Josephson结跳变电流在不同温度下的统计分布

本工作提出了一种通过对时间的测量来获得单个Josephson结的跳变电流统计分布P(I)的电路,该电路具有很好的精度和速度.利用该电路我们首次从4.2K到30mK的不同温度下,对NbN/AlN/AbN结和Nb/AlOx/Nb结的跳变电流统计分布进行了测量,得到了一系列P(I).在一定的温度范围内,P(I)和理论值符合的很好.该电路结合其他的实验手段,将在今后研究Josephson结其他物理性质(例如Rabi振荡)中发挥很好的作用.     

超导Josephson隧道结性能和超导量子比特电路的设计方案

超导Josephson隧道结是实现超导量子比特的基本元件。利用悬空掩膜和电子束斜蒸发相结合的工艺方法制备Al/Al2O3/Al超导Josephson隧道结,并且系统研究了底电极、上电极薄膜的厚度及氧化参数等工艺条件与隧道结超导电流密度Jc和面积归一化电阻Rc的关系。设计测量了三种方案的超导量子比特电路,通过对参数和结构的优化测出了较理想的量子比特(qubit)信号。     

基于超导量子干涉仪与介观LC共振器耦合电路的量子通信

量子计算如何在实验上实现一直受到广泛关注. 包括超导Josephson结的小量子器件(如超导量子干涉仪, SQUID)是实现量子计算的一种非常具有发展前景的物理系统. 本文通过对SQUID和介观LC共振器耦合电路系统的Cooper对数-相量子化机制的讨论, 合理地调制参数, 由此导出了该耦合电路在两能级近似下的J-C模型形式, 并提出了一种基于此模型的可实现量子信息传递的方案. 根据此方案可以利用介观LC共振器为数据总线来执行两SQUID间电荷量子比特的传递.     

Al/Al_2O_3/Al隧道结相位扩散的现象

超导约瑟夫森结是超导量子比特的核心元件,由约瑟夫森结组成的直流超导量子干涉器(DC-SQUID)由于其具有较高的探磁灵敏度,在超导磁通量子比特量子位测量中具有重要的作用.我们用铝为超导材料,采用电子束斜蒸发及静态氧化的方法制备了由两个Al/Al2O3/Al隧道结并联组成的DC-SQUID样品.将DC-SQUID样品置于20~1000mK范围内的不同温度下,对其跳变电流分布进行了测量.在50mK明显观测到了隧道结跳变电流机制由宏观量子隧穿到热激发的转变,并且在高于80mK时,观测到了相位扩散的现象.这对于隧道结相位动力学研究具有重要意义.     

约瑟夫森结跳变电流在微波辐照下的分布

在16 mK的极低温下,我们对电流偏置的Nb/AlOx/Nb结进行了跳变电流统计分布的测量.在没有外加微波的辐照时,分布曲线只有一个峰;在有外加微波的辐照时,其跳变电流的统计分布除了基峰外,会出现谐振峰,形成双峰结构.该谐振峰的位置和结的量子化的能级相关,其高度随外加微波功率的改变而改变.研究约瑟夫森结跳变电流在微波辐照下的分布,对于研究超导量子比特的能级量子化、Rabi振荡等具有重要的意义.     

通过设计量子库来调控量子比特间的Bell非定域性研究

量子系统间的Bell非定域性是一种比量子纠缠更为严格的量子关联,它在刻画多体量子关联有着不可或缺的作用.类似于量子纠缠,在开放两量子比特和三量子比特系统中的Bell非定域性可能会出现猝死现象.本文建议了一个可供选择的方案即在热库环境中通过增加辅助粒子来调控两量子比特和三量子比特间的Bell非定域性动力学.研究发现：通过调节辅助粒子数目,不仅两量子比特和三量子比特系统的Bell非定域性可以避免猝死现象的发生,而且在长时间极限下它能维持在一个较高水平.论文得到的结果将对多体量子系统间量子关联的调控和避免猝死现象等相关研究有积极的指导意义.     

通过四比特 态实现任意量子态的量子隐形传态方案

提出了一个将四比特|χ〉态作为量子通道实现任意单量子比特和两量子比特的量子态的隐形传送方案.该方案依赖于两个通信站点之间的纠缠.在这个方案里,我们给出了Alice的测量结果以及Bob进行的相应的幺正操作,计算结果表明,该隐形传送方案是完美的,也就是说它的成功概率可达到1.此外,该方案中用到的测量以及纠缠通道的制备在目前的技术下是完全可行的.因此,我们的方案有望在实验上实现.     

量子比特的普适远程翻转和克隆

提出一种把量子隐形传态、最佳普适量子比特翻转和最佳普适量子克隆三者结合起来的量子比特普适远程翻转和克隆方案.当发送者和处于不同地点的三个接收者共享一个特定的四粒子纠缠态作为量子信道时,通过发送者的Bell基测量、经典通信和各个接收者的局域幺正变换,一个接收者能够以2/3的最佳保真度得到一份原未知量子比特的正交补态,另外两个接收者能够分别以5/6的最佳保真度得到原未知量子比特的一份拷贝.此方案用较少的量子纠缠资源同时完成了未知量子比特的普适远程翻转和克隆,且其保真度分别达到了最佳.实现此方案的关键在于构造出发送者和接收者共享的特定四粒子纠缠态作为量子信道,分析了此特殊四粒子态内在的纠缠结构.     

噪声对超导量子比特测量系统的影响

详细介绍了噪声对超导量子比特测量系统的影响。测量系统通过对时间的测量来获得单个约瑟夫森结跳变电流统计分布P(I)。在测量电路中采取了一系列的降噪措施,使得实验所能测到的约瑟夫森结的最低等效温度进一步的降低。     

三量子比特系统中单个谐振子诱导的量子关联

我们考虑初始无关联并且与由一个谐振子构成的环境之间互相耦合的三量子比特系统。通过研究量子比特-环境的耦合强度以及量子比特初始态对量子关联的影响,我们发现环境可以诱导量子关联,提出并证明了四个命题阐述谐振子如何调控三个量子比特中量子关联的分布。给出了产生量子关联的条件。特别地,对于弱耦合,我们不但能够获得很多的量子关联,而且还使量子比特系统和环境始终处于分离态。一般地,量子关联动力学是很复杂 的,这是由于环境起着两个互相竞争的作用：一方面诱导出各个比特之间的量子关联;另一方面又使它们发生消相干,从而破坏量子关联。     

Interaction-free measurements with superconducting qubits

An interaction-free measurement protocol is described for a quantum circuit consisting of a superconducting qubit and a readout Josephson junction. By measuring the state of the qubit, one can ascertain the presence of a current pulse through the circuit at a previous time without any energy exchange between the qubit and the pulse.     

Low noise submillimeter SIS receiver with niobium nitride quasiparticle tunnel junctions

We have developed and tested a submillimeter waveguide SIS mixer with NbN-MgO-NbN quasiparticle tunnel junctions. The two junction array is integrated in a full NbN printed circuit. The NbN film critical temperature is 15 K and the junction gap voltage is 5 mV. The size of the junctions is 1.4 × 1.4 µm and Josephson critical current density is about 1.5 KA/cm² resulting in junction R_NC product about 40. The inductive tuning circuit in NbN is integrated with each junction in two junction array. A single non contacting backshort was tuned at each frequency in the mixer block.At 306 GHz the minimum DSB receiver noise temperature is as low as 230 K. The sources of the receiver noise and of the limits of the NbN SIS submillimeter mixer improvement are discussed.     

Tunneling spectroscopy of two-level systems inside a Josephson junction

We consider a two-level system (TLS) with energy level separation plankvOmega0 inside a Josephson junction. The junction is shunted by a resistor R and is voltage V biased. If the TLS modulates the Josephson energy and/or is optically active, it is Rabi driven by the Josephson oscillations in the running phase regime near the resonance 2eV=plankvOmega0. The Rabi oscillations, in turn, translate into oscillations of current and voltage that can be detected in noise measurements. This effect provides an option to fully characterize the TLS inside Josephson junction and to find the TLS's contribution to the decoherence when the junction is used as a qubit.     

Electron spectroscopy study in the NbN growth for NbN/AlN interfaces

NbN superconductor and wide band gap AlN thin films were deposited using sputtering at room temperature. Study of the nitride interfaces are forerunner to the growth Josephson junctions that are considered able to work in the terahertz frequency. We find that to be compatible with lithography technology and to have a high critical transition temperature, the substrate should not be overheated, and this means working in low power regime to limit the induced heating of the plasma. X-ray photoelectron spectroscopy and X-ray diffraction analysis were performed on samples deposited on crystalline, amorphous, flexible, and nanostructured substrates. The experimental results suggest us how to improve the deposition process in order to obtain the best nitride films as well as NbN/AlN/NbN trilayers for Josephson junction applications.     

Multiphoton transitions between energy levels in a current-biased Josephson tunnel junction

The escape of a current-biased Josephson tunnel junction from the zero-voltage state in the presence of weak microwave radiation is investigated experimentally at low temperatures. The measurements of the junction switching current distribution indicate the macroscopic quantum tunneling of the phase below a crossover temperature of T small star, filled approximately 280 mK. At temperatures below T small star, filled we observe both single-photon and multiphoton transitions between the junction energy levels by applying microwave radiation in the frequency range between 10 and 38 GHz to the junction. These observations reflect the anharmonicity of the junction potential containing only a small number of levels.     

Quantum dissociation of a vortex-antivortex pair in a long josephson junction

The thermal and the quantum dissociation of a single vortex-antivortex (VAV) pair in an annular Josephson junction is experimentally observed and theoretically analyzed. In our experiments, the VAV pair is confined in a pinning potential controlled by external magnetic field and bias current. The dissociation of the pinned VAV pair manifests itself in a switching of the Josephson junction from the superconducting to the resistive state. The observed temperature and field dependence of the switching current distribution is in agreement with the analysis. The crossover from the thermal to the macroscopic quantum tunneling mechanism of dissociation occurs at a temperature of about 100 mK. We also predict the specific magnetic field dependence of the oscillatory energy levels of the pinned VAV state.     

State tomography of capacitively shunted phase qubits with high fidelity

We introduce a new design concept for superconducting phase quantum bits (qubits) in which we explicitly separate the capacitive element from the Josephson tunnel junction for improved qubit performance. The number of two-level systems that couple to the qubit is thereby reduced by an order of magnitude and the measurement fidelity improves to 90%. This improved design enables the first demonstration of quantum state tomography with superconducting qubits using single-shot measurements.     

Flux qubit with a large loop size and tunable Josephson junctions

We present the design of a superconducting flux qubit with a large loop inductance. The large loop inductance is desirable for coupling between qubits. The loop is configured into a gradiometer form that could reduce the interference from environmental magnetic noise. A combined Josephson junction, i.e., a DC-SQUID is used to replace the small Josephson junction in the usual 3-JJ (Josephaon junction) flux qubit, leading to a tunable energy gap by using an independent external flux line. We perform numerical calculations to investigate the dependence of the energy gap on qubit parameters such as junction capacitance, critical current, loop inductance, and the ratio of junction energy between small and large junctions in the flux qubit. We suggest a range of values for the parameters.