Quantum transmission of atoms through a slab of superfluid helium

We describe a measurement of the transmission probability of 4He atoms through a freely suspended slab of superfluid 4He at low temperatures. In our experiment the slab is realized by using an array of parallel cylindrical holes of diameter 51 microm in a glass disc of thickness 190 microm. By controlling the chemical potential, the holes can be made to fill or empty with liquid, and the surface curvature varied. We have measured the transmission of atom beams, generated by a thin-film heater and detected with a sensitive bolometer, through this structure. The results show that the dominant transmission channel is atom-R+ roton-atom with a probability p approximately 0.12 and that R+ rotons can undergo total internal reflection at the free liquid surfaces.     

Quantum communication through an unmodulated spin chain

We propose a scheme for using an unmodulated and unmeasured spin chain as a channel for short distance quantum communications. The state to be transmitted is placed on one spin of the chain and received later on a distant spin with some fidelity. We first obtain simple expressions for the fidelity of quantum state transfer and the amount of entanglement sharable between any two sites of an arbitrary Heisenberg ferromagnet using our scheme. We then apply this to the realizable case of an open ended chain with nearest neighbor interactions. The fidelity of quantum state transfer is obtained as an inverse discrete cosine transform and as a Bessel function series. We find that in a reasonable time, a qubit can be directly transmitted with better than classical fidelity across the full length of chains of up to 80 spins. Moreover, our channel allows distillable entanglement to be shared over arbitrary distances.     

Quantum transport through a multi-quantum-dot-pair chain side-coupled with Majorana bound states

We investigate the quantum transport properties through a special kind of quantum dot(QD) system composed of a serially coupled multi-QD-pair(multi-QDP) chain and side-coupled Majorana bound states(MBSs) by using the Green functions method,where the conductance can be classified into two kinds:the electron tunneling(ET) conductance and the Andreev reflection(AR) one.First we find that for the nonzero MBS-QDP coupling a sharp AR-induced zero-bias conductance peak with the height of e~2/h is present(or absent) when the MBS is coupled to the far left(or the other) QDP.Moreover,the MBS-QDP coupling can suppress the ET conductance and strengthen the AR one,and further split into two sub-peaks each of the total conductance peaks of the isolated multi-QDPs,indicating that the MBS will make obvious influences on the competition between the ET and AR processes.Then we find that the tunneling rate ΓLis able to affect the conductances of leads L and R in different ways,demonstrating that there exists a ΓL-related competition between the AR and ET processes.Finally we consider the effect of the inter-MBS coupling on the conductances of the multi-QDP chains and it is shown that the inter-MBS coupling will split the zero-bias conductance peak with the height of e~2/h into two sub-peaks.As the inter-MBS coupling becomes stronger,the two sub-peaks are pushed away from each other and simultaneously become lower,which is opposite to that of the single QDP chain where the two sub-peaks with the height of about e~2/2h become higher.Also,the decay of the conductance sub-peaks with the increase of the MBS-QDP coupling becomes slower as the number of the QDPs becomes larger.This research should be an important extension in studying the transport properties in the kind of QD systems coupled with the side MBSs,which is helpful for understanding the nature of the MBSs,as well as the MBS-related QD transport properties.     

Seeing time-reversal transmission characteristics through kinetic anti-ferromagnetic Ising chain

As an example of our new approach to complex near-field (NF) scattering of electromagnetic waves, the time-reversal (TR) transmission process on an NF current-element array is mapped to the statistical process on a kinetic Ising transmission chain. Equilibrium statistical mechanics and non-equilibrium Monte Carlo (MC) dynamics help us to find signal jamming, aging, annihilating, creating, and TR symmetry breaking on the chain with inevitable background noises; and these results are general in NF systems where complex electromagnetic scattering arises.     

Improvement of signal transmission through spike-timing-dependent plasticity in neural networks

We explore the effects of spike-timing-dependent plasticity (STDP) on weak signal transmission in a noisy neural network. We first consider the network where an ensemble of independent neurons, which are subjected to a common weak signal, are connected in parallel to a single postsynaptic neuron via excitatory synapses. STDP can make the signal transmission more efficient, and this effect is more prominent when the presynaptic activities exhibit some correlations. We further consider a two-layer network where there are only couplings between two layers and find that postsynaptic neurons can fire synchronously under suitable conditions. Both the reliability and timing precision of neuronal firing in the output layer are remarkably improved with STDP. These results indicate that STDP can play crucial roles in information processing in nervous systems.Received: 23 March 2004, Published online: 12 July 2004PACS: 87.18.Sn Neural networks - 87.17.Aa Theory and modeling; computer simulation     

Quantum information transmission between two qubits through an intermediary photon gas

The theory of the quantum information transmission between two semiconductor two-level quantum dots as two qubits through an intermediary photon gas in a cavity is presented. The reduced density matrix of each two-level quantum dot is the quantum information encoded into this qubit. The quantum information exchange between two distant qubits imbedded in the photon gas is performed in the form of the mutual dependence of their reduced density matrices due to the interaction between the electrons in the qubits and the photon gas. The system of rate equations for the reduced density matrix of the two-qubit system is derived. From the solution of this system of equations it follows the mutual dependence of the reduced density matrices of two distant qubits.     

Non-Equilibrium Quantum Transport of Bosons through a Quantum Dot

The quantum dot coupled to reservoirs is known as a typical mesoscopic setup to manifest the quantum characteristics of particles in transport. In analogue to many efforts made on the study of electronic quantum dots in the past decades, we study the transport of bosons through such a device. We first generalize the formula which relates the current to the local properties of dot in the bosonic situation. Then, as an illustrative example, we calculate the local density of state and lesser Green function of the localized boson with a bosonic Fano-Anderson model. The current-voltage （I - V） behaviour at zero temperature is presented, and in the bosonic dot it is the I - V curve, in contrast to the differential conductance in the electronic dot, which is found to be proportional to the spectral function.     

Quantum transmission across a low-dimensional nanoscale diffuse junction

The transmission across a nano-scale diffuse junction between two metals is studied numerically for 1D wires and 2D ribbons within a tight-binding model. In such a situation, a junction electric potential must appear to equilibrate the electrochemical potential on each side of the junction. We compute the transmission under two very different potential distributions and we find that the typical transmission does not depend on the details of this distribution. The transmission is found to follow a scaling law. The variations of the 1D transmission are explained in a semi-classical way by considering the multiple reflections caused by the barriers due to the on-site energy mismatch between the two metals. Both the 1D and 2D results can also be partly interpreted in the frame of the Anderson theory of localisation.     

Quantum vibrational impurity embedded in a one-dimensional chain

We perform a fully quantum mechanical numerical calculation for the problem of a single electron (or excitation) propagating in an N-site one-dimensional chain in the presence of a single Holstein impurity. We compute the long-time averaged probability for finding the electron on the impurity site as a function of the nonlinearity parameter, defined in terms of the electron-phonon coupling strength and the oscillator frequency. The results, in the intermediate nonlinearity parameter range, differ substantially from the ones obtained through the use of the discrete nonlinear Schrödinger equation, even in the high-frequency regime.     

Fano resonance and wave transmission through a chain structure with an isolated ring composed of defects

We consider a discrete model that describes a linear chain of particles coupled to an isolated ring composed of N defects.This simple system can be regarded as a generalization of the familiar Fano-Anderson model.It can be used to model discrete networks of coupled defect modes in photonic crystals and simple waveguide arrays in two-dimensional lattices.The analytical result of the transmission coefficient is obtained,along with the conditions for perfect reflections and transmissions due to either destructive or constructive interferences.Using a simple example,we further investigate the relationship between the resonant frequencies and the number of defects N,and study how to affect the numbers of perfect reflections and transmissions.In addition,we demonstrate how these resonance transmissions and refections can be tuned by one nonlinear defect of the network that possesses a nonlinear Kerr-like response.     

Fano resonance and wave transmission through a chain structure with an isolated ring composed of defects

We consider a discrete model that describes a linear chain of particles coupled to an isolated ring composed of N defects. This simple system can be regarded as a generalization of the familiar Fano-Anderson model. It can be used to model discrete networks of coupled defect modes in photonic crystals and simple waveguide arrays in two-dimensional lattices. The analytical result of the transmission coefficient is obtained, along with the conditions for perfect reflections and transmissions due to either destructive or constructive interferences. Using a simple example, we further investigate the relationship between the resonant frequencies and the number of defects N, and study how to affect the numbers of perfect reflections and transmissions. In addition, we demonstrate how these resonance transmissions and refections can be tuned by one nonlinear defect of the network that possesses a nonlinear Kerr-like response.     

用三粒子纠缠态作量子信道远程操纵单比特旋转

提出用三粒子纠缠态作量子信道远程操纵单比特旋转的理论方案。首先，我们利用最大纠缠的GHZ态的性质远程操纵单比特旋转，其保真度和成功几率均为1。 我们还提出了两个用部分纠缠的GHZ态作量子信道实现保真度为1的远程操纵单比特旋转的方案。这些方案的特点是，两地之间还存在一第三者，他作为监控方参与量子远程操纵过程，特别地，当量子信道为部分纠缠态时，他能矫正被非理想量子信道致畸的量子态。除了GHZ型态外，我们还证明了W型态亦可用作量子信道远程操纵单比特旋转，但后者的成功几率总是小于前者。     

Quantum formalism via signal analysis

The general properties of signals permit a nonaxiomatic reconstruction of the quantum probability formalism independent of the standard Copenhagen interpretation of quantum mechanics. Performance standards are specified for candidate clock, signaller, and reflector devices, and it is shown that the resulting formalism forces identification of a probability- or intensity-like structure as the absolute square of an amplitude, the relative phases of amplitudes appearing explicitly in the probability composition law. Inequalities are produced which on one interpretation reduce to the Heisenberg relations, but it is pointed out that this reconstruction disqualifies position as an observable property of a signal.     

Quantum transport through a Z-shaped silicene nanoribbon

In this work,the electronic transport properties of Z-shaped silicene nanoribbon(ZsSiNR) structure are investigated.The calculations are based on the tight-binding model and Green's function method in Landauer-Biittiker formalism,in which the electronic density of states(DOS),transmission probability,and current-voltage characteristics of the system are calculated,numerically.It is shown that the geometry of the ZsSiNR structure can play an important role to control the electron transport through the system.It is observed that the intensity of electron localization at the edges of the ZsSiNR decreases with the increase of the spin-orbit interaction(SOI) strength.Also,the semiconductor to metallic transition occurs by increasing the SOI strength.The present theoretical results may be useful to design silicene-based devices in nanoelectronics.     

Quantum correlations in a long range interaction spin chain

We propose a new type of long range interaction spin chain. The quantum correlations such as quantum discord, entanglement, and structure factor are investigated in the thermal state with considering them both in zero temperature and finite temperature. Based on our results, we compare the differences and show the relations between the three types of quantum correlations in this long range interaction model.     

The non—local implementation of a CNOT gate and single—qubit rotation

We show how a CNOT gate and single-qubit rotation can be implemented non-locally,We also report on the quantitative relations between these quantum actions,entanglement and classical communication resources required in the implementation.     

Real-time video signal transmission over a terahertz communication link

A 2.4-m communication link operating at 3.9 THz based on a terahertz quantum cascade laser and a terahertz quantum well photodetector （THz QWP） are introduced. The lumped electrical models of THz QWP for small signals are presented. A discussion of the bandwidth limit of the detecting circuit is presented. Using direct on off-keying modulation and intensity detection, the transmission of digital video signal with a data rate of 2.5 Mb/s is demonstrated. Pseudo-random binary sequences are transmitted to investigate the bit-error rate （BER） at different rates. Result shows the error free transmission when the rate is below 5 Mb/s.     

Ultrahigh light transmission through a C-shaped nanoaperture

Optical resolution beyond the diffraction limit can be achieved by use of a metallic nanoaperture in a near-field optical system. Conventional nanoapertures have very low power throughput. Using a numerical finite-difference time domain method, we discovered a unique C-shaped aperture that provides approximately 3 orders of magnitude more power throughput than a conventional square aperture with a similar near-field spot size of approximately 0.1 lambda. Microwave experiments at 6 GHz quantitatively confirmed the simulated transmission enhancement. The high transmission of the C-aperture--or one of the related shapes--is linked to both a propagation mode in the aperture and local surface plasmons.