A theoretical framework is developed for scattering of scalar radiation from stationary, three-dimensional media with correlation functions of scattering potentials obeying $\mathcal{PT}$ symmetry. It is illustrated that, unlike in scattering from deterministic $\mathcal{PT}$-symmetric media, its stationary generalization involves two mechanisms leading to symmetry breaking in the statistics of scattered radiation, one stemming from the complex-valued medium realizations and the other stemming from the complex-valued degree of the medium's correlation.

We derive a general Hamiltonian that governs the interaction between an $N$-ion chain and an externally controlled laser field, where the ion motion is quantized and the laser field is considered beyond the plane-wave approximation. This general form not only explicitly includes terms that are used to drive ion-ion entanglement, but also a series of unwanted terms that can lead to quantum gate infidelity. We demonstrate the power of our expressivity of the general Hamiltonian by singling out the effect of axial mode heating and confirm this experimentally. We discuss pathways forward in furthering the trapped-ion quantum computational quality, guiding hardware design decisions.

To produce electrons with well-defined spin via photoionization, realizing orbital-resolved ionization is the prerequisite due to spin-orbital coupling. In this paper, we numerically and experimentally demonstrate that the electrons emitted from two original energy degenerated orbitals ($j=3/2, m=1$ and $j=3/2, m=-1$) of Xe atoms can be well separated in the energy domain through selectively achieving Freeman resonance during strong-field ionization using a circularly polarized laser field. This orbital resolved Freeman resonant ionization leads to strong spin sensitivity without the need of fine laser frequency and intensity tuning. This result enables the production of high-degree spin-polarized electrons by energy gating, which provides a unique degree of freedom for advancing many promising applications.

Here we study spin angular momentum (SAM) and orbital angular momentum (OAM) in counter propagating vectorial light fields, where the polarization forms a longitudinal standing wave structure, oscillating from vector to scalar. We outline the dynamics of the exchange between the electric and magnetic components in such vectorial longitudinal standing waves, highlighting regimes in which the individual components are constant along the $z$ axis. We confirm the theoretical findings with a simple experimental setup that reveals all the salient features, allowing us to measure the SAM and OAM densities, including through the “peaks” and “troughs” of the standing wave at subwavelength step intervals where polarization structure changes from homogeneous to nonhomogenous. The apparent spin-orbit coupling in such counterpropagating paraxial light is explained as an artefact of considering only one component of the field, electric or magnetic. This work generalizes previous findings based on traveling vectorial structured light waves, explains the apparent spin-orbit coupling in counterpropagating paraxial light, and broadens our understanding of angular momentum conservation in arbitrary structured light waves.

We measure the resonant forward scattering of light by a highly saturated atomic medium through the flashes emitted immediately after an abrupt extinction of the probe beam. The experiment is done in a dilute regime where the phenomena are well captured using the independent scattering approximation. Comparing our measurements to a model based on Maxwell-Bloch equations, our experimental results are consistent with contributions from only the elastic component, whereas the attenuation of the coherent transmission power is linked to the elastic and inelastic scatterings. In the large saturation regime and at the vicinity of the atomic resonance, we derive an asymptotic expression relating the elastic scattering power to the forward-scattered power.

We introduce a metric in a geometric way for measuring the distance between two quantum states and derive the instantaneous angular velocity of a quantum state's dynamical evolution, which has two components: $\mathrm{\Delta }{E}_t/\hslash$ in the longitudinal direction and ${\overline{E}}_t/\hslash$ in the latitudinal direction. By applying the instantaneous angular velocities in the dynamical evolution governed by a time-independent Hamiltonian, the fastest way for a pure state to evolve to a target state is discussed. As a consequence, we generalize the Mandelstam-Tamm and Margolus-Levitin bounds without using the Heisenberg uncertainty relation and point out the conditions for saturating the two bounds. The fastest dynamical evolution of a quantum system with a particular time-dependent Hamiltonian is also briefly discussed. The quantum features in the dynamical evolution are revealed through phase change and lifetime of the initial state.

Quantum walks on lattices can give rise to one-particle relativistic wave equations in the long-wavelength limit. In going to multiple particles, quantum cellular automata (QCA) are natural generalizations of quantum walks. In one spatial dimension, the quantum walk can be “promoted” to a QCA that, in the long-wavelength limit, gives rise to the Dirac quantum field theory (QFT) for noninteracting fermions. This QCA/QFT correspondence has both theoretical and practical applications, but there are obstacles to similar constructions in two or more spatial dimensions. Here we show that a method of construction employing distinguishable particles confined to the completely antisymmetric subspace yields a QCA in two spatial dimensions that gives rise to the two-dimensional (2D) Dirac QFT. Generalizing to 3D will entail some additional complications, but no conceptual barriers. We examine how this construction evades the “no-go” results in earlier work.

A complete treatment of open quantum systems made of identical particles has remained elusive due to the intrinsic problem of these particles being individually unaddressable. Filling this gap is essential for the full characterization of quantum networks which are typically built by identical subsystems. We provide a general framework which allows one to obtain the dynamics of $N$ noninteracting spatially indistinguishable particles locally coupled to separated environments. This framework is universal, being valid for both bosons and fermions and for any type of system-environment interaction. It is applied to study the dynamics of two identical qubits under paradigmatic Markovian noises, such as phase damping, depolarization, and amplitude damping. We find that spatial indistinguishability of identical qubits is a controllable inherent property of the system which protects exploitable quantum entanglement against detrimental noise.

Distributed quantum information stored over nodes within a quantum network are prone to node failures, motivating the need for coded quantum networks. We encode $k$ qubits of quantum information over an $n$-node network in a distributed fashion, with one qubit per node, using modified graph-state codes. We devise a procedure to recover the encoded quantum information in any given arbitrary network after a detected node fails. The recovery procedure is locally within four edges of the failed node. For a network with even number of nodes, we provide a class of scalable modified graph-state codes with rate $(n-2)/n$, saturating the quantum Singleton bound. This yields higher coding rates than Grassl et al.'s well-known erasure codes and has asymptotic rate 1. For odd node size $n$, we design the network links to obtain an $(n-3)/n$ rate-optimal modified graph state code.

We study the growth of genuine multipartite entanglement in random quantum circuit models, which include random unitary circuit models and the random Clifford circuit. We find that for the random Clifford circuit, the growth of multipartite entanglement remains slower in comparison to the random unitary case. However, the final saturation value of multipartite entanglement is almost the same in both cases. The behavior is then compared to the genuine multipartite entanglement obtained in random matrix product states with a moderately high bond dimension. We then relate the behavior of multipartite entanglement to other global properties of the system, viz., the delocalization of the many-body wave functions in Hilbert space. Along with this, we analyze the robustness of such highly entangled quantum states obtained through random unitary dynamics under weak measurements.

We analyze the geometry of positive and completely positive, trace-preserving Pauli maps that are fully determined by up to two distinct parameters. This includes five classes of symmetric and noninvertible Pauli channels. Using the Hilbert-Schmidt metric in the space of the Choi-Jamiołkowski states, we compute the relative volumes of entanglement breaking, time-local generated, and divisible channels. Finally, we find the shapes of the complete positivity regions in relation to the tetrahedron of all Pauli channels.

In this work, we report benchmark variational calculations of the five lowest doublet $S$-states of the ${\mathrm{C}}^{+}$ ion. The wave functions of this five-electron system are expanded in terms of 16 000 all-particle explicitly correlated Gaussians (ECGs) whose nonlinear variational parameters are subject to extensive optimization. The motion of the finite-mass nucleus is explicitly included in the Hamiltonian, while relativistic corrections to the energy levels are computed in the framework of the perturbation theory. Lowest-order quantum electrodynamics (QED) corrections are also estimated. The results obtained for the energy levels enable the determination of transition frequencies with the accuracy that approaches the available experimental data and may open up avenues for future determination of nuclear charge radii of carbon isotopes.

An earlier work of Dolmatov et al. [Phys. Rev. A 73, 013201 (2006)] on Ne inside a charged fullerene ($\mathrm{Ne}@{\mathrm{C}}_{60}^{\mathrm{q}=-3,-5}$) has revealed the presence of two unusually large confinement resonances termed Coulomb confinement resonances (CCRs); the first CCR is narrow and the second one is broad and their origins are attributed to the presence of charge on the fullerene surface. The present work extends this study to other subshells and also to other systems such as $\mathrm{Ar}@{\mathrm{C}}_{60}^{\mathrm{q}}$ and $\mathrm{Xe}@{\mathrm{C}}_{60}^{\mathrm{q}}$ using relativistic random phase approximation, but with an aim to investigate the genesis of the CCRs. Further, a detailed analysis of these resonances within a single active electron approximation unearths an interesting difference associated with the origin of these two CCRs. The photoionized electron is temporally trapped in the ${\mathrm{C}}_{60}$ confinement well for the case of narrow resonance and in the atomic well region for the case of broad resonance. Also, the present work shows that the broad resonance can occur even for the case of $q=0$ and is demonstrated for $\mathrm{Xe}@{\mathrm{C}}_{60}^{\mathrm{q}}$ as a test case.

We investigate theoretically how the duration of an isolated attosecond pulse (IAP) can be minimized by carefully selecting frequencies of high-order harmonic generation (HHG) spectra produced by ultrashort driving laser pulses. Based on numerical calculations of HHG by solving the time-dependent Schrödinger equation for a single H atom, we provide three strategies for generating shorter IAPs. First, when the high-frequency region of an HHG plateau is selected one should use frequencies below the cutoff. Second, for a wide HHG plateau the low-frequency region can produce shorter IAPs than the high-frequency region. Third, we propose a method of producing IAPs with transform-limited duration by selecting special frequency stripes across the entire HHG plateau. Analytic analyses show that how our strategies work is related to the (Fourier-transform) properties of an Airy function. We also carry out HHG calculations considering macroscopic effects by means of intensity averaging over the focal region. We find that our conclusions for single-atom calculations can still apply for macroscopic HHG spectra as long as they resemble single-atom spectra.

Atoms trapped in a red-detuned retroreflected Laguerre-Gaussian beam undergo orbital motion within rings whose centers are on the axis of the laser beam. We determine the wave functions, energies, and degeneracies of such quantum rotors (QRs), and the microwave transitions between the energy levels. We then show how such QR atoms can be used as highly accurate rotation sensors when the rings are singly occupied.

The absolute rate coefficients for dielectronic recombination (DR) of sodiumlike krypton ions were measured by employing the electron-ion merged-beam technique at the heavy-ion storage ring CSRm at the Institute of Modern Physics in Lanzhou, China. The measured DR spectrum covers the electron-ion collision energy range of 0–70 eV, encompassing all of the DR resonances due to $3s\to 3p$ and part of the DR resonances from $3s\to 3d\left(\mathrm{\Delta }n=0\right)$ and $3s\to 4l(\mathrm{\Delta }n=1)$ core excitations. A series of peaks associated with DR processes have been identified by the Rydberg formula. The experimental DR results are compared with the theoretical calculations using a relativistic configuration interaction flexible atomic code and the distorted-wave collision package autostructure. A very good agreement has been achieved between the experimental results and the theoretical calculations by considering the strong mixing among the low-energy resonances in both calculations. The experimentally derived DR spectrum is then convolved with a Maxwellian-Boltzmann distribution to obtain the temperature dependent plasma recombination rate coefficients and compared with previously available results from the literature. The present experimental result yields a precise plasma rate coefficients at the low temperature range up to $\sim 1\times {10}^6\mathrm{K}$ and the calculated data by Altun et al. [Z. Altun, A. Yumak, N. R. Badnell, S. D. Loch, and M. S. Pindzola, Astron. Astrophys. 447, 1165 (2006)] provide reliable plasma rate coefficients at high temperature range above $2\times {10}^6\mathrm{K}$.

Accurate calculations of $g_j$ factors of the ground state of the ${\mathrm{Cd}}^{+},$ ${\mathrm{Yb}}^{+},$ and ${\mathrm{Hg}}^{+}$ ions are presented by employing the normal order unrelaxed $\mathrm{\Lambda }$-approach relativistic coupled-cluster $\left(\mathrm{\Lambda }\text{−}\mathrm{RCC}\right)$ theory. Contributions from the quantum electrodynamics (QED) are estimated from the free electron QED contributions and the roles of electron correlation effects are analyzed with different cutoff of occupied and virtual active orbitals in the $\mathrm{\Lambda }$-RCC method. Our final $g_j$ factors come out to be 2.002291(4), 2.002798(113), and 2.003128(41) for the ${\mathrm{Cd}}^{+},$ ${\mathrm{Yb}}^{+},$ and ${\mathrm{Hg}}^{+}$ ions, respectively. Our result for ${\mathrm{Hg}}^{+}$ agrees up to the fourth decimal places with its experimental value 2.0031745(74) indicating that our calculations of the other ions are of similar accuracies. The understanding of roles of electron correlation effects from this study will be useful in improving accuracies of the $g_j$ factors in the considered ions further by adding more physical effects in the future and performing calculations with similar accuracies in other heavier atomic systems. The reported $g_j$ factors can also be used to scrutinize the background noise related to the stray magnetic fields in the laser-cooled microwave ion clocks using the above ions.

Penning ionization releases electrons in a state-selected Rydberg gas of nitric oxide entrained in a supersonic molecular beam. Subsequent processes of electron impact avalanche, bifurcation, and quench form a strongly coupled, spatially correlated, ultracold plasma of ${\mathrm{NO}}^{+}$ ions and electrons that exhibits characteristics of self-organized criticality. This plasma contains a residue of nitric oxide Rydberg molecules. A conventional fluid dynamics of ion-electron-Rydberg quasi-equilibrium predicts rapid decay to neutral atoms. Instead, the NO plasma endures for a millisecond or more, suggesting that quenched disorder creates a state of suppressed electron mobility. Supporting this proposition, a 60-MHz radio frequency field with a peak-to-peak amplitude less than 1 V ${\mathrm{cm}}^{-1}$ acts dramatically to mobilize electrons, causing the plasma to dissipate by dissociative recombination and Rydberg predissociation. An evident density dependence shows that this effect relies on collisions, giving weight to the idea of arrested relaxation as a cooperative property of the ensemble.

We theoretically investigate the emission of high-harmonic (HH) radiation in model crystals by bichromatic few-cycle driving pulses that are composed as the phase-coherent superposition of a mid-infrared fundamental pulse and its second harmonic. Adjusting the model-crystal parameters to reproduce the lowest band gap of MgO, we examine the extent to which distinct domains of the HH spectrum can be controlled and enhanced by tuning the temporal profile of the bichromatic driving laser electric field. We change the driving-pulse shape by varying its fundamental-versus-second-harmonic pulse amplitude ratio and delay, while keeping the energy of the driving laser pulse fixed. For suitable amplitude ratios and delays, we find an up to fivefold enhancement of the spectral HH yield and significant shifts of the HH cutoff frequency.

The phenomenon of quantum revivals resulting from the self-interference of wave packets has been observed in several quantum systems and utilized widely in spectroscopic applications. Here, we present a combined analytical and numerical study on the generation of orientational quantum revivals (OQRs) exclusively using a single-cycle terahertz pulse. As a proof of principle, we examine the scheme in the linear polar molecule HCN with experimentally accessible pulse parameters and obtain strong field-free OQR without requiring the condition of the sudden-impact limit. To visualize the involved quantum mechanism, we derive a three-state model using the Magnus expansion of the time-evolution operator. Interestingly, the terahertz pulse interaction with the electric-dipole moment can activate direct multiphoton processes, leading to OQR enhancements beyond that induced by a rotational ladder-climbing mechanism from the rotational ground state. We further show that the theoretical maximum degree of orientation (0.774) for three populated rotational states can be obtained with available terahertz pulses.