We design a setup to realize tunable topological phases in elastic photonic crystals. Using the Su-Schrieffer-Heeger (SSH) model as a canonical example, we show how a system can be continuously tuned across its topological phase transition by stretching. We examine the setup both analytically and numerically, showing how the phase transition point may be identified from the behavior of bulk modes. Our design principle is generic, as it can be applied to a variety of systems, and enables multiple new theoretical predictions to be experimentally tested by continuously strain-tuning system properties, such as the shape of the band structure and the topological invariant. In addition, it allows for cost-effective device fabrication, since a wide range of parameter space can be accessed on a single photonic crystal chip.

A self-Q-switch regime based on the dual-frequency beat effect with a microchip crystal cavity is demonstrated. Unlike the microchip based passively Q-switched laser, the pulses are generated without a saturable absorber. It is the microchip's dual-crystal structure along with the beat effect that leads to the Q value's modulation and offers the possibility for generating Q-switched pulses. A Nd:YAG microchip and a $\mathrm{LiTa}{\mathrm{O}}_3$ microchip compose the laser cavity by the optical cement method. The stress and thermal induced anisotropy of the Nd:YAG crystal offers the dual-frequency component and the $\mathrm{LiTa}{\mathrm{O}}_3$ microchip offers polarization selection for the beat effect. The dual-frequency component experiences periodically changed loss with the variation of the two frequencies’ phase difference, due to the polarization selection by the $\mathrm{LiTa}{\mathrm{O}}_3$ chip. The principle of the beat effect based self-Q-switch regime is analyzed and Q-switched pulses with a repetition rate of around 1 MHz are achieved experimentally. This phenomenon gives us a way to obtain pulsed lasers and might be interesting to those conducting research on Q-switch lasers and light-borne microwave generation.

We explore the optomechanically induced transparency (OMIT) in a parity-time-symmetric ($\mathcal{PT}$-symmetric) optomechanical system (OMS) in which the mechanical oscillator is coupled to another one via the Coulomb interaction. Calculating the transmission rate of the probe field, we find that double optomechanically induced absorption (OMIA) can be observed by modulating the Coulomb coupling strength with zero gain rate of the active cavity. The absorption window for the system in the $\mathcal{PT}$-symmetry-broken phase is deeper than that in other phases due to the strong field localization. When the gain rate is not zero, the output field of the system exhibits double inverted OMIT. Both splits of the double OMIA and double OMIT depend linearly on the strength of the Coulomb interaction. The inverted OMIT can also be changed into an absorption window and the system can achieve perfect absorption by modulating the Coulomb coupling strength. The transmission light at the double inverted OMIT is always fast light and the transmission light at $\delta ={\omega }_m$ is always of slow light, independent of the phase the system in (the $\mathcal{PT}$-symmetry phase or the $\mathcal{PT}$-symmetry-broken phase). The absolute value of the group delay at $\delta ={\omega }_m$ can be manipulated by controlling the strength of the Coulomb interaction. These observations provide us with a tool to control light propagation which might have potential application in quantum optical devices and quantum information networks.

We theoretically explore the applications of a nonlinear circuit QED system, where a charge qubit is inductively coupled to an LC resonator, in the photonic engineering and ultrastrong-coupling multiphoton quantum optics. An arbitrary Fock-state pulsed maser, where the artificial qubit plays the gain-medium role, is achieved via simply sweeping the gate-voltage bias. The resonantly pumped parametric qubit-resonator interface leads to a squeezed intraresonator field, which is utilizable for the quantum-limited microwave amplification. Moreover, upwards and downwards multiphoton quantum jumps may be observed in the driving-free steady-state system.

We propose an all-optical scheme to control the photon statistics using the hybrid quantum plasmonic system. With the aid of dressed-state-assisted quantum interference effects, it is shown that the photon correlations of a signal field can be continuously modulated from bunching to antibunching under the control of a pump field. Apart from the exact multimode model, a single-mode model and an analytical treatment are also provided to help us identify the roles of multimode coupling and quantum interference between probability amplitudes. The proposed scheme, in contrast to the cavity quantum electrodynamics methods, works well even in the bad cavity limit. These findings suggest that this composite system provides a feasible nanophotonic platform for active modulation of photon statistics and for future quantum devices.

An exact analytical treatment of the dynamical problem for time-dependent $2\times 2$ pseudo-Hermitian $\text{su}(1,1)$ Hamiltonians is reported. A class of exactly solvable and physically transparent scenarios are identified within both classical and quantum contexts. The class is spanned by a positive parameter $\nu$ that allows for distinguishing two different dynamical regimes. Our results are usefully employed for exactly solving a classical propagation problem in a guided wave optics scenario. The usefulness of our procedure in a quantum context is illustrated by defining and investigating the $\text{su}(1,1)$ “Rabi” scenario, bringing to light analogies and differences with the standard su(2) Rabi model. Our approach, applied to the generalized von Neumann equation describing open quantum systems through non-Hermitian Hamiltonians, succeeds in evidencing that the $\nu$-dependent passage from a real to a complex energy spectrum is generally unrelated to the existence of the two dynamical regimes.

With a perturbative treatment based on the Keldysh Green's function technique, we study the resonant enhancement of nonlinear interaction effects in a four-mode optomechanical ring. In such a system, we identify five distinct types of resonant scattering between unperturbed polariton modes, induced by the nonlinear optomechanical interaction. By computing the cavity density of states and optomechanical induced transparency signal, we find that the largest nonlinear effects are induced by a decay process involving the two phononlike polaritons. In contrast to the conventional two-mode optomechanical system, our proposed system can exhibit prominent nonlinear features even in the regime when the single-photon coupling is much smaller than the cavity damping.

The unavoidable interaction of quantum systems with their environment usually results in the loss of desired quantum resources. Suitably chosen system Hamiltonians, however, can, to some extent, counteract such detrimental decay, giving rise to the set of stabilizable states. Here we discuss the possibility to stabilize Gaussian states in continuous-variable systems. We identify necessary and sufficient conditions for such stabilizability and elaborate these on two benchmark examples, a single, damped mode and two locally damped modes. The obtained stabilizability conditions, which are formulated in terms of the states' covariance matrices, are, more generally, also applicable to non-Gaussian states, where they may similarly help to, e.g., discuss entanglement preservation and/or detection up to the second moments.

We study the Schwinger electron-positron pair production by a strong electromagnetic field of two colliding $e$-polarized laser pulses with a relative phase shift $\mathrm{\Psi }$. The spatiotemporal distribution of created pairs is very sensitive to this phase shift and to polarization of the pulses. We study this dependence in detail and demonstrate how it can be explained in terms of the underlying invariant field structure of the counterpropagating focused pulses.

We present high-accuracy relativistic coupled-cluster calculations of the $P$-odd interaction coefficient $W_A$ describing the nuclear anapole moment effect on the molecular electronic structure. The molecule under study, BaF, is considered a promising candidate for the measurement of the nuclear anapole moment, and the preparation for the experiment is now underway [E. Altuntaş et al., Phys. Rev. Lett. 120, 142501 (2018)]. The influence of various computational parameters (size of the basis set, treatment of relativistic effects, and treatment of electron correlation) on the calculated $W_A$ coefficient is investigated and a recommended value of $147.7\mathrm{Hz}$ with an estimated uncertainty of $1.5\%$ is proposed.

We report the results of our theoretical studies of the permanent electric dipole moments (PDMs) of alkaline-earth-metal monohydride (AEMH) molecules, in their $X{}^2{\mathrm{\Sigma }}^{+}$ state, using a variant of the relativistic coupled-cluster theory in the singles and doubles approximation. It is the most detailed evaluation to date of the correlation effects associated with this property for this class of molecules. The molecules that have been considered in the present work are BeH, MgH, CaH, SrH, and BaH. We find that for SrH and BaH, the correlation contributions to the PDM are significant, about 18% and 21%, respectively, at the coupled-cluster singles and doubles level. All the core electrons have been excited in our calculations. We highlight the details of the individual contributions of different classes of correlation effects. For all the above molecular candidates, we have also made a comprehensive analysis of the possible sources of error in our calculations.

The nuclear recoil effect on the $g$ factor of highly charged Li-like ions is evaluated in the range $Z=10–92$. The calculations are performed by using $1/Z$ perturbation theory. The one-electron recoil contribution is evaluated within the fully relativistic approach with the wave functions which account approximately for the screening effects. The two-electron recoil contributions of the first and higher orders in $1/Z$ are calculated within the Breit approximation by using a four-component approach.

Photoionization of an atom $A$ by a strong laser field in the presence of a spatially well-separated neighboring atom $B$ is considered. The laser field frequency is assumed to lie below the ionization potential of atom $A$ and be resonant with a dipole-allowed transition in atom $B$. In this situation, the ionization may occur either directly by multiphoton absorption from the laser field at the first atomic center or via an indirect pathway involving two-center electron-electron correlations, where the neighbor atom $B$ is first photoexcited and, afterwards, transfers its energy upon deexcitation radiationlessly to atom $A$. Considering monochromatic as well as bichromatic laser fields, we study various coupling regimes of the photoionization process and identify experimentally accessible parameter domains where the two-center channel is dominant.

F. Grusdt et al. [New J. Phys. 19, 103035 (2017)] recently made a renormalization-group study of a one-dimensional Bose polaron in cold atoms. Their study went beyond the usual Fröhlich description, which includes only single-phonon processes, by including two-phonon processes in which two phonons are simultaneously absorbed or emitted during impurity scattering [Y. E. Shchadilova et al., Phys. Rev. Lett. 117, 113002 (2016)]. We study this same beyond-Fröhlich model, but in the static impurity limit where the ground state is described by a multimode squeezed state instead of the multimode coherent state in the static Fröhlich model. We solve the system exactly by applying the generalized Bogoliubov transformation, an approach that can be straightforwardly adapted to higher dimensions. Using our exact solution, we obtain a polaron energy free of infrared divergences and construct analytically the polaron phase diagram. We find that the repulsive polaron is stable on the positive side of the impurity-boson interaction but is always thermodynamically unstable on the negative side of the impurity-boson interaction, featuring a bound state, whose binding energy we obtain analytically. We find that the attractive polaron is always dynamically unstable, featuring a pair of imaginary energies which we obtain analytically. We expect the multimode squeezed state to help with studies that go not only beyond the Fröhlich paradigm but also beyond Bogoliubov theory, just as the multimode coherent state has helped with the study of Fröhlich polarons.

We present a proposal to control and improve the nonclassicality of photons in a single-atom cavity-QED system with a strong coupling strength, where the atom is directly driven by external fields. Exploring the eigenvalues and the corresponding eigenstates of the system, we show that the dressed states can be optically manipulated by changing the control field intensity. In particular, tuning the control field frequency to be resonant with the one-photon excitation state, we show that the nonclassicality of cavity photons arising from the two-photon blockade can be actively controlled. We also show that there exists a magic control field Rabi frequency at which the two-photon blockade phenomenon can be significantly improved. The work presented here provides an optical method to control the statistical features of the cavity field that could find utility in nonclassical light generation, photon gateway operation, and exotic quantum-state generation.

The Markovian evolution of an open quantum system is characterized by a positive entropy production, while the global entropy gets redistributed between the system and the environment degrees of freedom. Starting from these premises, we analyze the entropy variation of an open quantum system in terms of two distinct relations: the Clausius inequality, that provides an intrinsic bound for the entropy variation in terms of the heat absorbed by the system, and an extrinsic inequality, which instead relates the former to the corresponding entropy increment of the environment. By modeling the thermalization process with a Markovian collisional model, we compare and discuss the two bounds, showing that the latter is asymptotically saturated in the limit of large interaction time. In this regime not only the reduced density matrix of the system reaches an equilibrium configuration, but it also factorizes from the environment degrees of freedom. This last result is proven analytically when the system-bath coupling is sufficiently strong and through numerical analysis in the weak-coupling regime.

We introduce a method to construct non-Markovian variants of completely positive (CP) dynamical maps, particularly, qubit Pauli channels. We identify non-Markovianity with the breakdown in CP divisibility of the map, i.e., appearance of a not-completely positive intermediate map. In particular, we consider the case of non-Markovian dephasing in detail. The eigenvalues of the Choi matrix of the intermediate map crossover at a point which corresponds to a singularity in the canonical decoherence rate of the corresponding master equation and thus to a momentary noninvertibility of the map. Thereafter, the rate becomes negative, indicating non-Markovianity. We quantify the non-Markovianity by two methods, one based on CP divisibility [Hall et al., Phys. Rev. A 89, 042120 (2014)], which does not require optimization but requires normalization to handle the singularity, and another method, based on distinguishability [Breuer et al. Phys. Rev. Lett. 103, 210401 (2009)], which requires optimization but is insensitive to the singularity.

In a work by Berta et al. [Phys. Rev. A 90, 062127 (2014)], uncertainty relations in the presence of quantum memory were formulated for mutually unbiased bases using conditional collision entropy. In this paper, we generalize their results to the mutually unbiased measurements. Our primary result is an equality between the amount of uncertainty for a set of measurements and the amount of entanglement of the measured state, both of which are quantified by the conditional collision entropy. Implications of this equality relation are discussed. We further show that similar equality relations can be obtained for generalized symmetric informationally complete measurements. We also derive an interesting equality for arbitrary orthogonal basis of the space of Hermitian, traceless operators.

Adaptive measurements have recently been shown to significantly improve the performance of quantum state and process tomography. Adaptive tomography is especially useful in complicated experiments with low outcome generation rate, since it allows one to extract maximal information from a limited amount of data. However, the existing methods either cannot be simply applied to high-dimensional systems or are prohibitively computationally expensive. Here we propose and experimentally implement a tomographic protocol specially designed for the reconstruction of high-dimensional quantum states. The protocol shows qualitative improvement in infidelity scaling with the number of measurements and is fast enough to allow for complete state tomography of states with dimensionality up to 36.

In a recent letter [Phys. Rev. Lett. 118, 253001 (2017)] we have described how studies of the recoil velocity distribution in the photodissociation of OCS in the energy interval 42 600–42 900 ${\mathrm{cm}}^{-1}$ revealed an unexpected behavior: the recoil velocity distribution of only the lowest-kinetic-energy photofragments exhibited rapid, resonantlike variations with energy and caused complete inversion of the recoil direction. Periodic orbit analysis and quantum nonadiabatic calculations unveiled the existence of a resonance state localized at large bending angles towards the exit of the dissociation channel. In this article, we present an extensive theoretical study and we show how the fingerprints of these resonances are identified by the analysis of the nonadiabatic transitions and the stereodynamics of photofragments trajectories. Additionally, the experimental study is extended to a second photolysis energy region, 43 300–43 650 ${\mathrm{cm}}^{-1}$, where a similar rapid variation of the recoil direction is detected. The energy separation between this second resonance region and the one previously reported is $\sim 800{\mathrm{cm}}^{-1}$, which is twice the calculated period of the localized resonant state, offering a second point of convergence between the experiment and the theory.