We propose a scheme to realize highly efficient and controllable surface polariton beam splitters with a hybrid system via the electromagnetically induced transparency (EIT) of quantum emitters doped near the interface between a metamaterial and a dielectric. By means of the destructive interference effects contributed by the EIT of the quantum emitters and by the electric-magnetic response of the metamaterial, we show that such beam splitters can work at very low light level and they have small quantum decoherence and low Ohmic loss; moreover, the propagation loss of the surface polaritons can be reduced further by using weak microwave fields and the dispersion effect of the surface polaritons can be balanced by using the EIT-enhanced Kerr effect. Therefore, the surface polariton beam splitters obtained here have high splitting efficiency and may be controlled actively. Our study opens a route not only for coherent and active control of low-loss surface polaritons but also for promising applications in optical information processing based on quantum hybrid systems.

Symmetry shares an entwined history with the structure of physical theory. We propose a consequence of symmetry towards the axiomatic derivation of Hilbert space quantum theory. We introduce the notion of information symmetry (IS) and show that it constraints the state-space structure in any physical theory. To this end, we study the minimal error binary state discrimination problem in the framework of generalized probabilistic theories. A theory is said to satisfy IS if the probability of incorrectly identifying each of two randomly prepared states is the same for both the states. It is found that this simple principle rules out several classes of theories while being perfectly compatible with quantum theory.

The energies and wave functions of the two-body Dirac equation for positronium are compared with those of the Pauli approximation and the Bethe-Salpeter equation. The unusual behavior of the ground-state wave function of the Dirac equation is explained in terms of anomalous bound-state solutions.

We address the problem of the persistence of entanglement of quantum light under mode transformations, where orthogonal modes define the parties between which quantum correlations can occur. Since the representation of a fixed photonic quantum state in different optical mode bases can substantially influence the entanglement properties of the said state, we devise a constructive method to obtain families of states with the genuine feature of remaining entangled for any choice of mode decomposition. In the first step, we focus on two-photon states in a bipartite system and optimize their entanglement properties with respect to unitary mode transformations. Applying a necessary and sufficient entanglement witness criteria, we are then able to prove that the class of constructed states is entangled for arbitrary mode decompositions. Furthermore, we provide optimal bounds to the robustness of the mode-independent entanglement under general imperfections. In the second step, we demonstrate the power of our technique by showing how it can be straightforwardly extended to higher-order photon numbers in multipartite systems, together with providing a generally applicable and rigorous definition of mode-independent separability and inseparability for mixed states.

In the work by Silva et al. [Phys. Rev. Lett. 114, 250401 (2015)], the sharing of nonlocality by multiple observers was demonstrated through the quantum violation of Clauser-Horne-Shimony-Halt inequality. We provide a scheme for the sharing of nonlocality and nontrivial preparation contextuality sequentially through the quantum violation of a family of Bell's inequalities, where Alice and Bob perform $2^{n-1}$ and $n$ numbers of measurements of dichotomic observables, respectively. For this, we consider that Alice always performs projective measurement and multiple Bobs sequentially perform unsharp measurement. We show that when Bob's choices of measurement settings are unbiased, a maximum of two Bobs can sequentially share the nonlocality through the violation of our inequalities. Further, we show that the local bound of the aforementioned family of inequalities gets reduced if nontrivial preparation noncontextuality assumptions are further imposed. Then there is a chance to share the nontrivial preparation contextuality for a greater number of Bobs than that of nonlocality. We demonstrate that the nontrivial preparation contextuality can be sequentially shared by arbitrary numbers of Bob for unbiased choices of his measurement settings.

A theoretical approach for the electron-impact vibronic excitation of molecular ions with low-lying excited electronic states is described. In this approach, the fixed-nuclear $R$-matrix method is employed to compute electron-ion scattering matrices in the Born-Oppenheimer approximation. A vibronic frame transformation and the closed-channel elimination procedure in a spirit of molecular quantum defect theory are employed to construct an energy-dependent scattering matrix describing interactions between vibronic channels of the target ion induced by the incident electron. The obtained scattering matrix accounts for Rydberg series of vibronic resonances in the collisional spectrum. The approach is applied to the ${\mathrm{CH}}^{+}$ ion of an astrophysical and technological interest. Cross sections for vibronic excitation for different combinations of initial and final vibronic states are computed. A good agreement between electronic-excitation cross sections, obtained using the quantum defect theory and in a direct $R$-matrix calculation, demonstrates that the present approach provides a reliable tool for determination of vibronic (de-)excitation cross sections for targets with low-energy electronic resonances. Such targets were difficult to treat theoretically using earlier methods.

We consider a homogeneous heteronuclear Bose mixture with contact interactions at the mean-field collapse, i.e., with interspecies attraction equal to the mean geometrical intraspecies repulsion. We show that the Lee-Huang-Yang (LHY) energy functional is accurately approximated by an expression that has the same functional form as in the homonuclear case. The approximated energy functional is characterized by two exponents, which can be treated as fitting parameters. We demonstrate that the values of these parameters which preserve the invariance under permutation of the two atomic species are exactly those of the homonuclear case. Deviations from the exact expression of the LHY energy functional are discussed quantitatively and a specific application is described.

Long-range effective methods are ubiquitous in physics and in quantum theory, in particular. Furthermore, the reliability of such methods is higher when the nature of short-ranged interactions need not be modeled explicitly. This may be necessary for two reasons: (1) there are interactions that occur over a short range that cannot be accurately modeled with a potential function and/or (2) the entire Hamiltonian loses its reliability when applied at short distances. This work is an investigation of the utility and consequences of omitting a finite region of space from quantum-mechanical analysis, accomplished by imposition of an artificial boundary behind which obscured short-ranged physical effects may operate. With this method, a free function of integration that depends on momentum is interpreted as a function encoding information needed to match a long-distance wave function to an appropriate state function on the other side of the boundary. Omitting part of the space from analysis implies that the strict unitarity requirement of quantum mechanics must be relaxed, since particles can actually propagate beyond the boundary. Strict orthogonality of eigenmodes and Hermiticity of the Hamiltonian must also be relaxed in this method; however, all of these canonical relations are obeyed when averaged over sufficiently long times. What is achieved, therefore, appears to be an effective long-wavelength theory, at least for stationary systems. As examples, the quantum defect theory of the one-dimensional Coulomb interaction is recovered, along with a different perspective of the inverse-square potential and the free particle, as well as the Wigner time delay associated with contact interactions. Potential applications of this method may include three-dimensional atomic systems and two-dimensional systems, such as graphene.

The physics of topological singularities, namely, exceptional points (EPs), has been a key to a wide range of intriguing and unique physical effects in non-Hermitian systems. In this context, exploration of the mutual interactions among the states in four-level systems around fourth-order EPs (EP4s) is lacking. Here we report a four-level parameter-dependent perturbed non-Hermitian Hamiltonian, mimicking quantum or wave-based systems, to explore the physical aspects of an EP4 analytically as well as numerically. The proposed Hamiltonian exhibits different orders of interaction schemes with the simultaneous presence of different higher-order EPs. Here an EP4 has been realized by mutual interaction between four states with proper parameter manipulation. We comprehensively investigate the dynamics of corresponding coupled eigenvalues with stroboscopic parametric variation in the vicinity of the embedded EP4 to establish a successive state-switching phenomenon among them, which proves to be robust even in the presence of different orders of EPs. Implementing the relation of the perturbation parameters with the coupling control parameters, we report a region to host multiple EP4s in a specific system. The chiral behavior of successive state exchange has also been established near the EP4. The proposed scheme, which is enriched with physical aspects of EP4s, should provide a unique light manipulation tool in any anisotropic multistate integrated system.

We report a theoretical and experimental study on the role of indistinguishability in the estimation of an interferometric phase. In particular, we show that the quantum Fisher information, which limits the maximum precision achievable in the parameter estimation, increases linearly with respect to the degree of indistinguishability between the input photons in a two-port interferometer, in the ideal case of a pure probe state. We experimentally address the role played by the indistinguishability for the case of two photons entering a polarization-based interferometer, where the degree of indistinguishability is characterized by the overlap between two spatial modes. The experimental results support the fact that, even in the presence of white noise, a quantum enhancement in the interferometric phase estimation can be obtained from a minimum degree of indistinguishability.

It was recently shown that, in general, the von Neumann spin entropy of fermionic particles is not invariant under Lorentz boosts. We show that an analogous result can be recovered (at the lowest order of $v^2/c^2$) using plain nonrelativistic quantum mechanics provided one uses that energy weighs: $E=m{c}^2$. This should (i) help to moderate the skepticism on the observer dependence of the spin entropy of fermionic particles, (ii) emphasize the “soft” relativistic nature of this result, and (iii) show that this is a particular case of a more general class of systems, since our calculation only assumes a nonrelativistic particle endowed with an internal degree of freedom.

We study the relation between the emergence of objectivity and qubit-environment entanglement generation. We find that although entanglement with the unobserved environments is irrelevant (since sufficiently strong decoherence can occur regardless), entanglement with the observed environments is crucial. In fact, the appearance of an objective qubit-observed-environment state is strictly impossible if their joint evolution does not lead to entanglement. Furthermore, if a single observer has access to a single environment (no macrofractions), then the required orthogonality of the observed environmental states comes only as a consequence of the system-environment state becoming strongly entangled (maximally entangled for the given initial occupation of the qubit if the environmental state is initially pure).

In this work, we explore the space of quantum states composed of $N$ particles. To investigate entanglement resistant to particle loss, we introduce the notion of $m$-resistant states. A quantum state is $m$ resistant if it remains entangled after losing an arbitrary subset of $m$ particles but becomes separable after losing any number of particles larger than $m$. We establish an analogy to the problem of designing a topological link consisting of $N$ rings such that, after cutting any $(m+1)$ of them, the remaining rings become disconnected. We present a constructive solution to this problem, which allows us to exhibit several $N$-particle states with the desired property of entanglement resistance to particle loss.

We analyze the geometrical properties of trace-preserving Pauli maps. Using the Choi-Jamiołkowski isomorphism, we express the Hilbert-Schmidt line and volume elements in terms of the eigenvalues of the Pauli map. We analytically compute the volumes of positive, trace-preserving Pauli maps and Pauli channels. In particular, we find the relative volumes of the entanglement-breaking Pauli channels as well as the channels that can be generated by a time-local generator. Finally, we show what part of the Pauli channels are $\mathrm{P}$- and $\mathrm{CP}$-divisible, which is related to the notion of Markovianity.

We calculate hyperfine structure intervals $\mathrm{\Delta }{E}^{hfs}\left(2P_{1/2}\right)$ and $\mathrm{\Delta }{E}^{hfs}\left(2P_{3/2}\right)$ for $P$ states in muonic ions of lithium, beryllium, and boron. To construct the particle interaction operator in momentum space we use the tensor method of the projection operators on states with definite quantum numbers of total atomic momentum $F$ and total muon momentum $j$. We take into account the vacuum polarization, relativistic, quadruple, and structure corrections of order ${\alpha }^4$, ${\alpha }^5$, and ${\alpha }^6$. The obtained numerical values of hyperfine splittings can be used for a comparison with future experimental data of the CREMA Collaboration.

We systematically investigate the non-Hermitian generalizations of the Landau-Zener (LZ) transition and the Landau-Zener-Stückelberg (LZS) interferometry. The LZ transition probabilities, or band populations, are calculated for a generic non-Hermitian model and their asymptotic behavior is analyzed. We then focus on non-Hermitian systems with a real adiabatic parameter and study the LZS interferometry formed out of two identical avoided level crossings. Four distinctive cases of interferometry are identified and the analytic formulas for the transition probabilities are calculated for each case. The differences and similarities between the non-Hermitian case and its Hermitian counterpart are emphasized. In particular, the geometrical phase originated from the sign change of the mass term at the two level crossings is still present in the non-Hermitian system, indicating its robustness against the non-Hermiticity. We further apply our non-Hermitian LZS theory to describing the Bloch oscillation in one-dimensional parity-time $\left(\mathcal{PT}\right)$ reversal symmetric non-Hermitian Su-Schrieffer-Heeger model and propose an experimental scheme to simulate such dynamics using photonic waveguide arrays. The Landau-Zener transition, as well as the LZS interferometry, can be visualized through the beam intensity profile and the transition probabilities measured by the center of mass of the profile.

A different mode of effective interaction of molecular rotational degrees of freedom with an intense, nonresonant, ultrashort laser pulse is explored. Transient nonadiabatic charge redistribution (TNCR) in larger molecules or molecular ions causes impulsive-torque interaction that replaces the traditional mechanism of molecular alignment based on perturbative interaction of the laser field with electronic subsystem as manifested in linear anisotropic polarizability or hyperpolarizability. We explore this alignment mechanism on a popular generic model of a tight-binding diatomic molecule. We consider the case of rotational wave-packet formation when a molecule is initially in the ground rotational state. The rotational wave packet emerging from the TNCR interaction consists of states with higher rotational quantum numbers, in comparison with the anisotropic-polarizability case, and the after-pulse alignment oscillations are out-of-phase with those resulting from the traditional interaction. The TNCR interaction mode is expected to play a major role when a strong laser field actually causes extensive nonresonant excitation and/or ionization of a molecule.

We investigate the Coulomb effect on the dynamics of atoms in a strong elliptical laser field by solving the three-dimensional time-dependent Schrödinger equation with and without the photoelectron core Coulomb interactions and find that the Coulomb effect plays an important role for the excitation and ionization processes, while it is less important for the high-order harmonic generation. Our calculation results can serve as a benchmark for other approximative methods. Looking at the photoelectron momentum distribution, we find that the Coulomb effect is important for the momentum distribution in the polarization plane and less important for the momentum distributions along the normal direction perpendicular to the polarization plane. Similar to the photoabsorption in a weak field, we demonstrate that the excitation and ionization can be treated in a unified way, which implies the low-energy photoelectron distributions can be connected with the populations of highly excited states.

It has been observed that in a pre-excited medium produced by femtosecond laser pulses, nonlinear interaction can be dramatically enhanced. The underlying mechanism is still under debate because of the lack of detailed information on the species, energy states, and temporal dynamics of the excited medium. Here, we report a dramatic enhancement of seventh-harmonic generation in pre-excited argon atoms. By measuring the evolution of the enhanced signal with the propagation distance, we are able to separate the contributions from the improved phase matching and the enhanced nonlinear susceptibility. The time- and spectrum-resolved measurements clearly show that generation of excited states is responsible for the enhanced nonlinear susceptibility. The maximum population of excited states on the picosecond and nanosecond timescales can be attributed to two possible pathways.

We report on the stable effective trapping of cold rubidium atoms by a method for a single optical trap in the near-optical resonant regime. An optical trap with the near-optical resonance condition consists of not only the dipole but also the radiative forces, while a trap using a far-off resonance dominates only the dipole force. We measure the spatial behaviors of the center-of-mass positions and the loading efficiencies of the trapped atoms in the near-optical resonant trap, by changing the detuning over the range of $-0.373$ to $-2.23$ THz from the ${\mathrm{D}}_2$ resonance. The time dependence of the spatial behavior in the trap indicates that the suitable positions for stably holding atoms are altered because the equilibrium condition between the optical dipole and the radiative forces depends on the trap laser detuning. The stable position, which is not a primary position of focus in the Gaussian beam optics, depends only on the laser detuning due to the change in the radiative force, while it is independent of the change in the laser intensity, which results in a balance between the radiative and dipole forces.