Collapse models postulate the existence of intrinsic noise which modifies quantum mechanics and is responsible for the emergence of macroscopic classicality. Assessing the validity of these models is extremely challenging because it is nontrivial to discriminate unambiguously their presence in experiments where other hardly controllable sources of noise compete to the overall decoherence. Here we provide a simple procedure that is able to probe the hypothetical presence of the collapse noise with a levitated nanosphere in a Fabry-Pérot cavity. We show that the stationary state of the system is particularly sensitive, under specific experimental conditions, to the interplay between the trapping frequency, the cavity size, and the momentum diffusion induced by the collapse models, allowing one to detect them even in the presence of standard environmental noises.

We report a systematic measurement of the ${\left(2p_{1/2}^{-1}3d_{3/2}\right)}_{J=1}$ and ${\left(2s_{1/2}^{-1}3p_{1/2}\right)}_{J=1}$ levels in 14 neonlike ions between ${\mathrm{Ba}}^{46+}$ and ${\mathrm{Pb}}^{72+}$ and document the effects of their avoided crossing near $Z=68$. Strong mixing affects the oscillator strengths over a surprisingly wide range of atomic numbers and leads to the vanishing of one transition two atomic numbers below the crossing. The crossing voids the otherwise correct expectation that the ${\left(2p_{1/2}^{-1}3d_{3/2}\right)}_{J=1}$ level energy is only weakly affected by quantum electrodynamics (QED). For about 10 atomic numbers surrounding the crossing, its QED contributions are anomalously large, attaining almost equality to those affecting the ${\left(2s_{1/2}^{-1}3p_{1/2}\right)}_{J=1}$ level. As a result, the accuracy of energy level calculations appears compromised near the crossing.

We propose an efficient method to construct shortcuts to adiabaticity through designing a substitute Hamiltonian to try to avoid the defect in which the speed-up protocols' Hamiltonian may involve terms which are difficult to realize in practice. We show that as long as the counterdiabatic coupling terms—even only some of them—have been nullified by the additional Hamiltonian, the corresponding shortcuts to the adiabatic process could be constructed and the adiabatic process would be sped up. As an application example, we apply this method to the popular Landau-Zener model for the realization of fast population inversion. The results show that in both Hermitian and non-Hermitian systems, we can design different additional Hamiltonians to replace the traditional counterdiabatic driving Hamiltonian to speed up the process. This method provides many choices for designing additional terms of the Hamiltonian such that one can choose a realizable model in practice.

We put forward an experimentally feasible scheme for heralded entanglement generation between two distant macroscopic mechanical resonators. The protocol exploits a hybrid quantum device, a qubit interacting with a mechanical resonator as well as a cavity mode, for each party. The cavity modes interfere on a beam splitter followed by suitable heralding detections, which postselect a hybrid entangled state with success probability 1/2. Subsequently, by local measurements on the qubits, a mechanically entangled coherent state can be achieved. The mechanical entanglement can be further verified via monitoring the entanglement of the qubit pair. The setup is envisioned as a test bed for sensing gravitational effects on the quantum dynamics of gravitationally coupled massive objects. As a concrete example, we illustrate the implementation of our protocol using the current circuit QED architectures.

Based on the exact dynamics of a two-qubit system and environment, we investigate system-environment (SE) quantum and classical correlations. The coupling is chosen to represent a dephasing channel for one of the qubits and the environment is a proper thermal bath. First we discuss the general issue of dilation for qubit phase damping. Based on the usual thermal bath of harmonic oscillators, we derive criteria of separability and entanglement between an initial $X$ state and the environment. Applying these criteria to initial Werner states, we find that entanglement between the system and environment is built up in time for temperatures below a certain critical temperature $T_{\mathrm{crit}}$. On the other hand, the total state remains separable during those short times that are relevant for decoherence and loss of entanglement in the two-qubit state. Close to $T_{\mathrm{crit}}$ the SE correlations oscillate between separable and entangled. Even though these oscillations are also observed in the entanglement between the two qubits, no simple relation between the loss of entanglement in the two-qubit system and the build-up of entanglement between the system and environment is found.

The high-resolution photoelectron spectra of $\mathrm{C}{\mathrm{o}}^{-}$ were obtained via the slow-electron velocity-map imaging method. The electron affinity of cobalt element was determined to be $5341.45\left(37\right)\mathrm{c}{\mathrm{m}}^{-1}$ or 662.256(46) meV. The fine structure of $\mathrm{C}{\mathrm{o}}^{-}$ was well resolved. The fine-structure intervals $\mathrm{C}{\mathrm{o}}^{-}\left({}^{3}F_4\right)-\mathrm{C}{\mathrm{o}}^{-}\left({}^{3}F_3\right)$ and $\mathrm{C}{\mathrm{o}}^{-}\left({}^{3}F_4\right)-\mathrm{C}{\mathrm{o}}^{-}\left({}^{3}F_2\right)$ were found to be $920.9\left(6\right)\mathrm{c}{\mathrm{m}}^{-1}$ and $1550.3\left(9\right)\mathrm{c}{\mathrm{m}}^{-1}$, respectively. The accuracy was improved by a factor of more than ten with respect to the previous laser photodetachment threshold measurement.

We present a detailed study of the radiative potential method [V. V. Flambaum and J. S. M. Ginges, Phys. Rev. A 72, 052115 (2005)], which enables the accurate inclusion of quantum electrodynamics (QED) radiative corrections in a simple manner in atoms and ions over the range $10\le Z\le 120$, where $Z$ is the nuclear charge. Calculations are performed for binding energy shifts to the lowest valence $s, p$, and $d$ waves over the series of alkali-metal atoms Na to E119. The high accuracy of the radiative potential method is demonstrated by comparison with rigorous QED calculations in frozen atomic potentials, with deviations on the level of 1%. The many-body effects of core relaxation and second- and higher-order perturbation theory on the interaction of the valence electron with the core are calculated. The inclusion of many-body effects tends to increase the size of the shifts, with the enhancement particularly significant for $d$ waves; for K to E119, the self-energy shifts for $d$ waves are only an order of magnitude smaller than the $s$-wave shifts. It is shown that taking into account many-body effects is essential for an accurate description of the Lamb shift.

A quantum-classical approach is developed to describe the strong-field molecular dynamics of ${{\mathrm{H}}_2}^{+}$, taking into account all degrees of freedom and simultaneously dissociation as well as ionization. The electron and nuclei are treated correlated, by propagating the nuclei stochastically on potential energy surfaces. It is demonstrated that Floquet surface hopping (FSH) is particularly well suited to describe the laser-driven dynamics. The method is tested against exact solutions of the time-dependent Schrödinger equation, where available. In addition, the FSH results are in excellent agreement with recent experimental data of the dissociation and ionization dynamics of ${{\mathrm{H}}_2}^{+}$. As an additional issue of this work, the primary importance of the focal volume average is worked out for the understanding of experimental results. It determines the gross features of the experimental spectra and provides also a natural explanation of the puzzling saturation effect in the dissociation spectra, observed experimentally. Future applications and further extensions of the method are discussed.

Nonsequential double ionization of Ar atoms in intense few-cycle laser pulses is studied by a classical ensemble method. The laser pulses are of trapezoidal shape with one cycle in both ramp on and ramp off. We obtain the cycle-resolved electron dynamics by increasing the optical cycles in the laser pulse one by one. We find that, at the higher laser intensity, the correlated-electron momentum distribution (CMD) in the three-cycle laser pulse exhibits two predominate structures in the first and third quadrants. They are formed by the electron pairs in which the second electron is knocked out by the returning electron in the second cycle. As the pulse duration increases, more electron pairs accumulate in the second and fourth quadrants of the CMDs. In these electron pairs, the second electron is first excited owing to collision with the returning electron and then is ionized by the laser field. By varying the peak intensity, we show the transition of the CMDs from anticorrelation to correlation in three-cycle laser pulses, which disproves that multiple collisions cause the transition.

The scattering of polarized light incident from one dielectric medium on its two-dimensional randomly rough interface with a second dielectric medium is studied. A reduced Rayleigh equation for the scattering amplitudes is derived for the case where p- or s-polarized light is incident on this interface, with no assumptions being made regarding the dielectric functions of the media. Rigorous, purely numerical, nonperturbative solutions of this equation are obtained. They are used to calculate the reflectivity and reflectance of the interface, the mean differential reflection coefficient, and the full angular distribution of the intensity of the scattered light. These results are obtained for both the case where the medium of incidence is the optically less dense medium and in the case where it is the optically more dense medium. Optical analogs of the Yoneda peaks observed in the scattering of x rays from metal surfaces are present in the results obtained in the latter case. Brewster scattering angles for diffuse scattering are investigated, reminiscent of the Brewster angle for flat-interface reflection, but strongly dependent on the angle of incidence. When the contribution from the transmitted field is added to that from the scattered field it is found that the results of these calculations satisfy unitarity with an error smaller than ${10}^{-4}$.

The Heisenberg-Robertson uncertainty relation quantitatively expresses the impossibility of jointly sharp preparation of incompatible observables. However, it does not capture the concept of incompatible observables because it can be trivial even for two incompatible observables. We experimentally demonstrate that the new stronger uncertainty relations proposed by Maccone and Pati [Phys. Rev. Lett. 113, 260401 (2014)] relating to the sum of variances are valid in a state-dependent manner and that the lower bound is guaranteed to be nontrivial when two observables are incompatible on the state of the system being measured. The behavior we find agrees with the predictions of quantum theory and obeys the new uncertainty relations even for the special states which trivialize the Heisenberg-Robertson relation. We realize a direct measurement model and perform an experimental investigation of the strengthened relations.

Recently a variant on Ramsey interferometry for coupled spin-$1/2$ systems was proposed to directly measure the retarded spin-spin Green's function. In conventional experimental situations, the spin system is initially in a nonequilibrium state before the Ramsey interferometry is performed, so we examine the nonequilibrium retarded spin-spin Green's functions within the transverse-field Ising model. We derive the lowest four spectral moments to understand the short-time behavior and we employ a Lehmann-like representation to determine the spectral behavior. We simulate a Ramsey protocol for a nonequilibrium quantum spin system that consists of a coherent superposition of the ground state and diabatically excited higher-energy states via a temporally ramped transverse magnetic field. We then apply the Ramsey spectroscopy protocol to the final Hamiltonian, which has a constant transverse field. The short time allows us to extract the initial transport of many-body correlations, while the long-time behavior relates to the excitation spectra of the Hamiltonian. Compressive sensing is employed in the data analysis to efficiently extract that spectra.

We uncover the nonsymmorphic symmetry and investigate its effects on the noncollinear band structures of a quasi-two-dimensional optical lattice with synthetic one-dimensional spin-orbit coupling and a tunable Zeeman field. The perpendicular Zeeman field breaks time-reversal symmetry and lifts the Kramers degeneracy which is protected by time-reversal and generalized inversion symmetries. Interestingly, we find that the degeneracy of Bloch bands on the border of the Brillouin zone is immune to the Zeeman field. This degeneracy, reminiscent of that in nonsymmorphic crystals, is protected by the hidden glide-plane symmetry that comprises a physical reflection involving both spatial and spin degrees of freedom followed by a nonprimitive lattice translation. Furthermore, we show that the band degeneracy can be lifted by the glide-plane-symmetry-breaking lattice potential. Finally, we propose to detect these effects by measuring a dynamical structure factor with optical Bragg spectroscopy.

We demonstrate that superpositions of coherent states and displaced Fock states, also referred to as generalized Schrödinger cat states, can be generated by application of a deformed Glauber displacement operator on number states, coherent states, and displaced Fock states. Based on such a deformed displacement operator we introduce a deformed version of the so-called Glauber photonic lattices. These novel lattices are endowed with alternating positive and negative coupling coefficients and give rise to classical analogs of Schrödinger cat states. Finally, we show that the analytic propagator of these new Glauber-Fock arrays explicitly contains the Wigner operator opening the possibility to emulate Wigner functions of the quantum harmonic oscillator in the classical domain.

The interplay between nonlinearity and the band structure of pristine honeycomb lattices is systematically explored. For that purpose, a theory of collective excitations valid for the first Brillouin zone of the lattice is developed. Closed-form expressions of two-dimensional excitations are derived for Bloch wave numbers beyond the high-symmetry points of the band structure. A description of the regions of validity of different nonlinear excitations in the first-Brillouin zone is given. We find that the unbounded nature of these excitations in nonlinear honeycomb latices is a signature of the strong influence of the Dirac cones in other parts of the band structure.

We investigate two-photon quantum interference in an opaque scattering medium that intrinsically supports a large number of transmission channels. By adaptive spatial phase modulation of the incident wave fronts, the photons are directed at targeted speckle spots or output channels. From ${10}^3$ experimentally available coupled channels, we select two channels and enhance their transmission to realize the equivalent of a fully programmable $2\times 2$ beam splitter. By sending pairs of single photons from a parametric down-conversion source through the opaque scattering medium, we observe two-photon quantum interference. The programed beam splitter need not fulfill energy conservation over the two selected output channels and hence could be nonunitary. Consequently, we have the freedom to tune the quantum interference from bunching (Hong-Ou-Mandel-like) to antibunching. Our results establish opaque scattering media as a platform for high-dimensional quantum interference that is notably relevant for boson sampling and physical-key-based authentication.

Nonclassical-state generation is an important component throughout experimental quantum science for quantum information applications and probing the fundamentals of physics. Here, we investigate permutations of quantum nondemolition quadrature measurements and single quanta addition or subtraction to prepare quantum superposition states in bosonic systems. The performance of each permutation is quantified and compared using several different nonclassicality criteria including Wigner negativity, nonclassical depth, and optimal fidelity with a coherent-state superposition. We also compare the performance of our protocol using squeezing instead of a quadrature measurement and find that the purification provided by the quadrature measurement can significantly increase the nonclassicality generated. Our approach is ideally suited for implementation in light-matter systems such as quantum optomechanics and atomic spin ensembles, and offers considerable robustness to initial thermal occupation.