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Majorana fermion exchange in strictly one-dimensional structures

EPL 2015年第1期110卷

作者： Chiu, Ching-Kai Vazifeh, M. M. Franz, M. Univ British Columbia Dept Phys & Astron Vancouver BC V6T 1Z1 Canada Univ British Columbia Quantum Matter Inst Vancouver BC V6T 1Z4 Canada

It is generally thought that the adiabatic exchange of two identical particles is impossible in one spatial dimension. Here we describe a simple protocol that permits the adiabatic exchange of two Majorana fermions in a one-dimensional topological superconductor wire. The exchange relies on the concept of "Majorana shuttle" whereby a pi domain wall in the superconducting order parameter which hosts a pair of ancillary majoranas delivers one zero mode across the wire while the other one tunnels in the opposite direction. The method requires some tuning of parameters and does not, therefore, enjoy full topological protection. The resulting exchange statistics, however, remain non-Abelian for a wide range of parameters that characterize the exchange. Copyright (C) EPLA, 2015

关键词： quantum computation architectures and implementations Fractional statistics systems (anyons, etc.) Phases: geometric, dynamic or topological

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Strain designed Josephson π-junction qubits with topological insulators

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EPL 2015年第5期110卷

作者： Benjamin, Colin Natl Inst Sci Educ & Res Bhubaneswar 751005 Orissa India

A Josephson qubit is designed via the application of a tensile strain to a topological insulator surface sandwiched between two s-wave superconductors. The strain applied leads to a shift in the Dirac point without changing the pre-existing conducting states, on the surface of a topological insulator. The strain applied can be tuned to form a pi-junction in such a structure. Combining two such junctions in a ring architecture leads to the ground state of the ring being in a doubly degenerate state -the "0" and "1" states of a qubit. A qubit designed this way is quite easily controlled via the tunable strain applied. We report on the conditions necessary to design such a qubit. Finally the operating time of a single-qubit phase gate is derived. Copyright (C) EPLA, 2015

关键词： quantum computation architectures and implementations Josephson devices Mechanical properties, surface strains

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All-optical router at single-photon level by interference

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EPL 2015年第6期111卷

作者： Yan, Wei-Bin Liu, Bao Zhou, Ling Fan, Heng Dalian Nationalities Univ Sch Phys & Mat Engn Liaoning Key Lab Optoelect Films & Mat Dalian 116600 Peoples R China Beijing Computat Sci Res Ctr CSRC Beijing 100084 Peoples R China Dalian Univ Technol Sch Phys & Optoelect Technol Dalian 116024 Peoples R China Chinese Acad Sci Inst Phys Beijing 100190 Peoples R China Collaborat Innovat Ctr Quantum Matter Beijing 100190 Peoples R China

The realization of the all-optical router with multiple input ports and output ports will have direct application to achieve the quantum network. We present a scheme of all-optical routing of coherent-state photons in a waveguide-cavity coupled system. Our router has four input ports and four output ports. The routing of photons is based on the interference. The outcomes show that the transport of the coherent-state photons injected through any input port can be controlled by the phases of the coherent-state photons injected through other input ports. This control can be realized at the single-photon level and requires no additional control fields. Therefore, the all-optical routing at single-photon level can be achieved. Copyright (C) EPLA, 2015

关键词： Cavity quantum electrodynamics, micromasers Scattering theory quantum computation architectures and implementations

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Fermionic computation is non-local tomographic and violates monogamy of entanglement

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EPL 2014年第2期107卷

作者： D'Ariano, G. M. Manessi, F. Perinotti, P. Tosini, A. Univ Pavia Dipartimento Fis QUIT Grp I-27100 Pavia Italy Ist Nazl Fis Nucl Sez Pavia I-27100 Pavia Italy

We show that the computational model based on local fermionic modes in place of qubits does not satisfy local tomography and monogamy of entanglement, and has mixed states with maximal entanglement of formation. These features directly follow from the parity superselection rule. We generalize quantum superselection rules to general probabilistic theories as sets of linear constraints on the convex set of states. We then provide a link between the cardinality of the superselection rule and the degree of holism of the resulting theory. Copyright (C) EPLA, 2014

关键词： Entanglement measures, witnesses, and other characterizations quantum computation architectures and implementations

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Topological phase transition and gate-controlled charge transfer in minimal topological-spin qubit circuit

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EPL 2014年第6期106卷

作者： Wang, Zhen-Hua Kuang, Xiao-Yu Zhong, Ming-Min Yang, Zai-Xiu Li, Hui Sichuan Univ Inst Atom & Mol Phys Chengdu 610065 Peoples R China

We propose a minimal topological-spin qubit circuit to investigate the non-Abelian rotations within the degenerate ground-state manifold, as well as the topological phase transition and charge transfer. From a quantum dot, the state of the Majorana system can be read out. Along with the splitting of the degenerate ground-state manifold of a topological qubit, a topological phase transition is observed, showing the existence of a Majorana fermion. By choosing the phase difference across the dots, the non-Abelian rotations and the required energetically degenerate state in the qubit can be achieved, which results in universal quantum computation. Moreover, on the basis of the measurement of the electron in quantum dots and topological qubit, this demonstrates that topologically protected tunnel braids between dots and MBSs are critical to non-Abelian rotations. Copyright (C) EPLA, 2014

关键词： Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems Electronic transport in nanoscale materials and structures quantum computation architectures and implementations

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Switching current distributions in Josephson junctions at very low temperatures

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EPL 2014年第6期107卷

作者： Blackburn, James A. Cirillo, Matteo Gronbech-Jensen, Niels Wilfrid Laurier Univ Waterloo ON N2L 3C5 Canada Univ Roma Tor Vergata Dipartimento Fis I-00133 Rome Italy Univ Roma Tor Vergata MINAS Lab I-00133 Rome Italy Univ Calif Davis Dept Appl Sci Davis CA 95616 USA CNR SPIN Inst Italy Genoa Italy

Swept bias experiments carried out on Josephson junctions yield the distributions of the probabilities of early switching from the zero-voltage state. Kramers' theory of the thermally activated escape from a one-dimensional potential is well known to fall short of explaining such experiments when the junctions are at millikelvin temperatures. We propose a simple revision of the theory which is shown to give extremely good agreement with experimental data. Copyright (C) EPLA, 2014

关键词： Tunneling phenomena, Josephson effects Josephson devices quantum computation architectures and implementations

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Disguising quantum channels by mixing and channel distance trade-off

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JOURNAL OF PHYSICS A-MATHEMATICAL AND THEORETICAL 2014年第10期47卷

作者： Fung, Chi-Hang Fred Chau, H. F. Univ Hong Kong Dept Phys Hong Kong Hong Kong Peoples R China Univ Hong Kong Ctr Theoret & Computat Phys Hong Kong Hong Kong Peoples R China

We consider the reverse problem of the distinguishability of two quantum channels, which we call the disguising problem. Given two quantum channels, the goal here is to make the two channels identical by mixing with some other channels with minimal mixing probabilities. This quantifies how much one channel can disguise as the other. In addition, the possibility to trade-off between the two mixing probabilities allows one channel to be more preserved (less mixed) at the expense of the other. We derive lower-and upper-bounds of the trade-off curve and apply them to a few example channels. Optimal trade-off is obtained in one example. We relate the disguising problem and the distinguishability problem by showing that the former can lower and upper bound the diamond norm. We also show that the disguising problem gives an upper-bound on the key generation rate in quantum cryptography.

关键词： quantum communication quantum computation architectures and implementations quantum cryptography and communication security quantum channels

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The decay of quantum correlations between quantum dot spin qubits and the characteristics of its magnetic-field dependence

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EPL 2014年第6期107卷

作者： Mazurek, P. Roszak, K. Horodecki, P. Univ Gdansk Inst Theoret Phys & Astrophys PL-80952 Gdansk Poland Natl Quantum Informat Ctr Gdansk PL-81824 Sopot Poland Wroclaw Univ Technol Inst Phys PL-50370 Wroclaw Poland Charles Univ Prague Fac Math & Phys Dept Condensed Matter Phys CR-12116 Prague Czech Republic Gdansk Univ Technol Fac Appl Phys & Math PL-80952 Gdansk Poland

We address the question of the role of quantum correlations beyond entanglement in the context of quantum magnetometry. We study the evolution of the rescaled variant of the geometric quantum discord of two electron-spin qubits interacting with an environment of nuclear spins via the hyperfine interaction. We have found that quantum correlations display a strong magnetic-field sensitivity which can be utilized for decoherence-driven measurements of the external magnetic field. The discord-based measurement is sensitive to a wider range of magnetic-field values than the entanglement-based measurement, including very small fields which are inaccessible via entanglement. Copyright (C) EPLA, 2014

关键词： Magnetic properties of nanostructures Decoherence, open systems, quantum statistical methods quantum computation architectures and implementations

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Period finding with adiabatic quantum computation

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EPL 2014年第5期105卷

作者： Hen, I. Univ So Calif Inst Informat Sci Marina Del Rey CA 90292 USA

We outline an efficient quantum-adiabatic algorithm that solves Simon's problem, in which one has to determine the "period", or xor mask, of a given black-box function. We show that the proposed algorithm is exponentially faster than its classical counterpart and has the same complexity as the corresponding circuit-based algorithm. Together with other related studies, this result supports a conjecture that the complexity of adiabatic quantum computation is equivalent to the circuit-based computational model in a stronger sense than the well-known, proven polynomial equivalence between the two paradigms. We also discuss the importance of the algorithm and its theoretical and experimental implications for the existence of an adiabatic version of Shor's integer factorization algorithm that would have the same complexity as the original algorithm. Copyright (C) EPLA, 2014

关键词： quantum algorithms, protocols, and simulations quantum computation architectures and implementations

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On the orthogonalization of quantum states

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EPL 2013年第1期103卷

作者： Xiao, Yong Yang, Guang Hebei Univ Coll Phys Sci & Technol Baoding 071002 Peoples R China Jiaozuo Univ Dept Basic Sci Jiaozuo 454000 Peoples R China

Some quantum states can dynamically evolve to an orthogonal state after a time interval, while some states can never evolve to their orthogonal states. In this paper, we investigate three issues on the orthogonalization properties of quantum states. First, we answer the question as to under what conditions an initial state can evolve to an orthogonal state. Second, we discuss the orthogonalization rate of quantum states, i.e., the total number of orthogonal states that the evolution can go through in unit time. Finally, we show that the Margolus-Levitin bound for orthogonalization time remains tight for superpositions of more than two eigenstates. Copyright (c) EPLA, 2013

关键词： quantum computation architectures and implementations Foundations of quantum mechanics, measurement theory

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