High-dimensional quantum state transfer in a noisy network environment

We propose and analyze an efficient high-dimensional quantum state transfer protocol in an XX coupling spin network with a hypercube structure or chain structure. Under free spin wave approximation, unitary evolution results in a perfect high-dimensional quantum swap operation requiring neither external manipulation nor weak coupling. Evolution time is independent of either distance between registers or dimensions of sent states, which can improve the computational efficiency. In the low temperature regime and thermodynamic limit, the decoherence caused by a noisy environment is studied with a model of an antiferromagnetic spin bath coupled to quantum channels via an Ising-type interaction. It is found that while the decoherence reduces the fidelity of state transfer, increasing intra-channel coupling can strongly suppress such an effect. These observations demonstrate the robustness of the proposed scheme.

Quantum Key Distribution Based on a Weak-Coupling Cavity QED Regime

We present a quantum key distribution scheme using a weak-coupling cavity QED regime based on quantum dense coding.Hybrid entanglement statesof photons and electrons are used to distribute information.We just need to transmit photons without storing them in the scheme.The electron confined in a quantum dot,which is embedded in a microcavity,is held by one of the legitimate users throughout the whole communication process.Only the polarization of a single photon and spin of electron measurements are applied in this protocol,which are easier to perform than collective-Bell state measurements.Linear optical apparatus,such as a special polarizing beam splitter in a circular basis and single photon operations,make it more flexible to realize under current technology.Its efficiency will approach 100%in the ideal case.The security of the scheme is also discussed.

Editorial:Celebrating the 30 Wonderful Year Journey of Chinese Physics B

The year 2022 marks the 30^(th)anniversary of Chinese Physics B.This editorial provides a brief history of the journal and introduces the anniversary theme collection comprising over 30 invited reviews and perspective articles from renowned scholars in various branches of physics.

Detection of Magnetic Gap in Topological Surface States of MnBi_(2)Te_(4)

Recently,intrinsic antiferromagnetic topological insulator MnBi_(2)Te_(4) has drawn intense research interest and leads to plenty of significant progress in physics and materials science by hosting quantum anomalous Hall effect,axion insulator state,and other quantum phases.An essential ingredient to realize these quantum states is the magnetic gap in the topological surface states induced by the out-of-plane ferromagnetism on the surface of MnBi_(2)Te_(4).However,the experimental observations of the surface gap remain controversial.Here,we report the observation of the surface gap via the point contact tunneling spectroscopy.In agreement with theoretical calculations,the gap size is around 50 me V,which vanishes as the sample becomes paramagnetic with increasing temperature.The magnetoresistance hysteresis is detected through the point contact junction on the sample surface with an out-of-plane magnetic field,substantiating the surface ferromagnetism.Furthermore,the non-zero transport spin polarization coming from the ferromagnetism is determined by the point contact Andreev reflection spectroscopy.Combining these results,the magnetism-induced gap in topological surface states of MnBi_(2)Te_(4) is revealed.

A High-Randomness and High-Stability Electronic Quantum Random Number Generator without Post Processing

Random numbers are one of the key foundations of cryptography.This work implements a discrete quantum random number generator(QRNG)based on the tunneling effect of electrons in an avalanche photo diode.Without any post-processing and conditioning,this QRNG can output raw sequences at a rate of 100 Mbps.Remarkably,the statistical min-entropy of the 8,000,000 bits sequence reaches 0.9944 bits/bit,and the min-entropy validated by NIST SP 800-90B reaches 0.9872 bits/bit.This metric is currently the highest value we have investigated for QRNG raw sequences.Moreover,this QRNG can continuously and stably output raw sequences with high randomness over extended periods.The system produced a continuous output of 1,174 Gbits raw sequence for a duration of 11,744 s,with every 8 Mbits forming a unit to obtain a statistical min-entropy distribution with an average value of 0.9892 bits/bit.The statistical min-entropy of all data(1,174 Gbits)achieves the value of0.9951 bits/bit.This QRNG can produce high-quality raw sequences with good randomness and stability.It has the potential to meet the high demand in cryptography for random numbers with high quality.

Nuclear magnetic resonance for quantum computing： Techniques and recent achievements

Rapid developments in quantum information processing have been made, and remarkable achievements have been obtained in recent years, both in theory and experiments. Coherent control of nuclear spin dynamics is a powerful tool for the experimental implementation of quantum schemes in liquid and solid nuclear magnetic resonance （NMR） system, especially in liquid-state NMR. Compared with other quantum information processing systems, the NMR platform has the advantages such as the long coherence time, the precise manipulation, and well-developed quantum control techniques, which make it possible to accurately control a quantum system with up to 12-qubits. Extensive applications of liquid-state NMR spectroscopy in quantum information processing such as quantum communication, quantum computing, and quantum simulation have been thoroughly studied over half a century. This article introduces the general principles of NMR quantum information processing, and reviews the new-developed techniques. The review will also include the recent achievements of the experimental realization of quantum algorithms for machine learning, quantum simulations for high energy physics, and topological order in NMR. We also discuss the limitation and prospect of liquid-state NMR spectroscopy and the solid-state NMR systems as quantum computing in the article.

Interaction region of magnon-mediated spin torques and novel magnetic states

We determine the region in which the magnon-mediated spin torques exist.This region can be controlled by the spin waves.In terms of stability analysis of magnetization dynamics based on the spin-wave background,we obtain the instability conditions of spin waves.With these results,we find the relationship between unstable regions and the formation of Akhmediev breather,Kuznetsov-Ma breather and rogue waves.We establish the phase diagram of some novel magnetic excitaions.

Mott Transition and Superconductivity in Quantum Spin Liquid Candidate NaYbSe2

The Mott transition is one of the fundamental issues in condensed matter physics,especially in the system with antiferromagnetic long-range order.However,such a transition is rare in quantum spin liquid(QSL)systems without long-range order.Here we report the experimental pressure-induced insulator to metal transition followed by the emergence of superconductivity in the QSL candidate NaYbSe2 with a triangular lattice of 4 f Yb^3+ions.Detail analysis of transport properties in metallic state shows an evolution from non-Fermi liquid to Fermi liquid behavior when approaching the vicinity of superconductivity.An irreversible structure phase transition occurs around 11 GPa,which is revealed by the x-ray diffraction.These results shed light on the Mott transition in the QSL systems.

Charge disturbance/excitation in the Raman virtual state revealed by ROA signal: A case study of pinane

The Raman mode intensities are used to extract the bond polarizabilities which are the indication of the charge disturbance/excitation of the Raman virtual state. A classical formula based on the electric and magnetic dipolar coupling among the charges on the atoms is developed which relates the charges and vibrational amplitudes of the atoms in a normal mode to the Raman optical activity(ROA) mode signatures. By fitting with the experimental ROA signatures, we are able to elucidate the scaling parameter which relates the bond polarizability to the electric charge. The result shows that around40% of the charges in pinane are involved in the Raman process under 532 nm laser excitation.

From magnetically doped topological insulator to the quantum anomalous Hall effect

Quantum Hall effect （QHE）, as a class of quantum phenomena that occur in macroscopic scale, is one of the most important topics in condensed matter physics. It has long been expected that QHE may occur without Landau levels so that neither external magnetic field nor high sample mobility is required for its study and application, Such a QHE free of Landau levels, can appear in topological insulators （TIs） with ferromagnetism as the quantized version of the anomalous Hall effect, i.e., quantum anomalous Hall （QAH） effect. Here we review our recent work on experimental realization of the QAH effect in magnetically doped TIs. With molecular beam epitaxy, we prepare thin films of Cr-doped （Bi,Sb）2Te3 TIs with well- controlled chemical potential and long-range ferromagnetic order that can survive the insulating phase. In such thin films, we eventually observed the quantization of the Hall resistance at h/e2 at zero field, accompanied by a considerable drop in the longitudinal resistance. Under a strong magnetic field, the longitudinal resistance vanishes, whereas the Hall resistance remains at the quantized value. The realization of the QAH effect provides a foundation for many other novel quantum phenomena predicted in TIs, and opens a route to practical applications of quantum Hall physics in low-power-consumption electronics.

Elastic scattering of surface states on three-dimensional topological insulators

Topological insulators as a new type of quantum matter materials are characterized by a full insulating gap in the bulk and gapless edge/surface states protected by the time-reversal symmetry. We propose that the interference patterns caused by the elastic scattering of defects or impurities are dominated by the surface states at the extremal points on the constant energy contour. Within such a formalism, we summarize our recent theoretical investigations on the elastic scattering of topological surface states by various imperfections, including non-magnetic impurities, magnetic impurities, step edges, and various other defects, in comparison with the recent related experiments in typical topological materials such as BiSb alloys, Bi2Te3, and Bi2Se3 crystals.

Accurate electron affinity of atomic cerium and excited states of its anion

Electron affinities(EA)of most lanthanide elements still remain unknown owing to their relatively lower EA values and the fairly complicated electronic structures.In the present work,we report the high-resolution photoelectron spectra of atomic cerium anion Ce−using the slow electron velocity-map imaging method in combination with a cold ion trap.The electron affinity of Ce is determined to be 4840.62(21)cm^-1 or 0.600160(26)eV.Moreover,several excited states of Ce(^4H9/2,^4I9/2,^2H9/2,^2G9/2,^2G7/2,^4H13/2,^2F5/2,and ^4I13/2)are observed.

Quantum Anomalous Hall Multilayers Grown by Molecular Beam Epitaxy

Quantum anomalous Hall（QAH） effect is a quantum Hall effect that occurs without the need of external magnetic field. A system composed of multiple parallel QAH layers is an effective high Chern number QAH insulator and the key to the applications of the dissipationless chiral edge channels in low energy consumption electronics. Such a QAH multilayer can also be engineered into other exotic topological phases such as a magnetic Weyl semimetal with only one pair of Weyl points. This work reports the first experimental realization of QAH multilayers in the superlattices composed of magnetically doped（Bi,Sb）2Te3 topological insulator and Cd Se normal insulator layers grown by molecular beam epitaxy. The obtained multilayer samples show quantized Hall resistance h/Ne2, where h is Planck＇s constant, e is the elementary charge and N is the number of the magnetic topological insulator layers, resembling a high Chern number QAH insulator. The QAH multilayers provide an excellent platform to study various topological states of matter.

Molecular beam epitaxy growth of quantum devices

The inherent fragility and surface/interface-sensitivity of quantum devices demand fabrication techniques under very clean environment.Here,I briefly introduces several techniques based on molecular beam epitaxy growth on pre-patterned substrates which enable us to directly prepare in-plane nanostructures and heterostructures in ultrahigh vacuum.The molecular beam epitaxy-based fabrication techniques are especially useful in constructing the high-quality devices and circuits for solid-state quantum computing in a scalable way.

A Scheme for Simulation of Quantum Gates by Abelian Anyons

Anyons can be used to realize quantum computation, because they are two-level systems in two dimensions. In this paper, we propose a scheme to simulate single-qubit gates and CNOT gate using Abelian anyons in the Kitaev model. Two pairs of anyons （six spins） are used to realize single-qubit gates, while ten spins are needed for the CNOT gate. Based on these quantum gates, we show how to realize the Grover algorithm in a two-qubit system.

Modulation of Step Heights of Thin Pb Films by the Quantum Size Effect Observed by Non-Contact Atomic Force Microscopy

Using a home-made Q-plus sensor,simultaneous scanning tunneling microscopy (STM) and atomic force microscopy (AFM) measurements were performed on the wedge-shaped Pb islands grown on Si(111)-7 × 7.Atomic resolved AFM images were observed.The contrast of AFM topography shows no dependence on the sample bias (tip is grounded),while the simultaneously obtained tunneling current image exhibits strong bias dependence due to quantum well states (QWS).Furthermore,In the AFM mode,neighboring Pb films with one monolayer (ML) thickness difference within the same Pb island show the same apparent height,which means that the apparent step heights of Pb films oscillate with a bilayer periodicity,being consistent with previous observations by helium atom scattering,x-ray diffraction,and STM.The possible reasons underlying the oscillation of apparent step heights in AFM topography are discussed.

Quantum teleportation of particles in an environment

We discuss the teleportation of particles in an environment of an N-body system.In this case,we can change a many-body system into an arbitrary shape in space by teleporting some or all the constituent particles,and thus we call the quantum teleportation under this circumstance as quantum tele-transformation(QTT).The particular feature of QTT is that the wave function of the internal degrees of freedom remains the same,while the spatial wave function experiences a drastic change.The notion of QTT provides conceptual and pedagogical convenience for quantum information processing.In view of QTT,teleportation is the change of a single particle in space,while entanglement swapping is the change of one particle of an entangled pair.

Visualizing the in-Gap States in Domain Boundaries of Ultra-Thin Topological Insulator Films

Ultra-thin topological insulators provide a platform for realizing many exotic phenomena such as the quantum spin Hall effect,and quantum anomalous Hall effect.These effects or states are characterized by quantized transport behavior of edge states.Experimentally,although these states have been realized in various systems,the temperature for the edge states to be the dominating channel in transport is extremely low,contrary to the fact that the bulk gap is usually in the order of a few tens of milli-electron volts.There must be other in-gap conduction channels that do not freeze out until a much lower temperature.Here we grow ultra-thin topological insulator Bi_(2)Te_(3) and Sb_(2)Te_(3)films by molecular beam epitaxy and investigate the structures of domain boundaries in these films.By scanning tunneling microscopy and spectroscopy we find that the domain boundaries with large rotation angles have pronounced in-gap bound states,through which one-dimensional conduction channels are suggested to form,as visualized by spatially resolved spectroscopy.Our work indicates the critical role played by domain boundaries in degrading the transport properties.

Predicting Quantum Many-Body Dynamics with Transferable Neural Net works

Advanced machine learning(ML)approaches such as transfer learning have seldom been applied to approximate quantum many-body systems.Here we demonstrate that a simple recurrent unit(SRU)based efficient and transferable sequence learning framework is capable of learning and accurately predicting the time evolution of the one-dimensional(ID)Ising model with simultaneous transverse and parallel magnetic fields,as quantitatively corroborated by relative entropy measurements between the predicted and exact state distributions.At a cost of constant computational complexity,a larger many-body state evolution is predicted in an autoregressive way from just one initial state,without any guidance or knowledge of any Hamiltonian.Our work paves the way for future applications of advanced ML methods in quantum many-body dynamics with knowledge only from a smaller system.

Noise-Induced Entanglement Transition in One-Dimensional Random Quantum Circuits

A random quantum circuit is a minimally structured model to study entanglement dynamics of many-body quantum systems.We consider a one-dimensional quantum circuit with noisy Haar-random unitary gates using density matrix operator and tensor contraction methods.It is shown that the entanglement evolution of the random quantum circuits is properly characterized by the logarithmic entanglement negativity.By performing exact numerical calculations,we find that,as the physical error rate is decreased below a critical value p;≈0.056,the logarithmic entanglement negativity changes from the area law to the volume law,giving rise to an entanglement transition.The critical exponent of the correlation length can be determined from the finite-size scaling analysis,revealing the universal dynamic property of the noisy intermediate-scale quantum devices.