We have systematically investigated the optical analog of the inverse Klein tunneling effect—a quantum relativistic effect emerging from the Dirac equation and that is inverse to the well-known Klein tunneling effect. Inverse Klein tunneling can be observed if an electron moves through an inverse potential step from one region with high potential into the adjacent region with low potential. We propose to mimic this effect by launching a Dirac soliton around Dirac points in binary waveguide arrays where the inverse potential step can be easily created by introducing an offset for refractive indices of the waveguides. We obtain the analytical formulas for the transmission coefficient of a plane wave through the inverse potential step in the discrete model with binary waveguide arrays and in the continuous model. As expected, around the Dirac points these two theoretical results are in good agreement. We also show that the theoretical transmission coefficient in binary waveguide arrays is in excellent agreement with the numerical results obtained directly via the beam propagation simulation method. This verification confirms the validity of our model and theoretical results on inverse Klein tunneling in binary waveguide arrays. Some important relations for the transmission coefficient while comparing the inverse Klein tunneling effect and the Klein tunneling effect are also provided.

Like energy and momentum, optical helicity is a fundamental dynamical property of light. In the prevalent plane wave and paraxial description of light the optical helicity is directly proportional to the degree of circular polarization, being zero for both linearly and unpolarized fields. Here it is shown that the nonparaxial fields generated by tightly focused optical vortices which have the phase factor $exp\left(i\ell \varphi \right)$ possess a contribution to the optical helicity density that is completely independent of the polarization state of the source paraxial field. In stark contrast to what is known in classical optics with plane waves and paraxial light, the physical consequence is that unpolarized light can exhibit optical activity and chiral light-matter interactions.

Spatiotemporal extreme events are interesting phenomena, both from a fundamental point of view, as manifestations of complexity in dynamical systems, and for their possible applications in different research fields. Here, we present some recent results for extreme events in spatially extended semiconductor laser systems (broad-area vertical cavity surface-emitting lasers) with coherent injection. We study the statistics of spatiotemporal intensity peaks occurring in the transverse $(x,y)$ section of the field perpendicular to the light propagation direction and identify regions in the parameter space where extreme events are more likely to occur. Searching for precursors of these phenomena, we concentrate, on the one hand, on the spatiotemporal dynamics of the field phase and, in particular, on the presence of optical vortices in the vicinity of an extreme event. On the other hand, we focus on the laser gain dynamics and the phase-space trajectories of the system close to the occurrence of an extreme event. Both these complementary approaches are successful and allow us to shed some light on potential prediction strategies.

We analyze the collective light scattering by cold atoms in free space of a pump-laser beam possessing orbital angular momentum. We derive a set of coupled multiparticle equations for the atomic motion in which the vacuum-mode field is adiabatically eliminated. The resulting equations describe collective recoil as due to either transfer of linear momentum or orbital angular momentum. For a transverse annular atomic distribution the initial equilibrium with uniform atomic phases and no scattered field is unstable. The atoms are set in rotation and bunched in phase at different harmonics depending on the pump azimuthal index $\ell$ and on the ring radius.

For a generic semiclassical laser dynamics in the complex Ginzburg-Landau form, we develop a Bogoliubov approach for the computation of the laser emission linewidth. Our method provides a unifying perspective of the treatments by Henry and Petermann: both broadening mechanisms are ascribed to the nonorthogonality of the Bogoliubov modes, which live in a space with doubled degrees of freedom. As an example of application, the method allows one to study the interplay of drive dissipation, interactions, and spatial inhomogeneity typical of polariton condensates. The traditional theory of the Henry and Petermann factors is found to fail dramatically in the presence of sizable optical nonlinearities stemming from polariton-polariton interactions. Even in a strong confining potential, the intrinsically multimode nature of the density fluctuations has to be included in order to quantitatively describe phase diffusion in the manner of Henry.

In this paper, we investigate the effect of gyromagnetic terms on the topology of isofrequency surfaces. The diagonal terms of the permeability tensor are known to govern the topology of isofrequency surfaces. However, here we show that the off-diagonal gyrotropic terms of the material tensor also govern the existence of isofrequency surfaces. The relative strength of the gyrotropic term with respect to the permeability term decides the number of isofrequency surfaces which can exist in a gyromagnetic medium. We show that gyrotropy can either suppress an isofrequency surface or support an otherwise nonexistent isofrequency surface. Moreover, the dependence of isofrequency surfaces on gyrotropy is linked to the photonic spin of these isofrequency surfaces. Further, the photonic-spin profile indicates that the photonic spin is locked to the material and not the direction of momentum. This leads to a conflict between spin-momentum locking and material-locked photonic spin, which can be exploited in waveguiding structures to get an asymmetric mode profile and gyrotropy-induced cutoff.

Radiation reaction (RR) is the oldest still-unsolved problem in electrodynamics. In addition to conceptual difficulties in its theoretical formulation, the requirement of exceedingly large charge accelerations has thus far prevented its unambiguous experimental identification. Here, we show how measurable RR effects in a laser-electron interaction can be achieved through the use of flying focus pulses (FFPs). By allowing the focus to counterpropagate with respect to the pulse phase velocity, a FFP overcomes the intrinsic limitation of a conventional laser Gaussian pulse (GP) that limits its focus to a Rayleigh range. For an electron initially also counterpropagating with respect to the pulse phase velocity, an extended interaction length with the laser peak intensity is achieved in a FFP. As a result, the same RR deceleration factors are obtained, but at FFP laser powers orders of magnitude lower than for ultrashort GPs with the same energy. This renders the proposed setup much more stable than those using GPs and allows for more accurate in situ diagnostics. Using the Landau-Lifshitz equation of motion, we show numerically and analytically that the capability of emerging laser systems to deliver focused FFPs will allow for a clear experimental identification of RR.

We catalog known optical moiré lattices and uncover exotic lattice configurations following a geometric analog of the ancient sieve of Eratosthenes algorithm for finding prime numbers. Rich dynamics of Bose-Einstein condensates loaded into these optical lattices is revealed from numerical simulations of time-of-flight interference patterns. What sets this method apart is the ability to tune the periodicity of the optical lattices without changing the wavelength of the laser, yet maintaining the local potential at the individual lattice sites. In addition, we discuss the ability to spatially translate the optical lattice through applying a structured phase only.

We present a generalized quantum scattering circuit which can be used to perform a noninvasive quantum measurement and implement it on NMR qubits. Such a measurement is a key requirement for testing temporal noncontextual inequalities. We use this circuit to experimentally demonstrate the violation of the Peres-Mermin inequality on a three-qubit NMR quantum information processor. Further, we experimentally construct a Bell-type inequality corresponding to the temporal Klyachko-Can-Binicioglu-Shumovsky (KCBS) inequality, and demonstrate that the maximum quantum correlation is achieved for four two-point correlation functions involved in this inequality. The experimental violation of all the inequalities matches well with the theoretically predicted values, within experimental errors.

Thermodynamic quantities, such as heat and work, are not functions of state, but rather of the process undergone by a physical system. Assessing them can therefore be difficult, since it requires probing the system at least twice. This is even more so when these quantities are to be assessed at the stochastic level. In this paper we show how to obtain optimal estimates of thermodynamic quantities solely from indirect measurement unravelings of an auxiliary system. The method always yields the true average, and the mean-squared error of the prediction is directly proportional to how well the method estimates the variance. As an application, we study energy fluctuations in a driven system, and in an avoided crossing work protocol.

We follow a generalized kinematic approach to compute the geometric phases acquired in both unitary and dissipative Jaynes-Cummings models, which provide a fully quantum description for a two-level system interacting with a single mode of the (cavity) electromagnetic field, in a perfect or dissipative cavity, respectively. In the dissipative model, the nonunitary effects arise from the outflow of photons through the cavity walls and the incoherent pumping of the two-level system. Our approach allows to compare the geometric phases acquired in these models, leading to an exhaustive characterization of the corrections introduced by the presence of the environment. We also provide geometric interpretations for the observed behaviors. When the resonance condition is satisfied, we show the geometric phase is robust, exhibiting a vanishing correction under a nonunitary evolution. This fact is supported with a geometrical explanation as well.

Bound entanglement was shown to be activated [P. Horodecki et al., Phys. Rev. Lett. 82, 1056 (1999)] in the sense that the entanglement of a spatially separated two-qutrit system can be increased with nonzero probability via a sufficiently large number of preshared bound-entangled states, local three-level controlled operations, and classical communications. Here we present a local quantum Zeno scheme for activating bound entanglement which is based only on single-particle rotations and threshold measurements. In our scheme, neither a large number of bound-entangled states nor controlled operations are required, and classical communication is required only once at the end of the protocol. We show that a single bound-entangled state is sufficient for increasing the negativity of the target entangled state from 0.11 to 0.17, and by using four more bound-entangled states, negativity can be made greater than 0.42 and the fidelity to the maximally entangled state increases from 0.3 to 0.41, 0.50, 0.59, and 0.61. We believe our results are important not only for quantum technologies but also for a better understanding of quantum entanglement.

Geometric gates that use the global property of the geometric phase is believed to be a powerful tool to realize fault-tolerant quantum computation. However, for singlet-triplet qubits in semiconductor quantum dot, the low Rabi frequency of the microwave control leads to overly long gating time, and thus the constructing geometric gate suffers more from the decoherence effect. Here we investigate the key issue of whether the fast geometric gate can be realized for singlet-triplet qubits without introducing an extra microwave-driven pulse, while maintaining the high-fidelity gate operation at the same time. We surprisingly find that both the single- and two-qubit geometric gates can be implemented via only modulating the time-dependent exchange interaction of the Hamiltonian, which can typically be on the order of $\sim \mathrm{GHz}$, and thus the corresponding gate time is of several nanoseconds. Furthermore, the obtained geometric gates are superior to their counterparts, i.e., the conventional dynamical gates for singlet-triplet qubits, with a relatively high fidelity surpassing 99%. Therefore our scheme is particularly applied to singlet-triplet qubits to obtain fast and high-fidelity geometric gates. Our scheme can also be extended to other systems without a microwave drive.

Twists are defects in the lattice which can be utilized to perform computations on encoded data. Twists have been studied in various classes of topological codes like qubit and qudit surface codes, qubit color codes, and qubit subsystem color codes. They are known to exhibit projective non-Abelian statistics, which is exploited to perform encoded gates. In this paper, we initiate the study of twists in qudit color codes over the odd prime alphabet. Specifically, we present a systematic construction of twists in qudit color codes that permute both charge and color of the excitations. We also present a mapping between generalized Pauli operators and strings in the lattice. Making use of the construction, we give protocols to implement generalized Clifford gates using charge-and-color-permuting twists.

Implementation of many quantum information protocols requires an efficient initialization of the quantum register. In the present paper, we optimize a population trapping protocol for initializing a hybrid spin register associated with a single nitrogen-vacancy (NV) center in diamond. We initialize the quantum register by polarizing the electronic and nuclear spins of the NV with a sequence of microwave, radio-frequency, and optical pulses. We use a rate equation model to explain the distribution of population under the effect of the optical pulses. The model is compared to the experimental data obtained by performing partial quantum state tomography. To further increase the spin polarization, we propose a recursive protocol with optimized optical pulses. We also discuss the role of the relative values of the nuclear- and electronic-spin pumping rate in achieving the maximum degree of spin polarization.

We analyze the scattering problem of identical bosonic particles confined on a spherical surface. At low scattering energies and for a radius much larger than the healing length, we express the contact interaction strength in terms of the $s$-wave scattering length. Adopting this relation, we are then able to regularize the zero-point energy of the spherical Bose gas and to obtain its equation of state, which includes the corrections due to the finite radius of the sphere and coincides with the flat-case result in the infinite-radius limit. We also provide a microscopic derivation of the superfluid density of the system, reproducing a result postulated in a previous work. Our results are relevant for modeling the ongoing microgravity experiments with two-dimensional bubble-trapped Bose-Einstein condensates.

The controlled creation of dark-bright (DB) soliton trains in multicomponent Bose-Einstein condensates (BECs) is a topic of ongoing interest. In this work we generalize earlier findings on the creation of dark soliton trains in single-component BECs [A. Romero-Ros et al., Phys. Rev. A 103, 023329 (2021)] to two-component BECs. By choosing suitable filled box-type initial configurations (FBTCs) and solving the direct scattering problem for the defocusing vector nonlinear Schrödinger equation with nonzero boundary conditions we obtain analytical expressions for the DB soliton solutions produced by a general FBTC. It is found that the size of the initial box and the amount of filling directly affect the number, size, and velocity of the solitons, while the initial phase determines the parity (even or odd) of the solutions. Our analytical results are compared to direct numerical integration of the coupled Gross-Pitaevskii equations, both in the absence and in the presence of a harmonic trap, and an excellent agreement between the two is demonstrated.

The spectral phase of high-order harmonic and attosecond pulses is typically shaped by the interaction of the recollision electron with the strong field in the continuum. However, the phase of the transition-moment coupling bound and continuum states can be significant in shaping the emitted radiation. It has commonly been assumed that the propagation and recombination steps of the recollision process can be described independently. Here, we investigate the effect that the transition moment has recollision trajectories by incorporating the transition-moment phase into the Lewenstein model of recollision. We then use our model to investigate the all-optical measurement of the transition-moment phase around the Cooper minimum in argon and the spectral minimum due to two-center interference in a diatomic molecular system. Our results indicate that, while all-optical methods are generally sensitive to the transition-moment phase, they are insensitive to the phase shifts due to two-center interference and ionic structure. Thus, we have resolved the apparent discrepancy between studies with conflicting conclusions regarding the sensitivity of all-optical approaches to the transition-moment phase. Our work demonstrates that all-optical measurements focus on photorecombination time delays attributable to electronic structure and dynamics. Our method will allow any laboratory capable of generating attosecond pulses to perform these measurements, even at wavelengths at which the single-photoionization cross section becomes small.