A long-standing problem in quantum-orbit theory has been exactly which solutions of the saddle-point equations to include in the decomposition of the ionization or harmonic-generation amplitude. Up to now, solutions corresponding to a negative travel time have always been discarded. For the case of an elliptically polarized driving laser field, we show that certain solutions with a negative travel time are relevant and have to be included, in addition to the customary orbits with positive travel times, in order to achieve good agreement with the result of a numerical evaluation. In fact, these solutions are responsible for a pronounced qualitative effect in the high-order above-threshold ionization amplitude: a feature with the shape of a coffee bean split along the direction of the major polarization axis, which dominates the velocity map especially for long wavelength. We also discuss the electron trajectories in complex space and time that correspond to these orbits.

A major challenge for the realization of useful universal quantum computers is achieving high fidelity two-qubit entangling gate operations. However, calibration errors can affect the quantum gate operations and limit their fidelity. To reduce such errors it is desirable to have an analytical understanding and quantitative predictions of the effects that miscalibrations of gate parameters have on the gate performance. In this work, we study a systematic perturbative expansion in miscalibrated parameters of the Mølmer-Sørensen entangling gate, which is widely used in trapped-ion quantum processors. Our analytical treatment particularly focuses on systematic center-line detuning miscalibrations. Via a unitary Magnus expansion, we compute the gate evolution operator, which allows us to obtain relevant key properties such as relative phases, electronic populations, quantum state purity and fidelities. These quantities, subsequently, are used to assess the performance of the gate using the fidelity of entangled states as performance metric. We verify the predictions from our model by benchmarking them against measurements in a trapped-ion quantum processor. The method and the results presented here can help design and calibrate high-fidelity gate operations of large-scale quantum computers.

A useful approach to characterize and identify quantum phase transitions lies in the concept of multipartite entanglement. In this paper, we consider well-known measures of multipartite (global) entanglement, i.e., average linear entropy of one-qubit and two-qubit reduced density matrices, in order to study topological quantum phase transition (TQPT) in the Kitaev Toric code Hamiltonian with a nonlinear perturbation. We provide an exact mapping from aforementioned measures in the above model to internal energy and energy-energy correlations in the classical Ising model. Accordingly, we find that the global entanglement shows a continuous and sharp transition from a maximum value in the topological phase to zero in the magnetized phase in a sense that its first-order derivative diverges at the transition point. In this regard, we conclude that not only can the global entanglement serve as a reasonable tool to probe quantum criticality at TQPTs, but it also can reveal the highly entangled nature of topological phases. Furthermore, we also introduce a conditional version of global entanglement which becomes maximum at the critical point. Therefore, regarding a general expectation that multipartite entanglement reaches maximum value at the critical point of quantum many-body systems, our result proposes that the conditional global entanglement can be a good measure of multipartite entanglement in TQPTs.

Double ionization of benzene initiates a competition between its fragmentation driven by Coulomb repulsion and dication stabilization, in which the molecule undergoes a strong geometric rearrangement. We find experimentally that stabilization following an electron-impact-induced double-ionization is remarkably sensitive to the isotopic replacement of just one ${}^{13}\mathrm{C}$ atom in the benzene ring. This result has no analog in dications of smaller molecules. The large reactivity of medium-size hydrocarbon dications points towards an alternative route to imprint isotopic signatures in astrophysical processes.

In collisions involving projectiles with bound electrons, partial or total screening of their nuclear charge must be considered. Accordingly, a projectile dynamic effective charge, as a function of the collision momentum transfer, is included in the state-of-the-art continuum distorted wave with eikonal initial state model. This correction, applied in the exit channel, recovers a two-center description from collisions with neutral projectiles. Also, an improvement of both target and projectile ionization double differential cross sections is achieved, especially at the binary encounter and electron capture to the continuum peaks. The results calculated by our analytical model are shown in accordance with previous theoretical and experimental work.

New ($e,2e$) differential cross-section measurements from xenon are presented as the scattering geometry changes from coplanar to the perpendicular plane. Measurements were taken at energies of 60 eV, 80 eV, and 100 eV above the ionization potential. These new data are compared to measurements collected for the first four noble gases and to that from xenon at lower energies. The ratio of relative cross sections between xenon and helium have been measured using a xenon-helium mixture. With this data absolute differential cross sections of xenon can be determined once the helium cross sections have been calculated.

We consider a one-dimensional spinless Fermi gas coupled to an optical cavity. Using exact diagonalization, we numerically study the model and find that the system can undergo the ergodic-nonergodic transition, which depends on the cavity-fermion coupling strength and the fermion-fermion interacting strength. We demonstrate that the cavity-fermion coupling strength, which breaks the integrability of the system, can lead to the ergodic phase in the strong-interaction regime. It is shown that the ergodic-nonergodic transition is characterized by static statistical methods, including level-spacing distribution and the average ratio of consecutive level spacing. To experimentally observe the transition, we also study the dynamic properties of the system, including the out-of-time-ordered correlator, the time evolution of the occupancy imbalance, and the survival probability, which manifest different behaviors in different phases. In addition, we find that even though the system contains long-range interactions, it does not exhibit information fast scrambling whether in the ergodic phase or the nonergodic phase.

The sensitivity of a refractive index sensor based on an optical bound state in the continuum is considered. Applying Zel'dovich perturbation theory we derived an analytic expression for bulk sensitivity of an all-dielectric sensor utilizing symmetry protected in-$\mathrm{\Gamma }$ optical bound states in a dielectric grating. The upper sensitivity limit is obtained. A recipe is proposed for obtaining the upper sensitivity limit by optimizing the design of the grating. It is shown that the maximal sensitivity can be achieved regardless to the permittivity of the constituent dielectric of the system. The results are confirmed through direct numerical simulations.

We explore experimentally a quantum metamaterial based on a superconducting chip with 25 frequency-tunable transmon qubits coupled to a common coplanar resonator. The collective bright and dark modes are probed via the microwave response, i.e., by measuring the transmission amplitude of an external microwave signal. All qubits have individual control and readout lines. Their frequency tunability allows the number $N$ of resonantly coupled qubits to change and a disorder in their excitation frequencies to be introduced with preassigned distributions. While increasing $N$, we demonstrate the expected $N^{1/2}$ scaling law for the energy gap (Rabi splitting) between bright modes around the cavity frequency. By introducing a controllable disorder and averaging the transmission amplitude over a large number of realizations, we demonstrate a decay of mesoscopic fluctuations which mimics an approach towards the thermodynamic limit. The collective bright states survive in the presence of disorder when the strength of individual qubit coupling to the cavity dominates over the disorder strength.

In this paper we propose a kind of all-solid-state and a hybrid electrically and thermally tunable multifunctional metasurface array based on the graphene pattern and vanadium dioxide configuration, which can realize the functions of band-stop filtering, beam steering, and beam splitting. When ${\mathrm{VO}}_2$ is in the dielectric state, this metasurface can be used as an angle-insensitive filter that can realize the function of the dual-frequency adjustable band-stop filtering by changing the Fermi level of graphene in the range of 4.11–6.51 THz. When ${\mathrm{VO}}_2$ is in the metal state, this metasurface can be used as a terahertz phased array device. By changing the Fermi level of graphene and the phase distribution, it is proved that this metasurface can achieve a specific deflection angle, a continuous scanning range of 0–$68.{37}^{\circ }$, or beam splitting at angles of $\pm 25.{56}^{\circ }$ and $\pm 59.{29}^{\circ }$. Our work shows great potential in the field of multifunctional integrated chips.

We consider a general version of the phenomenon of more nonlocality with less entanglement within the framework of the unambiguous (i.e., conclusive) quantum state discrimination problem under local quantum operations and classical communication. We show that although the phenomenon was obtained before for two qutrits, it can also be observed for two qubits, whereas still being at the single-copy level. We establish that the phenomenon is intrinsically connected to the concept of unextendible entangled bases in the two-qubit case. In the process, we demonstrate a hierarchy of nonlocality among sets of two-qubit orthogonal pure states where the “nonlocality” is in the sense of a difference between global and local abilities of quantum state discrimination. We present a complete characterization of two-qubit pure orthogonal state sets of cardinality three with respect to their nonlocality in terms of unambiguous local distinguishability, the status for other cardinalities being already known. The results are potentially useful for secure quantum communication technologies with an optimal amount of resources.

We propose an approach based on random matrix theory to calculate the temporal second-order intensity correlation function $g^{\left(2\right)}\left(t\right)$ of the radiation emitted by random lasers and random fiber lasers. The multimode character of these systems, with a relevant degree of disorder in the active medium, and a large number of random scattering centers substantially hinder the calculation of $g^{\left(2\right)}\left(t\right)$. Here, we apply in a photonic system the universal statistical properties of Ginibre's non-Hermitian random matrix ensemble to obtain $g^{\left(2\right)}\left(t\right)$. Excellent agreement is found with time-resolved measurements for several excitation powers of an erbium-based random fiber laser. We also discuss the extension of the random matrix approach to address the statistical properties of general disordered photonic systems with various Hamiltonian symmetries.

We compare single-photon qubits and hybrid qubits as information carriers through quantum teleportation using a Gaussian continuous-variable channel. A hybrid qubit in our study is in the form of entanglement between a coherent state and a single photon. We find that hybrid qubits outperform photonic qubits when coherent amplitudes of the hybrid qubits are as low as $\alpha \lesssim 1$, while single-photon qubits yield better results for larger amplitudes. We analyze further the effect of photon losses and observe that the overall character of teleportation for different qubits remains the same although the teleportation fidelities are degraded by photon losses. Our work provides a comparative look at practical quantum information processing with different types of qubits.

We show that entangled measurements provide an exponential advantage in sample complexity for Pauli channel estimation, which is both a fundamental problem and a practically important subroutine for benchmarking near-term quantum devices. The specific task we consider is to simultaneously learn all the eigenvalues of an $n$-qubit Pauli channel to $\pm \varepsilon$ precision. We give an estimation protocol with an $n$-qubit ancilla that succeeds with high probability using only $O(n/{\varepsilon }^2)$ copies of the Pauli channel, while proving that any ancilla-free protocol (possibly with adaptive control and channel concatenation) would need at least $\mathrm{\Omega }\left(2^{n/3}\right)$ rounds of measurement. We further study the advantages provided by a small number of ancillas. For the case that a $k$-qubit ancilla ($k\le n$) is available, we obtain a sample complexity lower bound of $\mathrm{\Omega }\left(2^{(n-k)/3}\right)$ for any nonconcatenating protocol, and a stronger lower bound of $\mathrm{\Omega }\left(n{2}^{n-k}\right)$ for any nonadaptive, nonconcatenating protocol, which is shown to be tight. We also show how to apply the ancilla-assisted estimation protocol to a practical quantum benchmarking task in a noise-resilient and sample-efficient manner, given reasonable noise assumptions. Our results provide a practically interesting example for quantum advantages in learning and also bring insights for quantum benchmarking.

Quantum channels, in general, will disturb quantum states and will lead to decoherence (i.e., information leakage to the environment). For bipartite systems, local channels often cause more decoherence for global states than for local states due to correlations. Intuitively, the difference between these two kinds of decoherence (i.e., global and local ones) may be regarded as a quantifier characterizing certain aspects of correlations in the global bipartite states. Based on this idea, we introduce a quantifier of correlations (relative to a local channel) as the coherence difference and investigate its basic properties. We probe and quantify correlations relative to various channels including the unitary channels, the twirling channels, the projective measurements, and the weak measurements. In particular, we show that both product states and some natural classical-quantum states can be operationally characterized in terms of local channels: The product states are just those states with vanishing coherence difference relative to the twirling channel induced by the unitary group, whereas these natural classical-quantum states are just those states with vanishing coherence difference relative to the twirling channel induced by the subgroup of the unitary group that does not disturb the local states. We further illustrate the results by various examples.

We investigate dynamics of a vortex ring encountering a coaxial spherical obstacle in Bose-Einstein condensates. For a perfect spherical obstacle, the vortex ring would first expand and then shrink when passing over the obstacle. For the case of a periodically modified spherical obstacle, the circular vortex ring is deformed to be gear shaped during the passing-over process. This gear-shaped vortex ring can evolve into a helical Kelvin-wave state, or split into several smaller vortex rings. In the latter situation, these split vortex rings would travel some distances forwards and then reconnect with each other, forming one new forward-moving Kelvin wave, or two Kelvin waves of different sizes, with the bigger one moving forwards and the smaller one moving backwards.

In this article we calculate the energy of an impurity weakly coupled to a spin-1/2 fermionic superfluid. We show that the divergences resulting from three-body physics can only be cured using a proper description of the excitations of the many-body background. We highlight the crucial role played by interactions between quasiparticles which are overlooked within Bardeen-Cooper-Schrieffer mean-field theory of fermionic superfluidity. By contrast, we prove that their addition using the random phase approximation allows us to regularize the energy of the impurity. Finally, we show that these beyond mean-field corrections should be observable by the analysis of the frequency shift of the impurity center-of-mass oscillations in an external confining potential.

We demonstrate an intriguing giant optical beam shift in a tilted polarizer system analogous to the Imbert-Fedorov shift in partial reflection around the Brewster's angle of incidence. We explain this giant shift using a generalized theoretical treatment for beam shift in a tilted uniaxial anisotropic material incorporating the three-dimensional orientation of its optic axis in the laboratory frame. A regulated control and tunability of both the magnitude and direction of the shifts are demonstrated further by wisely tuning the input polarization and the corresponding orientation angles. These findings open up the possibility of precise micron-level beam steering using a simple linear polarizer.