The quantum steering ellipsoid visualizes the set of all qubit states that can be steered by measuring on another correlated qubit in the Bloch picture. Together with local reduced states, it provides a faithful geometric characterization of the underlying two-qubit state so that almost all nonclassical state features can be reflected in its geometric properties. Consequently, the various types of quantum ellipsoids with different geometric properties form an ellipsoid zoo, which, in this paper, is experimentally verified via measurements on many polarization-path photonic states. By generating two-qubit states with high fidelity, the corresponding ellipsoids are constructed to certify the presence of entanglement, one-way Einstein-Podolsky-Rosen steering, discord, and steering incompleteness. It is also experimentally verified that the steering ellipsoid can be reconstructed from using the 12 vertices of the icosahedron as measurement directions. Our results aid progress in applying the quantum steering ellipsoid to reveal nonclassical features of the multiqubit system.

The phenomenon of confinement is well known in high-energy physics and can also be realized for low-energy domain-wall excitations in one-dimensional quantum spin chains. A bound state consisting of two domain walls can behave like a meson, and recently, Vovrosh et al. [PRX Quantum 3, 040309 (2022)] demonstrated that a pair of mesons could dynamically form a metastable confinement-induced bound state (consisting of four domain walls) akin to a hadronic state. However, the protocol discussed by Vovrosh et al. involving the use of interactions with characteristically nonmonotonic distance dependence is not easy to come by in nature, thus posing a challenge for its experimental realization. In this regard, Rydberg atoms can provide the required platform for simulating confinement-related physics. We exploit the flexibility offered by interacting Rydberg-dressed atoms to engineering modified spin-spin interactions for the one-dimensional transverse-field Ising model. Our numerical simulations show how Rydberg-dressed interactions can give rise to a variety of effective potentials that are suitable for hadron formation, which opens the possibility of simulating confinement physics with Rydberg platforms as a viable alternative to current trapped-ion experiments.

This paper presents an experimental platform designed to facilitate quantum logic control of polar molecular ions in a segmented ring ion trap, paving the way for precision measurements. This approach focuses on achieving near-unity state preparation and detection, as well as long spin-precession coherence. A distinctive aspect lies in separating state preparation and detection conducted in a static frame from parity-selective spin precession in a rotating frame. Moreover, the method is designed to support spatially and temporally coincident measurements on multiple ions prepared in states with different sensitivity to the new physics of interest. This provides powerful techniques to probe and minimize potential sources of systematic error. While the primary focus of this paper is on detecting the electron's electric dipole moment (eEDM) using ${}^{232}\mathrm{ThF}^{+}$ ions, the proposed methodology holds promise for broader applications, particularly with ion species that exhibit enhanced sensitivity to the nuclear magnetic quadruple moment (nMQM).

Understanding of strong-field sequential double ionization (SDI) of molecules by a highly intense infrared laser pulse could be the key to observing electron motion in molecules in the attosecond to femtosecond timescale. Based on a novel density matrix approach for SDI (DM-SDI), a recent theoretical study [Yuen and Lin, Phys. Rev. A 109, L011101 (2024)] has shown that SDI can be used as a probe to monitor the changes in vibronic coherence in homonuclear diatomic molecules. In this article, we extend the DM-SDI model to general molecules that could possess permanent dipole moments and arbitrary symmetry. We apply the model to SDI of a water molecule and identify the formation pathway of individual dication states. We further deduce that kinetic energy release spectra from two- and three-body fragments could carry the signature of vibronic coherence between the lowest two states of the water cation. Our results suggest that observables from the SDI probe can be interpreted intuitively with only the knowledge of electronic structures of populated ionic states, making the SDI probe to be a highly desirable probing scheme for vibronic coherence in generic molecules.

We develop the analytical theory that describes simultaneous transmission of several photons through a waveguide coupled to a $\mathrm{\Lambda }$-type atom. We show that after transmission of a short few-photon pulse, the final state of the atom and all the photons is a genuine multipartite entangled state belonging to the $W$ class. The parameters of the input pulse are optimized to maximize the efficiency of three- and four-partite $W$-state production. The probability to obtain the canonical $W$ state of many photons is shown to be larger than $1/\mathrm{e}$.

The giant atom regime where the wavelength of the phonon field is smaller than the atomic size opens up new opportunities for exploring exotic phenomena and developing powerful quantum technologies. Here, we explore the radiation dynamics of a hybrid system consisting of a superconducting giant atomic ensemble and an acoustic waveguide. In particular, we show how to design the delayed feedback of the slow acoustic field on the giant atomic ensemble. This modulates the phonon superradiant dynamics into engineerable revival modes and unconventional scaling laws with respect to the number of giant atoms. These are a distinctive feature of time-delayed collective processes that have no analog in other settings. The recipes we provide for harnessing collective giant atoms can be exported to other platforms, laying the foundation for a variety of quantum simulations and applications.

We present an approach for eliminating the gauge freedom for derivative couplings, enabling nonadiabatic dynamics in the presence of geometric phase effects. This approach relies on a bottom-up construction of a parametric quantum Hamiltonian in terms of functions of a dynamical variable. These functions can be associated with real- and imaginary-valued contributions to the Hamiltonian in a given diabatic basis. By minimizing variable-dependent fluctuations of the imaginary functions we identify a set of diabatic bases that recover the real-valued gauge commonly used for topologically trivial systems. This minimization, however, also confines the gauge freedom in the topologically nontrivial case, opening a path towards finding gauge-invariant derivative couplings under geometric phase effects. Encouraging results are presented for fewest-switches surface-hopping calculations of a nuclear wave packet traversing an avoided-crossing region, for which fully gauge-invariant derivative couplings are found.

Quantum contextuality is usually verified through establishing the violation of appropriate inequalities constructed based on the assumption of generalized noncontextuality. Recently, the connection between anomalous weak values (AWVs) and the generalized contextuality under specific experimental conditions has been proposed. Experimentally, only the connection between the real part of the AWVs and the generalized contextuality is verified in a single-photon experimental system. In this paper, we conduct weak measurement experiments in a single ${}^{40}{\mathrm{Ca}}^{+}$ ion system under specific experimental conditions. The incompatibility between quantum mechanics and noncontextual ontological models (NCOMs) is verified by both the real and imaginary parts of the AWVs, which provides a more general test of the connection between the quantum contextuality and AWVs.

Entanglement is a key property of quantum computing that separates it from its classical counterpart; however, its exact role in the performance of quantum algorithms, especially variational quantum algorithms, is not well understood. In this work, we utilize tensor network methods to systematically probe the role of entanglement in the working of two variational quantum algorithms, the quantum approximate optimization algorithm (QAOA) and quantum neural networks (QNNs), on prototypical problems under controlled entanglement environments. We find that for the max-cut problem solved using QAOA, the fidelity as a function of entanglement is highly dependent on the number of layers, layout of edges in the graph, and edge density, generally exhibiting that a high number of layers indicates a higher resilience to truncation of entanglement. This is in contrast to previous studies based on no more than four QAOA layers which show that the fidelity of QAOA follows a scaling law for circuits. Contrarily, in the case of QNNs, trained circuits with high test accuracies are underpinned by higher entanglement, with any enforced limitation in entanglement resulting in a sharp decline in test accuracy. This is corroborated by the entanglement entropy of these circuits which is consistently high suggesting that, unlike QAOA, QNNs may require quantum devices capable of generating highly entangled states. Overall, our work provides a deeper understanding of the role of entanglement in the working of variational quantum algorithms which may help to implement these algorithms on NISQ-era quantum hardware in a way that maximizes their accuracies.

We investigate the form required for the time-dependent Rabi frequencies involved in a procedure capable to protect the action of quantum gates on an atomic qutrit by means of external fields continuously decoupling the system from the environmental noise. Several simulations are considered to protect the action of quantum-gate models, including randomly chosen ones. We argue that the requirements for the Rabi frequencies could be nowadays experimentally met. We also investigate the transition from one gate operation to another, including protecting a qutrit memory state. We finally apply our methodology to protect from noise the application of an algorithm capable of distinguishing the parity of permutations of three elements.

In this paper, we aim to broaden the spectrum of possible applications of quantum computers and use their capabilities to investigate effects in cavity quantum electrodynamics (cavity QED). Interesting application examples are material properties, multiphoton effects such as superradiance, systems with strong field-matter coupling, and others. For QED applications, experimental studies are challenging, and classical simulations are often expensive. Therefore, exploring the capabilities of quantum computers is of interest. Below we present a methodology for simulating a phase transition in a pair of coupled cavities that permit photon hopping. We map the spin and boson systems to separate parts of the register and use first-order Trotterization to time-propagate the wave function. The order parameter, the observable for the phase transition, is calculated by measuring the number operator and its square. We introduce a boson-to-qubit mapping to facilitate a multiphoton, multiatom case study. Our mapping scheme is based on the inverse Holstein-Primakoff transformation. In the multiphoton regime, boson operators are expressed via higher-spin operators which are subsequently mapped on a circuit using Pauli operators. We use a Newton series expansion to enable rigorous treatment of the square root operator. We reproduce the results of classical simulations of a phase transition with a noiseless six-qubit simulation. We find that the simulation can be performed with a modest amount of quantum resources. Finally, we perform simulations on noisy emulators and find that mitigation techniques are essential to distinguish signal from noise.

We numerically investigate the resembling pulse regimes in mode-locked fiber lasers through hybrid filtering implemented using super-Gaussian and sinusoidal spectral filters (SSFs). By modulating the saturation energy and filter bandwidth of the SSF, several stable regimes of resembling pulses including single, bounded, overlapped double resembling, and bounded triple resembling pulses can be produced. We explore the pulse characteristics of different single and bounded resembling pulses, such as the pulse intervals, chirps, optical spectra, and regime formation. During the transition between different mode-locked regimes, chaotic and pulsating pulses emerge, similar to the scenario of transitions induced by a single filter. Further study shows that synchronous and asynchronous power oscillations are produced in the pulse intensity and optical spectra when the laser operates at three typical pulsating states and one chaotic state. This paper reveals the effect of hybrid filtering on multipulse dynamics and proposes a method for modulating the mode-locking resembling pulses in fiber lasers.

A focusing system such as a single lens or a spherical mirror imparts intrinsic transverse orbital angular momentum (OAM) to spatiotemporal (ST) coupled fields, the ST intensity distribution of which presents ST covariance. This fact may greatly simplify the experimental setups used to date to impart transverse OAM. We evaluate analytically the imparted transverse OAM as a function of the focal length and the covariance. The focused fields with transverse OAM include elliptical ST vortices and rotating pulses without any ST phase singularity such as the “lighthouse” pulse. We provide closed-form, analytical expressions for these fields valid at any propagation distance from the focusing system, which are of interest in applications such as the interaction of these fields with matter. In general, focusing of ST coupled fields with intensity covariance generates mixed fields with ST vortices and rotating pulse fronts, where one or another feature dominates depending on the input field.

We investigate the nonlinear frequency conversion of nondiffracting optical modes via a nondegenerate four-wave mixing (FWM) process in rubidium vapor. In this process, two strong control fields and a weak probe field mutually interact via a four-level double-$\mathrm{\Lambda }$ type atomic system to produce a low frequency weak Stokes field which is a phase conjugate to probe. We show that any arbitrary mode such as Airy, Bessel, Mathieu, and Weber beams encoded initially in the spatial envelope of the probe field are efficiently transferred to the FWM Stokes field by satisfying the phase-matching condition. Interestingly, we found that the transmitted intensity profiles of the Stokes beam are identical to that of the probe beam, while the phase profile is complementary. The phase conjugation property of output beams is revealed by interfering them with a copropagating plane wave. We further found the fidelity in terms of structural similarity between two transmitted modes of about $99\%$, which substantiates the high efficiency of the FWM process. The efficient transfer and frequency conversion of such diffraction-free beams may have potential applications in nonlinear optical and quantum technologies.

We report on the possibility of teleportation of a quantum particle, a distinctly different phenomenon from the teleportation of a quantum state through entanglement. With the first meaning, teleportation is theoretically possible by placing the particle initially at rest (with a certain uncertainty) out of any equilibrium point of a potential well or barrier and by frequently monitoring whether the particle remains at rest. This quantum Zeno dynamics inhibits acceleration, and features disappearance from the classical turning point and appearance in another turning point, if there is any other, with a probability that approaches unity by increasing the frequency of the measurements. This phenomenon has all the ingredients attributed in science fiction to teleportation: The particle is always at rest, cannot be found in the path between the two turning points, and saves travel time. We discuss the feasibility, in principle, of teleportation of electrons, protons, and other particles, and conclude its increasing impracticability as the particle gets heavier. Nevertheless, our paper establishes a basis for further studies in which the projective and instantaneous measurements are replaced by more realistic models.

The presence of long-lived oscillations in the expectation values of local observables after quantum quenches has recently attracted considerable attention in relation to weak ergodicity breaking. Here, we focus on an alternative mechanism that gives rise to such oscillations in a class of systems that support kinematically protected gapped excitations at zero temperature. An open question in this context is whether such oscillations will ultimately decay. We provide strong support for the decay hypothesis by considering spin models that can be mapped to systems of weakly interacting fermions, which in turn are amenable to an analysis by standard methods based on the Bogoliubov–Born–Green–Kirkwood–Yvon (BBGKY) hierarchy. We find that there is a time scale beyond which the oscillations start to decay that grows as the strength of the quench is made small.

We present a consistent formalism to describe the dynamics of hybrid systems with mixed classical and quantum degrees of freedom. The probability function of the system, which, in general, will be a combination of the classical distribution function and the quantum density matrix, is described in terms of its corresponding moments. We then define a hybrid Poisson bracket, such that the dynamics of the moments is ruled by an effective Hamiltonian. In particular, a closed formula for the Poisson brackets between any two moments for an arbitrary number of degrees of freedom is presented, which corrects previous expressions derived in the literature for the purely quantum case. This formula is of special relevance for practical applications of the formalism. Finally, we study the dynamics of a particular hybrid system given by two coupled oscillators, one being quantum and the other classical. Due to the coupling, specific quantum and classical properties are transferred between different sectors. In particular, the quantum sector is allowed to violate the uncertainty relation, though we explicitly show that there exists a minimum positive bound of the total uncertainty of the hybrid system.