We theoretically investigate the creation of squeezed states of a Bose-Einstein condensate (BEC) trapped in a magnetic double-well potential. The number or phase squeezed states are created by modulating the tunnel coupling between the two wells periodically with twice the Josephson frequency, i.e., through parametric amplification. Simulations are performed with the multiconfigurational time-dependent Hartree method for bosons. We employ optimal control theory to bring the condensate to a complete halt at a final time, thus creating a highly squeezed state (squeezing factor of 0.12, ${\xi }_S^2=-18$ dB) suitable for atom interferometry.

We demonstrate experimentally that a broad-area laserlike optical oscillator (a nondegenerate photorefractive oscillator) with structured injected signal displays two-phase patterns. The technique [de Valcárcel and Staliunas, Phys. Rev. Lett. 105, 054101 (2010)] consists in spatially modulating the injection, so that its phase alternates periodically between two opposite values, i.e., differing by $\pi$.

We study the two-color nonreciprocal effects of transmission and reflection in cold atoms driven into the tripod configuration and confined in a moving optical lattice. Our numerical results show that a very high contrast of the forward-backward transmission up to around 92% (reflection up to around 85%) is observable near the sharp edges of two tunable photonic band gaps at lattice speeds of several meters per second. Such two-color optical nonreciprocity is attained in fact by breaking the time-reversal symmetry with asymmetric Doppler shifts and can be dynamically manipulated by varying the driving and coupling field detunings, the probe pulse length, the atomic lattice velocity, etc.

We show that an arbitrary spatial distribution of complex refractive index decrement inside an object can be exactly represented as a sum of two “monomorphous” complex distributions, i.e., distributions with the ratios of the real part to the imaginary part being constant throughout the object. A priori knowledge of constituent materials can be used to estimate the global lower and upper boundaries for this ratio. This “monomorphous decomposition” approach can be viewed as an extension of the successful phase-retrieval method, based on the transport of intensity equation, that was previously developed for monomorphous (homogeneous) objects, such as, e.g., objects consisting of a single material. We demonstrate that the monomorphous decomposition can lead to more stable methods for phase retrieval using the transport of intensity equation. Such methods may find application in quantitative in-line phase-contrast imaging and phase-contrast tomography.

Lossless filtering of a single coherent (Schmidt) mode from spatially multimode radiation is a problem crucial for optics in general and for quantum optics in particular. It becomes especially important in the case of nonclassical light that is fragile to optical losses. An example is bright squeezed vacuum generated via high-gain parametric down conversion or four-wave mixing. Its highly multiphoton and multimode structure offers a huge increase in the information capacity provided that each mode can be addressed separately. However, the nonclassical signature of bright squeezed vacuum, photon-number correlations, are highly susceptible to losses. Here we demonstrate lossless filtering of a single spatial Schmidt mode by projecting the spatial spectrum of bright squeezed vacuum on the eigenmode of a single-mode fiber. Moreover, we show that the first Schmidt mode can be captured by simply maximizing the fiber-coupled intensity. Importantly, the projection operation does not affect the targeted mode and leaves it usable for further applications.

Bell's theorem states that quantum mechanics is not a locally causal theory. This state is often interpreted as nonlocality in quantum mechanics. Toner and Bacon [Phys. Rev. Lett. 91, 187904 (2003)] have shown that a shared random-variable theory augmented by one bit of classical communication exactly simulates the Bell correlation in a singlet state. In this paper, we show that in Toner and Bacon protocol, one of the parties (Bob) can deduce another party's (Alice) measurement outputs, if she only informs Bob of one of her own outputs. Afterwards, we suggest a nonlocal version of Toner and Bacon protocol wherein classical communications is replaced by nonlocal effects, so that Alice's measurements cause instantaneous effects on Bob's outputs. In the nonlocal version of Toner and Bacon's protocol, we get the same result again. We also demonstrate that the same approach is applicable to Svozil's protocol.

In quantum theory, a physical observable is represented by a Hermitian operator as it admits real eigenvalues. This stems from the fact that any measuring apparatus that is supposed to measure a physical observable will always yield a real number. However, the reality of an eigenvalue of some operator does not mean that it is necessarily Hermitian. There are examples of non-Hermitian operators that may admit real eigenvalues under some symmetry conditions. In general, given a non-Hermitian operator, its average value in a quantum state is a complex number and there are only very limited methods available to measure it. Following standard quantum mechanics, we provide an experimentally feasible protocol to measure the expectation value of any non-Hermitian operator via weak measurements. The average of a non-Hermitian operator in a pure state is a complex multiple of the weak value of the positive-semidefinite part of the non-Hermitian operator. We also prove an uncertainty relation for any two non-Hermitian operators and show that the fidelity of a quantum state under a quantum channel can be measured using the average of the corresponding Kraus operators. The importance of our method is shown in testing the stronger uncertainty relation, verifying the Ramanujan formula, and measuring the product of noncommuting projectors.

Evolutional entanglement production is defined as the amount of entanglement produced by the evolution operator. This quantity is analyzed for systems whose Hamiltonians are characterized by spin operators. The evolutional entanglement production at the initial stage grows quadratically in time. For longer times, it oscillates, being quasiperiodic or periodic depending on the system parameters.

Stopping powers of antiprotons in ${\text{H}}_2$ and ${\text{H}}_2\text{O}$ targets are calculated using a semiclassical time-dependent convergent close-coupling method. In our approach the ${\text{H}}_2$ target is treated using a two-center molecular multiconfiguration approximation, which fully accounts for the electron-electron correlation. Double-ionization and dissociative ionization channels are taken into account using an independent-event model. The vibrational excitation and nuclear scattering contributions are also included. The ${\text{H}}_2\text{O}$ target is treated using a neonization method proposed by C. C. Montanari and J. E. Miraglia [J. Phys. B 47, 015201 (2014)], whereby the ten-electron water molecule is described as a dressed Ne-like atom in a pseudospherical potential. Despite being the most comprehensive approach to date, the results obtained for ${\text{H}}_2$ only qualitatively agree with the available experimental measurements.

The electron and photon scattering data of an atom are crucial for many scientific fields, including plasma physics, astrophysics, and so on. For high enough but nonrelativistic incident energies, the first Born approximation is applicable for calculating these data, in which the key physics quantity is the generalized oscillator strength (GOS). In high-energy electron impact excitation processes, atoms will be excited into various excited states including strongly perturbed Rydberg and adjacent continuum states. How to calculate these quantities of a nontrivial many-electron atom rapidly and accurately is still a great challenge. Based on our eigenchannel $R$-matrix method $R$-eigen, we further extend it to calculate the GOS of a whole channel in an atom, which includes all Rydberg and adjacent continuum states. The $J^{\pi }=1^{-}$ states of argon are chosen as an illustrating example. The calculation results are in good agreement with the available benchmark absolute experimental measurements. The calculated eigenchannel GOS matrix elements are smooth functions of the excitation energy and momentum transfer. From such smooth eigenchannel GOS matrix elements, we can obtain the GOS of any specific excited state through multichannel quantum defect theory, e.g., infinite Rydberg (including a strongly perturbed one), autoionization, and continuum states.

Fast radiative cooling of anthracene cations ${\left({\mathrm{C}}_{14}{\mathrm{H}}_{10}\right)}^{+}$ is studied with a compact electrostatic storage device, the Mini-Ring. The time evolution of the internal energy distribution of the stored ions is probed in a time range from 3 to 7 ms using laser-induced dissociation with 3.49-eV photons. The population decay rate due to radiative emission is measured to vary from 25 to $450{\mathrm{s}}^{–1}$ as a function of the excitation energy in the range from 6 to 7.4 eV. After corrections of the infrared emission effect via vibrational transitions, the fluorescence emission rate due to electronic transitions from thermally excited electronic states is estimated and compared with a statistical molecular approach. In the considered internal energy range, the radiative cooling process is found to be dominated by the electronic transition, in good agreement with our previous work [S. Martin et al., Phys. Rev. Lett. 110, 063003 (2013)] focused on a narrower energy range.

We study a problem of nonadiabatic superfluid dynamics of spin-orbit-coupled neutral fermions in two spatial dimensions. We focus on the two cases when the out-of-equilibrium conditions are initiated by either a sudden change of the pairing strength or the population imbalance. For the case of a zero-population imbalance and within the mean-field approximation, the nonadiabatic evolution of the pairing amplitude in a collisionless regime can be found exactly by employing the method of Lax vector construction. Our main finding is that the presence of the spin-orbit coupling significantly reduces the region in the parameter space where a steady state with periodically oscillating pairing amplitude is realized. For the collisionless dynamics initiated by a sudden disappearance of the population imbalance, we obtain an exact expression for the steady-state pairing amplitude. In the general case of quenches to a state with finite population imbalance, we show that there is a region in the steady-state phase diagram where at long times the pairing amplitude dynamics is governed by the reduced number of the equations of motion in full analogy with the exactly integrable case.

Semiconductor microcavities with artificial single-photon emitters have become one of the backbones of semiconductor quantum optics. In many cases, however, technical and physical issues limit the study of optical fields to incoherently excited systems. We analyze the model of an incoherently driven two-level system in a single-mode cavity. The specific structure of the applied master equation yields a recurrence relation for the steady-state values of correlations of the intracavity field and the emitter. We provide boundary conditions that permit a systematic solution which is numerically less demanding than standard methods. The method allows us to directly infer reasonable cutoff conditions from the system parameters. Different cavity systems from previous experiments are analyzed in terms of field correlation functions which can be measured via homodyne correlation measurements. We find that nonclassical correlations occur in systems of moderate quantum-dot–cavity coupling rather than strong coupling. Our boundary conditions also allow us to derive analytical results for the overall quantum state and its higher-order moments. We obtain very good approximations for the full quantum state of the field in terms of the characteristic functions. It turns out that for every physically reasonable set of system parameters, the state of the intracavity field is nonclassical.

In this work we present a theoretical model of the tunneling time and the tunneling process (in attosecond experiment for the He atom). Our model is supported with physical reasoning leading to a relation which performs an excellent estimation for the tunneling time in attosecond and strong-field experiments, where we address the important case of the He atom. Our tunneling time estimation is found by utilizing the time-energy uncertainty relation and represents a quantum clock. The tunneling time is also featured as the time of passage through the barrier similar to Einstein's photon-box Gedanken experiment. Our work tackles an important case study for the theory of time in quantum mechanics and is very promising for the search for a (general) time operator in quantum mechanics. The work can be seen as a fundamental step in dealing with the tunneling time in strong-field and ultrafast science and is appealing for more elaborate treatments using quantum wave-packet dynamics and especially for complex atoms and molecules.

We investigate the effect of noisy channels in a classical information transfer through a multipartite state which acts as a substrate for the distributed quantum dense coding protocol between several senders and two receivers. The situation is qualitatively different from the case with one or more senders and a single receiver. We obtain an upper bound on the multipartite capacity which is tightened in the case of the covariant noisy channel. We also establish a relation between the genuine multipartite entanglement of the shared state and the capacity of distributed dense coding using that state, both in the noiseless and the noisy scenarios. Specifically, we find that, in the case of multiple senders and two receivers, the corresponding generalized Greenberger-Horne-Zeilinger states possess higher dense coding capacities as compared to a significant fraction of pure states having the same multipartite entanglement.

We present two quantum union bounds for sequential projective measurements. These bounds estimate the disturbance accumulation and probability of outcomes when the measurements are performed sequentially. These results are based on a trigonometric representation of quantum states and should have wide application in quantum information theory for information-processing tasks such as communication and state discrimination, and perhaps even in the analysis of quantum algorithms.

Quantum entanglement is analyzed thoroughly in the case of the ground and lowest states of two-electron axially symmetric quantum dots under a perpendicular magnetic field. The individual-particle and the center-of-mass representations are used to study the entanglement variation at the transition from interacting to noninteracting particle regimes. The mechanism of symmetry breaking due to the interaction, which results in the states with symmetries related to the latter representation only being entangled even at the vanishing interaction, is discussed. The analytical expression for the entanglement measure based on the linear entropy is derived in the limit of noninteracting electrons. It reproduces remarkably well the numerical results for the lowest states with the magnetic quantum number $M\ge 2$ in the interacting regime. It is found that the entanglement of the ground state is a discontinuous function of the field strength. A method to estimate the entanglement of the ground state, characterized by the quantum number $M$, with the aid of the magnetic-field dependence of the addition energy is proposed.

We investigate multiqubit permutation-symmetric states with maximal entropy of entanglement. Such states can be viewed as particular spin states, namely anticoherent spin states. Using the Majorana representation of spin states in terms of points on the unit sphere, we analyze the consequences of a point-group symmetry in their arrangement on the quantum properties of the corresponding state. We focus on the identification of anticoherent states (for which all reduced density matrices in the symmetric subspace are maximally mixed) associated with point-group-symmetric sets of points. We provide three different characterizations of anticoherence and establish a link between point symmetries, anticoherence, and classes of states equivalent through stochastic local operations with classical communication. We then investigate in detail the case of small numbers of qubits and construct infinite families of anticoherent states with point-group symmetry of their Majorana points, showing that anticoherent states do exist to arbitrary order.