Based on the work done by an electromagnetic field on an atomic or molecular electronic system, a general gauge-invariant formulation of transient absorption spectroscopy is presented within the semiclassical approximation. Avoiding multipole expansions, a computationally viable expression for the spectral response function is derived from the minimal-coupling Hamiltonian of an electronic system interacting with one or more laser pulses described by a source-free, enveloped electromagnetic vector potential. With a fixed-basis expansion of the electronic wave function, the computational cost of simulations of laser-driven electron dynamics beyond the dipole approximation is the same as simulations adopting the dipole approximation. We illustrate the theory by time-dependent configuration interaction and coupled-cluster simulations of core-level absorption and circular dichroism spectra.

Kerr instability can be exploited to amplify visible, near-infrared, and midinfrared ultrashort pulses. We use the results of Kerr instability amplification theory to inform our simulations amplifying few-cycle pulses. We show that the amplification angle dependence is simplified to the phase-matching condition of four-wave mixing when the intense pump is considered. Seeding with few-cycle pulses near the pump leads to broadband amplification without spatial chirp, while longer pulses undergo compression through amplification. Pumping in the midinfrared leads to multioctave spanning amplified pulses with single-cycle duration not previously predicted. We discuss limitations of the amplification process and optimizing pump and seed parameters to maximize amplification.

We show that a laser at threshold can be utilized to generate the class of coherent and transform-limited waveforms ${(vt-z)}^m{e}^{i(kz-\omega t)}$ at optical frequencies. We derive these properties analytically and demonstrate them in semiclassical time-domain laser simulations. We then utilize these waveforms to expand other waveforms with high modulation frequencies and demonstrate theoretically the feasibility of complex-frequency coherent absorption at optical frequencies, with efficient energy transduction and cavity loading. This approach has potential applications in quantum computing, photonic circuits, and biomedicine.

The rotating-wave approximation (discarded of counter-rotating interactions) to light-matter interactions is widely used in the quantum electrodynamics Hamiltonian; however, its validity has long been a matter of debate. In this article, we explore the importance of counter-rotating interactions in the quantum dynamics of multiple molecules in complex dielectric environments within the framework of macroscopic quantum electrodynamics. Through our analytical analysis, we find that, in the weak coupling regime, the lack of counter-rotating interactions results in the absence of the Casimir-Polder potentials (medium-assisted energy shifts) in the ground-state molecules and a significant deviation of the intermolecule dipole-dipole interactions. Notably, the deviation of intermolecule dipole-dipole interaction can reach up to 50 percent in the near-field zone (short intermolecule distance). We also conduct a case study on the population dynamics of a pair of identical molecules above a plasmonic surface. Through analytical and numerical analysis, it is revealed that the lack of counter-rotating interactions can profoundly affect the dynamics, especially the oscillatory pattern due to intermolecule dipole-dipole interactions, of the molecules in both strong and weak coupling regimes, emphasizing the need for careful consideration when making the rotating-wave approximation in a multiple-molecule system coupled with quantum light.

Quantum light sources play a vital role in various aspects of quantum information science, but on-demand highly efficient generation of arbitrary multiphoton states which can be easily integrated is still challenging. Here, we propose a chip-integrable scheme to deterministically generate a group of $n$ photons with very high fidelity based on the long-range collective interaction between the emitters mediated by the waveguide modes. The $n$ photons are shown to be emitted in a bundle while two successive $n$-photon bundles tend to be antibunched and can behave as an $n$-photon gun. Our results here can find applications in areas such as chip-integrated quantum information processing and quantum metrology.

We present an inequality on the purity of a bipartite state depending solely on the length difference of the local Bloch vectors. For two qubits this inequality is tight for all marginal states and so extends the previously known solution for the two-qubit marginal problem. With this inequality we construct a three-dimensional Bloch model of the two-qubit quantum state space in terms of Bloch lengths, providing a pleasing visualization of this high-dimensional state space. This allows to characterize quantum states by a strongly reduced set of parameters and to investigate the interplay between local properties of the marginal systems and global properties encoded in the correlations.

We investigate the influence of geometry on the preservation of quantum coherence in spin clusters subjected to a thermal environment. Assuming weak interspin coupling, we explore the various buffer network configurations that can be embedded in a plane. Our findings reveal that the connectivity of the buffer network is crucial in determining the preservation duration of quantum coherence in an individual central spin. Specifically, we observe that the maximal planar graph yields the longest preservation time for a given number of buffer spins. Interestingly, our results demonstrate that the preservation time does not consistently increase with an increasing number of buffer spins. Employing a quantum master equation in our simulations, we further demonstrate that a tetrahedral geometry comprising a four-spin buffer network provides optimal protection against environmental effects.

Triple differential cross sections are presented for the electron impact ionization of a $3p$ shell of argon, using a model called 3CWZ. Within this model, the projectile as well as the ejected electrons are represented by Coulomb waves with variable charges $Z$($r$) instead of an effective charge; the postcollision interaction (PCI) is also included and treated exactly at all orders of perturbation theory. Our results are compared with recent experiments and other theoretical predictions in a range of several kinematics.

We theoretically investigate the critical velocity for dissipationless motion of a two-dimensional superfluid past a static potential barrier of large width. The circular-shaped barrier provides a comprehensive analytical framework for the critical speed, for which we derive closed-form expressions using the hydraulic approximation, the hodograph method, and Janzen-Rayleigh expansions of the velocity potential. These analytical estimates are shown to be in good agreement with the numerical results of an imaginary-time integration of the full wave equation. In contrast to most of the state of the art, our study is not restricted to an impenetrable potential barrier nor to a quartic interaction Hamiltonian, which enables realistic modeling of recent experiments with atomic Bose-Einstein condensates and paraxial superfluids of light in two dimensions.

We investigate both analytically and numerically the optical resonant radiation of bright, dark, and Peregrine solitons in a phase-matched second-harmonic generation process, when they undergo a large temporal walk-off. By virtue of approximate analytical soliton solutions, we obtain simple yet universal formulas for their resonant radiation frequencies, all of which show a good agreement with numerical simulations. We unveil that the optical resonant radiation occurs solely when the second-harmonic component has a normal group-velocity dispersion, and the temporal walk-off tends to stabilize the soliton evolution, by reducing the resonant radiation even in the phase-matched regime. These findings have significantly generalized previous results and will shed more light on the mechanism behind the generation of dispersive waves in quadratic media.

We present experimentally and numerically accessible quantities that can be used to differentiate among various families of random entangled states. To this end, we analyze the entanglement properties of bipartite reduced states of a tripartite pure state. We introduce a ratio of simple polynomials of low-order moments of the partially transposed reduced density matrix, and we show that this ratio takes well-defined values in the thermodynamic limit for various families of entangled states. This allows us to sharply distinguish entanglement phases in a way that can be understood from a quantum information perspective based on the spectrum of the partially transposed density matrix. We analyze in particular the entanglement phase diagram of Haar-random states, states resulting from the evolution of chaotic Hamiltonians, stabilizer states (which are outputs of Clifford circuits), matrix-product states, and fermionic Gaussian states. We show that for Haar-random states, the resulting phase diagram resembles the one obtained via the negativity, and that for all the cases mentioned above, a very distinctive behavior is observed. Our results can be used to experimentally test necessary conditions for different types of mixed-state randomness in quantum states formed in quantum computers and programmable quantum simulators.

Overcoming the issue of qubit-frequency fluctuations is essential to realizing stable and practical quantum computing with solid-state qubits. Static $ZZ$ interaction, which causes a frequency shift of a qubit depending on the state of neighboring qubits, is one of the major obstacles to integrating fixed-frequency transmon qubits. Here we propose and experimentally demonstrate $ZZ$-interaction-free single-qubit-gate operations on a superconducting transmon qubit by utilizing a semianalytically optimized pulse based on a perturbative analysis. The gate is designed to be robust against slow qubit-frequency fluctuations. The robustness of the optimized gate spans a few megahertz, which is sufficient to suppress the adverse effects of the $ZZ$ interaction. Our result paves the way for an efficient approach to overcoming the issue of $ZZ$ interaction without any additional hardware overhead.

The long-range part of the interatomic interactions plays a substantial role in the collisional dynamics of ultracold gases. Here, we report on the calculation of the isotropic and anisotropic $C_6$ coefficients characterizing the van der Waals interaction between dysprosium or erbium atoms in the two lowest energy levels and the ground-state alkali-metal (Li, Na, K, Rb, Cs, Fr) or alkaline-earth-metal (Be, Mg, Ca, Sr, Ba) atoms. The calculations are done using the integral of dynamic dipole polarizabilities at so-called imaginary frequencies of the two interacting atoms. For all atom pairs, we find that the isotropic $C_6$ coefficients are two or three orders of magnitude larger than the anisotropic ones. Those coefficients are essential for modeling collisional properties of heteronuclear quantum mixtures containing highly magnetic dysprosium or erbium atoms and alkali-metal or alkaline-earth-metal atoms.

We study theoretically the reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) in strongly polar molecules. The time-dependent energy of a polar molecule in the infrared (IR) field gives rise to an additional dipole-laser coupling contribution to the sideband delay. In a time-independent picture this translates to the initial state becoming a linear combination of IR-dressed states. We extend the recently developed time-independent molecular $R$-matrix method to include the additional interfering ionization pathways arising from the IR dressed initial state and obtain very good agreement with a reference nonperturbative time-dependent RABBITT simulation. We discuss the asymptotic behavior of such ionization amplitudes and recover a known approximate asymptotic formula for the dipole-laser coupling delay derived earlier in the context of attosecond streaking. At low photon energies the dipole-laser coupling contributes significantly even in an unoriented molecular sample. Finally, we show that in photodetachment of polar singly charged negative ions the sideband delay is asymptotically proportional only to Wigner delay.

In the theory of multiphoton ionization, the effects of relativity and spatial dependence in the laser field are typically neglected for the sake of computational feasibility, as well as being too small to measure. As advances in high-intensity, short-wavelength lasers are developed, ultimately it becomes necessary to investigate the impact of relativistic and/or beyond-dipole (nondipole) corrections, as they are no longer small enough to neglect. Using an ab initio approach and the time-dependent Dirac equation to simulating the ionization process in an exact nondipole treatment, we study the high-intensity multiphoton ionization of hydrogen by a short x-ray laser pulse in the relativistic regime, yielding the kinetic-energy spectrum of the photoelectron, and observe a shift in the energy of the emitted electron as induced by relativistic and nondipole effects. Overall, relativistic effects are seen to give rise to a positive energy shift (blueshift), while nondipole effects cause a corresponding negative shift (redshift), and in aggregate the relativistic and nondipole ionization result in a tiny redshift. Considering only photoelectrons that are emitted along the laser polarization direction, the blueshift is recovered, and the shift may be explained in terms of the relativistic mass shift of the electron in the oscillating field.

We consider the formation of cold ground-state polar molecules in a low vibrational level by laser fields. Starting from a pair of cold colliding atoms of dissimilar species, we propose a strategy consisting of three steps. In the first step, a pump pulse induces the molecule formation by photoassociating the atomic pair in a high or intermediate vibrational level of an excited electronic molecular state. This step is followed by a dump pulse in an intermediate vibrational level of the ground state. The last step corresponds to the vibrational cooling or vibrational stabilization process. Taking advantage of the permanent dipole moment, an infrared chirped pulse induces downward transitions among the vibrational levels of the ground electronic state, reaching the ground vibrational level. We perform the strategy with with fixed-shaped pulses and also with an optimized chirped pulse for the stabilization step. To perform the optimization, we introduce an optimal control technique in which the optimization of the chirping is carried out in the time domain. This methodology has the benefit of limiting the pulse amplitude while focusing only on the frequency of the field. The proposed scheme is an alternative to the use of two pairs of pump-dump pulses or to the direct photoassociation and vibrational stabilization in the ground state.

In cosmological evolution, it is the homogeneous scalar field (inflaton) that drives the universe to expand isotropically and to generate standard model particles. However, to simulate cosmology, atomic gas research has focused on the dynamics of Bose-Einstein condensates (BEC) with continuously applied forces. In this paper we argue that a complementary approach needs also to be pursued; we thus consider the analog BEC experiments in a nondriven and naturally closed atomic system. We implement this using a BEC in an optical lattice which, after a quench, freely transitions from an unstable to a stable state. This dynamical evolution displays the counterpart “preheating,” “reheating,” and “thermalization” phases of cosmology. Importantly, our studies of these analog processes yield tractable analytic models. Additionally, of great utility to the cold atom community, such understanding elucidates the dynamics of nonadiabatic condensate preparation. Indeed, the dynamical processes discussed here are generic and in future cold atom reequilibration experiments it will be important to observe both the preheating stage, corresponding to a fragmented condensate, and the reheating stage, corresponding to a particle cloud.