In this article, we investigate the efficiency of photonic state tomography in the presence of fiber attenuation. The theoretical formalism of the photon loss is provided by implementing methods from the theory of open quantum systems. The quantum state is reconstructed from photon counts obtained for symmetric informationally complete positive operator-valued measures. The number of photons that reach the detectors is numerically modeled by the binomial distribution, which describes the loss of light caused by the medium. This approach allows us to study the quality of state tomography versus the length of the fiber. In particular, we focus on entangled qubits and qutrits, which are sent through fibers of different lengths. The amount of entanglement detected by the measurement scheme is quantified and presented on graphs. The results demonstrate how the quality of photonic tomography depends on the distance between the source and the receiver.

We provide an analytical formula for the relative intensities of hyperfine components of rovibrational transitions in the ${\mathrm{H}}_2, {\mathrm{T}}_2$, and para-${\mathrm{D}}_2$ isotopologues of hydrogen. The resulting expression is essentially an algebraic factor independent of vibrational quantum numbers. For other molecular hydrogen isotopologues, we show that it is possible to distinguish a certain class of hyperfine components which hold a similar property. The results presented here can be used as an approximation for rovibrational transitions in other diatomic molecules in a ${}^1\mathrm{\Sigma }$ electronic state with smaller rotational constant.

Electron-impact excitation from the ground state to the excited energy level $2p_{3/2}$ of hydrogenlike ions and subsequent Lyman-${\alpha }_1$ $(2p_{3/2}\to 1s_{1/2})$ radiative decay are investigated using the relativistic distorted-wave method. Special attention is paid to the linear polarization and angular distribution of the Lyman-${\alpha }_1$ line and also to the effects of the Breit interaction. To this aim, detailed calculations are performed for hydrogenlike ${\mathrm{Ti}}^{21+}$, ${\mathrm{Mo}}^{41+}$, ${\mathrm{Ba}}^{55+}$, and ${\mathrm{Au}}^{78+}$ ions. It is found that the presently obtained (partial cross sections and) linear polarization agree excellently with other theoretical and experimental results available for low-$Z{\mathrm{Ti}}^{21+}$ ions within the experimental uncertainties. Moreover, the Lyman-${\alpha }_1$ line is found to be less linearly polarized and less anisotropic due to the contribution of the Breit interaction. Such effects of the Breit interaction behave more prominently for higher-$Z$ ions and higher impact electron energies, respectively. For instance, the Breit interaction qualitatively changes the polarization behavior and angular emission pattern of the Lyman-${\alpha }_1$ line from high-$Z{\mathrm{Au}}^{78+}$ ions at the impact energy of about 4.2 times the corresponding excitation threshold.

The contribution of bound electrons to the electronic stopping power is modeled by using the shellwise local plasma approximation (SLPA). An improvement to this formalism is proposed by including the Levine-Mermin dielectric function (SLPA-LM). This approach considers a more realistic description of the bound electron response by facing the problem of the damping in the collective excitations. We introduce a local damping that depends on the density of electrons of each shell, keeping the full-theoretical characteristic of the SLPA. We implement the present model to obtain the stopping power for metal targets of period-6 elements of the periodic table (Hf, Ta, W, Os, and Pt) and we analyze the importance of the SLPA-LM description of heavy multielectronic targets. Furthermore, we investigated metal targets of group VI of the periodic table (Cr, Mo, and W) to inspect the dependence of the SLPA-LM with the number of subshells.

We present precise values of electric polarizabilities for the ground state of Zn due to second-order dipole and quadrupole interactions, and due to third-order dipole-quadrupole interactions. These quantities are evaluated in the linear response theory framework by employing a relativistic version of the normal coupled-cluster (NCC) method. The calculated dipole polarizability value is compared with available experimental and other theoretical results, including those obtained using the ordinary coupled-cluster (CC) methods in both finite-field and expectation value evaluation approaches. We also give a term-by-term comparison of contributions from our CC and NCC calculations in order to show differences in the results from these two methods. Moreover, we present results from other lower-order methods to understand the role of electron correlation effects in the determination of the above quantities. A machine-learning-based scheme to generate optimized basis functions for atomic calculations is developed and applied here. From the analysis of the dipole polarizability result, the accuracy of the calculated quadrupole and third-order polarizability values is ascertained, for which no experimental values are currently available.

Tunable attosecond transient absorption spectroscopy is an ideal tool for studying and manipulating autoionization dynamics in the continuum. We investigate near-resonant two-photon couplings between the bright $3s^{-1}4p$ and dark $3s^{-1}4f$ autoionizing states of argon that lead to Autler-Townes-like interactions, forming entangled light-matter states or polaritons. We observe that one-photon couplings with intermediate dark states play an important role in this interaction, leading to the formation of multiple polaritonic branches whose energies exhibit avoided crossings as a function of the dressing-laser frequency. Our experimental measurements and theoretical essential-state simulations show good agreement and reveal how the delay, frequency, and intensity of the dressing pulse govern the properties of autoionizing polariton multiplets. These results demonstrate new pathways for quantum control of autoionizing states with optical fields.

We calculate properties of dipolar interacting ultracold molecules or Rydberg atoms in a semisynthetic three-dimensional configuration—one synthetic dimension plus a two-dimensional real-space optical lattice or periodic microtrap array—using the stochastic Green's function quantum Monte Carlo method. Through a calculation of thermodynamic quantities and appropriate correlation functions, along with their finite-size scalings, we show that there is a second-order transition to a low-temperature phase in which two-dimensional “sheets” form in the synthetic dimension of internal rotational or electronic states of the molecules or Rydberg atoms, respectively. Simulations for different values of the interaction $V$, which acts between atoms or molecules that are adjacent both in real and synthetic space, allow us to compute a phase diagram. We find a finite-temperature transition at sufficiently large $V$ as well as a quantum phase transition—a critical value $V_c$ below which the transition temperature vanishes.

Light transport in a disordered ensemble of resonant atoms placed in a waveguide is found to be very sensitive to the sizes of the cross section of a waveguide. Based on a self-consistent quantum microscopic model treating atoms as coherent radiating dipoles, we have shown that the nature of radiation transfer changes from Anderson localization regime in a single-mode waveguide to a traditional diffuse transfer in a multimode one. Moreover, the transmittance magnitude undergoes complicated steplike nonmonotonic dependence on the transverse sizes of a waveguide.

In the recent past, optical tweezers incorporating a stratified medium have been exploited to generate complex translational and rotational dynamics in mesoscopic particles due to the coupling between the spin and orbital angular momentum of the light, generated as a consequence of the tight focusing of light by a high numerical aperture objective lens into the stratified medium. Here, we consider an optical tweezers system with a tilted stratified medium (direction of stratification at an angle with the axis of the incident beam), and show that for input circularly polarized Gaussian beams, the resulting spin-orbit interaction deeply influences the generation of transverse spin angular momentum (TSAM) and Belinfante momentum of light, and allows additional control on their magnitude. Importantly, the TSAM generated in our system consists of both of the orthogonal components, which is in sharp contrast to the case of evanescent waves and surface plasmons, where only one of the TSAM components is generated. The broken symmetry due to the tilt ensures that depending upon the helicity of the input beam, the magnitude of the mutually orthogonal components of the TSAM depends entirely on the tilt angle. This may prove to be an effective handle in the exotic spin-controlled manipulation of particles in experiments.

Although attosecond transient absorption spectroscopy (ATAS) has been widely used in the study of ultrafast dynamics of atoms, molecules, and solids, there is a need for further theoretical exploration of macroscopic effects beyond the response of single atoms. Here, the extreme ultraviolet (XUV) absorbance of He atoms in the presence of a delayed near-infrared (NIR) pulse is studied theoretically, from the limit of dilute-gas medium to high densities. By numerically solving the time-dependent Schrödinger equation coupled with Maxwell's equations, we elucidate the macroscopic signatures in the absorption spectrum of an optically dense medium, including previously recognized XUV spectral pulse shaping, a complex spectral splitting, and a persistent quantum beating, based on the energies of the states involved. An analytical expression for optical density is deduced directly to pinpoint the physical origins of the various absorption features in a quantitative way, which helps one grasp the intuitive picture of the macroscopic absorption spectrum and provides guidance to explore applications such as optical pulse shaping and XUV molecular quantum beat spectroscopy.

The collisionless Boltzmann equation is used to describe the intensity correlations in partially saturated and fully saturated regimes in terms of photon distribution function in the phase space. Explicit expression for fourth moment of the light fields is obtained for the case of moderate and strong turbulence. Such expression consists of two terms accounting for two regions in the phase space that independently contribute to the correlation function. It is shown that the present solution agrees with previous results for the fully saturated regime. Additionally it embodies the effect of partially saturated radiation where the correlations of photon trajectories are important and the magnitude of the scintillation index is well above the unity. The fourth moment is used to study the fluctuations of transmittance, which consider the effect of finite detector aperture.

We investigate a scheme to guide a weak vector beam using optically written waveguides inside atomic vapor, while controlling its polarization rotation. The atoms are prepared in the four-level tripod configuration in the presence of an external magnetic field. The three transitions of the tripod system are driven by a strong control beam and the two orthogonal polarization components of a weak vector beam (VB). A suitable spatial intensity profile and the detuning of the control field facilitate the generation of an optical fiberlike refractive index gradient across the atomic vapor. This enables waveguiding of the weak VB for a couple of Rayleigh lengths. Further, the magnetic-field-induced anisotropy of the tripod system creates a difference in the refractive indices of its two components. This is the reason behind the polarization shaping of VB. The mechanism of efficient guiding of VB with controllable polarization may have important applications in high-density optical communication and quantum information.

Hong-Ou-Mandel (HOM) interference is a key phenomenon for understanding quantum interference and a basic tool for quantum information and metrological applications. By introducing the nonlinear frequency-conversion (NFC) technique into HOM interferometers, more phenomena could be found and more functions could be developed. Here, tunable quantum spatial beating is observed in a HOM interferometer based on the postfilter scheme, where two NFC modules function as tunable band-pass filters. Degenerate IR photon pairs at 1079.00 nm are generated based on the noncritical phase-matching spontaneous parametric down-conversion process and are up-converted to the visible band around 633.40 nm by the NFC modules before the detectors. We show that in addition to the filtering effect, the NFC can also improve the photon detection efficiency based on silicon avalanched detector, increasing the coherence length, and therefore improving the heralding efficiency and visibility. Our results demonstrate the advantages of the NFC technique in HOM interference, which could also be generalized to other quantum interference.

Superconducting nanowire single-photon detectors (SNSPDs) are efficient measurement devices used for counting single photons. The field of their applications covers experimental quantum-optical studies, optical quantum computing, quantum communication, and others. After registering a photon by such a detector, the next one cannot be registered during the dead time, and after that this ability is smoothly restored. We have included this feature into the photodetection theory and introduced the corresponding photocounting formula. In the regime of continuous-wave detection, the photocounting statistics nonlinearly depends on the density operator due to a memory effect of previous measurement time windows. The considered examples demonstrate the strong influence of the relaxation process and the memory effect on the resulting photocounting statistics of the SNSPDs.

Generalized cat states represent arbitrary superpositions of coherent states, which are of great importance in various quantum information processing protocols. Here we demonstrate a versatile approach to creating generalized itinerant cat states in the microwave domain, by reflecting coherent-state photons from a microwave cavity containing a superconducting qubit. We show that with a coherent control of the qubit state, a full control over the coherent-state superposition can be realized. The prepared cat states are verified through quantum state tomography of the qubit-state-dependent reflection photon field. We further quantify quantum coherence in the prepared cat states based on the resource theory, revealing a good experimental control on the coherent-state superpositions. The photon number statistic and the squeezing properties are also analyzed. Remarkably, fourth-order squeezing is observed in the experimental states. Those results open up new possibilities of applying generalized cat states for the purpose of quantum information processing.

We consider a quantum system with a time-independent Hamiltonian parametrized by a set of unknown parameters $\alpha$. The system is prepared in a general quantum state by an evolution operator that depends on a set of unknown parameters $P$. After the preparation, the system evolves in time, and it is characterized by a time-dependent observable $\mathcal{O}\left(t\right)$. We show that it is possible to obtain closed-form expressions for the gradients of the distance between $\mathcal{O}\left(t\right)$ and a calculated observable with respect to $\alpha , P$, and all elements of the system density matrix, whether for pure or mixed states. These gradients can be used in projected gradient descent to infer $\alpha , P$, and the relevant density matrix from dynamical observables. We combine this approach with random phase wave function approximation to obtain closed-form expressions for gradients that can be used to infer population distributions from averaged time-dependent observables in problems with a large number of quantum states participating in dynamics. The approach is illustrated by determining the temperature of molecular gas (initially, in thermal equilibrium at room temperature) from laser-induced time-dependent molecular alignment.

We study a variant of quantum circuit complexity, the binding complexity: Consider an $n$-qubit system divided into two sets of $k_1, k_2$ qubits each ($k_1\le k_2$) and gates within each set are free; what is the least cost of two-qubit gates “straddling” the sets for preparing an arbitrary quantum state, assuming no ancilla qubits allowed? First, our work suggests that, without making assumptions on the entanglement spectrum, $\mathrm{\Theta }\left(2^{k_1}\right)$ straddling gates always suffice. We then prove any $\text{U}\left(2^n\right)$ unitary synthesis can be accomplished with $\mathrm{\Theta }\left(4^{k_1}\right)$ straddling gates. Furthermore, we extend our results to multipartite systems, and show that any $m$-partite Schmidt decomposable state has binding complexity linear in $m$, which hints its multiseparable property. This result not only resolves an open problem posed by Vijay Balasubramanian, who was initially motivated by the complexity = volume conjecture in quantum gravity, but also offers realistic applications in distributed quantum computation in the near future.

In theoretical machine learning, the statistical complexity is a notion that measures the richness of a hypothesis space. In this work, we apply a particular measure of statistical complexity, namely, the Rademacher complexity, to the quantum circuit model in quantum computation and study how the statistical complexity depends on various quantum circuit parameters. In particular, we investigate the dependence of the statistical complexity on the resources, depth, width, and the number of input and output registers of a quantum circuit. To study how the statistical complexity scales with resources in the circuit, we introduce a magic resource measure based on the $(p,q)$ group norm, which quantifies the amount of magic resource in the quantum channels associated with the circuit. These dependencies are investigated in the following two settings: (i) where the entire quantum circuit is treated as a single quantum channel, and (ii) where each layer of the quantum circuit is treated as a separate quantum channel. The bounds we obtain can be used to constrain the capacity of quantum neural networks in terms of their depths and widths as well as the resources in the network.