocol between remote stationary qubits using single-photon reflection in a photonic network is explored theoretically. The nodes of the network consist of single qubits that are typically represented by the spin of a color center, each localized in a separate optical cavity and linked to other nodes via photonic links such as optical fibers. We derive a model applicable to a wide range of parameters and scenarios to describe the nodes and the local spin-photon interaction accounting for the pulsed (finite-bandwidth) nature of flying single photons while optimizing the rate and fidelity. We investigate entanglement generation between remote qubits and tailor protocols to a variety of physical implementations with different properties. Of particular interest is the regime of weak coupling and low cooperativity between the spin and cavity, which is relevant in the cases of the nitrogen- and silicon-vacancy centers in diamond. We also take into account the variability of the properties between realistic (stationary) nodes.

We demonstrate a machine-learning-based approach that can learn the time-dependent electronic excitation dynamics of small molecules subjected to ion irradiation. Ensembles of neural networks are trained on data generated by time-dependent density-functional theory to relate atomic positions to occupations of molecular orbitals. New data are incrementally and efficiently added to the training data using an active learning process, thereby improving model accuracy. Predicted changes in orbital occupations made by the neural network ensemble are found to have errors and one-standard-deviation uncertainties which are two orders of magnitude smaller than the typical values of the orbital occupation numbers. The trained neural network ensembles demonstrate a limited ability to generalize to molecules not used to train the models. In such cases, the models are able to identify key qualitative features, but struggle to match the quantitative values. The machine-learning procedure developed here is potentially broadly applicable and has the potential to enable study of broad ranges of both materials and dynamical processes by drastically lowering the computational cost and providing surrogate model for multiscale simulations.

Heavy atoms play a critical role in scientific research and applications by providing a better understanding of fundamental physical phenomena. The use of a slowed atomic beam to extend the interaction time is expected to enhance measurement sensitivity, resolution, and precision. Here, we theoretically demonstrate a rapid and short-distance deceleration of heavy atoms by using a continuous stimulated force with optical phase compensation and frequency chirping. The scheme effectively overcomes the spatial inhomogeneity of the stimulated force on atoms during deceleration. Our approach employs the numerical solutions of the optical Bloch equations for two-level atoms driven by time-dependent fields, and simulates atom trajectories using a three-dimensional Monte Carlo method. Concerning a buffer-gas-cooled radium atom beam with an initial velocity of 130 m/s, from our simulations it can be decelerated to 5 m/s within 5 cm distance using 40 mW laser power, achieving a deceleration force approximately 35 times stronger than the radiation force and covering an effective velocity range over eight times that of the radiation force. In addition, the deceleration of other heavy atoms, such as francium and thallium, also are applicable. This approach offers an efficient tool for rapidly slowing and cooling complex atoms and molecules, facilitating future precision measurements.

The method proposed by Khairulin et al. [JETP Lett. 117, 652 (2023)] for converting the linearly polarized radiation of high-order harmonics of an optical field into the circularly polarized in an optically modulated active medium of a plasma-based x-ray laser has been developed. This method is extended to the case of finite width of spectral lines (finite pulse duration) of high-harmonic radiation and nonlinear interaction of radiation with matter. The transformation of the polarization of the harmonic field is achieved by introducing a phase shift of π/2 between its polarization components, parallel and orthogonal to the polarization of the modulating field, through resonant dispersion on the spectral wings of the induced gain lines of the active medium and is accompanied by an increase in the power of high harmonics. It is shown that for harmonics with a finite bandwidth, the polarization of the output radiation is nonuniform in time. The reasons for such nonuniformity are analyzed and the conditions for its minimization are found. The influence of nonlinearity and amplified spontaneous emission of the active medium on the high-harmonic polarization is investigated. The possibilities for the experimental implementation of this effect in a neonlike active medium of a plasma x-ray laser based on $\mathrm{T}{\mathrm{i}}^{12+}$ ions with an unperturbed wavelength of the inverted transition at 32.6 nm are analyzed.

Recently, the local friendliness (LF) no-go theorem has attracted considerable interest due to its deep foundational implications. This no-go theorem applies to scenarios which combine Bell experiments with Wigner's friend-type setups, containing spacelike separated superobservers assumed to be capable of performing quantum operations on a local observer, also known as their “friend.” The theorem shows that a set of assumptions termed “local friendliness”—weaker than the set of assumptions required for proving Bell's theorem—constrains the space of probabilistic behaviors in extended Wigner's friend scenarios to the “local friendliness (LF) polytope,” and that quantum correlations can, in principle, violate those constraints. In this work, we extend the study of local friendliness to multipartite scenarios—with arbitrary but finite numbers of superobservers, friends, measurements, and outcomes—and fully characterize the corresponding LF polytopes, their relationships with each other, with the Bell polytope, the no-signaling polytope, and the quantum set of correlations. In particular, we identify, via constructive proofs, all the canonical LF scenarios where the LF polytope coincides with the Bell polytope—hence identifying all the scenarios where “genuine LF inequalities” may arise. Among other results, we also show that whenever the LF polytope is distinct from the Bell and the no-signaling polytope, it also is neither contained in nor contains the quantum set of correlations. Apart from the foundational motivation of better understanding the constraints from local friendliness, our results may also find use in a priori unrelated contexts of device-independent information processing, where the LF polytopes have also been shown to arise, under the name of “partially deterministic polytopes.”

We show that quantum computers can be used for producing large $n$-partite nonlocality, thereby providing a method to benchmark them. The main challenges to overcome are as follows: (i) The interaction topology might not allow arbitrary two-qubit gates. (ii) Noise limits the Bell violation. (iii) The number of combinations of local measurements grows exponentially with $n$. To overcome (i), we point out that graph states that are compatible with the two-qubit connectivity of the computer can be efficiently prepared. To mitigate (ii), we note that for specific graph states, there are $n$-partite Bell inequalities whose resistance to white noise increases exponentially with $n$. To address (iii) for any $n$ and any connectivity, we introduce an estimator that relies on random sampling. As a result, we propose a method for producing $n$-partite Bell nonlocality with unprecedented large $n$. This allows one, in return, to benchmark nonclassical correlations regardless of the number of qubits or the connectivity. We test our approach by using a simulation for a noisy IBM quantum computer, which predicts $n$-partite Bell nonlocality for at least $n=24$ qubits.

Unsupervised machine learning is one of the main techniques employed in artificial intelligence. We introduce an algorithm for quantum-assisted unsupervised data clustering using the self-organizing feature map, a type of artificial neural network. The complexity of our algorithm scales as $O\left(LN\right)$, in comparison to the classical case which scales as $O\left(LMN\right)$, where $N$ is the number of samples, $M$ is the number of randomly sampled cluster vectors, and $L$ is the number of the shifts of cluster vectors. We perform a proof-of-concept demonstration of one of the central components on the IBM quantum computer and show that it allows us to reduce the number of calculations in the number of clusters. Our algorithm exhibits exponential decrease in the errors of the distance matrix with the number of runs of the algorithm.

The standard way to generate many-body quantum correlations is via a dynamical protocol: an initial product state is transformed by interactions that generate nonclassical correlations at later times. Here, we show that many-body Bell correlations are inherently present in the eigenstates of a variety of spin-1/2 chains. In particular, we show that the eigenstates and thermal states of the collective Lipkin-Meshkov-Glick model possess many-body Bell correlations. We demonstrate that the Bell correlations present in the eigenstates of the Lipkin-Meshkov-Glick model can take on quantized values that change discontinuously with variations in the total magnetization. Finally, we show that these many-body Bell correlations persist even in the presence of both diagonal and off-diagonal disorder.