Dynamical decoupling represents an active approach towards the protection of quantum memories and quantum gates. Because dynamical decoupling operations can interfere with a system's own time evolution, the protection of quantum gates is more challenging than that of quantum states. In this work, we put forward a simple but general approach towards the realization of higher-order protection of quantum gates and further execute the first cloud-based demonstration of dynamical-decoupling-protected quantum gates at the first order and the second order. The central idea of our approach is to engineer (hence regain the control of) the gate Hamiltonian in coordination with higher-order dynamical decoupling sequences originally proposed for the protection of quantum memories. The physical demonstration on an IBM quantum processor indicates the effectiveness and potential of our approach on noisy intermediate scale quantum computers.

Two-qubit gates are a fundamental constituent of a quantum computer and typically its most challenging operation. In a trapped-ion quantum computer, this is typically implemented with laser beams which are modulated in amplitude, frequency, phase, or a combination of these. The required modulation becomes increasingly more complex as the quantum computer becomes larger, complicating the control hardware design. Here we develop a simple method to essentially remove the pulse-modulation complexity by engineering the normal modes of the ion chain. We experimentally test the feasibility of the method in a three-ion chain. This opens up the possibility to trade off complexity between the design of the trapping fields and the optical control system, which will help scale the ion-trap quantum computing platform.

Quantum transducers are essential for connecting hybrid quantum systems with distinct frequencies, pivotal for hybrid quantum networks. A deterministic quantum transducer protocol is proposed for the upconversion of single phonons in the microwave domain to optical photons. Utilizing a hybrid optomechanical crystal coupled to a three-level emitter, the protocol deterministically upconverts an itinerant phonon into an optical photon, when an auxiliary optical cavity mode is driven by a single photon. Furthermore, the deterministic upconversion transducer can also be achieved by driving the emitter with a classical laser pulse, rather than driving the optical cavity with a single photon. The intracavity photon numbers in both protocols are significantly less than one during the operation, thus overcoming the problem of parasitic heating within radiation-pressure coupling of hybrid optomechanical systems. These outstanding features make the protocols particularly useful for the realization of large-scale distributed hybrid quantum networks.

We propose to realize a mechanical dynamical Casimir effect (MDCE) in a hybrid optomechanical system consisting of a cavity mode, a low-frequency mechanical oscillator, and a two-level atomic system. Described by the effective Hamiltonian, the mechanical energy is directly converted to the photons through a three-wave-mixing mechanism. It is not a quantum simulation of a parametric DCE such as in superconducting circuits. Using a master-equation approach, we analyze the system dynamics in various regimes with respect to the ratio of the effective coupling strength and the loss rate of the system. The dynamics under the strong-coupling regime confirms various three-wave-mixing processes for creating photons by annihilation of mechanical and atomic excitations. Under the weak-coupling regime, a continuous production of photons can be demonstrated by driving both the mechanical oscillator and atom. By virtue of an atom of tunable frequency, our method avoids using the high-frequency mechanical oscillator, which is required for the conventional DCE in optomechanical systems under the double-photon resonance yet is out of reach of experiment. It is found that the mechanical frequency can be about two orders smaller than the cavity mode in yielding a remarkable flux of DCE photons.

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.