Analysis and Improvement of an Efficient Controlled Quantum Secure Direct Communication and Authentication Protocol

The controlled quantum secure direct communication(CQSDC)with authentication protocol based on four particle cluster states via quantum one-time pad and local unitary operations is cryptanalyzed.It is found that there are some serious security issues in this protocol.An eavesdropper(Eve)can eavesdrop on some information of the identity strings of the receiver and the controller without being detected by the selective-CNOT-operation(SCNO)attack.By the same attack,Eve can also steal some information of the secret message that the sender transmits.In addition,the receiver can take the same kind of attack to eavesdrop on some information of the secret message out of the control of the controller.This means that the requirements of CQSDC are not satisfied.At last,we improve the original CQSDC protocol to a secure one.

A Self-Powered Nanogenerator for the Electrical Protection of Integrated Circuits from Trace Amounts of Liquid

With the increase in the use of electronic devices in many different environments,a need has arisen for an easily implemented method for the rapid,sensitive detection of liquids in the vicinity of electronic components.In this work,a high-performance power generator that combines carbon nanoparticles and TiO2 nanowires has been fabricated by sequential electrophoretic deposition(EPD).The opencircuit voltage and short-circuit current of a single generator are found to exceed 0.7 V and 100μA when 6μL of water was applied.The generator is also found to have a stable and reproducible response to other liquids.An output voltage of 0.3 V was obtained after 244,876,931,and 184μs,on exposure of the generator to 6μL of water,ethanol,acetone,and methanol,respectively.The fast response time and high sensitivity to liquids show that the device has great potential for the detection of small quantities of liquid.In addition,the simple easily implemented sequential EPD method ensures the high mechanical strength of the device.This compact,reliable device provides a new method for the sensitive,rapid detection of extraneous liquids before they can impact the performance of electronic circuits,particularly those on printed circuit board.

Phase-controlled coherent population trapping in superconducting quantum circuits

We investigate the influences of the-applied-field phases and amplitudes on the coherent population trapping behavior in superconducting quantum circuits. Based on the interactions of the microwave fields with a single A-type three-level fluxonium qubit, the coherent population trapping could be obtainable and it is very sensitive to the relative phase and amplitudes of the applied fields. When the relative phase is tuned to 0 or π, the maximal atomic coherence is present and coherent population trapping occurs. While for the choice of π/2, the atomic coherence becomes weak. Meanwhile, for the fixed relative phase π/2, the value of coherence would decrease with the increase of Rabi frequency of the external field coupled with two lower levels. The responsible physical mechanism is quantum interference induced by the control fields, which is indicated in the dressed-state representation. The microwave coherent phenomenon is present in our scheme, which will have potential applications in optical communication and nonlinear optics in solid-state devices.

Hamiltonian Characterization for Dipolar Coupled Spin Systems

Measuring the Hamiltonian of dipolar coupled spin systems is usually a difficult task due to the high complexity of their spectra. Currently, molecules with unknown geometrical structure and low symmetry are extremely tedious or impossible to analyze by sheer spectral fitting. We present a novel method that addresses the problem of spectral analysis and report experimental results of extracting, by spectral fitting, the parameters of an oriented 6-spin system with very low symmetry in structure, without using apriori knowledge or assumptions on the molecular geometry or order parameters. The advantages of our method are achieved with the use of a new spectral analysis algorithm non-assigned frequency optimization of NMR spectra （NAFONS） and by the use of simplified spectra obtained by transition selective pulses. This new method goes beyond the limit of spectral analysis for dipolar coupled spin systems and is helpful for related fields, such as quantum computation and molecular structure analysis.

Preparation of pseudo-pure states for NMR quantum computing with one ancillary qubit

Quantum state preparation plays an equally important role as quantum operations and measurements in quantum information processing. The previous methods for initialization require either an exponential number of experiments, or cause signal reduction or place restrictions on molecular structures. In this study, we propose three types of quantum circuits for preparing the pseudo-pure states of(n-1) qubits in the n-coupled Hilbert space, which simply needs the assistance of one ancilla spin and two different experiments independent of n. Most importantly, our methods work well on homo-nuclear and hetero-nuclear molecules without the reduction of signals in the gradient field. As a proof-of-principle demonstration, we experimentally prepared the pseudo-pure states of heteronuclear 2-qubit and homonuclear 4-qubit molecules using a nuclear magnetic resonance quantum information processor.

Testing a quantum error-correcting code on various platforms

Quantum error correction plays an important role in fault-tolerant quantum information processing.It is usually difficult to experimentally realize quantum error correction,as it requires multiple qubits and quantum gates with high fidelity.Here we propose a simple quantum error-correcting code for the detected amplitude damping channel.The code requires only two qubits.We implement the encoding,the channel,and the recovery on an optical platform,the IBM Q System,and a nuclear magnetic resonance system.For all of these systems,the error correction advantage appears when the damping rate exceeds some threshold.We compare the features of these quantum information processing systems used and demonstrate the advantage of quantum error correction on current quantum computing platforms.

A Memory-efficient Simulation Method of Grover’s Search Algorithm

Grover’s search algorithm is one of the most significant quantum algorithms,which can obtain quadratic speedup of the extensive search problems.Since Grover's search algorithm cannot be implemented on a real quantum computer at present,its quantum simulation is regarded as an effective method to study the search performance.When simulating the Grover's algorithm,the storage space required is exponential,which makes it difficult to simulate the high-qubit Grover’s algorithm.To this end,we deeply study the storage problem of probability amplitude,which is the core of the Grover simulation algorithm.We propose a novel memory-efficient method via amplitudes compression,and validate the effectiveness of the method by theoretical analysis and simulation experimentation.The results demonstrate that our compressed simulation search algorithm can help to save nearly 87.5%of the storage space than the uncompressed one.Thus under the same hardware conditions,our method can dramatically reduce the required computing nodes,and at the same time,it can simulate at least 3 qubits more than the uncompressed one.Particularly,our memory-efficient simulation method can also be used to simulate other quantum algorithms to effectively reduce the storage costs required in simulation.

Quantum information-holding single-photon router based on spontaneous emission

In this paper, we propose a single-photon router via the use of a four-level atom system coupled with two one-dimensional coupled-resonator waveguides. A single photon can be directed from one quantum channel into another by atomic spontaneous emission. The coherent resonance and the photonic bound states lead to the perfect reflection appearing in the incident channel.The fidelity of the atom is related to the magnitude of the coupling strength and can reach unit when the coupling strength matches g_a = g_b. This shows that the transfer of a single photon into another quantum channel has no influence on the fidelity at special points.

Quantum state tomography for generic pure states

We examine the problem of whether a multipartite pure quantum state can be uniquely determined by its reduced density matrices.We show that a generic pure state in three party Hilbert space HA■HB■HC, where dim(HA) = 2 and dim(HB) = dim(HC), can be uniquely determined by its reduced states on subsystems HA■HB and HA■HC. Then, we generalize the conclusion to the case that dim(H_1) > 2. As a corollary, we show that a generic N-qudit pure quantum state is uniquely determined by only two of its[(N+1)/2]-particle reduced density matrices. Furthermore,our results indicate a method to uniquely determine a generic N-qudit pure state of dimension D = d^N with only O(D) local measurements, which is an improvement compared to the previous known approach that uses O(D log^2 D) or O(D log D) local measurements.

Complete Assignment and Benchmarking of an 8-Qubit System

In this paper, the assignment of acomplex 8-spin-half system （7,7-dichloro-6-oxo-2-tio-bicycle [3.2.0] heptane-4-carboxlic acid） using nuclear magnetic resonance （NMR） techniques is presented and the hamiltonian obtained, was used to demonstrate universal control. The system has 313C and 51H,in our work, we carried out traditional 1-D and 2-D experiments and also made use of coherent control together with simulation to get the full hamiltonian of this weakly coupled system. Spin-echo J-resolved 2-D experiments were used to obtain the heteronuclear and homonuclear coupling values; COSY45 experiments were used to obtain the signs of homonuclear coupling constants. The signs of heteronuclear coupling constants were obtained using the polarization transfer method. All the data obtained in the experiments were used in the simulation of the 1-D spectra and then optimized using the least square fitting method. After obtaining the full hamiltonian of the 8-spin system, we used it in QIP, prepared pseudopure states and implemented 1-qubit and 2-qubit gates on one of its 6-qubit subsystems.

Measurement-device-independent quantum key distribution with hyper-encoding

Measurement device-independent quantum key distribution(MDI-QKD) protocols are immune to all possible attacks on the photon detectors during quantum communication, but their key generation rates are low compared with those of other QKD schemes.Increasing each individual photon’s channel capacity is an efficient way to increase the key generation rate, and high-dimensional(HD) encoding is a powerful tool for increasing the channel capacity of photons. In this paper, we propose an HD MDI-QKD protocol with qudits hyper-encoded in spatial mode and polarization degrees of freedom(DOFs). In the proposed protocol, keys can be generated using the spatial mode and polarization DOFs simultaneously. The proposed protocol is unconditionally secure,even for weak coherent pulses with decoy states. The proposed MDI-QKD protocol may be useful for future quantum secure communication applications.

NMRCloudQ： a quantum cloud experience on a nuclear magnetic resonance quantum computer

Cloud-based quantum computing is anticipated to be the most useful and reachable form for public users to experience with the power of quantum. As initial attempts, IBM Q has launched influential cloud services on a superconducting quantum processor in 2016, but no other platforms has followed up yet. Here,we report our new cloud quantum computing service – NMRCloud Q(http://nmrcloudq.com/zh-hans/),where nuclear magnetic resonance, one of the pioneer platforms with mature techniques in experimental quantum computing, plays as the role of implementing computing tasks. Our service provides a comprehensive software environment preconfigured with a list of quantum information processing packages,and aims to be freely accessible to either amateurs that look forward to keeping pace with this quantum era or professionals that are interested in carrying out real quantum computing experiments in person. In our current version, four qubits are already usable with in average 99.10% single-qubit gate fidelity and 97.15% two-qubit fidelity via randomized benchmaking tests. Improved control precisions as well as a new seven-qubit processor are also in preparation and will be available later.

Optical neural network quantum state tomography

Quantum state tomography(QST)is a crucial ingredient for almost all aspects of experimental quantum information processing.As an analog of the“imaging”technique in quantum settings,QST is born to be a data science problem,where machine learning techniques,noticeably neural networks,have been applied extensively.We build and demonstrate an optical neural network(ONN)for photonic polarization qubit QST.The ONN is equipped with built-in optical nonlinear activation functions based on electromagnetically induced transparency.The experimental results show that our ONN can determine the phase parameter of the qubit state accurately.As optics are highly desired for quantum interconnections,our ONN-QST may contribute to the realization of optical quantum networks and inspire the ideas combining artificial optical intelligence with quantum information studies.

Quantum state and process tomography via adaptive measurements

We investigate quantum state tomography(QST) for pure states and quantum process tomography(QPT) for unitary channels via adaptive measurements. For a quantum system with a d-dimensional Hilbert space, we first propose an adaptive protocol where only 2d. 1 measurement outcomes are used to accomplish the QST for all pure states. This idea is then extended to study QPT for unitary channels, where an adaptive unitary process tomography(AUPT) protocol of d2+d.1measurement outcomes is constructed for any unitary channel. We experimentally implement the AUPT protocol in a 2-qubit nuclear magnetic resonance system. We examine the performance of the AUPT protocol when applied to Hadamard gate, T gate(/8 phase gate), and controlled-NOT gate,respectively, as these gates form the universal gate set for quantum information processing purpose. As a comparison, standard QPT is also implemented for each gate. Our experimental results show that the AUPT protocol that reconstructing unitary channels via adaptive measurements significantly reduce the number of experiments required by standard QPT without considerable loss of fidelity.

Fast adiabatic method for measuring topological Chern number

Topological phases play an increasingly central role in condensed matter physics[1,2]and fault-tolerant quantum computation[3].The global nature can be characterized by certain topological invariants,many among them can be defined as the integrals of some geometric quantities.A well-known example is the Chern number[4].It is the integral of Berry curvature over a surface without boundary and is thus closely related to Berry phase[5].

Inverse Design of Strained Graphene Surfaces for Electron Control

This paper is devoted to the inverse design of strained graphene surfaces for the control of electrons in the semi-classical optical-like regime.Assuming that charge carriers are described by the Dirac equation in curved-space and exploiting the fact that wave propagation can be described by ray-optics in this regime,a general computational strategy is proposed in order to find strain fields associated with a desired effective refractive index profile.The latter is first determined by solving semi-classical trajectories and by optimizing a chosen objective functional using a genetic algorithm.Then,the graded refractive index corresponding to the strain field is obtained by using its connection to the metric component in isothermal coordinates.These coordinates are evaluated via numerical quasiconformal transformations by solving the Beltrami equation with a finite volume method.The graphene surface deformation is finally optimized,also using a genetic algorithm,to reproduce the desired index of refraction.Some analytical results and numerical experiments are performed to illustrate the methodology.

Observing quantum coherence from photons scattered in free-space

Quantum channels in free-space,an essential prerequisite for fundamental tests of quantum mechanics and quantum technologies in open space,have so far been based on direct line-of-sight because the predominant approaches for photon-encoding,including polarization and spatial modes,are not compatible with randomly scattered photons.Here we demonstrate a novel approach to transfer and recover quantum coherence from scattered,non-line-of-sight photons analyzed in a multimode and imaging interferometer for time-bins,combined with photon detection based on a 8×8 single-photon-detector-array.The observed time-bin visibility for scattered photons remained at a high 95%over a wide scattering angle range of−45^(0) to+45^(0),while the individual pixels in the detector array resolve or track an image in its field of view of ca.0.5°.Using our method,we demonstrate the viability of two novel applications.Firstly,using scattered photons as an indirect channel for quantum communication thereby enabling non-line-of-sight quantum communication with background suppression,and secondly,using the combined arrival time and quantum coherence to enhance the contrast of low-light imaging and laser ranging under high background light.We believe our method will instigate new lines for research and development on applying photon coherence from scattered signals to quantum sensing,imaging,and communication in free-space environments.

Quantum non-demolition phonon counter with a hybrid optomechnical system

A phonon counting scheme based on the control of polaritons in an optomechanical system is proposed. This approach permits us to measure the number of phonons in a quantum non-demolition(QND) manner for arbitrary modes not limited by the frequency matching condition as in usual photon-phonon scattering detections. The performance on phonon number transfer and quantum state transfer of the counter are analyzed and simulated numerically by taking into account all relevant sources of noise.

Spin-preserving chiral photonic crystal mirror

Chirality refers to a geometric phenomenon in which objects are not superimposable on their mirror image.Structures made of nanoscale chiral elements can exhibit chiroptical effects,such as dichroism for left-and right-handed circularly polarized light,which makes these structures highly suitable for applications ranging from quantum information processing and quantum optics to circular dichroism spectroscopy and molecular recognition.At the same time,strong chiroptical effects have been challenging to achieve even in synthetic optical media,and chiroptical effects for light with normal incidence have been speculated to be prohibited in thin,lossless quasi-two-dimensional structures.Here,we report an experimental realization of a giant chiroptical effect in a thin monolithic photonic crystal mirror.Unlike conventional mirrors,our mirror selectively reflects only one spin state of light while preserving its handedness,with a near-unity level of circular dichroism.The operational principle of the photonic crystal mirror relies on guided-mode resonance(GMR)with a simultaneous excitation of leaky transverse electric(TE-like)and transverse magnetic(TM-like)Bloch modes in the photonic crystal slab.Such modes are not reliant on the suppression of radiative losses through long-range destructive interference,and even small areas of the photonic crystal exhibit robust circular dichroism.Despite its simplicity,the mirror strongly outperforms earlier reported structures and,contrary to a prevailing notion,demonstrates that near-unity reflectivity contrast for opposite helicities is achievable in a quasi-two-dimensional structure.

计算的极限

计算深刻地影响着人们的日常生活和生产活动,也推动了诸多其他科学领域的发展和变革.本文从几个不同的方面探讨计算的能力和极限.从计算的模型和丘奇图灵论题,到P和NP问题的深远影响及量子计算对传统计算的冲击,我们深入讨论了对计算极限的理解.