The quantum level population behavior of two coupled flux qubits depending on the external driving field characteristics is studied. Explicit expressions for the multiphoton transition probabilities at an arbitrary control field amplitude are obtained for the case of small tunnel splitting energies. We describe the controllable features of their formation and thereby the creation or destruction of entanglement on the direct interlevel transition and during the transition through intermediate states by system bias tuning. We found a feature of the qubit population inversion that results in the independence of the resonance positions from the qubit coupling strength. Using the Floquet-Markov equation, we numerically demonstrate that the positions of multiphoton resonances are stable to dissipative processes.

Quantum computation has seen rapid progress in the past years. Due to implementation complexity and cost, cloud-based quantum computing is strongly believed to be the most feasible framework for individual users. Universal blind quantum computing (UBQC) provides the protocol for secure delegation of arbitrary quantum computations and has received significant attention. However, one of the major challenges in UBQC is a secure and reliable transmission of a quantum state over a long distance. Herein, we improve the efficiency of UBQC using alternative photon sources. Our analysis shows that a heralded single-photon source can significantly improve the transmitted distance of UBQC and a modified coherent source can reduce the required transmitted number of pulses. Furthermore, we study the finite-data-size effect, which is a crucial for real-world applications. Our proposed scheme can be a promising alternative for the future development of UBQC.

Neutral atoms are promising for large-scale quantum computing, but accurate neutral-atom entanglement depends on large Rydberg interactions which strongly limit the interatomic distances. Via a phase accumulation in detuned Rabi cycles enabled by a Rydberg interaction of similar magnitude to the Rydberg Rabi frequency, we study a controlled-phase gate with an arbitrary phase and extend it to the controlled-not gate. The gates need only three steps for coupling one Rydberg state, depend on an easily accessible van der Waals interaction that naturally arises between distant atoms, and have no rotation error in the weak interaction regime. Importantly, they can work with very weak interactions so that well-separated qubits can be entangled. The gates are sensitive to the irremovable fluctuation of Rydberg interactions, but can still have a fidelity over 98% with realistic position fluctuation of qubits separated over 20 $\mu \mathrm{m}$.

In a combined experimental and theoretical study we investigate the $7dnl$ and $8pnl$ ($n\ge 11, l=9–12$) doubly excited planetary states of Sr. The experimental spectrum was obtained using a five-photon resonant excitation scheme. The method of configuration interaction with exterior complex scaling was used to compute the energy-level structure and dynamics of the two highly excited electrons from first principles. Good quantitative agreement was obtained with the spectra we recorded, and the theoretical calculations shed light on their complex structure and the signatures of electron correlations therein. The two-electron probability densities we calculated reveal the strongly correlated angular motion of the two electrons in the $7dnl$ and $8pnl$ planetary states, and confirm quantitatively the predictions of the frozen-planet approximation describing electron dynamics as the polarization of the fast inner electron by the electric field of the outer “frozen” electron.

Measured and calculated positron binding energies are presented for a range of hydrocarbons with up to six carbon atoms (viz., methane, acetylene, ethylene, ethane, propane, butane, and hexane) and their chlorinated counterparts. Both experiment and theory confirm the large effect that the chlorine atoms have on the positron binding energy and the strong sensitivity of the binding energy to the exact position of the chlorine atoms. The experimental binding energies have been obtained by measuring positron resonant annihilation using a trap-based positron beam. The calculations are performed using the previously developed model-correlation-potential method [A. R. Swann and G. F. Gribakin, J. Chem. Phys. 149, 244305 (2018)]. The overall trends are discussed with regard to the molecular polarizability, dipole moment, and geometry. Good agreement between theory and experiment is found, with the exception of the chlorinated ethylenes and chlorinated hexane. Calculations of the electron-positron annihilation rate in the bound state are also presented.

We develop an analytic-gradient-based method for relativistic coupled-cluster calculations of effective electric field ${\mathcal{E}}_{\text{eff}}$ with improved efficiency and robustness over the previous state of the art. The enhanced capability to calculate this time-reversal-symmetry-violation sensitivity parameter enables efficient screening of candidate molecules for the electron electric dipole moment (eEDM) search. As examples, the $|{\mathcal{E}}_{\text{eff}}|$ values of metal methoxides including ${\mathrm{BaOCH}}_3$, ${\mathrm{YbOCH}}_3$, and ${\mathrm{RaOCH}}_3$ are shown to be as large as those of the corresponding fluorides and hydroxides, which supports the recent proposal of using these symmetric-top molecules to improve the sensitivity of eEDM measurements. The computational results also show that molecules containing late actinide elements, NoF, NoOH, LrO, and ${\mathrm{LrOH}}^{+}$, exhibit particularly large $|{\mathcal{E}}_{\text{eff}}|$ values of around 200 GV/cm.

We study theoretically x-ray absorption and ionization spectra of an atom or molecule by two coherent x-ray pulses that show a relative phase shift resulting in a time delay of the pulse envelopes. We demonstrate that the phase modulation of the spectra is shifted with respect to the phase oscillation comb of the x-ray double pulse. The reason for this shift is the dynamics of the process defined by the interplay of the delay time, the pulse width, the detuning, and the lifetime of the core-excited state.

We study interactions between multiple temporal pairs of slits in coherent control of photoionization of $S$-state atoms using a pulse train of $N+1$ pairs evenly delayed in time by $\tau$, in which the two pulses in a pair with a delay ${\tau }_0$ are counter-rotating circularly polarized. For interacting two double-slit experiments, while Ramsey interference between two identical Archimedean spirals yields pairs of principal spirals, interference of two spirals with opposite handedness does not lead to spirals, but instead to crocodile-eye-like patterns with nictares. For more than two interacting experiments, the resulting patterns turn out just to be those two reference patterns modulated by different kinds of $N$-dependent time-energy Fraunhofer functions exhibiting diffraction-grating-like patterns.

We present a method to estimate the backward rescattering time of a tunnel-ionized electron in an intense laser pulse. The idea of the classical backpropagation method is applied to determine the backward rescattering time and its dependence on various parameters such as the ionization potential of the parent ion, its spatial profile and the peak intensity and wavelength of the driving laser field. The accurate estimation of the rescattering time would provide critical information in analyzing ultrafast rescattering dynamics in applications such as high-order harmonic generation, above-threshold ionization, and laser-induced electron diffraction experiments. We find that the backward rescattering time is significantly affected by the peak intensity of the laser pulse. The rescattering time changes by a few hundred attoseconds when the peak intensity varies from $1\times {10}^{14}$ to $5\times {10}^{14}\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$.

Systems consisting of cold atoms trapped near photonic crystal waveguides have recently emerged as an exciting platform for quantum atom-light interfaces. Such a system enables realization of tunable long-range interactions between internal states of atoms (spins), mediated by guided photons. Currently, experimental platforms are still limited by low filling fractions, where the atom number is much smaller than the number of sites at which atoms can potentially be trapped. Here, we show that this regime in fact enables interesting many-body quantum phenomena, which are typically associated with short-range disordered systems. As an example, we show how the system can realize the so-called “random singlet phase” (RSP), in which all atoms pair into entangled singlets, but the pairing occurs over a distribution of ranges as opposed to nearest neighbors. We use a renormalization group method to obtain the distribution of spin entanglement in the RSP, and show how this state can be approximately reached via adiabatic evolution from the ground state of a noninteracting Hamiltonian. We also discuss how experimentally this RSP can be observed. We anticipate that this work will accelerate the route toward the exploration of strongly correlated matter in atom-nanophotonics interfaces, by avoiding the requirement of perfectly filled lattices.

Optical cavities are an enabling technology of modern quantum science: From their essential role in the operation of lasers to applications as flywheels in atomic clocks and interaction-enhancing components in quantum optics experiments, developing a quantitative understanding of the mode shapes and energies of optical cavities has been crucial for the growth of the field. Nonetheless, the standard treatment using paraxial quadratic optics fails to capture the influence of optical aberrations present in modern cavities with high finesse, small waist, and/or many degenerate modes. In this work, we compute the mode spectrum of optical resonators, allowing for both nonparaxial beam propagation and beyond-quadratic mirrors and lenses. Generalizing prior works, we develop a complete theory of resonator aberrations, including intracavity lenses, nonplanar geometries, and arbitrary mirror forms. Harnessing these tools, we reconcile the near absence of aberration in the work of Schine et al. [N. Schine et al., Nature (London) 534, 671 (2016)] with the strongly evident aberrations in the seemingly similar cavity of Clark et al. [L. W. Clark et al., Nature (London) 582, 41 (2020)]. We further validate our approach by comparison to a family of nonplanar lens cavities realized in the laboratory, finding good quantitative agreement. This work opens prospects for cavities with smaller waists and more degenerate modes.

Highly accurate and robust control of quantum operations is vital for the realization of error-correctible quantum computation. In this paper, we show that the robustness of high-precision controls can be remarkably enhanced through sampling-based stochastic optimization of a risk-sensitive (RS) loss function. Following the stochastic gradient-descent direction of this loss function, the optimization is guided to penalize poor-performance uncertainty samples in a tunable manner. We propose two algorithms, which are termed as the RS GRAPE and the adaptive RS GRAPE. Their effectiveness is demonstrated by numerical simulations, which is shown to be able to achieve high-control robustness while maintaining high fidelity.

We present a time-dependent quantum algorithm for nuclear inelastic scattering in the time-dependent basis function on qubits approach. This algorithm aims to quantum simulate a subset of the nuclear inelastic scattering problems that are of physical interest in which the internal degrees of freedom of the reaction system are excited by time-dependent external interactions. We expect that our algorithm will enable an exponential speedup in simulating the dynamics of the subset of the inelastic scattering problems, which would also be advantageous for the applications to more complicated scattering problems. For a demonstration problem, we solve for the Coulomb excitation of the deuteron, where the quantum simulations are performed with IBM Qiskit.

Quantum entanglement is at the heart of quantum information sciences and quantum technologies. In the optical domain, the most common type of quantum entanglement is polarization entanglement, which is usually created in a postselection manner involving destructive photon detection and thus hindering further applications which require readily available entanglement resources. In this work, we propose a scheme to prepare multipartite entangled states of polarized photons in a heralded manner, i.e., without postselection. We exploit the quantum scissors technique to truncate a given continuous-variable entanglement into the target entangled states which are of hybrid discrete-continuous or solely discrete types. We consider two implementations of the quantum scissors: one modified from the original quantum scissors [Pegg et al., Phys. Rev. Lett. 81, 1604 (1998)] using single photons and linear optics and the other designed here using a type-II two-mode squeezer. We clarify the pros and cons of these two implementations as well as discussing practical aspects of the entanglement preparation. Our work illustrates an interface between various types of optical entanglement and the proposed quantum scissors techniques could serve as alternative methods for heralded generation of polarization entanglement.

We show an implementation of a $\kappa$-deformed Dirac equation in tight-binding arrays of photonic waveguides. This is done with a special configuration of couplings extending to second-nearest neighbors. Geometric manipulations can control these evanescent couplings. A careful study of wave packet propagation is presented, including the effects of deformation parameters on Zitterbewegung or trembling motion. In this way, we demonstrate how to emulate the effects of a flat noncommutative spacetime—i.e., $\kappa$-Minkowski spacetime—in simple experimental setups.

We demonstrate the optomechanically induced Faraday and splitting effects caused by optical mode conversion in a double-cavity optomechanical system. In the scheme, the polarization states of propagating light fields can be arbitrarily tailored to achieve optical Faraday rotation successfully via optomechanical interaction. Meanwhile, the energy-level splitting of the degenerate optical modes leads to the separation of two orthogonal polarization optical modes and the manipulation of spin angular momentum of photons. Moreover, the polarized output component orthogonal to the control field can be manipulated by properly choosing the tunneling interaction between two optical cavities, which enables the complete polarization control of the probe field. Using the polarized control field and optical gain in the auxiliary cavity, we can effectively improve the efficiency of optical Faraday rotation. Finally, we study how to achieve different optical control for different polarization states, which provides an effective method for the implementation of optical isolators in cavity optomechanical systems. Our scheme may benefit future realization of flexible controllability for the vector properties and spin angular momentum of photons, and can promote potential applications in optical nonreciprocal transport, routers, isolators, and circulators.

We present a method for the direct measurement of the Wigner characteristic function of a thermalizing harmonic oscillator that is completely inaccessible for control or measurement. The strategy employs a recently proposed probe-measurement-based scheme [Phys. Rev. Lett. 122, 110406 (2019)] which relies on the pulsed control of a two-level probe. We generalize this scheme to the case of a nonunitary time evolution of the target harmonic oscillator, describing its thermalization through contact to a finite-temperature environment, given in the form of a Lindblad master equation. This generalization is achieved using a superoperator formalism and yields analytical expressions for the direct measurement of the characteristic function, accounting for the decoherence during the measurement process.

The electric-field-dependent $g$ factor and the electron electric dipole moment (eEDM)-induced Stark splittings for the lowest rotational levels of ${}^{207,208}\mathrm{PbF}$ are calculated. Observed and calculated Zeeman shifts for ${}^{207}\mathrm{PbF}$ are found to be in very good agreement. It is shown that the ${}^{207}\mathrm{PbF}$ hyperfine sublevels provide a promising system for the eEDM search and related experiments.

The interplay between interparticle interactions and topological features may result in unusual phenomena. Interestingly, interactions may induce topological features in an originally trivial system, as we illustrate for the case of a one-dimensional two-component Hubbard model in which one component is subjected to Su-Schrieffer-Heeger dimerization, whereas the other one is not. We show that due to intercomponent interactions the topological properties of one component are induced in the originally trivial one. Although for large interactions topological inheritance may be readily explained by on-site pairing, we show that the threshold for full inheritance occurs at weak interactions, for which the components are not yet paired. We illustrate this inheritance by discussing both bulk and edge properties, as well as dynamical observables as mean chiral displacement and charge pumping.