The theory of frame distortion in polyatomic molecules is reconsidered with the aid of a diagonal Herman-Wallis-type concept applied to nominally forbidden pure rotational transitions. Watson's formula for electric dipole forbidden transition moments is obtained using the formalism of extraneous quantum numbers [K. V. Kazakov, Quantum Theory of Anharmonic Effects in Molecules (Elsevier, Amsterdam, 2012)]. A similar consideration is permitted to derive the transition moments for magnetic dipole absorption in account of rovibrational interaction. The modified rotational selection rules and some numerical estimates are obtained by taking trial examples of highly symmetric molecules. The distortion electric dipole spectra of methane simulated for various temperatures are compared with the pertinent experimental data. Particular attention is given to trihydrium, for which rovibrational effects in both electric and magnetic dipole spectra are examined. It is shown that, first, the distortion magnetic moment of ${{\mathrm{H}}_3}^{+}$ is generated solely by electronic motion if the $g$ factor is diagonal, while the nuclear motion manifests itself through the off-diagonal inclusion of $g$, and, second, the smallness of this magnetic contribution to the line strength is quenched by the strong dependence on the rotational quantum number.

Ion mobilities of ${\mathrm{Lr}}^{+}$ $(Z=103)$ and of its lighter chemical homolog ${\mathrm{Lu}}^{+}$ $(Z=71)$ in helium were calculated for the ground state ${}^{1}S_0$ and the lowest metastable state ${}^{3}D_1$. To this end we applied the multireference configuration interaction method to calculate the ion-atom interaction potentials in the different states. The Gram-Charlier approach to solving the Boltzmann equation was used to deduce the mobilities of the different electronic states, based on the calculated interaction potentials. We found that the zero-field ion mobilities are similar for the ${\mathrm{Lr}}^{+}$ and ${\mathrm{Lu}}^{+}$ ions. In addition, the ion mobilities of the different states are substantially different for temperatures above $100\mathrm{K}$. The relative differences between the mobilities of the ground and excited states at room temperature are about $15\%$ and $13\%$ for ${\mathrm{Lu}}^{+}$ and ${\mathrm{Lr}}^{+}$ ions, respectively, which should be sufficiently large enough to enable laser resonance chromatography of these ions.

We propose an optical control scheme for driving a polar diatomic molecule from a high-lying vibrational level to a target low-lying one, within the same electronic state. The scheme utilizes an infrared chirped laser pulse with an analytical shape, whose parameters are optimized by means of a heuristic formulation of quantum optimal control based on a genetic algorithm. We illustrate this methodology computationally for a KRb Feshbach molecule in the lowest triplet electronic state.

Exceptional points emerge in the complex eigenspectra of non-Hermitian systems, and give rise to rich critical behaviors. An outstanding example is the chiral state transfer, where states can swap under an adiabatic encircling around the exceptional point, but only along one direction. In dissipative quantum systems, such exceptional-point encirclings are often accompanied by decoherence, whose impact is beyond the description of non-Hermitian Hamiltonians. In this paper, we study in detail the effects of dephasing on the encircling dynamics, adopting the full Lindblad master equation. Introducing experimentally relevant quantum-jump processes that account for dephasing, we show that gaps emerge in the eigenspectra landscape of the corresponding Liouvillian superoperator. It follows that the chiral state transfer does not take place in the adiabatic limit, since the system always adiabatically follows the quasisteady state of the Liouvillian regardless of the encircling direction. Nevertheless, the chirality is restored at intermediate encircling times, where the dynamics is nonadiabatic in both encircling directions, distinct from the typical chiral state transfer in non-Hermitian systems. While our results are applicable to several recent experiments, we examine a recent cold-atom experiment in particular, and show that the observed long-time chirality is limited to the special encircling path therein. Our paper provides further insight into the chiral state transfer under experimental conditions, and is helpful for controlling open-system dynamics from the perspective of non-Hermitian physics.

Weyl points exist in a fascinating topological state of matter with linear band crossings analogous to magnetic monopoles. Tremendous efforts have been devoted to investigating fermionic topological materials with Weyl points in the single-particle band dispersion. It remains elusive for realizing interaction-induced Weyl points, especially for bosons. Motivated by recent experimental progress in ultracold atoms, we propose a scheme to create Weyl points for Bogoliubov excitations of a bosonic superfluid in a three-dimensional cubic optical lattice. The unique design of the lattice leads to interaction-induced time-reversal symmetry breaking for a $p$-orbital superfluid, which in turn induces Weyl Bogoliubov excitations. Analogous to Weyl semimetals of electronic systems, the superfluid also supports topologically protected edge modes due to the bulk-boundary correspondence.

We investigate the Meissner currents of interacting bosons subjected to a staggered artificial gauge field in a three-leg ribbon geometry, realized by spin-tensor–momentum coupled spin-1 atoms in a one-dimensional optical lattice. By calculating the current distributions using the state-of-the-art density-matrix renormalization-group method, we find a rich phase diagram containing interesting Meissner and vortex phases, where the currents are mirror symmetric with respect to the middle leg (i.e., they flow in the same direction on the two boundary legs, opposite to that on the middle leg), leading to spin-tensor-type Meissner currents, which is very different from previously observed chiral edge currents under uniform gauge field. The currents are uniform along each leg in the Meissner phase and form vortex-antivortex pairs in the vortex phase. Moreover, the system also supports a polarized phase that spontaneously breaks the mirror symmetry, whose ground states are degenerate with currents either being uniform or forming vortex-antivortex pairs. We also discuss the experimental schemes for probing these phases. Our work provides useful guidance for ongoing experimental research on synthetic flux ribbons and paves the way for exploring novel many-body phenomena therein.

We present a numerical study of the active harmonic mode locking (HML) of quantum cascade lasers (QCLs) in the terahertz (THz) spectral range. This is a recently experimentally demonstrated technique based on the modulation of the bias current of the laser by radio-frequency (rf) injection, allowing for the generation of multiple short pulses per round-trip in the QCL cavity. We first study the Fabry-Pérot (FP) configuration, unraveling the impact of both laser intrinsic parameters and rf injection on the characteristics of the generated harmonic pulses. Our study demonstrates that the key parameter for the achievement of ultrashort pulses is the QCL gain bandwidth: by pushing this quantity beyond 1 THz, we reproduce second-order ultrashort harmonic pulses with a duration of 2.5 ps. This suggests that the combination of the HML technique with the design of optimized cavities providing a larger gain, such as multistack active regions, can allow for the generation of narrow pulses approaching the picosecond limit in THz QCLs. Then a direct comparison with the experiment is presented, which reveals that the introduction of an unmodulated section in the laser cavity is crucial to considerably reduce the duration of the generated pulses. Finally, motivated by recent observations of high-contrast localized structures such as optical solitons in free-running ring QCLs, we investigate the formation of harmonic pulses in these unidirectional cavities. We make a comparative analysis aimed at understanding if this configuration can guarantee an improvement in the performance of the HML with respect to the FP case. We find that the intrinsic properties of ring QCLs, such as unidirectionality and the absence of spatial hole burning, lead to a 1.5-ps pulse duration, promoting these devices as strong candidates for the generation of ultrashort pulses in the THz range.

The role of a chiral quantum-dot molecule (QDM) system in controlling and enhancing the Goos-Hänchen (GH) shift is revealed here. The proposed setup is a four-level atomic system excited by incoherent pump and probe fields where the electron tunneling is prominent in affecting the GH shift upon the reflection and transmission of probe beams. The GH shifts in the chiral QDM system are calculated analytically by using stationary phase theory, whereas the dynamics of the quantum-dot system are derived by the density-matrix method. It is demonstrated that the GH shifts through transmission and reflection light beams are strongly influenced due to right-circularly polarized (RCP) and left-circularly polarized light. The GH shifts through RCP light are enhanced in the presence of chiral QDM systems, and these shifts could become either highly negative or highly positive, sensitively depending on the electron tunneling and chiral coefficients. This proposal can be viewed as a theoretical approach to the potential application of GH shifts in nano-optoelectronic sensors.

Quantum theory allows for nonlocality without entanglement. Notably, there exist bipartite quantum measurements consisting of only product eigenstates, yet they cannot be implemented via local quantum operations and classical communication. In the present work, we show that a measurement exhibiting nonlocality without entanglement can be certified in a device-independent manner. Specifically, we consider a simple quantum network and construct a self-testing procedure that is robust to noise. This result also demonstrates that genuine network quantum nonlocality can be obtained using only nonentangled measurements. From a more general perspective, our work establishes a connection between the effect of nonlocality without entanglement and the area of Bell nonlocality.

We study a one-dimensional system subjected to a linearly varying imaginary vector potential, which is described by the single-particle continuous Schrödinger equation and is analytically solved. The eigenenergy spectrum is found to be real under open boundary condition (OBC) but forms a parabola in the complex energy plane under periodic boundary condition (PBC). The eigenstates always exhibit a modulated Gaussian distribution and are all pinned on the same position, which is determined by the imaginary vector potential and boundary conditions. These behaviors are in sharp contrast to the non-Hermitian skin effect (NHSE) in systems with constant imaginary vector potential, where the eigenstates are exponentially distributed under OBC but become extended under PBC. We further demonstrate that even though the spectrum under PBC is an open curve, the Gaussian type of NHSE still has a topological origin and is characterized by a nonvanishing winding number in the PBC spectrum. The energies interior to the parabola can support localized edge states under semi-infinite boundary condition. The corresponding tight-binding lattice models also show similar properties, except that the PBC spectrum forms closed loops. Our work opens a door for the study of quantum systems with spatially varying imaginary vector potentials.

We study the geometric Uhlmann phase of mixed states at finite temperature in a system of two coupled spin-$\frac{1}{2}$ particles driven by a magnetic field applied to one of the spins. In the parameter space of temperature and coupling, we show the emergence of two topological Uhlmann phase transitions when the magnetic field evolves around the equator, where a winding number can characterize each temperature range. For small couplings, the width of the temperature gap of the nontrivial phase is roughly the critical temperature $T_c$ of one-dimensional fermion systems with two-band Hamiltonians. The first phase transition in the low-temperature regime and small values of the coupling corresponds to the peak of the Schottky anomaly of the heat capacity, typical of a two-level system in solid-state physics involving the ground and first excited states. The second phase transition occurs at temperatures very close to the second maximum of the heat capacity associated with a multilevel system. We also derive analytical expressions for the thermal Uhlmann phase for both subsystems, showing that they exhibit phase transitions. In the driven subsystem, for minimal $g$, a topological phase transition appears at $T_c$ again. However, for larger values of $g$, the transitions occur at lower-temperature values, and they disappear when the coupling reaches a critical value $g_c$. The latter is not the case for the undriven subsystem, where at low temperatures, a single phase transition occurs at $g_c$. Nevertheless, as the temperature rises, we demonstrate the emergence of two phase transitions defining a coupling gap, where the phase is nontrivial and vanishes as the temperature reaches a critical value.

Intermediate-scale quantum devices are becoming more reliable and may soon be harnessed to solve useful computational tasks. At the same time, common classical methods used to verify their computational output become intractable due to a prohibitive scaling of required resources with system size. Inspired by recent experimental progress, here we describe and analyze efficient cross-platform verification protocols for quantum states and show how these can be used to verify computations. We focus on the pairwise comparison between distant nodes of a quantum network, identify the most promising protocols, and discuss possible implementations in laboratory settings. As a proof of principle, we implement basic versions of these schemes on available quantum processors.

We show how to estimate a broad class of multipartite entanglement measures from Bell-basis measurement data. In addition to lowering the experimental requirements relative to previously known methods of estimating these measures, our proposed scheme also enables a simpler analysis of the number of measurement repetitions required to achieve an $\epsilon$-close approximation of the measures, which we provide for each. We focus our analysis on the recently introduced concentratable entanglements [Beckey et al., Phys. Rev. Lett. 127, 140501 (2021)] because many other well-known multipartite entanglement measures are recovered as special cases of this family of measures. We extend the definition of the concentratable entanglements to mixed states and show how to construct lower bounds on the mixed state concentratable entanglements that can also be estimated using only Bell-basis measurement data. Finally, we demonstrate the feasibility of our methods by classically simulating their implementation on a noisy Rydberg atom quantum computer.

Readout errors on near-term quantum computers can introduce significant error to the empirical probability distribution sampled from the output of a quantum circuit. These errors can be mitigated by classical postprocessing given the access of an experimental response matrix that describes the error associated with the measurement of each computational basis state. However, the resources required to characterize a complete response matrix and to compute the corrected probability distribution scale exponentially with the number of qubits, $n$. In this work, we modify standard matrix inversion techniques using perturbative approximations with significantly reduced complexity and bounded error when the likelihood of high-order bit-flip events is strongly suppressed. Given a characteristic error rate $q$, we discuss a method to recover the probability of the all-zeros bit string $p_0$ by sampling only a small subspace of the response matrix before inverting readout error, resulting in a relative speedup of $\text{poly}[2^n/(\begin{array}{c}n\\ w\end{array}\left)\right]$, which we motivate using a simplified error model for which the approximation incurs only $O\left(q^w\right)$ error for some integer $w$. We then provide a generalized technique to efficiently recover full output distributions with $O\left(q^w\right)$ error in the perturbative limit. These approximate techniques for readout-error correction may greatly accelerate near-term quantum computing applications.

In a practical continuous-variable quantum-key-distribution (CVQKD) system, it is vital to accurately evaluate and then compensate for the phase drifts of the signals, so that the involved system can achieve better performance and stability. In this paper, based on the long short-term memory network (LSTM) model, an automatic phase compensation approach of the CVQKD system is proposed. The LSTM model is first trained to predict the phase drift value of the quantum signal relative to the local oscillator over time. Then, the predicted phase drift value can be used by Alice to reconstruct her data. Finally, Alice and Bob can obtain the raw key, so that the CVQKD system can achieve enhancements in terms of performance and stability. The experimental results indicate that the proposed LSTM-based automatic phase compensation algorithm can accurately predict the phase drift value and perform phase compensation instead of real-time phase drift measurement, which improves the performance of the CVQKD system without requiring any additional quantum resources and extra experimental hardware.

Multiphoton split states, where each photon is in a different spatial mode, represent an essential resource for various quantum applications, yet their efficient characterization remains an open problem. Here, we formulate the general structure of their reduced spatial density matrices and identify the number of real and complex-valued independent coefficients, which in particular completely determine the distinguishability of all photons. Then, we show that this density matrix can be fully characterized by measuring correlations after photon interference in a static integrated circuit, where the required number of outputs scales subquadratically versus the number of photons. We present optimized circuit designs composed of segmented coupled waveguides, representing a linear optical neural network, which minimize the reconstruction error and facilitate robustness to fabrication deviations.

Due to turbulence and tracking errors, free-space optical channels involving mobile transceivers are characterized by a signal's partial loss or complete erasure. This work presents an error-correction protocol capable of protecting a signal passing through such channels by encoding it with an ancillary entangled bipartite state. Beyond its ability to offer protection under realistic channel conditions, our protocol has the ability to transmit both a quantum state and a classical symbol using the same encoded state. We show how the protocol can improve the fidelity of transmitted coherent states over a wide range of losses and erasure probabilities relative to nonencoded direct transmission. In addition, the use of ancillary non-Gaussian entangled bipartite states in the signal encoding is considered, showing how this can increase performance. Finally, we briefly discuss the application of our protocol to the transmission of more complex input states, such as multimode entangled states.

We measured the spontaneous decays of internally hot copper and silver dimer anions, ${\mathrm{Cu}}_2^{-}$ and ${\mathrm{Ag}}_2^{-}$, stored in one of the two ion-beam storage rings of the Double Electrostatic Ion Ring Experiment (DESIREE) at Stockholm University. A coincidence detection technique was utilized enabling essentially background-free measurements of ${\mathrm{Cu}}_2^{-}\to$ Cu $+ {\mathrm{Cu}}^{-}$ and ${\mathrm{Ag}}_2^{-}\to$ Ag $+ {\mathrm{Ag}}^{-}$ fragmentation rates. Furthermore, the total rates of neutral decay products (monomers and dimers) were measured and the relative contributions of fragmentation and electron emission (${\mathrm{Cu}}_2^{-}\to {\mathrm{Cu}}_2 + e^{-}$ and ${\mathrm{Ag}}_2^{-}\to {\mathrm{Ag}}_2 + e^{-}$) were deduced as functions of storage time. Fragmentation is completely dominant at early times. However, after about 20 ms of storage, electron emission is observed and becomes the leading decay path after 100 ms for both dimer anions. The branching ratios between fragmentation and electron emission (vibrationally assisted autodetachment processes) are very nearly the same for ${\mathrm{Cu}}_2^{-}$ and ${\mathrm{Ag}}_2^{-}$ throughout the present storage cycle of 10 seconds. This is surprising considering the difference between the electron affinities of the neutral dimers, ${\mathrm{Cu}}_2$ and ${\mathrm{Ag}}_2$, and the difference between the ${\mathrm{Cu}}_2^{-}$ and the ${\mathrm{Ag}}_2^{-}$ dissociation energies.

We obtain continuum molecular wavefunctions for open-shell molecules in the Hartree-Fock framework. We do so while accounting for the singlet or triplet total spin symmetry of the molecular ion, that is, of the open-shell orbital and the initial orbital where the electron ionizes from. Using these continuum wavefunctions, we obtain the dipole matrix elements for a core electron that ionizes due to single-photon absorption by a linearly polarized x-ray pulse. After ionization from the x-ray pulse, we control or streak the electron dynamics using a circularly polarized infrared (IR) pulse. For a high-intensity IR pulse and photon energies of the x-ray pulse close to the ionization threshold of the $1\sigma$ or $2\sigma$ orbitals, we achieve control of the angle of escape of the ionizing electron by varying the phase delay between the x-ray and IR pulses. For a low-intensity IR pulse, we obtain final electron momenta distributions on the plane of the circularly polarized IR pulse and we find that many features of these distributions correspond to the angular patterns of electron escape solely due to the x-ray pulse.

The formation of a Raman-induced temporal waveguide is demonstrated by launching short pump and probe pulses inside a photonic crystal fiber. The pump pulse creates a fundamental soliton whose speed changes continuously owing to its deceleration through the Raman-induced red shift of its spectrum. The spectrum of the probe pulse is blue-shifted to ensure that the two pulses move at the same speed and follow the same trajectory over the entire length of the fiber. The output wavelengths of the pump and probe pulses depend on the peak power of input pump pulses and their measured values agree with the predictions based on the dispersion data. Numerical modeling also shows good agreement with the experimental results.