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Decoherence of quantum gates based on Aharonov-Anandan phases in a multistep scheme

EPL 2013年第4期102卷 40011-40011页

作者： Li, Xin Shi, Yu Fudan Univ State Key Lab Surface Phys Shanghai 200433 Peoples R China Fudan Univ Dept Phys Shanghai 200433 Peoples R China Kunming Univ Sci & Technol Fac Sci Kunming 650093 Peoples R China

We study quantum decoherence of single-qubit and two-qubit Aharonov-Anandan (AA) geometric phase gates realized in a multistep scheme. Each AA gate is also compared with the dynamical phase gate performing the same unitary transformation within the same time period and coupled with the same environment, which is modeled as harmonic oscillators. It is found that the fidelities and the entanglement protection of the AA phase gates are enhanced by the states being superpositions of different eigenstates of the environmental coupling, and the noncommutativity between the qubit interaction and the environmental coupling. Copyright (C) EPLA, 2013

关键词： quantum computation architectures and implementations Phases: geometric, dynamic or topological quantum error correction and other methods for protection against decoherence

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Intrinsic decoherence and Rabi oscillation damping of Mn²⁺ and Co²⁺ electron spin qubits in bulk ZnO

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EPL 2013年第4期104卷 47005-47004页

作者： Benzid, K. Chetoui, A. Maamache, M. Turek, P. Tribollet, J. Univ Strasbourg UDS CNRS Inst ChimUMR 7177 F-67081 Strasbourg France Univ Ferhat Abbas Setif 1 Dept Phys Lab Phys Quant & Systemes Dynam Setif Algeria

We demonstrate by pulse EPR that two electron spin qubits in bulk ZnO, the Mn2+ and the Co2+ spin qubits, which have, respectively, long (T-2(6K) = 178 mu s) and short (T-2(1.7K) = 9 mu s) transverse spin coherence time T-2 at low temperature, have however very short and similar Rabi oscillation damping times, on the order of TR approximate to 250 ns at low temperature. A detailed study of Mn2+ spin qubits has shown that the main contribution to the Rabi oscilation damping rate is temperature independent and proportional to the Rabi frequency. This main contribution to the damping rate during coherent microwave manipulation of spins is interpreted as due to the changes of the dipolar couplings induced by the long microwave pulse used in this kind of EPR nutation experiment. Strategies are suggested for overcoming this problem of Rabi oscillation overdamping in future spin-based quantum computers. Copyright (C) EPLA, 2013

关键词： Electron paramagnetic resonance and relaxation quantum computation architectures and implementations Iron group (3

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Majorana fermions in a periodically driven semiconductor-superconductor Heterostructure

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EPL 2013年第2期104卷

作者： Wu, C. C. Sun, J. Huang, F. J. Li, Y. D. Liu, W. M. Chinese Acad Sci Inst Phys Beijing Natl Lab Condensed Matter Phys Beijing 100190 Peoples R China Yunnan Univ Sch Phys Sci & Technol Kunming 650000 Peoples R China

We propose a new approach to create Majorana fermions at the edge of a periodically driven semiconductor-superconductor Heterostructure. We calculate the quasi-energy spectrum of the periodically driven Heterostructure by using Floquet's theory. When the interaction between different Brillouin zones of quasi-energy is neglected, one Majorana fermion with zero quasi-energy can be created at each edge of the Heterostructure when the ratio of driven amplitude and driven frequency is larger than a minimum. Furthermore, Majorana fermions with nonzero quasi-energy emerge when the driven frequency is low and the interaction between different Brillouin zones of quasi-energy needs to be considered. We also discuss the experimental protocol of creating Majorana fermions in the periodically driven Heterostructure. Copyright (C) EPLA, 2013

关键词： Fermions in reduced dimensions (anyons, composite fermions, Luttinger liquid, etc.) quantum computation architectures and implementations Proximity effects, Andreev reflection, SN and SNS junctions

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quantum speed limit and optimal evolution time in a two-level system

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EPL 2013年第4期104卷 40005-40004页

作者： Poggi, P. M. Lombardo, F. C. Wisniacki, D. A. Univ Buenos Aires Fac Ciencias Exactas & Nat Dept Fis Juan Jose Giambiagi RA-1428 Buenos Aires DF Argentina Univ Buenos Aires Fac Ciencias Exactas & Nat IFIBA RA-1428 Buenos Aires DF Argentina

quantum mechanics establishes a fundamental bound for the minimum evolution time between two states of a given system. Known as the quantum speed limit (QSL), it is a useful tool in the context of quantum control, where the speed of some control protocol is usually intended to be as large as possible. While QSL expressions for time-independent Hamiltonians have been well studied, the time-dependent regime has remained somewhat unexplored, albeit being usually the relevant problem to be compared with when studying systems controlled by external fields. In this paper we explore the relation between optimal times found in quantum control and the QSL bound, in the (relevant) time-dependent regime, by discussing the ubiquitous two-level Landau-Zener-type Hamiltonian. Copyright (C) EPLA, 2013

关键词： quantum systems with finite Hilbert space quantum computation architectures and implementations

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Readout of a qubit array via a single transmission line

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EPL 2011年第4期96卷

作者： Jerger, M. Poletto, S. Macha, P. Huebner, U. Lukashenko, A. Il'ichev, E. Ustinov, A. V. Karlsruhe Inst Technol Inst Phys D-76128 Karlsruhe Germany DFG Ctr Funct Nanostruct CFN D-76128 Karlsruhe Germany Inst Photon Technol D-07702 Jena Germany

Frequency-selective readout for superconducting qubits opens up the way towards scaling qubit circuits without increasing the number of measurement lines. Here we demonstrate the readout of an array of 7 flux qubits located on the same chip using a single measurement line. Each qubit is placed near an individual lambda/4 resonator which, in turn, is coupled to a common microwave transmission line. We performed spectroscopy and coherent manipulation of all qubits and determined their parameters in a single measurement run. Copyright (C) EPLA, 2011

关键词： quantum computation architectures and implementations Superconducting device characterization, design, and modeling

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Simulation of anyonic fractional statistics of Kitaev's toric model in circuit QED

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EPL 2011年第2期93卷

作者： Xue, Zheng-Yuan S China Normal Univ Lab Quantum Informat Technol ICMP Guangzhou 510006 Guangdong Peoples R China S China Normal Univ Sch Phys & Telecommun Engn Guangzhou 510006 Guangdong Peoples R China

Since the anyonic excitations in the Kitaev toric model are perfectly localized quasiparticles, it is possible to generate dynamically the ground state and the excitations of the model Hamiltonian to simulate the anyonic interferometry. We propose a scheme in circuit QED to simulate the interferometry. The qubit-cavity interaction can be engineered to realize effective state control as well as the controlled dynamics of qubits, which are sufficient to prepare the ground states, create and remove the anyonic excitation, and simulate the anyonic interferometry. The simplicity and high fidelity of the operations used open the very promising possibility of simulating fractional statistics of anyons in a macroscopic material in the near future. Copyright (C) EPLA, 2011

关键词： quantum computation architectures and implementations quantum state engineering and measurements Josephson devices

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Implementation of the Deutsch-Jozsa algorithm violates nonlocal realism

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EUROPEAN PHYSICAL JOURNAL D 2010年第3期56卷 441-444页

作者： Nagata, K. Obihiro Univ Agr & Vet Med Obihiro Hokkaido Japan

The theoretical resource state for the implementation of the Deutsch-Jozsa algorithm is a multiqubit pure uncorrelated state. We show that N-qubit pure uncorrelated quantum states cannot admit rotationally invariant nonlocal realistic theories with a violation factor of 3(N). We find the violation factor 3(N)when the measurement setup is entire range of settings for each of the observers, that is, considering rotationally invariant nonlocal realistic theories along with the property of a correlation function in the quantum theory. The implementation of the Deutsch-Jozsa algorithm theoretically relying on N-qubit pure uncorrelated states rules out rotationally invariant nonlocal realism with a violation factor of 3(N)in an ideal case. Our analysis relies on the property of theoretical resource states for the algorithm. We cannot simulate the Deutsch-Jozsa algorithm by using rotationally invariant nonlocal realistic theories due to the property of theoretical resource states for the algorithm.

关键词： quantum computation architectures and implementations Entanglement and quantum nonlocality Formalism

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Theory of the electron and nuclear spin coherence times of shallow donor spin qubits in isotopically and chemically purified zinc oxide

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EUROPEAN PHYSICAL JOURNAL B 2009年第4期72卷 531-540页

作者： Tribollet, J. Rue Victor Hugo F-33400 Talence France

In this article, I present a theoretical study of the electron and nuclear spin coherence times of shallow donor spin qubits in zinc oxide (ZnO) at low temperature. The influence of different spin-phonon processes as well as different spin-spin processes on the spin coherence time of shallow donors in ZnO is considered, both in the case of an electron spin qubit and in the case of a nuclear spin qubit encoded on a shallow donor. It is estimated that the electron spin coherence time of an isolated indium shallow donor in natural quasi-intrinsic ZnO is on the order of hundreds of microseconds, limited by the nuclear spectral diffusion process. The electron spin coherence time of an isolated indium shallow donor can be extended to few milliseconds in isotopically and chemically purified quasi-intrinsic ZnO. In this optimal case, the electron spin coherence time of an isolated indium shallow donor is only limited by a spin-lattice decoherence process. It is also estimated that the nuclear spin coherence time of an isolated indium shallow donor in natural quasi-intrinsic ZnO is on the order of hundreds of milliseconds, limited by the nuclear spectral diffusion process. The nuclear spin coherence time of an isolated indium shallow donor can be extended to few seconds in isotopically and chemically purified quasi-intrinsic ZnO. In this optimal case, the nuclear spin coherence time of an isolated indium shallow donor is only limited by the cross relaxation decoherence process. This study thus shows the great potential of electron and nuclear spin qubits encoded on shallow donors in isotopically and chemically purified quasi-intrinsic ZnO for the implementation of quantum processor and/or quantum memories.

关键词： quantum computation architectures and implementations Electron paramagnetic resonance and relaxation

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Deterministic generation of entangled photons by cavity-assisted interactions through adiabatic passage

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EUROPEAN PHYSICAL JOURNAL D 2009年第2期51卷 275-282页

作者： Zhang, X. L. Zhang, Y. X. Feng, M. Chongqing Univ Ctr Modern Phys Chongqing 400044 Peoples R China Chongqing Univ Dept Phys Chongqing 400044 Peoples R China Chinese Acad Sci Wuhan Inst Phys & Math State Key Lab Magnet Resonance & Atom & Mol Phys Wuhan 430071 Peoples R China

We propose a deterministic scheme for generation of highly entangled photon states using a high-Q two-mode optical cavity and the dark state evolution. Because of the adiabatic operation, our proposal is robust to ambient noise, and the relevant dynamics is insensitive to the randomness of moderate fluctuations regarding experimental parameters. Our scheme not only works deterministically, but also has the advantage of achieving highly entangled photons by adiabatically increasing or decreasing the Rabi frequencies regarding the classical driving pulses, which would be practical in real implementation. Our scheme can also be extended to generation of multiphoton entanglement.

关键词： quantum computation architectures and implementations quantum communication quantum state engineering and measurements

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quantum computation in a decoherence-free subspace with superconducting devices

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EUROPEAN PHYSICAL JOURNAL D 2009年第1期55卷 223-228页

作者： Xue, Z. -Y. Zhu, S. L. Wang, Z. D. 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 S China Normal Univ Lab Quantum Informat Technol ICMP Guangzhou 510006 Guangdong Peoples R China S China Normal Univ SPTE Guangzhou 510006 Guangdong Peoples R China

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.

关键词： quantum computation architectures and implementations quantum state engineering and measurements Josephson devices

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