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We study the problem of vortex nucleation in rotating two-dimensional Bose–Einstein condensate confined in a harmonic trap. We show that, within the Gross–Pitaevskii theory with the boundary condition of vanishing of the order parameter at infinity, topological defects nucleation occurs via the creation of vortex-antivortex pairs far from the cloud center, where the modulus of the order parameter is small. Then vortices move toward the center of the cloud and antivortices move in the opposite direction but never disappear. We also discuss the role of surface modes. [ABSTRACT FROM AUTHOR]

An investigation is made into the accuracy of the theoretical description of the field dependence of the equilibrium magnetization M(H) of type-II superconductors with Ginzburg-Landau parameters κ>1 in various approximations for magnetic fields H much less than the upper critical field H[sub c2]. It is found that taking the spatial distribution of the order parameter into account has a substantial effect on the behavior of the magnetization M(H) even at the very lowest fields in the close vicinity of the lower critical field H[sub c1]. The mechanisms through which and the degree to which the field dependence M(H) is influenced by such factors as the self-energy of the vortex, the accuracy of description of the spatial distribution of the vortex field, the suppression of the order parameter due to the overlapping of the vortices, and the use of the principle of superposition of fields are investigated for fields in the interval H[sub c1]≼H«H[sub c2]. In this field interval an analytical expression for M(H) is obtained by direct evaluation of the lattice sums with allowance for the influence of the structure of the vortex cores. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The vibrational contribution μ[sub v](H) to the dynamic magnetic permeability μ[sub ac][sup ′] is calculated with allowance for the spatial variation of the order parameter in the magnetic field region H«H[sub c2], where the properties of type-II superconductors are usually described in the London approximation. The features of the behavior of the analytical relations μ[sub v](H) are analyzed for the case when the vortices execute small oscillations about their equilibrium positions corresponding to the thermodynamic-equilibrium vortex lattice. It is shown that taking into account the suppression of the order parameter in the vortex cores leads to substantial differences in the behavior of μ[sub v](H) in comparison with the analogous dependence obtained in the London approximation. It is found that the main factor responsible for these differences is a change in shape of the effective potential well in which the vibrational motion of the subsurface vortices takes place. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We point out that realization of quantum communication protocols in programmable quantum computers provides a deep benchmark for capabilities of real quantum hardware. Particularly, it is prospective to focus on measurements of entropy-based characteristics of the performance and to explore whether a "quantum regime" is preserved. We perform proof-of-principle implementations of superdense coding and quantum key distribution BB84 using 5- and 16-qubit superconducting quantum processors of IBM Quantum Experience. We focus on the ability of these quantum machines to provide an efficient transfer of information between distant parts of the processors by placing Alice and Bob at different qubits of the devices. We also examine the ability of quantum devices to serve as quantum memory and to store entangled states used in quantum communication. Another issue we address is an error mitigation. Although it is at odds with benchmarking, this problem is nevertheless of importance in a general context of quantum computation with noisy quantum devices. We perform such a mitigation and noticeably improve some results. [ABSTRACT FROM AUTHOR]

The influence of individual impurities of Fe on the electronic properties of topological insulator Bi2Se3 is studied by Scanning Tunneling Microscopy. The microscope tip is used in order to remotely charge/discharge Fe impurities. The charging process is shown to depend on the impurity location in the crystallographic unit cell, on the presence of other Fe impurities in the close vicinity, and on the overall doping level of the crystal. We present a qualitative explanation of the observed phenomena in terms of tip-induced local band bending. Our observations evidence that the specific impurity neighborhood and the position of the Fermi energy with respect to both the Dirac point and bulk bands have to be taken into account when considering the electron scattering on the disorder in topological insulators. [ABSTRACT FROM AUTHOR]

We consider a hybrid digital-analog quantum computing approach, which allows implementing any quantum algorithm without standard two-qubit gates. This approach is based on the always-on interaction between qubits, which can provide an alternative to such gates. We show how digital-analog approach can be applied to simulate the dynamics of fermionic systems, in particular, the Fermi-Hubbard model, using fermionic SWAP network and refocusing technique. We concentrate on the effects of connectivity topology, the spread of interaction constants as well as on errors of entangling operations. We find that an optimal connectivity topology of qubits for the digital-analog simulation of fermionic systems of arbitrary dimensionality is a chain for spinless fermions and a ladder for spin 1/2 particles. Such a simple connectivity topology makes digital-analog approach attractive for the simulation of quantum materials and molecules., (© 2022 IOP Publishing Ltd.)