Quantum computation in a decoherence-free subspace with superconducting devices

We propose a scheme to implement quantum computation in decoherence-free subspace with superconducting devices inside a cavity by unconventional geometric manipulation. Universal single-qubit gates in encoded qubit can be achieved with cavity assisted interaction. A measurement-based two-qubit Controlled-Not gate is produced with parity measurements assisted by an auxiliary superconducting device and followed by prescribed single-qubit gates. The measurement of currents on two parallel devices can realize a projective measurement, which is equivalent to the parity measurement on the involved devices.     

Universal quantum computation in decoherence-free subspace with neutral atoms

We show how realistic cavity-assisted interaction between neutral atoms and coherent optical pulses, and measurement techniques, combined with optical transportation of atoms, allow for a universal set of quantum gates acting on decoherence--free subspace in a deterministic way. The logical qubits are immunized to the dominant source of decoherece-dephasing, while the influences of additional errors are shown by numerical simulations. We analyze the performance and stability of all required operations and emphasize that all techniques are feasible with current experimental technology.     

Fault-tolerant quantum computation via exchange interactions

Quantum computation can be performed by encoding logical qubits into the states of two or more physical qubits, and control of effective exchange interactions and possibly a global magnetic field. This "encoded universality" paradigm offers potential simplifications in quantum computer design since it does away with the need to control physical qubits individually. Here we show how encoded universality schemes can be combined with fault-tolerant quantum error correction, thus establishing the scalability of such schemes.     

Distributed quantum computation via optical fibers

We investigate the possibility of realizing effective quantum gates between two atoms in distant cavities coupled by an optical fiber. We show that highly reliable swap and entangling gates are achievable. We exactly study the stability of these gates in the presence of imperfections in coupling strengths and interaction times and prove them to be robust. Moreover, we analyze the effect of spontaneous emission and losses and show that such gates are very promising in view of the high level of coherent control currently achievable in optical cavities.     

Distributed quantum computing in decoherence-free subspace via adiabatic passage

A scheme is proposed to implement distributed quantum computation in decoherence-free subspaces (DFSs) via adiabatic passage. The logical single-qubit is encoded in two atoms trapped in a single-mode cavity and the cavities are connected by an optical fiber. Our scheme is immune from the decoherence due to dephasing in virtue of encoding scheme and the decoherence due to spontaneous emission from excited states as the system in our scheme evolves along a dark state. Furthermore, the decoherence due to photon decay is greatly suppressed since the fiber mode remains in a vacuum state and the populations of the cavities’ modes being excited can be negligible under certain condition. It is shown that the minimum fidelity of the resultant gate operation for an arbitrary input state could be over 0.97.     

Quantum superdense coding via cavity-assisted interactions

Quantum superdense coding (QSC) is an example of how entanglement can be used to minimize the number of carriers of classical information. This paper proposes two schemes for implementing QSC by means of cavity assisted interactions with single-photon pulses. The schemes are insensitive to the cavity decay and the thermal field, thus it might be realizable based on the current cavity QED techniques.     

Time reversible evolution via nonadiabatic coupling in adiabatic dark subspace

We propose a method for the creation of arbitrary superposition of N atomic states using generalized stimulated Raman adiabatic passage (STIRAP) techniques with laser fields coupling each one of N lower states to a single upper state in a (N+1)-level atomic system. (N-1) dark states that are composed of N lower states span a dark subspace. In the adiabatic limit, the dark and bright subspaces are decoupled, thus the nonadiabatic interaction within this dark subspace dominates the evolution of the system. Different from general methods to create our required coherent superposition state, in a reverse way, here we consider the required state as the starting point of evolution dynamics, and utilize laser fields to drive it into a single lower state step by step. Time reverse pulses of laser fields return the single lower state back to our required coherent superposition state based on time reversal symmetry. In principle, the computationally simple method allows the case with a large value of N. Based on the STIRAP techniques, it is robust against small variations of parameters of laser pulses and is immune to spontaneous radiation.     

Measurement-only topological quantum computation via anyonic interferometry

We describe measurement-only topological quantum computation using both projective and interferometrical measurement of topological charge. We demonstrate how anyonic teleportation can be achieved using “forced measurement” protocols for both types of measurement. Using this, it is shown how topological charge measurements can be used to generate the braiding transformations used in topological quantum computation, and hence that the physical transportation of computational anyons is unnecessary. We give a detailed discussion of the anyonics for implementation of topological quantum computation (particularly, using the measurement-only approach) in fractional quantum Hall systems.     

Sparse representation-based image restoration via nonlocal supervised coding

Sparse representation (SR) and nonlocal technique (NLT) have shown great potential in low-level image processing. However, due to the degradation of the observed image, SR and NLT may not be accurate enough to obtain a faithful restoration results when they are used independently. To improve the performance, in this paper, a nonlocal supervised coding strategy-based NLT for image restoration is proposed. The novel method has three main contributions. First, to exploit the useful nonlocal patches, a nonnegative sparse representation is introduced, whose coefficients can be utilized as the supervised weights among patches. Second, a novel objective function is proposed, which integrated the supervised weights learning and the nonlocal sparse coding to guarantee a more promising solution. Finally, to make the minimization tractable and convergence, a numerical scheme based on iterative shrinkage thresholding is developed to solve the above underdetermined inverse problem. The extensive experiments validate the effectiveness of the proposed method.     

Loss tolerance in one-way quantum computation via counterfactual error correction

We introduce a scheme for fault tolerantly dealing with losses (or other "leakage" errors) in cluster state computation that can tolerate up to 50% qubit loss. This is achieved passively using an adaptive strategy of measurement--no coherent measurements or coherent correction is required. Since the scheme relies on inferring information about what would have been the outcome of a measurement had one been able to carry it out, we call this counterfactual error correction.     

Coupled flow-polymer dynamics via statistical field theory: Modeling and computation

Field-theoretic models, which replace interactions between polymers with interactions between polymers and one or more conjugate fields, offer a systematic framework for coarse-graining of complex fluids systems. While this approach has been used successfully to investigate a wide range of polymer formulations at equilibrium, field-theoretic models often fail to accurately capture the non-equilibrium behavior of polymers, especially in the early stages of phase separation. Here the “two-fluid” approach serves as a useful alternative, treating the motions of fluid components separately in order to incorporate asymmetries between polymer molecules. In this work we focus on the connection of these two theories, drawing upon the strengths of each of the approaches in order to couple polymer microstructure with the dynamics of the flow in a systematic way. For illustrative purposes we work with an inhomogeneous melt of elastic dumbbell polymers, though our methodology will apply more generally to a wide variety of inhomogeneous systems. First we derive the model, incorporating thermodynamic forces into a two-fluid model for the flow through the introduction of conjugate chemical potential and elastic strain fields for the polymer density and stress. The resulting equations are composed of a system of fourth order PDEs coupled with a non-linear, non-local optimization problem to determine the conjugate fields. The coupled system is severely stiff and with a high degree of computational complexity. Next, we overcome the formidable numerical challenges posed by the model by designing a robust semi-implicit method based on linear asymptotic behavior of the leading order terms at small scales, by exploiting the exponential structure of global (integral) operators, and by parallelizing the non-linear optimization problem. The semi-implicit method effectively removes the fourth order stability constraint associated with explicit methods and we observe only a first order time-step restriction. The algorithm for solving the non-linear optimization problem, which takes advantage of the form of the operators being optimized, reduces the overall simulation time by several orders of magnitude. We illustrate the methodology with several examples of phase separation in an initially quiescent flow.     

Generation of entanglement in finite spin systems via adiabatic quantum computation

For a finite XY chain and a finite two-dimensional Ising lattice, it is shown that the paramagnetic ground state is adiabatically transformed to the Greenberger-Horne-Zeilinger state in the ferromagnetic phase by changing slowly the external magnetic field. It is found that the fidelity between the Greenberger-Horne-Zeilinger state and an adiabatically evolved state depends on the interpolation schemes as well as the energy gap between the ground and exited states. A possibility whether quantum phase transitions can be simulated on adiabatic quantum computation is discussed.     

Coalescence analysis for evolving foams via optical flow computation on projection image sequences

A novel image‐processing procedure is proposed for the analysis of sequences of two‐dimensional projection images. Sudden events like the merging of bubbles in an evolving foam can be detected and spatio‐temporally located in a given projection image sequence. The procedure is based on optical flow computations extended by a forward–backward check for each time step. Compared with prior methods, efficient suppression of noise or false events is achieved owing to uniform foam motion, and the reliability of detection is thus increased. The applicability of the proposed procedure in combination with synchrotron radiography is illustrated by a series of characteristic studies of foams of different kind. First, the detection of single‐bubble collapses in aqueous foams is considered. Second, a spatial distribution of coalescence events in metals foamed in casting molds is estimated. Finally, the structural stability of polymer foams containing admixed solid nanoparticles is examined.     

Distributed quantum computation with superconducting qubit via LC circuit using dressed states

A scheme is proposed where two superconducting qubits driven by a classical field interacting separately with two distant LC circuits connected by another LC circuit through mutual inductance,are used for implementing quantum gates.By using dressed states,quantum state transfer and quantum entangling gate can be implemented.With the help of the time-dependent electromagnetic field,any two dressed qubits can be selectively coupled to the data bus (the last LC circuit),then quantum state can be transferred from one dressed qubit to another and multi-mode entangled state can also be formed.As a result,the promising perspectives for quantum information processing of mesoscopic superconducting qubits are obtained and the distributed and scalable quantum computation can be implemented in this scheme.     

Nonadiabatic geometric quantum computation protected by dynamical decoupling via the XXZ Hamiltonian

Nonadiabatic geometric quantum computation protected by dynamical decoupling combines the robustness of nonadiabatic geometric gates and the decoherence-resilience feature of dynamical decoupling. Solid-state systems provide an appealing candidate for the realization of nonadiabatic geometric quantum computation protected dynamical decoupling since the solid-state qubits are easily embedded in electronic circuits and scaled up to large registers. In this paper, we put forward a scheme of nonadiabatic geometric quantum computation protected by dynamical decoupling via the XXZ Hamiltonian, which not only combines the merits of nonadiabatic geometric gates and dynamical decoupling but also can be realized in a number of solid-state systems, such as superconducting circuits and quantum dots.     

Passive and temporally continuous optical spectral analyzer of RF signals via wavelength coding

A novel approach realizing an optical spectrum analyzer for photonic detection of an unknown RF carrier signals is presented. The described module may be part of an electronic warfare system in which detection of a narrow band RF signal is required. Moreover, The RF signal is characterized by an unknown time varying carrier frequency embedded in wide band noise. The system uses a passive, fiber based photonic configuration. It allows the spectrum mapping of an incoming electronic RF signal modulated on an optical carrier. The spectral analyzer configuration uses a finite impulse response (FIR) filter that is realized by two different optical paths of parallel fibers which generate a spectral notch filter. Hence, a wavelength coding is realized by chromatic dispersion such that each wavelength is filtered by a different FIR filter. Therefore, the energy at a WDM demux output channels is actually proportional to the spectrum of the input RF signal. This spectral mapping is obtained without lose of temporal RF information.     

One-step implementing three-qubit phase gate via manipulating rf SQUID qubits in the decoherence-free subspace with respect to cavity decay

We present a scheme for implementing a three-qubit phase gate via manipulating rf superconducting quantum interference device (SQUID) qubits in the decoherence-free subspace with respect to cavity decay. Through appropriate changes of the coupling constants between rf SQUIDs and cavity, the scheme can be realized only in one step. A high fidelity is obtained even in the presence of decoherence.     

波束域加权子空间拟合算法

将阵元域高分辨加权子空间拟合算法(WSF算法)推广到了波束域中,利用声系统接收基阵波束输出信息估计目标方位.由于波束输出可以看作是,一个阵元数为波束个数的虚拟阵的阵元输出,而在大多数的水声系统中,参与确定某个来向的信号方位的波束数通常远小于阵元数,从而算法的运算量得以大大降低。针对某水声系统接收基阵波束输出所作的计算机仿真结果表明,波束域WSF算法应用到实际的水声系统时,可以保持其在阵元域中估计目标方位的优越性能。