We show that the performances of a Stern-Gerlach apparatus can be improved by using a magnetic field profile for the atomic spin evolution designed with the shortcut-to-adiabaticity technique. Interestingly, it can be made more compact, for atomic beams propagating at a given velocity, and more resilient to a dispersion in velocity in comparison with the results obtained with a standard uniform rotation of the magnetic field. Our results are obtained using a reverse-engineering approach based on Lewis-Riesenfeld invariants. We discuss quantitatively the advantages offered by our configuration in terms of the resources involved and show that it drastically enhances the fidelity of the quantum state transfer achieved by the Stern-Gerlach device.

A constructive theoretical platform for the description of quantum space-time crystals uncovers for $N$ interacting and ring-confined rotating particles the existence of low-lying states with proper space-time crystal behavior. The construction of the corresponding many-body trial wave functions proceeds first via symmetry breaking at the mean-field level followed by symmetry restoration using projection techniques. The ensuing correlated many-body wave functions are stationary states and preserve the rotational symmetries, and at the same time they reflect the point-group symmetries of the mean-field crystals. This behavior results in the emergence of sequences of select magic angular momenta $L_m$. For angular-momenta away from the magic values, the trial functions vanish. Symmetry breaking beyond the mean-field level can be induced by superpositions of such good-$L_m$ many-body stationary states. We show that superposing a pair of adjacent magic angular momenta states leads to formation of special broken-symmetry states exhibiting quantum space-time-crystal behavior. In particular, the corresponding particle densities rotate around the ring, showing undamped and nondispersed periodic crystalline evolution in both space and time. The experimental synthesis of such quantum space-time-crystal wave packets is predicted to be favored in the vicinity of ground-state energy crossings of the Aharonov-Bohm-type spectra accessed via an externally applied, natural or synthetic, magnetic field. These results are illustrated here for Coulomb-repelling fermionic ions and for a lump of contact-interaction attracting bosons.

We describe AlGaAs photonic-crystal architectures that simultaneously realize strong exciton-photon coupling, long polariton lifetime, and room-temperature polariton Bose-Einstein condensation (BEC). Strong light trapping, induced by a 3D photonic band gap (PBG), leads to peak field intensity 20 times as large as that in an AlGaAs Fabry-Pérot microcavity and exciton-photon coupling as large as 20 meV (i.e., vacuum Rabi splitting 40 meV). The strong exciton-photon coupling, small polariton effective mass, and long polariton lifetime lead to possible realizations of equilibrium room-temperature BEC. We also consider the influence of polarization degeneracy and symmetry breaking in the ground state on the BEC-onset temperature and condensate fraction. Woodpile and slanted-pore PBG structures that break X-Y symmetry facilitate larger condensate fractions at moderate temperatures. The effects of electronic and photonic disorder are marginal, thanks to the 3D photonic band gap.

We present a general method for the modeling of semiconductor lasers such as a vertical-cavity surface-emitting laser and a vertical-external-cavity surface-emitting laser containing multiple quantum wells and involving anisotropies that may reveal (i) a local linear birefringence due to the strain field at the surface or (ii) a birefringence in quantum wells due to phase amplitude coupling originating from the reduction of the biaxial $D_{2d}$ symmetry group to the $C_{2v}$ symmetry group at the III-V ternary semiconductor interfaces. From a numerical point of view, a scattering S-matrix recursive method is implemented using a gain or amplification tensor derived analytically from the Maxwell-Bloch equations. It enables one to model the properties of the emission (threshold, polarization, and mode splitting) from the laser with multiple quantum well active zones by searching for the resonant eigenmodes of the cavity. The method is demonstrated on real laser structures and is presently used for the extraction of optical permittivity tensors of surface strain and quantum wells in agreement with experiments. The method can be generalized to find the laser eigenmodes in the most general case of circular polarized pumps (unbalance between the spin-up and spin-down channels) and/or dichroism allowing an elliptically polarized light emission as recently demonstrated experimentally when the linear birefringence is almost compensated [Joly et al., Opt. Lett. 42, 651 (2017)].

Plasmonics allows manipulating light at the nanoscale, but has limitations due to the static nature of nanostructures and lack of reconfigurability. We propose and theoretically analyze a room-temperature liquid-metal nanodroplet that changes its shape, and therefore dynamically reconfigures the plasmon resonance spectrum, due to capillary oscillations. We show the possibility to control the capillary oscillation frequency of the nanodroplet and to drive the oscillations electrically or mechanically. Employed as a dynamically reconfigurable nanoantenna, the nanodroplet may find applications in sensors, imaging, microscopy, and medicine.

Binary coherent state alphabets can be represented in a two-dimensional Hilbert space. We capitalize this formal connection between the otherwise distinct domains of qubits and continuous variable states to map binary phase-shift keyed coherent states onto the Bloch sphere and to derive their quantum-optimal clones. We analyze the Wigner function and the cumulants of the clones, and we conclude that optimal cloning of binary coherent states requires a nonlinearity above second order. We propose several practical and near-optimal cloning schemes and compare their cloning fidelity to the optimal cloner.

In continuous-variable quantum key distribution, the loss and excess noise of the quantum channel are key parameters that determine the secure key rate and the maximal distribution distance. We investigate the imperfect quantum state preparation in Gaussian modulation coherent-state protocol both theoretically and experimentally. We show that the Gaussian distribution characteristic of the prepared states in phase space is broken due to the incorrect calibration of the working parameters for the amplitude modulator and phase modulator. This further causes a significant increase of the excess noise and misestimate of the channel loss. To ensure an accurate estimate of the quantum channel parameters and achieve a reliable quantum key distribution, we propose and demonstrate two effective schemes to calibrate the working parameters of the modulators.

The (im)possibility of local distinguishability of orthogonal multipartite quantum states still remains an intriguing question. Beyond ${\mathbb{C}}^3\otimes {\mathbb{C}}^3$, the problem remains unsolved even for maximally entangled states (MESs). So far, the only known condition for the local distinguishability of states is the well-known orthogonality preservation (OP). Using an upper bound on the locally accessible information for bipartite states, we derive a very simple necessary condition for any set of pairwise orthogonal MESs in ${\mathbb{C}}^d\otimes {\mathbb{C}}^d$ to be perfectly locally distinguishable. It is seen that particularly when the number of pairwise orthogonal MES states in ${\mathbb{C}}^d\otimes {\mathbb{C}}^d$ is equal to $d$, then this necessary condition, along with the OP condition, imposes more constraints (for said states to be perfectly locally distinguishable) than the OP condition does. When testing this condition for the local distinguishability of all sets of four generalized Bell states in ${\mathbb{C}}^4\otimes {\mathbb{C}}^4$, we find that it is not only necessary but also sufficient to determine their local distinguishability. This demonstrates that the aforementioned upper bound may play a significant role in the general scenario of local distinguishability of bipartite states.

The lifetimes and decay channels of ultralong-range Rydberg molecules created in a dense Bose-Einstein condensate are examined by monitoring the time evolution of the Rydberg population using field ionization. Studies of molecules with values of the principal quantum number, $n$, in the range $n=49$ to $n=72$ that contain tens to hundreds of ground-state atoms within the Rydberg electron orbit show that their presence leads to marked changes in the field ionization characteristics. The Rydberg molecules have lifetimes of $\sim 1-5\mu \mathrm{s}$, their destruction being attributed to two main processes: formation of ${\mathrm{Sr}}_2{}^{+}$ ions through associative ionization and dissociation induced through $L$-changing collisions. The observed loss rates are consistent with a reaction model that emphasizes the interaction between the Rydberg core ion and its nearest-neighbor ground-state atom. The measured lifetimes place strict limits on the time scales over which studies involving Rydberg species in cold, dense atomic gases can be undertaken and limit the coherence times for such measurements.

A multicenter distorted-wave method is used to study the electron impact single ionization from the outer (${1t}_2$) and inner (${2a}_1$) valence orbitals of a ${\mathrm{CH}}_4$ molecule. The triple differential cross sections are calculated in coplanar asymmetric kinematics with the scattered electron energy at 500 eV and the ejected electron energy at 12, 37, and 74 eV, respectively. The nuclear term in the ionization transition amplitude is fully included, and the calculated results well reproduce the experimental measurements, especially for the ${2a}_1$ orbital, where excellent agreement has been achieved.

A detailed understanding of the dissociative electron ionization dynamics of $\mathrm{S}{\mathrm{F}}_6$ is important in the modeling and tuning of dry-etching plasmas used in the semiconductor manufacture industry. This paper reports a crossed-beam electron ionization velocity-map imaging study on the dissociative ionization of cold $\mathrm{S}{\mathrm{F}}_6$ molecules, providing complete, unbiased kinetic energy distributions for all significant product ions. Analysis of these distributions suggests that fragmentation following single ionization proceeds via formation of $\mathrm{S}{{\mathrm{F}}_5}^{+}$ or $\mathrm{S}{{\mathrm{F}}_3}^{+}$ ions that then dissociate in a statistical manner through loss of F atoms or ${\mathrm{F}}_2$, until most internal energy has been liberated. Similarly, formation of stable dications is consistent with initial formation of $\mathrm{S}{{\mathrm{F}}_4}^{2+}$ ions, which then dissociate on a longer time scale. These data allow a comparison between electron ionization and photoionization dynamics, revealing similar dynamical behavior. In parallel with the ion kinetic energy distributions, the velocity-map imaging approach provides a set of partial ionization cross sections for all detected ionic fragments over an electron energy range of 50–100 eV, providing partial cross sections for ${\mathrm{S}}^{2+}$, and enables the cross sections for $\mathrm{S}{{\mathrm{F}}_4}^{2+}$ from $\mathrm{S}{\mathrm{F}}^{+}$ to be resolved.

We describe procedures for converging on and characterizing zero-energy Feshbach resonances that appear in scattering lengths for ultracold atomic and molecular collisions as a function of an external field. The elastic procedure is appropriate for purely elastic scattering, where the scattering length is real and displays a true pole. The regularized scattering length procedure is appropriate when there is weak background inelasticity, so that the scattering length is complex and displays an oscillation rather than a pole, but the resonant scattering length $a_{\mathrm{res}}$ is close to real. The fully complex procedure is appropriate when there is substantial background inelasticity and the real and imaginary parts of $a_{\mathrm{res}}$ are required. We demonstrate these procedures for scattering of ultracold ${}^{85}\mathrm{Rb}$ in various initial states. All of them can converge on and provide full characterization of resonances, from initial guesses many thousands of widths away, using scattering calculations at only about ten values of the external field.

Electrons in plasmas produced by next-generation ultraintense lasers ($I>5\times {10}^{22}\mathrm{W}/{\mathrm{cm}}^2$) can be spin polarized to a high degree (10%–70%) by the laser pulses on a femtosecond time scale. This is due to electrons undergoing spin-flip transitions as they radiate $\gamma$-ray photons, preferentially spin polarizing in one direction. Spin polarization can modify the radiation reaction force on the electrons, which differs by up to 30% for opposite spin polarizations. Consequently, the polarization of the radiated $\gamma$-ray photons is also modified: the relative power radiated in the $\sigma$ and $\pi$ components increases and decreases by up to 30%, respectively, potentially reducing the rate of pair production in the plasma by up to 30%.

The many-electron weak-field asymptotic theory (ME-WFAT) for static tunneling ionization [O. I. Tolstikhin et al., Phys. Rev. A 89, 013421 (2014)] is applied to diatomic molecules. In the ME-WFAT, the dependence of the ionization rate on the molecular orientation with respect to the static field direction is determined by the structure factor, which in turn depends on the asymptotic tail of the Dyson orbital. We extract the latter by the time-dependent generalized-active-space configuration-interaction method [S. Bauch et al., Phys. Rev. A 90, 062508 (2014)], which takes into account electron correlation effects systematically. Results for the orientation-dependent structure factor are presented for ${\mathrm{H}}_2$ and LiH. Compared to mean-field Hartree-Fock results, the inclusion of electron-electron correlation affects the structure factor, and hence the rate, even for these simple systems.

Fully quantum and classical calculations on a helium atom with two excited, radially localized Rydberg wave packets are performed. The differences between classical and quantum methods are compared for a wide range of principal quantum numbers to study the validity of the classical method for low-lying states. The effects of fast terahertz single-cycle pulses on an atomic system with one or two Rydberg wave packets are also studied using classical equations of motion. These results suggest that single-cycle pulses can be used as time-resolved probes to detect motion of the wave packets and to investigate autoionization properties.

We present a table-top quantum estimation protocol to measure the gravitational acceleration $g$ by using an optomechanical cavity. In particular, we exploit the nonlinear quantum light-matter interaction between an optical field and a massive mirror acting as mechanical oscillator. The gravitational field influences the system dynamics affecting the phase of the cavity field during the interaction. Reading out such a phase carried by the radiation leaking from the cavity, we provide an estimate of the gravitational acceleration through interference measurements. Contrary to previous studies, having adopted a fully quantum description, we are able to propose a quantum analysis proving the ultimate bound to the estimability of the gravitational acceleration and verifying optimality of homodyne detection. Noticeably, thanks to the light-matter decoupling at the measurement time, no initial cooling of the mechanical oscillator is demanded in principle.

A study on the spatial characteristics of a near-field optical trap is presented. For analysis, a plasmonic near-field trap consisting of a C-shaped engraving is considered. Numerical simulations are performed to calculate the optical force exerted on a spherical nanoparticle by the trap. A Brownian dynamics model is used to simulate a large number of independent trajectories of a nanoparticle submerged in the optical force field. Statistical analysis is performed on the trajectory data to calculate the trapping probability at different points in space. The points with equal trapping probabilities are enclosed in a surface to visualize the influence domain of the trap. The metric capturing range is defined and calculated from the spatial extent of such surfaces. The possible applications of the defined metric are discussed. Some design examples from the literature are also analyzed and are found to be consistent with the proposed analysis.

We demonstrate tunable pulling and pushing optical forces on plasmonic nanostructures at plasmon singularity and Fano resonance. The plasmonic nanostructure containing a spherical core with optical gain and a metallic shell shows much larger optical pulling force than a pure gain sphere. When the size of the nanostructure is beyond the quasistatic limit, one can obtain large field enhancement and giant pulling force at the emerged quadrupole mode. The introduction of an optical pump compensates the dissipative loss from the metal shell, thus enabling strong coupling between a narrow quadrupole mode and a board dipole mode, giving rise to Fano resonance. The giant negative forces originate from the reversal of the electric field at Fano resonance, which leads to pulling forces on bound currents and charges. Meanwhile, the separation of the Lorentz force helps to reveal the nature of the pulling forces in the gain system. We have shown that by applying the Lorentz force density formula, it is possible to obtain the correct value of the force inside our complex inhomogeneous structure made up of dispersive and lossy metamaterial irrespective of the electromagnetic momentum density. Our results provide a practical way to manipulate nanoparticles and give deep insight into light-matter interaction.