We study the collective effects that emerge in waveguide quantum electrodynamics where several (artificial) atoms are coupled to a one-dimensional superconducting transmission line. Since single microwave photons can travel without loss for a long distance along the line, real and virtual photons emitted by one atom can be reabsorbed or scattered by a second atom. Depending on the distance between the atoms, this collective effect can lead to super- and subradiance or to a coherent exchange-type interaction between the atoms. Changing the artificial atoms transition frequencies, something which can be easily done with superconducting qubits (two levels artificial atoms), is equivalent to changing the atom-atom separation and thereby opens the possibility to study the characteristics of these collective effects. To study this waveguide quantum electrodynamics system, we extend previous work and present an effective master equation valid for an ensemble of inhomogeneous atoms driven by a coherent state. Using input-output theory, we compute analytically and numerically the elastic and inelastic scattering and show how these quantities reveal information about collective effects. These theoretical results are compatible with recent experimental results using transmon qubits coupled to a superconducting one-dimensional transmission line [van Loo et al. (unpublished)].

In quantum optics a pure state is considered classical, relative to the statistics of photodetection, if and only if it is a coherent state. A different and newer notion of nonclassicality is based on modal entanglement. One example that relates these two notions is the Hong-Ou-Mandel effect, where modal entanglement is generated by a beamsplitter from the nonclassical photon-number state $|1\rangle \otimes |1\rangle$. This suggests that beamsplitters or, more generally, linear-optical networks are mediators of the two notions of nonclassicality. In this Brief Report, we show the following: Given a nonclassical pure-product-state input to an $N$-port linear-optical network, the output is almost always mode entangled; the only exception is a product of squeezed states, all with the same squeezing strength, input to a network that does not mix the squeezed and antisqueezed quadratures. Our work thus gives a necessary and sufficient condition for a linear network to generate modal entanglement from pure-product inputs, a result that is of immediate relevance to the boson-sampling problem.

Transitionless quantum driving achieves adiabatic evolution in a hurry, using a counterdiabatic Hamiltonian to stifle nonadiabatic transitions. Here this shortcut to adiabaticity is cast in terms of a generator of adiabatic transport. This yields a classical analog of transitionless driving, and provides a strategy for constructing quantal counterdiabatic Hamiltonians. As an application of this framework, exact classical and quantal counterdiabatic terms are obtained for a particle in a box and for even-power-law potentials in one degree of freedom.

A charged-particle cloud in a Paul trap is a nonlinear system that exhibits deterministic heating, i.e., conversion of energy provided by the radio-frequency (rf) drive of the trap into the disordered, gaslike motions of the trapped particles. In the presence of weak damping clouds reach an equilibrium state in which heating and cooling balance. For large damping, a cloud may collapse into a crystal. While the rf heating power of a given cloud depends sensitively on its particle number, its equilibrium size, the damping, and the control parameters of the trap, we find that scaling the heating power of the cloud with respect to its associated Coulomb-decoupled, non-interacting-gas state, and its equilibrium size with respect to its crystal size, results in a single, universal heating curve, independent of particle number and trap control parameters.

The photolysis of the hydronium cation H${}_3$O${}^{+}$ has been studied at the extreme ultraviolet wavelengths of $35.56\pm 0.24$ nm ($34.87\pm 0.24$ eV) and $21.85\pm 0.17$ nm ($56.74\pm 0.44$ eV) using a crossed ion-photon beam setup at the free-electron laser FLASH. Coincidence photoelectron and photofragment spectroscopy was performed at 21.85 nm, where both inner and outer valence ionization are allowed, and revealed that the XUV photolysis of H${}_3$O${}^{+}$ is by far dominated by ionization of outer valence electrons forming the ${}^1{A}_1$ and ${}^{2}E$ states of the dication H${}_3$O${}^{2+}$. The dications were found to dissociate into the channels H${}_2$O${}^{+}$+H${}^{+}$ ($72\pm 4\%$), OH${}^0$+2H${}^{+}$ ($18\pm 6\%$), and OH${}^{+}$+H${}^{+}$+H${}^0$ ($10\pm 1\%$). A kinematic analysis of the H${}_2$O${}^{+}$+H${}^{+}$ channel after photoabsorption at 35.56 nm (where only outer valence ionization is possible) showed dissociation into excited states of the water radical ion, where the ${}^1{A}_1$ state breaks up into the linear $\tilde{A}{}^2{A}_1$ state of H${}_2$O${}^{+}$ and the ${}^{2}E$ state decays into the strongly bent $\tilde{B}{}^2{B}_2$ state. Finally, from the ${}^{2}E$ state of H${}_3$O${}^{2+}$, dissociation into OH${}^0(X{}^2\Pi$)+2H${}^{+}$ was identified to occur with a near linear dissociation geometry.

Interacting two-component Fermi gases are at the heart of our understanding of macroscopic quantum phenomena such as superconductivity. The changing nature of the interaction is expected to head to novel quantum phases. Here we study the ground state of a two-component fermionic gas in a harmonic potential with dipolar and contact interactions. Using a variational Wigner function we present the phase diagram of the system with equal but opposite values of the magnetic moment. We identify the second-order phase transition from paramagnetic to ferronematic phases. Moreover, we show the impact of the experimentally relevant magnetic field on the stability and magnetization of the system. We also investigate a two-component Fermi gas with large but almost equal values of the magnetic moment to study how the interplay between contact and dipolar interactions affects the stability properties of the mixture. To be specific we discuss experimentally relevant parameters for ultracold ${}^{161}$Dy.

We measure the Gouy phase anomaly for matter waves using in-line holography to retrieve the full complex field of an astigmatic electron wave function. Sequential phase shifts of π/2 rad are observed for electron trajectories along the optic axis that pass through each line-focus caustic of subnanometer transverse width. Our observations demonstrate that anomalous phase shifts of matter waves in the vicinity of caustics can be robustly measured using phase retrieval, extending the current scope of singular electron optics.

We consider the far-zone spectrum generated by finite, statistically homogeneous, planar electromagnetic sources having three electric field components. We introduce an electromagnetic scaling law which ensures spectral invariance, i.e., that the normalized far-zone spectrum is the same in any paraxial or nonparaxial direction and equal to that of the source. The scaling law for electromagnetic fields presented in this work is more general than that put forward in a previous publication. In particular, we show paraxially that even though the individual field components do not obey the scalar scaling law, spectral invariance may be achieved, and conversely, that spectral invariance may not hold even if the field components separately follow the scalar scaling law.

We consider a vacuum in which all of the wavelengths of vacuum fluctuations in a preferred inertial frame are longer than a given minimum length $\hslash /\Lambda$. This paper studies spinor QED in such momentum-cutoff vacuums, in particular, the Lorentz anomalies which appear in the radiative corrections that result from vacuum fluctuations in the continuum limit $\Lambda \to \infty .$ A gauge-invariant momentum-cutoff generating functional $Z_{\Lambda }[A,J]$ in the background-field formalism is given, from which well-defined radiative correction terms as well as renormalization constants can be derived, at least in the form of a loop-expansion series. Using the conventional Lorentz-covariant renormalized perturbation procedure, one-loop and two-loop (for photon self-energy only) calculations are carried out in detail. We find that the non-Lorentz-covariant terms in one-loop and two-loop radiative corrections converge to nonvanishing terms in the limit $\Lambda \to \infty .$ The physical meaning as well as some of the phenomenological consequences of these Lorentz anomalous terms are discussed.

We discuss entanglement generation in a closed system of one or two atomic quantum dots (qubits) coupled via Raman transitions to a pool of cold interacting bosons. The system exhibits rich entanglement dynamics, which we analyze in detail in an exact quantum mechanical treatment of the problem. The bipartite setup of only one atomic quantum dot coupled to a pool of bosons turns out to be equivalent to two qubits which easily get entangled from an initial product state. We show that both the number of bosons in the pool and the boson-boson interaction crucially affect the entanglement characteristics of the system. The tripartite system of two atomic quantum dots and a pool of bosons reduces to a qubit-qutrit-qubit realization. We consider entanglement possibilities of the pure system as well as of reduced ones by tracing out one of the constituents, and show how the entanglement can be controlled by varying system parameters. We demonstrate that the qutrit, as expected, plays a leading role in entangling the two qubits and the maximum entanglement depends in a nontrivial way on the pool characteristics.

We study stationary entanglement between three harmonic oscillators which are dipole coupled to a one-dimensional or a three-dimensional bosonic environment. The analysis of the open-system dynamics is performed with generalized quantum Langevin equations which we solve exactly in a Fourier representation. The focus lies on Gaussian bipartite and tripartite entanglement induced by the highly non-Markovian interaction mediated by the environment. This environment-induced interaction represents an effective many-party interaction with a spatial long-range feature: A main finding is that the presence of a passive oscillator is detrimental for stationary two-mode entanglement. Furthermore, our results indicate that the environment-induced entanglement mechanism corresponds to uncontrolled feedback which is predominantly coherent at low temperatures and for moderate oscillator-environment coupling as compared to the oscillator frequency.

We study the local unitary equivalence of arbitrary dimensional multipartite quantum mixed states. We present a necessary and sufficient criterion of the local unitary equivalence for general multipartite states based on matrix realignment. The criterion is shown to be operational, even for particularly degenerated states, by detailed examples. In addition, explicit expressions of the local unitary operators are constructed for locally equivalent states. Complementary to the criterion, an alternative approach based on the partial transposition of matrices is also given, which makes the criterion more effective in dealing with generally degenerated mixed states.

We investigate effective noise channels for encoded quantum systems with and without active error correction. Noise acting on physical qubits forming a logical qubit is thereby described as a logical noise channel acting on the logical qubits, which leads to a significant decrease of the effective system dimension. This provides us with a powerful tool to study entanglement features of encoded quantum systems. We demonstrate this framework by calculating lower bounds on the lifetime of distillable entanglement and the negativity for encoded multipartite qubit states with different encodings. At the same time, this approach leads to a simple understanding of the functioning of (concatenated) error correction codes.

A method is proposed for extracting ionization amplitudes from the solution of the time-dependent Schrödinger equation (TDSE) describing a system in a time-dependent external field. The method is a hybrid of two earlier developed methods, the time-dependent surface flux method and the method using the propagated wave packet as the source term in a time-independent driven Schrödinger equation with a field-free Hamiltonian. It is demonstrated that the method combines the advantages of the earlier ones and allows the extraction of ionization amplitudes by solving the TDSE within a small spatial domain (with the boundary conditions provided by exterior complex scaling) and a time interval, not exceeding the external field pulse duration.

We study the applicability of the Zhu-Nakamura theory to a class of time-dependent quantum mechanical level-crossing models called superparabolic level-glancing models. The phenomenon of a level glancing, being on the borderline between a proper crossing of energy levels and an avoided crossing, is also an important special case between the two different approximative expressions in the Zhu-Nakamura theory. It is seen that the application of the theory to these models is not straightforward. We discuss some possible causes of these difficulties and also compare the approximative formulas of Zhu-Nakamura theory to those obtained by the generalization of the Dykhne-Davis-Pechukas theory.

We investigate the effects of interatomic interactions and expansion on the distortion of interference fringes of a pair of initially well-separated, but coherent, condensate clouds trapped in a harmonic trap. The distortion of interference fringes, which can lead to the spontaneous formation of vortices in the atom clouds, depends crucially on two relevant parameters: the center-of-mass velocity and peak density of the initial state. We identify three qualitatively distinct regimes for the interfering condensates: collision, expansion, and merging, by the spatial and temporal features of the fringe spacings. Using a comprehensive set of numerical simulations based on the Gross-Pitaevskii equation, we specify the crossovers between these regimes and propose the optimal system parameters required for dynamical instabilities and vortex creation.

We suggest a time-dependent mean-field hydrodynamic model for a binary dipolar boson-fermion mixture to study the stability and collapse of fermions in the ${}^{164}$Dy-${}^{161}$Dy mixture. The condition of stability of the dipolar mixture is illustrated in terms of phase diagrams. A collapse is induced in a disk-shaped stable binary mixture by jumping the interspecies contact interaction from repulsive to attractive by the Feshbach resonance technique. The subsequent dynamics is studied by solving the time-dependent mean-field model including three-body loss due to molecule formation in boson-fermion and boson-boson channels. Collapse and fragmentation in the fermions after subsequent explosions are illustrated. The anisotropic dipolar interaction leads to anisotropic fermionic density distribution during collapse. This study is carried out in three-dimensional space using realistic values of dipolar and contact interactions.

We generalize Greenberger-Horne-Zeilinger (GHZ) theorem to an arbitrary number of $D$-dimensional systems. Contrary to conventional approaches using compatible composite observables, we employ incompatible and concurrent observables, whose common eigenstate is still a generalized GHZ state. It is these concurrent observables which enable one to prove a genuinely $N$-partite and $D$-dimensional GHZ theorem. Our principal idea is illustrated for a four-partite system with $D$ which is an arbitrary multiple of 3. By extending to $N$ qudits, we show that GHZ theorem holds as long as $N$ is not divisible by all nonunit divisors of $D$, smaller than $N$.

The loss of coherence in a general open quantum system interacting with a bosonic environment is analyzed. The reservoir is initially in a thermal state. The reduced dynamics is described by a non-Markovian time-local master equation. We consider spectral densities that are sub- or super-Ohmic at low frequencies and arbitrarily shaped at high frequencies. In the super-Ohmic regime, for noninteger frequency powers larger than 2, long time survival of coherence appears. In the latter regime, at vanishing temperature, the asymptotic amount of surviving coherence is stabilized to its initial value, up to a phase factor, by properly increasing the bandwidth and decreasing the low-frequency profile of the spectral density. For noninteger positive frequency powers less than 2, stretched exponential-like decoherence is found over long times. The relaxations to the asymptotic configurations become arbitrarily slow by approaching the frequency power 2 of the super-Ohmic regime. The same dependence on temperature, spectral density, and scale frequency appears for purity and concurrence of two qubits and coherence of a qubit.

We experimentally investigate high-order-harmonic generation (HHG) of diatomic molecules (O${}_2$, N${}_2$) and their companion atoms (Xe, Ar, respectively, which are of nearly the same ionization potentials as the corresponding molecules) with midinfrared femtosecond laser pulses. It is found that the cutoff of O${}_2$ extends to higher energy as compared to that of Xe, and the extension of cutoff energy of O${}_2$ depends on the driver wavelength. Our theoretical analysis shows that the ionization suppression induced by two-center interference should be responsible for the harmonic cutoff extension of O${}_2$.