Double-dot interferometer for quantum measurement of Majorana qubits and stabilizers

Motivated by the need of quantum measurement of Majorana qubits and surface-code stabilizers, we analyze the performance of a double-dot interferometer under the influence of environment noise. The double-dot setup design allows accounting for the full multiple tunneling process between the dots through the Majorana island, within a master equation approach. In the co-tunneling regime, which results in a Majorana-mediated effective coupling between the dots, the master equation approach allows us to obtain analytic solutions for the measurement currents. The measurement quality,characterized by figures of merit such as the visibility of measurement signals, is carried out in regard to the unusual decoherence effect rather than ‘which-path’ dephasing. The results obtained in this work are expected to be useful for future experiments of Majorana qubit and stabilizer measurements.

Band structures of strained kagome lattices

Materials with kagome lattices have attracted significant research attention due to their nontrivial features in energy bands.We theoretically investigate the evolution of electronic band structures of kagome lattices in response to uniaxial strain using both a tight-binding model and an antidot model based on a periodic muffin-tin potential.It is found that the Dirac points move with applied strain.Furthermore,the flat band of unstrained kagome lattices is found to develop into a highly anisotropic shape under a stretching strain along y direction,forming a partially flat band with a region dispersionless along ky direction while dispersive along kx direction.Our results shed light on the possibility of engineering the electronic band structures of kagome materials by mechanical strain.

Adaptive semi-empirical model for non-contact atomic force microscopy

Non-contact atomic force microscope is a powerful tool to investigate the surface topography with atomic resolution.Here we propose a new approach to estimate the interaction between its tips and samples,which combines a semi-empirical model with density functional theory(DFT)calculations.The generated frequency shift images are consistent with the experiment for mapping organic molecules using CuCO,Cu,CuCl,and CuO_(x)tips.This approach achieves accuracy close to DFT calculation with much lower computational cost.

Analytical formula describing the non-saturating linear magnetoresistance in inhomogeneous conductors

The effective-medium theory(EMT)has proved successful in modeling the non-saturating linear magnetoresistance induced by inhomogeneity.However,calculating magnetoresistance using the EMT usually involves solving coupled integral equations which have no analytical solutions,and therefore,it is still difficult to directly compare the predictions of EMT with experimental data.Here we demonstrate that the linear magnetoresistance predicted by the EMT can be either exactly formulated or well approximated by a simple analytical equationΔ_(ρ)/ρ_(0)=√k^(2)B^(2)+a^(2)-a in a number of known situations.The relations between the EMT parameters and the phenomenological parameters k and a are evaluated.Our results provide a convenient and effective method for extracting the EMT parameters from experimental data.

Collectively induced transparency and absorption in waveguide quantum electrodynamics with Bragg atom arrays

Collective quantum states, such as subradiant and superradiant states, are useful for controlling optical responses in many-body quantum systems. In this work, we study novel collective quantum phenomena in waveguide-coupled Bragg atom arrays with inhomogeneous frequencies.For atoms without free-space dissipation, collectively induced transparency is produced by destructive quantum interference between subradiant and superradiant states. In a large Bragg atom array, multi-frequency photon transparency can be obtained by considering atoms with different frequencies. Interestingly, we find collectively induced absorption(CIA) by studying the influence of free-space dissipation on photon transport. Tunable atomic frequencies nontrivially modify decay rates of subradiant states. When the decay rate of a subradiant state equals to the free-space dissipation, photon absorption can reach a limit at a certain frequency. In other words, photon absorption is enhanced with low free-space dissipation, distinct from previous photon detection schemes. We also show multi-frequency CIA by properly adjusting atomic frequencies. Our work presents a way to manipulate collective quantum states and exotic optical properties in waveguide quantum electrodynamics(QED) systems.

Improved Reall-Santos method for AdS black holes in general 4-derivative gravities

For asymptotically flat black holes,Reall-Santos method is a convenient tool to compute leading higher derivative corrections to the thermodynamic quantities without actually solving the modified field equations.However,there are subtleties in its generalization to asymptotically AdS black holes with general higher derivative corrections.First of all,it is necessary to know all the higher derivative holographic counterterms and the surface terms implementing the variational principle and subtracting the divergence.One then needs to solve for the modified AdS radius and rescale the time coordinate in an appropriate way such that the induced metric on the conformal boundary of AdS black hole is not modified.We observe that Reall-Santos method can be directly applied to a particular 4-derivative gravity model,known as the Einstein-Weyl gravity,which does not modify the AdS radius and requires only the Gibbons-Hawking-York term and holographic counterterms for the 2-derivative theory.We thus suggest that to compute the thermodynamic quantities of AdS black holes in general 4-derivative theories of gravity,one simply needs to transform it to a Einstein-Weyl gravity with identical thermodynamic variables by appropriate field redefinitions.We explicitly verify this proposal with spherically-symmetric and static charged black holes in Einstein-Maxwell theory extended with generic 4-derivative interactions.

Selective linear etching of monolayer black phosphorus using electron beams

Point and line defects are of vital importance to the physical and chemical properties of certain two-dimensional(2D)materials.Although electron beams have been demonstrated to be capable of creating single-and multi-atom defects in 2D materials,the products are often random and difficult to predict without theoretical inputs.In this study,the thermal motion of atoms and electron incident angle were additionally considered to study the vacancy evolution in a black phosphorus(BP)monolayer by using an improved first-principles molecular dynamics method.The P atoms in monolayer BP tend to be struck away one by one under an electron beam within the displacement threshold energy range of 8.55-8.79 eV,which ultimately induces the formation of a zigzag-like chain vacancy.The chain vacancy is a thermodynamically metastable state and is difficult to obtain by conventional synthesis methods because the vacancy formation energy of 0.79 eV/edge atom is higher than the typical energy in monolayer BP.Covalent-like quasi-bonds and a charge density wave are formed along the chain vacancy,exhibiting rich electronic properties.This work proposes a theoretical protocol for simulating a complete elastic collision process of electron beams with 2D layers and will facilitate the establishment of detailed theoretical guidelines for experiments on 2D material etching using focused high-energy electron beams.

Cross correlation mediated by distant Majorana zero modes with no overlap

Existing studies via shot noise calculation conclude that the cross correlation between the currents in the two leads connected by a pair of Majorana zero modes(MZMs)vanishes when their coupling energy𝜖∈_(M)→0.Motivated by the intrinsic nature of nonlocality of the MZMs,we revisit this important problem and propose an experimental scheme to demonstrate the nonvanishing cross correlation even at the limit𝜖∈_(M)→0.The proposed scheme employs the Andreevprocess-associated branch circuit currents,which are theoretically obtained by applying a decomposition analysis for the total currents while are accessible directly in practical measurement.For different bias voltage setup,we find intriguing results of both negative and positive correlations and carry out simple physical understanding using a quantum jump technique.Importantly,combining together with the evidence of the zero-bias-peak of conductance,the nonlocal cross correlation predicted in this work can help to confirm the existence of the nonlocal MZMs.

Influence of Thermal Radiation on Hot Cluster Decay Rates and Abundances

The influence of radiative cooling on the unimolecular decay rates of free, hot clusters and molecules with unspecified excitation energies is quantified. Two different regimes, dedined by the magnitude of the energy of the photons emitted, are identified and the boundary between them is given. The boundary is determined in terms of the photon emission rate constants and thermal properties of the particles. Also the abundance spectra are calculated for the continuous cooling case, corresponding to small photon energies. The two regimes correspond to continuous cooling and single photon quenching of the unimolecular decay. The radiative effect can be parametrized by a redefinition of the time each individual cluster has available to undergo evaporation, expressed by an effective radiative time constant.

Quasi-One-Dimensional Free-Electron-Like States Selected by Intermolecular Hydrogen Bonds at the Glycine/Cu(100) Interface

We carry out ab initio density functional theory calculations to study manipulation of electronic structures of selfassembled molecular nanostructures on metal surfaces by investigating the geometric and electronic properties of glycine molecules on Cu(100).It is shown that a glycine monolayer on Cu(100)forms a two-dimensional hydrogen-bonding network between the carboxyl and amino groups of glycine using a first principles atomistic calculation on the basis of a recently found structure.This network includes at least two hydrogen-bonding chains oriented roughly perpendicular to each other.Through molecule–metal electronic hybridization,these two chains selectively hybridized with the two isotropic degenerate Cu(100)surface states,leading to two anisotropic quasi-one-dimensional surface states.Electrons occupying these two states can near-freely move from a molecule to its adjacent molecules directly through the intermolecular hydrogen bonds,rather than mediated by the substrate.This results in the experimentally observed anisotropic free-electron-like behavior.Our results suggest that hydrogen-bonding chains are likely candidates for charge conductors.

Narrow Waveguide Based on Ferroelectric Domain Wall

Ferroelectric materials are spontaneous symmetry breaking systems that are characterized by ordered electric polarizations.Similar to its ferromagnetic counterpart,a ferroelectric domain wall can be regarded as a soft interface separating two different ferroelectric domains.Here we show that two bound state excitations of electric polarization(polar wave),or the vibration and breathing modes,can be hosted and propagate within the ferroelectric domain wall.In particular,the vibration polar wave has zero frequency gap,thus is constricted deeply inside ferroelectric domain wall,and can even propagate in the presence of local pinnings.The ferroelectric domain wall waveguide as demonstrated here offers a new paradigm in developing ferroelectric information processing units.

A Second-Order Semi-Implicit Method for the Inertial Landau-Lifshitz-Gilbert Equation

Electron spins in magnetic materials have preferred orientations collectively and generate the macroscopic magnetization.Its dynamics spans over a wide range of timescales from femtosecond to picosecond,and then to nanosecond.The Landau-Lifshitz-Gilbert(LLG)equation has been widely used in micromagnetics simulations over decades.Recent theoretical and experimental advances have shown that the inertia of magnetization emerges at sub-picosecond timescales and contributes significantly to the ultrafast magnetization dynamics,which cannot be captured intrinsically by the LLG equation.Therefore,as a generalization,the inertial LLG(iLLG)equation is proposed to model the ultrafast magnetization dynamics.Mathematically,the LLG equation is a nonlinear system of parabolic type with(possible)degeneracy.However,the iLLG equation is a nonlinear system of mixed hyperbolic-parabolic type with degeneracy,and exhibits more complicated structures.It behaves as a hyperbolic system at sub-picosecond timescales,while behaves as a parabolic system at larger timescales spanning from picosecond to nanosecond.Such hybrid behaviors impose additional difficulties on designing efficient numerical methods for the iLLG equation.In this work,we propose a second-order semiimplicit scheme to solve the iLLG equation.The second-order temporal derivative of magnetization is approximated by the standard centered difference scheme,and the first-order temporal derivative is approximated by the midpoint scheme involving three time steps.The nonlinear terms are treated semi-implicitly using one-sided interpolation with second-order accuracy.At each time step,the unconditionally unique solvability of the unsymmetric linear system is proved with detailed discussions on the condition number.Numerically,the second-order accuracy of the proposed method in both time and space is verified.At sub-picosecond timescales,the inertial effect of ferromagnetics is observed in micromagnetics simulations,in consistency with the hyperbolic property of the iLLG model;at nanosecond timescales,the results of the iLLG model are in nice agreements with those of the LLG model,in consistency with the parabolic feature of the iLLG model.

Influence of quantum correction on black hole shadows, photon rings, and lensing rings

We calculate photon sphere r_(ph) and critical curve b_(c) for a quantum corrected Schwarzschild black hole,finding that they violate universal inequalities proved for asymptotically flat black holes that satisfy the null energy condition in the framework of Einstein gravity.This violation seems to be a common phenomenon when considering quantum modification of Einstein gravity.Furthermore,we study the shadows,lensing rings,and photon rings in the quantum corrected Schwarzschild black hole.The violation leads to a larger bright lensing ring in the observational appearance of the thin disk emission near the black hole compared with the classical Schwarzschild black hole.Our analysis may provide observational evidence for the quantum effect of general relativity.

Theory for frequent measurements of spontaneous emissions in a non-Markovian environment: Beyond the Lorentzian spectrum

The measurement-result-conditioned evolution of a system(e.g., an atom) with spontaneous emissions of photons is described by the quantum trajectory(QT) theory. In this work we generalize the associated QT theory from an infinitely wide bandwidth Markovian environment to the finite bandwidth non-Markovian environment. In particular, we generalize the treatment for an arbitrary spectrum, which is not restricted by the specific Lorentzian case. We rigorously prove the general existence of a perfect scaling behavior jointly defined by the bandwidth of the environment and the time interval between successive photon detections.For a couple of examples, we obtain analytic results to facilitate the QT simulations based on the Monte-Carlo algorithm. For the case where the analytical result is not available, a numerical scheme is proposed for practical simulations.

Modified Bogoliubov-de Gennes treatment for Majorana conductances in three-terminal transports

We consider a two-lead(three-terminal) setup of non-local transport through Majorana zero modes(MZMs) and construct a Majorana master equation(which is also valid for small bias voltages). We first present representative results of current and then show that only a modified Bogoliubov-de Gennes(Bd G) treatment can consistently recover the same results. Based on the interplay of the two approaches, we reveal the existence of non-vanishing channels of teleportation and crossed Andreev reflections even at the limit ∈;→ 0(zero coupling energy of the MZMs), which leads to new predictions for the height of the zero-bias-peak of the local conductance and the ∈;-scaling behavior of the teleportation conductance, for verification by experiments.

Cachazo-Svrcek-Witten rules for tree-level gluonic amplitudes revisited

We provide a new proof of Cachazo-Svrcek-Witten rules for tree-level gluonic amplitudes.As a key step,we explicitly demonstrate the cancellation of spurious poles originating from the maximally helicity violating vertices in these rules.To achieve this,we introduce specially-defined two-off-shell-line sub-amplitudes and examine their residues at spurious poles.

Negative corrections to black hole entropy from string theory

We report for the first time that in heterotic string compactified on 4-torus or equivalently IIA string compactified on K3,the leading α′corrections to the rotating black string entropy at fixed conserved charges can be negative.This further implies that the correction to the mass of extremal rotating string is positive,opposite to the standard expectation from the weak gravity conjecture.Our result suggests that the validity of positivity of entropy shift due to higher order operators depends on other factors omitted previously in the effective field theory analysis.

Twin cogenesis

We investigate a cogenesis mechanism within the twin Higgs setup that can naturally explain the nature of dark matter,the cosmic coincidence puzzle,little hierarchy problem,leptogenesis,and the tiny neutrino masses.Three heavy Majorana neutrinos are introduced to the standard model sector and the twin sector respectively,which explain the tiny neutrino masses and generate the lepton asymmetry and the twin lepton asymmetry at the same time.The twin cogenesis mechanism applies to any viable twin Higgs model without an explicit Z_(2)breaking in the leptonic sector and evading theΔN_(eff) constraint.We illustrate the twin cogenesis mechanism using the neutrino-philic twin two Higgs doublet model,a newly proposed model to lift the twin neutrino masses with spontaneous Z_(2)breaking.The dark photon with a Stueckelberg mass O(10)MeV ensures the energy in the twin sector as well as the symmetric component of twin sector particles can be depleted.The lightest twin baryons are the dark matter candidates with masses of approximately 5.5 GeV,which explains naturally the amount of dark matter and visible matter in the Universe are of the same order.We also demonstrate twin cogenesis in the fraternal twin Higgs setup,in which the dark matter candidate is the twin bottom bound stateΩ’_(b’b’b’).

Holographic operator product expansion of loop operators in the N=4~SO(N) super Yang-Mills theory

In this study,we compute the correlation functions of Wilson(-'t Hooft)loops with chiral primary operators in the N=4 supersymmetric Yang-Mills theory with SO(N)gauge symmetry,which has a holographic dual description of the Type IIB superstring theory on the AdS_(5)×Rp^(5)background.Specifically,we compute the coefficients of the chiral primary operators in the operator product expansion of Wilson loops in the fundamental representation,Wilson-'t Hooft loops in the symmetric representation,Wilson loops in the anti-fundamental representation,and Wilson loops in the spinor representation.We also compare these results to those of the N=4 SU(N)super Yang-Mills theory.

Explaining the W boson mass anomaly and dark matter with a U(1) dark sector

The W boson mass recently reported by the CDF collaboration shows a deviation from the standard model prediction with an excess at the 7σ level.We investigate two simple extensions of the standard model with an extra U(1) dark sector.One is the U(1)xextension,where the U(1)xgauge field mixes with the standard model through gauge kinetic terms.The other is a general U(1)AY+Bqextension of the standard model.Fitting various experimental constraints,we find that the U(1)xextension with only kinetic mixing can enhance the W boson mass by10 MeV at most.The U(1)AY+Bqextension can easily generate a 77 MeV enhancement of the Wboson mass and also offer a viable dark matter candidate with a mass ranging from several hundred GeV to TeV,which may be detected by future dark matter direct detection experiments with improved sensitivities.