A conceptually simple and experimentally prevalent class of entanglement witnesses, known as fidelity witnesses, detects entanglement via a state's fidelity with a pure reference state. While existence proofs guarantee that a suitable witness can be constructed for every entangled state, such assurances do not apply to fidelity witnesses. Recent results have found that entangled states that cannot be detected by a fidelity witness, known as unfaithful states, are exceedingly common among bipartite states. In this paper, we show that even among two-qubit states, which are the simplest of all entangled states, unfaithful states can be created through a suitable application of decoherence and filtering to a Bell state. We also show that the faithfulness is not monotonic to entanglement, as measured by the concurrence. Finally, we experimentally verify our predictions using polarization-entangled photons, and specifically we demonstrate a situation in which an unfaithful state is brought to faithfulness at the expense of further reducing the entanglement of the state.

Quantum non-Markovianity of channels can be produced by mixing Markovian channels, as observed recently by various authors. We consider an analogous question of whether singularities of the channel can be produced by mixing nonsingular channels, i.e., ones that lack them. Here we answer the question in the negative in the context of qubit Pauli channels. On the other hand, mixing channels with a singularity can lead to the elimination of singularities in the resultant channel. We distinguish between two types of singular channels, which lead under mixing to broadly quite different properties of the singularity in the resultant channel. The connection to non-Markovianity (in the sense of completely positive indivisibility) is pointed out. These results impose nontrivial restrictions on the experimental realization of noninvertible quantum channels by a process of channel mixing.

The $S\text{−},$ $P\text{−},$ and $D\text{−}\mathrm{wave}$ elastic scattering of electrons by atomic helium with the scattering energy between 0.136 and $16.463\mathrm{eV}$ is studied using the confined variational method. Our calculated phase shifts improve the best published results by at least one order of magnitude. The finite nuclear mass effect on the $S$-wave phase shift at very low scattering energy between 0.022 and $0.067\mathrm{eV}$ is investigated, and the accurate $S$-wave scattering length is determined to be $1.1723a_0$, where $a_0$ is the Bohr radius. The results reported here can serve as a benchmark for other theoretical and experimental studies.

We carried out calculations of the energies, hyperfine structure constants, and electric-dipole transition amplitudes for the low-lying states of ${\mathrm{Ba}}^{+}$ in the framework of the relativistic linearized coupled-cluster single double and coupled-cluster single double (valence) triple methods. Taking into account that an iterative inclusion of the valence triples into consideration is a complicated and computationally demanding process, we study the effects of computational restriction on the final results. We also present a detailed study of various corrections to all calculated properties, and we use our results to formulate several broad rules that can be used in future calculations of the elements where experimental data are scarce and correct theoretical predictions are highly important.

We present a numerical study of the resonant high-harmonic generation by tin ions in an elliptically polarized laser field along with a simple analytical model revealing the mechanism and main features of this process. We show that the yield of the resonant harmonics behaves anomalously with the fundamental field ellipticity, namely, the drop of the resonant harmonic intensity with the fundamental ellipticity is much slower than for high harmonics generated through the nonresonant mechanism. Moreover, we study the polarization properties of high harmonics generated in elliptically polarized field and show that the ellipticity of harmonics near the resonance is significantly higher than for ones far off the resonance. This introduces a prospective way to create a source of the quasimonochromatic coherent XUV with controllable ellipticity potentially up to circular.

We present a sequence of driven-dissipative protocols for controlling cold atoms in tilted optical lattices. These experimentally accessible examples are templates that demonstrate how dissipation can be used to manipulate quantum many-body systems. We consider bosonic atoms trapped in a tilted optical lattice, immersed in a superfluid bath, and excited by coherent Raman lasers. With these ingredients, we are able to controllably transport atoms in the lattice and produce self-healing quantum states: a Mott insulator and the topologically ordered spin-1 Affleck-Kennedy-Lieb-Tasaki state.

Low-energy collisions of Rydberg atom-ion systems are investigated theoretically. We present the parameter space associated with suitable approaches for the dynamics of Rydberg atom-ion collisions, i.e., quantum, Langevin, and classical exchange regimes, showing that for the lowest reachable temperatures a classical treatment is appropriate. A quasiclassical trajectory method is used to study charge exchange cross sections for ${\mathrm{Li}}^{*}\text{−}{\mathrm{Li}}^{+}$ and ${\mathrm{Li}}^{*}\text{−}{\mathrm{Cs}}^{+}$ at collision energies down to $1\mathrm{K}$. For cold collisions we find the charge exchange cross section deviating from the $n^4$ geometric scaling. Furthermore, for low-energy collisions, we find both an influence of the ionic core repulsion as well as variations for two different models used for describing the electron-core potential.

The superfluidity of low-temperature bosons is well established in the collisional regime. In the collisionless regime, however, the presence of superfluidity is not yet fully clarified, in particular in lower spatial dimensions. Here, we compare the Vlasov-Landau equation, which does not take into account the superfluid nature of the bosonic system, with the Andreev-Khalatnikov equations, which instead explicitly contain a superfluid velocity. We show that recent experimental data of the sound mode in a two-dimensional collisionless Bose gas of ${}^{87}\mathrm{Rb}$ atoms are in good agreement with both theories, but the sound damping is better reproduced by the Andreev-Khalatnikov equations below the Berezinskii-Kosterlitz-Thouless critical temperature $T_c$, while above $T_c$ the Vlasov-Landau results are closer to the experimental ones. For one-dimensional bosonic fluids, where experimental data are not yet available, we find larger differences between the sound velocities predicted by the two transport theories and, also in this case, the existence of a superfluid velocity reduces the sound damping.

Bound states in the continuum (BICs) in a periodic structure sandwiched between two homogeneous media have interesting properties and useful applications in photonics. The topological nature of BICs was previously revealed based on a topological charge related to the far-field polarization vector of the surrounding resonant states. Recently, it was established that when symmetry-protected BICs (with a nonzero topological charge) are destroyed by generic symmetry-breaking perturbations, pairs of circularly polarized resonant states (CPSs) emerge and the net topological charge is conserved. A periodic structure can also support propagating BICs with a nonzero wave vector. These BICs are not protected by symmetry in the sense of symmetry mismatch, but they need symmetry for their robust existence. Based on a highly accurate computational method for a periodic array of slightly noncircular cylinders, we show that a propagating BIC is typically destroyed by a structural perturbation that breaks only the in-plane inversion symmetry, and when this happens, a pair of CPSs of opposite handedness emerge so that the net topological charge is conserved. We also study the generation and annihilation of CPSs when a structural parameter is varied. It is shown that two CPSs with opposite topological charge and the same handedness, connected to two BICs or in a continuous branch from one BIC, may collapse and become a CPS with a zero charge. Our study clarifies the important connection between symmetry and topological charge conservation.

We study the effect of frequent projective measurements on the dynamics of quantum self-sustaining systems by considering the prototypical example of the quantum van der Pol oscillator. Quantum fluctuations are responsible for phase diffusion which progressively blurs the semiclassical limit-cycle dynamics and synchronization, either to an external driving or between two coupled self-sustained oscillators. We show that by subjecting the system to repeated measurements of heterodyne type at an appropriate repetition frequency one can significantly suppress phase diffusion without spoiling the semiclassical dynamics. This quantum Zeno-like effect may be effective in the case of either one or two coupled van der Pol oscillators, and we discuss its possible implementation in the case of trapped ions.

We characterize entanglement subject to its definition over real and complex, composite quantum systems. In particular, a method is established to assess quantum correlations with respect to a selected number system, illuminating the deeply rooted, yet rarely discussed question of why quantum states are described via complex numbers. With our experiment, we then realize two-photon polarization states that are entangled with respect to the notion of two rebits, comprising two two-level systems over real numbers. At the same time, the generated states are separable with respect to two complex qubits. Among other results, we reconstruct the best approximation of the generated states in terms of a real-valued, local expansion and show that this yields an incomplete description of our data. Conversely, the generated states are shown to be fully decomposable in terms of tensor-product states with complex wave functions. Thereby, we probe paradigms of quantum physics with modern theoretical tools and experimental platforms that are relevant for applications in quantum information science and technology and connected to the fundamentals of the quantum description of nature.

The sensitivity of quantum systems to external disturbances is a fundamental problem for the implementation of functional quantum devices, quantum information, and computation. Based on remarkable experimental progress in optics and ultracold gases, we study the consequences of a short-time (instantaneous) noise while an intensity-dependent phase acquisition is associated with a qubit propagating on an $N$ cycle. By employing quantum coherence measures, we report emerging unstable regimes in which quantum walks arise, such as self-focusing and breathing dynamics. Our numerical and analytical results unveil appropriate settings which favor the stable regime, with the asymptotic distribution surviving for weak nonlinearities and disappearing in the thermodynamic limit with $1/N$. The diagram showing the threshold between different regimes reveals the quantum gates close to Pauli-Z as more noise tolerant. As we move towards the Pauli-X quantum gate, such aptness dramatically decreases and the threshold to the self-focusing regime becomes almost unavoidable. Quantum gates close to Hadamard exhibit an unusual aspect, in which an increment of the nonlinear strength can remove the dynamics from the self-focusing regime.

A thermally isolated quantum system undergoes unitary evolution by interacting with an external work source. The two-point energy measurement (TPM) protocol defines the work exchanged between the system and the work source by performing ideal energy measurements on the system before and after the unitary evolution. However, the ideal energy measurements used in the TPM protocol ultimately result from a unitary coupling with a measurement apparatus, which requires an interaction with an external work source. For the TPM protocol to be self-consistent, we must be able to perform the TPM protocol on the compound of the system plus the apparatus, thus revealing the total work distribution, such that when ignoring the apparatus degrees of freedom, we recover the original TPM work distribution for the system of interest. In the present paper, we show that such self-consistency is satisfied as long as the apparatus is initially prepared in an energy eigenstate. Moreover, we demonstrate that if the apparatus Hamiltonian is equivalent to the “pointer observable,” then (i) the total work distribution will satisfy the “unmeasured” first law of thermodynamics for all system states and system-only unitary processes, and (ii) the total work distribution will be identical to the system-only work distribution for all system states and system-only unitary processes if and only if the unmeasured work due to the unitary coupling between the system and apparatus is zero for all system states.

We propose an extension of coherent control using laser-synthesized free-electron matter waves. In contrast to coherent control schemes exploiting optical coherences to steer the dynamics of matter waves, we analyze the opposite and investigate the control of quantum light emission driven by laser-sculpted coherent free-electron matter waves. We apply this concept to the control of entangled photon-pair emission in electron-atom collisions, in which the incident electron wave packet, colliding with a target atom $B$, is engineered by interferometric resonantly enhanced multiphoton ionization of a parent atom $A$. Each ionization pathway leads to electron wave packets that coherently interfere during temporal evolution in the continuum. Their mutual coherence can be controlled by adjusting the relative phases or time delays of the frequency components of the ionizing field contributing to the interfering pathways. We report coherent control of entangled photon-pair generation in radiative photocascade emission upon decay of the target atom after inelastic excitation triggered by the collision with the synthesized electron wave packet.

Recently the study of the resonance-enhanced below-threshold harmonic generation (BTHG) of atoms and molecules has received considerable attention. BTHG, with spatially coherent and high conversion efficiency, can provide a potential way to produce powerful vacuum-ultraviolet light sources. However, the resonance effects associated with molecular BTHG have become more complex to study due to the presence of the extra internuclear degree of freedom. Here we perform an ab initio study and demonstrate the multichannel-resolved dynamics in resonance-enhanced BTHG of ${\mathrm{H}}_2^{+}$ molecular ions by combining a synchrosqueezing time-frequency transform with an extended semiclassical simulation. We find that BTHG of ${\mathrm{H}}_2^{+}$ on resonance is enabled only near the zero-laser field region and the multichannel interference is responsible for enhanced BTHG. The time-frequency spectra in the vicinity of the resonance exhibit a periodic split structure with oscillating laser pulses and can be explained by the individual dynamical behavior of multiple channels in BTHG.

Bose-Einstein condensates in annular geometries have received significant attention due to their potential use as interferometers for inertial sensing and other applications. We systematically study the critical velocity of a barrier for vortex formation in such a geometry. More significantly, we are able to show that the details of the critical velocity can be captured by a simple analytic solution which can be considered the atomtronic analog of the usual nomographic equations for electronic circuit components. Experimentally useful nomograms can be plotted from the main result of this paper, from the analytic expression abbreviated as $\mathrm{\Delta }$, whose accuracy has been validated via full simulations. It is a function of the potential parameters only and it can be used to directly determine parameter regimes for a given application.

It is important to dope the antiferromagnetic state while preserving low entropy, in the quantum simulation of the Fermi-Hubbard model. In a recent work [Phys. Rev. Lett. 123, 233603 (2019)], we proposed a quantum adiabatic doping protocol using an incommensurate lattice potential, where its feasibility is demonstrated with a certain particle-doping fraction. Here, we carry out a systematic study of quantum adiabatic doping for a wide range of doping fractions including both particle doping and hole doping, with both commensurate and incommensurate cases considered. We find that there is still a localizationlike slowing-down problem even at commensurate fillings, and that it becomes less harmful in the hole-doped regime. With interactions, the adiabatic preparation is found to be more efficient because the interaction effect destabilizes localization. For both free and interacting cases, we find adiabatic doping has a better performance in the hole-doped regime than the particle-doped regime. We also investigate adiabatic doping starting from a half-filling Mott insulator, which is found to be more efficient for certain filling fractions.

We study the entanglement entropy and spectrum between components in binary Bose-Einstein condensates in $d$ spatial dimensions. We employ effective field theory to show that the entanglement spectrum exhibits an anomalous square-root dispersion relation in the presence of intercomponent tunneling (Rabi coupling) and a gapped dispersion relation in its absence. These spectral features are associated with the emergence of long-range interactions in terms of the superfluid velocity and the particle density in the entanglement Hamiltonian. Our results demonstrate that unusual long-range interactions can be emulated in a subsystem of multicomponent BECs that have only short-range interactions. We also find that for a finite Rabi coupling the entanglement entropy exhibits a volume-law scaling with subleading logarithmic corrections originating from the Nambu-Goldstone mode and the symmetry restoration for a finite volume.

The influence of optical feedback on semiconductor lasers has been a widely studied field of research due to fundamental interests as well as the optimization of optical data transmission and computing. Recent publications have shown that it is possible to induce a periodic pulselike output in quantum-dot and quantum-well laser diodes based on the locking of the external cavity modes and the relaxation oscillation frequency. We present an in-depth analysis of this effect. We choose submonolayer quantum dots as a gain system, as these provide a relatively strong amplitude-phase coupling, which has proven to be very beneficial for these locking effects to occur. By introducing an alternative theoretical model, we can correctly reproduce the essential features of the gain system and validate them by comparison to our experimental results. From this starting point we can further explore how the staircase behavior of the oscillation frequency with increasing pump current can be influenced by changing various laser parameters. The staircase behavior is induced by a reordering of the Hopf bifurcations giving birth to the regular pulselike oscillations.