We design optimal interferometric schemes for the implementation of two-qubit linear-optical quantum filters diagonal in the computational basis. The filtering is realized by the interference of the two photons encoding the qubits in a multiport linear-optical interferometer, followed by conditioning on presence of a single photon in each output port of the filter. The filter thus operates in the coincidence basis, similarly to many linear-optical unitary quantum gates. The implementation of a filter with linear optics may require an additional overhead in terms of a reduced overall success probability of the filtering and the optimal filters are those that maximize the overall success probability. We discuss in detail the case of symmetric real filters and extend our analysis also to asymmetric and complex filters.

Quantum metrology is the use of genuinely quantum properties such as entanglement as a resource to outperform classical sensing strategies. Typically, entanglement is created by implementing gate operations or inducing many-body interactions. However, existing sensing schemes with these approaches require accurate control of the probe system such as switching on and off the interaction among qubits, which can be challenging for practical applications. Here, we propose an entanglement-enhanced sensing scheme with an always-on nearest-neighbor interaction between qubits. We adopt the transverse field Ising chain as the probe system, making use of the so-called quantum domino dynamics for the generation of the entangled states. In addition to the advantage that our scheme can be implemented without controlling the interactions, it only requires initialization of the system, projective measurements on a single qubit, and control of the uniform magnetic fields. We can achieve an improved sensitivity beyond the standard quantum limit even under the effect of realistic decoherence.

While Efimov physics in ultracold atoms is usually modeled with an isolated Feshbach resonance, many real world resonances appear in close vicinity to each other and are therefore overlapping. Here we derive a realistic model based on the mutual coupling of an open channel and two closed molecular channels while neglecting short-range physics as permitted by the narrow character of the considered resonances. The model is applied to three distinct scenarios with experimental relevance. We show that the effect of overlapping resonances is manifested most strikingly at a narrow resonance in whose vicinity there is a slightly narrower one. In this system the Efimov ground state extends not only over the scattering length zero crossing between the two resonances but also over the pole of the second resonance to finally meet the dissociation threshold below it. In the opposite scenario, when a narrow resonance is considered in the vicinity of a slightly broader one, we observe that the Efimov features are pushed to lower binding energies and smaller scattering lengths by a significant factor facilitating their experimental investigation. Both scenarios are compared with the case of two narrow resonances which are far enough away from each other to be effectively decoupled. In this case the two-channel model results are recovered. Finally, we analyze the rich excitation spectrum of the system and construct and explain its nodal pattern.

In this study, we used differential geometry to develop an effective field theory to study the behavior of light propagation in distorted photonic crystals (DPCs) of averagely homogeneous refractive index media. To study the light-ray trajectories in DPCs, we derived a geodesic equation based on the principle of least action by defining the metric tensor in terms of the lattice-position distortion. The geodesic equation implies that the lattice-position distortion can curve the trajectory. We present multiple exact solutions for the trajectory for simple distortion under the condition of having the same values of the lattice point filling factor at each unit cell. These solutions explicitly demonstrate that the light is only bent through the introduction of lattice distortion, and that these results are consistent with the finite-difference time-domain simulation results.

We investigate the interaction of laser pulses carrying orbital angular momentum (OAM) with a symmetry-broken ladder-type quantum coupling scheme involving three internal states. A weak probe beam acts on the lower leg of the ladder scheme, while a control beam of higher intensity drives the upper leg. In contrast to natural atoms, such a model with broken symmetry allows generating a sum-frequency signal beam between the most upper and lower quantum states, forming a cyclic closed-loop configuration of light-matter interaction. We propose situations for the efficient transfer of optical vortices to the generated signal beam via a nonlinear three-wave mixing process. It is demonstrated that the exchange process can occur both in the electromagnetically induced transparency (EIT) and the Autler-Townes splitting (ATS) regimes. The transition between the EIT and ATS conversion schemes can smoothly happen by simply tuning the knob of the control field. It is shown that the ATS regime is considerably more favorable than the EIT to achieve maximum energy conversion efficiency between light beams carrying the OAM. The results may provide an applications-based perspective to the ongoing research centered on vortex conversion-based comparisons between the ATS and EIT.

We have performed single-active-electron numerical calculations of neon- and argonlike atoms interacting with short, intense, circularly polarized laser pulses at wavelengths between 10 nm and 1000 nm. Our results reveal a surprisingly large change by a factor of about 100 in the ionization ratio of electrons in initial states counter-rotating with respect to the field over corotating electrons in the previously unexplored intermediate few-photon ionization regime. The physical mechanism behind this observation is related to resonant enhanced ionization via states close in energy to the initial states. These doorway states are accessible exclusively from the initial state for counter-rotating electrons within the respective wavelength regime. The results may open a new route for controlling the generation of spin-polarized electrons by ultrashort laser pulses.

The scattering amplitude in simple quantum graphs is a well-known process which may be highly complex. In this work, motivated by the Shannon entropy, we propose a methodology that associates a graph with a scattering entropy, which we call the average scattering entropy. It is defined by taking into account the period of the scattering amplitude which we calculate using the Green's function procedure. We first describe the methodology on general grounds and then exemplify our findings considering several distinct groups of graphs. We go on and investigate other possibilities, one that contains groups of graphs with the same number of vertices, with the same degree, and the same number of edges, with the same length, but with distinct topologies and with different entropies. Another possibility we investigate contains graphs of the fishbone type, where the scattering entropy depends on the boundary conditions on the vertices of degree 1, with the corresponding values decreasing and saturating very rapidly as we increase the number of elementary structures in the graphs.

Quantum uncertainty relations are formulated in terms of relative entropy between distributions of measurement outcomes and suitable reference distributions with maximum entropy. This type of entropic uncertainty relation can be applied directly to observables with either discrete or continuous spectra. We find that a sum of relative entropies is bounded from above in a nontrivial way, which we illustrate with some examples.

Nitrogen vacancy (NV) centers in diamond have attracted considerable recent interest for use in quantum sensing, promising increased sensitivity for applications ranging from geophysics to biomedicine. Conventional sensing schemes involve monitoring the change in red fluorescence from the NV center under green laser and microwave illumination. Due to the strong fluorescence background from emission in the NV triplet state and low relative contrast of any change in output, sensitivity is severely restricted by a high optical shot noise level. Here, we propose a means to avoid this issue, by using the change in green pump absorption through the diamond as part of a semiconductor external cavity laser run close to the lasing threshold. We show that theoretical sensitivity to the magnetic field on the pT/$\sqrt{\mathrm{Hz}}$ level is possible using a diamond with an optimal density of NV centers. We discuss the physical requirements and limitations of the method, particularly the role of amplified spontaneous emission near threshold and explore realistic implementations using current technology.

Characterization of quantum devices received from unknown providers is a significant primary task for any quantum information processing protocol. Self-testing protocols are designed for this purpose of certifying quantum components from the observed statistics under a set of minimal assumptions. Here we propose a self-testing protocol for certifying binary Pauli measurements employing the violation of a Leggett-Garg inequality. The scenario based on temporal correlations does not require entanglement, a costly and fragile resource. Moreover, unlike previously proposed self-testing protocols in the prepare and measure scenario, our approach requires neither dimensional restrictions, nor other stringent assumptions on the type of measurements. We further analyze the robustness of this hitherto unexplored domain of self-testing of measurements.

The angular distribution of the anions formed in the dissociative electron attachment (DEA) to ${\mathrm{H}}_2, {\mathrm{D}}_2$, and HD is studied for the 10-eV resonance using the velocity slice imaging technique. The angular distributions of ${\mathrm{H}}^{-}$ and ${\mathrm{D}}^{-}$ from HD are found to be identical and very similar to that of ${\mathrm{H}}^{-}$ (${\mathrm{D}}^{-}$) from ${\mathrm{H}}_2$ (${\mathrm{D}}_2$) indicating that the distinct dissociation limits for ${\mathrm{H}}^{-}$ and ${\mathrm{D}}^{-}$ channels and the small but finite dipole moment of HD do not affect the DEA process. The angular distributions suggest that the main contribution for this resonance is from the capture of $s$ and $d$ partial waves of the attaching electron confirming the resonance to be a ${}^2{{\mathrm{\Sigma }}_g}^{+}$ state. The relative amplitudes of the two partial waves as a function of electron energy also indicate the contribution from a higher-lying ${}^2{{\mathrm{\Sigma }}_g}^{+}$ state through possible predissociation at higher electron energies.

Rogue wave in ultrafast fiber lasers is a hot research topic of nonlinear optics, which is generally monitored at the fixed cavity positions, despite its dramatic intra-cavity evolution. The shot-to-shot statistical properties change significantly if the chaotic pulse experiences strong changes along the cavity, which is rarely considered in the past. Here, we simulate the dynamic evolution of the rogue wave in the dispersion-managed fiber laser. Attributing to the temporal structure change induced by the dispersion, the rogue wave within the noise-like pulse appears in a limited cavity region while disappears in the rest parts. It is also demonstrated that the dispersion is capable of manipulating rogue waves in the pulse trains outside the cavity. Our results offer insightful views on the properties of chaotic waves in ultrafast lasers.

We consider an optical resonator containing a photonic crystal fiber and driven coherently by an injected beam. This device is described by a generalized Lugiato-Lefever equation with fourth-order dispersion. We use an asymptotic approach to derive interaction equations governing the slow time evolution of the coordinates of two interacting dissipative solitons. We show that Cherenkov radiation induced by positive fourth-order dispersion leads to a strong increase of the interaction force between the solitons. As a consequence, a large number of equidistant soliton bound states in the phase space of the interaction equations can be stabilized. We show that the presence of even small spectral filtering not only dampens the Cherenkov radiation at the soliton tails and reduces the interaction strength, but can also affect the bound state stability.

We develop a method to characterize arbitrary superpositions of light orbital angular momentum (OAM) with high fidelity by using astigmatic transformation and machine-learning processing. In order to identify each superposition unequivocally, we combine two intensity measurements. The first one is the direct image of the input beam, which is invariant for positive and negative OAM components. The second one is an image obtained using an astigmatic transformation, which allows distinguishing between positive and negative topological charges. Samples of these image pairs are used to train a convolution neural network and achieve high-fidelity recognition of arbitrary OAM superpositions.

Following our work [Phys. Rev. Lett. 125, 020401 (2020)], we discuss a semiclassical description of one-dimensional quantum tunneling through multibarrier potentials in terms of complex time. We start by defining a complex-extended continuum level density of unbound systems and show its relation to a complex time shift of the transmitted wave. While the real part of the level density and time shift describes the passage of the particle through classically allowed coordinate regions, the imaginary part is connected with an instantonlike picture of the tunneling through forbidden regions. We describe singularities in the real and imaginary parts of the level density and time shift caused by stationary points of the tunneling potential, and show that they represent a dual extension of excited-state quantum phase transitions from bound to continuum systems. Using the complex scaling method, we numerically verify the predicted effects in several tunneling potentials.

We consider an open quantum system of $N$ interacting spin-1/2 particles on a $d$-dimensional lattice whose time evolution is (weakly) symmetric under the action of a finite group $G$ that is a subgroup of all the possible permutations of the lattice sites. We define a neural network representation for the system density operator that explicitly accounts for the system symmetries, and we show that its properties are preserved when using variational techniques such those recently used in the optimization of restricted-Boltzmann machines (RBMs). As a proof of principle, we demonstrate the validity of our approach by determining the steady state structure of the one-dimensional dissipative XYZ model in the presence of a uniform magnetic field and uniform dissipation. We first show a direct comparison between the results obtained by optimizing a RBM representation for the density operator and those obtained by means of the invariant neural network defined in the present paper, and then we show results concerning the convergence to the unique steady state configuration when using our approach only. Numerical results support the conjecture that the invariant Ansatz defined in this paper is more expressive that a standard RBM and can be optimized with lesser numerical efforts.

Using the quantum teleportation in continuous variables as a test scheme, we compare two entangled transformations—mixing of signals on a beam splitter and by the controlled-$Z$ (cz) operation. We evaluate these transformations in terms of the errors added to teleported oscillators. We have shown that the cz operation leads to the lower error of teleportation. This error can be further reduced by choosing appropriate weight coefficients for the cz transforms. We have compared the errors of the theoretical cz scheme and its practical implementation in the optical design. Although the cz optical scheme adds intrinsic noise to the overall transformation, it is nevertheless possible to specify the parameters which provides a gain in comparison with the traditional teleportation protocol.

The analogy between quantum chemistry and light-front quantum field theory, first noted by Wilson, serves as motivation to develop light-front quantum simulation of quantum field theory. We demonstrate how calculations of hadron structure can be performed on noisy intermediate-scale quantum devices within the basis light-front quantization (BLFQ) framework. Within BLFQ, relativistic quantum field theories take a form that permits direct application of methods for digital quantum simulation of quantum chemistry, which can be readily scaled into the quantum advantage regime. We calculate the light-front wave functions of pions using an effective light-front Hamiltonian in a basis representation on a current quantum processor. We use the variational quantum eigensolver to find the ground-state energy and the corresponding wave function, which is subsequently used to calculate pion mass radius, decay constant, elastic form factor, and charge radius.

The single, double, triple, and quadruple Auger decays of ${\mathrm{Ar}}^{+}\left(2s^{-1}\right)$ are studied, based on many-body perturbation theory. The level-to-level investigation indicates that the double Auger primarily comes from the cascade process involving the Coster-Kronig decay. Moreover, the complex triple Auger and quadruple Auger decay are expressed by the multistep approach, i.e., the combination of cascade and direct processes. On the other hand, by employing the separate orbital sets for optimizing the single-electron wave functions, our ion yield ratios (${\mathrm{Ar}}^{2+}:{\mathrm{Ar}}^{3+}:{\mathrm{Ar}}^{4+}:{\mathrm{Ar}}^{5+}=3.5:88.7:7.5:0.26$) agree quite well with the recent experimental data (3:89:8:0.3) [Lablanquie et al., Phys. Chem. Chem. Phys. 13, 18355 (2011)]. Moreover, the single, double, and triple Auger spectra, as well as lifetime, are all in accord with the measurements of Lablanquie et al.