The response of the level anticrossing signal to a quasiresonant radio-frequency field, which appears in a zero magnetic field at nitrogen-vacancy (NV) color centers in diamond, is investigated. It is shown that the complex structure of this response can be explained by the Autler-Townes splitting. The possibility of controlling the parameters of the level anticrossing signal is considered. It is shown that the slope of the central resonance recorded in this structure upon low-frequency modulation of the external magnetic field can be 2.3 times higher than the slope of the resonance recorded in the absence of a radio-frequency field. Conclusions are drawn about the nature of the level anticrossing effect arising in zero field in NV color centers in diamond.

It is shown that electron backscattering can enhance high-order-harmonic generation in periodic systems with broken translational symmetry. Paradigmatically, we derive for a finite chain of atoms the harmonic cutoff due to electrons backscattered from the edges of the chain and demonstrate a maximum in the harmonic yield if twice the quiver amplitude of the driven electrons equals the chain length. For an intuitive understanding of our quantum results we develop a refined semiclassical trajectory model with finite electron-hole separation after tunneling. We demonstrate that the same “tunnel exit” also holds for interband harmonics in conventional periodic solid-state systems.

We propose an ultracold atom analog of false-vacuum decay using all three states of a spin-1 Bose gas. We consider a one-dimensional system with both radio-frequency and optical Raman couplings between internal states. An advantage of our proposal is the lack of a time-modulated coupling, which can lead to instabilities. Within the elaborate phase structure of the system, we identify an effective Klein-Gordon field and use Gross-Pitaevskii simulations within the truncated Wigner approximation to model the decay of a metastable state. We examine the dependence of the rate of vacuum decay on particle density for ${}^7\mathrm{Li}$ and ${}^{41}\mathrm{K}$ and find reasonable agreement with instanton methods.

We propose observable bounds for Gaussian illumination to maximize the signal-to-noise ratio, which minimizes the discrimination error between the presence and absence of a low-reflectivity target using Gaussian states. The observable bounds are achieved with mode-by-mode measurements. In the quantum regime using a two-mode squeezed vacuum state, our observable receiver outperforms the other feasible receivers whereas it cannot approach the quantum Chernoff bound. The corresponding observable cannot be implemented with heterodyne detections due to the additional vacuum noise. In the classical regime using a thermal state, a receiver implemented with a photon number difference measurement approaches its bound regardless of the signal mean photon number, while it asymptotically approaches the classical bound in the limit of a huge idler mean photon number.

We use contemporary quantum computers to experimentally investigate quantum steering of an open quantum system by measurements on its environment. On three IBM quantum processors we distinguish a qubit as the open system and perform pairwise interactions with multiple environment ancillas, following a collision model approach. Different measurement strategies on the ancillas lead to different state ensembles of the open system, which are reconstructed by employing state tomography. The amount of steering within the resulting assemblages is quantified with the help of a semidefinite program. We successfully observe the presence of quantum steering in our experimental simulations and can discriminate the different performance qualities and noise levels of the selected quantum devices.

In this work, we exploit the idea of composite pulses to achieve robust population inversion in a three-level quantum system. The scheme is based on the modulation of the coupling strength, while the other physical parameters remain unchanged. The composite pulses sequence is designed by vanishing high-order error terms, and can compensate the systematic errors to any desired order. In particular, this scheme keeps a good performance under the disturbance of waveform deformations. This trait ensures that population inversion can be nearly obtained even when the pulse sequence has a short jump delay. As an example, we employ the designed composite pulse sequence to prepare the $W$ state in a robust manner in the superconducting circuits. The numerical results show that the fidelity can still maintain a high level in a decoherence environment.

The quantum approximate optimization algorithm (QAOA) is a quantum-classical hybrid algorithm intending to find the ground state of a target Hamiltonian. Theoretically, QAOA can obtain the approximate solution if the quantum circuit is deep enough. Actually, the performance of QAOA decreases practically if the quantum circuit is deep since near-term devices are not noise free, and the errors caused by noise accumulate as the quantum circuit increases. In order to reduce the depth of quantum circuits, we propose an ansatz dubbed as “Shortcuts to QAOA” (S-QAOA), S-QAOA provides shortcuts to the ground state of the target Hamiltonian by including more two-body interactions and releasing the parameter freedoms. To be specific, besides the existing $ZZ$ interaction in the QAOA ansatz, other two-body interactions are introduced in the S-QAOA ansatz such that the approximate solutions could be obtained with smaller circuit depth. Considering the Max-Cut problem and the Sherrington-Kirkpatrick model, numerically computation shows the $YY$ interaction has the best performance. The reason for this might arise from the counterdiabatic effect generated by $YY$ interaction. On top of this, we release the freedom of parameters of two-body interactions, which a priori do not necessarily have to be fully identical, and numerical results show that it is worth paying the extra cost of having more parameter freedom since one has a greater improvement on success rate.

In order to quantify the relative performance of different test-bed quantum computing devices, it is useful to benchmark them using a common protocol. While some benchmarks rely on the performance of random circuits and are generic in nature, here we instead propose and implement a practical, application-based benchmark. In particular, our protocol calculates the energy of the ground state in the single-particle subspace of a one-dimensional (1D) Fermi Hubbard model, a problem which is efficient to solve classically. We provide a quantum ansatz for the problem that is provably able to probe the full single-particle subspace for a general-length 1D chain and scales efficiently in number of gates and measurements. Finally, we demonstrate and analyze the benchmark performance on superconducting and ion-trap test-bed hardware from three hardware vendors and with up to 24 qubits.

Within the Floquet theory of periodically driven quantum systems, the nonlinear single-spin dynamics under pulse of a circularly polarized electromagnetic field is analyzed. It is demonstrated that the field, first, lifts the spin degeneracy and, second, the field-induced spin splitting is accompanied by the photon emission at the spitting frequency. This two-stage process leads, particularly, to the polarization of spins along the angular momentum of the circularly polarized field. As a result, the pulse-induced magnetization appears, what can be observed in state-of the-art measurements.

The limit of decreasing laser frequency cannot be considered independently from nondipole effects due to the increase in the laser-induced continuum electron speed in this limit. Therefore, in this work, tunneling ionization in the adiabatic limit is considered for an effective field that includes effects beyond the electric-dipole term to first order in $1/c$, with $c$ being the speed of light. The nondipole term describes the interaction resulting from the electric dipole-induced velocity of the electron and the magnetic-field component of the laser. The impact of this term on the ionization rate, tunnel exit point, momentum at the tunnel exit, and electron dynamics is discussed. In the appropriate limit, the results of a nondipole strong-field approximation approach and those of the strict adiabatic limit, where time and field strength are parameters, are discussed. The nondipole strong-field approximation approach is used to identify nonadiabatic modifications of the initial conditions. The results open up an avenue to include nondipole effects in the initial tunneling ionization step in semiclassical models of strong-field and attosecond physics.

We propose a scheme based on perturbation theory to reconstruct a molecular density matrix (DM) using the pump-probe state-resolved energy spectra. By applying the scheme to an ${\mathrm{O}}_2^{+}$ system produced via strong-field ionization, we demonstrate that the reconstruction scheme can retrieve the populations and coherences of vibrational-electronic (vibronic) states while maintaining good robustness. We further extend the strong-field transient ionization model to include the rotational degree of freedom. The evolution of the vibronic-state DM can be recovered, benefiting from the population stabilities between adjacent fractional revivals. The retrieved signals quantitatively capture the distinguishing decay features of the vibronic coherences. Our strategy paves the way toward understanding elaborate vibronic-state DMs in molecular systems.

We present a systematic study of a mobile impurity immersed in a three-dimensional Fermi sea of fermions at finite temperature, by using the standard non-self-consistent many-body $T$-matrix theory that is equivalent to a finite-temperature variational approach with the inclusion of one-particle-hole excitation. The impurity spectral function is determined in the real-frequency domain, avoiding any potential errors due to the numerical analytic continuation in previous $T$-matrix calculations and the small spectral broadening parameter used in variational calculations. In the weak-coupling limit, we find that the quasiparticle decay rate of both attractive and repulsive polarons does not increase significantly with increasing temperature, and therefore Fermi polarons may remain well defined far above Fermi degeneracy. In contrast, near the unitary limit with strong coupling, the decay rate of Fermi polarons rapidly increases and the quasiparticle picture breaks down close to the Fermi temperature. We analyze in detail the recent ejection and injection radio-frequency (rf) spectroscopy measurements, performed at Massachusetts Institute of Technology (MIT) and at the European Laboratory for Non-Linear Spectroscopy (LENS), respectively. We show that the momentum average of the spectral function, which is necessary to account for the observed rf spectroscopy, has a sizable contribution to the width of the quasiparticle peak in spectroscopy. As a result, the measured decay rate of Fermi polarons could be significantly larger than the calculated quasiparticle decay rate at zero momentum. By taking this crucial contribution into account, we find that there is a reasonable agreement between theory and experiment for the lifetime of Fermi polarons in the strong-coupling regime, as long as they remain well defined.

We perform an extensive numerical study on the breathing dynamics of a few-body Bose polaron setup in a one-dimensional species-selective harmonic trap. The dynamics is triggered by a quench of the impurity trap. The excitation of the background majority atoms is mediated via the majority-impurity interaction. The breathing spectrum is obtained for different numbers of majority particles, several values of the majority-component interaction strengths, and trap ratios. It is further compared to the breathing spectrum of a particle-balanced few-body Bose-Bose mixture. In particular, for equal postquench traps the employed protocol allows to couple states of different center-of-mass parity in contrast to species-symmetric trap quenches. Among the participating eigenstates we identify one having odd center-of-mass parity and even global parity. The breathing frequency induced by this state is a monotonically decreasing function of the coupling parameter. Importantly, in order to be numerically observable, it requires the entanglement between the species to be taken into account. We demonstrate this by comparing the numerically exact results obtained by means of the multilayer multiconfiguration time-dependent Hartree method for mixtures to the ones of a species mean-field ansatz. The entanglement-sensitive breathing frequency persists also for unequal postquench traps where the center of mass cannot be decoupled. Finally, we analyze the impact of global parity symmetry on the breathing dynamics by initializing a state of odd global parity. We evidence a striking resemblance to the breathing spectrum of the ground state, but find also some additional modes.

A weakly interacting Bose gas with a single static impurity is studied in one dimension under the assumption that its phase coherence length is longer than the system size. The local spectral density is evaluated and an analytic expression valid for positive strengths of the impurity coupling to the bosons, at all distances and at all frequencies, is obtained using the Bogoliubov–de Gennes approximation.

We describe a synthetic three-leg Hall ladder system with a tunable magnetic flux for neutral ${}^{173}\mathrm{Yb}$ atoms in a one-dimensional optical lattice. The ladder legs are formed by three hyperfine ground spin states of the atoms, and the complex interleg links are generated through Raman couplings between the spin states using multiple laser beams. The effective magnetic flux through a ladder plaquette $\varphi$ is controlled by the angles of the Raman laser beams with the lattice axis. We investigate the quench dynamics of the Hall ladder system for $\varphi \approx \frac{\pi }{3},\frac{\pi }{2},$ and $\frac{2\pi }{3}$ after a sudden application of the Raman coupling in various interleg link configurations. The semiclassical trajectory of the atoms in the plane of the spin composition and lattice position exhibits the characteristic motion of the effective magnetic field. In a tube configuration with the three legs cyclically linked, the quench evolution was observed to be substantially damped, which is attributed to the random flux threading the Hall tube.

The nonlinear spectral broadening of a near-infrared laser pulse propagating through pre-aligned molecules is investigated experimentally in a pump-probe scheme. The spectral shift of over 100 nm as well as spectral shaping is achieved via tuning the experimental conditions such as pump-probe delay, gas pressure, and pulse energy. Numerical simulation based on the nonlinear Schrödinger equation shows that the experimentally observed spectral tuning is due to the competition between different nonlinear effects, i.e., stimulated Raman scattering, the Kerr effect, and molecular alignment. Adjusting the pump-probe delay properly can decouple different nonlinear processes and allows for spectral tuning in a controlled way. This scheme is demonstrated to be attractive for the generation of few-cycle laser pulses with a frequency-tunable spectrum, broadband pulse shaping, and many strong field applications in the visible range.

We theoretically investigate $\mathcal{PT}$ symmetry, induced mechanical lasing, and force sensing in an optically levitated nanoparticle with coupled oscillation modes. The coupling in the levitated system is created by the modulation of an asymmetric optical potential in the plane transverse to the beam trapping the nanoparticle. We show that such a coupling can lead to $\mathcal{PT}$-symmetric mechanical behavior for experimentally realistic parameters. Further, by examining the phonon dynamics and the second-order coherence of the nanoparticle modes, we determine that induced mechanical lasing is also possible. Finally, we demonstrate that tunable ultrasensitive force sensing ($\sim zN/\sqrt{Hz}$) can be engineered in the system. Our studies represent an advance in the fields of coherent manipulation of coupled degrees of freedom of levitated mechanical oscillators and their application for sensing.

We investigate the performance of a symmetrical two-dimensional electromagnetically induced grating produced in a four-level $N$-type atomic scheme, which interacts with a weak probe field and two simultaneous coupling fields: a two-dimensional standing wave and a composite optical vortex beam. Based on the Maxwell wave equation, we study numerically the behavior of the amplitude, the phase modulations, as well as the probe field diffraction intensities of different order under various conditions for the coupling field detunings and the orbital angular momentum of the Laguerre-Gaussian field. The different orders of diffraction are altered when the azimuthal angle of the composite vortex light changes, thus producing a two-dimensional symmetric grating which transfers the probe energy to higher orders of diffraction. A detailed analysis of the probe field energy transfer to these different orders proves the possibility for direct control over the performance of the grating.

A multiple scattering model of a quantum particle interacting with a random Lorentz gas of fixed point scatterers is established in an Euclidean space of arbitrary dimension. At the core of the model, the scattering amplitude for the point scatterers is derived in detail and expressed in terms of the scattering length. The fundamental properties of the model, such as the cross section and the scattering matrix, are calculated. In addition, the model is shown to verify the optical theorem and thus probability conservation. Finally, the differential and total cross sections are numerically computed in two situations whether the Lorentz gas is smaller or larger than the mean free path. A distinct Airy diffraction peak is obtained for a large enough number of scatterers. This observation is related to the extinction paradox.

The multiple scattering model of a quantum particle in a random Lorentz gas consisting of fixed point scatterers is considered in arbitrary dimension. An efficient method is developed to numerically compute the map of the density of scattering resonances in the complex plane of the wave number without finding them one by one. The method is applied to two collision models for the individual scatterers, namely a resonant model, and a nonresonant hard-sphere model. The results obtained with the former are compared to the literature. In particular, the spiral arms surrounding the single-scatterer resonance are identified as proximity resonances. Moreover, the hard-sphere model is used to reveal previously unknown structures in the resonance density. Finally, it is shown how Anderson localization affects the distribution of resonance widths, especially in the one-dimensional case.