基于金刚石氮-空位色心自旋系综与超导量子电路混合系统的量子节点纠缠

量子纠缠是实现量子计算和量子通信的核心基础,本文提出了在金刚石氮-空位色心(NV centers)自旋系综与超导量子电路耦合的混合系统中实现两个分离量子节点之间纠缠的理论方案.在该混合系统中,把金刚石NV centers自旋系综和与之耦合的超导共面谐振器视为一个量子节点,两个量子节点之间通过一个空的超导共面谐振器连接.具有较长相干时间的NV centers自旋系综作为一个量子存储器,用于制备、存储和发送量子信息;易于外部操控的超导量子电路可执行量子逻辑门操作,快速调控量子信息.为了实现两个分离量子节点之间的纠缠,首先对系统的哈密顿量进行正则变换,将其等价为两个NV centers自旋系综与同一个超导共面谐振器之间的JC耦合;然后采用NV centers自旋-光子混合比特编码的方式,通过调节超导共面谐振器的谐振频率,精确控制体系演化时间,高保真度地实现了两个分离量子节点之间的量子纠缠.本方案还可以进一步扩展和集成,用于构建多节点纠缠的分布式量子网络.     

Long plasma channels and high-voltage discharges induced by strong picosecond laser pulses

The formation of long plasma channels and laser-induced high-voltage discharges are demonstrated by focusing infrared picosecond laser pulses in air. Based on measurements of the channel conductivity, the maximum electron density in excess of 1014cm-3is estimated. The plasma channels are good conductors, through which long-air-gap high-voltage discharges are triggered. The breakdown voltages show large drops but the discharging paths are not well guided: in this, the plasma spots distributed along the channel might play an important role.     

Current induced intermediate state in thin film superconductors

High-resolution magneto-optical studies of the current-induced intermediate state in superconducting lead films indicate a magnetic structure consisting of chains of flux tubes rapidly moving along channels from both edges to the center of the strip. At increasing levels of the transport current new flux tube channels are created abruptly, resulting in relatively large resistive voltage steps.     

Induced current of high temperature superconducting loops by combination of exciting coil and thermal switch

With its commercialization, the second-generation (2G) high temperature superconducting (HTS) RE—Ba—Cu—O (REBCO, RE is rare earth) tape is extensively applied to the superconducting magnets in the high magnetic fields. However, unlike low temperature superconducting (LTS) magnets, the HTS magnet cannot operate in the persistent current mode (PCM) due to the immature superconducting soldering technique. In this paper, an exciting method for two HTS sub-loops, so-called charging and load loops, is proposed by flux pump consisting of exciting coil and controllable thermal switch. Two HTS sub-loops are made of an REBCO tape with two slits. An exciting coil with iron core is located in one sub-loop and is supplied with a triangular waveform current so that magnetic field is generated in another sub-loop. The influence of magnetic flux on induced current in load loop is presented and verified in experiment at 77 K. The relationship between the induced magnetic flux density and the current on the sub-loops having been calibrated, magnetic flux density, and induced current are obtained. The results show that the HTS sub-loops can be excited by a coil with thermal switch and the induced current increases with magnetic flux of exciting coil increasing, which is promising for persistent current operation mode of HTS magnets.     

超导磁体的磁通泵供电原理研究

磁通泵是给超导磁体供电的装置.本文主要介绍磁通泵的国内外发展情况,论述磁通泵的运行原理,计算磁通泵的运行参数和运行损耗.     

超导线圈中的感应电流

通过和普通导体线圈比较,从超导线圈内磁通量保持不变的性质出发,讨论了不同情况下超导线圈中感应电流的大小、方向和持续时间等方面的特点.     

Superconductivity in the exactly solvable model of pseudogap state: The absence of self-averaging

The features of the superconducting state are studied in the simple exactly solvable model of the pseudogap state induced by fluctuations of the short-range “dielectric” order in the model of the Fermi surface with “hot” spots. The analysis is carried out for arbitrary short-range correlation lengths ξ_corr. It is shown that the superconducting gap averaged over such fluctuations differs from zero in a wide temperature range above the temperature T _c of the uniform superconducting transition in the entire sample, which is a consequence of non-self-averaging of the superconducting order parameter over the random fluctuation field. In the temperature range T>T _c, superconductivity apparently exists in individual regions (drops). These effects become weaker with decreasing correlation length ξ_corr; in particular, the range of existence for drops becomes narrower and vanishes as ξ_corr → 0, but for finite values of ξ_corr, complete self-averaging does not take place.     

Modelling of DC Electric Fields Induced by RF Sheath in Front of ICRF Antenna

Reducing the ICRH (ion cyclotron range frequency) antenna-plasma interaction is one of the key points for reaching very long tokamak discharges. One problem which limits such discharges, is the appearance of hot spots on the surface of the antenna: Radio Frequency (RF) sheaths modify the properties of the edge plasma by rectifying the RF potential along open magnetic field lines and can induce hot spots. This paper investigates the corrections to sheath potentials introduced by the interactions between adjacent flux tubes. Our theoretical study started from an oscillating double Langmuir probe model, in which a transverse influx of current was included. This model was confronted with 1D PIC simulations along a magnetic field line, and demonstrated that current exchanges can decrease mean potentials. A 2D electrostatic fluid code was then developed, which couples adjacent flux tubes in a poloidal cross section with collisional conductivity or polarization currents. It showed that transverse currents are able to smooth structures smaller than a characteristic size in the sheath potential maps (results for Tore Supra). These computed rectified potentials can be used to obtain the DC electric fields in front of the antenna. And then, it gives an estimate of the particle drift and the energy flux on the antenna structure, which can explain hot spots.     

Low temperature phase transition in the chain-like compounds NbSe3 and TaSe3

New transitions have been observed at low temperature by electrical measurements on the linear trichalcogenides NbSe₃ and TaSe₃. The resistivity of TaSe₃ drops near zero below 1.5 K which may indicate a superconducting transition. The resistivity of NbSe₃ drops below 2.2 K by 30 to 75% of its residual value at 4.2 K. However for TaSe₃ and NbSe₃ initial susceptibility measurements do not show any increase in diamagnetism indicating a flux expulsion. In addition the transitions below the critical temperatures are strongly non-linear with current density higher than 10^-3 A mm^-2.     

Superconducting state evolution with applied magnetic flux in mesoscopic rings

The magnetic flux dependence of the vortex state for small mesoscopic superconducting rings surrounded by a medium is investigated by the phenomenological Ginzburg-Landau theory. The influences of the ring size and the surface superconductivity on the free energy and total supercurrent are studied. For narrow rings, the persistent current evolves towards a periodic behaviour with magnetic flux. The complete paramagnetic or diamagnetic state, corresponding to positive or negative current flowing in the whole ring, can occur. A remarkable intermittent superconducting behaviour for the ground-state transition is found when the strength of surface-suppressed superconductivity is enlarged or the ring size is decreased. Consequently, a pure superconducting state with positive total current can be obtained.     

低温/高温复合超导体中的电流分布的研究

低温/高温复合超导体是将高温超导体部分取代复合实用低温超导体中的金属稳定材料或两者直接复合成一体,这种复合超导体具有稳定性高,工程电流密度大等优点.本文对NbTi/Bi2223高温复合超导体中的电流分布进行了理论研究,并得出液氦温度下,如果复合超导导体中,低温超导体和高温超导体具有相同的临界电流,超导体正常运行时低温超导体中的电流大于高温超导体中的电流,两者之比随运行电流的升高而降低.     

一种电流引线的热力学计算方法

从超导磁体气冷电流引线的热平衡方程出发,对电流引线进行分段,提出了一种较为精确计算电流引线长横比及由电流引线末端流入低温容器热量的计算方法;电流引线中氦气流阻是设计电流引线时一个很重要的参数,由于电流引线片形状很复杂,计算其中氦气流阻比较好的方法是采用CFD软件Fluent。氦气模型单元数很庞大,因此对氦气模型进行了简化和分段,相邻两段模型间采用流量和压力边界条件进行耦合。     

Observation of the external-ac-current-induced dc voltage proportional to the steady current in superconducting loops

A dc voltage induced by an external ac current was observed in a system of asymmetric aluminum loops at temperatures corresponding to 0.95–0.98 of the superconducting transition temperature. The voltage magnitude and sign change periodically in a magnetic field with a period corresponding to the magnetic flux quantum through the loop. The amplitude of these oscillations depends nonmonotonically on the amplitude of ac current and is almost independent of its frequency in the range from 100 Hz to 1 MHz. The observed phenomenon is interpreted as the result of displacing the loop into a dynamic resistive state by the external current, where the loop is “switched” back and forth between the closed superconducting state with a nonzero steady current and the nonclosed state with a nonzero resistance along the loop circle. It is shown that voltages are summed up in a system of loops connected in series. For systems with one, three, and twenty loops, the voltage reaches 10, 40, and 300 μ V, respectively.     

Quasiparticle relaxation in optically excited high-Q superconducting resonators

The quasiparticle relaxation time in superconducting films has been measured as a function of temperature using the response of the complex conductivity to photon flux. For tantalum and aluminum, chosen for their difference in electron-phonon coupling strength, we find that at high temperatures the relaxation time increases with decreasing temperature, as expected for electron-phonon interaction. At low temperatures we find in both superconducting materials a saturation of the relaxation time, suggesting the presence of a second relaxation channel not due to electron-phonon interaction.     

Vertical Josephson interferometers for quantum computation

We characterize a niobium-based vertical Josephson interferometer which we propose to include in a superconducting loop for applications to quantum computation using flux qubits. The most interesting feature of this device is that the Josephson current is precisely modulated by a small transversal magnetic field parallel to superconducting loop plane from a maximum to zero, with fine control and precision. This device can be used to independently control the off-diagonal Hamiltonian terms of flux qubits and/or to control the flux transfer function of a superconducting transformer for inter-qubits coupling.     

Observation of flux qubit states with the help of a superconducting differential double contour interferometer

The critical current of a new structure, the superconducting differential double contour interferometer (DDCI), investigated recently, depends on the parity of the sum of quantum numbers of the two superconducting loops connected in two points by two Josephson junctions. The theory confirms that the DDCI structure can be used for the ideal readout of quantum states of the flux qubit. Large jumps in the critical current and voltage enables to observe continuously the change in time the state of the flux qubit. Such observations can have fundamental importance for the investigation of macroscopic quantum systems with strongly discrete spectrum such as the flux qubit. The DDCI structure can also be used for precise measurement of a very weak magnetic field.     

Sudden critical current drops induced in S/F structures

In the search for new physical properties of S/F structures, we have found that the superconductor critical current can be controlled by the domain state of the neighboring ferromagnet. The superconductor is a thin wire of thickness d_s ≈2ξ_S. Nb/Co and Nb/Py (Permalloy Ni₈₀Fe₂₀) bilayer structures were grown with a significant magnetic anisotropy. Critical current measurements of Nb/Co structures with ferromagnet thickness d_F > 30 nm show sudden drops in two very defined steps when the measurements are made along the hard axes direction (i.e. current track parallel to hard anisotropy axes direction). These drops disappear when they are made along the easy axis direction or when the ferromagnet thickness is below 30 nm. The drops are accompanied by vortex flux flow. In addition magnetorestistance measurements close to T_C show a sharp increase near saturation fields of the ferromagnet. Similar results are reproduced in Nb/Py bilayer structure with the ferromagnet thickness d_F ~ 50 nm along the easy anisotropy axes. These results are explained as being due to spontaneous vortex formation and flow induced by Bloch domain walls of the ferromagnet underneath. We argue these Bloch domain walls produce a 2D vortex-antivortex lattice structure.     

Spectroscopy and coherent manipulation of single and coupled flux qubits

Measurements of three-junction flux qubits, both single flux qubits and coupled flux qubits, using a coupled direct current superconducting quantum interference device （dc-SQUID） for readout are reported. The measurement procedure is described in detail. We performed spectroscopy measurements and coherent manipulations of the qubit states on a single flux qubit, demonstrating quantum energy levels and Rabi oscillations, with Rabi oscillation decay time TRabi =- 78 ns and energy relaxation time T~ = 315 ns. We found that the value of TRabi depends strongly on the mutual inductance between the qubit and the magnetic coil. We also performed spectroscopy measurements on inductively coupled flux qubits.     

缓慢退磁的超导圆盘中的俘获场

本文提出了一个系统的理论框架计算通过场冷过程磁化的超导圆盘中的俘获场和温度分布.此计算基于电流运动方程,考虑到电流密度是温度和磁场的函数,考虑到在正常区和超导区由磁通运动产生的热耗散.计算结果和最近不同研究组的实验结果一致.     

Superconductivity in the pseudogap state in the hot-spot model: Ginzburg-Landau expansion

Peculiarities of the superconducting state (s and d pairing) are considered in the model of the pseudogap state induced by short-range order fluctuations of the dielectric (AFM (SDW) or CDW) type, which is based on the model of the Fermi surface with “hot spots.” A microscopic derivation of the Ginzburg-Landau expansion is given with allowance for all Feynman diagrams in perturbation theory in the electron interaction with short-range order fluctuations responsible for strong scattering in the vicinity of hot spots. The superconducting transition temperature is determined as a function of the effective pseudogap width and the correlation length of short-range order fluctuations. Similar dependences are derived for the main parameters of a superconductor in the vicinity of the superconducting transition temperature. It is shown, in particular, that the specific heat jump at the transition point is considerably suppressed upon a transition to the pseudogap region on the phase diagram.