In recent decades, a great variety of research and applications concerning Bell nonlocality have been developed with the advent of quantum information science. Providing that Bell nonlocality can be revealed by the violation of a family of Bell inequalities, finding maximal Bell violation (MBV) for unknown quantum states becomes an important and inevitable task during Bell experiments. In this paper we introduce a self-guided method to find MBVs for unknown states using a stochastic gradient ascent algorithm (SGA), by parametrizing the corresponding Bell operators. For three investigated systems (two qubit, three qubit, and two qutrit), this method can ascertain the MBV of general two-setting inequalities within 100 iterations. Furthermore, we prove SGA is also feasible when facing more complex Bell scenarios, e.g., $d$-setting $d$-outcome Bell inequality. Moreover, compared to other possible methods, SGA exhibits significant superiority in efficiency, robustness, and versatility.

We theoretically investigate the high-harmonic generation (HHG) from periodical structures driven by an intense laser pulse. Including the full bands, the single electron time-dependent Schrödinger equation is numerically solved in the velocity gauge using Bloch states to obtain the emission spectra. The contributions from different crystal sites are identified using the localized Wannier functions and time-frequency analysis. It is found that the cutoff energy of harmonics is depending on the migration distance of the electron and the instantaneous laser field strength when the electron transports into the related sites. We show that the coherence among different sites during electron propagation is crucial for HHG from solids which can be taken advantage to control the individual site contribution to the certain frequency range of the total harmonic spectra.

A recent experiment at the European Laboratory for Non-Linear Spectroscopy (LENS) on a three-dimensional (3D) Fermi gas has reported a negative effective mass ($m^{*}<0$) for Fermi polarons in the strongly repulsive regime. There naturally arises a question of whether the negative $m^{*}$ is a precursor of the instability towards phase separation (or itinerant ferromagnetism). In this paper, we make use of the exact solutions to study the ground state and excitation properties of repulsive Fermi polarons in one dimension, which also can exhibit a negative $m^{*}$ in the super-Tonks-Girardeau regime. By analyzing the total spin, quasimomentum distribution, and pair correlations, we conclude that the negative $m^{*}$ is irrelevant to the instability towards ferromagnetism or phase separation but rather an intrinsic feature of collective excitations for fermions in the strongly repulsive regime. Surprisingly, for large and negative $m^{*}$, such an excitation is accompanied with a spin-density modulation when the majority fermions move closer to the impurity rather than being repelled far away, contrary to the picture of phase separation. These results shed light on the recent observation of negative $m^{*}$ in the 3D repulsive Fermi polarons.

We study the average intensity-intensity correlations signal at the output of a two-mode squeezing device with $|N\rangle \otimes |\alpha \rangle$ as the two input modes. We show that the input photon number can be resolved from the average intensity-intensity correlations. In particular, we show jumps in the average intensity-intensity correlations signal as a function of input photon number $N$. Therefore, we propose that such a device may be deployed as a photon-number-resolving detector at room temperature with high efficiency.

In a recent article [Phys. Rev. A 94, 052128 (2016)], the authors compute the predictions of two collapse models on the transition probabilities of neutral mesons. Notably, they claim to find an influence on the decay rates and attempt to prove that a new parameter $\theta \left(0\right)$ is required to fully characterize the noise of standard collapse models. These two claims are incorrect and motivated by flawed computations. This Comment derives the correct transition probabilities exactly from the master equation, explains how they could be computed perturbatively in a safe way, and finally shows where the main mistake of the authors of the original article was made.

Quantum-mechanical spin is often thought of in terms of classical angular momentum. In fact spin is defined by its commutation relations and the spin and orbital angular momentum operators are very different. Here we solve the Dirac equation in a rotating frame of reference and create a localized wave packet from the resulting wave functions to examine the consequences of the rotational motion. We highlight some unexpected effects in the properties of the wave packet and show that the spin operator and orbital angular momentum operator describe different aspects of the rotational properties of the wave packet. It is also observed that neither quantum-mechanical spin nor orbital angular momentum can be fully understood within an inertial frame.

Measurement-based quantum computing enables universal quantum computing with only adaptive single-qubit measurements on certain many-qubit states, such as the graph state, the Affleck-Kennedy-Lieb-Tasaki (AKLT) state, and several tensor-network states. Finding new resource states of measurement-based quantum computing is a hard task, since for a given state there are exponentially many possible measurement patterns on the state. In this paper, we consider the problem of deciding, for a given state and a set of unitary operators, whether there exists a way of measurement-based quantum computing on the state that can realize all unitaries in the set, or not. We show that the decision problem is QCMA-hard (where QCMA stands for quantum classical Merlin Arthur), which means that finding new resource states of measurement-based quantum computing is harder than quantum computing itself [unless BQP (bounded-error quantum polynomial time) is equal to QCMA]. We also derive an upper bound of the decision problem: the problem is in a quantum version of the second level of the polynomial hierarchy.

The resource theoretic measure of quantum coherence is basis dependent, and the amount of coherence contained in a state is different in different bases. We obtained analytical solutions for the maximum coherence by optimizing the reference basis and highlighted the essential role of the mutually unbiased bases (MUBs) on attaining the maximum coherence. Apart from the relative entropy of coherence, we showed that the MUBs are optimal for the robustness of coherence, the coherence weight, and the modified skew information measure of coherence for any state. Moreover, the MUBs are optimal for all the faithful coherence measures if the state is pure or is of the single qubit. We also highlighted an upper bound for the $l_1$ norm of coherence and compared it with the other bounds as well as the maximum one attainable by optimizing the reference basis.

We demonstrate a quasioptical cycling for the $X(v=0)\to A(v^{\prime }=0)$ transition and a radiative force-induced deflection on the buffer-gas-cooled BaF molecular beam. The laser-induced fluorescence enhancement with additional sidebands and a polarization modulation scheme indicates that the hyperfine states and the Zeeman sublevels are closed. The quasioptical cycling by repumping the $X(v=1)\to A(v^{\prime }=0)$ leads to an $\sim 0.8-\mathrm{mm}$ deflection of the beam via scattering $\sim 150$ photons per molecule, in good agreement with the predictions from our multilevel rate equation model. Further improvement by closing the leakage $X(v=2)$ and $\mathrm{\Delta }$ state allows scattering of thousands of photons and laser cooling and slowing of BaF.

We theoretically demonstrate that the terahertz-spectrum generation in three-color laser-gas interactions can be effectively manipulated by tuning the laser wavelengths. We show that by positive and negative detuning of one of the color laser wavelengths in the three-color laser pulse, the terahertz spectral range can be greatly expanded, and in particular, the high spectral frequency components emerge. However, the total terahertz energy is maintained at a relatively high level and can even significantly increase. Moreover, we also show that the middle and high spectral frequency components of the terahertz spectrum can be effectively tuned by modulating the relative wavelength of the three-color laser pulses. Finally, our analysis shows that the ionization events play an important role in the intense and shaped terahertz generation.

We propose a physical scheme for the realization of two-dimensional topological odd-parity superfluidity in a spin-independent bond-centered square optical lattice based upon interband fermion pairing. The $D_4$ point-group symmetry of the lattice protects a quadratic band crossing, which allows one to prepare a Fermi surface of spin-up fermions with odd parity close to the degeneracy point. In the presence of spin-down fermions with even parity populating a different energetically well-separated band, odd-parity pairing is favored. Strikingly, as a necessary prerequisite for pairing, both Fermi surfaces can be tuned to match well. As a result, topological superfluid phases emerge in the presence of merely $s$-wave interaction. Due to the $Z_2$ symmetry of these odd-parity superfluids, we infer their topological features simply from the symmetry and the Fermi-surface topology as confirmed numerically.

We theoretically investigate superfluidity in a strongly interacting Fermi gas confined to two dimensions at finite temperature. Using a Gaussian pair fluctuation theory in the superfluid phase, we calculate the superfluid density and determine the critical temperature and chemical potential at the Berezinskii-Kosterlitz-Thouless transition. We propose that the transition can be unambiguously demonstrated in cold-atom experiments by stirring the superfluid Fermi gas using a red-detuned laser beam, to identify the characteristic jump in the local Landau critical velocity at the superfluid-normal interface, as the laser beam moves across the cloud.

We investigate theoretically the ground-state property of a two-dimensional array of superconducting circuits including the on-site superconducting qubits (SQs) with weak anharmonicity. In particular, we analyze the influence of this anharmonicity on the Mott-insulator to superfluid quantum phase transition. The complete ground-state phase diagrams are presented under the mean-field approximation. Interestingly, the anharmonicity of SQs affects the Mott lobes enormously, and the single excitation Mott lobe disappears when the anharmonicity becomes zero. Our results can be used to guide the implementations of quantum simulations using the superconducting circuits, which have nice integrating and flexibility.

Effects of Fano resonances on optical scattering force upon a high-index dielectric nanoparticle radiated by a plane wave are theoretically investigated. The results demonstrate that five different types of Fano resonances appear in the force and are expressed in a unified way. The magnetic (electric) -based cascades of Fano resonances dominate the peaks of the force. The magnetic (electric) -based conventional Fano resonances and the unconventional Fano resonances suppress the peaks of the force for particular large nanoparticles. All types of Fano resonances together cause broad dips of the force and even a near-zero force effect at particular radii. Finally, by investigating the effects of positive and negative indexes, loss, and gain of the particle on the force, it is shown that negative force arises only as the appropriate gain is introduced in the particle. Our results are important for not only deep understanding of electromagnetic interactions in the optical force but also stable optical manipulation of the dielectric nanoparticle.

The encoding of quantum information in photonic time-bin qubits is apt for long-distance quantum communication schemes. In practice, due to technical constraints such as detector response time, or the speed with which copolarized time-bins can be switched, other encodings, e.g., polarization, are often preferred for operations like state detection. Here, we present the conversion of qubits between polarization and time-bin encodings by using a method that is based on an ultrafast optical Kerr shutter and attain efficiencies of 97% and an average fidelity of $0.827\pm 0.003$ with shutter speeds near 1 ps. Our demonstration delineates an essential requirement for the development of hybrid and high-rate optical quantum networks.

We propose an efficient scheme for the generation of tunable optical waveguides based on atomic vapor in an $N$-type configuration. We exploit both control-field-induced transparency and Kerr-field-induced absorption to produce a flexible probe transparency window otherwise not feasible. We employ a suitable spatial profile of control and Kerr beams to create a high contrast refractive index modulation that holds the key to guiding a weak narrow probe beam. Further, we numerically demonstrate that high-contrast tunable waveguide permits the propagation of different modes of probe beam to several Rayleigh lengths without diffraction. This efficient guiding of narrow optical beam may have important applications in large-density image processing and high-resolution imaging.

We investigate terahertz emission from two-color femtosecond-laser-induced microplasmas. Under the strongest focusing conditions, microplasmas are shown to act as point sources for broadband terahertz-to-far-infrared radiation, where the emission bandwidth is determined by the gas pressure. Semi-analytical modeling allows us to identify scaling laws with respect to the plasma dimensions. We find that the optical-to-THz conversion efficiency depends crucially on the focusing conditions. We use this insight to demonstrate by means of Maxwell-consistent three-dimensional simulations that, for only $10\mu \mathrm{J}$ laser pulse energy a conversion efficiency well above ${10}^{-4}$ can be achieved.