Josephson Effects in Three-Band Superconductors with Broken Time-Reversal Symmetry

In superconductors with three or more bands, time-reversal symmetry (TRS) may be broken in the presence of repulsive interband couplings, resulting in a pair of degenerate states characterized by opposite chiralities. We consider a Josephson junction between a three-band superconductor with broken TRS and a single-band superconductor. Phenomena such as asymmetric critical currents, subharmonic Shapiro steps and symmetric Fraunhhofer patterns are revealed theoretically. Existing experimental results are discussed in terms of the present work.Comment: 7 pages, 4 figures, Appl. Phys. Lett., in pres

Fractional Flux Plateau in Magnetization Curve of Multicomponent Superconductor Loop

Time-reversal symmetry (TRS) may be broken in superconductors with three or more condensates interacting repulsively, yielding two degenerate states specified by chirality of gap functions. We consider a loop of such superconductor with two halves occupied by the two states with opposite chiralities. Fractional flux plateaus are found in magnetization curve associated with free-energy minima, where the two domain walls between the two halves accommodate different inter-component phase kinks leading to finite winding numbers in a part of the whole condensates around the loop. Fractional flux plateaus form pairs with their heights related to the flux quantum {\Phi}0 = hc/2e. This phenomenon is a clear evidence of time-reversal symmetry broken (TRSB) superconductivity, which in a general point of view provides a novel chance to explore relative phase difference, phase kink and soliton in ubiquitous multi-component superconductivity such as that in iron pnicitides.Comment: 8 pages, 7 figure

Vortices with Fractional Flux Quanta in Multi-Band Superconductors

In superconductors with three or more components, time-reversal symmetry may be broken when the inter-component couplings are repulsive, leading to a superconducting state with two-fold degeneracy. When prepared carefully there is a stable domain wall on a constriction which connects two bulks in states with opposite chiralities. Applying on external magnetic field, vortices in different components dissociate with each other, resulting in a ribbon shape distribution of magnetic field at the position of domain wall.Comment: 4 pages, 4 figures, to appear on Journal of Superconductivity and Novel Magnetis

Landau-Zener-St\"uckelberg Interferometry for Majorana Qubit

Stimulated by a very recent experiment observing successfully two superconducting states with even- and odd-number of electrons in a nanowire topological superconductor as expected from the existence of two end Majorana quasiparticles (MQs) [Albrecht \textit{et al.}, Nature \textbf{531}, 206 (2016)], we propose a way to manipulate Majorana qubit exploiting quantum tunneling effects. The prototype setup consists of two one-dimensional (1D) topological superconductors coupled by a tunneling junction which can be controlled by gate voltage. We show that, upon current injection, the time evolution of superconducting phase difference at the junction induces an oscillation in energy levels of the Majorana parity states, whereas the level-crossing is avoided by a small coupling energy of MQs in the individual 1D superconductors. This results in a Landau-Zener-St\"{u}ckelberg (LZS) interference between the Majorana parity states. Adjusting the current pulse and gate voltage, one can build a LZS interferometry which provides an arbitrary manipulation of the Majorana qubit. The LZS rotation of Majorana qubit can be monitored by the microwave radiated from the junction

CHAOS: an SDN-based Moving Target Defense System

The static nature of current cyber systems has made them easy to be attacked and compromised. By constantly changing a system, Moving Target Defense (MTD) has provided a promising way to reduce or move the attack surface that is available for exploitation by an adversary. However, the current network- based MTD obfuscates networks indiscriminately that makes some networks key services, such as web and DNS services, unavailable, because many information of these services has to be opened to the outside and remain real without compromising their usability. Moreover, the indiscriminate obfuscation also severely reduces the performance of networks. In this paper, we propose CHAOS, an SDN (Software-defined networking)-based MTD system, which discriminately obfuscates hosts with different security levels in a network. In CHAOS, we introduce a Chaos Tower Obfuscation (CTO) method, which uses a Chaos Tower Structure (CTS) to depict the hierarchy of all the hosts in an intranet and provides a more unpredictable and flexible obfuscation method. We also present the design of CHAOS, which leverages SDN features to obfuscate the attack surface including IP obfuscation, ports obfuscation, and fingerprint obfuscation thereby enhancing the unpredictability of the networking environment. We develop fast CTO algorithms to achieve a different degree of obfuscation for the hosts in each layer. Our experimental results show that a network protected by CHAOS is capable of decreasing the percentage of information disclosure effectively to guarantee the normal flow of traffic

Quantum Magnetism in Wannier-Obstructed Mott Insulators

We develop a strong coupling approach towards quantum magnetism in Mott insulators for Wannier obstructed bands. Despite the lack of Wannier orbitals, electrons can still singly occupy a set of exponentially-localized but nonorthogonal orbitals to minimize the repulsive interaction energy. We develop a systematic method to establish an effective spin model from the electron Hamiltonian using a diagrammatic approach. The nonorthogonality of the Mott basis gives rise to multiple new channels of spin-exchange (or permutation) interactions beyond Hartree-Fock and superexchange terms. We apply this approach to a Kagome lattice model of interacting electrons in Wannier obstructed bands (including both Chern bands and fragile topological bands). Due to the orbital nonorthogonality, as parameterized by the nearest neighbor orbital overlap $g$, this model exhibits stable ferromagnetism up to a finite bandwidth $W\sim U g$, where $U$ is the interaction strength. This provides an explanation for the experimentally observed robust ferromagnetism in Wannier obstructed bands. The effective spin model constructed through our approach also opens up the possibility for frustrated quantum magnetism around the ferromagnet-antiferromagnet crossover in Wannier obstructed bands.Comment: 21 pages, 11 figure

Spintronics at Nanoscale: Flat-Band Ferromagnetism in Armchair Nanoribbons and Nanotubes

We study the electronic correlation effects in armchair nanoribbon and nanotube using weak-coupling approach and non-Abelian density-matrix renormalization-group method. We show that upon appropriate doping, the system exhibits a new type of flat-band ferromagnetism, different from the well-known Milke-Tasaki one. The strongly correlated ground state consists of intrinsic magnetic moments of flat-band states and itinerant carriers of dispersive bands, and the exchange coupling between them yields a ferromagnetism. The resultant ferromagnetic state with metallic conductivity has a potential in spintronics applications at nanoscale.Comment: 5 pages, 4 figures, introduction part rewritte

Entire Space Multi-Task Model: An Effective Approach for Estimating Post-Click Conversion Rate

Estimating post-click conversion rate (CVR) accurately is crucial for ranking systems in industrial applications such as recommendation and advertising. Conventional CVR modeling applies popular deep learning methods and achieves state-of-the-art performance. However it encounters several task-specific problems in practice, making CVR modeling challenging. For example, conventional CVR models are trained with samples of clicked impressions while utilized to make inference on the entire space with samples of all impressions. This causes a sample selection bias problem. Besides, there exists an extreme data sparsity problem, making the model fitting rather difficult. In this paper, we model CVR in a brand-new perspective by making good use of sequential pattern of user actions, i.e., impression -> click -> conversion. The proposed Entire Space Multi-task Model (ESMM) can eliminate the two problems simultaneously by i) modeling CVR directly over the entire space, ii) employing a feature representation transfer learning strategy. Experiments on dataset gathered from Taobao's recommender system demonstrate that ESMM significantly outperforms competitive methods. We also release a sampling version of this dataset to enable future research. To the best of our knowledge, this is the first public dataset which contains samples with sequential dependence of click and conversion labels for CVR modeling.Comment: accept by SIGIR-201

Unraveling of a generalized quantum Markovian master equation and its application in feedback control of a charge qubit

In the context of a charge qubit under continuous monitoring by a single electron transistor, we propose an unraveling of the generalized quantum Markovian master equation into an ensemble of individual quantum trajectories for stochastic point process. A suboptimal feedback algorism is implemented into individual quantum trajectories to protect a desired pure state. Coherent oscillations of the charge qubit could be maintained in principle for an arbitrarily long time in case of sufficient feedback strength. The effectiveness of the feedback control is also reflected in the detector's noise spectrum. The signal-to-noise ratio rises significantly with increasing feedback strength such that it could even exceed the Korotkov-Averin bound in quantum measurement, manifesting almost ideal quantum coherent oscillations of the qubit. The proposed unraveling and feedback protocol may open up the prospect to sustain ideal coherent oscillations of a charge qubit in quantum computation algorithms.Comment: 10 pages, 5 figre

Experimental Ultra-small Longitudinal Phase Estimation via Weak Measurement Amplification

Weak value amplification has been applied to various small physical quantities estimation, however there still lacks a practical feasible protocol to amplify ultra-small longitudinal phase, which is of importance in high precision measurement. Very recently, a different amplification protocol within the framework of weak measurements is proposed to solve this problem, which is capable of measuring any ultra-small longitudinal phase signal that conventional interferometry tries to do. Here we experimentally demonstrate this weak measurements amplification protocol of ultra-small longitudinal phase and realize one order of magnitude amplification in the same technical condition, which verifies the validity of the protocol and show higher precision and sensitivity than conventional interferometry. Our results significantly broaden the area of applications of weak measurements and may play an important role in high precision measurements