Beam splitters are not-free operations with regard to quantum coherence. As a consequence, they can create coherence from both coherent and incoherent states. We investigate the increase in coherence produced by cascades of beam splitters. To this end, we construct two different configurations and analyze different sequences of input states.

Measuring an observable which does not commute with the Hamiltonian of a quantum system usually modifies the mean energy of this system. In an autonomous measurement scheme, coupling the system to a quantum meter, the system's energy change must be compensated by the meter's energy change. Here, we theoretically study such an autonomous meter-system dynamics: a qubit interacting dispersively with a light pulse propagating in a one-dimensional waveguide. The phase of the light pulse is shifted, conditioned to the qubit's state along the $z$ direction, while the orientation of the qubit Hamiltonian is arbitrary. As the interaction is dispersive, photon number is conserved so that energy balance has to be attained by spectral deformations of the light pulse. Building on analytical and numerical solutions, we reveal the mechanism underlying this spectral deformation and display how it compensates for the qubit's energy change. We explain the formation of a three-peak structure of the output spectrum and we provide the conditions under which this is observable.

The role of completely positive–indivisibility (CP-indivisibility) and incompatibility as valuable resources for various information-theoretic tasks is widely acknowledged. This study delves into the intricate relationship between CP-divisibility and channel compatibility. Our investigation focuses on the behavior of incompatibility robustness of quantum channels for a pair of generic dynamical maps. We show that the incompatibility robustness of channels is monotonically nonincreasing for a pair of generic CP-divisible dynamical maps. Further, our explicit study of the behavior of incompatibility robustness with time for some specific dynamical maps reveals nonmonotonic behavior in the CP-indivisible regime. Additionally, we propose a measure of CP-indivisibility based on the incompatibility robustness of quantum channels. Our investigation provides valuable insights into the nature of quantum dynamical maps and their relevance in information-theoretic applications.

The electromagnetic trapping of ion chains can be regarded as a process of nontrivial entangled quantum state preparation within Hilbert spaces of the local axial motional modes. To begin uncovering properties of this entanglement resource produced as a by-product of conventional ion-trap quantum information processing, the quantum continuous-variable formalism is herein utilized to focus on the leading-order entangled ground state of local motional modes in the presence of a quadratic trapping potential. The decay of entanglement between disjoint subsets of local modes is found to exhibit features of entanglement structure and responses to partial measurement reminiscent of the free massless scalar field vacuum. With significant fidelities between the two, even for large system sizes, a framework is established for initializing quantum field simulations by “imaging” extended entangled states from natural sources, rather than building correlations through deep circuits of few-body entangling operators. By calculating probabilities in discrete Fock subspaces of the local motional modes, we present considerations for locally transferring these predistributed entanglement resources to the qudits of ion internal energy levels, improving this procedure's anticipated experimental viability.

Nonclassical features of quantum systems have the potential to strengthen the way we currently exchange information. In this paper, we explore this enhancement on the most basic level of single particles. To be more precise, we compare how well multiparty information can be transmitted to a single receiver using just one classical or quantum particle. Our approach is based on a multiple-access communication model in which messages can be encoded into a single particle that is coherently distributed across multiple spatial modes. Theoretically, we derive lower bounds on the accessible information in the quantum setting that strictly separate it from the classical scenario. This separation is found whenever there is more than one sender, and also when there is just a single sender who has a shared phase reference with the receiver. When there is only one sender, the separation is impossible without a shared phase reference due to the famous Holevo's bound. Experimentally, we present a proof-of-principle demonstration of such quantum advantage in single-particle communication by implementing a multiport interferometer with messages being encoded along the different trajectories. Specifically, we consider a two-sender communication protocol built by a three-port optical interferometer. In this scenario, the rate sum achievable with a classical particle is upper bounded by one bit, while we experimentally observe a rate sum of $1.0152\pm 0.0034$ bits in the quantum setup.

In this work, we report triple differential cross sections for the single ionization of ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{C}}_4{\mathrm{H}}_8\mathrm{O}$ by electron impact at intermediate to low impact energies, calculated within the framework of the CDW-EIS model considering two different approximations to the continuum electrons' interaction with the molecular ion in the final state. These calculations are benchmarked with recently reported experimental data and two state-of-the-art theoretical models. Present results suggest that a spherical average of the anisotropic potential associated to the residual ion improves the description of these collision processes for electron emission into the scattering and perpendicular planes, with respect to approximating it as a single center of charge $+1$, whereas full-perpendicular emission is best described by this simpler approximation.

For electronic systems with multireference (MR) character, Kohn-Sham density-functional theory (KS-DFT) with the conventional exchange-correlation (xc) energy functionals can lead to incorrect spin densities and related properties. For example, for ${\mathrm{H}}_2$ dissociation, the spin-restricted and spin-unrestricted solutions obtained with the same xc energy functional in KS-DFT can be distinctly different, yielding the unphysical spin-symmetry breaking effects in the spin-unrestricted solutions. Recently, thermally-assisted-occupation density-functional theory (TAO-DFT) has been shown to resolve the aforementioned spin-symmetry breaking, when the fictitious temperature is properly chosen. In this work, a response theory based on TAO-DFT is developed to demonstrate that TAO-DFT with a sufficiently large fictitious temperature can always resolve the unphysical spin-symmetry breaking in MR systems. To further support this, TAO-DFT calculations with various fictitious temperatures are performed for the dissociation of ${\mathrm{H}}_2, {\mathrm{N}}_2, {\mathrm{He}}_2$, and ${\mathrm{Ne}}_2$ as well as the twisted ethylene.

Theoretical characterization of the photoionization mechanism in atoms and molecules necessitates a precise depiction of the ionization continuum. However, the accurate inclusion of the correlated electronic wave function at short range, particularly for intricate multicenter molecules, poses a significant challenge. In this study, we have devised a multicenter continuum-state approach by solving the coupled-channel Schrödinger equation. This approach enables the computation of molecular frame photoelectron angular distributions (MFPADs) induced by both plane-wave and twisted photons. To elucidate our methodology, we present calculations focusing on the ${\mathrm{CO}}_2$ C 1s orbital. Our computations exhibit favorable agreements with experimental observations and established theoretical frameworks for plane-wave photoionization. For the twisted-photon ionization, the MFPADs demonstrate independence from the orbital angular momentum (OAM) of the twisted light, provided no restrictions are imposed on the impact parameters. The MFPADs exhibit varying dependence on the opening angle for varying propagation directions of twisted photons, when considering linearly polarized vectors lying within the cone surface. Additionally, for a comprehensive comprehension of the OAM-dependent photoionization process, we investigate the MFPADs resulting from placing the molecule at the phase singularity in the twisted center.

We investigate the transition from a dark soliton to vortices in a Bose-Einstein condensate, achieved through the adiabatic manipulation of external potentials. Our primary focus is on the fundamental transition process, where a single dark soliton generates a single vortex with a $\pm 1$ topological charge in a symmetric configuration. We reveal the emergence of this transition near the condition that the transverse characteristic length exceeds twice the healing length of the condensate, where some higher excited states are always involved in the transition process. The two topological charges of a fundamental vortex are generated with equal probability, and the final charge can be effectively manipulated by introducing some weak asymmetric operations. Our results provide an alternative approach for generating vortices with controllable charges and numbers from dark solitons, and open an avenue for studying the interactions of vortices based on dark-soliton-permitted systems.

Regarding the high-order harmonic generation (HHG) of solids, the intra- and interband contributions have been disputed for over a decade now. The prior scheme in this context has been limited to resolve the temporal HHG profile. In this work we propose that the real-space intra- and intercell dynamics in crystal structure respectively correspond to the reciprocal-space inter- and intraband dynamics. Thus, we utilize the polarization-resolved symmetry of the HHG yield to clarify its dominant term among the inter- and intraband contributions. For simplicity, we necessarily exclude the impact of the propagation effect and therefore take the monolayer ${\mathrm{MoS}}_2$ as the representative target material. Robustness of this correspondence scheme has been demonstrated by distinguishing the dominant contribution in the parallel HHG spectra. According to the change of the symmetry in the polarization-resolved harmonic yields, in the case of the tuning carrier distribution the transition of the dominant term between intra- and interband contributions has been verified. This work paves an alternative way to resolve the intra- and interband dynamics based on the structural symmetries of the atomic layer materials.

We show that a passive dispersive reflector integrated into a semiconductor laser can be used to tailor the laser dynamics for the generation of ultrashort pulses as well as stable dual-mode lasing. We analyze the stability using a general model that applies to any laser with frequency-dependent mirror losses. Finally, we present a generalization of the Fano laser concept, which provides a flexible platform for tailoring the mirror dispersion for self-pulsing. In addition to functioning as a design guideline, our model also accounts for several results in the literature.

Quantum state tomography is the standard technique for reconstructing a quantum state from experimental data. In the regime of finite statistics, experimental data cannot give perfect information about the quantum state. A common way to express this limited knowledge is by providing confidence regions in the state space. Though other confidence regions were previously proposed, they are either too wasteful to be of practical interest, cannot easily be applied to general measurement schemes, or are too difficult to report. Here we construct confidence regions that solve these issues, as they have an asymptotically optimal sample cost and good performance for realistic parameters, are applicable to any measurement scheme, and can be described by an ellipsoid in the space of Hermitian operators. Our construction relies on a vector Bernstein inequality and bounds with high probability the Hilbert–Schmidt norm error of sums of multinomial samples transformed by linear maps.

It is known that general convertibility of bipartite entangled states is not possible to arbitrary error without some classical communication. While some tradeoffs between communication cost and conversion error have been proven, these bounds can be very loose. In particular, there are many cases in which tolerable error might be achievable using zero-communication protocols. In this work we address these cases by deriving the optimal fidelity of pure quantum state conversions under local unitaries as well as local operations and shared randomness. We also use these results to explore catalytic conversions between pure states using zero communication.

Quantum homomorphic encryption hides quantum information from unauthorized participants while still allowing them to perform calculations on encrypted quantum information. In this paper, we propose a cross quantum homomorphic encryption scheme based on the butterfly network, in which every receiver can receive the evaluated quantum state with perfect fidelity. The entangled states between senders of the butterfly network are preshared, and the Bell basis measurements are used to encrypt the quantum plaintext. With both classical and quantum channels, the homomorphic evaluation of quantum plaintext can be completed on the butterfly network perfectly and crossly. Our scheme solves a bottleneck problem of cross evaluation at the bottleneck nodes, and improves the evaluation efficiency and total network throughput. In addition, we have designed quantum circuits according to different quantum evaluations with a butterfly network, which provide a basic blueprint for experimental implementation. The security of the scheme is also analyzed.

We study transport in the one-dimensional mass-imbalanced Fermi-Hubbard model in the high-temperature limit, focusing on the case of strong interactions. Prior theoretical and experimental investigations have revealed unconventionally long transport timescales, with complications due to strong finite-size effects. We compute the dynamical current-current correlation function directly in the thermodynamic limit using infinite tensor network techniques. We show that transport in the strong-imbalance limit is dominated by AC resonances, which we compute with an analytic expansion. We study the dephasing of these resonances with mass imbalance, $\eta$. In the small-imbalance limit, the model is nearly integrable. We connect these unusual limits by computing the DC conductivity and transport decay time as a function of $\eta$ and the interaction strength $U/t$. We propose an experimental protocol to measure these correlation functions in cold atom experiments.

We consider two-dimensional spin-polarized dipolar Fermi gases confined in a double-layer system and calculate the momentum transfer between the layers as a function of temperature to investigate the transport properties of the system. We use the Hubbard approximation to describe the correlation effects and the screening between the dipoles within a single layer. The effective interlayer interaction between the dipoles across the layers is obtained by the random-phase approximation. We calculate the interaction strength and the layer separation distance dependence of the drag rate, and we show that there is a critical distance below which the system is unstable. In addition, we calculate the typical behavior of the collective modes related to the density fluctuations.

The continuous spontaneous localization (CSL) model is the most studied among collapse models, which describes the breakdown of the superposition principle for macroscopic systems. Here, we derive an upper bound on the parameters of the model by applying it to the rotational noise measured in a recent short-distance gravity experiment [Lee et al., Phys. Rev. Lett. 124, 101101 (2020)]. Specifically, considering the noise affecting the rotational motion, we found that despite being a tabletop experiment the bound is only one order of magnitude weaker than that from LIGO for the relevant values of the collapse parameter. Further, we analyze possible ways to optimize the shape of the test mass to enhance the collapse noise by several orders of magnitude and eventually derive stronger bounds that can address the unexplored region of the CSL parameters space.

The reduction and distortion of quantum correlations in the presence of classical noise leads to varied levels of inefficiency in the availability of entanglement as a resource for quantum information processing protocols. While generically minimizing required entanglement for mixed quantum states remains challenging, a class of many-body Gaussian quantum states (denoted $\mathcal{N}\mathrm{IC}$) is here identified that exhibits two-mode bipartite entanglement structure, resembling that of pure states, for which the logarithmic negativity entanglement measure remains invariant upon inclusion of the classical correlations and optimal entanglement resources can be clearly quantified. This subclass is found to be embedded within a broader class of many-body Gaussian states (denoted $\mathcal{N}$-SOL) that retain two-mode entanglement structure for detection processes. These two entanglement classes are relevant in theoretical and experimental applications from the scalar field vacuum to the local axial motional modes of trapped ion chains. Utilizing the subspace that heralds inseparability in response to partial transposition, a minimum noise filtering process is designed to be necessary, sufficient, and computable for determining membership in these classes of entanglement structure. Application of this process to spacelike regions of the free scalar field vacuum is found to improve resource upper bounds, providing new understanding of the entanglement required for the quantum simulation of quantum fields as observed by arrays of local detectors.

The quantum Zeno effect is a distinctive phenomenon in quantum mechanics, describing the nontrivial effect of frequent projective measurements on hindering the evolution of a quantum system. However, when subjected to environmental noise, the quantum system may dissipate, and the quantum Zeno effect no longer works. This research starts from the physical mechanism for the decay of the quantum Zeno effect in the presence of noise and investigates the effect of coherent quantum controls on mitigating the decrease of the survival probability that the system stays in the initial state induced by the noise. We derive the decay rate of the survival probability with and without coherent quantum controls in general, and show that when the frequency of the projective measurements is large but finite, proper coherent controls by sufficiently strong Hamiltonians can be designed to decrease the decay rate of the survival probability. A two-level quantum system suffering from typical unitary and nonunitary noise is then considered to demonstrate the effect of the proposed coherent quantum control scheme in protecting the quantum Zeno effect against the noise. The decay rate of the survival probability is obtained in the presence of noise, and the control Hamiltonian is further optimized analytically to minimize the decay rate by a variational approach. The evolution paths of the quantum system with the optimal coherent controls are illustrated numerically for different scenarios to explicitly show how the coherent control scheme works in lowering the decay of survival probability.