We employ a recently developed three-dimensional semiclassical model to identify nondipole effects in triple ionization of Ne driven by infrared laser pulses at intensities where electron-electron correlation prevails. This model fully accounts for the Coulomb interaction of each electron with the core and avoids artificial autoionization by employing effective Coulomb potentials to describe the interaction between bound electrons. Using the effective Coulomb potential for bound-bound electrons (ECBB) model, we identify a prominent signature of nondipole effects. Namely, the component along the direction of light propagation of the average sum of the final electron momenta is large and positive. That is, we identify a positive momentum offset, absent in the dipole approximation. We find that this positive momentum offset stems mostly from the momentum change due to the magnetic field. To better understand this momentum change, we also develop a simple model for the motion of an electron inside an electromagnetic field. This simple model accounts for the effect of the Coulomb forces only as a sharp change in the momentum of the electron during recollision. We show that the momentum change due to the magnetic field is related to the sharp change in momentum during recollision for the recolliding electron as well as to the time of recollision for both the recolliding and bound electrons. Hence, we demonstrate that the final electron momentum offset probes the strength of a recollision and hence the degree of correlation in multielectron ionization.

Using the hydrogen atom as a prototype, multiphoton excitation and ionization are simultaneously investigated with 400–800-nm laser fields. By tuning the relative phase $\varphi$ between the two color components, the interference between different excitation channels results in the enhancement or suppression of the excitation probability and then determines the subsequent ionization. Moreover, the $\varphi$-dependent excitation and ionization are related to the relative intensity of the 400–800-nm fields. When the laser intensity of the 800-nm field is $5\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$, the excitation and ionization probabilities present similar fluctuation behavior as a function of $\varphi$. With the increase of the 800-nm laser intensity, more multiphoton excitation and ionization channels open, and the ponderomotive energy begins to have an influence, making the two oscillatory behaviors differ from each other. The coexistence of excitation and ionization channels can be identified by decomposing the probabilities to the components with different angular quantum numbers, which can trace how the system absorbs multiphotons in strong laser fields and further control the ultrafast scenarios in atoms and even molecules.

We study ultrafast charge rearrangement in dissociating 2-iodopropane $\left(2\text{−}{\mathrm{C}}_3{\mathrm{H}}_7\mathrm{I}\right)$ using site-selective core ionization at the iodine atom. Clear signatures of electron transfer between the neutral propyl fragment and multiply charged iodine ions are observed in the recorded delay-dependent ion momentum distributions. The detected charge-transfer pathway is only favorable within a small (few angstroms), charge-state-dependent spatial window located at C-I distances longer than that of the neutral ground-state molecule. These results offer insights into the physics underpinning charge transfer in isolated molecules and pave the way for a different class of time-resolved studies.

Polarons, which arise from the self-trapping interaction between electrons and lattice distortions in a solid, have been known and extensively investigated for nearly a century. Nevertheless, the study of polarons continues to be an active and evolving field, with ongoing advancements in both fundamental understanding and practical applications. Here, we present a microscopic model that exhibits a diverse range of dynamic behavior, arising from the intricate interplay between two excitation-phonon coupling terms. The derivation of the model is based on an experimentally feasible Rydberg-dressed system with dipole-dipole interactions, making it a promising candidate for realization in a Rydberg atoms quantum simulator for excitation dynamics interacting with optical phonons. Remarkably, our analysis reveals a growing asymmetry in Bloch oscillations, leading to a macroscopic transport of nonspreading excitations under a constant force. Finally, we demonstrate the robustness of our findings against on-site random potential.

We investigate numerically and experimentally the properties of a two-color optical fiber taper trap, for which the evanescent field of the modes in the fiber taper give rise to a trapping potential with both radial confinement and confinement along the fiber axis. Experimentally, we use the technique to confine colloidal nanoparticles near the surface of an optical fiber taper, and show that the trapping position of the particles is adjustable by controlling the relative power of two modes in the fiber. We also demonstrate a proof-of-principle application by trapping quantum dots together with gold nanoparticles in a configuration where the trapping fields double as the excitation field for the quantum dots. This scheme will allow the positioning of quantum emitters in order to adjust coupling to resonators combined with the fiber taper.

A form of quantum control is proposed by applying twisted light, also known as optical vortex beams, to drive ultranarrow atomic transitions in neutral Ca, Mg, Yb, Sr, Hg, and Cd bosonic isotopes. This innovative all-optical spectroscopic method introduces spatially tailored electric and magnetic fields to fully rewrite atomic selection rules, reducing simultaneously probe-induced frequency shifts and additional action of external ac and dc field distortions. A twisted-light focused probe beam produces strong longitudinal electric and magnetic fields along the laser propagation axis, which opens the ${}^{1}S_0\to {}^{3}P_0$ doubly forbidden clock transition with a high $E1M1$ two-photon excitation rate. This long-lived clock transition is thus immune to nonscalar electromagnetic perturbations. Zeeman components of the $M2$ magnetic quadrupole ${}^{1}S_0\to {}^{3}P_2$ transition considered for quantum computation and simulation are now selectively driven by transverse or longitudinal field gradients with vanishing electric fields. These field gradients are manipulated by the mutual action of orbital and spin angular momentum of the light beam and are used in presence of tunable vector and tensor polarizabilities. A combination of these two different twisted-light induced clock transitions within a single quantum system, at the same magic wavelength and in presence of a common thermal environment, significantly reduces systematic uncertainties. Furthermore, it generates an optical synthetic frequency which efficiently limits the blackbody radiation shift and its variations at room temperature. Engineering light-matter interaction by optical vortices merged with composite pulses will ultimately benefit experimental atomic and molecular platforms targeting an optimal coherent control of quantum states, reliant quantum simulation, novel approaches to atomic interferometry, and precision tests of fundamental theories in physics and high-accuracy optical metrology.

Compass states deliver a sub-Planck phase-space structure in the sense that sensitivity to phase-space displacement is superior to the sensitivity of displacing the vacuum state in any direction, but this sensitivity is anisotropic: better sensitivity for some directions of phase-space displacement versus others. Here we introduce generalized compass states as superpositions of $n$ compass states, with each oriented by $\pi /2n$ with respect to its predecessor. Specifically, we derive Wigner functions for these generalized compass states and approximate closed-form expressions for overlaps between generalized compass states and their displaced counterparts. Furthermore, we show that generalized compass states, in the limit $n\to \infty$, display isotropic sensitivity to phase-space displacement in any direction.

The nonlocality of single-photon states has been analyzed from several different but interrelated perspectives. In this article we propose a demonstration based on the electromagnetic energy density observable and on the antilocal property of the frequency operator $\mathrm{\Omega }=c{(-\mathrm{\Delta })}^{1/2}$. The present proof is based on the standard quantization of the electromagnetic field, which can be formulated equivalently in the momentum representations or in the position representations of Landau and Peierls [Z. Phys. 62, 188 (1930)] and of Białynicki-Birula [edited by E. Wolf (Elsevier, Amsterdam, 1996)]. Our proof extends to all single-photon states the results of Białynicki-Birula, which were formulated for two particular classes of states, those involving a uniform localization [Phys. Rev. Lett. 80, 5247 (1998)] or alternatively states that are electrically or magnetically localized [Phys. Rev. A 79, 032112 (2009)]. Our approach is formulated in terms of Knight's definition of strict localization [J. Math. Phys. 2, 459 (1961)], based on the comparison of expectation values of single-photon states of local observables with those of the vacuum.

Coherent-population trapping (CPT) is a multilevel quantum coherence phenomenon of promising applications in atomic clocks and magnetometers. In particular, multipulse CPT Ramsey interferometry is a powerful tool for improving the performance of CPT atomic clocks. Most studies on multipulse CPT Ramsey interferometry consider periodic pulse sequence and time-independent detuning. However, to further improve the accuracy and precision, one may modify the spectrum symmetry which involves a pulse sequence with time-dependent detuning or phase shift. Here we theoretically analyze the multipulse CPT Ramsey interferometry under arbitrary pulse sequences of time-dependent detuning and obtain a general analytical formula. Using our formula, we analyze the popular CPT Ramsey interferometry schemes such as two-pulse symmetric and antisymmetric spectroscopy and multipulse symmetric and antisymmetric spectroscopy. Under periodic pulses, we quantitatively obtain the influences of pulse width, pulse period, pulse number, and Rabi frequency. Furthermore, we examine the impact of CPT pulses and Rabi frequency on the light shift in both two-pulse and multipulse CPT Ramsey interference. Our theoretical results can guide the experimental design to improve the performance of atomic clocks via multipulse CPT Ramsey interferometry.

We report absolute frequency measurements on the three excited-state transitions connecting the $2p^2\left({}^{3}P\right)3s{}^2{P}_{1/2,3/2}$ and $2p^2\left({}^{3}P\right)3p{}^2{D}_{3/2,5/2}^{\circ }$ states in neutral ${}^{14}\mathrm{N}$ using saturated absorption spectroscopy in a glass cell filled with a mixture of nitrogen and argon gas and driven with a radio-frequency electric field. The absolute transition frequencies are found to be 319 288 834(150), 319 085 445(125), and 316 795 808(140) MHz for the 1/2–3/2, 3/2–5/2, and 3/2–3/2 transitions, respectively. This work represents the saturated absorption measurements for these transitions. These results are approximately a factor of 5 improvement over previous results, despite being conservative estimates. We found that the overlapping hyperfine structure prevented fitting individual spectral features, presenting difficulties for possible future tests of quantum electrodynamics in seven-electron systems. A better understanding of the amplitude behavior of a merged hyperfine structure will allow the extrapolation of the center of gravity frequencies for these transitions to sub-MHz precision using our collected data.

Signatures for strong-field quantum electrodynamics are determined for collisions between round ultrarelativistic leptonic beams in the quantum limit of beamstrahlung. In the low-disruption regime, we derive the integrated beamstrahlung photon spectrum that features a characteristic peak close to the beam energy. The conditions to precisely observe this peak experimentally are given regarding the beam parameters. Moreover, the effects of electron-positron pair creation and beam disruption on the photon spectrum are discussed and explored with three-dimensional particle-in-cell QED simulations. The photon spectrum is associated with the emission of ultrashort and highly collimated $\gamma$-ray beams with a peak spectral brightness exceeding ${10}^{30}\mathrm{photons}/(\mathrm{s}{\mathrm{mm}}^2{\mathrm{mrad}}^{2}0.1\%\mathrm{BW})$ at the 100-GeV level of photon energy (close to the beam energy).

Manipulating the spectral line shape exhibits great potential in realizing active optical circuits with switching, sensing, and modulation capabilities. Exploring unusual line shapes, such as Fano resonance and electromagnetically induced transparency (EIT), has attracted substantial interest. Conventional methods of engineering the spectral line shape have limited tunability and face challenges in multiplexing different spectral line shapes. Here, we propose and numerically demonstrate a mechanism to tailor the transmission line shape almost at will by exploiting the interference of frequency-dependent chiral dipolar states in two helix particles sitting above a dielectric waveguide. We show that, by tuning the polarization of the chiral dipoles and exploiting transverse spin-orbit interaction, one can control the asymmetric Pancharatnam-Berry geometric phase for the excited guided waves propagating in opposite directions. The interference of the guided waves, respectively, excited by the two particles can give rise to transmissions with various line shapes, including Lorentzian-like, antiresonance-like, Fano-like, and EIT-like line shapes, which carry an intriguing property of line-shape-momentum locking, i.e., the transmissions in opposite directions have different line shapes. Our findings open possibilities for multiplexed and multifunctional nanophotonic designs with unprecedented capability of spectral line shaping. The proposed structures can be conveniently integrated with optical circuits for on-chip applications.

We study the open relaxation dynamics of an asymmetric dipole that is ultrastrongly coupled to a single electromagnetic cavity mode. By using a thermalizing master equation for the whole interacting system we derive a phase diagram of the Liouvillian gap. It emerges that the ultrastrong coupling inhibits the system's relaxation toward the equilibrium state due to an exponential suppression of the dipole tunneling rate. However, we find that polaronic multiphoton resonances restore fast relaxation by a cavity-mediated dipole resonant tunneling process. Aside from the numerical evidence, we develop a fully analytical description by diagonalizing the Rabi model through a generalized rotating-wave approximation, valid in the so-called polaron frame. The relaxation physics of such ultrastrong-coupling systems is then reduced to a multiphoton polaron version of the standard textbook dressed states picture. At the end we discuss an extension to a multiwell dipole that can set the basis of a cascaded resonant tunneling setup in the ultrastrong coupling regime.

A two-dimensional (2D) material, formed, for example, by a self-assembled molecular monolayer or by a single layer of a van der Walls material, can couple efficiently with photonic nanocavities, potentially reaching the strong-coupling regime. The coupling can be modeled using classical harmonic-oscillator models or cavity quantum electrodynamics Hamiltonians that often neglect the direct dipole-dipole interactions within the monolayer. Here we diagonalize the full Hamiltonian of the system, including these direct dipole-dipole interactions. The main effect on the optical properties of a typical 2D system is simply to renormalize the effective energy of the bright collective excitation of the monolayer that couples with the nanophotonic mode. On the other hand, we show that for situations of extreme field confinement, large transition dipole moments, and low losses, fully including the direct dipole-dipole interactions is critical to correctly capture the optical response, with many collective states participating in it. To quantify this result, we propose a simple equation that indicates the condition for which the direct interactions strongly modify the optical response.

As a general framework, matrix exponential dimensionality reduction (MEDR) deals with the small-sample-size problem that appears in linear dimensionality-reduction (DR) algorithms. High complexity is the bottleneck in this type of DR algorithm because one has to solve a large-scale matrix exponential generalized eigenproblem. To address it, here we design a general quantum algorithm framework for MEDR based on the block-encoding technique. This framework is configurable, that is, by selecting suitable methods to design the block encodings of the data matrices, a series of efficient quantum algorithms can be derived from this framework. Specifically, by constructing the block encodings of the data matrix exponentials, we solve the generalized eigenproblem and then obtain the digital-encoded quantum state corresponding to the compressed low-dimensional data set, which can be directly utilized as the input state for other quantum machine learning tasks to overcome the curse of dimensionality. As applications, we apply this framework to four linear DR algorithms and design their quantum algorithms, which all achieve a polynomial speedup in the dimension of the sample over their classical counterparts.

Quantum machine learning has attracted considerable interest due to its potential to improve certain learning tasks. In conventional quantum machine learning, the output is the expectation value of a preselected observable, and the projective measurement forces a quantum circuit to run many times to obtain the output with reasonable precision. In this work, we propose a protocol to utilize the adiabatic quantum evolution to execute quantum learning tasks, in which the output is obtained by the adiabatic weak measurement rather than the projective measurement. In comparison to previous protocols, we use only a single-shot measurement and therefore avoid the measurement repetition in the previous protocols. Moreover, our protocol allows us to extract the expectation values of multiple observables without disrupting the concerned quantum states.

Establishing the limits of precision in the estimation of parameters for noisy quantum channels probed by qubits is important for many areas of quantum information, such as quantum sensing, computation, and communication. Here we consider the estimation of parameters characterizing a general class of noisy Pauli channels. We show that two entangled qubits, such that only one of them probes the channel, may lead, under an entangling measurement, to strong enhancement of the precision in the estimation, as compared to the precision corresponding to sending the pair, entangled or not, through the channel. We prove that entanglement plays an essential role, as does the entangling detection procedure, consisting in projecting the final state onto a Bell-state basis. We also prove that quantum advantage is obtained only when the output state, after interaction with the sample, is not entangled anymore. This behavior has striking similarities with quantum illumination, where initial entanglement of probe and ancilla beams, followed by an entangling measurement, lead to enhancement of the sensitivity of photodetection, even after the output beams are disentangled. Similarities and differences with ghost imaging are also discussed.