Description of the evolution of the state of “josephson atoms” in the context of the informational interpretation of quantum mechanics

The evolution of a quantum system is informationally interpreted and used to describe decay in the coherent states of Josephson atoms, including qubits based on two- and three-junction superconducting quantum interferometers. The reduced Lindblad equation is employed to examine the measurement procedure of the Josephson qubit state and the influence of a measuring device on its coherent-state decay.     

Macroscopic coherent rectification in Andreev interferometers

We investigate nonlinear transport through quantum coherent metallic conductors contacted to superconducting components. We find that in certain geometries, the presence of superconductivity generates a large, finite-average rectification effect. Specializing to Andreev interferometers, we show that the direction and magnitude of rectification can be controlled by a magnetic flux tuning the superconducting phase difference at two contacts. In particular, this results in the breakdown of an Onsager reciprocity relation at finite bias. The rectification current is macroscopic in that it scales with the linear conductance, and we find that it exceeds 5% of the linear current at sub-gap biases of a few tens of microelectronvolts.     

Pertubation analysis of the dynamical behavior of two-junction interferometers

The dynamical properties of symmetric quantum interferometers with equal junctions of negligible capacitance have been studied by means of perturbation analysis in the limit of small values of the parameter β. In this limit, two characteristic time constants arise. These quantities may be linked to two different dynamical processes in the system: the first is related to the time evolution of the average superconducting phase difference across the two junctions; the second defines the time scale for flux motion. The response of the system to constant and time-dependent externally applied magnetic fields is considered and a general perturbed solution for the average superconducting phase difference and the fluxon number variable is derived to first order in β.     

Singlet two-electron states in superconducting materials based on iron pnictides

Two-electron states with a zero pulse for the symmetry group of superconducting materials based on iron pnictides are considered. Cases are established for the introduction of one or two additional quantum numbers, notably, the internal quantum number, which characterizes single-electron states, or an additional quantum number, i.e. the irreducible representation index of the intermediate group. The octet structure of zeros observed in photoelectron spectra and the phase difference between the two-electron states on various Fermi surfaces are interpreted.     

Entanglement dynamics in quantum operations using superconducting phase qubits

The decoherence of quantum states in superconducting phase qubits under the impact of quantum noise is considered. The creating, maintaining, and breaking of entanglement in quantum operations important in creating full-scale quantum computers based on superconducting elements are analyzed.     

Thin spectrum states in bulk superconductors and superconducting grains

We show how a local pairing model for superconductivity can be used to describe the symmetry breaking mechanism in exact analogy to the cases of quantum crystals and antiferromagnets. We find that there are low energy states associated with the symmetry breaking process which are not influenced by the Anderson-Higgs mechanism. The presence of these ‘thin spectrum’ states in qubits based on superconducting material leads to a maximum time for which such qubits can remain quantum coherent. We also show how the charging energy of superconducting quantum dots may give the thin spectrum states a finite energy gap, impeding the spontaneous breaking of phase symmetry.     

铌基超导量子比特及辅助器件的制备

近年来超导量子计算的研究方兴未艾,随着谷歌宣布首次实现“量子优势”,这一领域的研究受到了人们进一步的广泛关注.超导量子比特是具有量子化能级、量子态叠加和量子态纠缠等典型量子特性的宏观器件,通过电磁脉冲信号控制磁通量、电荷或具有非线性电感和无能量耗散的约瑟夫森结上的位相差,可对量子态进行精确调控,从而实现量子计算和量子信息处理.超导量子比特有着诸多方面的优势,很有希望成为普适量子计算的核心组成部分.以铌或其他硬金属(如钽等)为首层大面积材料制备的超导量子比特及辅助器件(简称铌基器件)拥有其独特的优点以及进一步发展的空间,目前已引起越来越多的兴趣.本文将介绍常见的多种超导量子比特的基本构成和工作原理,进而按照器件加工的一般顺序,从基片选择和预处理、薄膜生长、图形转移、刻蚀和约瑟夫森结的制备等方面详细介绍铌基超导量子比特及其辅助器件的多种制备工艺,为超导量子比特的制备提供一个可借鉴的清晰的工艺过程.最后,介绍若干制备铌基超导量子比特与辅助器件的具体例子,并对器件制备的工艺与方法的优化做展望.     

Frequency-coded quantum key distribution

We report an intrinsically stable quantum key distribution scheme based on genuine frequency-coded quantum states. The qubits are efficiently processed without fiber interferometers by fully exploiting the nonlinear interaction occurring in electro-optic phase modulators. The system requires only integrated off-the-shelf devices and could be used with a true single-photon source. Preliminary experiments have been performed with weak laser pulses and have demonstrated the feasibility of this new setup.     

平面自旋压缩态的产生与原子干涉的机理

在玻色-爱因斯坦凝聚体中实现自旋压缩和量子纠缠, 对于提高原子干涉测量相位灵敏度和原子钟精度有着非常重要的意义. 基于一种新的平面自旋分量的不确定性关系, 介绍了如何利用两分量玻色-爱因斯坦凝聚系统中原子间相互作用提供的非线性效应和原子内部能级间线性耦合, 实现量子平面自旋压缩(挤压)和模式纠缠. 描述了一项关于平面压缩态的理论工作, 该工作利用哈密顿量的精确对角化求解系统基态, 优化非线性作用和线性耦合强度比值, 使得包含平均自旋方向在内的两个正交自旋分量的不确定度同时压缩, 因此在平面上所有相位角度的涨落都受到压制, 而在与该平面垂直的第三个自旋分量方向反压缩. 利用传统自旋压缩判定纠缠, 只能判断多个不可分辨的原子处于纠缠态, 而平面自旋压缩可以检测两个可区分模式(比如, 原子内态)间的纠缠, 从而在不同模式间进行量子信息处理. 同时, 为实现超越标准量子极限的原子干涉相位精密测量, 传统方式是利用单个自旋分量压缩, 但需要对待测相位角度有很好的估计, 或者可以进行多次测量以逐渐逼近可获得的最大压缩极限, 这就要求量子态可以被精确的重复制备. 而利用平面自旋压缩, 对任意未知相位角度只需要测量两个垂直自旋分量就可以实现高的相位测量灵敏度.     

Intrinsically stable phase-modulated polarization encoding system for quantum key distribution

We demonstrate experimentally an intrinsically stable polarization coding and decoding system composed of optical-fiber Sagnac interferometers with integrated phase modulators for quantum key distribution. An interference visibility of 98.35% can be kept longtime during the experiment without any efforts of active compensation for coding all four desired polarization states.     

Spin Ensembles Coupled to Superconducting Resonators: A Scalable Architecture for Solid-State Quantum Computing

A design is proposed for scalable solid-state quantum computing, which is based on collectively enhanced magnetic coupling between nitrogen-vacancy center ensembles and superconducting transmission line resonators interconnected by current-biased Josephson junction superconducting phase qubit. In this hybrid system, we realize distant multi-qubit controlled phase gate operations and generate distant multi-qubit entangled W-like states, being indispensable resource to quantum computation. Our proposed architecture consists of solid-state spin ensembles and circuit QED, and could achieve quantum computing in a solid-state environment with high-fidelity and scalable way. The experimental feasibility is discussed, and the implementation efficiency is demonstrated numerically.     

Nanoscale superconducting quantum bits

Various physical systems were proposed for quantum information processing. Among those nanoscale devices appear most promising for integration in electronic circuits and large-scale applications. We discuss Josephson junction circuits in two regimes where they can be used for quantum computing. These systems combine intrinsic coherence of the superconducting state with control possibilities of single-charge circuits. In the regime where the typical charging energy dominates over the Josephson coupling, the low-temperature dynamics is limited to two states differing by a Cooper-pair charge on a superconducting island. In the opposite regime of prevailing Josephson energy, the phase (or flux) degree of freedom can be used to store and process quantum information. Under suitable conditions the system reduces to two states with different flux configurations. Several qubits can be joined together into a register. The quantum state of a qubit register can be manipulated by voltage and magnetic field pulses. The qubits are inevitably coupled to the environment. However, estimates of the phase coherence time show that many elementary quantum logic operations can be performed before the phase coherence is lost. In addition to manipulations, the final state of the qubits has to be read out. This quantum measurement process can be accomplished using a single-electron transistor for charge Josephson qubits, and a d.c.-SQUID for flux qubits. Recent successful experiments with superconducting qubits demonstrate for the first time quantum coherence in macroscopic systems.     

Spin Ensembles Coupled to Superconducting Resonators：A Scalable Architecture for Solid-State Quantum Computing

A design is proposed for scalable solid-state quantum computing, which is based on collectively enhanced magnetic coupling between nitrogen-vacancy center ensembles and superconducting transmission line resonators interconnected by current-biased Josephson junction superconducting phase qubit. In this hybrid system, we realize distant multi-qubit controlled phase gate operations and generate distant multi-qubit entangled W-like states, being indispensable resource to quantum computation. Our proposed architecture consists of solid-state spin ensembles and circuit QED, and could achieve quantum computing in a solid-state environment with high-fidelity and scalable way. The experimental feasibility is discussed, and the implementation efficiency is demonstrated numerically.     

Quantum Key Distribution System with Six Polarization States Encoded by Phase Modulation

An intrinsically stable quantum key distribution system （QKD） with six polarization states encoded by phase modulation is introduced. The encoder and decoder are in the same structures that consist of two polarizing Sagnac interferometers connected in tandem. The six polarization states are determined and distinguished by different sets of phase shifts induced by two respective electrically-driven integrated phase modulators. A mean visibility of interference fringes is kept stable at 97.58% for an hour＇s performance. Theoretical and experimental analyses show that the proposed QKD system features intrinsically stability immune from environment fluctuation.     

Coexistence of s-wave superconductivity and antiferromagnetism

We study the phase diagram of a new model that exhibits a first order transition between s-wave superconducting and antiferromagnetic phases. The model, a generalized Hubbard model augmented with competing spin-spin and pair-pair interactions, was investigated using the projector quantum Monte Carlo method. Upon varying the Hubbard U from attractive to repulsive, we find a first order phase transition between superconducting and antiferromagnetic states.     

Trapped electron coupled to superconducting devices

We propose to couple a trapped single electron to superconducting structures located at a variable distance from the electron. The electron is captured in a cryogenic Penning trap using electric fields and a static magnetic field in the tesla range. Measurements on the electron will allow investigating the properties of the superconductor such as vortex structure, damping and decoherence. We propose to couple a superconducting microwave resonator to the electron in order to realize a circuit QED-like experiment, as well as to couple superconducting Josephson junctions or superconducting quantum interferometers (SQUIDs) to the electron. The electron may also be coupled to a vortex which is situated in a double well potential, realized by nearby pinning centers in the superconductor, acting as a quantum mechanical two level system that can be controlled by a transport current tilting the double well potential. The electron may also be coupled to a single vortex, thus hybridizing an elementary excitation of a superconductor and an elementary particle.     

Quantum jumps between macroscopic quantum states of a superconducting qubit coupled to a microscopic two-level system

We report the observation of quantum jumps between macroscopic quantum states in a superconducting phase qubit coupled to the two-level systems in the Josephson tunnel junction, and all key features of quantum jumps are confirmed in the experiments. Moreover, quantum jumps can be used to calibrate such two-level systems, which are believed to be one of the main decoherence sources in Josephson devices.     

Model of a proposed superconducting phase slip oscillator: a method for obtaining few-photon nonlinearities

We theoretically investigate a driven oscillator with the superconducting inductance subject to quantum phase slips (QPS). We find uncommon nonlinearities in the proposed device: they oscillate as a function of the number of photons N with a local period of the order of √N. We prove that such nonlinearities result in multiple metastable states encompassing few photons and study oscillatory dependence of various responses of the oscillator. Such nonlinearities enable new possibilities for quantum manipulation of photon states and very sensitive measurements to confirm the coherence of phase slips.     

Tolerance-enhanced SU(1,1) interferometers using asymmetric gain

SU(1,1) interferometers play an important role in quantum metrology. Previous studies focus on various inputs and detection strategies with symmetric gain. In this paper, we analyze a modified SU(1,1) interferometer using asymmetric gain. Two vacuum states are used as the input and on-off detection is performed at the output. In a lossless scenario, symmetric gain is the optimal selection and the corresponding phase sensitivity can achieve the Heisenberg limit as well as the quantum Cramer-Rao bound. In addition, we analyze the phase sensitivity with symmetric gain in the lossy scenario. The phase sensitivity is sensitive to internal losses but extremely robust against external losses. We address the optimal asymmetric gain and the results suggest that this method can improve the tolerance to internal losses. Our work may contribute to the practical development of quantum metrology.