Quantum control for time-dependent noise by inverse geometric optimization

Quantum systems are exceedingly difficult to engineer because they are sensitive to various types of noises.In particular,timedependent noises are frequently encountered in experiments but how to overcome them remains a challenging problem.In this work,we propose a flexible robust control technique to resist time-dependent noises based on inverse geometric optimization working in the filter-function formalism.The basic idea is to parameterize the control filter function geometrically and minimize its overlap with the noise spectral density.This then effectively reduces the noise susceptibility of the controlled system evolution.We show that the proposed method can produce high-quality robust pulses for realizing desired quantum evolutions under realistic noise models.Also,we demonstrate this method in examples including dynamical decoupling and quantum sensing protocols to enhance their performances.

Finding Ordered State in a Disordered Jungle

A rich portfolio of emergent phenomena has been discovered in twisted two-dimensional(2D)moirésystems,including strongly correlated insulators,[1]superconductivity,[2]integer and fractional Chern insulators(ChIs),[3-5]magnetism,[6]and interfacial ferroelectricity.

Two-dimensional transport and strong spin-orbit interaction in SrMnSb_2

We have carried out magneto-transport measurements for single crystal SrMnSb2. Clear Shubnikov-de Haas oscil- lations were resolved at relatively low magnetic field around 4 T, revealing a quasi-2D Fermi surface. We observed a development of quantized plateaus in Hall resistance （Rxy） at high pulsed fields up to 60 T. Due to the strong 2D confine- ment and layered properties of the samples, we interpreted the observation as bulk quantum Hall effect that is contributed by the parallel 2D conduction channels. Moreover, the spin degeneracy was lifted leading to Landau level splitting. The presence of anisotropic g factor and the formation of the oscillation beating pattern reveal a strong spin-orbit interaction in the SrMnSb2 system.

Nanoscale Impact Ionization and Electroluminescence in a Biased Scanning-Tunneling-Microscope Junction

Electroluminescence from a p-type Ga As(110)surface was induced by tunneling electrons in a scanning tunneling microscope under both polarities of bias voltage.The optical spectra exhibit a polarity-independent luminescence peak at 1.47 eV resulting from the exciton recombination.However,the quantum yield of photon emission at negative bias voltage is two orders of magnitude weaker than that at positive bias voltage.Moreover,the luminescence at negative bias voltage shows the linear dependence of bias voltage,distinct from the rapid rise due to resonant electron injection at positive bias.Furthermore,the threshold bias voltage for electroluminescence at negative bias is nearly twice the bandgap of Ga As,not simply satisfying the energy conservation for the creation of an electron–hole pair.Through theoretical calculation,we propose an impact ionization model to nicely explain the newly observed electroluminescence at negative bias voltage.We believe that this mechanism of impact ionization could be readily applied to other nanoscale optoelectronics including 2D semiconductors and 1D nanostructures.

Dynamical-invariant-based holonomic quantum gates:Theory and experiment

Among existing approaches to holonomic quantum computing,the adiabatic holonomic quantum gates(HQGs)suffer errors due to decoherence,while the non-adiabatic HQGs either require additional Hilbert spaces or are difficult to scale.Here,we report a systematic,scalable approach based on dynamical invariants to realize HQGs without using additional Hilbert spaces.While presenting the theoretical framework of our approach,we design and experimentally evaluate single-qubit and two-qubits HQGs for the nuclear magnetic resonance system.The single-qubit gates acquire average fidelity 0.9972 by randomized benchmarking,and the controlled-NOT gate acquires fidelity 0.9782 by quantum process tomography.Our approach is also platform-independent,and thus may open a way to large-scale holonomic quantum computation.

Optical bound states in the continuum in periodic structures:mechanisms,effects,and applications

Optical bound states in the continuum(BICs)have recently stimulated a research boom,accompanied by demonstrations of abundant exotic phenomena and applications.With ultrahigh quality(Q)factors,optical BICs have powerful abilities to trap light in optical structures from the continuum of propagation waves in free space.Besides the high Q factors enabled by the confined properties,many hidden topological characteristics were discovered in optical BICs.Especially in periodic structures with well-defined wave vectors,optical BICs were discovered to carry topological charges in momentum space,underlying many unique physical properties.Both high Q factors and topological vortex configurations in momentum space enabled by BICs bring new degrees of freedom to modulate light.BICs have enabled many novel discoveries in light-matter interactions and spin-orbit interactions of light,and BIC applications in lasing and sensing have also been well explored with many advantages.In this paper,we review recent developments of optical BICs in periodic structures,including the physical mechanisms of BICs,explored effects enabled by BICs,and applications of BICs.In the outlook part,we provide a perspective on future developments for BICs.

Absence of metallicity and bias-dependent resistivity in low-carrier-density EuCd_(2)As_(2)

EuCd_(2)As_(2)was theoretically predicted to be a minimal model of Weyl semimetals with a single pair of Weyl points in the ferromagnet state.However,the heavily p-doped Eu Cd_(2)As_(2)crystals in previous experiments prevent direct identification of the semimetal hypothesis.Here,we present a comprehensive magneto-transport study of high-quality Eu Cd_(2)As_(2)crystals with ultralow bulk carrier density(10^(13)cm^(-3)).In contrast to the general expectation of a Weyl semimetal phase,Eu Cd_(2)As_(2)shows insulating behavior in both antiferromagnetic and ferromagnetic states as well as surface-dominated conduction from band bending.Moreover,the application of a dc bias current can dramatically modulate the resistance by over one order of magnitude,and induce a periodic resistance oscillation due to the geometric resonance.Such nonlinear transport results from the high nonequilibrium state induced by an electrical field near the band edge.Our results suggest an insulating phase in Eu Cd_(2)As_(2)and put a strong constraint on the underlying mechanism of anomalous transport properties in this system.

Antisymmetric planar Hall effect in rutile oxide films induced by the Lorentz force

The conventional Hall effect is linearly proportional to the field component or magnetization component perpendicular to a film. Despite the increasing theoretical proposals on the Hall effect to the in-plane field or magnetization in various special systems induced by the Berry curvature, such an unconventional Hall effect has only been experimentally reported in Weyl semimetals and in a heterodimensional superlattice. Here, we report an unambiguous experimental observation of the antisymmetric planar Hall effect(APHE) with respect to the in-plane magnetic field in centrosymmetric rutile RuO_(2) and IrO_(2) single-crystal films. The measured Hall resistivity is found to be linearly proportional to the component of the applied in-plane magnetic field along a particular crystal axis and to be independent of the current direction or temperature. Both the experimental observations and theoretical calculations confirm that the APHE in rutile oxide films is induced by the Lorentz force. Our findings can be generalized to ferromagnetic materials for the discovery of anomalous Hall effects and quantum anomalous Hall effects induced by in-plane magnetization. In addition to significantly expanding knowledge of the Hall effect, this work opens the door to explore new members in the Hall effect family.

Diffractive neural networks with improved expressive power for gray-scale image classification

In order to harness diffractive neural networks(DNNs)for tasks that better align with real-world computer vision requirements,the incorporation of gray scale is essential.Currently,DNNs are not powerful enough to accomplish gray-scale image processing tasks due to limitations in their expressive power.In our work,we elucidate the relationship between the improvement in the expressive power of DNNs and the increase in the number of phase modulation layers,as well as the optimization of the Fresnel number,which can describe the diffraction process.To demonstrate this point,we numerically trained a double-layer DNN,addressing the prerequisites for intensitybased gray-scale image processing.Furthermore,we experimentally constructed this double-layer DNN based on digital micromirror devices and spatial light modulators,achieving eight-level intensity-based gray-scale image classification for the MNIST and Fashion-MNIST data sets.This optical system achieved the maximum accuracies of 95.10%and 80.61%,respectively.

Large Dynamical Axion Field in Topological Antiferromagnetic Insulator Mn2Bi2Te5

The dynamical axion field is a new state of quantum matter where the magnetoelectric response couples strongly to its low-energy magnetic fluctuations.It is fundamentally different from an axion insulator with a static quantized magnetoelectric response.The dynamical axion field exhibits many exotic phenomena such as axionic polariton and axion instability.However,these effects have not been experimentally confirmed due to the lack of proper topological magnetic materials.Combining analytic models and first-principles calculations,here we predict a series of van der Waals layered Mn2Bi2Te5-related topological antiferromagnetic materials that could host the long-sought dynamical axion field with a topological origin.We also show that a large dynamical axion field can be achieved in antiferromagnetic insulating states close to the topological phase transition.We further propose the optical and transport experiments to detect such a dynamical axion field.Our results could directly aid and facilitate the search for topological-origin large dynamical axion field in realistic materials.

Nonrelativistic and nonmagnetic terahertz-wave generation via ultrafast current control in anisotropic conductive heterostructures

Precise and ultrafast control over photo-induced charge currents across nanoscale interfaces could lead to important applications in energy harvesting,ultrafast electronics,and coherent terahertz sources.Recent studies have shown that several relativistic mechanisms,including inverse spin-Hall effect,inverse Rashba–Edelstein effect,and inverse spin-orbit-torque effect,can convert longitudinally injected spinpolarized currents from magnetic materials to transverse charge currents,thereby harnessing these currents for terahertz generation.However,these mechanisms typically require external magnetic fields and exhibit limitations in terms of spin-polarization rates and efficiencies of relativistic spin-to-charge conversion.We present a nonrelativistic and nonmagnetic mechanism that directly utilizes the photoexcited high-density charge currents across the interface.We demonstrate that the electrical anisotropy of conductive oxides RuO2 and IrO2 can effectively deflect injected charge currents to the transverse direction,resulting in efficient and broadband terahertz radiation.Importantly,this mechanism has the potential to offer much higher conversion efficiency compared to previous methods,as conductive materials with large electrical anisotropy are readily available,whereas further increasing the spin-Hall angle of heavy-metal materials would be challenging.Our findings offer exciting possibilities for directly utilizing these photoexcited high-density currents across metallic interfaces for ultrafast electronics and terahertz spectroscopy.

Tunable charge density wave in TiS3 nanoribbons

Recently, modifications of charge density wave（CDW） in two-dimensional（2D） show intriguing properties in quasi-2D materials such as layered transition metal dichalcogenides（TMDCs）. Optical, electrical transport measurements and scanning tunneling microscopy uncover the enormous difference on the many-body states when the thickness is reduced down to monolayer. However, the CDW in quasi-one-dimensional（1D） materials like transition metal trichalcogenides（TMTCs） is yet to be explored in low dimension whose mechanism is likely distinct from their quasi-2D counterparts.Here, we report a systematic study on the CDW properties of titanium trisulfide（TiS3）. Two phase transition temperatures were observed to decrease from 53 K（103 K） to 46 K（85 K） for the bulk and 〈 15-nm thick nanoribbon, respectively,which arises from the increased fluctuation effect across the chain in the nanoribbon structure, thereby destroying the CDW coherence. It also suggests a strong anisotropy of CDW states in quasi-1D TMTCs which is different from that in TMDCs.Remarkably, by using back gate of-30 V ～ 70 V in 15-nm device, we can tune the second transition temperature from110 K（at-30 V） to 93 K（at 70 V） owing to the altered electron concentration. Finally, the optical approach through the impinging of laser beams on the sample surface is exploited to manipulate the CDW transition, where the melting of the CDW states shows a strong dependence on the excitation energy. Our results demonstrate TiS3 as a promising quasi-1D CDW material and open up a new window for the study of collective phases in TMTCs.

Temperature-switching logic in MoS2 single transistors

Due to their unique characteristics,two-dimensional(2D)materials have drawn great attention as promising candidates for the next generation of integrated circuits,which generate a calculation unit with a new working mechanism,called a logic transistor.To figure out the application prospects of logic transistors,exploring the temperature dependence of logic characteristics is important.In this work,we explore the temperature effect on the electrical characteristic of a logic transistor,finding that changes in temperature cause transformation in the calculation:logical output converts from‘AND’at 10 K to‘OR’at 250 K.The transformation phenomenon of temperature regulation in logical output is caused by energy band which decreases with increasing temperature.In the experiment,the indirect band gap of MoS2 shows an obvious decrease from 1.581 eV to 1.535 eV as the temperature increases from 10 K to 250 K.The change of threshold voltage with temperature is consistent with the energy band,which confirms the theoretical analysis.Therefore,as a promising material for future integrated circuits,the demonstrated characteristic of 2D transistors suggests possible application for future functional devices.

Quantum precision measurement of two-dimensional forces with 10^(-28)-Newton stability

High-precision sensing of vectorial forces has broad impact on both fundamental research and technological applications such as the examination of vacuum fluctuations and the detection of surface roughness of nanostructures.Recent years have witnessed much progress on sensing alternating electromagnetic forces for the rapidly advancing quantum technology-orders of magnitude improvement has been accomplished on the detection sensitivity with atomic sensors,whereas such high-precision measurements for static electromagnetic forces have rarely been demonstrated.Here,based on quantum atomic matter waves confined by a two-dimensional optical lattice,we perform precision measurement of static electromagnetic forces by imaging coherent wave mechanics in the reciprocal space.The lattice confinement causes a decoupling between real-space and reciprocal dynamics,and provides a rigid coordinate frame for calibrating the wavevector accumulation of the matter wave.With that we achieve a stateof-the-art sensitivity of 2.30(8)×10^(-26) N/√Hz.Long-term stabilities on the order of 10^(-28) N are observed in the two spatial components of a force,which allows probing atomic Van der Waals forces at one millimeter distance.As a further illustrative application,we use our atomic sensor to calibrate the control precision of an alternating electromagnetic force applied in the experiment.Future developments of this method hold promise for delivering unprecedented atom-based quantum force sensing technologies.

Visualization of tunnel magnetoresistance effect in single manganite nanowires

We reported a study of tunnel magnetoresistance(TMR)effect in single manganite nanowire via the combination of magnetotransport and magnetic force microscopy imaging.TMR value up to 290%has been observed in single(La1-yPry)1-x CaxMnO3 nanowires with varying width.We find that the TMR effect can be explained in the scenario of opening and blockade of conducting channels from inherent magnetic domain evolutions.Our findings provide a new route to fabricate TMR junctions and point towards future improvements in complex oxide-based TMR spintronics.

Ultrafast magnetization enhancement and spin current injection in magnetic multilayers by exciting the nonmagnetic metal

A systematic investigation of spin injection behavior in Au/FM(FM=Fe and Ni)multilayers is performed using the superdiffusive spin transport theory.By exciting the nonmagnetic layer,the laser-induced hot electrons may transfer spin angular momentum into the adjacent ferromagnetic(FM)metals resulting in ultrafast demagnetization or enhancement.We find that these experimental phenomena sensitively depend on the particular interface reflectivity of hot electrons and may reconcile the different observations in the experiment.Stimulated by the ultrafast spin currents carried by the hot electrons,we propose the multilayer structures to generate highly spin-polarized currents for the development of future ultrafast spintronics devices.The spin polarization of the electric currents carried by the hot electrons can be significantly enhanced by the joint effects of bulk and interfacial spin filtering.Meanwhile,the intensity of the generated spin current can be optimized by varying the number of repeated stacking units and the thickness of each metallic layer.

Preparing quantum states by measurement-feedback control with Bayesian optimization

The preparation of quantum states is crucial for enabling quantum computations and simulations.In this work,we present a general framework for preparing ground states of many-body systems by combining the measurement-feedback control process(MFCP)with machine learning techniques.Specifically,we employ Bayesian optimization(BO)to enhance the efficiency of determining the measurement and feedback operators within the MFCP.As an illustration,we study the ground state preparation of the one-dimensional Bose−Hubbard model.Through BO,we are able to identify optimal parameters that can effectively drive the system towards low-energy states with a high probability across various quantum trajectories.Our results open up new directions for further exploration and development of advanced control strategies for quantum computations and simulations.

Experimental quantum simulation of a topologically protected Hadamard gate via braiding Fibonacci anyons

Topological quantum computation(TQC)is one of the most striking architectures that can realize fault-tolerant quantum computers.In TQC,the logical space and the quantum gates are topologically protected,i.e.,robust against local disturbances.The topological protection,however,requires complicated lattice models and hard-to-manipulate dynamics;even the simplest system that can realize universal TQC-the Fibonacci anyon system—lacks a physical realization,let alone braiding the non-Abelian anyons.Here,we propose a disk model that can simulate the Fibonacci anyon system and construct the topologically protected logical spaces with the Fibonacci anyons.Via braiding the Fibonacci anyons,we can implement universal quantum gates on the logical space.Our disk model merely requires two physical qubits to realize three Fibonacci anyons at the boundary.By 15 sequential braiding operations,we construct a topologically protected Hadamard gate,which is to date the least-resource requirement for TQC.To showcase,we implement a topological Hadamard gate with two nuclear spin qubits,which reaches 97.18%fidelity by randomized benchmarking.We further prove by experiment that the logical space and Hadamard gate are topologically protected:local disturbances due to thermal fluctuations result in a global phase only.As a platform-independent proposal,our work is a proof of principle of TQC and paves the way toward fault-tolerant quantum computation.

Realization of skyrmion shift register

The big data explosion demands novel data storage technology. Among many different approaches, solitonic racetrack memory devices hold great promise for accommodating nonvolatile and low-power functionalities. As representative topological solitons, magnetic skyrmions are envisioned as potential information carriers for efficient information processing. While their advantages as memory and logic elements have been vastly exploited from theoretical perspectives, the corresponding experimental efforts are rather limited. These challenges, which are key to versatile skyrmionic devices, will be studied in this work. Through patterning concaved surface topography with designed arrays of indentations on standard Si/SiO_(2) substrates, we demonstrate that the resultant non-flat energy landscape could lead to the formation of hexagonal and square skyrmion lattices in Ta/CoFeB/MgO multilayers. Based on these films, one-dimensional racetrack devices are subsequently fabricated, in which a long-distance deterministic shifting of skyrmions between neighboring indentations is achieved at room temperature. Through separating the word line and the bit line, a prototype shift register device, which can sequentially generate and precisely shift complex skyrmionic data strings, is presented. The deterministic writing and longdistance shifting of skyrmionic bits can find potential applications in transformative skyrmionic memory,logic as well as the in-memory computing devices.

量子极限下的能量耗散

Energy dissipation is of fundamental interest and crucial importance in quantum systems. However,whether energy dissipation can emerge without backscattering inside topological systems remains a question. As a hallmark, we propose a microscopic picture that illustrates energy dissipation in the quantum Hall(QH) plateau regime of graphene. Despite the quantization of Hall, longitudinal, and two-probe resistances(dubbed as the quantum limit), we find that the energy dissipation emerges in the form of Joule heat. It is demonstrated that the non-equilibrium energy distribution of carriers plays much more essential roles than the resistance on energy dissipation. Eventually, we suggest probing the phenomenon by measuring local temperature increases in experiments and reconsidering the dissipation typically ignored in realistic topological circuits.