In this paper, we propose a protocol to prepare $W$ states with superconducting quantum interference devices by using dressed states. Through choosing a set of dressed states suitably, the protocol can be used to accelerate the adiabatic passages while additional couplings are unnecessary. Moreover, we can optimize the evolution of the system with the restraint to the populations of the intermediate states by choosing suitable control parameters. Numerical simulations show that the protocol is robust against the parameter variations and decoherence mechanisms. Furthermore, the protocol is faster and more robust against the dephasing compared with that by the adiabatic passages. As for the Rabi frequencies of pulses designed by the method, they can be expressed by the linear superpositions of Gaussian functions, which does not increase difficulty in the experiments. In addition, the protocol could be controlled and manipulated easily in experiments with a circuit quantum electrodynamics system.

The interplay of noise and quantum coherence in transport gives rise to rich dynamics relevant for a variety of systems. In this work, we put forward a proposal for an experiment testing noise-induced transport in the vibrational modes of a chain of trapped ions. We focus on the case of transverse modes, considering multiple-isotope chains and an “angle trap,” where the transverse trapping varies along the chain. This variation induces localization of the motional modes and therefore suppresses transport. By suitably choosing the action of laser fields that couple to the internal and external degrees of freedom of the ions, we show how to implement effective local dephasing on the modes, broadening the vibrational resonances. This leads to an overlap of the local mode frequencies, giving rise to a pronounced increase in the transport of excitations along the chain. We propose an implementation and measurement scheme which require neither ground-state cooling nor low heating rates, and we illustrate our results with a simulation of the dynamics for a chain of three ions.

We study the electron-nuclear coupled dynamics for oriented ${\mathrm{HeH}}^{+}$ molecules in strong laser fields numerically and analytically. At small orientation angles, the asymmetric molecule tends to stretch to distances larger than the equilibrium separation and strong even harmonics are emitted. We show that the permanent dipole of the system plays an important role in the vibrational dynamics of the nuclear wave packet. The nuclear motion and the molecular structure can be read from the spectra and ellipticity of odd-even high harmonics. Our results also have implications for strong-field ionization of the asymmetric system.

We investigate the polarization properties of high harmonics generated with the bichromatic counter-rotating circularly polarized (BCCP) laser fields by numerically solving the time-dependent Schrödinger equation (TDSE). It is found that the helicity of the elliptically polarized harmonic emission is reversed at particular harmonic orders. Based on the time-frequency analysis and the classical three-step model, the correspondence between the positions of helicity reversions and the classical trajectories of continuum electrons is established. It is shown that the electrons ionized at one lobe of laser field can be divided into different groups based on the different lobes they recombine at, and the harmonics generated by adjacent groups have opposite helicities. Our study performs a detailed analysis of high harmonics in terms of electron trajectories and depicts a clear and intuitive physical picture of the HHG process in BCCP laser fields.

In this work we present an ab initio nonperturbative theory for the description of the two-electron dynamics of a spherical quantum dot (QD) in THz and mid-IR ultrashort laser fields. We have approximated the QD's confinement potential to have a Gaussian-like spatial dependence and have calculated its electronic structure and photoionization one-electron cross sections with an ab initio configuration-interaction approach. We have also calculated the ionization yields following the interaction with a laser pulse by numerically solving the time-dependent Schrödinger equation. All results show a strong dependence on the dot size. We have also identified resonances, of the Fano-type, in the photoionization spectrum with properties varying along with the QD size.

We theoretically investigate the dynamics of a gas of strongly interacting Rydberg atoms subject to a time-domain Ramsey interferometry protocol. The many-body dynamics is governed by an Ising-type Hamiltonian with long-range interactions of tunable strength. We analyze and model the contrast degradation and phase accumulation of the Ramsey signal and identify scaling laws for varying interrogation times, ensemble densities, and ensemble dimensionalities.

A challenge in precision measurement with squeezed spin state arises from the spin dephasing due to stray magnetic fields. To suppress such environmental noises, we employ a continuous driving protocol, rotary echo, to enhance the spin coherence of a spin-1 Bose-Einstein condensate in stray magnetic fields. Our analytical and numerical results show that the coherent and the squeezed spin states are preserved for a significantly long time, compared to the free induction decay time, if the condition $h\tau =m\pi$ is met with $h$ the pulse amplitude and $\tau$ pulse width. In particular, both the spin average and the spin squeezing, including the direction and the amplitude, are simultaneously fixed for a squeezed spin state. Our results point out a practical way to implement quantum measurements based on a spin-1 condensate beyond the standard quantum limit.

We demonstrate, both numerically and analytically, that it is possible to deterministically generate two photons from one and only one photon. We characterize the output two-photon field and make our calculations close to reality by including losses. Our proposal relies on real or artificial three-level atoms with a cyclic transition strongly coupled to a one-dimensional waveguide. We show that almost perfect down-conversion, with efficiency over $99\%$, is reachable using state-of-the-art waveguide QED architectures such as photonic crystals or superconducting circuits. In particular, we sketch an implementation in circuit QED, where the three-level atom is a transmon.

We propose a device architecture capable of direct quantum coherent electro-optical conversion of microwave-to-optical photons. The hybrid system consists of a planar superconducting microwave circuit coupled to an integrated whispering-gallery-mode microresonator made from an electro-optical material. We show that by exploiting the large vacuum electric field of the planar microwave resonator, electro-optical (vacuum) coupling strengths $g_0$ as large as $\sim 2\pi O(10-100)$ kHz are achievable with currently available technology—a more than 3 orders of magnitude improvement over prior designs and realizations. Operating at millikelvin temperatures, such a converter would enable high-efficiency conversion of microwave-to-optical photons. We analyze the added noise and show that maximum quantum coherent conversion efficiency is achieved for a multiphoton cooperativity of unity which can be reached with optical power as low as $O\left(1\right)$ mW.

Photonic switches are one kind of key devices in quantum optical networks and have been demonstrated previously through either manipulating the cavity-atom interactions [Phys. Rev. Lett. 111, 193601 (2013)] or driving the additional levels of the switch atoms [Phys. Rev. Lett. 106, 113601 (2011)]. In this paper, we propose an alternative configuration generated by three empty cavities coupled in series to serve as an ideal photonic switch of the photons along the waveguide, i.e., photons are completely transmitted (reflected) if the three cavities are (are not) exactly resonant. It is shown that the quality of such a detuning-sensitive photonic switch is insensitive to the cavity-cavity, cavity-waveguide, and cavity-atom coupling strengths. The feasibility of the devices is also demonstrated with the current waveguide photonic technique.

By breaking the spin symmetry in the relativistic domain, a powerful tool in physical sciences was lost. In this work, we examine an alternative of spin symmetry for systems described by the many-electron Dirac-Coulomb Hamiltonian. We show that the square of many-electron operator ${\mathcal{K}}_{+}$, defined as a sum of individual single-electron time-reversal (TR) operators, is a linear Hermitian operator which commutes with the Dirac-Coulomb Hamiltonian in a finite Fock subspace. In contrast to the square of a standard unitary many-electron TR operator $\mathcal{K}$, the ${\mathcal{K}}_{+}^2$ has a rich eigenspectrum having potential to substitute spin symmetry in the relativistic domain. We demonstrate that ${\mathcal{K}}_{+}$ is connected to $\mathcal{K}$ through an exponential mapping, in the same way as spin operators are mapped to the spin rotational group. Consequently, we call ${\mathcal{K}}_{+}$ the generator of the many-electron TR symmetry. By diagonalizing the operator ${\mathcal{K}}_{+}^2$ in the basis of Kramers-restricted Slater determinants, we introduce the relativistic variant of configuration state functions (CSF), denoted as Kramers CSF. A new quantum number associated with ${\mathcal{K}}_{+}^2$ has potential to be used in many areas, for instance, (a) to design effective spin Hamiltonians for electron spin resonance spectroscopy of heavy-element containing systems; (b) to increase efficiency of methods for the solution of many-electron problems in relativistic computational chemistry and physics; (c) to define Kramers contamination in unrestricted density functional and Hartree-Fock theory as a relativistic analog of the spin contamination in the nonrelativistic domain.

We explore a previously unknown connection between two important problems in physics, i.e., quantum macroscopicity and the quantum phase transition. We devise a general and computable measure of quantum macroscopicity that can be applied to arbitrary spin states. We find that a macroscopic quantum superposition of an extremely large size arises during the quantum phase transition of the transverse Ising model in contrast to some seeming macroscopic quantum phenomena such as superconductivity, superfluidity, and Bose-Einstein condensates. Our result may be an important step forward in understanding macroscopic quantum properties of many-body systems.

We propose a simple setup for the conversion of multipartite entangled states in a quantum network with restricted access. The scheme uses nonlocal operations to enable the preparation of states that are inequivalent under local operations and classical communication, but most importantly does not require full access to the states. It is based on a flexible linear optical conversion gate that uses photons, which are ideally suited for distributed quantum computation and quantum communication in extended networks. In order to show the basic working principles of the gate, we focus on converting a four-qubit entangled cluster state to other locally inequivalent four-qubit states, such as the Greenberger-Horne-Zeilinger and symmetric Dicke states. We also show how the gate can be incorporated into extended graph state networks and can be used to generate variable entanglement and quantum correlations without entanglement but nonvanishing quantum discord.

We present a theory illuminating the crossover from strong-field tunneling ionization to weak-field multiphoton ionization in the interaction of a classical laser field with a hydrogen atom. A simple formula is derived in which the ionization amplitude appears as a product of two separate amplitudes. The first describes the initial polarization of the atom by virtual multiphoton absorption and the second the subsequent tunneling out of the polarized atom. Tunneling directly from the ground state and multiphoton absorption without tunneling appear naturally as the limits of the theory.

We investigate spatial adiabatic passage of a Bose-Einstein condensate in a triple-well potential within the three-mode approximation. By rewriting the dynamics in the so-called time-dependent dark-dressed basis, we analytically derive the optimal conditions for the nonlinear parameter and the on-site energies of each well to achieve a highly efficient condensate transport. We show that the nonlinearity yields a high-efficiency plateau for the condensate transport as a function of the on-site energy difference between the outermost wells, favoring the robustness of the transport. We also analyze the case of different nonlinearities in each well, which, for certain parameter values, leads to an increase of the width of this plateau.

Optomechanical arrays made of structured flexible dielectrics are a promising system for exploring quantum and many-body optomechanical phenomena. We generalize investigations of the optomechanical properties of periodic arrays of one-dimensional scatterers in optical resonators to the case of vibrating membranes whose thickness is not necessarily small with respect to the optical wavelength of interest. The array optical transmission spectrum and its optomechanical coupling with a linear Fabry-Perot cavity field are investigated both analytically and numerically.

We theoretically investigate the effect of long-range dipole-dipole interaction (LRDDI) on a superradiant laser (SL). This effect is induced from the atom-photon interaction in the dissipation process. In the bad-cavity limit usually performed to initiate SL, we demonstrate that cavity photon number oscillates as an interparticle distance of the atoms varies. Similarly the atom-atom coherence alternates with signs, showing critical transitions alternatively in SL operations. This suggests a complexity of the collective effect emerging in a large ensemble of atoms. Therefore this effect in a SL cannot be simply interpreted by only a part of the whole ensemble. We numerically solve for a steady-state SL including the spatially dependent LRDDI, and locate the maximal cavity photon number and the minimal spectral linewidth respectively at the optimal atomic separations in the setting of an equidistant atomic array. The scaling of a finite number of atoms shows that a steady-state SL outperforms the one without LRDDI, which allows for probing narrow atomic transitions and is potentially useful for precision measurements and next-generation optical clocks.

We develop a finite-temperature hydrodynamic approach for a harmonically trapped one-dimensional quasicondensate and apply it to describe the phenomenon of frequency doubling in the breathing-mode oscillations of the quasicondensate momentum distribution. The doubling here refers to the oscillation frequency relative to the oscillations of the real-space density distribution, invoked by a sudden confinement quench. By constructing a nonequilibrium phase diagram that characterizes the regime of frequency doubling and its gradual disappearance, we find that this crossover is governed by the quench strength and the initial temperature rather than by the equilibrium-state crossover from the quasicondensate to the ideal Bose gas regime. The hydrodynamic predictions are supported by the results of numerical simulations based on a finite-temperature $c$-field approach and extend the utility of the hydrodynamic theory for low-dimensional quantum gases to the description of finite-temperature systems and their dynamics in momentum space.

Nonlocal correlations arising from measurements on tripartite entangled states can be classified into two groups, one genuinely three-way nonlocal and other local with respect to some bipartition. Still, whether a genuinely tripartite entangled quantum state can exhibit genuine three-way nonlocality remains a challenging problem as far as measurement context is concerned. Here we introduce an approach in this regard. We consider three tripartite quantum states, none of which is genuinely three-way nonlocal in a specific Bell scenario (three parties, two measurements per party, two outcomes per measurement), but they can exhibit genuine three-way nonlocality when the initial states are subjected to stochastic local operations and classical communication. So, genuine three-way nonlocality is a resource which can be revealed by using a sequence of measurements.

The Thomson backscatter spectra by an electron moving in the resonance acceleration regime are derived analytically and computed numerically in the simultaneous presence of a superintense laser field and a strong uniform magnetic field. The dependences of fundamental frequency of harmonic spectra on the laser intensity and magnetic resonance parameter are examined carefully. By calculating the emission of a single electron in a circularly polarized laser field of plane-wave form and a constant external magnetic field, the scale invariance of the radiation spectra is evident in terms of harmonic orders. The scaling law of backscattered spectra is exhibited remarkably for the laser intensity as well as for the initial axial momentum of the electron when the cyclotron frequency of the electron approaches the laser frequency. The results indicate that the magnetic resonance parameter plays an important role in the strength of emission. And the found rich features of backscattering spectra may be useful to the tunability of the radiation source.