To probe ultrafast molecular dynamics, a high spatial-temporal resolution is of crucial importance. Different from the usual pump-probe method, here we present a probing scheme to clock the molecular dynamics with subcycle precision by investigating the correlated electron-nuclear energy spectra in a multicycle laser pulse. The molecular dynamics in the tunneling ionization regime are numerically simulated in a two-dimensional ${\mathrm{H}}_2^{+}$ non-Born-Oppenheimer model. It is found that the asymmetry between the yields of electrons ionized in opposite directions along the laser polarization axis shows peaks and valleys alternately as a function of the nuclear energy. Our results show that the temporally resolved asymmetry of the ionization of electrons can be used as a clock to probe the nuclear Coulomb explosion channel with subcycle resolution and extract the corresponding molecular dynamics accurately. This method can be used even when the intensity averaging of the focused laser beam is considered.

Quantum information processing with geometric features of quantum states may provide promising noise-resilient schemes for quantum metrology. In this work, we theoretically explore phase-space geometric Sagnac interferometers with trapped atomic clocks for rotation sensing, which could be intrinsically robust to certain decoherence noises and reach high precision. With the wave guide provided by sweeping ring traps, we give criteria under which the well-known Sagnac phase is a pure or unconventional geometric phase with respect to the phase space. Furthermore, corresponding schemes for geometric Sagnac interferometers with designed sweeping angular velocity and interrogation time are presented, and the experimental feasibility is also discussed. Such geometric Sagnac interferometers are capable of saturating the ultimate precision limit given by the quantum Cramér-Rao bound.

We study the propagation of entanglement after quantum quenches in the nonintegrable paramagnetic quantum Ising spin chain. Tuning the parameters of the system, we observe a sudden increase in the entanglement production rate, which we show to be related to the appearance of new quasiparticle excitations in the postquench spectrum. We argue that the phenomenon is the nonequilibrium version of the well-known Gibbs paradox related to mixing entropy and demonstrate that its characteristics fit the expectations derived from the quantum resolution of the paradox in systems with a nontrivial quasiparticle spectrum.

We demonstrate that the integration of a phase-change material (PCM) in one of the two microresonators of a photonic circuit, coupled to a bus waveguide, can lead to unidirectional Fano resonances and to the emergence of a unidirectional transmission window. The phase change is caused by light-induced heating and is accompanied by an abrupt increase in the extinction coefficient of the PCM resonator. Due to the photonic circuit asymmetry, the critical value of the input light intensity triggering the phase change is strongly dependent on the input light direction. The latter determines the unidirectional nature of the emerging transmission window. This effect can be utilized in on-chip magnetic-free isolators and $Q$ switches.

We propose a quantum optical device to experimentally realize quantum processes, which perform the regularization of the—in general highly singular—Glauber–Sudarshan $P$ functions of arbitrary quantum states before their application and/or measurement. This allows us to produce a broad class of nonclassical states with regular $P$ functions, also called nonclassicality quasiprobabilities. For this purpose, the input states are combined on highly transmissive beam splitters with specific Gaussian or non-Gaussian classical states. We study both balanced and unbalanced homodyne detections for the direct sampling of the output states of the implemented processes, which requires no further regularization or state-reconstruction. By numerical simulations we demonstrate the feasibility of our approach and we outline the generalization to multimode light.

When a two-level system (TLS) is coupled to an electromagnetic resonator, its transition frequency changes in response to the quantum vacuum fluctuations of the electromagnetic field, a phenomenon known as the Lamb shift. Remarkably, by replacing the TLS by a harmonic oscillator, normal-mode splitting leads to a quantitatively similar shift, without taking quantum fluctuations into account. In a weakly anharmonic system, lying in between the harmonic oscillator and a TLS, the origins of such shifts can be unclear. An example of this is the dispersive shift of a transmon qubit in circuit quantum electrodynamics (QED). Although often referred to as a Lamb shift, the dispersive shift observed in spectroscopy in circuit QED could contain a significant contribution from normal-mode splitting that is not driven by quantum fluctuations, raising the question: how much of this shift is quantum in origin? Here we treat normal-mode splitting separately from shifts induced by quantum vacuum fluctuations in the Hamiltonian of a weakly anharmonic system, providing a framework for understanding the extent to which observed frequency shifts can be attributed to quantum fluctuations.

For the two-mode and $(N-1)-\mathrm{bosonic}$ Bose-Hubbard quantum system a less usual phase transition controlled by the parameter $\varepsilon$ representing the on-site energy difference is studied. In the literature the parameter is considered either real ($\varepsilon >0$) or purely imaginary (with, say, $\gamma =\mathrm{Im}\varepsilon >0$), so the phase transition is analyzed here at the interface $\varepsilon =\gamma =0$. The evolution in the $\gamma -\mathrm{controlled}$ phase is required unitary so that the main task for the theory is found in the (quasi-)Hermitization of the Hamiltonian, achieved by a suitable amendment of the inner product in Hilbert space, $\langle ·|·\rangle \to \langle ·|\mathrm{\Theta }|·\rangle$. In the most relevant domain of small $\gamma$ the linearized Hilbert-space metric $\mathrm{\Theta }\left(\gamma \right)$ [constrained by the requirement ${lim}_{\gamma \to 0}\mathrm{\Theta }\left(\gamma \right)=I$ of the smoothness of the change of the Hilbert space at the phase transition] is constructed in closed form. Beyond the phase-transition instant, several forms of the systematic non-numerical recurrent construction of the exact metrics $\mathrm{\Theta }\left(\gamma \right)$ are also shown user friendly and feasible, at the not too large matric dimensions $N$ at least.

The entanglement and quantum correlation measures have been investigated for the ground state of a spin chain with a Kitaev-type exchange interactions on alternating bonds, along with a transverse magnetic field. There is a macroscopic degeneracy in the ground state for zero magnetic field, implying a quantum critical point. In this model, many entanglement and correlation measures, except the macroscopic multispecies entanglement measure, do not show true singular behavior in the vicinity of the critical point, as seen in the transverse-Ising model ground state and related models. We compute the pair concurrence measure of entanglement, the pair quantum discord to track the quantum correlations, and a global entanglement measure and a multispecies entanglement measure to investigate multiparty entanglement, both analytically and numerically. The nearest-neighbor concurrence shows a peak structure as a function of magnetic field, near the critical point, for various values of the ratio of interaction strengths, but its derivative does not show a singular behavior close to the critical point. A similar behavior is shown by the quantum discord and the global entanglement. The multispecies entanglement shows a clear signature of the critical point behavior, and the first-order derivative shows a divergence near zero magnetic field, for equal interaction strengths.

We propose a protocol for quantum networking based on deterministic quantum state transfer between distant memory nodes using photon-number superposition states (PNSS). In the suggested scheme, the quantum nodes are single atoms confined in high-finesse optical cavities linked by photonic channels. The quantum information written in a superposition of atomic Zeeman states of sending system is faithfully mapped through cavity-assisted Raman scattering onto PNSS of linearly polarized cavity photons. The photons travel to the receiving cavity, where they are coherently absorbed with unit probability creating the same superposition state of the second atom, thus ensuring high-fidelity transfer between distant nodes. We develop this approach at first for photonic qubit and show that this superposition state is no less reliably protected against the propagation losses compared to the single-photon polarization states, whereas the limitation associated with the delivery of more than one photon does not affect the process fidelity. Then, by preserving the advantages of qubits, we extend the developed technique to the case of state transfer by photonic qutrit, which evidently possesses more information capacity. This reliable and efficient scheme promises also a successful distribution of entanglement over long distances in quantum networks.

We consider online detection strategies for identifying a change point in a stream of quantum particles allegedly prepared in identical states. We show that the identification of the change point can be done without error via sequential local measurements while attaining the optimal performance bound set by quantum mechanics. In this way, we establish the task of exactly identifying a quantum change point as an instance where local protocols are as powerful as global ones. The optimal online detection strategy requires only one bit of memory between subsequent measurements, and it is amenable to experimental realization with current technology.

A measure of total correlations cannot increase under deterministic local operations. We show that, for any number of systems, this condition alone does not guarantee the existence of maximally correlated states. Namely, there is no state that simultaneously maximizes all the measures satisfying it. If, in addition, the measures do not increase with probability unity under local measurements, then such states exist for two systems. They are the maximally entangled states. For a larger number of systems, it depends on their Hilbert-space dimensions.

The difference in the mean-square nuclear charge radius of xenon isotopes was measured utilizing a method based on extreme ultraviolet spectroscopy of highly charged Na-like ions. The isotope shift of the Na-like $D1$ ($3s{}^{2}S_{1/2}-3p{}^{2}P_{1/2}$) transition between the ${}^{124}\mathrm{Xe}$ and ${}^{136}\mathrm{Xe}$ isotopes was experimentally determined using the electron-beam ion-trap facility at the National Institute of Standards and Technology. The mass-shift and the field-shift coefficients were calculated with enhanced precision by the relativistic many-body perturbation theory and multiconfiguration Dirac-Hartree-Fock method. The mean-square nuclear charge radius difference was found to be $\delta {\langle r^2\rangle }^{136,124}=0.269\left(42\right){\mathrm{fm}}^2$. Our result has smaller uncertainty than previous experimental results and agrees with the literature values.

We apply attosecond transient absorption spectroscopy (ATAS) to explore the effects of a nonzero permanent dipole on electron dynamics at the subfemtosecond scale, exemplified in the polar LiF molecule. In contrast with nonpolar systems, a familiar feature of the ATA spectra—the light-induced structures—are observed adjacent to a bright state. Moreover, a previously unobserved ladder structure is identified. The observations are analyzed in the context of a model based on fixed-nuclei adiabatic states, supported by full numerical simulations. Analytic calculations originating in the adiabatic model shed light on the nature and origins of the findings.

High-order harmonic generation in two-color ($\omega -2\omega$) counter-rotating circularly polarized laser fields opens the path to generate isolated attosecond pulses and attosecond pulse trains with controlled ellipticity. The generated harmonics have alternating helicity, and the ellipticity of the generated attosecond pulse depends sensitively on the relative intensities of two adjacent, counter-rotating harmonic lines. For the $s$-type ground state, such as in helium, the successive harmonics have nearly equal amplitude, yielding isolated attosecond pulses and attosecond pulse trains with linear polarization, rotated by ${120}^{\circ }$ from pulse to pulse. In this work, we suggest a solution to overcome the limitation associated with the $s$-type ground state. It is based on modifying the three propensity rules associated with the three steps of the harmonic generation process: ionization, propagation, and recombination. We control the first step by seeding high-order harmonic generation with XUV light tuned well below the ionization threshold, which generates virtual excitations with the angular momentum corotating with the $\omega$ field. We control the propagation step by increasing the intensity of the $\omega$ field relative to the $2\omega$ field, further enhancing the chance of the $\omega$ field being absorbed versus the $2\omega$ field, thus favoring the emission corotating with the seed and the $\omega$ field. We demonstrate our proposed control scheme using a helium atom as a target and solving a time-dependent Schrödinger equation in two and three dimensions.

Although frequency-shift gravity-gradient compensation is currently the technique with the most potential to test the weak equivalence principle with atom interferometry at high precision, it faces an unavoidable obstacle on how to cancel the coupling between the Raman-pulse-duration effect and the gravity gradient related to the atomic initial kinematics. According to the Einstein equivalence principle, we develop a method to calculate this coupling effect in a space-stretching free-falling frame, based on which an improved frequency-shift gravity-gradient compensation technique is put forward to make high-accuracy weak-equivalence-principle tests with the microscopic particles, such as at the level of ${10}^{-14}$, more possible.

We present a numerical study of the interaction quench dynamics in a superfluid ultracold Fermi gas confined in a three-dimensional cigar-shaped harmonic trap. In the present paper, we investigate the amplitude mode of the superfluid order parameter after interaction quenches, which start deep in the BCS phase and end in the BCS-BEC-crossover regime. To this end, we exploit the Bogoliubov–de Gennes formalism, which takes the confinement potential explicitly into account and provides a microscopic fully coherent description of the system. We find an anharmonic nonlinear oscillation of the modulus of the superfluid order parameter, i.e., of the Higgs mode. This oscillation persists for large times with only a small amplitude modulation being visible. We connect the frequency and the mean value of this oscillation with the breaking of Cooper pairs in the superfluid phase. Additionally, we demonstrate that the occurrence of this persistent oscillation is connected to the onset of chaotic dynamics in our model. Finally, we calculate an interaction quench phase diagram of the Higgs mode for quenches on the BCS side of the BCS-BEC crossover and discuss its properties as a function of the aspect ratio of the cigar-shaped trap.

Flat bands and dynamical localization of binary mixtures of Bose-Einstein condensates, with spin-orbit coupling subjected to a deep optical lattice which is shaking in time and to a periodic time modulation of the Zeeman field, are investigated. In contrast with usual dynamical localization in the absence of spin-orbit coupling, we find that to fully suppress the tunneling in the system the optical lattice shaking is not enough, and a proper tuning of the spin-orbit term, achievable via the Zeeman field modulation, is also required. This leads to a sequence of Zeeman parameter values where energy bands become flat, the tunneling in the system is suppressed, and the dynamical localization phenomenon occurs. Exact wave functions at the dynamical localization points show that the binary mixture localizes on a dimer with the two components occupying different sites. This type of localization occurs in exact form also for the ground state of the system at the dynamical localization points in the presence of nonlinearity and remains valid, although in approximate form, for a wide range of the Zeeman parameter around these points. The possibility of observing the above phenomena in real experiments is also briefly discussed.

We study the time-dependent occupation of an impurity state hybridized with a continuum of extended or localized states. Of particular interest is the return probability, which gives the long-time limit of the average impurity occupation. In the extended case, the return probability is zero unless there are bound states of the impurity and continuum. We present exact expressions for the return probability of an impurity state coupled to a lattice, and show that the existence of bound states depends on the dimension of the lattice. In a disordered lattice with localized eigenstates, the finite extent of the eigenstates results in a nonzero return probability. We investigate different parameter regimes numerically by exact diagonalization, and show that the return probability can serve as a measure of the localization length in the regime of weak hybridization and disorder. Possible experimental realizations with ultracold atoms are discussed.

We investigate a system of fermions trapped in a quasiperiodic potential from an open quantum system theory perspective, designing a protocol in which an impurity atom (a two-level system) is coupled to a trapped fermionic cloud described by the noninteracting Aubry-André model. The Fermi system is prepared in a charge-density-wave state before it starts its relaxation. In this work we focus our attention on the time evolution of the impurity in such an out-of-equilibrium environment and study whether the induced dynamics can be classified as Markovian or non-Markovian. We find how the localized phase of the Aubry-André model displays evidence of strong and stable memory effects and can be considered as a controllable and robust non-Markovian environment.

We demonstrate that achieving zero backward scattering (ZBS), i.e., the first Kerker condition, allows for optical tweezing of high-index microspheres, which cannot be trapped using standard techniques. For this purpose, we propose an alternative material platform based on composite metamaterials. By tuning the volume filling fraction of inclusions and the microsphere radius, stable trapping can be achieved provided that ZBS is combined with the condition for destructive interference between the fields reflected at the external and internal interfaces of the microsphere when located at the focal point. By using the Mie-Debye-spherical aberration theory, we also show that the ZBS condition is even more useful in realistic, standard optical tweezer setups, in which spherical aberration is unavoidable due to refraction at the interface between the glass slide and the water-filled sample. Altogether, our findings not only pave the way for possible new trapping designs and applications but also unveil the role of backscattering in the physics of optical tweezers.