541 research outputs found
Monotonicity of Phaselocked Solutions in Chains and Arrays of Nearest-Neighbor Coupled Oscillators
Double quantum dot with integrated charge sensor based on Ge/Si heterostructure nanowires
Coupled electron spins in semiconductor double quantum dots hold promise as
the basis for solid-state qubits. To date, most experiments have used III-V
materials, in which coherence is limited by hyperfine interactions. Ge/Si
heterostructure nanowires seem ideally suited to overcome this limitation: the
predominance of spin-zero nuclei suppresses the hyperfine interaction and
chemical synthesis creates a clean and defect-free system with highly
controllable properties. Here we present a top gate-defined double quantum dot
based on Ge/Si heterostructure nanowires with fully tunable coupling between
the dots and to the leads. We also demonstrate a novel approach to charge
sensing in a one-dimensional nanostructure by capacitively coupling the double
dot to a single dot on an adjacent nanowire. The double quantum dot and
integrated charge sensor serve as an essential building block required to form
a solid-state spin qubit free of nuclear spin.Comment: Related work at http://marcuslab.harvard.edu and
http://cmliris.harvard.ed
Resonant Adiabatic Passage with Three Qubits
We investigate the non-adiabatic implementation of an adiabatic quantum
teleportation protocol, finding that perfect fidelity can be achieved through
resonance. We clarify the physical mechanisms of teleportation, for three
qubits, by mapping their dynamics onto two parallel and mutually-coherent
adiabatic passage channels. By transforming into the adiabatic frame, we
explain the resonance by analogy with the magnetic resonance of a spin-1/2
particle. Our results establish a fast and robust method for transferring
quantum states, and suggest an alternative route toward high precision quantum
gates
Quantum computing with antiferromagnetic spin clusters
We show that a wide range of spin clusters with antiferromagnetic
intracluster exchange interaction allows one to define a qubit. For these spin
cluster qubits, initialization, quantum gate operation, and readout are
possible using the same techniques as for single spins. Quantum gate operation
for the spin cluster qubit does not require control over the intracluster
exchange interaction. Electric and magnetic fields necessary to effect quantum
gates need only be controlled on the length scale of the spin cluster rather
than the scale for a single spin. Here, we calculate the energy gap separating
the logical qubit states from the next excited state and the matrix elements
which determine quantum gate operation times. We discuss spin cluster qubits
formed by one- and two-dimensional arrays of s=1/2 spins as well as clusters
formed by spins s>1/2. We illustrate the advantages of spin cluster qubits for
various suggested implementations of spin qubits and analyze the scaling of
decoherence time with spin cluster size.Comment: 15 pages, 7 figures; minor change
Extreme Type-II Superconductors in a Magnetic Field: A Theory of Critical Fluctuations
A theory of critical fluctuations in extreme type-II superconductors
subjected to a finite but weak external magnetic field is presented. It is
shown that the standard Ginzburg-Landau representation of this problem can be
recast, with help of a novel mapping, as a theory of a new "superconductor", in
an effective magnetic field whose overall value is zero, consisting of the
original uniform field and a set of neutralizing unit fluxes attached to
fluctuating vortex lines. The long distance behavior is related to
the anisotropic gauge theory in which the original magnetic field plays the
role of "charge". The consequences of this "gauge theory" scenario for the
critical behavior in high temperature superconductors are explored in detail,
with particular emphasis on questions of 3D XY vs. Landau level scaling,
physical nature of the vortex "line liquid" and the true normal state, and
fluctuation thermodynamics and transport. A "minimal" set of requirements for
the theory of vortex-lattice melting in the critical region is also proposed
and discussed.Comment: 28 RevTeX pages, 4 .ps figures; appendix A added, additional
references, streamlined Secs. IV and V in response to referees' comment
Ventricular beat detection in single channel electrocardiograms
BACKGROUND: Detection of QRS complexes and other types of ventricular beats is a basic component of ECG analysis. Many algorithms have been proposed and used because of the waves' shape diversity. Detection in a single channel ECG is important for several applications, such as in defibrillators and specialized monitors. METHODS: The developed heuristic algorithm for ventricular beat detection includes two main criteria. The first of them is based on steep edges and sharp peaks evaluation and classifies normal QRS complexes in real time. The second criterion identifies ectopic beats by occurrence of biphasic wave. It is modified to work with a delay of one RR interval in case of long RR intervals. Other algorithm branches classify already detected QRS complexes as ectopic beats if a set of wave parameters is encountered or the ratio of latest two RR intervals RR(i-1)/RR(i )is less than 1:2.5. RESULTS: The algorithm was tested with the AHA and MIT-BIH databases. A sensitivity of 99.04% and a specificity of 99.62% were obtained in detection of 542014 beats. CONCLUSION: The algorithm copes successfully with different complicated cases of single channel ventricular beat detection. It is aimed to simulate to some extent the experience of the cardiologist, rather than to rely on mathematical approaches adopted from the theory of signal analysis. The algorithm is open to improvement, especially in the part concerning the discrimination between normal QRS complexes and ectopic beats
Coulomb Blockade in a Silicon/Silicon-Germanium Two-Dimensional Electron Gas Quantum Dot
We report the fabrication and electrical characterization of a single
electron transistor in a modulation doped silicon/silicon-germanium
heterostructure. The quantum dot is fabricated by electron beam lithography and
subsequent reactive ion etching. The dot potential and electron density are
modified by laterally defined side gates in the plane of the dot. Low
temperature measurements show Coulomb blockade with a single electron charging
energy of 3.2 meV.Comment: Typos corrected; to appear in Appl. Phys. Let
H5N1 Whole-Virus Vaccine Induces Neutralizing Antibodies in Humans Which Are Protective in a Mouse Passive Transfer Model
BACKGROUND: Vero cell culture-derived whole-virus H5N1 vaccines have been extensively tested in clinical trials and consistently demonstrated to be safe and immunogenic; however, clinical efficacy is difficult to evaluate in the absence of wide-spread human disease. A lethal mouse model has been utilized which allows investigation of the protective efficacy of active vaccination or passive transfer of vaccine induced sera following lethal H5N1 challenge. METHODS: We used passive transfer of immune sera to investigate antibody-mediated protection elicited by a Vero cell-derived, non-adjuvanted inactivated whole-virus H5N1 vaccine. Mice were injected intravenously with H5N1 vaccine-induced rodent or human immune sera and subsequently challenged with a lethal dose of wild-type H5N1 virus. RESULTS: Passive transfer of H5N1 vaccine-induced mouse, guinea pig and human immune sera provided dose-dependent protection of recipient mice against lethal challenge with wild-type H5N1 virus. Protective dose fifty values for serum H5N1 neutralizing antibody titers were calculated to be ≤1∶11 for all immune sera, independently of source species. CONCLUSIONS: These data underpin the confidence that the Vero cell culture-derived, whole-virus H5N1 vaccine will be effective in a pandemic situation and support the use of neutralizing serum antibody titers as a correlate of protection for H5N1 vaccines
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