We study magnetic solitons, solitary waves of spin polarization (i.e., magnetization), in binary Bose-Einstein condensates in the presence of Rabi coupling. We show that the system exhibits two types of magnetic solitons, called $2\pi$ and $0\pi$ solitons, characterized by a different behavior of the relative phase between the two spin components. $2\pi$ solitons exhibit a $2\pi$ jump of the relative phase, independent of their velocity, the static domain wall explored by Son and Stephanov being an example of such $2\pi$ solitons with vanishing velocity and magnetization. $0\pi$ solitons instead do not exhibit any asymptotic jump in the relative phase. Systematic results are provided for both types of solitons in uniform matter. Numerical calculations in the presence of a one-dimensional harmonic trap reveal that a $2\pi$ soliton evolves in time into a $0\pi$ soliton, and vice versa, oscillating around the center of the trap. Results for the effective mass, the Landau critical velocity, and the role of the transverse confinement are also discussed.

We propose and experimentally implement an efficient procedure based on entanglement swapping to determine the Bell nonlocality measure of Horodecki et al. [Phys. Lett. A 200, 340 (1995)] and the fully entangled fraction of Bennett et al. [Phys. Rev. A 54, 3824 (1996)] of an arbitrary two-qubit polarization-encoded state. The nonlocality measure corresponds to the amount of the violation of the Clauser-Horne-Shimony-Holt (CHSH) optimized over all measurement settings. By using simultaneously two copies of a given state, we measure directly only six parameters. This is an experimental determination of these quantities without quantum state tomography or continuous monitoring of all measurement bases in the usual CHSH inequality tests. We analyze how well the measured degrees of Bell nonlocality and other entanglement witnesses (including the fully entangled fraction and a nonlinear entropic witness) of an arbitrary two-qubit state can estimate its entanglement. In particular, we measure these witnesses and estimate the negativity of various two-qubit Werner states. Our approach could especially be useful for quantum communication protocols based on entanglement swapping.

We combine near-deterministic preparation of a single atom with Raman sideband cooling, to create a push-button mechanism to prepare a single atom in the motional ground state of tightly focused optical tweezers. In the two-dimensional (2D) radial plane, we achieve a large ground-state fidelity for the entire procedure (loading and cooling) of $\sim 0.73$, while the ground-state occupancy is $\sim 0.88$ for realizations with a single atom present. For 1D axial cooling, we attain a ground-state fraction of $\sim 0.52$. The combined 3D cooling provides a ground-state population of $\sim 0.11$. Our Raman sideband cooling variation is indifferent to magnetic field fluctuations, allowing widespread unshielded experimental implementations. Our work provides a pathway towards a range of coherent few-body experiments.

Nondestructive weak measurements (WMs) made on a quantum particle are useful in order to extract information as the particle evolves from a prepared state to a finally detected state. The physical meaning of this information has been open to debate, particularly in view of the apparent discontinuous trajectories of the particle recorded by WMs. In this work we investigate the properties of vanishing weak values for projection operators as well as general observables. We then analyze the implications when inferring the past of a quantum particle. We provide a nonoptical example for which apparent discontinuous trajectories are obtained by WMs. Our approach is compared to previous results.

Bound entanglement, being entangled yet not distillable, is essential to our understanding of the relations between nonlocality and entanglement besides its applications in certain quantum information tasks. Recently, bound entangled states that violate a Bell inequality have been constructed for a two-qutrit system, disproving a conjecture by Peres that bound entanglement is local. Here we construct this kind of nonlocal bound entangled state for all finite dimensions larger than two, making possible their experimental demonstration in most general systems. We propose a Bell inequality, based on a Hardy-type argument for nonlocality, and a steering inequality to identify their nonlocality. We also provide a family of entanglement witnesses to detect their entanglement beyond the Bell inequality and the steering inequality.

Continuous spontaneous localization (CSL) is a model that captures the effects of a class of extensions to quantum theory which are expected to result from quantum gravity and is such that wave-function collapse is a physical process. The rate of such a process could be very much lower than the upper bounds set by searches to date and yet still modify greatly the interpretation of quantum mechanics and solve the quantum measurement problem. Consequently experiments are sought to explore this. We describe an experiment that has the potential to extend sensitivity to CSL by many orders of magnitude. The method is to detect heating of the motion of charged macroscopic objects confined in a Paul trap. We discuss the detection and the chief noise sources. We find that CSL with standard parameters could be observed using a vibration-isolated ion trap of size 1 cm at ultralow pressure with optical interferometric detection.

We provide a very simple case showing that the weak form of the Heisenberg limit can be beaten while the prior information is improved without bias.

The strong subadditivity inequality of von Neumann entropy relates the entropy of subsystems of a tripartite state ${\rho }_{ABC}$ to that of the composite system. Here, we define ${\mathit{T}}^{\left(a\right)}\left({\rho }_{ABC}\right)$ as the extent to which ${\rho }_{ABC}$ fails to satisfy the strong subadditivity inequality $S\left({\rho }_B\right)+S\left({\rho }_C\right)\le S\left({\rho }_{AB}\right)+S\left({\rho }_{AC}\right)$ with equality and investigate its properties. In particular, by introducing auxiliary subsystem $E$, we consider any purification $|{\psi }_{ABCE}\rangle$ of ${\rho }_{ABC}$ and formulate ${\mathit{T}}^{\left(a\right)}\left({\rho }_{ABC}\right)$ as the extent to which the bipartite quantum correlations of ${\rho }_{AB}$ and ${\rho }_{AC}$, measured by entanglement of formation and quantum discord, change under the transformation $B\to BE$ and $C\to CE$. Invariance of quantum correlations of ${\rho }_{AB}$ and ${\rho }_{AC}$ under such transformation is shown to be a necessary and sufficient condition for vanishing ${\mathit{T}}^{\left(a\right)}\left({\rho }_{ABC}\right)$. Our approach allows one to characterize, intuitively, the structure of states for which the strong subadditivity is saturated. Moreover, along with providing a conservation law for quantum correlations of states for which the strong subadditivity inequality is satisfied with equality, we find that such states coincide with those that the Koashi-Winter monogamy relation is saturated.

We propose fast phase gates of single nuclear spins interacting with single electron spins. The gate operation utilizes geometric phase shifts of the electron spin induced by fast and slow rotating fields; the path difference depending on nuclear-spin states enables nuclear phase shifts. The gate time is inversely proportional to the frequency of the slow rotating field. As an example, we use nitrogen-vacancy centers in diamond, and show, in principle, the phase-gate time orders of magnitude to be shorter than previously reported. We also confirmed the robustness of the gate against decoherence and systematic errors.

We analyze the operation of a four-qubit Bacon-Shor code with simultaneous continuous measurement of noncommuting gauge operators. The error syndrome in this case is monitored via time-averaged cross-correlators of the output signals. We find the logical error rate for several models of decoherence, and also find the termination rate for this quantum error detecting code. The code operation is comparable to that based on projective measurements when the collapse time scale due to continuous measurements is an order of magnitude less than the time period between the projective measurements. An advantage of the continuous-measurement implementation is the absence of time dependence in the code operation, with passive continuous monitoring of the error syndrome.

Network centrality has important implications well beyond its role in physical and information transport analysis; as such, various quantum-walk-based algorithms have been proposed for measuring network vertex centrality. In this work, we propose a continuous-time quantum walk algorithm for determining vertex centrality, and show that it generalizes to arbitrary graphs via a statistical analysis of randomly generated scale-free and Erdős-Rényi networks. As a proof of concept, the algorithm is detailed on a four-vertex star graph and physically implemented via linear optics, using spatial and polarization degrees of freedoms of single photons. This paper reports a successful physical demonstration of a quantum centrality algorithm.

The weak-field asymptotic theory (WFAT) and the Born-Oppenheimer approximation (BOA) provide two general approaches to the theory of tunneling ionization of molecules enabling one to treat the internuclear motion quantum mechanically on an equal footing with the electronic motion. The approaches are based on the asymptotic expansions in field strength and electron-to-nuclear mass ratio, respectively. The problem is that they yield quite different formulas for the ionization rate, which causes an ambiguity for applications in strong-field physics. A recent experimental observation of the isotope effect in tunneling ionization of hydrogen molecules [X. Wang et al., Phys. Rev. Lett. 117, 083003 (2016)] motivates us to address this theoretical ambiguity. The WFAT and BOA have different but overlapping regions of applicability in the field strength—nuclear mass plane. We link the approaches analytically, in particular showing that the rate formulas in fact coincide in the overlap region where they both apply, and compare their predictions quantitatively.

We investigate above-threshold ionization (ATI) of homonuclear diatomic molecules by the so-called bicircular field using the improved molecular strong-field approximation. Bicircular field is a two-color laser field having coplanar circularly polarized counter-rotating components of frequencies $r\omega$ and $s\omega$, with $r$ and $s$ integers. Our analysis includes the high-energy part of the corresponding spectra, i.e., high-order ATI (HATI). The obtained molecular (H)ATI spectra are more complicated than the corresponding atomic spectra. We have identified four symmetries which are satisfied in (H)ATI of homonuclear diatomic molecules. Two of these symmetries are general rotational symmetries valid both for direct and rescattered HATI electrons. The remaining two symmetries are reflection symmetries valid only for the direct ATI electrons. Analytical proof of these symmetries is also given. These symmetries are illustrated using numerical examples of HATI spectra of the ${\mathrm{N}}_2$ molecule for various molecular orientations.

We study the superconducting properties of population-imbalanced ultracold Fermi mixtures in one-dimensional optical lattices that can be effectively described by the spin-imbalanced attractive Hubbard model in the presence of a Zeeman magnetic field. We use the mean-field theory approach to obtain the ground-state phase diagrams including some unconventional superconducting phases such as the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase, and the $\eta$ phase (an extremal case of the FFLO phase), both for the case of a fixed chemical potential and for a fixed number of particles. It allows us to determine optimal regimes for the FFLO phase as well as $\eta$-pairing stability. We also investigate the evolution from the weak coupling (BCS-like limit) to the strong coupling limit of tightly bound local pairs (BEC) with increasing attraction, at $T=0$. Finally, the obtained results show that in spite of the occurrence of the Lifshitz transition induced by an external magnetic field, the superconducting state can still exist in the system, at higher magnetic field values.

We present the results of asymptotic and numerical analysis of dissipative Kerr solitons in whispering gallery mode microresonators influenced by higher-order dispersive terms leading to the appearance of a dispersive wave (Cherenkov radiation). Combining direct perturbation method with the method of moments we find expressions for the frequency, strength, spectral width of the dispersive wave, and soliton velocity. Mutual influence of the soliton and dispersive wave was studied. The formation of the dispersive wave leads to a shift of the soliton spectrum maximum from the pump frequency (spectral recoil), while the soliton displaces the dispersive wave spectral peak from the zero dispersion point.

Efficient manipulation of light with sound in subwavelength-sized volumes is important for applications in photonics, phononics, and biophysics, but remains elusive. We theoretically demonstrate the control of light with MHz-range ultrasound in a subwavelength, 300-nm-wide water-filled hole with a 100-nm-radius air bubble. Ultrasound-driven pulsations of the bubble modulate the effective refractive index of the hole aperture, which gives rise to spectral tuning of light transmission through the hole. This control mechanism opens up novel opportunities for tunable acousto-optic and optomechanical metamaterials, and all-optical ultrasound transduction.

In the article by Ma et al. [Phys. Rev. A 94, 052709 (2016)], $\gamma$-ray spectra for positron annihilation on molecules were calculated in the independent-particle approximation with the positron wave function set to unity. Based on comparisons with experimental data, they concluded that inner valence electrons play a dominant role in positron annihilation. These conclusions are incorrect and resulted from fallacious analysis that ignored the known effect of the positron wave function on the spectra.

In reply to the Comment of Green et al. [Phys. Rev. A 95, 036701 (2017).] on our paper [Phys. Rev. A 94, 052709 (2016)], we reconfirm that all the conclusions are based on the observation and the comparisons of the theoretical and experimental data. One criticism of Green et al. [Phys. Rev. A 95, 036701 (2017).] concerns the positrophilic electrons and the inner valence electrons. The inner valence electrons or positrophilic electrons show most agreeable widths with the corresponding experimental measurements due to their narrowest momentum distributions for all 59 molecules. However, we agree with the criticism of Green et al. [Phys. Rev. A 95, 036701 (2017).] and reconfirm that this agreement does not represent the dominance of the inner valence in the annihilation process. In this Reply, we will clarify the difference between agreement and dominance and illustrate with some figures. Another criticism is about the approximation used in our paper. We emphasize that the averaged discrepancy of 34.2% for these molecules of the theoretical $\gamma$-ray spectra from the experimental measurements is due to the neglect of the positron-electron correlations, vibrational couplings, virtual-state formation, even tunneling of core electrons not the neglect of the positron wave function. In this Reply, we will show, even in this zero-order approximation, these positron-induced effects in the electron-positron annihilation process of molecules can also be analyzed with more corrections and explanations.

We propose a versatile scheme to slow supersonically cooled molecules using a decelerating potential well, obtained by steering a focusing laser beam onto a pair of spinning reflective mirrors under a high-speed brake. The longitudinal motion of molecules in the moving red-detuned light field is analyzed and their corresponding phase-space stability is investigated. Trajectories of $\mathrm{C}{\mathrm{H}}_4$ molecules under the influence of the potential well are simulated using the Monte Carlo method. For instance, with a laser beam of power 20 kW focused onto a spot of waist radius 40–100 μm, corresponding to a peak laser intensity on the order of $\sim {10}^8\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$, a $\mathrm{C}{\mathrm{H}}_4$ molecule of $\sim 250\mathrm{m}/\mathrm{s}$ can be decelerated to $\sim 10\mathrm{m}/\mathrm{s}$ over a distance of a few centimeters on a time scale of hundreds of microseconds.