2,159 research outputs found
Fission Decay Rates Determined from a Quantal Transport Equation
The decay of a metastable system is described by extending Kramers' method to
the quantal regime. For temperatures above twice the crossover value we recover
the result known from applying Euclidean path integrals to solvable models. Our
derivation is not restricted to a linearly coupled heat bath of oscillators,
and thus applicable to nuclear systems.Comment: 10 pages, Plain Tex, TU-Muenchen preprin
Phase diffusion and charging effects in Josephson junctions
The supercurrent of a Josephson junction is reduced by phase diffusion. For
ultrasmall capacitance junctions the current may be further decreased by
Coulomb blockade effects. We calculate the Cooper pair current by means of
time-dependent perturbation theory to all orders in the Josephson coupling
energy and obtain the current-voltage characteristic in closed form in a range
of parameters of experimental interest. The results comprehend phase diffusion
of the coherent Josephson current in the classical regime as well as the
supercurrent peak due to incoherent Cooper pair tunneling in the strong Coulomb
blockade regime.Comment: 4 pages, 3 figures, RevTe
Low noise buffer amplifiers and buffered phase comparators for precise time and frequency measurement and distribution
Extremely low noise, high performance, wideband buffer amplifiers and buffered phase comparators were developed. These buffer amplifiers are designed to distribute reference frequencies from 30 KHz to 45 MHz from a hydrogen maser without degrading the hydrogen maser's performance. The buffered phase comparators are designed to intercompare the phase of state of the art hydrogen masers without adding any significant measurement system noise. These devices have a 27 femtosecond phase stability floor and are stable to better than one picosecond for long periods of time. Their temperature coefficient is less than one picosecond per degree C, and they have shown virtually no voltage coefficients
Lissajous curves and semiclassical theory: The two-dimensional harmonic oscillator
The semiclassical treatment of the two-dimensional harmonic oscillator
provides an instructive example of the relation between classical motion and
the quantum mechanical energy spectrum. We extend previous work on the
anisotropic oscillator with incommensurate frequencies and the isotropic
oscillator to the case with commensurate frequencies for which the Lissajous
curves appear as classical periodic orbits. Because of the three different
scenarios depending on the ratio of its frequencies, the two-dimensional
harmonic oscillator offers a unique way to explicitly analyze the role of
symmetries in classical and quantum mechanics.Comment: 9 pages, 3 figures; to appear in Am. J. Phy
Tunneling into Multiwalled Carbon Nanotubes: Coulomb Blockade and Fano Resonance
Tunneling spectroscopy measurements of single tunnel junctions formed between
multiwalled carbon nanotubes (MWNTs) and a normal metal are reported. Intrinsic
Coulomb interactions in the MWNTs give rise to a strong zero-bias suppression
of a tunneling density of states (TDOS) that can be fitted numerically to the
environmental quantum-fluctuation (EQF) theory. An asymmetric conductance
anomaly near zero bias is found at low temperatures and interpreted as Fano
resonance in the strong tunneling regime.Comment: 4 pages, 4 figure
Identification of Coulomb blockade and macroscopic quantum tunneling by noise
The effects of Macroscopic Quantum Tunneling (MQT) and Coulomb Blockade (CB)
in Josephson junctions are of considerable significance both for the
manifestations of quantum mechanics on the macroscopic scale and potential
technological applications. These two complementary effects are shown to be
clearly distinguishable from the associated noise spectra. The current noise is
determined exactly and a rather sharp crossover between flux noise in the MQT
and charge noise in the CB regions is found as the applied voltage is changed.
Related results hold for the voltage noise in current-biased junctions.Comment: 6 pages, 3 figures, epl.cls include
A one-channel conductor in an ohmic environment: mapping to a TLL and full counting statistics
It is shown that a one-channel mesoscopic conductor in an ohmic environment
can be mapped to the problem of a backscattering impurity in a
Tomonaga-Luttinger liquid (TLL). This allows to determine non perturbatively
the effect of the environment on curves, and to find an exact
relationship between dynamic Coulomb blockade and shot noise. We investigate
critically how this relationship compares to recent proposals in the
literature. The full counting statistics is determined at zero temperature.Comment: 5 pages, 2 figures, shortened version for publication in Phys. Rev.
Let
Spin-orbital Kondo decoherence by environmental effects in capacitively coupled quantum dot devices
Strong correlation effects in a capacitively coupled double quantum-dot setup
were previously shown to provide the possibility of both entangling spin-charge
degrees of freedom and realizing efficient spin-filtering operations by static
gate-voltage manipulations. Motivated by the use of such a device for quantum
computing, we study the influence of electromagnetic noise on a general
spin-orbital Kondo model, and investigate the conditions for observing
coherent, unitary transport, crucial to warrant efficient spin manipulations.
We find a rich phase diagram, where low-energy properties sensitively depend on
the impedance of the external environment and geometric parameters of the
system. Relevant energy scales related to the Kondo temperature are also
computed in a renormalization-group treatment, allowing to assess the
robustness of the device against environmental effects.Comment: 13 pages, 13 figures. Minor modifications in V
Density of states in a superconductor carrying a supercurrent
We have measured the tunneling density of states (DOS) in a superconductor
carrying a supercurrent or exposed to an external magnetic field. The pair
correlations are weakened by the supercurrent, leading to a modification of the
DOS and to a reduction of the gap. As predicted by the theory of
superconductivity in diffusive metals, we find that this effect is similar to
that of an external magnetic field.Comment: To be published in Physical Review Letter
High Frequency Quantum Admittance and Noise Measurement with an On-chip Resonant Circuit
By coupling a quantum detector, a superconductor-insulator-superconductor
junction, to a Josephson junction \textit{via} a resonant circuit we probe the
high frequency properties, namely the ac complex admittance and the current
fluctuations of the Josephson junction at the resonant frequencies. The
admittance components show frequency dependent singularities related to the
superconducting density of state while the noise exhibits a strong frequency
dependence, consistent with theoretical predictions. The circuit also allows to
probe separately the emission and absorption noise in the quantum regime of the
superconducting resonant circuit at equilibrium. At low temperature the
resonant circuit exhibits only absorption noise related to zero point
fluctuations, whereas at higher temperature emission noise is also present.Comment: 15 pages, 15 figure
- …