3,977 research outputs found

    Use of composite rotations to correct systematic errors in NMR quantum computation

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    We implement an ensemble quantum counting algorithm on three NMR spectrometers with 1H resonance frequencies of 500, 600 and 750 MHz. At higher frequencies, the results deviate markedly from naive theoretical predictions. These systematic errors can be attributed almost entirely to off-resonance effects, which can be substantially corrected for using fully-compensating composite rotation pulse sequences originally developed by Tycko. We also derive an analytic expression for generating such sequences with arbitrary rotation angles.Comment: 8 pages RevTex including 7 PostScript figures (18 subfigures

    Robust Logic Gates and Realistic Quantum Computation

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    The composite rotation approach has been used to develop a range of robust quantum logic gates, including single qubit gates and two qubit gates, which are resistant to systematic errors in their implementation. Single qubit gates based on the BB1 family of composite rotations have been experimentally demonstrated in a variety of systems, but little study has been made of their application in extended computations, and there has been no experimental study of the corresponding robust two qubit gates to date. Here we describe an application of robust gates to Nuclear Magnetic Resonance (NMR) studies of approximate quantum counting. We find that the BB1 family of robust gates is indeed useful, but that the related NB1, PB1, B4 and P4 families of tailored logic gates are less useful than initially expected.Comment: 6 pages RevTex4 including 5 figures (3 low quality to save space). Revised at request of referee and incorporting minor corrections and updates. Now in press at Phys Rev

    Experimental investigation of non-uniform heating on flow boiling instabilities in a microchannels based heat sink

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    Two-phase flow boiling in microchannels is one of the most promising cooling technologies able to cope with high heat fluxes generated by the next generation of central processor units (CPU). If flow boiling is to be used as a thermal management method for high heat flux electronics it is necessary to understand the behaviour of a non-uniform heat distribution, which is typically the case observed in a real operating CPU. The work presented is an experimental study of two-phase boiling in a multi-channel silicon heat sink with non-uniform heating, using water as a cooling liquid. Thin nickel film sensors, integrated on the back side of the heat sinks were used in order to gain insight related to temperature fluctuations caused by two-phase flow instabilities under non-uniform heating. The effect of various hotspot locations on the temperature profile and pressure drop has been investigated, with hotspots located in different positions along the heat sink. It was observed that boiling inside microchannels with non-uniform heating led to high temperature non-uniformity in transverse direction

    Equivalent qubit dynamics under classical and quantum noise

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    We study the dynamics of quantum systems under classical and quantum noise, focusing on decoherence in qubit systems. Classical noise is described by a random process leading to a stochastic temporal evolution of a closed quantum system, whereas quantum noise originates from the coupling of the microscopic quantum system to its macroscopic environment. We derive deterministic master equations describing the average evolution of the quantum system under classical continuous-time Markovian noise and two sets of master equations under quantum noise. Strikingly, these three equations of motion are shown to be equivalent in the case of classical random telegraph noise and proper quantum environments. Hence fully quantum-mechanical models within the Born approximation can be mapped to a quantum system under classical noise. Furthermore, we apply the derived equations together with pulse optimization techniques to achieve high-fidelity one-qubit operations under random telegraph noise, and hence fight decoherence in these systems of great practical interest.Comment: 5 pages, 2 figures; converted to PRA format, added Fig. 2, corrected typo

    High fidelity one-qubit operations under random telegraph noise

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    We address the problem of implementing high fidelity one-qubit operations subject to time dependent noise in the qubit energy splitting. We show with explicit numerical results that high fidelity bit flips and one-qubit NOT gates may be generated by imposing bounded control fields. For noise correlation times shorter than the time for a pi-pulse, the time optimal pi-pulse yields the highest fidelity. For very long correlation times, fidelity loss is approximately due to systematic error, which is efficiently tackled by compensation for off-resonance with a pulse sequence (CORPSE). For intermediate ranges of the noise correlation time we find that short CORPSE, which is less accurate than CORPSE in correcting systematic errors, yields higher fidelities. Numerical optimization of the pulse sequences using gradient ascent pulse engineering results in noticeable improvement of the fidelities for the bit flip and marginal improvement for the NOT gate.Comment: 7 pages, 6 figure

    A precise CNOT gate in the presence of large fabrication induced variations of the exchange interaction strength

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    We demonstrate how using two-qubit composite rotations a high fidelity controlled-NOT (CNOT) gate can be constructed, even when the strength of the interaction between qubits is not accurately known. We focus on the exchange interaction oscillation in silicon based solid-state architectures with a Heisenberg Hamiltonian. This method easily applies to a general two-qubit Hamiltonian. We show how the robust CNOT gate can achieve a very high fidelity when a single application of the composite rotations is combined with a modest level of Hamiltonian characterisation. Operating the robust CNOT gate in a suitably characterised system means concatenation of the composite pulse is unnecessary, hence reducing operation time, and ensuring the gate operates below the threshold required for fault-tolerant quantum computation.Comment: 9 pages, 8 figure

    Generalization of short coherent control pulses: extension to arbitrary rotations

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    We generalize the problem of the coherent control of small quantum systems to the case where the quantum bit (qubit) is subject to a fully general rotation. Following the ideas developed in Pasini et al (2008 Phys. Rev. A 77, 032315), the systematic expansion in the shortness of the pulse is extended to the case where the pulse acts on the qubit as a general rotation around an axis of rotation varying in time. The leading and the next-leading corrections are computed. For certain pulses we prove that the general rotation does not improve on the simpler rotation with fixed axis.Comment: 6 pages, no figures; published versio

    Laboratory and Field Studies of Resistance of Crab Apple Clones to Rhagoletis pomonella (Diptera: Tephritidae)

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    Oviposition and larval survival of Rhagoletis pomonella (Walsh) varied significantly among fruit from 25 crab apple speciesand clones evaluated in field and laboratory studies. In general, the relative oviposition preference and larval survival was similar in fruit infested naturally in the field and fruit tested in the laboratory. Flies oviposited more in clones with larger fruit, although this relationship was more pronounced in laboratory tests when fruit was infested by laboratory-reared flies than in fruit infested in the field by wild flies. ‘Aldenhamensis,' ‘Fuji,' ‘Vilmorin,' Malus zumi calocarpa Rehd., and M. hupehensis (Pamp) Rehd. fruit was not infested in the field, but flies oviposited in fruit of all 25 species and clones in choice tests in the laboratory. Eggs hatched but larvae did not survive in fruit of ‘Henry F. DuPont,' ‘Frettingham,' ‘Fuji,' ‘Sparkler,' M. hupehensis, and M. zumi calocarpa. Larval mortality was very high in fruit from ‘Vilmorin,' ‘Sparkler,' ‘NA 40298,' ‘Henrietta Crosby,' ‘Golden Gem,' ‘Almey,' M. baccata L. (Borkh.), and M. sikktmensis (Hook.) Koehn
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