We report the observation of diffraction cancellation for visible beams with widths from tens of wavelengths down to fractions of the optical wavelength. The phenomenon is observed at the transition from diffraction- to antidiffraction-dominated beam propagation, triggered by a thermal shock in a photorefractive nanodisordered lithium-enriched potassium-tantalate-niobate (KTN:Li) crystal. Here beams propagate without distortion, independently of intensity and size. Confirming the main prediction of the scale-free-optics model, we find a single unified light behavior that spans across the entire hierarchy of standard optical spatial scales, from wide plane-wave-like beams that obey geometrical optics down to ultranarrow beams with widths of the order of a single wavelength.

Recent theoretical studies of stimulated Brillouin scattering (SBS) in nanoscale devices have led to intense research effort dedicated to the demonstration and application of this nonlinearity in on-chip systems. The key feature of SBS in integrated photonic waveguides is that small, high-contrast waveguides are predicted to experience powerful optical forces on the waveguide boundaries, which are predicted to further boost the SBS gain that is already expected to grow dramatically in such structures because of the higher mode confinement alone. In all recent treatments, the effect of radiation pressure is included separately from the scattering action that the acoustic field exerts on the optical field. In contrast to this, we show here that the effects of radiation pressure and motion of the waveguide boundaries are inextricably linked. Central to this insight is a new formulation of the SBS interaction that unifies the treatment of light and sound, incorporating all relevant interaction mechanisms—radiation pressure, waveguide boundary motion, electrostriction, and photoelasticity—from a rigorous thermodynamic perspective. Our approach also clarifies important points of ambiguity in the literature, such as the nature of edge effects with regard to electrostriction and of body forces with respect to radiation pressure. This new perspective on Brillouin processes leads to physical insight with implications for the design and fabrication of SBS-based nanoscale devices.

We study the coherent effect of the Casimir-Polder interaction on the oscillations of two-photon driven atoms. We find that, for oscillations between two degenerate states in $\mathrm{\Lambda }$ configuration, shifts on the Rabi frequency may be induced by nonadditive level shifts. For oscillations between two Rydberg states in ladder configuration, shifts on the Rabi frequency may be induced by the effective renormalization of the laser interaction.

The measurement fidelity of superconducting transmon and Xmon qubits is partially limited by the qubit energy relaxation through the resonator into the transmission line, which is also known as the Purcell effect. One way to suppress this energy relaxation is to employ a filter which impedes microwave propagation at the qubit frequency. We present semiclassical and quantum analyses for the bandpass Purcell filter realized by E. Jeffrey et al. [Phys. Rev. Lett. 112, 190504 (2014)]. For typical experimental parameters, the bandpass filter suppresses the qubit relaxation rate by up to two orders of magnitude while maintaining the same measurement rate. We also show that in the presence of a microwave drive the qubit relaxation rate further decreases with increasing drive strength.

Ionization energies for the ground state of berylliumlike ions with nuclear charge numbers in the range $16\le Z\le 96$ are rigorously evaluated. The calculations merge the ab initio QED treatment in the first and second orders of the perturbation theory in the fine-structure constant $\alpha$ with the third- and higher-order electron-correlation contributions evaluated within the Breit approximation. The nuclear recoil and nuclear polarization effects are taken into account. The accuracy of the ionization energies obtained has been significantly improved in comparison with previous calculations.

We theoretically study the control of the interferences of multiple tunneling photoelectron wave packets in both temporal and spatial domains by an orthogonally polarized two-color laser pulse. Profound carpetlike and stripelike interference patterns can be turned on or off in the momentum spectra using a weak streaking field at half the frequency of a strong fundamental field. The modulations of the interference patterns with respect to the relative phase between the two frequency components are well recaptured by both a semiclassical interference model and an ab initio simulation with numerically solving the time-dependent Schrödinger equation. We highlight the importance of the ionic Coulomb potential on the photoelectron angular distributions of atoms in the orthogonally polarized two-color pulses. It is shown that the interference induced by the forward rescattering trajectories is enhanced while the contribution of the direct trajectories is suppressed. This study offers alternative routes toward probing and controlling the ultrafast ionization dynamics of atoms and molecules.

We apply time-dependent $R$-matrix theory to study inner-shell ionization of C atoms in ultrashort high-frequency light fields with a photon energy between 170 and 245 eV. At an intensity of ${10}^{17}$ $W/{\mathrm{cm}}^2$, ionization is dominated by single-photon emission of a $2\ell$ electron, with two-photon emission of a $1s$ electron accounting for about 2–3% of all emission processes, and two-photon emission of $2\ell$ contributing about 0.5–1%. Three-photon emission of a $1s$ electron is estimated to contribute about 0.01–0.03%. Around a photon energy of 225 eV, two-photon emission of a $1s$ electron, leaving ${\mathrm{C}}^{+}$ in either $1s2s2p^3$ or $1s2p^4$, is resonantly enhanced by intermediate $1s2s^{2}2p^3$ states. The results demonstrate the capability of time-dependent $R$-matrix theory to describe inner-shell ionization processes including rearrangement of the outer electrons.

We consider a two-leg Bose-Hubbard ladder in the presence of a magnetic flux. We make use of Gross-Pitaevskii, Bogoliubov, bosonization, and renormalization group approaches to reveal a structure of ground-state phase diagrams in a weak-coupling regime relevant to cold atom experiments. It is found that, except for a certain flux $\varphi =\pi$, the system shows different properties as changing hoppings, which also leads to a quantum phase transition similar to the ferromagnetic $XXZ$ model. This implies that population-imbalance instability occurs for certain parameter regimes. On the other hand, for $\varphi =\pi$, it is shown that an umklapp process caused by commensurability of a magnetic flux stabilizes a superfluid with chirality and the system does not experience such a phase transition.

The fully covariant formulation of transformation optics is used to find the configuration of a cloaking device operating in an expanding universe modeled by a Friedmann-Lemaître-Robertson-Walker space-time. This space-time cloak is used as a platform for probing the covariant formulation of transformation optics, thereby rigorously enhancing the conceptual understanding of the theory. By studying the problem in both comoving and physical coordinates, we explicitly demonstrate the preservation of general covariance of electrodynamics under the transformation optics procedure. This platform also enables a detailed study of the various transformations that arise in transformation optics. We define a corporeal transformation as the “transformation” of transformation optics and distinguish it from coordinate and frame transformations. We find that corporeal transformations considered in the literature have generally been restricted to a subset of all possible corporeal transformations, providing a potential mechanism for increased functionality of transformation optics.

Coherent feedback control of quantum systems has demonstrable advantages over measurement-based control, but so far there has been little work done on coherent estimators and more specifically coherent observers. Coherent observers are input the coherent output of a specified quantum plant and are designed such that some subset of the observer's and plant's expectation values converge in the asymptotic limit. We previously developed a class of mean tracking (MT) observers for open harmonic oscillators that only converged in mean position and momentum; here we develop a class of covariance matrix tracking (CMT) coherent observers that track both the mean and the covariance matrix of a quantum plant. We derive necessary and sufficient conditions for the existence of a CMT observer and find that there are more restrictions on a CMT observer than there are on a MT observer. We give examples where we demonstrate how to design a CMT observer and show that it can be used to track properties like the entanglement of a plant. As the CMT observer provides more quantum information than a MT observer, we expect it will have greater application in future coherent feedback schemes mediated by coherent observers. Investigation of coherent quantum estimators and observers is important in the ongoing discussion of quantum measurement because they provide an estimation of a system's quantum state without explicit use of the measurement postulate in their derivation.

The spontaneous parity-time $\left(\mathcal{PT}\right)$ symmetry breaking is discussed in non-Hermitian $\mathcal{PT}$-symmetric Kitaev and extended Kitaev models whose Hermiticity is broken by the presence of two conjugated imaginary potentials $\pm i\gamma$ at two end sites. In the case of the non-Hermitian Kitaev model, a spontaneous $\mathcal{PT}$-symmetry breaking transition $\left(S\mathcal{PT}BT\right)$ occurs at a certain ${\gamma }_c$ in the topologically trivial phase (TTP) region, similar to that of the Su-Schrieffer-Heeger (SSH) model. However, unlike the SSH model, the system also undergoes such a transition in the topologically nontrivial phase (TNP) region. We study an extended Kitaev model by combining the superconducting pairing in the Kitaev model and the staggered hopping in the SSH model. This model contains three different topological phases: the TTP, the Kitaev-like TNP, and the SSH-like TNP. For the non-Hermitian extended Kitaev model, a $S\mathcal{PT}BT$ occurs in the Kitaev-like TNP region, as well as in part of the TTP and SSH-like TNP regions, whereas the $\mathcal{PT}$ symmetry is broken for an arbitrary nonzero $\gamma$ in the rest of the TTP and SSH-like TNP regions. Therefore, we can conclude that there is no universal correlation between topological properties and the $S\mathcal{PT}BT$.

In the Hermitian regime, the Berry phase is always a real number. It may be imaginary for a non-Hermitian system, which leads to amplitude amplification or attenuation of an evolved quantum state. We study the dynamics of the non-Hermitian Rice-Mele model driven by a time-dependent external field. The exact results show that it can have a full real spectrum for any value of the field. Several rigorous results are presented for the Berry phase with respect to the varying field. We demonstrate that the Berry phase is the same complex constant for any initial state in a single sub-band. Numerical simulation indicates that the amplitude control of a state can be accomplished by a quasiadiabatic process within a short time.

A special configuration of graph state stabilizers, which contains only Pauli ${\sigma }_X$ operators, is studied. The vertex sets $\xi$ associated with such configurations are defined as what we call $X$ chains of graph states. The $X$ chains of a general graph state can be determined efficiently. They form a group structure such that one can obtain the explicit representation of graph states in the $X$ basis via the so-called $X$-chain factorization diagram. We show that graph states with different $X$-chain groups can have different probability distributions of $X$-measurement outcomes, which allows one to distinguish certain graph states with $X$ measurements. We provide an approach to find the Schmidt decomposition of graph states in the $X$ basis. The existence of $X$ chains in a subsystem facilitates error correction in the entanglement localization of graph states. In all of these applications, the difficulty of the task decreases with increasing number of $X$ chains. Furthermore, we show that the overlap of two graph states can be efficiently determined via $X$ chains, while its computational complexity with other known methods increases exponentially.

Experimental observations of quantum walks in one dimension have provided many exciting applications in quantum computing, while recent theoretical investigation of single-phase defect in these system points towards interesting phenomena associated with bound states. Here we obtain analytical solutions of a quantum walk with a general quantum coin in one dimension with multiple defects, with predictions on the condition for zero reflectance for scattering state and the existence of an analogy to the Ramsauer effect for multiple defects. We also show the transition from the zero-reflectance state to the bound state can provide a method for preparing the quantum walk in a bound state. Applications to systems similar to thin film optics are suggested.

Macroscopic devices that probe and control elementary quantum systems are subjected to environment-induced decoherence and typically consist of a large number of particles that cannot be precisely controlled and individually accessed. These two factors limit the control of quantum systems by an open macroscopic device. Here, we investigate a faithful and robust implementation of specific elementary quantum operations under such conditions. We propose a procedure for quantum restoration of a single qubit that is resilient to particle loss and decoherence of the macroscopic device and compare it with a similarly robust procedure for indirect quantum detection of the qubit state by measurement on the macroscopic device. Complementary to detection, the restoration recovers the original state of the system qubit by quantum erasing which decouples the system from the device. Perfect restoration is possible even if the particles of the device are only classically correlated. We experimentally witness the robustness of quantum detection and restoration for a two-qubit device represented by the quantum state of photons.

The Mott scattering of high-energetic twisted electrons by atoms is investigated within the framework of the first Born approximation and Dirac's relativistic equation. Special emphasis is placed on the angular distribution and longitudinal polarization of the scattered electrons. In order to evaluate these angular and polarization properties we consider two experimental setups in which the twisted electron beam collides with either a single well-localized atom or macroscopic atomic target. Detailed relativistic calculations have been performed for both setups and for the electrons with kinetic energy from 10 to 1000 keV. The results of these calculations indicate that the emission pattern and polarization of outgoing electrons differ significantly from the scattering of plane-wave electrons and can be very sensitive to the parameters of the incident twisted beam. In particular, it is shown that the angular- and polarization-sensitive Mott measurements may reveal valuable information about both the transverse and longitudinal components of the linear momentum and the projection of the total angular momentum of twisted electron states. Thus, the Mott scattering emerges as a diagnostic tool for the relativistic vortex beams.

We present an investigation of resonances in positron-sodium scattering using the complex scaling method. For the target sodium atoms, the interaction between the core and outer electron is treated using two different types of analytical model potentials. Explicitly correlated Gaussian wave functions are used to represent the correlation effects between the outer electron, the positron, and the ${\text{Na}}^{+}$ core. $S-, P-$, and $D\text{-wave}$ resonances with natural parity have been calculated for energies extending up to the positronium $n=2$ formation threshold. Resonance states for unnatural parities $P^e$ and $D^0$ have been calculated for energies extending up to the positronium $n=3$ threshold. Below both positronium thresholds we have for each symmetry identified a dipole series of resonances, with binding energies scaling in good agreement with expectations from an analytical calculation. The presented results are compared with other theoretical calculations.

We study the quantum dynamics generated by the two-axis countertwisting Hamiltonian from an initial spin coherent state in a spin-$1/2$ ensemble. A characteristic feature of the two-axis countertwisting Hamiltonian is the existence of four neutrally stable and two saddle unstable fixed points. The presence of the latter is responsible for a high level of squeezing. The squeezing is accompanied by the appearance of several quantum states of interest in quantum metrology with Heisenberg-limited sensitivity, and we show fidelity functions for some of them. We present exact results for the quantum Fisher information and the squeezing parameter. Although the overall time evolution of both changes strongly with the number of particles, we find that they have regular dynamics for short times. We explain scaling with the system size by using a Gaussian approach.