Using discrete and continuous variable subsystems, hybrid approaches to quantum information could enable more quantum computational power for the same physical resources. Here, we propose a hybrid scheme that can be used to generate the necessary Gaussian and non-Gaussian operations for universal continuous variable quantum computing in trapped ions. This scheme utilizes two linear spin-motion interactions to generate a broad set of nonlinear effective spin-motion interactions including one- and two-mode squeezing, beam splitter, and trisqueezing operations in trapped ion systems. We discuss possible experimental implementations using laser-based and laser-free approaches.

Studies of high-$n, 120\lesssim n\lesssim 160$, strontium $5sng, 5snh$, and $5sni$ Rydberg states are reported. Measurements using microwave spectroscopy provide the quantum defects for these states and furnish a valuable test of quantum defects reported for lower-$\ell$ states in the high-$n$ limit. With increasing $\ell$ the relativistic spin-orbit interaction becomes comparable to the electron-electron interaction resulting in the breakdown of the standard $LS$-coupling scheme. Heavier alkaline-earth quasi-two-electron systems differ significantly from a true two-electron system such as helium because indirect spin-orbit effects are dramatically enhanced. We explore the high-$n$ limit of this transition between $LS$ and $jj$ coupling. For high-$\ell$ states the quantum defects are determined predominantly by the polarizability of the inner electrons.

In a series of papers we present a comprehensive investigation of the four-body proton-helium differential scattering problem using the two-center wave-packet convergent close-coupling approach in the intermediate energy region where coupling between various channels is important. The approach uses correlated two-electron wave functions for the helium target. For comparison, a recently developed method that reduces the target to an effective single-electron system is also used. In this paper we present calculations of angular differential cross sections for elastic-scattering, target excitation, and electron-capture processes. The results of the two-electron and effective single-electron methods exhibit a good level of agreement. They also agree well with available experimental data. It is concluded that both versions of the wave-packet convergent close-coupling approach are capable of providing a realistic differential picture of all interdependent and interconnected binary processes taking place in proton-helium collisions at intermediate energies.

We use nonlinear optimal control simulation to systematically examine how to suppress wave-packet spreading with strong nonresonant near-infrared (NIR) laser pulses through a case study of a vibrational wave packet in the $B$ state of ${\mathrm{I}}_2$. As the degree of spreading of the vibrational wave packet is controllable by adjusting the pump pulse shape, the vibrational wave packet may be regarded as a prototype system to study spreading suppression by using nonresonant laser pulses. We quantitatively define spreading suppression in terms of the probability of finding the wave packet in the analytically defined, initially excited state at every vibrational period. From optimal control simulation and simulation by using model Gaussian pulse trains that mimic optimal pulses, we conclude that a simple periodic NIR pulse train without highly tuned temporal widths, amplitudes, or irradiation timings can almost perfectly stop the wave-packet spreading over a long control period, provided that the degree of spreading is not too large.

Understanding the intracavity field dynamics in passive microresonator systems has already been intriguing. It becomes fascinating when the system is complex, such as a concentric dual microring resonator that exhibits avoided mode crossing (AMC). In this work, we present a systematic study of intracavity oscillatory field dynamics near AMC in a concentric silicon-nitride microring resonator with the help of coupled Lugiato-Lefever equation. We identify two regions where the hybrid modes are strongly coupled and weakly coupled based on their eigenfrequency separation, which originate from mode coupling near AMC. We calculate the overlap integral to identify the cutoff pump wavelength region to observe the existence of the two hybrid modes. In a strongly coupled hybrid mode region, we observe intracavity power oscillation and transfer of energy between the two hybrid modes in a periodic manner. We also evaluate the nonidentical phase variation of these two modes. In a weakly coupled hybrid mode region, power oscillation and energy transfer between modes reduces significantly, whereas their phases vary in almost identical fashion. We validate our numerical findings with the semianalytical variational method, leading to an in-depth understanding of the mode-coupling-induced dynamics. We finally analyze the polarization properties of the field confined in the coupled system. Exploiting the Stokes parameters and Jones vector, we deduce a polarization-evolving state and a polarization-locked state in strongly coupled region and weakly coupled region, respectively.

In this paper, an alternative approach for constructing Lagrangians for driven and undriven linearly damped systems is proposed, by introducing a redefined time coordinate and an associated coordinate transformation to ensure that the resulting Lagrangian satisfies the Helmholtz conditions. The approach is applied to canonically quantize the damped harmonic oscillator and although it predicts an energy spectrum that decays at the same rate to previous models, unlike those approaches it recovers the classical critical damping condition, which determines transitions between energy eigenstates, and is therefore consistent with the correspondence principle. It is also demonstrated how to apply the procedure to a driven damped harmonic oscillator.

In this paper, we study measures of quantum non-Markovianity based on the conditional mutual information. We obtain such measures by considering multiple parts of the total environment such that the conditional mutual information can be defined in this multipartite setup. The benefit of this approach is that the conditional mutual information is closely related to recovery maps and Markov chains; we also point out its relations with the change in distinguishability. We study along the way the properties of leaked information, which is the conditional mutual information that can be back flowed, and we use this leaked information to show that the correlated environment is necessary for the nonlocal memory effect.

A reasonable quantum information theory for fermions must respect the parity superselection rule to comply with the special theory of relativity and the no-signaling principle. This rule restricts the possibility of any quantum state to have a superposition between even- and odd-parity fermionic states. thereby characterizing the set of physically allowed fermionic quantum states. Here we introduce the physically allowed quantum operations, in congruence with the parity superselection rule, that map the set of allowed fermionic states onto itself. We first introduce unitary and projective measurement operations of the fermionic states. We further extend the formalism to general quantum operations in the forms of Stinespring dilation, operator-sum representation, and axiomatic completely positive and trace-preserving maps. We explicitly show the equivalence between these three representations of fermionic quantum operations. We discuss the possible implications of our results in characterization of correlations in fermionic systems.

Lyman transitions of ${\mathrm{Xe}}^{53+*}$ produced by nonradiative electron capture in collisions of 95, 146, and 197 ${\mathrm{MeV/u}\mathrm{Xe}}^{54+}$ ions with Kr and Xe atoms have been studied experimentally. It shows that the captured electrons preferentially populate lower states. The intensity ratios $I\left(\mathrm{Ly}\text{−}\beta \right)/I\left(\mathrm{Ly}\text{−}\alpha \right)$ and $I\left(\mathrm{Ly}\text{−}\gamma \right)/I\left(\mathrm{Ly}\text{−}\alpha \right)$ are found to decrease with increasing projectile velocity, and the ratios for Xe decrease faster than those of Kr. The obtained results are compared with the relative cross sections calculated with the relativistic eikonal approximation. For both targets, the theoretical results overestimate the experimental ratio $I\left(\mathrm{Ly}\text{−}\beta \right)/I\left(\mathrm{Ly}\text{−}\alpha \right)$ but underestimate $I\left(\mathrm{Ly}\text{−}\gamma \right)/I\left(\mathrm{Ly}\text{−}\alpha \right).$ Moreover, the theory gives parallel decreasing curves for the two targets, which does not agree with the present experiment. In order to well understand the experimental findings, a more sophisticated theoretical approach is urgently required.

A UV-pump x-ray-probe (UVX-PP) experiment for probing UV initiated dissociation of the water molecule with long pulses is proposed on the basis of theoretical simulations. With long overlapping UV and x-ray pulses, we can obtain high-spectral-resolution x-ray-absorption or ionization spectra, containing detailed information about dynamics in the intermediate valence-excited state. The dynamics can be influenced with frequency detuning of the pump UV pulse, which acts as a camera shutter by regulating the duration of the UVX-PP process. Thereby, this UVX-PP setup gives access to ultrafast dynamics of the nuclear wave packet without experimentally challenging requirements of ultrashort pulses and controlled delay times. In a case study of the water molecule, we focus on the lowest UVX-PP channel ($1b_1\to 4a_1, 1a_1\to 1b_1$) where both intermediate valence-excited and final core-excited states are dissociative. The variation of the UVX-PP duration, controlled by the UV detuning, and different dispersion laws of so-called molecular and atomic bands allow one to study the dynamics of fragmentation of the water molecule in the intermediate state. A feature is that the long lifetime of intermediate valence-excited states opens a door for studies of photoinduced dissociation of polyatomic molecules with heavy fragments.

We demonstrate that an ultracold many-body bosonic ensemble confined in a one-dimensional double-well potential can exhibit chaotic dynamics due to the presence of a single impurity. The nonequilibrium dynamics is triggered by a quench of the impurity-Bose interaction and is illustrated via the evolution of the population imbalance for the bosons between the two wells. While the increase of the postquench interaction strength always facilitates the irregular motion for the bosonic population imbalance, it becomes regular again when the impurity is initially populated in the highly excited states. Such an integrability to chaos (ITC) transition is fully captured by the transient dynamics of the corresponding linear entanglement entropy, whose infinite-time-averaged value additionally characterizes the edge of the chaos and implies the existence of an effective Bose-Bose attraction induced by the impurity. To elucidate the physical origin for the observed ITC transition, we perform a detailed spectral analysis for the mixture with respect to both the energy spectrum as well as the eigenstates. Specifically, two distinguished spectral behaviors upon a variation of the interspecies interaction strength are observed. While the avoided level crossings take place in the low-energy spectrum, the energy levels in the high-energy spectrum possess a bandlike structure and are equidistant within each band. This leads to a significant delocalization of the low-lying eigenvectors, which, in turn, accounts for the chaotic nature of the bosonic dynamics. By contrast, those highly excited states bear a high resemblance to the noninteracting integrable basis, which explains the recovery of the integrability for the bosonic species. Finally, we discuss the induced Bose-Bose attraction as well as its impact on the bosonic dynamics.

We consider the quantum Rabi model as an open system that is subject to dissipation, dephasing, and sinusoidal qubit driving. One can change to an interaction picture where the qubit-driving term disappears at the expense of changing the free energy of the qubit which becomes time dependent. If the driving frequency is large with respect to the rest of the parameters with the exception of the driving strength, then one can obtain an effective Hamiltonian that accurately describes the dynamics of the system. The driving has two effects: the qubit-transition frequency is changed and the qubit has reduced dephasing. The driving strength can be chosen so that the qubit-transition frequency is reduced, made equal to zero, or even made negative so that the excited and ground states of the qubit are interchanged. Therefore, sinusoidal qubit driving offers another method to control the qubit-transition frequency and to reduce qubit dephasing. Adjusting the driving strength allows one to consider a qubit with degenerate energy levels. Not taking dissipation into account, the evolution operator of the qubit-harmonic oscillator system is given by a linear combination of the orthogonal projectors onto the eigenstates of ${\widehat{\sigma }}_x$ followed by the evolution operator of a forced harmonic oscillator, the harmonic oscillator can be prepared in such a way that it is always found in a Schrödinger cat state, and the transition probability of the qubit can exhibit a collapse-revival behavior. In addition, the Born-Markov-secular master equation is deduced and the effects of dissipation are presented. In particular, smaller ultrastrong-coupling values are preferable over larger ultrastrong-coupling values and deep strong-coupling values in order to have long-lived, easily distinguishable Schrödinger cat states because the decoherence rate is inversely proportional to the square of the coupling. Finally, the qubit-harmonic oscillator system can be prepared in highly entangled states that are stable under dissipation.

We investigate the single-photon scattering spectra of a giant atom coupled to a one-dimensional waveguide via multiple connection points or a continuous coupling region. Using a full quantum mechanical method, we obtain the general analytic expressions for the single-photon scattering coefficients, which are valid in both the Markovian and the non-Markovian regimes. We summarize the influences of the nondipole effects, mainly caused by the phases accumulated by photons traveling between coupling points, on the scattering spectra. We find that under the Markovian limit, the phase decay is detuning independent, resulting in Lorentzian line shapes characterized by the Lamb shifts and the effective decay rates, while in the non-Markovian regime, the accumulated phases become detuning dependent, giving rise to non-Lorentzian line shapes, characterized by multiple side peaks and total transmission points. Another interesting phenomenon in the non-Markovian regime is the generation of a broad photonic band gap by a single giant atom. We further generalize the case of discrete coupling points to the continuum limit with atom coupling to the waveguide via a continuous area, and analyze the scattering spectra for some typical distributions of coupling strength.

We present a study on the dissociation of a weakly bound molecular ion in the presence of an external time-dependent electric field based on a full quantum treatment of the dynamics. We focus on the dissociation dynamics of a molecular ion in a Paul trap relevant for atom-ion hybrid traps. Our results show that a weakly bound molecular ion survives in a Paul trap giving a theoretical ground to previous experimental findings [A. Krükow et al. Phys. Rev. Lett. 116, 193201 (2016) and A. Mohammadi et al., Phys. Rev. Research 3, 013196 (2021)]. In particular, we find that weakly bound molecular ions are more likely to survive in traps with a large rf frequency. Similarly, we show that applying an electric field ramp is an efficient method to state-selectively detect weakly bound molecular ions, analogous to the well-known selective field ionization technique applied in Rydberg atoms.

Generalized thermalization is a process that occurs in integrable systems in which unitary dynamics, e.g., following a quantum quench, results in states in which observables after equilibration are described by generalized Gibbs ensembles. Here we discuss an emergent eigenstate construction that allows one to built emergent local Hamiltonians of which one eigenstate captures the entire generalized thermalization process following a global quantum quench. Specifically, we study the emergent eigenstate that describes the quantum dynamics of hard-core bosons in one dimension for which the initial state is a density wave, and this state evolves under a homogeneous Hamiltonian.

Quantum theory shows a striking nonclassical feature of contextuality, which is also a crucial resource for quantum information processing. In this paper, we study the dependence of violating noncontextuality inequalities against the dimension of quantum systems. To this end, we consider the restrictions imposed on relations among measurements by limitation of dimensions. Based on the graph-theoretic approach to quantum contextuality, we construct noncontextuality inequalities using multiple mutually orthogonal projectors which are unique to high-dimensional quantum systems. We report two typical classes of dimension-dependent noncontextuality inequalities for one of which a violation implies the dimension of the quantum system is higher than a threshold, and for the other, different amounts of violations correspond to different thresholds of dimension. Our paper is expected to inspire more dimension-dependent noncontextuality inequalities.

Repulsion, induced from quantum vacuum fluctuations, for an anisotropically polarizable atom on the symmetry axis of an anisotropically polarizable annular disk is studied. There exist two torsion-free points on each side of the annular disk, where the interaction energy is orientation independent. The position of the second of the two torsion-free points, on either side of the disk, is shown to determine the orientation dependence of the atom at distances far from the plate, revealing a geometrical dependence. In the ring limit of the annular disk, an additional region of repulsion emerges.

In two celebrated experiments of quantum mechanics, the Stern-Gerlach (SG) and Heisenberg microscope (HM) experiments, the electron spin $s_z$ and spatial $\mathit{r}$ coordinates are measured separately. In this paper, the combined $\mathrm{SG}+\mathrm{HM}$ experiment is proposed to measure the full electron coordinate $\mathit{x}=\{\mathit{r},s_z\}$. To this end, noninterfering and (virtually) simultaneous SG and HM experiments are proposed to apply to the individual fragments A and B of an entangled dissociating system $\mathrm{A}\text{−}\mathrm{B}$. The theoretical description of a spin-collapsed and partially spatially collapsed state of the $\mathrm{SG}+\mathrm{HM}$ experiment is given for the prototype “perfectly” entangled system, the dissociating ${\mathrm{H}}_2$ molecule.

Electron spins in semiconductor quantum dots are a promising platform for qubits. An essential step toward quantum error correction with spin qubits requires the ability to read out states and identify errors in the two-spin logical basis in a fast and high-fidelity way. Here we propose a scheme for spin singlet-triplet readout in a double quantum dot, with a superconducting resonator serving as the detector. Making use of a spin driving pulse sequence, this procedure can read out the spin states with a fidelity of $99.9\%$ in a time 100 ns. Interestingly, this method makes use of the spin driving pulse to adjust the resonator signal field, leading to significant improvement of the spin readout. In addition, this method can be directly applied to multiplexed readout of qubits and repeated readout to identify logical errors. These results pave the way for the implementation of quantum error correction and scalable quantum processor with spin qubits.