For waveguides with two separate regions of anomalous dispersion, it is possible to create a quasistable two-color solitary wave. In this paper, we consider how those waves interact with dispersive radiation, i.e., both the generation of Cherenkov radiation and the scattering of incident dispersive waves. We derive the analytic resonance conditions and verify them through numeric experiments. We also report incident radiation driving the internal oscillations of the soliton during the scattering process in the case of an intense incident radiation. We generalize the resonance conditions for the case of an oscillating soliton and demonstrate how one can use the scattering process to probe and excite an internal mode of two-color soliton molecules.

Dynamical encircling of the exceptional point (EP) reveals a number of intriguing physical phenomena and their potential applications. To enrich the manipulations of optical systems in experiments, we study the dynamic encircling of the EP, that is, the state transfer process, in a largely detuned multimode optomechanical system. The process of state transfer has been investigated with different factors regarding the location of the start point, orientation, and initial state of the trajectories around the EP in the parameter space. The results show that nonreciprocal and chiral topological energy transfers between two optical modes are performed successfully by tuning the effective optomechanical coupling into a multimode system with large detuning. Moreover, the evolution speed of the system parameters is also discussed. Our work demonstrates the fundamental physics of the EP in the large detuning domain of a multimode optomechanical system and provides an alternative for manipulating optical modes in non-Hermitian systems.

We demonstrate theoretically that a few-cycle vortex beam with high peak power can be generated by self-compression of a midinfrared femtosecond vortex beam in thin fused silica plates. We first investigate the requirements of solitonlike pulse self-compression of the vortex beam on a laser wavelength and parameters of the thin plates based on three preconditions of sufficient spectral broadening, appropriate dispersion compensation, and inhibition of multiple filamentation. Our simulations confirm the existence of the solitonlike pulse self-compression regime of the vortex beam and demonstrate that the 3-$\mu \mathrm{m}$ femtosecond vortex beam with an input duration of 90 fs can be compressed to 15.1 fs with the vortex characteristics preserved. The conversion efficiency is as high as 91.5%, and the peak power reaches 0.18 TW. Our work is instructive for experiments generating high-peak-power few-cycle vortex beams and will help with obtaining isolated attosecond light vortices, opening an alternative perspective in ultrafast science.

We propose a scheme to transfer the spatial intensity as well as phase information encoded initially in the spatial profile of a weak probe field onto a newly generated Stokes field in a nonlinear process of four-wave mixing (FWM). The FWM process is explored in a gaseous medium consisting of atoms modeled as a three-level system in the $\mathrm{\Lambda }$ configuration. We found that different orders of Hermite-Gaussian and Laguerre-Gaussian modes carried by a probe beam are effectively transferred to a Stokes beam. Interestingly, the transferred intensity of the Stokes beam is a clone of the probe beam, whereas the phase profile is conjugate to the probe. The phase distribution of the transmitted modes at the exit of the medium is explored by superimposing them on a copropagating plane wave. Moreover, the parametric amplification due to the nonlinear process of FWM prevents any information loss due to linear absorption and permits us to work at high optical depths. We found a structural similarity between transferred images of about $99\%$, which provides clear evidence for the successful transfer of information in the FWM process.

We investigate the strong-field ionization of atomic hydrogen in a few-cycle elliptically polarized infrared pulse by solving the time-dependent Schrödinger equation. The dependence of the photoelectron momentum distribution on the pulse intensity, ellipticity, length, envelope, and carrier envelope phase is analyzed. In particular, we explain the variation of the electron offset angle with asymptotic electron energy through the combined action of the field and the Coulomb potential, and demonstrate that low-ellipticity pulses make it possible to access the electron release time.

We report on the realization of a degenerate mixture of ultracold fermionic lithium and chromium atoms. Based on an all-optical approach, with an overall duty cycle of about 13 s, we produce large and degenerate samples of more than $2\times {10}^5{}^6\mathrm{Li}$ atoms and ${10}^5{}^{53}\mathrm{Cr}$ atoms, with both species exhibiting normalized temperatures of about $T/T_F=0.25$. Additionally, through the exploitation of a crossed bichromatic optical dipole trap, we can controllably vary the density and degree of degeneracy of the two components almost independently and widely tune the lithium-to-chromium density ratio. Our ${}^6\mathrm{Li}\text{−}{}^{53}\mathrm{Cr}$ Fermi mixture opens the way to the investigation of a variety of exotic few- and many-body regimes of quantum matter and it appears as an optimally suited system to realize ultracold paramagnetic polar molecules, characterized by both electric and magnetic dipole moments. Ultimately, our strategy also provides an efficient pathway to produce dipolar Fermi gases, or spin mixtures, of ultracold ${}^{53}\mathrm{Cr}$ atoms.

A phenomenon of topological directed amplification of certain initial perturbations is revealed theoretically to emerge in a class of asymptotically stable skin-effect lattices described by non-normal Toeplitz operators $H_g$ with positive “numerical ordinate” $\omega \left(H_g\right)>0$. Non-normal temporal evolution, even in the presence of global dissipation, is shown to manifest a counterintuitive transient phase of edge-state amplification—a behavior, drastically different from the asymptote, that spectral analysis of $H_g$ fails to directly reveal. A consistent description of the effect is provided by the general tool of “pseudospectrum,” and a quantitative estimation of the maximum power amplification is provided by the Kreiss constant. A recipe to determine an optimal initial condition that will attain maximum amplification power is given by singular value decomposition of the propagator $e^{-i{H}_{g}t}$. It is further predicted that the interplay between non-normality and nonlinearity in a skin-effect laser array can facilitate narrow-emission spectra with scalable stable-output power.

We develop and test a method that integrates many-electron weak-field asymptotic theory (ME-WFAT) [O. I. Tolstikhin, L. B. Madsen, and T. Morishita, Phys. Rev. A 89, 013421 (2014)] in the integral representation (IR) into the density-functional-theory (DFT) framework. In particular, we present modifications of the integral formula in the IR ME-WFAT to incorporate the potential terms unique to DFT. By solving an adiabatic rate equation for the angle-resolved ionization yield in our DFT-based ME-WFAT method, we show that the results are in excellent agreement with those of real-time time-dependent density-functional-theory (TDDFT) simulations for NO, OCS, $\mathrm{C}{\mathrm{H}}_3\mathrm{Br}$, and $\mathrm{C}{\mathrm{H}}_3\mathrm{Cl}$ interacting with one- and two-color laser fields with a fundamental wavelength of 800 nm. This agreement is significant because the WFAT calculations take only a small fraction of the time of full TDDFT calculations. These results suggest that in the wavelength region commonly used in strong-field experiments (800 nm and longer), our DFT-based WFAT treatment can be used to rapidly screen for the ionization properties of a large number of molecules as a function of alignment or orientation between the molecule and the strong field.

I present a different approach to Rayleigh-Schrödinger perturbation theory, based on Laplace transforms and polynomial theory, yielding an iterative expression for the perturbative expansion of the energy of the nondegenerate ground state of a quantum system, which easily lends itself to symbolic computation. A stochastic interpretation of the various perturbative corrections naturally leads to a resummation scheme that is equivalent to the reptation quantum Monte Carlo method and that actually provided the original motivation to its development in the late 1990s.

In the noisy intermediate-scale quantum era, an important goal is the conception of implementable algorithms that exploit the rich dynamics of quantum systems and the high dimensionality of the underlying Hilbert spaces to perform tasks while prescinding from noise-proof physical systems. An emerging class of quantum learning machines is that based on the paradigm of quantum kernels. Here, we study how dissipation and decoherence affect their performance. We address this issue by investigating the expressivity and the generalization capacity of these models within the framework of kernel theory. We introduce and study the effective kernel rank, a figure of merit that quantifies the number of independent features a noisy quantum kernel is able to extract from input data. Moreover, we derive an upper bound on the generalization error of the model that involves the average purity of the encoded states. Thereby we show that decoherence and dissipation can be seen as an implicit regularization for quantum kernel machines. As an illustrative example, we report exact finite-size simulations of machines based on chains of driven-dissipative quantum spins to perform a classification task, where the input data are encoded into the driving fields and the quantum physical system is fixed. We determine how the performance of noisy kernel machines scales with the number of nodes (chain sites) as a function of decoherence and examine the effect of imperfect measurements.

Data loading with shallow quantum circuits is a highly desirable ingredient for efficient execution of many quantum algorithms before large-scale quantum error corrections with full fault tolerance become readily available. The need for efficient data loading is especially urgent for the study of quantum machine learning. In this work, we propose a protocol that only uses a parameterized shallow quantum circuit without ancilla qubits for loading data into the amplitude of a quantum state with high fidelity. We term this data-loading method Hamiltonian-based data loading (HDL), which comprises two stages. First, the HDL algorithm identifies a Hamiltonian $\widehat{H}$ whose unique ground state $|\psi \rangle$ represents the normalized data $\vec{x}$ in the form of amplitude encoding. Next, the target state is reconstructed with a parameterized quantum circuit by utilizing methods like variational quantum eigensolver to minimize energy. In this work, we provide three convincing examples to demonstrate the effectiveness of HDL for loading an $N$-dimensional classical data with minimal quantum resources ($O\left[\mathrm{poly}\left({log}_{2}N\right)\right]$-depth quantum circuit without ancilla qubits). Our approach is particularly useful for quantum hardware without large-scale error corrections, and shall benefit the development of quantum machine learning algorithms.

This work examines the problem of learning an unknown von Neumann measurement of dimension $d$ from a finite number of copies. To obtain a faithful approximation of the given measurement, we are allowed to use it $N$ times. Our main goal is to estimate the asymptotic behavior of the maximum value of the average fidelity function $F_d$ for a general $N\to 1$ learning scheme. We show that $F_d=1-\mathrm{\Theta }\left(\frac{1}{N^2}\right)$ for arbitrary but fixed dimension $d$. In addition to that, we compared various learning schemes for $d=2$. We observed that the learning scheme based on deterministic port-based teleportation is asymptotically optimal but performs poorly for low $N$. In particular, we discovered a parallel learning scheme, which despite its lack of asymptotic optimality, provides a high value of the fidelity for low values of $N$ and uses only two-qubit entangled memory states.

We describe the setup and characterization of a cryogenic multipole radio-frequency (RF) ion trap that enables the accumulation and cooling of mass-selected ions before injection into the SAPHIRA storage ring. To characterize the RF trap setup, we use ${\mathrm{OH}}^{-}$ anions and explore the threshold photodetachment cross section measured after storage in SAPHIRA as a probe of the rotational temperature. Beyond the temperature of the ion trap assembly, cooled to 6 K, the final rotational temperature of the ${\mathrm{OH}}^{-}$ ions is strongly influenced by the density of cooled He and the actual number of trapped ions while much less affected (possibly unaffected) by the time-varying field of the trap. To obtain rotationally cold ${\mathrm{OH}}^{-}$ ions, the RF trap must be operated with low He density and a low number of ions. High He densities, corresponding to a strong coupling of the trapped ions and He gas, lead to a significant rotational heating of the trapped ion ensemble, and the He density seems to limit the actual reachable rotational temperature. We demonstrate that cold ions in the RF trap remain cold for at least 30 ms in the SAPHIRA (300 K) storage ring at a base pressure of $\sim 8\times {10}^{-9}\mathrm{mbar}$.

Motivated by recent work on the non-Hermitian skin effect in the bosonic Kitaev-Majorana model, we study the quantum criticality of interacting bosonic Kitaev-Hubbard models on a chain and a two-leg ladder. In the hard-core limit, we show exactly that the non-Hermitian skin effect disappears via a transformation from hard-core bosonic models to spin-1/2 models. We also show that hard-core bosons can engineer the Kitaev interaction, the Dzyaloshinskii-Moriya interaction and the compass interaction in the presence of the complex hopping and pairing terms. Importantly, quantum criticalities of the chain with a three-body constraint and unconstrained soft-core bosons are investigated by the density matrix renormalization-group method. This paper reveals the effect of many-body interactions on the non-Hermitian skin effect and highlights the power of bosons with pairing terms as a probe for the engineering of interesting models and quantum phase transitions.

The large-scale expansion dynamics of quantum gases is a central tool for ultracold gas experiments and poses a significant challenge for theory. In this work we provide an exact reformulation of the Gross-Pitaevskii equation for the ultracold Bose gas in a coordinate frame that adaptively scales with the system size during evolution, enabling simulations of long evolution times during expansion or similar large-scale manipulation. Our approach makes no hydrodynamic approximations, is not restricted to a scaling ansatz, harmonic potentials, or energy eigenstates, and can be generalized readily to noncontact interactions via the appropriate stress tensor of the quantum fluid. As applications, we simulate the expansion of the ideal gas, a cigar-shaped condensate in the Thomas-Fermi regime, and a linear superposition of counterpropagating Gaussian wave packets. We recover known scaling for the ideal gas and Thomas-Fermi regimes, and identify a linear regime of aspect-ratio preserving free expansion; analysis of the scaling dynamics equations shows that an exact, aspect-ratio invariant, free expansion does not exist for nonlinear evolution. Our treatment enables exploration of nonlinear effects in matter-wave dynamics over large scale-changing evolution.

The sonic analog of Hawking radiation can now be experimentally recreated in Bose-Einstein condensates that contain an acoustic black hole. In these experiments the signal strength and approximate analog Hawking temperature increase for denser condensates, which however also suffer increased atom losses from inelastic collisions. To determine how these affect analog Hawking radiation, we numerically simulate creation of the latter in a Bose-Einstein condensate in the presence of atomic losses, including nonunitary quantum field dynamics using the truncated Wigner approximation. In particular we explore modifications of density-density correlations through which the radiation has been analyzed so far. We find no evidence that losses directly alter the basic picture of the analog Hawking effect; instead all consequences that we find are indirect: Losses increase the contrast of the correlation signal, which we attribute to condensate heating by the losses, in turn leading to a component of stimulated radiation in addition to the spontaneous one. Other indirect consequences are the modification of the white-hole instability pattern and a change of slope of the Hawking tongue.

Self-testing, which refers to device-independent characterization of the state and the measurement, enables the security of a quantum information processing task certified independently of the operation performed inside the devices. Quantum states lie at the core of self-testing as key resources. However, for different entangled states, usually, different measurement settings should be taken in self-testing recipes. This may lead to the redundancy of measurement resources. In this work, we use fixed two-binary measurements and answer the question of which states can be self-tested with the same settings. By investigating the structure of generalized tilted-Clauser-Horne-Shimony-Holt Bell operators with the sum-of-squares decomposition method, we show that a family of two-qubit entangled states can be self-tested with the same measurement settings. The robustness analysis indicates that our scheme is feasible for practical experiment instruments. Moreover, our results can be applied to various quantum information processing tasks.