基于超导量子比特网络的Grover搜索算法实现方案

首先,提出了一个改进超导电路结构,此结构能实现任意两个量子比特的相互作用而非近邻作用,长程作用是实现量子计算所必需的,此结构能用目前的技术制作。其次,基于此结构提出了Grover搜索算法实现的物理方案。由于能实现任意两量子比特之间的控制相位门,所以多比特Grover搜索算法也能实现,以满足各种量子计算的需要。此方案是一个基于电流控制的超导电荷比特网络结构的可扩展及易实现的Grover搜索算法实现方案。     

Quantum interfaces using nanoscale surface plasmons

The strong coupling between individual optical emitters and propagating surface plasmons confined to a conducting nanotip make this system act as an ideal interface for quantum networks, through which a stationary qubit and a flying photon (surface plasmon) qubit can be interconverted via a Raman process. This quantum interface paves the way for many essential functions of a quantum network, including sending, receiving, transferring, swapping, and entangling qubits at distributed quantum nodes as well as a deterministic source and an efficient detector of a single-photon. Numerical simulation shows that this scheme is robust against experimental imperfections and has high fidelity. Furthermore, being smaller this interface would significantly facilitate the scalability of quantum computers.     

Scalable ion trap quantum computing without moving ions

A hybrid quantum computing scheme is studied where the hybrid qubit is made of an ion trap qubit serving as the information storage and a solid-state charge qubit serving as the quantum processor, connected by a superconducting cavity. In this paper, we extend our previous work [CITE] and study the decoherence, coupling and scalability of the hybrid system. We present our calculations of the decoherence of the coupled ion-charge system due to the charge fluctuations in the solid-state system and the dissipation of the superconducting cavity under laser radiation. A gate scheme that exploits rapid state flips of the charge qubit to reduce decoherence by the charge noise is designed. We also study a superconducting switch that is inserted between the cavity and the charge qubit and provides tunable coupling between the qubits. The scalability of the hybrid scheme is discussed together with several potential experimental obstacles in realizing this scheme.     

Nanophotonic waveguidance in quantum networks - A simulation approach for quantum state transfer

A cavity-assisted Raman process can initialize the inter-conversion of stationary spin qubits and flying photon qubits in quantum channels. The qubit transmission essentially requires the implementation of special laser fields to excite atoms at the transmitting node of the quantum cavity. The flying qubit is ultimately absorbed at the receiving node of the channel to regenerate the original spin state of the nanodot. The present paper deals with the phenomena involved in such nanophotonic waveguidance by the process of rigorous simulation, and it is reported that the results obtained by implementing suitable transmission protocol reflect well the reliable transfer/entanglement of the quantum states of the nanodot qubit.     

Regulation of Entanglement and Geometric Quantum Discord of Hybrid Superconducting Qubits for Circuit QED

We investigate the dynamic evolution behaviors of entanglement and geometric quantum discord (GQD) of hybrid qubits in a circuit QED system. Under certain initial conditions, interactions between qubits and that between the qubit and the cavity can suppress the decay of entanglement and GQD of qubits. Under the initial condition |ψ _Q (0)〉 _I =sinα|↓↑〉+cosα|↑↓〉, such decay was avoided by increasing the coupling strength ratio between qubits and that between the qubit and the cavity. Under the aforementioned condition, the survival time of entanglement and GQD was prolonged by decreasing the coupling strength ratio between qubits and that between the qubit and the cavity.     

Eliminating interactions between non-neighboring qubits in the preparation of cluster states in quantum molecules

We propose a scheme to eliminate the effect of non-nearest-neighbor qubits in preparing cluster state with double-dot molecules. As the interaction Hamiltonians between qubits are Ising-model and mutually commute, we can get positive and negative effective interactions between qubits to cancel the effect of non-nearest-neighbor qubits by properly changing the electron charge states of each quantum dot molecule. The total time for the present multi-step cluster state preparation scheme is only doubled for one-dimensional qubit chain and tripled for two-dimensional qubit array comparing with the time of previous protocol leaving out the non-nearest-neighbor interactions.     

Simple schemes for generation of W-type multipartite entangled states and realization of quantum- information concentration

We propose simple schemes for generating W-type multipartite entangled states in cavity quantum electrodynamics (CQED). Our schemes involve a largely detuned interaction of Λ-type three-level atoms with a single-mode cavity field and a classical laser, and both the symmetric and asymmetric W states can be created in a single step. Our schemes are insensitive to both the cavity decay and atomic spontaneous emission. With the above system, we also propose a scheme for realizing quantum-information concentration which is the reverse process of quantum cloning. In this scheme, quantum-information originally coming from a single qubit, but now distributed into many qubits, is concentrated back to a single qubit in only one step.     

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.     

Engineering decoherence in Josephson persistent-current qubits

We discuss the relaxation and dephasing rates that result from the control and the measurement setup itself in experiments on Josephson persistent-current qubits. For control and measurement of the qubit state, the qubit is inductively coupled to electromagnetic circuitry. We show how this system can be mapped on the spin-boson model, and how the spectral density of the bosonic bath can be derived from the electromagnetic impedance that is coupled to the qubit. Part of the electromagnetic environment is a measurement apparatus (DC-SQUID), that is permanently coupled to the single quantum system that is studied. Since there is an obvious conflict between long coherence times and an efficient measurement scheme, the measurement process is analyzed in detail for different measurement schemes. We show, that the coupling of the measurement apparatus to the qubit can be controlled in situ. Parameters that can be realized in experiments today are used for a quantitative evaluation, and it is shown that the relaxation and dephasing rates that are induced by the measurement setup can be made low enough for a time-resolved study of the quantum dynamics of Josephson persistent-current qubits. Our results can be generalized as engineering rules for the read-out of related qubit systems. Received 4 September 2002 Published online 27 January 2003 RID="a" ID="a"Present address: Department of Physics, Harvard University, 17 Oxford Street, Cambridge, MA 02138, USA RID="b" ID="b"Present address: Sektion Physik and CeNS, Ludwig-Maximilians Universit?t, Theresienstr. 37, 80333 Munich, Germany e-mail: wilhelm@theorie.physik.uni-muenchen.de     

Coupling nitrogen-vacancy centers in diamond to superconducting flux qubits

We propose a method to achieve coherent coupling between nitrogen-vacancy (NV) centers in diamond and superconducting (SC) flux qubits. The resulting coupling can be used to create a coherent interaction between the spin states of distant NV centers mediated by the flux qubit. Furthermore, the magnetic coupling can be used to achieve a coherent transfer of quantum information between the flux qubit and an ensemble of NV centers. This enables a long-term memory for a SC quantum processor and possibly an interface between SC qubits and light.     

Preparation of multi-photon Fock states and quantum entanglement properties in circuit QED

We demonstrate the controllable generation of multi-photon Fock states in circuit quantum electrodynamics （circuit QED）. The external bias flux regulated by a counter can effectively adjust the bias time on each superconducting flux qubit so that each flux qubit can pass in turn through the circuit cavity and thereby avoid the effect of decoherence. We further investigate the quantum correlation dynamics of coupling superconducting qubits in a Fock state. The results reveal that the lower the photon number of the light field in the number state, the stronger the interaction between qubits is, then the more beneficial to maintaining entanglement between qubits it will be.     

Quantum Logic Gates and Cluster States with Four-level Atoms in Cavity QED

We propose a model to implement the two-qubit quantum logic gates, i.e., the quantum phase gate and the Controlled-NOT gate, and generate the atomic qubits cluster states with a large detuned interaction between four-level atoms and a single-mode cavity field. In the presented protocol, the quantum information is encoded on the stable ground states of the atoms, and the effect of decoherence from atomic spontaneous emission is negligible. In addition, the interaction between atoms and the cavity is large detuned, and the cavity is only virtually excited. Therefore, the scheme is insensitive to the cavity decay. The experimental feasibility of our proposal is also discussed.     

Controllable anisotropic exchange coupling between spin qubits in quantum dots

The exchange coupling between quantum dot spin qubits is isotropic, which restricts the types of quantum gates that can be formed. Here, we propose a method for controlling anisotropic interactions between spins arranged in a bus geometry. The symmetry is broken by an external magnetic field, resulting in XXZ-type interactions that can efficiently generate maximally entangled Greenberger-Horne-Zeilinger states or universal gate sets for exchange-only quantum computing. We exploit the XXZ couplings to propose a qubit scheme, based on double dots.     

Generation of an N-qubit phase gate via atom--cavity nonidentical coupling

A scheme for approximate generation of an N-qubit phase gate is proposed in cavity QED based on nonidentical coupling between the atoms and the cavity. The atoms interact with a highly detuned cavity-field mode, but quantum information does not transfer between the atoms and cavity field, and thus the cavity decay is negligible. The gate time does not rise with an increase in the number of qubits. With the choice of a smaller odd number l （related to atom-cavity coupling constants）, the phase gate can be generated with a higher fidelity and a higher success probability in a shorter time （the gate time is much shorter than the atomic radiative lifetime and photon lifetime）. When the number of qubits N exceeds certain small values, the fidelity and success probability rise slowly with an increase in the number of qubits N. When N→∞, the fidelity and success probability infinitely approach 1, but never exceed 1.     

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.     

Topological quantum memory interfacing atomic and superconducting qubits

We propose a scheme to manipulate a topological spin qubit which is realized with cold atoms in a one-dimensional optical lattice. In particular, by introducing a quantum opto-electro-mechanical interface, we are able to first transfer a superconducting qubit state to an atomic qubit state and then to store it into the topological spin qubit. In this way, an efficient topological quantum memory could be constructed for the superconducting qubit. Therefore, we can consolidate the advantages of both the noise resistance of the topological qubits and the scalability of the superconducting qubits in this hybrid architecture.     

腔QED中超导量子干涉仪概率克隆机的物理实现

提出一个量子概率克隆机的物理实现方案,该方案首先将高Q腔中的两个超导量子干涉仪分别作为初始比特和目标比特,腔模作为测量比特,通过腔模和经典微波脉冲与超导量子干涉仪的多种相互作用实现量子概率克隆机的幺正演化;然后将腔模态映射到另一个超导量子干涉仪上,通过对该超导量子干涉仪磁通量的测量完成状态坍缩,从而以最优的成功概率实现量子态的精确克隆.本方案采用双光子拉曼共振过程加快单比特门的操作速率,并且总操作时间远小于自发辐射和腔模衰变时间,因而在实验上是可行的.     

Implementation of a two-qubit controlled-rotation gate based on unconventional geometric phase with a constant gating time

We propose an alternative scheme to implement a two-qubit controlled-R (rotation) gate in the hybrid atom-CCA (coupled cavities array) system. Our scheme results in a constant gating time and, with an adjustable qubit-bus coupling (atom-resonator), one can specify a particular rotation R on the target qubit. We believe that this proposal may open promising perspectives for networking quantum information processors and implementing distributed and scalable quantum computation.     

Charge qubit purification by an electronic feedback loop

We propose the manipulation of an isolated qubit by a simple instantaneous closed-loop feedback scheme in which a time-dependent electronic detector current is directly back-coupled into qubit parameters. As a specific detector model, we employ a capacitively coupled single-electron transistor. We demonstrate the stabilization of pure delocalized qubit states above a critical detector-qubit coupling. This electronic purification is independent of the initial qubit state and is accomplished after few electron jumps through the detector. Our simple scheme can be used for the efficient and robust initialization of solid-state qubits in quantum computational algorithms at arbitrary temperatures.     

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。