Dependence of the angle-integrated probability of the ionization of a polarized atom by a few-cycle circularly polarized (CP) pulse on the rotation direction of the pulse electric field is analyzed. Numerical simulations of the ionization of excited states of the hydrogen atom reveal that the circular dichroism effect, i.e., the difference of the probability for right- and left-hand circular polarizations of the laser pulse, exhibits oscillatory behavior vanishing for some values of the photoelectron energy. This feature is most pronounced for intermediate (on the atomic scale) CP pulse intensities and very short pulse durations. We have applied the nonstationary perturbation theory (PT) in all orders to analyze the observed effects. Results of our treatment demonstrate that PT can provide an adequate qualitative description of the ionization by ultrashort pulses whose intensities are comparable to the atomic ones only upon taking into account ionization channels involving the virtual photon emission.

We report the formation of $\mathrm{S}{\mathrm{F}}_6^{-}$ in a molecular beam containing coexpanded NO and $\mathrm{S}{\mathrm{F}}_6$ by direct electron transfer from laser-excited Rydberg states of NO, and electron attachment from ultracold plasma induced by Penning ionization or photoionization of NO. At short times of up to 2 µs after laser excitation, we find that the dominant pathway for near-zero-energy electron attachment to $\mathrm{S}{\mathrm{F}}_6$ molecules forming $\mathrm{S}{\mathrm{F}}_6^{-}$ ions is from Rydberg electron transfer from excited Rydberg states, within a few $\mathrm{c}{\mathrm{m}}^{-1}$ of the Rydberg limit. $\mathrm{S}{\mathrm{F}}_6^{-}$ ion production occurs by electron transfer from NO Rydberg states with a principal quantum number $n\ge 165$. At this principal quantum number threshold, the Rydberg reaction range is ∼15 µm. For lower-lying Rydberg states of NO, no $\mathrm{S}{\mathrm{F}}_6^{-}$ ions are detected despite the efficiency of $\mathrm{S}{\mathrm{F}}_6$ as an electron scavenger of weakly bound electrons. We hypothesize that $\mathrm{S}{\mathrm{F}}_6^{-}$ anions formed in closer proximity to a $\mathrm{N}{\mathrm{O}}^{+}$ cation core may undergo efficient back-electron transfer.

Within a joint experimental and theoretical research project, double photoionization of the He-like ${\mathrm{B}}^{3+}$ ion by a single photon was studied in the energy range from approximately 350 to 1160 eV. With the parent-ion beam in the experiment containing both $1s^2{}^{1}S$ ground-state (90.9%) and $1s2s{}^{3}S$ metastable ${\mathrm{B}}^{3+}$ ions (9.1%), two contributions to the apparent single- and double-ionization cross sections, ${\sigma }_{34}^{\text{app}}$ and ${\sigma }_{35}^{\text{app}}$, respectively, are involved in the measurements. Ratios $Y(5+)/Y(4+)$ of double- over single-ionization yields were experimentally determined. By multiplying the measured ratios by the known cross section ${\sigma }_{34}^{\text{app}}$, the apparent cross section ${\sigma }_{35}^{\text{app}}$ for double ionization was inferred. Using the information thus obtained, the measured yield of ${\mathrm{B}}^{5+}$ ions produced from hydrogenlike ${\mathrm{B}}^{4+}$ could also be normalized. The results show good agreement with theoretical benchmark cross sections ${\sigma }_{45}$ for single photoionization of ${\mathrm{B}}^{4+}$. Convergent close-coupling calculations were performed to determine double-photoionization cross sections ${\sigma }_{35}$ for ${\mathrm{B}}^{3+}\left(1s^2\right)$ and ${\mathrm{B}}^{3+}\left(1s2s\right)$ ions. The result for the ground state agrees very well with theoretical data obtained previously with an intermediate-energy $R$-matrix approach. Within the systematic uncertainty of the normalized experimental cross sections, calculated and measured data are in agreement with one another.

Spin-orbit coupling (SOC) in ultracold atoms is engineered by light-atom interaction, such as two-photon Raman transitions between two Zeeman spin states. In this paper, we propose and experimentally realize chiral Raman coupling to generate SOC in ultracold atomic gases, which exhibits high quantization axis direction dependence. Chiral Raman coupling for SOC is created by chiral light-atom interaction, in which a circularly polarized electromagnetic field generated by two Raman lasers interacts with two Zeeman spin states $\delta m_F=\pm 1$ (chiral transition). We present a simple scheme of chiral one-dimensional (1D) Raman coupling by employing two Raman lasers at an intersecting angle ${90}^{\circ }$ with the proper polarization configuration. In this case, Raman coupling for SOC exists in one direction of the magnetic quantization axis and disappears in the opposite direction. Then we extend this scheme into a chiral two-dimensional (2D) optical square Raman lattice configuration to generate the 1D SOC. There are two orthogonal 1D SOCs, which exist in the positive and negative directions of the magnetic quantization axis respectively. This case is compared with 2D SOC based on the nonchiral 2D optical Raman lattice scheme for studying the topological energy band. This paper broadens the horizon for understanding chiral physics and simulating topological quantum systems.

We study the out-of-equilibrium dynamics of the Bose-Hubbard model for two-component bosons using a strong-coupling approach within the closed-time-path formalism and develop an effective theory for the action of this problem. We obtain equations of motion for the superfluid order parameters of both boson species for both the superfluid and Mott-insulating phases and study these in the low-frequency, long-wavelength limit during a quantum quench for various initial conditions. We find that an additional degree of freedom for bosons leads to a richer phase diagram and out-of-equilibrium dynamics than the single-component situation.

Bosonic quantum error correction codes encode logical qubits in the Hilbert space of one or multiple harmonic oscillators. A prominent class of bosonic codes is that of Gottesman-Kitaev-Preskill (GKP) codes of which implementations have been demonstrated with trapped ions and microwave cavities. In this paper, we investigate theoretically the preparation and error correction of a GKP qubit in a vibrational mode of a neutral atom stored in an optical dipole trap. This platform has recently shown remarkable progress in simultaneously controlling the motional and electronic degrees of freedom of trapped atoms. The protocols we develop make use of motional states and, additionally, internal electronic states of the trapped atom to serve as an ancilla qubit. We compare optical tweezer arrays and optical lattices and find that the latter provide more flexible control over the confinement in the out-of-plane direction, which can be utilized to optimize the conditions for the implementation of GKP codes. Concretely, the different frequency scales that the harmonic oscillators in the axial and radial lattice directions exhibit and a small oscillator anharmonicity prove to be beneficial for robust encodings of GKP states. Finally, we underpin the experimental feasibility of the proposed protocols by numerically simulating the preparation of GKP qubits in an optical lattice with realistic parameters.

We propose an implementation of bivariate bicycle codes [S. Bravyi et al., Nature (London) 627, 778 (2024)] based on long-range Rydberg gates between stationary neutral atom qubits. An optimized layout of data and ancilla qubits reduces the maximum Euclidean communication distance needed for nonlocal parity-check operators. An optimized Rydberg gate pulse design enables $\mathsf{CZ}$ entangling operations with fidelity $\mathcal{F}>0.999$ at a distance greater than $12\mu \mathrm{m}$. The combination of optimized layout and gate design leads to a quantum error correction cycle time of $\sim 1.28\mathrm{ms}$ for a [[144,12,12]] code, which is nearly a factor-of-two improvement over previous designs.

Cross sections for electron scattering on the ground state of LiH are calculated using the molecular convergent close-coupling method. The fixed-nuclei approximation is utilized, and calculations are performed at the mean internuclear separation of $3.06a_0$ for projectile energies ranging from 0.1 to 500 eV. A model-potential approach is utilized to treat the LiH molecule as a two-electron system, and good agreement is found with the literature for various properties of the structure model. Cross sections are presented for elastic scattering, excitation to several electronic states, and ionization. Comparison is made with existing results, and qualitative agreement is found with small close-coupling calculations at low projectile energies. Good agreement is found with first-order results at high projectile energies.

This study theoretically investigates the angular distribution of core-level photoem ission time delay within a molecular frame. This phenomenon can be measured with the advancement of attosecond pulsed lasers and metrology. Our focus is on homonuclear diatomic molecules. The two-center interference patterns observed in the gerade and ungerade core-level Molecular-Frame Photoelectron Angular Distributions (MFPAD) of homonuclear diatomic molecules demonstrate symmetry breaking with respect to the direction of light propagation, attributed to the nondipole (multipole) effect. This nondipole effect reflects the zeptosecond time discrepancy in the initiation of photoelectron waves at the two absorbing atoms. Our study delves into the photoemission time delay resulting from the nondipole effect through the introduction of a theoretical model. We reveal that when considering the contributions from the gerade and ungerade delocalized states in incoherent sums, the two-center interference terms cancel each other in both the MFPADs and photoemission time delays. However, a residual term persists showcasing the nondipole effect in the photoemission time delays. Furthermore, by expanding the scattering state of photoelectrons using the Multiple Scattering theory, we demonstrate the significant role played by the scattering of photoelectrons at the molecular potential in describing the photoemission time delays of homonuclear diatomic molecules. Additionally, we establish that the direct wave contribution does not exhibit angular dependence in the photoemission time delays. Next, we apply our theoretical model to a nitrogen molecule, demonstrating the energy- and angular-dependent characteristics of the MFPADs and photoemission time delays through both analytical and numerical approaches. The incoherent sums of the MFPADs in both forward and backward directions exhibit equal intensity, whereas the incoherent sums of the photoemission time delays show a slight variation of a few hundred zeptoseconds compared with numerical calculations using a multiple scattering code. Our findings demonstrate that the photoemission time delay serves as a valuable tool for investigating attosecond and zeptosecond photoionization dynamics, providing insights that may be overlooked in the MFPAD analysis. This study provides a basis for theoretical investigations into photoionization phenomena in nano systems containing numerous identical atoms, such as large polyatomic molecules and condensed matter, enabling the examination of photoemission time delays and nondipole effects.

It is well known that the Aubry-André model lacks mobility edges due to its energy-independent self-duality but may exhibit edge states. When duality is broken, we show that mobility regions arise and nontrivial topological phases emerge. By varying the degree of duality breaking, we identify mobility regions and establish a connection between Aubry-André atomic wires with fermions and quantum Hall systems for a family of Hamiltonians that depends on the relative phase of laser fields, viewed as a synthetic dimension. Depending on the filling factor and the degree of duality breaking, we find three classes of nontrivial phases: a conventional topological insulator, a conventional topological Aubry-André insulator, and an unconventional (hybrid) topological Aubry-André insulator. Finally, we discuss appropriate Chern numbers that illustrate the classification of topological phases of localized fermions in atomic wires.

Spontaneous symmetry breaking is a phenomenon of an alteration of a state symmetry without a change in the system symmetry. A transition from a state with unbroken symmetry to a state with broken symmetry leads to a qualitative change in system properties and respective states are called symmetric and nonsymmetric phases of the system. Usually, the change in the system parameters is necessary for the transition between symmetric and nonsymmetric phases. In this paper, we consider the behavior of an optomechanical system with an exceptional point under the influence of noise. We demonstrate that below the exceptional point, $\mathcal{PT}$-symmetric and non-$\mathcal{PT}$-symmetric phases can coexist. In this regime, a noise can lead to random transitions between the symmetric and nonsymmetric phases. These transitions are accompanied by the abrupt changes in the intensities of photon and phonon modes. The predicted phenomenon opens up the possibility of studying the kinetics of phase transitions caused by spontaneous breaking of $\mathcal{PT}$ symmetry.

The interplay between information-scrambling Hamiltonians and local continuous measurements hosts platforms for exotic measurement-induced phase transition in out-of-equilibrium steady states. Here, we consider such transitions under the addition of local random white noise and measurement inefficiency in an XX spin chain. We identify a nonmonotonic dependence on the local noise strength in both the averaged entanglement and operator correlations, specifically the subsystem parity variance. While the nonmonotonicity persists at any finite efficiency for the operator correlations, it disappears at finite inefficiency for the entanglement. The analysis of scaling with the system size in a finite-length chain indicates that, at finite efficiency, this effect leads to distinct measurement-induced phase transitions for operator correlations and entanglement. Our result hints at a difference between area-law entanglement scaling and Zeno-localized phases for inefficient monitoring.

Despite its simplicity and strong theoretical guarantees, adiabatic state preparation has received considerably less interest than variational approaches for the preparation of low-energy electronic structure states. Two major reasons for this are the large number of gates required for Trotterizing time-dependent electronic structure Hamiltonians, as well as discretization errors heating the state. We show that a recently proposed randomized algorithm [E. Granet and H. Dreyer, npj Quantum Inf. 10, 82 (2024)], which implements exact adiabatic evolution without heating and with far fewer gates than Trotterization, can overcome this problem. We develop three methods for measuring the energy of the prepared state in an efficient and noise-resilient manner, yielding chemically accurate results on a four-qubit molecule in the presence of realistic gate noise, without the need for error mitigation. These findings suggest that adiabatic approaches to state preparation could play a key role in quantum chemistry simulations both in the era of noisy as well as error-corrected quantum computers.

Deterministic quantum computation with one qubit (DQC1) is of significant theoretical and practical interest due to its computational advantages in certain problems, despite its subuniversality with limited quantum resources. In this work, we introduce parametrized DQC1 as a quantum machine learning model. We demonstrate that the gradient of the measurement outcome of a DQC1 circuit with respect to its gate parameters can be computed directly using the DQC1 protocol. This allows for gradient-based optimization of DQC1 circuits, positioning DQC1 as a quantum protocol for both training and inference. We then analyze the expressivity of the parametrized DQC1 circuits characterizing the set of learnable functions, and show that DQC1-based machine learning (ML) is as powerful as quantum neural networks based on universal computation. Our findings highlight the potential of DQC1 as a practical and versatile platform for ML, capable of rivaling more complex quantum computing models while utilizing simpler quantum resources.

We consider a periodically driven system where the high-frequency driving protocol consists of a sequence of potentials switched on and off at different instants within a period. We explore the possibility of introducing an adiabatic modulation of the driving protocol by considering a slow evolution of the instants when the sequence of potentials is switched on and off. We examine how this influences the long-term dynamics of periodically driven quantum systems. By assuming that the slow and fast timescales in the problem can be decoupled, we derive the stroboscopic (effective) Hamiltonian for a four-step driving sequence up to the first order in perturbation theory. We then apply this approach to a spin-$1/2$ system, where the adiabatic modulation of the driving protocol is chosen to produce an evolving emergent magnetic field that interacts with the spin. We study the emergence of diabolical points and diabolical loci in the parameter space of the effective Hamiltonian. Further, we study the topological properties of the maps of the adiabatic paths in the parameter space to the eigenspace of the effective Hamiltonian. In effect, we obtain a technique for tuning the topological properties of the eigenstates by selecting different adiabatic evolutions of the driving protocol, characterized by distinct paths in parameter space. This technique can be applied to any periodic driving protocol to achieve desirable topological effects.

It is well known in quantum machine learning that the form of the feature map has a significant effect on the operation and performance of those tasks being undertaken. It is essential to investigate how to generate an effective feature map within the limitations of the number of qubits and quantum gates to apply in noisy intermediate-scale quantum devices. In this paper, we explore how the qubit connectivity impacts the performance of quantum extreme reservoir computation (QERC) for image classification using the XY Hamiltonian. We find that networks of qubits with simple connectivity generate an effective feature map, whereas a network with high connectivity does not since the XY Hamiltonian on it yields a unitary map with a periodic property. The existence of the periodicity can be fully determined by the underlying connectivity and the overall symmetry of the Hamiltonian. However, the symmetries in the XY Hamiltonians can contribute to a better generalization for QERC. We further find that it is possible to design QERC to give this high generalization property while maintaining high accuracy. Our work suggests that Hamiltonian dynamics with simple connectivity or symmetry can be exploited as a computational resource in quantum machine learning.

Generating stable entanglement by mimicking a squeezed environment plays an important role in quantum metrology and quantum information. Here, we propose an efficient scheme for generating stable entanglement of two boron-vacancy (VB) defects in a suspended circular hexagonal boron nitride ($h$-BN) membrane via a two-phonon drive. Compared to other reported methods, our scheme only needs a linear resource and thus is more convenient for experimental implementation. In the presence of a gradient magnetic field, the electronic spin degree of freedom of the VB defect can be coupled to the mechanical mode of $h$-BN membrane through spin-phonon interaction. The coupling strength is closely related to the geometry of the magnetic field and the parameters of the membrane, which allows us to realize different coupling regimes by choosing the parameter appropriately. After applying a time-dependent pump to the $h$-BN membrane, we can modulate its spring constant and obtain a tunable two-phonon drive to the mechanical mode. We then investigate the dissipative coupling between two VB spins in the condition of a large dissipation limit, showing that the stable entanglement can be generated under a two-phonon drive. In addition, we also take the spin dephasing into consideration to explore the robustness of our scheme. We then extend this scheme to multiparticle systems, which shows that the entanglement degree deepens with the increase of VB spin number. This work offers a fascinating platform for generating stable entanglement with VB spins in two-dimensional $h$-BN.

$\mathrm{\Lambda }$, $V$, $\mathrm{\Xi }$ (ladder), and other three-level quantum systems with one forbidden transition (referred to here as $\mathrm{\Lambda }$-type systems) play an important role in quantum physics. Various applications require manipulation of such systems using as a control a shaped laser field. In this work, we study how the degeneracy of energy states or of Bohr frequencies in these systems affects the efficiency or difficulty of finding the optimal shape of the control field. For this, we adopt the notion of higher-order traps, which was introduced in Pechen and Tannor, Phys. Rev. Lett. 106, 120402 (2011), where second-/third-order traps were discovered for a $\mathrm{\Lambda }$-atom with one forbidden transition and with nondegenerate energy levels. We theoretically study the control of such systems with and without degeneracy in their eigenstates and Bohr frequencies, and we investigate numerically using GRAPE and l-BFGS algorithms how this degeneracy influences the efficiency of optimizing the control laser field. We find that the degeneracy of the Bohr frequencies in the $\mathrm{\Xi }$ system, which makes the system energy levels symmetrically distributed, leads to the appearance of a seventh-order trap with a more significant attracting domain resulting in a more difficult optimization, while the degeneracy of energy states in generic $\mathrm{\Lambda }$-type systems does not lead to an increase of the order of the zero control trap compared to the nondegenerate case. We also find that when not only are the Bohr frequencies degenerate in the system $\mathrm{\Xi }$ but also the dipole moments for the two allowed transitions coincide (in this case, the $\mathrm{\Xi }$ system is not controllable), then true traps arise in the quantum control landscape. In particular, the constant zero control becomes a trap.

Digitizing an adiabatic evolution is a strategy able to combine the good performance of gate-based quantum processors with the advantages of adiabatic algorithms, thus providing a hybrid model for efficient quantum information processing. In this work we develop validity conditions for high-fidelity digital adiabatic tasks. To this end, we assume a digitizing process based on the Suzuki-Trotter decomposition, which allows us to introduce a digitized adiabatic theorem. As a consequence of this theorem, we show that the performance of such a hybrid model is limited by the fundamental constraints on the adiabatic theorem validity, even in ideal quantum processors. We argue how our approach predicts the existence of intrinsic nonadiabatic errors reported by Barends et al. [Nature (London) 534, 222 (2016)] through an empirical study of digital annealing. In addition, our approach allows us to explain the existence of a scaling of the number of Suzuki-Trotter blocks for the optimal digital circuit with respect to the optimal adiabatic total evolution time, as reported by Mbeng et al. [Phys. Rev. B 100, 224201 (2019)] through robust numerical analysis of digital annealing. We illustrate our results through two examples of digitized adiabatic algorithms, namely, the two-qubit exact-cover problem and the three-qubit adiabatic factorization of the number 21.