237 research outputs found
Phase measurement of photon-assisted tunneling through a quantum dot
Recent double-slit interference experiments have demonstrated the possibility
of probing the phase of the complex transmission coefficient of a quantum dot
via the Aharonov-Bohm effect. We propose an extension of these experiments: an
ac voltage imposed on the side gate with the concomitant photonic sidebands
leads to additional structure both in the amplitude and in the phase of the
Aharonov-Bohm signal. Observation of these effects would be a definitive proof
of coherent absorption and reemission of photons from the ac source.Comment: 6 pages using latex2e and EuroPhys.sty. Uses epsf to include 5
figures (submitted to Europhys. Lett.
Modeling Evolution of Crosstalk in Noisy Signal Transduction Networks
Signal transduction networks can form highly interconnected systems within
cells due to network crosstalk, the sharing of input signals between multiple
downstream responses. To better understand the evolutionary design principles
underlying such networks, we study the evolution of crosstalk and the emergence
of specificity for two parallel signaling pathways that arise via gene
duplication and are subsequently allowed to diverge. We focus on a sequence
based evolutionary algorithm and evolve the network based on two physically
motivated fitness functions related to information transmission. Surprisingly,
we find that the two fitness functions lead to very different evolutionary
outcomes, one with a high degree of crosstalk and the other without.Comment: 18 Pages, 16 Figure
Quantum-Dot Cascade Laser: Proposal for an Ultra-Low-Threshold Semiconductor Laser
We propose a quantum-dot version of the quantum-well cascade laser of Faist
et al. [Science {\bf 264}, 553 (1994)]. The elimination of single phonon decays
by the three-dimensional confinement implies a several order-of-magnitude
reduction in the threshold current. The requirements on dot size (10-20nm) and
on dot density and uniformity [one coupled pair of dots per (180nm)^3 with 5%
nonuniformity] are close to current technology.Comment: 8 pages, REVTEX 3.0, 3 compressed postscript figure
Exponential sensitivity of noise-driven switching in genetic networks
Cells are known to utilize biochemical noise to probabilistically switch between distinct gene expression states. We demonstrate that such noise-driven switching is dominated by tails of probability distributions and is therefore exponentially sensitive to changes in physiological parameters such as transcription and translation rates. However, provided mRNA lifetimes are short, switching can still be accurately simulated using protein-only models of gene expression. Exponential sensitivity limits the robustness of noise-driven switching, suggesting cells may use other mechanisms in order to switch reliably
Time-dependent transport in interacting and non-interacting mesoscopic systems
We consider a mesoscopic region coupled to two leads under the influence of
external time-dependent voltages. The time dependence is coupled to source and
drain contacts, the gates controlling the tunnel- barrier heights, or to the
gates which define the mesoscopic region. We derive, with the Keldysh
nonequilibrium Green function technique, a formal expression for the fully
nonlinear, time-dependent current through the system. The analysis admits
arbitrary interactions in the mesoscopic region, but the leads are treated as
noninteracting. For proportionate coupling to the leads, the time-averaged
current is simply the integral between the chemical potentials of the
time-averaged density of states, weighted by the coupling to the leads, in
close analogy to the time-independent result of Meir and Wingreen (PRL {\bf
68}, 2512 (1992)). Analytical and numerical results for the exactly solvable
non-interacting resonant-tunneling system are presented.Comment: 42 pages, 13 figures (available either as ps-files, or as FAX, upon
request), RevTex 3.
Hidden long evolutionary memory in a model biochemical network
We introduce a minimal model for the evolution of functional
protein-interaction networks using a sequence-based mutational algorithm, and
apply the model to study neutral drift in networks that yield oscillatory
dynamics. Starting with a functional core module, random evolutionary drift
increases network complexity even in the absence of specific selective
pressures. Surprisingly, we uncover a hidden order in sequence space that gives
rise to long-term evolutionary memory, implying strong constraints on network
evolution due to the topology of accessible sequence space.Comment: 20 Pages, 14 Figure
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