448 research outputs found
Controlling nematodes in gardens
"Nematodes cause serious damage to gardens in Southeast Missouri. These pests can occur in other Missouri areas but are less common there. Nematodes are a greater problem where there are long, warm growing seasons and lighter, sandier soils."--First page.H.F. DiCarlo (Department of Horticulture), James A. Wrath (Plant Pathology, College of Agriculture)Revised 5/90/8
Computing prime factors with a Josephson phase qubit quantum processor
A quantum processor (QuP) can be used to exploit quantum mechanics to find
the prime factors of composite numbers[1]. Compiled versions of Shor's
algorithm have been demonstrated on ensemble quantum systems[2] and photonic
systems[3-5], however this has yet to be shown using solid state quantum bits
(qubits). Two advantages of superconducting qubit architectures are the use of
conventional microfabrication techniques, which allow straightforward scaling
to large numbers of qubits, and a toolkit of circuit elements that can be used
to engineer a variety of qubit types and interactions[6, 7]. Using a number of
recent qubit control and hardware advances [7-13], here we demonstrate a
nine-quantum-element solid-state QuP and show three experiments to highlight
its capabilities. We begin by characterizing the device with spectroscopy.
Next, we produces coherent interactions between five qubits and verify bi- and
tripartite entanglement via quantum state tomography (QST) [8, 12, 14, 15]. In
the final experiment, we run a three-qubit compiled version of Shor's algorithm
to factor the number 15, and successfully find the prime factors 48% of the
time. Improvements in the superconducting qubit coherence times and more
complex circuits should provide the resources necessary to factor larger
composite numbers and run more intricate quantum algorithms.Comment: 5 pages, 3 figure
Performance-optimized hierarchical models predict neural responses in higher visual cortex
Logical-qubit operations in an error-detecting surface code
We realize a suite of logical operations on a distance-two logical qubit
stabilized using repeated error detection cycles. Logical operations include
initialization into arbitrary states, measurement in the cardinal bases of the
Bloch sphere, and a universal set of single-qubit gates. For each type of
operation, we observe higher performance for fault-tolerant variants over
non-fault-tolerant variants, and quantify the difference through detailed
characterization. In particular, we demonstrate process tomography of logical
gates, using the notion of a logical Pauli transfer matrix. This integration of
high-fidelity logical operations with a scalable scheme for repeated
stabilization is a milestone on the road to quantum error correction with
higher-distance superconducting surface codes.Comment: 16 pages, 9 figures, 2 table
Noiseless nonreciprocity in a parametric active device
Nonreciprocal devices such as circulators and isolators belong to an
important class of microwave components employed in applications like the
measurement of mesoscopic circuits at cryogenic temperatures. The measurement
protocols usually involve an amplification chain which relies on circulators to
separate input and output channels and to suppress backaction from different
stages on the sample under test. In these devices the usual reciprocal symmetry
of circuits is broken by the phenomenon of Faraday rotation based on magnetic
materials and fields. However, magnets are averse to on-chip integration, and
magnetic fields are deleterious to delicate superconducting devices. Here we
present a new proposal combining two stages of parametric modulation emulating
the action of a circulator. It is devoid of magnetic components and suitable
for on-chip integration. As the design is free of any dissipative elements and
based on reversible operation, the device operates noiselessly, giving it an
important advantage over other nonreciprocal active devices for quantum
information processing applications.Comment: 17 pages, 4 figures + 12 pages Supplementary Informatio
Efficient creation of multipartite entanglement in flux qubits
We investigate three superconducting flux qubits coupled in a loop. In this
setup, tripartite entanglement can be created in a natural, controllable, and
stable way. Both generic kinds of tripartite entanglement -the W type as well
as the GHZ type entanglement- can be identified among the eigenstates. We also
discuss the violation of Bell inequalities in this system and show the impact
of a limited measurement fidelity on the detection of entanglement and quantum
nonlocality.Comment: 15 pages, 7 figures; extended sections on coupling strength, system
preparation, and entanglement detectio
Charge fluctuations in open chaotic cavities
We present a discussion of the charge response and the charge fluctuations of
mesoscopic chaotic cavities in terms of a generalized Wigner-Smith matrix. The
Wigner-Smith matrix is well known in investigations of time-delay of quantum
scattering. It is expressed in terms of the scattering matrix and its
derivatives with energy. We consider a similar matrix but instead of an energy
derivative we investigate the derivative with regard to the electric potential.
The resulting matrix is then the operator of charge. If this charge operator is
combined with a self-consistent treatment of Coulomb interaction, the charge
operator determines the capacitance of the system, the non-dissipative
ac-linear response, the RC-time with a novel charge relaxation resistance, and
in the presence of transport a resistance that governs the displacement
currents induced into a nearby conductor. In particular these capacitances and
resistances determine the relaxation rate and dephasing rate of a nearby qubit
(a double quantum dot). We discuss the role of screening of mesoscopic chaotic
detectors. Coulomb interaction effects in quantum pumping and in photon
assisted electron-hole shot noise are treated similarly. For the latter we
present novel results for chaotic cavities with non-ideal leads.Comment: 29 pages, 13 figures;v.2--minor changes; contribution for the special
issue of J. Phys. A on "Trends in Quantum Chaotic Scattering
Residual Kondo effect in quantum dot coupled to half-metallic ferromagnets
We study the Kondo effect in a quantum dot coupled to half-metallic
ferromagnetic electrodes in the regime of strong on-dot correlations. Using the
equation of motion technique for nonequilibrium Green functions in the slave
boson representation we show that the Kondo effect is not completely suppressed
for anti-parallel leads magnetization. In the parallel configuration there is
no Kondo effect but there is an effect associated with elastic cotunneling
which in turn leads to similar behavior of the local (on-dot) density of states
(LDOS) as the usual Kondo effect. Namely, the LDOS shows the temperature
dependent resonance at the Fermi energy which splits with the bias voltage and
the magnetic field. Moreover, unlike for non-magnetic or not fully polarized
ferromagnetic leads the only minority spin electrons can form such resonance in
the density of states. However, this resonance cannot be observed directly in
the transport measurements and we give some clues how to identify the effect in
such systems.Comment: 15 pages, 8 figures, accepted for publication in J. Phys.: Condens.
Mat
Detailed Structure of a CDW in a Quenched Random Field
Using high resolution x-ray scattering, we have measured the structure of the
Q_1 CDW in Ta-doped NbSe_3. Detailed line shape analysis of the data
demonstrates that two length scales are required to describe the phase-phase
correlation function. Phase fluctuations with wavelengths less than a new
length scale are suppressed and this is identified with the amplitude
coherence length. We find that xi_a* = 34.4 \pm 10.3 angstroms. Implications
for the physical mechanisms responsible for pinning are discussed.Comment: revtex 3.0, 3 postscript uuencoded figure
Rescaling multipartite entanglement measures for mixed states
A relevant problem regarding entanglement measures is the following: Given an
arbitrary mixed state, how does a measure for multipartite entanglement change
if general local operations are applied to the state? This question is
nontrivial as the normalization of the states has to be taken into account.
Here we answer it for pure-state entanglement measures which are invariant
under determinant 1 local operations and homogeneous in the state coefficients,
and their convex-roof extension which quantifies mixed-state entanglement. Our
analysis allows to enlarge the set of mixed states for which these important
measures can be calculated exactly. In particular, our results hint at a
distinguished role of entanglement measures which have homogeneous degree 2 in
the state coefficients.Comment: Published version plus one important reference (Ref. [39]
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