Controllably Coupling Superconducting Charge and Flux Qubits by Using Nanomechanical Resonator

We propose an effective mechanism to couple superconducting charge and flux qubits by using a quantized nanomechanical resonator. The coupling between the charge and flux qubits can be controlled by the external flux of the charge qubit. Under the strong coupling limit, an iSWAP gate can be generated by this scheme. The experimental feasibility in our scheme is also presented.     

Entanglement reciprocation between two charge qubits and two-cavity field

We propose a simple scheme to generate two-mode entangled coherent state in two separated cavities and realize the entanglement reciprocation between the superconducting charge qubits and continuous-variable system. By measuring the state of charge qubits, we find that the entanglement of two charge qubits, which are initially prepared in the maximally entangled state, can be transferred to the two-cavity field, and at this time the two-cavity field is in the entangled coherent state. We also find that the entanglement can be retrieved back to the two charge qubits after measuring the state of the two-cavity field.     

Entanglement reciprocation between two charge qubits and two-cavity field

We propose a simple scheme to generate two-mode entangled coherent state in two separated cavities and realize the entanglement reciprocation between the superconducting charge qubits and continuous-variable system. By measuring the state of charge qubits, we find that the entanglement of two charge qubits, which are initially prepared in the maximally entangled state, can be transferred to the two-cavity field, and at this time the two-cavity field is in the entangled coherent state. We also find that the entanglement can be retrieved back to the two charge qubits after measuring the state of the two-cavity field.      

Linear Entropy and Entanglement in Two-Charge-Qubit Circuits

We consider a model that contains two coupled superconducting charge qubits by sharing a large Josephson junction. We examine the dynamical properties of the linear entropy of two qubits and the probability of both qubits being in an excited state. The results show that the initial mean photon number, the initial phase of the field and the relative phase of the two qubits＇ levels play an important role in the evolution of the linear entropy of the two qubits and the probability.     

Entangling two uncoupled flux qubits via their sequential interaction with a quantized electromagnetic field

A theoretical scheme for the generation of maximally entangled states of two superconducting flux qubits via their sequential interaction with a monochromatic quantum field is presented. The coupling of the qubits with the quantized field can be tuned on and off resonance by modulating the effective Josephson energy of each qubit via an externally applied magnetic flux. The system operates in such a way as to transfer the entanglement from a bipartite field-qubit subsystem to the two qubits. This scheme is attractive in view of the implementation of practical quantum processing systems.     

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.     

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.     

Synthesis of maximally entangled mixed states and disentanglement in coupled Josephson charge qubits

We analyze a controllable generation of maximally entangled mixed states of a circuit containing two-coupled superconducting charge qubits. Each qubit is based on a Cooper pair box connected to a reservoir electrode through a Josephson junction. Illustrative variational calculations were performed to demonstrate the effect on the two-qubits entanglement. At sufficiently deviation between the Josephson energies of the qubits and/or strong coupling regime, maximally entangled mixed states at certain instances of time is synthesized. We show that entanglement has an interesting subsequent time evolution, including the sudden death effect. This enables us to completely characterize the phenomenon of entanglement sharing in the coupling of two superconducting charge qubits, a system of both theoretical and experimental interest.     

Producing cluster states in charge qubits and flux qubits

We propose a method to efficiently generate cluster states in charge qubits, both semiconducting and superconducting, as well as flux qubits. We show that highly entangled cluster states can be realized by a "one-touch" entanglement operation by tuning gate bias voltages for charge qubits. We also investigate the robustness of these cluster states for nonuniform qubits, which are unavoidable in solid-state systems. We find that quantum computation based on cluster states is a promising approach for solid-state qubits.     

Berry phase in superconducting charge qubits interacting with a cavity field

We propose a method for analyzing Berry phase for a multi-qubit system of superconducting charge qubits interacting with a microwave field. By suitably choosing the system parameters and precisely controlling the dynamics, novel connection found between the Berry phase and entanglement creations.     

Switching from positive to negative absorption with electromagnetically induced transparency in circuit quantum electrodynamics

We present a theoretical study of electromagnetically induced transparency(EIT) in a superconducting quantum circuit with a tunable V-shaped energy spectrum derived from two superconducting Josephson charge qubits coupled with each other through a superconducting quantum interference device. Using the density matrix formalism and the steady-state approximation, we obtain the analytical expressions of the first-order matrix element associated with the absorption and dispersion of the probe field for two different V-type schemes. Our results show that, for this superconducting quantum system, it is possible to realize a remarkable phenomenon that dynamic conversion between EIT and EIT with amplification without population inversion. Such a unique optical feature has potential applications in quantum optical devices and quantum information processing.     

超导量子比特实验的开端

我们对超导量子比特领域的科学背景、历史起源和早期发展做简要评述.莱格特(Anthony J. Leggett)为这个领域打下了理论基础.克拉克(John Clarke)和他的两个学生马丁尼(John Martinis)和德沃雷(Michel H. Devoret)最早通过偏电流约瑟夫森结,首次观察到约瑟夫森结的量子行为.后来德沃雷实现了电荷量子比特叠加态、电荷-磁通混合量子比特的拉比共振和其他演化及投影测量.中村泰信(Yasunobu Nakamura)首先实现电荷量子比特的量子叠加和拉比振荡,还参与莫伊(J. E. Mooij)组实现了磁通量子比特的拉比振荡和读出.     

Multipartite entanglement in the interaction system between a single-mode microwave cavity field and superconducting charge qubits

This paper proposes a method of generating multipartite entanglement through using d.c. superconducting quantum interference devices (SQUID) inside a standing wave cavity. In this scheme, the d.c. SQUID works in the charge region. It is shown that, a large number of important multipartite entangled states can be generated by a controllable interaction between a cavity field and qubits. It is even possible to produce entangled states involving different cavity modes based on the measurement of charge qubits states. After such superpositions states are created, the interaction can be switched off by the classical magnetic field through the SQUID, and there is no information transfer between the cavity field and the charge qubits.     

Unconventional Geometric Quantum Gate in Circuit QED

We propose a scheme to implement an unconventional geometric phase gate in circuit QED, i.e. two superconducting charge qubits inside a superconducting transmission line resonator. The quantum operation depends only on global geometric features, and thus is insensitive to the state of the cavity mode.     

Charge Qubit Storage and Its Engineered Decoherence via Microwave Cavity

We study the entanglement of the superconducting charge qubit with the quantized electromagnetic field in a microwave cavity. It can be controlled dynamically by a classical external field threading the SQUID within the charge qubit. Utilizing the controllable quantum entanglement, we can demonstrate the dynamic process of the quantum storage of information carried by charge qubit. On the other hand, based on this engineered quantum entanglement, we can also demonstrate a progressive decoherence of charge qubit with quantum jump due to the coupling with the cavity field in quasi-classical state.     

Implementation of a Quantum Conditional Phase Gate for the Quantum Fourier Transform in Circuit QED

It is well known that multiple superconducting charge qubits coupled to a transmission line resonator can be controlled to achieve quantum logic gates between two arbitrary qubits. We propose a scheme to realize a quantum conditional phase gate with a geometric property by circuit electrodynamics, and it is applied naturally to reaJize the quantum Fourier transform with high fidelity. It is also demonstrated that the application is feasible and considerable under the present experimental technology.     

Entanglement Dynamics of Three Superconducting Charge Qubits Coupled to a Cavity

We investigate the entanglement dynamics of a quantum system consisting of three superconducting charge qubits (SCQs) interacting with a microwave field. For separable and entangled states of the SCQs, the evolutions are studied under various photon numbers of cavity field. The results show that the amplitude and period of the bipartite entanglement square concurrences can be controlled by the choice of initial states of SCQs and photon numberof cavity field, respectively. This simple model of a quantum register allows us to understand the dynamic process of the quantum storage of information carried by charge qubit.     

Non-Classical Correlations and Transfer of Quantum Information in a Superconducting Qubit System with Dynamical Decoupling Pulses

We investigate the dynamics of non-classical correlations(entanglement and quantum discord) of the system consisting of two non-interacting superconducting qubits coupling with a common data bus, where the system is driven by the dynamical decoupling pulses. It is found that the non-classical correlations between two superconducting qubits can be increased by appling a train of dynamical decoupling pulses. Furthermore, we also explore the influence of the dynamical decoupling pulses on the information flowing between superconducting qubits and data bus by making use of the trace distance. It is shown that the dynamical decoupling pulses can protect quantum information of two superconducting qubits and force information to flow back to the superconducting qubits from the data bus.

     

Shortcut-based quantum gates on superconducting qubits in circuit QED

Construction of optimal gate operations is significant for quantum computation. Here an efficient scheme is proposed for performing shortcut-based quantum gates on superconducting qubits in circuit quantum electrodynamics (QED). Two four-level artificial atoms of Cooper-pair box circuits, having sufficient level anharmonicity, are placed in a common quantized field of circuit QED and are driven by individual classical microwaves. Without the effect of cross resonance, one-qubit NOT gate and phase gate in a decoupled atom can be implemented using the invariant-based shortcuts to adiabaticity. With the assistance of cavity bus, a one-step SWAP gate can be obtained within a composite qubit-photon-qubit system by inversely engineering the classical drivings. We further consider the gate realizations by adjusting the microwave fields. With the accessible decoherence rates, the shortcut-based gates have high fidelities. The present strategy could offer a promising route towards fast and robust quantum computation with superconducting circuits experimentally.     

New features of total correlations in coupled Josephson charge qubits with intrinsic decoherence

Adopting the framework of two-coupled superconducting charges model, we derive a general formula for the total correlation function between the two charge qubits initially prepared in a mixed state. The impact of the different parameters of the system is explicitly investigated. We have identified a class of two-qubit states that have rich dynamics when the deviation between the characteristic energies of the charge qubits tends to a minimum value. Present calculations show that the total correlation decreases abruptly to zero in a finite time due to the influence of the decoherence.