Implementing a noise protected logical qubit in methyl groups via microwave irradiation

We propose a proof-of-principle experiment to encode one logical qubit in noise protected subspace of three identical spins in a methyl group. The symmetry analysis of the wavefunction shows that this fermionic system exhibits a symmetry correlation between the spatial degree of freedom and the spin degree of freedom. We show that one can use this correlation to populate the noiseless subsystem by relying on the interaction between the electric dipole moment of the methyl group with a circularly polarized microwave field. Logical gates are implemented by controlling both the intensity and phase of the applied field.     

Dynamics of tripartite quantum correlations and decoherence in flux qubit systems under local and non-local static noise

We investigate the dynamics of entanglement, decoherence and quantum discord in a system of three non-interacting superconducting flux qubits (fqubits) initially prepared in a Greenberger–Horne–Zeilinger (GHZ) state and subject to static noise in different, bipartite and common environments, since it is recognized that different noise configurations generally lead to completely different dynamical behavior of physical systems. The noise is modeled by randomizing the single fqubit transition amplitude. Decoherence and quantum correlations dynamics are strongly affected by the purity of the initial state, type of system–environment interaction and the system–environment coupling strength. Specifically, quantum correlations can persist when the fqubits are commonly coupled to a noise source, and reaches a saturation value respective to the purity of the initial state. As the number of decoherence channels increases (bipartite and different environments), decoherence becomes stronger against quantum correlations that decay faster, exhibiting sudden death and revival phenomena. The residual entanglement can be successfully detected by means of suitable entanglement witness, and we derive a necessary condition for entanglement detection related to the tunable and non-degenerated energy levels of fqubits. In accordance with the current literature, our results further suggest the efficiency of fqubits over ordinary ones, as far as the preservation of quantum correlations needed for quantum processing purposes is concerned.     

Effect of quantum noise on deterministic joint remote state preparation of a qubit state via a GHZ channel

Quantum Information Processing - Quantum secure communication brings a new direction for information security. As an important component of quantum secure communication, deterministic joint remote...     

Robust incoherent control of qubit systems via switching and optimisation

A robust incoherent quantum control scheme via projective measurements plus unitary transformations is proposed for driving a qubit system from an unknown initial mixed state to an arbitrary target pure state. This scheme consists of two main steps: projective measurement on the initial mixed state and optimal control between two pure states. The first step projects the initial state into an eigenstate of the qubit system by projective measurement and guarantees that the proposed scheme is robust to different initial mixed states. The second step finds a set of suitable optimal controls to drive the qubit system from the conditional eigenstate to the target pure state. The connection between the two steps is accomplished by a switching strategy. To accomplish the second step, two approaches are presented in detail. These approaches are time-optimal transition with unbounded control and bang-bang control with minimal switches. The minimal time and minimal number of switches in these approaches can be calculated by simple analytical expressions. The proposed approaches provide two relatively straightforward optimal design methods.     

Disentanglement and decoherence from classical non-Markovian noise: random telegraph noise

We calculate the two-qubit disentanglement due to classical random telegraph noise using the quasi-Hamiltonian method. This allows us to obtain analytical results even for strong coupling and mixed noise, important when the qubits have tunable working point. We determine when entanglement sudden death and revival occur as functions of qubit working point, noise coupling strength and initial state entanglement. For extended Werner states, we show that the concurrence is related to the difference of two functions: one is related to dephasing and the other longitudinal relaxation. A physical interpretation based on the generalized Bloch vector is given: revival only occurs for strongly-coupled noise and comes from the angular motion of the vector.     

Precision protection through multi-body indirect correlations and the reconstruction of stable probe qubit system

The obstacle restricting us to generalize the ideal quantum metrology scheme to practice is the decay of the precision of estimation induced by the environment. When the parameter to be estimated is the phase parameter of atomic state, the loss of precision is caused by the spontaneous emission of the atom, which is the result of the interaction between the atom and the background quantum fields. Since the probe qubit constructed by the ground state and excited state of single atom is sensitive to the environment, we have provided schemes to use two proper states of n proper arrangement atoms as the new stable qubit states instead of the usual ground and excited state of single atom, to reduce the decay rate of the precision of estimation through the multi-atom indirect correlations.     

Stabilization via static output feedback

Necessary and sufficient conditions for the existence of a stabilizing static output feedback gain matrix are presented. It is shown that any such gain matrix is the solution of some linear quadratic control problem where the cost functional has a suitable cross term     

Deterministic remote preparation of an arbitrary qubit state using a partially entangled state and finite classical communication

Quantum Information Processing - We propose a deterministic remote state preparation (RSP) scheme for preparing an arbitrary (including pure and mixed) qubit, where a partially entangled state and...     

Quantum correlations of identical particles subject to classical environmental noise

In this work, we propose a measure for the quantum discord of indistinguishable particles, based on the definition of entanglement of particles given in Wiseman and Vaccaro (Phys Rev Lett 91:097902, 2003. doi: 10.1103/PhysRevLett.91.097902). This discord of particles is then used to evaluate the quantum correlations in a system of two identical bosons (fermions), where the particles perform a quantum random walk described by the Hubbard Hamiltonian in a 1D lattice. The dynamics of the particles is either unperturbed or subject to a classical environmental noise—such as random telegraph, pink or brown noise. The observed results are consistent with those for the entanglement of particles, and we observe that on-site interaction between particles have an important protective effect on correlations against the decoherence of the system.     

Natural path planning using wavelet noise in static environment

In a constrained virtual environment, motion of a character has to be controlled with precision to avoid collisions. For modeling motion of a character representing some real‐life object, it is important to generate paths that look natural. A number of path planning algorithms have been introduced to solve real‐time path queries in an environment. However, in all such algorithms, a fixed path is returned always for the same start and goal positions in the environment. Recently, Perlin noise has been used to add variations to a path so that it appears more natural. However, because of pure random nature of Perlin noise function, often unnatural path occurs. In this paper, we present a new approach to natural path planning by adding wavelet noise to a path generated using subdivision‐based Corridor Map Method. Because wavelet noise is almost perfectly band limited and provides good details with minimal aliasing effects, the resulting path becomes smoother and more natural. Moreover, by appropriately choosing the levels of down/up sampling in the wavelet noise generation algorithm, frequency of wavelet noise can be adjusted. This serves as an effective tool in bringing variations in path as per the requirement. Copyright © 2012 John Wiley & Sons, Ltd.     

Coexistence of qubit effects

Two quantum events, represented by positive operators (effects), are coexistent if they can occur as possible outcomes in a single measurement scheme. Equivalently, the corresponding effects are coexistent if and only if they are contained in the ranges of a single (joint) observable. Here we give several equivalent characterizations of coexistent pairs of qubit effects. We also establish the equivalence between our results and those obtained independently by other authors. Our approach makes explicit use of the Minkowski space geometry inherent in the four-dimensional real vector space of selfadjoint operators in a two-dimensional complex Hilbert space.     

Genre as noise: noise in genre

Given a specific information need, documents of the wrong genre can be considered as noise. From this perspective, genre classification helps to separate relevant documents from noise. Orthographic errors represent a second, finer notion of noise. Since specific genres often include documents with many errors, an interesting question is whether this “micro-noise” can help to classify genre. In this paper we consider both problems. After introducing a comprehensive hierarchy of genres, we present an intuitive method to build specialized and distinctive classifiers that also work for very small training corpora. Special emphasis is given to the selection of intelligent high-level features. We then investigate the correlation between genre and micro noise. Using special error dictionaries, we estimate the typical error rates for each genre. Finally, we test if the error rate of a document represents a useful feature for genre classification.     

Controlled remote state preparation of arbitrary two and three qubit states via the Brown state

In this paper, several new protocols for the controlled remote state preparation (CRSP) by using the Brown state as the quantum channel are proposed. Firstly, we propose a CRSP protocol of an arbitrary two qubit state. Then, the CRSP protocol of an arbitrary three qubit state, which has rarely been considered by the previous papers, is investigated. The coefficients of the prepared states can be not only real, but also complex. To design these protocols, some useful and general measurement bases are constructed, which can greatly reduce the restrictions for the coefficients of the prepared states. The security analysis is provided in detail. Moreover, receiver??s all recovery operations are summarized into a concise formula.     

Simultaneous stabilization via static output feedback and statefeedback

In this paper, the simultaneous stabilization problem is considered using the matrix inequality approach. Some necessary and sufficient conditions for simultaneous stabilizability of strictly proper multi-input/multi-output (MIMO) plants via static output feedback and state feedback are obtained in the form of coupled algebraic Riccati inequalities. It is shown that any such stabilizing feedback gain is the solution of some coupled linear quadratic control problems where every cost functional has a suitable cross term. A heuristic iterative algorithm based on the linear matrix inequality technique is presented to solve the coupled matrix inequalities. The effectiveness of the approach is illustrated by numerical examples     

SIP协议实现中消息解析的研究

在满足SIP协议被设计为可灵活扩展的文本协议的要求的基础上,分析了传统SIP消息解析器中基于字符串比较操作来判断消息头字段类型对整体SIP消息解析性能的影响.结合SIP协议栈实现过程中的对可维护性和可扩展性要求,深入研究了逐行解析消息头字段的基本流程和静态查找表的基本工作原理,提出了借助开源工具Gperf自动化构建高效静态查找表以实现头字段类型查询的方法,并实现了SIP消息头类型的快速匹配的目的.该方法改善了SIP消息解析器的性能和扩展性.     

Quantum dynamics of superconducting nano-circuits: phase qubit, charge qubit and rhombi chains

We have studied different quantum dynamics of superconducting nano-circuits with Josephson junctions. A dc SQUID, when it is strongly decoupled from the environment, demonstrates two-level and multilevel dynamics. We have realized a two qubits coupled circuit based on a dc SQUID in parallel with an asymmetric Cooper pair transistor (ACPT). The ACPT behaves as a charge qubit. Its asymmetry produces a strong tunable coupling with the dc SQUID which is used to realize entangled states between the two qubits and new read-out of the charge qubit based on adiabatic quantum transfer. We have measured the current–phase relations of different rhombi chains in the presence or absence of quantum fluctuations which confirm theoretical predictions.     

Simultaneous linear-quadratic optimal control design via static output feedback

In this paper, the problem of designing a fixed static output feedback control law which minimizes an upper bound on linear quadratic (LQ) performance measures for r distinct MIMO plants is addressed using linear matrix inequality (LMI) technique. An iterative LMI algorithm is proposed to obtain the solution. Examples are used to demonstrate its effectiveness. Copyright ©1999 John Wiley & Sons, Ltd.     

Fixed-structure robust controller synthesis via decentralized static output feedback

This paper describes and illustrates a unified methodology for robust, fixed-structure controller synthesis. The approach is based upon direct fixed-structure controller synthesis using a decentralized static output feedback formulation as a general framework for representing a large class of controller structures. Scaled Popov bounds for the real structured singular value are used to account for real parameter uncertainty and provide the means for optimizing a worst-case ℋ︁₂ cost bound with respect to the free parameters of the controller. Quasi-Newton optimization algorithms are used to solve the resulting numerical optimization problem. Initial stability multiplier and scaling matrices needed in scaled Popov synthesis are obtained by solving an LMI feasibility problem. Using both centralized and decentralized controller structures, numerical results are obtained for a 16th-order acoustic duct model with uncertain damped natural frequencies and for a two-dimensional beam-spring model with uncertain actuator locations. © 1998 John Wiley & Sons, Ltd.     

Block noninteracting control with stability via static state feedback

Necessary and sufficient conditions for the existence of a static state feedback that achieves noninteraction and internal stability are obtained. This is accomplished by first characterizing the set of all controllability subspaces (distributions) that can arise as solutions to the noninteracting control problem. This characterization is then used to identify a fixed internal dynamics that is common to every noninteractive closed loop. The stability properties of this dynamics is shown to be the key factor in the problem of achieving noninteracting control with internal stability.     

Protection of quantum Fisher information in entangled states via classical driving

Quantum entanglement can offer a quadratic enhancement in the precision of parameter estimation. We here study the protection of quantum Fisher information (QFI) of the phase parameter in entangled-atom states within the framework of independently dissipative environments and driven individually by classical fields. It is shown that the QFI of the phase parameter can be protected effectively only when the classical fields that drive all atoms are suitably strong, and if one of them vanishes or is very weak, then the ability of protection loses, no matter how strong the other driving fields are. We also study the evolution of fidelity of the entangled state itself and find that though the protections of QFI and quantum states are two different notions, the method can also be used to protect quantum states effectively when the driving fields are suitably strong.