We discover a pronounced temporal shift of the peak of an optical pulse upon total internal reflection of the pulse from a sharp temporal boundary propagating in a homogeneous, isotropic, weakly dispersive linear medium. We derive an analytical expression for this shift and juxtapose the discovered effect to the spatial Goos-Hänchen shift occurring on reflection of a beam from an interface separating two homogeneous, isotropic, conservative linear media. In particular, we show that, in contrast to the spatial shift, the sign of the temporal shift is dictated by that of the group-velocity mismatch between the pulse and the temporal boundary, implying the possibility of a delay or advancement of the pulse upon reflection. Our analytical results, which are in excellent agreement with our numerical simulations, shed light on the fundamental aspects of the interaction of wave packets with temporal boundaries in material media.

We investigate the defect suppression effect in an inhomogeneous trapped atomic Bose gas which is quenched into a superfluid phase. We find that the spontaneous defect production is relatively more suppressed in the sample's outer region with higher atomic density gradient. By measuring the spatial distribution of created defects for various quench times, we show that the power-law scaling of the local defect density with the quench time is enhanced in the outer region, which is consistent with the Kibble–Zurek mechanism including the causality effect due to the spatial inhomogeneity of the system. This work opens an avenue in the study of nonequilibrium phase transition dynamics using the defect position information.

We consider the orientation of linear and symmetric-top molecules induced by laser and delayed terahertz (THz) pulses at high rotational temperatures (up to room temperature). We introduce an echo-assisted approach in which the achieved transient molecular orientation is an order of magnitude higher than the orientation produced by a single THz pulse. The laser pulse first dissects the wide molecular phase-space distribution into multiple narrow strips (filaments), each being cold and evolving separately. A subsequent THz pulse causes a substantial transient orientation of the individual filaments, which leads to an enhanced orientation of the whole molecular ensemble at later times via the echo mechanism. This enhanced degree of orientation is important in attosecond science, chemical reaction control, ultrafast molecular imaging, and other domains of physics.

It is commonly assumed that the most correct description of the electromagnetic world is the abstract one, and that topological constructs, such as lines of force are not covariant. In this Letter, we show that for a $y$-invariant system with a $p$-polarized electromagnetic field, it is possible to construct absolute (i.e., Lorentz invariant) lines in $(t,x,z)$ space-time, which we call electric spaghettis (ESs). The electromagnetic field is fully described by the ES topology that transcends the limit between space and time, plus a new invariant, the characteristic parameter $\eta$. In a Fabry-Perot resonant slit grating, three ES patterns can be distinguished, corresponding to three regions: null straight lines in plane-wave regions, rounded rhombi in the interference region inside the grating, and Bernoulli's logarithmic spiral patterns—the first ever described fractals—in the funneling region.

Quantum information science and intense laser-matter interaction are two apparently unrelated fields. Here, we introduce the notion of quantum information theory to intense laser-driven processes by providing the quantum mechanical description of measurement protocols for high-order harmonic generation in atoms. This allows one to conceive new protocols for quantum state engineering of light. We explicitly evaluate conditioning experiments on individual optical field modes and provide the corresponding quantum operation for coherent states. The associated positive-operator-valued measures are obtained and give rise to the quantum theory of measurement for the generation of high-dimensional entangled states and coherent-state superposition with controllable nonclassical features on the attosecond timescale. This establishes the use of intense laser-driven processes as an alternative platform for quantum information processing.

We propose an efficient tomography method to address the reconstruction complexity of any general quantum coherent-optical detectors. By extracting the linear loss from the entire detection system, we obtain the effective positive-operator-valued measure in matrix representations with a much smaller size. We apply this method to reconstruct a typical coherent detector: the weak-field homodyne detector. The method also highlights the effects of linear loss on nonclassical features of coherent optical detectors.

We study a system of $N$ nonrelativistic particles which form a near-threshold resonance. Assuming no subset of these particles can form a bound state, the resonance can only decay through an “explosion” into $N$ particles. We find that the decay width of the resonance scales as $E^{\mathrm{\Delta }-5/2}$ in the limit when the energy $E$ of the resonance goes to zero, where $\mathrm{\Delta }$ is the ground-state energy of a system of $N$ particles in a spherical harmonic trap with unit frequency. The formula remains valid when some pairs of final particles have zero-energy $s$-wave resonance, but the Efimov effect is not present. In the limit of large $N$, we show that the final particles follow a Maxwell-Boltzmann distribution if they are bosons and a semicirclelike law if they are fermions. We expect our general result to be applicable to various systems that exist in nature. In particular, we argue that metastable ${}^3\mathrm{He}$ droplets exist with the lifetime varying over many orders of magnitude ranging from a fraction of a nanosecond to values greatly exceeding the age of the Universe.

Polaritons arise due to a light-matter interaction and have been amply investigated for atoms and molecules in a quantum field. By increasing the coupling between the atom/molecule and the cavity, the upper polariton penetrates into the continuum (i.e., ionization continuum of atoms or molecules or dissociation continuum of molecules) within the standard two-level approach. We investigate what happens to the polaritons in reality. We first show that the upper polariton cannot enter the continuum if the atomic or molecular system itself does not support metastable states, often called resonances, and instead becomes a giant diffuse bound polariton which can be extended in space to any size one wishes. Then, we show how external perturbations can enable such a diffuse polariton to penetrate into the continuum and turn it into a metastable polariton, i.e., into a polaritonic resonance. In order to achieve these results the coupling of the bound states to the continuum induced by the quantum light has to be taken into account. We discuss how such giant diffuse bound polaritons as well as polaritonic resonances including their finite lifetime can be calculated and present explicit numerical examples. The results provide a complete picture of what happens to the upper polariton in the vicinity of the continuum and may be utilized to enhance ionization or dissociation inside the cavity.

We study the consequences of matter-wave revivals associated with many-body quantum interferences on physical observables for an ultracold bosonic system in an optical lattice. For the Bose-Hubbard dimer, we show that the interplay between weak intersite tunneling and strong on-site interactions can lead to the quantum dynamics of a density wave displaying several features not found in the mean-field limit. We demonstrate in particular the influence of these phenomena on occupancy oscillations and coherence. We furthermore observe resurgent revivals and a (anti)synchronization of revival peaks and occupancy oscillation peaks, which are the signatures of cooperative matter-wave interference effects that alternate between constructive and destructive features leading to the peak revival behaviors. The impact of such many-body quantum interference phenomena in various experimentally accessible observables is presented, paving the way for their detection in future ultracold-atom experiments.

Atomic Rydberg interactions allow one to create atom-light entanglement that can be used for diverse applications in quantum information science. The interaction-induced dephasing of collective atomic states is often the dominant contribution to the entanglement generation process in atomic ensembles. Although the mechanism has been used widely, its dynamics has not been previously observed, while its consequences have sometimes been ascribed instead to the presence of the excitation blockade. Here we report a study of the temporal evolution of an initially unentangled Rydberg spin wave into an (entangled) Dicke state. By comparing our observations to results of numerical simulations, we elucidate how the interaction-induced dephasing is responsible for entanglement generation in many-atom settings. These results have relevance to broad classes of applications for collective atomic systems, including driving of collective atomic qubits, on-demand generation of single photons, and preparation of entangled states involving atoms or light.

We propose counterintuitive topological pumps induced by disorder in noninteracting and interacting systems. We show an extrinsic topological pump driven by the on-site quasiperiodic potential for a two-loop sequence, where the disorder nonequivalently suppresses the topology of two pump loops. Moreover, we find an intrinsic topological pump induced by the hopping quasiperiodic disorder from a trivial single-loop pump in the clean limit, dubbed the topological Anderson-Thouless pump (TATP) as a dynamical analog of topological Anderson insulators. We demonstrate that the mechanism of the TATP is the disorder-induced shift of gapless critical points and the TATP can even appear in the dynamic disorder and interacting cases. We also extend the TATP to higher-order topological systems with disorder-induced quantized corner transport.

Discriminating between ballistic and diffuse components of light propagating through highly scattering media is not only important for imaging purposes but also for investigating the fundamental diffusion properties of the medium itself. Massively developed to this end over the past 20 years, nonlinear temporal gating remains limited to $\sim {10}^{-10}$ transmission factors. Here, we report nonlinear time-gated measurements of highly scattered femtosecond pulses with transmission factors as low as $\approx {10}^{-12}$. Our approach is based on the third-order nonlinear cross-correlation of femtosecond pulses, a standard diagnostic used in high-power laser science, applied to the study of fundamental light scattering properties.

The use of simple quantum processes promises to provide numbers that no physical observer could predict, but in practice, unwanted noise and imperfect devices can compromise fundamental randomness and protocol security. The ultimate random number generators take advantage of quantum nonlocality to certify unpredictability—including to an adversary—even in the absence of trust in devices. We demonstrate a generator of public or private randomness based on the quantum steering task. We use polarization-entangled photon pairs to certify and extract randomness in a one-sided device-independent framework, with the detection loophole closed. Our work enables nonlocality-based certified randomness generation in environmental regimes where fully-device-independent protocols are not feasible.

We propose and experimentally explore a method for realizing frustrated lattice models using a Bose-Einstein condensate held in an optical square lattice. A small lattice distortion opens up an energy gap such that the lowest band splits into two. Along the edge of the first Brillouin zone for both bands, a nearly flat energy-momentum dispersion is realized. For the excited band, a highly degenerate energy minimum arises. By loading ultracold atoms into the excited band, a classically frustrated $XY$ model is formed, describing rotors on a square lattice with competing nearest and next-nearest tunneling couplings. Our experimental optical lattice provides a regime where a fully coherent Bose-Einstein condensate is observed and a regime where frustration is expected. If we adiabatically tune from the condensate regime to the regime of frustration, the momentum spectra show a complete loss of coherence. Upon slowly tuning back to the condensate regime, coherence is largely restored. Good agreement with model calculations is obtained.

In this Letter we consider the problem of the certification of quantum measurements with an arbitrary number of outcomes. We propose a simple scheme for certifying any set of $d$-outcome projective measurements which do not share any common invariant proper subspace, termed here genuinely incompatible, and the maximally entangled state of two qudits. For our purpose, we focus on a simpler scenario, termed as a one-sided device-independent scenario where the resource employed for certification is quantum steering. We also study the robustness of our self-testing statements for a certain class of genuinely incompatible measurements including mutually unbiased bases which are essential for several quantum information-theoretic tasks such as quantum cryptography.

A superconducting qubit in a waveguide behaves as a pointlike nonlinear element. If irradiated with nearly resonant microwave pulses, the qubit undergoes quantum evolution and generates coherent fields at sideband frequencies due to elastic scattering. This effect is called quantum wave mixing (QWM), and the number of emerged side components depends on the number of interacting photons. By driving a superconducting qubit with short pulses with alternating carrier frequencies, we control the maximal number of photons simultaneously interacting with a two-level system by varying the number and duration of applied pulses. Increasing the number of pulses results in consecutive growth of the order of nonlinearity, which manifests in additional coherent side peaks appearing in the spectrum of scattered radiation while the whole spectrum maintains its asymmetry.

We study the performance of approximate invariant-based inverse engineering, in which we design approximate dynamics so that it realizes a target process, determine its parameter schedule by using invariant-based inverse engineering, and control a true system according to it. We find that the worst-case performance of this control scheme can be evaluated by using the designed dynamics. By using a quantum speed limit formula, we derive a lower bound for overlap between the true dynamics and approximate dynamics. Notably, it can be calculated without knowledge of the true dynamics. We apply this bound to the above control scheme. We also find that the present result may be used for control scheduling.

We study the properties of intermediate four-atom complexes formed in bimolecular collisions, which may have the critical role for understanding of losses in experiments with ultracold alkali-metal molecules. We show that the variation in the nuclear spin–spin and quadrupole couplings may be strong enough to couple different rotational manifolds resulting in an increase in the density of states and lifetimes of the collision complexes. We estimate the nuclear spin–rotation, spin–spin, and quadrupole coupling constants for bialkali four-atom complexes using ab initio quantum-chemical methods and model the reaction kinetics using an extended rate equation approach. We also reveal that the interaction-induced variation of the electron spin–nuclear spin couplings may explain the recently observed long lifetime of alkali-metal three-atom complexes formed in atom-molecule collisions. Our results can be helpful for interpreting recent experimental data obtained with nonreactive systems which reported unexpectedly large loss rates and for designing future experiments utilizing polar molecules for the purpose of precision measurements and quantum technologies.

Mechanical rotation and oscillation have far lower frequencies than light does; thus they are not coupled to each other conventionally. In this Letter, we show that the torsional mechanical oscillation of an optical cavity can be coupled to the optical modes by introducing birefringence, which produces nondegenerate modes in the cavity: ordinary and extraordinary rays. Twisting the cavity mixes them and modulates the electromagnetic energy. We find the torsional optomechanical Hamiltonian by quantizing the total energy and reveal that the torsional oscillation can be resonantly driven by light.

We study the spectral stability of a two-dimensional discrete Schrödinger equation on a square lattice, in the simultaneous presence of a fractional Laplacian and $\mathcal{PT}$ symmetry. For that purpose, we compute the plane-wave spectrum in closed form, as a function of the gain/loss parameter and the fractional exponent. Examination of the spectrum reveals that an increase of the gain/loss parameter favors the early appearance of complex eigenvalues, that is, the onset of a broken $\mathcal{P}T$ symmetry. On the other hand, as the fractional exponent decreases from unity, at a critical value a gap opens up separating the upper and lower bands, and the spectrum becomes real. A further decrease of the exponent increases the width of the gap and the system remains in the $\mathcal{PT}$-symmetric phase down to a vanishing value of the fractional exponent. An examination of the density of states and the participation ratio reinforces these observations and leads one to conclude that, unlike the standard, nonfractional case where the binary lattice is always in the broken $\mathcal{PT}$ phase, for the fractional case it is possible to have a symmetric $\mathcal{P}\mathcal{T}$ phase in the presence of a finite gain/loss parameter and a small enough fractional exponent.