Recent breakthrough experiments on dipolar condensates have reported the creation of supersolids, including two-dimensional arrays of quantum droplets. Droplet arrays are, however, not the only possible nontrivial density arrangement resulting from the interplay of mean-field instability and quantum stabilization. Several other possible density patterns may occur in trapped condensates at higher densities, including the so-called honeycomb supersolid, a phase that exists, as it is also the case of a triangular droplet supersolid, in the thermodynamic limit. We show that compared to droplet supersolids, honeycomb supersolids have a much-enhanced superfluid fraction while keeping a large density contrast, and constitute in this sense a much better dipolar supersolid. However, in contrast to droplet supersolids, quantized vortices cannot be created in a honeycomb supersolid without driving a transition into a so-called labyrinthic phase. We show that the reduced moment of inertia, and with it the superfluid fraction, can be however reliably probed by studying the dynamics following a scissorslike perturbation.

The zero energy of a one-dimensional semi-infinite non-Hermitian lattice with nontrivial spectral topology may disappear when we introduce boundaries to the system. While the corresponding zero-energy state can be considered as a quasi-edge state for the finite lattice with a long survival time, any small disruption (noise) in the initial form of the quasi-edge state can significantly shorten the survival time. Here, by tailoring the couplings at one edge we form an exceptional point allowing for a topological phase transition and the stabilization of the quasi-edge state in a finite-size lattice with open edges. Such a small modification in the lattice does not require closing and opening of the band gap and opens the door for experimental realization of such robust zero-energy edge states.

Controlling light scattering by nanoparticles is fundamentally important for the understanding and the control of light with photonic nanostructures, as well as for nanoparticle scattering itself, including Mie scattering. Here, we theoretically and numerically investigate the possibility to manipulate nanoparticle scattering by wave-front shaping that was initially developed to control light scattered by large numbers of nanoparticles in nanophotonic media. By employing a scattering matrix analysis, we find that even a single nanoparticle supports multiple strongly scattering eigenchannels, suggesting wave-front shaping as a promising tool to manipulate scattered light of a single nanoparticle. By sending in shaped wave fronts, we selectively excite eigenchannels, as is apparent from the distinct field distributions. These scattering eigenchannels are related to different resonant leaky modes of the scatterer, that reveal remarkable localized “hot spots” where the field is substantially enhanced. Moreover, we investigate the backscattered spectra; we send in wave fronts relevant for a particular eigenchannel, and observe that the backscattered spectrum reveals not only the excited channel but also several others. This result points to the existence of short- and long-range spectral correlations for an eigenchannel. Our paper offers a flexible tool to manipulate light scattering of a single nanoparticle, and thus opens possibilities to control field patterns and light-matter interactions in a nanoparticle, as well as to explore new features of nanoparticle scattering such as the spectral correlation and temporal response of light scattered by nanoscatterers, including Mie spheres.

We investigate the multimode Jaynes-Cummings model. The population of the excited state of an atom is expressed by an integrodifferential equation. The exact solution is found in an idealized case by taking the infinite limit of the number of modes. The solution shows a mathematical reason why the kinklike behavior of the excited-state population appears. We also confirm that numerical calculations agree with the exact solution.

Quantifying the optical activity of a sample requires the precise measurement of the rotation of the plane of linear polarization of the transmitted light. Central to this notion is that the sample needs to be exposed to light of a defined polarization state. We show that by using a polarization-entangled photon source we can measure optical activity whilst illuminating a sample with unpolarized light. This not only allows for low light measurement of optical activity but also allows for the analysis of samples that would otherwise be perturbed if subjected to polarized light.

When collecting spectroscopic data on at least four isotopes, nonlinearities in the King plot are a possible sign of physics beyond the standard model. In this work, an improved approach to the search for hypothetical new interactions with isotope-shift spectroscopy of few-electron ions is presented. Careful account is taken of the small nuclear corrections to the energy levels and the gyromagnetic factors, which cause deviations from King linearity within the standard model and are hence a possible source of confusion. In this approach, the experimental King nonlinearity is not compared to the vanishing prediction of the standard model at the leading order, but to the calculated full standard model contribution to King nonlinearity. This makes searching for beyond-the-standard-model physics with King linearity analysis possible in a high-precision experimental regime, avoiding confusion. The bounds which can be set on beyond-the-standard-model parameters remain limited by the uncertainties on the small standard model nuclear corrections which cause King nonlinearity. Direct comparison between theory and experiment on a single pair of isotopes is advocated as a more suitable approach for few-electron ions.

In natural processes involving weak interactions, a violation of spatial parity conservation should appear. Although the parity-violation effects are expected to be observable in molecular systems, their tiny magnitude has prevented their detection to date. We present a theoretical analysis of four-component relativistic nuclear-spin-dependent parity-violating nuclear spin-rotation and NMR shielding tensors in a set of tetrahedral chiral molecules. This work emphasizes the significant contribution of the ligands and the electronic structure of the chiral center in enhancing these effects, paving the way for the targeted design of promising molecules for measurements.

We present a device-independent analysis of controlled quantum teleportation where the receiver is not trusted. We show that the notion of genuine tripartite nonlocality allows us to certify control power in such a scenario. By considering a specific adversarial attack strategy on a device characterized by depolarizing noise, we find that control power is a monotonically increasing function of genuine tripartite nonlocality. These results are relevant for building practical quantum communication networks and also shed light on the role of nonlocality in multipartite quantum information processing.

Interconversions of $W$ and Greenberger-Horne-Zeilinger states in various physical systems have recently been attracting considerable attention. We address this problem in the fairly general physical setting of qubit arrays with long-ranged (all-to-all) Ising-type qubit-qubit interaction, which are simultaneously acted upon by transverse Zeeman-type global control fields. Motivated in part by a recent Lie-algebraic result that implies state-to-state controllability of such a system for an arbitrary pair of states that are invariant with respect to qubit permutations, we present a detailed investigation of the state-interconversion problem in the three-qubit case. The envisioned interconversion protocol has the form of a pulse sequence that consists of two instantaneous ($\delta$-shaped) control pulses, each of them corresponding to a global qubit rotation, and an Ising-interaction pulse of finite duration between them. Its construction relies heavily on the use of the (four-dimensional) permutation-invariant subspace (symmetric sector) of the three-qubit Hilbert space. In order to demonstrate the viability of the proposed state-interconversion scheme, we provide a detailed analysis of the robustness of the underlying pulse sequence to systematic errors, i.e., deviations from the optimal values of its five characteristic parameters.

Two-component dipolar condensates are now experimentally producible, and we theoretically investigate the nature of supersolidity in this system. We predict the existence of a binary supersolid state in which the two components form a series of alternating domains, producing an immiscible double supersolid. Remarkably, we find that a dipolar component can even induce supersolidity in a nondipolar component. In stark contrast to single-component dipolar supersolids, alternating-domain supersolids do not require quantum stabilization, and the number of crystal sites is not strictly limited by the condensate populations, with the density hence being substantially lower. Our results are applicable to a wide range of dipole moment combinations, marking an important step towards long-lived bulk supersolidity.

We present a study concerning the dynamics of dense and harmonic coherent regimes in quantum cascade lasers (QCLs) in a Fabry–Perot configuration emitting in the terahertz (THz) spectral region. Firstly, we study the device in free-running operation, reproducing the main features of the self-generated optical frequency combs and harmonic frequency combs in this spectral range, commenting on the points of difference from the midinfrared region and finding excellent agreement with the most recent experimental evidence. Then, we analyze the THz-QCL dynamics under radio-frequency injection, with a focus on the effect of the modulation of the current on the degree of locking of the system, and we perform a systematic investigation aimed to provide a procedure for the generation of a train of pulses with short duration and high contrast. Furthermore, we extend our study to the generation of sequences of pulses with repetition frequency, which is a multiple of the free-spectral range (FSR) of the laser cavity, reproducing harmonic mode locking of the laser, a method which has been recently demonstrated in experiments with THz QCLs, and the results of which are particulary promising for a large stream of applications ranging from optical communication to imaging.

Multipass cells (MPCs) recently emerged as a new femtosecond pulse compression technique for lasers with high pulse energy. While most of the work focused on the compression of near-infrared laser systems so far, we address the case of holmium-based gain materials, which offer an enormous potential in terms of pulse energy, yet can only host a very narrow bandwidth in the 2-$\mu \text{m}$ wavelength range. Compression into the few-cycle regime, therefore, requires at least a hundredfold spectral broadening. Specifically, we investigate the utility of four different solid-state materials in the MPC, namely, fused silica, sapphire, YAG, and diamond. Spectral broadening dynamics is numerically investigated using the $2D+1$-unidirectional pulse propagation equation. To this end, we put particular emphasis on the thermal properties of the nonlinear optical material, which turn out as a critical issue above 2-$\mu \text{m}$ wavelength. Solving the heat equation for each of the materials, we then estimate the maximum temperature at beam center. These considerations show that outside the near infrared, thermodynamic parameters of nonlinear optical materials become at least equally important as their optical properties. Among the four materials under test, in particular, diamond stands out as it combines highly favorable thermal properties with a large optical nonlinearity. Finally, a concomitant slow degradation of spatial coherence is monitored. Our findings provide an effective guideline for the design of high pulse-energy compressors at wavelengths of $2\mu \text{m}$ and beyond.

We investigate the localization behavior of the non-Hermitian tightly bound state in the presence of asymmetric tunneling and external uniform fields in various dimensions, where the discrete translational symmetry and the inversion symmetry are broken. We demonstrate that the non-Hermitian skin effects can still exist and compete with Wannier-Stark localization induced by the uniform fields. The total localization is not only related to the size of the system, but the net tunneling strength as well. For the two-dimensional lattices with square shape boundaries, the uniform fields can pull back the population of the states at the corner and the fading stripe localization can be observed as a compromise between the two-dimensional non-Hermitian skin effects and the uniform fields. The experimental simulation of the non-Hermitian chain with uniform fields can be achieved by the methods of electrical circuits.

We derive a Markovian master equation for driven open quantum systems based on the Lewis-Riesenfeld-invariants theory, which is available for arbitrary driving protocols. The role of the Lewis-Riesenfeld invariants is to help us bypass the time-ordering obstacle in expanding the propagator of the free dynamics, such that the Lindblad operators in our driven Markovian master equation can be determined easily. We also illustrate that, for the driven open quantum systems, the spontaneous emission and the thermal excitation induce the transitions between eigenstates of the Lewis-Riesenfeld invariant, but not the system Hamiltonian's. As an example, we present the driven Markovian master equation for a driven two-level system coupled to a heat reservoir. By comparing to the exactly solvable models, the availability of the driven Markovian master equation is verified. Meanwhile, the adiabatic limit and inertial limit of the driven Markovian master equation are also discussed, which result in the same Markovian master equations as those presented before in the corresponding limits.

In quantum metrology, the parameter estimation accuracy is bounded by quantum Fisher information. In this paper we present coherence measures in terms of (quantum) Fisher information by directly considering the postselective nonunitary parametrization process. This coherence measure demonstrates the apparent operational meaning by the exact connection between coherence and parameter estimation accuracy. We also discuss the distinction between our coherence measure and the quantum Fisher information subject to unitary parametrization. The analytic coherence measure is given for qubit states.

Noisy intermediate-scale quantum computers are useful for various tasks such as state preparation and variational quantum algorithms. However, the non-Euclidean quantum geometry of parameterized quantum circuits is detrimental for these applications. Here we introduce the natural parameterized quantum circuit (NPQC) that can be initialized with a Euclidean quantum geometry. The initial training of variational quantum algorithms is substantially sped up as the gradient is equivalent to the quantum natural gradient. Further, we show how to estimate the parameters of the NPQC by sampling the circuit, which could be used for benchmarking or calibrating NISQ hardware. For a general class of quantum circuits, the NPQC has the minimal quantum Cramér-Rao bound, which highlights its potential for quantum metrology. Finally, we show how to generate arbitrary superpositions of two states with the NPQCs for state preparation tasks. Our results can be used to enhance currently available quantum processors.

We initiate a systematic study to provide upper bounds on device-independent keys, secure against a nonsignaling adversary (NSDI). We employ the idea of “squashing” on the secrecy monotones and show that squashed secrecy monotones are the upper bounds on the NSDI key. Our technique for obtaining upper bounds is based on the nonsignaling analog of quantum purification: the complete extension. As an important instance of an upper bound, we construct a measure of nonlocality called “squashed nonlocality.” Using this bound, we identify numerically a certain domain of two binary inputs and two binary outputs of nonlocal devices for which the squashed nonlocality is zero. Therefore one can not distill a secure key from these nonlocal devices via a considered (standard) class of operations. Showing a connection of our approach to the one in New J. Phys. 8, 126 (2006), we provide, to our knowledge, the tightest known upper bound in the (3,2,2,2) scenario. Moreover, we formulate a security condition, equivalent to known ones, for the considered class of protocols. To achieve this, we introduce a nonsignaling norm that constitutes an analogy to the trace norm used in the security condition of the quantum key distribution.