超导量子比特

随着2019年谷歌成功实现了"量子优势",超导量子计算的研究正引起人们更加广泛的关注.超导量子比特是拥有量子化能级、量子态叠加和量子态纠缠等量子力学特性的宏观器件,目前被广泛应用于量子物理、原子物理、量子光学、量子化学、量子模拟和量子计算等诸多领域中.本文将重点讨论位相、电荷、传输子以及磁通型超导量子比特的基本原理及其...     

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.     

高稳定性Tl2Ba2CaCu2Ox薄膜DC-SQUID

与YBCO薄膜相比,Tl2Ba2CaCu2Ox薄膜具有更高的Tc和更好的抗湿性能,所以Tl2Ba2CaCu2Ox薄膜器件不仅在液氮温度下具有更好的稳定性,而且在室温下具有更长的存放寿命.我们用磁控溅射的方法分别在24°和36.8°的STO双晶衬底上外延出Tl2Ba2CaCu2Ox薄膜并制备成晶界结DC-SQUID,比较了二者由于晶界夹角不同而产生的性能差异,并给出可工作在95K的DC-SQUID干涉曲线;对工作在77K的Tl2Ba2CaCu2Ox和YBCO薄膜双晶DC-SQUID,比较了二者在噪声特性及临界电流和结电阻随温度变化等特性,并对结果作了讨论.     

Shapiro Steps in Flux-Trapped Surface Intrinsic Junctions of Bi2Sr2CaCu2O8＋δ

Microwave-field responses of the surface intrinsic Josephson junctions (IJJs) of Bi2Sr2CaCu2O8 δ superconductors are investigated. The IJJs are fabricated using an in situ low-temperature cleavage technique, which leads to the well-characterized surface CuO2 double layers and surface junctions. For the surface junctions in the largejunction limit, usually no Shapiro steps appear when a microwave field is applied. It is found that when the junctions are in a flux-trapped state, which is produced by a pulsed current and in which the critical current is significantly suppressed, clear Shapiro steps can be observed. These results are important for the study of the microwave-field properties of vortex-carrying IJJs and may find their use in device applications.     

Fiske steps in surface intrinsic Josephson junctions of Bi2Sr2CaCu2O8+δ single crystals

Current－voltage characteristics of the surface intrinsic Josephson junctions under a magnetic field parallel to CuO_2 planes have been studied experimentally. Fiske steps in such a junction have been observed. The Swihart velocity for the junction is found to be 7.2×10^5m/s, which corresponds to the velocity mode of a single junction.     

本征约瑟夫森结跳变电流分布的量子修正

在约瑟夫森结跳变电流统计分布的理论拟合过程中,通常考虑的是宏观量子隧穿与热激活这两种过程. 对Bi₂Sr₂CaCu₂O_8+δ表面本征约瑟夫森结的结果分析表明,在宏观量子隧穿与热激活的交界区域内,若考虑量子修正能使实验曲线与理论曲线符合得更好. 这种较为完整的拟合方法,对研究本征约瑟夫森器件中的宏观量子现象及其在超导量子比特中的应用具有积极的意义. 关键词： 约瑟夫森结 跳变电流分布 量子修正     

Quantum Spontaneous Magnetization in Single-Junction Superconducting π-Rings

Based upon the calculations of energy levels and quantum states, we discuss the probability of quantum spontaneous magnetization flux as a function of the screen parameter β for the superconducting ring containing one Josephson π-junction, and compare the result with that in the classical case. Our results indicate that there is significant difference in the magnetization behaviour aroundβ = 1 between the quantum and classical situations.     

电子型掺杂高温超导体La2-xCexCuO4飞秒时间分辨动力学研究

利用飞秒时间分辨光抽运探测技术研究了电子型掺杂La_2-xCe_xCuO₄(LCCO)高温超导材料的准粒子超快动力学过程.得到低温(T<0.7T_c)、转变温度附近(0.7T_c≤T≤T_c)和高温(T>T_c)三个温区内的动力学行为.研究发 关键词： 电子型掺杂高温超导体 飞秒时间分辨 准粒子 声子瓶颈     

超导薄膜磁场穿透深度的精确测量

磁场穿透深度λ是超导体的一个基本参数,目前,一般测试穿透深度的实验只能给出其变化量Δλ＝λ（Ｔ）－λ（０）,而不能得到λ的绝对值．并且由于测量精度的限制,用给定的理论模型拟合实验结果而得到的λ也有很大的不确定性．本文对此进行了详细的分析,并用双线圈互感法研究了超导薄膜穿透深度的精确测量,并给出了磁控溅射Ｎｂ膜的测量结果．我们的研究表明,这一方法能较为准确地给出λ的绝对值,从而避免了以往的测量及拟合所导致的不确定性．基于ＢＣＳ理论并考虑样品有限的电子平均自由程后,理论计算结果与我们的测量结果吻合较好     

Tunneling spectra of underdoped Bi_2Sr_2Ca_(1-x)Y_xCu_2O_(8+δ) intrinsic Josephson junctions

<正>Mesa-structured submicron intrinsic Josephson junctions are successfully fabricated and well characterized on underdoped Bi_2Sr_2Ca_(1-x)Y_xCu-2O_(8+δ) single crystals with a T_c of 80 K.Tunneling spectra at the temperatures ranging from 4.2 K to 295 K are measured.A pulse technique is used to reduce sample heating for the measurement near pseudogap opening temperature T~*~280 K.Our experimental results show that the superconducting gap,the peak-dip separation,and the pseudogap opening temperature are all increased as compared with those from near optimally doped samples,which requires further theoretical analysis in the future.