Terahertz time-domain spectroscopy is increasingly used in many fields of research. For strongly absorbing materials with refraction index close to 1, optical parameters at terahertz frequencies are most conveniently quantified using transmission measurements through thin samples. Unfortunately, extracting optical parameters from raw data implies the use and/or development of complicated numerical data processing procedures. In this work we present an efficient computational procedure for extracting the optical parameters in very thin samples $(\lesssim 100\mu \text{m})$ from transmission terahertz time-domain spectroscopy. In our procedure, we are able to successfully remove from raw data the Fabry-Perot interference effects, which are commonly recognized to be the leading cause of inaccuracy in the extracted parameters, introducing fictitious oscillations in their frequency dependence. The procedure is based on the Davidenko method to identify the roots of complex functions used to numerically solve the implicit equation obtained by equating the experimental and theoretical transfer functions. The advantage of the method is the possibility of obtaining the roots using the numerical solution of a system of real differential equations using standard mathematical packages. In addition, we show that complete removal of the Fabry-Perot oscillations is achieved by including in the computational procedure, besides the sample thickness, the instrumental error on the starting instant of the terahertz signal sampling. This error could be common to many terahertz time-domain systems, especially those using optical fibers. This correction is necessary in general to preserve the terahertz spectroscopic features in the extracted optical parameters for strongly absorbing materials with refraction index close to 1, such as water, biological matter, and several organic materials.

Assuming the standard axioms for quaternionic quantum theory and a spatially localized scattering interaction, the $S$ matrix in quaternionic quantum theory is complex valued, not quaternionic. Using the standard connections between the $S$ matrix, the forward scattering amplitude for electromagnetic wave scattering, and the index of refraction, we show that the index of refraction is necessarily complex, not quaternionic. This implies that the recent optical experiment of Procopio et al. [Nat. Commun. 8, 15044 (2017)] based on the Peres proposal does not test for hypercomplex or quaternionic quantum effects arising within the standard Hilbert space framework. Such a test requires looking at near zone fields, not radiation zone fields.

Quantum states of light having a Wigner function with negative values represent a key resource in quantum communication and quantum information processing. Here we present the generation of such a state at the telecommunication wavelength of 1550 nm. The state is generated by means of photon subtraction from a weakly squeezed vacuum state and is heralded by the “click” of a single photon counter. Balanced homodyne detection is applied to reconstruct the Wigner function, also yielding the state's photon-number distribution. The heralding photons are frequency up-converted to 532 nm to allow for the use of a room-temperature (silicon) avalanche photodiode. The Wigner function reads $W(0,0)=-0.063\pm 0.004$ at the origin of phase space, which certifies negativity with more than 15 standard deviations.

We study the thermalization of an ensemble of $N$ elementary, arbitrarily complex, quantum systems, mutually noninteracting but coupled as electric or magnetic dipoles to a blackbody radiation. The elementary systems can be all the same or belong to different species, distinguishable or indistinguishable, located at fixed positions or having translational degrees of freedom. Even if the energy spectra of the constituent systems are nondegenerate, as we suppose, the ensemble unavoidably presents degeneracies of the energy levels and/or of the energy gaps. We show that, due to these degeneracies, a thermalization analysis performed by the popular quantum optical master equation reveals a number of serious pathologies, possibly including a lack of ergodicity. On the other hand, a consistent thermalization scenario is obtained by introducing a Lindblad-based approach, in which the Lindblad operators, instead of being derived from a microscopic calculation, are established as the elements of an operatorial basis with squared amplitudes fixed by imposing a detailed balance condition and requiring their correspondence with the dipole transition rates evaluated under the first-order perturbation theory. Due to the above-mentioned degeneracies, this procedure suffers a basis arbitrariness, which, however, can be removed by exploiting the fact that the thermalization of an ensemble of noninteracting systems cannot depend on the ensemble size. As a result, we provide a clear-cut partitioning of the thermalization time into dissipation and decoherence times, for which we derive formulas giving the dependence on the energy levels of the elementary systems, the size $N$ of the ensemble, and the temperature of the blackbody radiation.

We study the properties of Hamiltonians defined as the generators of transfer matrices in quasi-one-dimensional waveguides. For single- or multimode waveguides obeying flux conservation and time-reversal invariance, the Hamiltonians defined in this way are non-Hermitian, but satisfy symmetry properties that have previously been identified in the literature as “pseudo-Hermiticity” and “anti-$\mathcal{P}\mathcal{T}$ symmetry”. We show how simple one-channel and two-channel models exhibit transitions between real, imaginary, and complex eigenvalue pairs.

Anderson localization is a consequence of coherent interference of multiple scattering events in the presence of disorder, which leads to an exponential suppression of the transmission. The decay of the transmission is typically probed at a given energy or frequency. Here we show that this decay affects the dynamics of a qubit coupled to the disordered system and we express the relaxation rate of the qubit in terms of the localization properties. Conversely, adding static disorder to a channel coupled to a qubit will reduce the decoherence rate of the qubit. Hence, when designing electrodes that couple to qubits, it is possible to improve their performance by adding impurities to the channel.

In this paper we consider the excitation of one quantum dipole by another in the deep quantum limit. We use a full quantum mechanical theory to describe the interaction of the dipoles through the electromagnetic field. Our nonperturbative analytical calculations result in the exact solution. We show that minimal quantum uncertainty of the dipole oscillation amplitudes, taken at different times, have a retarded character. It is demonstrated that the commutator of the dipole oscillation amplitudes becomes nonzero inside the light cone only. Moreover, due to radiation in free space the value of the commutator has a global maximum.

The single-electron detachment (SED) cross sections of $\mathrm{F}{\mathrm{e}}^{-}$, $\mathrm{R}{\mathrm{u}}^{-}$, $\mathrm{N}{\mathrm{i}}^{-}$, $\mathrm{P}{\mathrm{d}}^{-}$, and $\mathrm{P}{\mathrm{t}}^{-}$ in collisions with inert-gas atoms are measured by using the growth-rate method. The projectile energy ranges from 10 to 30 keV. The cross sections increase as the impact velocity increases. For anions in the same element group, the data can be fitted into a straight line. The positions of fitting lines for different groups are different. In our velocity region, the intersections of fitting lines for different inert gases appear in the Cu group and the Ni group, but not in the Fe group. A model based on inelastic collision is used to explain the experimental results. Some conclusions are reached by analyzing the experimental data. The electron configuration of the anion plays an important role in SED cross sections. The mass relationship between the anion projectile and the inert-gas atom target is also an essential factor. The ionization energy of the target also has an influence on the SED cross sections.

Two-photon ionization of xenon was investigated theoretically for exciting-photon energies from 6.7 to 11.5 eV, which results in the ionization of Xe between $5p_{1/2}$ (13.43 eV) and $5s$ (23.40 eV) thresholds. We describe the extension of a previously developed computational technique for the inclusion of relativistic effects to calculate energies of intermediate resonance state and cross sections for two-photon ionization. Reasonable consistency of cross sections calculated in length and velocity form was obtained only after considering many-electron correlations. Agreement between calculated and measured resonance energies is found when core polarization was additionally included in the calculations. The presently computed two-photon photoionization cross sections of Xe are compared with Ar cross sections in our previous work. Photoelectron angular distribution parameters calculated here indicate that intermediated resonances strongly influence photoelectron angular distribution of Xe.

Experimental and theoretical investigations of the wavelength dependence of the nonsequential double ionization (NSDI) process of the xenon atom in intense laser fields are reported. The observed wavelength dependence of the ratio ${\mathrm{Xe}}^{2+}/{\mathrm{Xe}}^{+}$ deviates significantly from the prediction of the three-step model of ionization (sometimes called the “simple-man” model), both in the slope of the overall decrease with increasing wavelength and in the existence of some pronounced humps. A semiclassical model calculation reproduces the changing slope of the overall decrease, which is due to the interplay of wave-packet diffusion, Coulomb focusing, and a closely related Coulomb defocusing effect of the liberated electron. The hump is beyond the scope of the semiclassical model.

We study the Bose-Einstein condensation of photons in a plasma, where we include the cases of both transverse photons and plasmons. We consider four-wave mixing processes of photon and plasmon modes in a relativistic isotropic plasma to determine the coupling constant to lowest order. We further show that photon condensation is possible in an unbounded plasma because, in contrast with other optical media, plasmas introduce an effective photon mass. This guarantees the existence of a finite chemical potential and a critical temperature, which is calculated for both transverse photons and plasmons. By considering four-wave mixing processes, we derive the interactions between the photons in the condensate. We also study the elementary excitations (or Bogoliubov modes) of the condensed photon and plasmon gases, and determine the respective dispersion relations. Finally, we discuss the kinetics of photon condensation via inverse Compton scattering between the photons and the electrons in the plasma.

An innovative way to produce quantum Hall ribbons in a cold atomic system is to use $M$ hyperfine states of atoms in a one-dimensional optical lattice to mimic an additional “synthetic dimension.” A notable aspect here is that the SU($M$) symmetric interaction between atoms manifests as “infinite ranged” along the synthetic dimension. We study the many-body physics of fermions with SU($M$) symmetric attractive interactions in this system using a combination of analytical field theoretic and numerical density-matrix renormalization-group methods. We uncover the rich ground-state phase diagram of the system, including unconventional phases such as squished baryon fluids, shedding light on many-body physics in low dimensions. Remarkably, changing the parameters entails interesting crossovers and transition; e.g., we show that increasing the magnetic field (that produces the Hall effect) converts a “ferrometallic” state at low fields to a “squished baryon superfluid” (with algebraic pairing correlations) at high fields. We also show that this system provides a unique opportunity to study quantum phase separation in a multiflavor ultracold fermionic system.

In nonlinear fiber systems operating in pulsed regime, the pumping excitation is no longer time extended (cw regime) but localized in time with a finite duration. In this case, standard linear stability analysis (often called modulational instability), as it stands, fails to describe the linear time evolution inside the fiber and one has to reformulate the problem as an initial value problem leading to convective and absolute instabilities. We show, in normal dispersion regime, that the dynamical evolution of a localized probe in a highly birefringent optical fiber results in a modulational instability splitting into convective and absolute instabilities. We have also characterized the splitting in terms of coupling of the group-velocity mismatch between the two polarized states and the total injected pump power. We evidenced a transition from absolute to convective regime and characterized the qualitative difference of the dynamics in the two regimes. The results of the numerical integration of the governing equations are in excellent agreement with our analytical findings and may contribute to understanding the complex dynamics appearing in the time domain of the supercontinuum generation that plays a key role in the formation of optical rogue waves.

We study the properties of bisqueezed tripartite Gaussian states created by two spontaneous parametric down-conversion processes that share a common idler. We give a complete description of the quantum correlations across all partitions, as well as of the genuine multipartite entanglement, obtaining analytical expressions for most of the quantities of interest. We find that the state contains genuine tripartite entanglement, in addition to the bipartite entanglement among the modes that are directly squeezed. We also investigate the effect of homodyne detection of the photons in the common idler mode, and analyze the final reduced state of the remaining two signal modes. We find that this measurement leads to a conversion of the coherence of the two signal modes into entanglement, a phenomenon that can be regarded as a redistribution of quantum resources between the modes. The applications of these results to quantum optics and circuit quantum electrodynamics platforms are also discussed.

How to effectively construct robust quantum gates for time-varying noise is a very important but still outstanding problem. Here we develop a systematic method to find pulses for quantum gate operations robust against both low- and high-frequency (comparable to the qubit transition frequency) stochastic time-varying noise. Our approach, taking into account the noise properties of quantum computing systems, can output single smooth pulses in the presence of multisources of noise. Furthermore, our method can be applied to different system models and noise models and will make essential steps toward constructing high-fidelity and robust quantum gates for fault-tolerant quantum computation. Finally, we discuss and compare the gate operation performance by our method with that by the filter-transfer-function method.

Quantum walks and random walks bear similarities and divergences. One of the most remarkable disparities affects the probability of finding the particle at a given location: typically, almost a flat function in the first case and a bell-shaped one in the second case. Here I show how one can impose any desired stochastic behavior (compatible with the continuity equation for the probability function) on both systems by the appropriate choice of time- and site-dependent coins. This implies, in particular, that one can devise quantum walks that show diffusive spreading without losing coherence as well as random walks that exhibit the characteristic fast propagation of a quantum particle driven by a Hadamard coin.

We study the two-body scattering problem in the zero-range approximation with a sinusoidally driven scattering length and calculate the relation between the mean value and amplitude of the drive for which the effective scattering amplitude is resonantly enhanced. In this manner we arrive at a family of curves along which the effective scattering length diverges but the nature of the corresponding Floquet-induced resonance changes from narrow to wide. Remarkably, on these curves the driving does not induce heating. To study the effect of these resonances on the three-body problem we consider one light and two heavy particles with driven heavy-light interaction in the Born–Oppenheimer approximation and find that the Floquet driving can be used to tune the three-body and inelasticity parameters.

Fulde-Ferrell (FF) Larkin-Ovchinnikov (LO) phases were proposed for superconductors or superfluids in a strong magnetic field. With the experimental progress in ultracold atomic systems, topological FFLO phases have also been put forward since it is a natural consequence of realizable spin-orbital coupling (SOC). In this paper, we theoretically investigate a triangular lattice model with SOC and an in-plane field. By constructing the phase diagram, we show that it produces gapped and gapless topological FF phases with Chern numbers $C=\pm 1,\pm 2,-3$, and $-4$. We get the phase boundaries by the change in the sign of the Pfaffian. The chiral edge states for different topological FF phases also are elucidated.

We provide a theory for quantum-optical realizations of the open Dicke model with internal, atomic spin states subject to spontaneous emission with rate $\gamma$. This introduces a second decay channel for excitations to irreversibly dissipate into the environment, in addition to the photon loss with rate $\kappa$, which is composed of individual atomic decay processes and a collective atomic decay mechanism. The strength of the latter is determined by the cavity geometry. We compute the mean-field nonequilibrium steady states for spin and photon observables in the long-time limit, $t\to \infty$. Although $\gamma$ does not conserve the total angular momentum of the spin array, we argue that our solution is exact in the thermodynamic limit for the number of atoms $N\to \infty$. In light of recent and upcoming experiments realizing superradiant phase transitions using internal atomic states with pinned atoms in optical lattices, our work lays the foundation for the pursuit of a new class of open quantum magnets coupled to quantum light.

We explore the optomechanically induced absorption (OMIA) in a parity-time- ($\mathcal{PT}$-) symmetric optomechanical system (OMS). By numerically calculating the Lyapunov exponents, we find out the stability border of the $\mathcal{PT}$-symmetric OMS. The results show that in the $\mathcal{PT}$-symmetric phase the system can be either stable or unstable depending on the coupling constant and the decay rate. In the $\mathcal{PT}$-symmetric broken phase the system can have a stable state only for small gain rates. By calculating the transmission rate of the probe field, we find that there is an inverted optomechanically induced transparency (OMIT) at $\delta =-{\omega }_M$ and an OMIA at $\delta ={\omega }_M$ for the $\mathcal{PT}$-symmetric optomechanical system. At each side of $\delta =-{\omega }_M$ there is an absorption window due to the resonance absorption of the two generated supermodes. Comparing with the case of optomechanics coupled to a passive cavity, we find that the active cavity can enhance the resonance absorption. The absorption rate at $\delta ={\omega }_M$ increases as the coupling strength between the two cavities increases. Our work provides us with a promising platform for controlling light propagation and light manipulation in terms of $\mathcal{PT}$ symmetry, which might have potential applications in quantum information processing and quantum optical devices.