查看更多>>摘要：Non-orthogonality in non-Hermitian quantum systems gives rise to tremendous exotic quantum phenomena,which can be fundamentally traced back to non-unitarity.In this paper,we introduce an interesting quantity(denoted as η)as a new variant of the Petermann factor to directly and efficiently measure non-unitarity and the associated non-Hermitian physics.By tuning the model parameters of underlying non-Hermitian systems,we find that the discontinuity of both η and its first-order derivative(denoted as ∂η)pronouncedly captures rich physics that is fundamentally caused by non-unitarity.More concretely,in the 1D non-Hermitian topo-logical systems,two mutually orthogonal edge states that are respectively localized on two boundaries become non-orthogonal in the vicinity of discontinuity of η as a function of the model parameter,which is dubbed"edge state transition".Through theoretical analysis,we identify that the appearance of edge state transition indicates the existence of exceptional points(EPs)in topological edge states.Regarding the discontinuity of ∂η,we investigate a two-level non-Hermitian model and establish a connection between the points of discontinuity of ∂η and EPs of bulk states.By studying this connection in more general lattice models,we find that some models have discontinuity of ∂η,implying the existence of EPs in bulk states.
查看更多>>摘要：We present a novel class of Rydberg-mediated nuclear-spin entanglement in divalent atoms with global laser pulses.First,we show a fast nuclear-spin controlled phase gate of an arbitrary phase realizable either with two laser pulses when assisted by Stark shifts,or with three pulses.Second,we propose to create an electrons-nuclei-entangled state,which is named a super bell state(SBS)for it mimics a large Bell state incorporating three small Bell states.Third,we show a protocol to create a three-atom elec-trons-nuclei entangled state which contains the three-body W and Green-berger-Horne-Zeilinger(GHZ)states simultaneously.These protocols possess high intrinsic fidelities,do not require single-site Rydberg address-ing,and can be executed with large Rydberg Rabi frequencies in a weak,Gauss-scale magnetic field.The latter two protocols can enable measure-ment-based preparation of Bell,hyperentangled,and GHZ states,and,specifically,SBS can enable quantum dense coding where one can share three classical bits of information by sending one particle.
查看更多>>摘要：Creation of stable intrinsically anisotropic self-bound states with embedded vorticity is a challenging issue.Previously,no such states in Bose-Einstein condensates(BECs)or other physical settings were known.Dipolar BEC suggests a unique possibility to predict stable two dimensional anisotropic vortex quantum droplets(2D-AVQDs).We demonstrate that they can be created with the vortex axis oriented perpendicular to the polarization of dipoles.The stability area and characteristics of the 2D-AVQDs in the parameter space are revealed by means of analytical and numerical meth-ods.Further,the rotation of the polarizing magnetic field is considered,and the largest angular velocities,up to which spinning 2D-AVQDs can follow the rotation in clockwise and anti-clockwise directions,are found.Collisions between moving 2D-AVQDs are studied too,demonstrating formation of bound states with a vortex-antivortex-vortex structure.A stability domain for such stationary bound states is identified.Unstable dipolar states,that can be readily implemented by means of phase imprinting,quickly transform into robust 2D-AVQDs,which suggests a straightforward possibility for the creation of these states in the experi-ment.
查看更多>>摘要：Recently universal dynamic scaling is observed in several systems,which exhibit a spatiotemporal self-similar scaling behavior,analogous to the spatial scaling near phase transition.The latter one arises from the emer-gent continuous scaling symmetry.Motivated by this,we investigate the possible relation between the scaling dynamics and the continuous scaling symmetry in this paper.We derive a theorem that the scaling invariance of the quenched Hamiltonian and the initial density matrix can lead to the universal dynamic scaling.It is further demonstrated both in a two-body system analytically and in a many-body system numerically.For the latter one,we calculate the dynamics of quantum gases quenched from the zero interaction to a finite interaction via the non-equilibrium high-temperature virial expansion.A dynamic scaling of the momentum distribution appears in certain momentum-time windows at unitarity as well as in the weak interacting limit.Remarkably,this universal scaling dynamics persists approximately with smaller scaling exponents even if the scaling symmetry is fairly broken.Our findings may offer a new perspective to interpret the related experiments.We also study the Contact dynamics in the BEC-BCS crossover.Surprisingly,the half-way time displays a maxi-mum near unitarity while some damping oscillations occur on the BEC side due to the dimer state,which can be used to detect possible two-body bound states in experiments.
查看更多>>摘要：Optimization problems are prevalent in various fields,and the gradient-based gradient descent algorithm is a widely adopted optimization method.However,in classical computing,computing the numerical gradient for a function with d variables necessitates at least d+1 function evaluations,resulting in a computational complexity of O(d).As the number of variables increases,the classical gradient estimation methods require substantial resources,ultimately surpassing the capabilities of classical computers.Fortunately,leveraging the principles of superposition and entanglement in quantum mechanics,quantum computers can achieve genuine parallel computing,leading to exponential acceleration over classical algorithms in some cases.In this paper,we propose a novel quantum-based gradient calculation method that requires only a single oracle calculation to obtain the numerical gradient result for a multivariate function.The complexity of this algorithm is just O(1).Building upon this approach,we successfully implemented the quantum gradient descent algorithm and applied it to the variational quantum eigensolver(VQE),creating a pure quantum vari-ational optimization algorithm.Compared with classical gradient-based optimization algorithm,this quantum optimization algorithm has remark-able complexity advantages,providing an efficient solution to optimization problems.The proposed quantum-based method shows promise in enhancing the performance of optimization algorithms,highlighting the potential of quantum computing in this field.
查看更多>>摘要：We study the local quantum Fisher information(LQFI)in the mixed-spin Heisenberg XXZ chain.Both the maximal and minimal LQFI are studied and the former is essential to determine the accuracy of the quantum parameter estimation,the latter can be well used to characterize the discord-type quantum correlations.We investigate the effects of the temperature and the anisotropy parameter on the maximal LQFI and thus on the accuracy of the parameter estimation.Then we make use of the minimal LQFI to study the discord-type correlations of different site pairs.Different dimensions of the subsystems cause different values of the mini-mal LQFI which reflects the asymmetry of the discord-type correlation.In addition,the site pairs at different positions of the spin chains have different minimal LQFI,which reveals the influence of the surrounding spins on the bipartite quantum correlation.Our results show that the LQFI obtained through a simple calculation process provides a convenient way to investigate the discord-type correlation in high-dimensional systems.
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