482 research outputs found
Charting the circuit QED design landscape using optimal control theory
With recent improvements in coherence times, superconducting transmon qubits
have become a promising platform for quantum computing. They can be flexibly
engineered over a wide range of parameters, but also require us to identify an
efficient operating regime. Using state-of-the-art quantum optimal control
techniques, we exhaustively explore the landscape for creation and removal of
entanglement over a wide range of design parameters. We identify an optimal
operating region outside of the usually considered strongly dispersive regime,
where multiple sources of entanglement interfere simultaneously, which we name
the quasi-dispersive straddling qutrits (QuaDiSQ) regime. At a chosen point in
this region, a universal gate set is realized by applying microwave fields for
gate durations of 50 ns, with errors approaching the limit of intrinsic
transmon coherence. Our systematic quantum optimal control approach is easily
adapted to explore the parameter landscape of other quantum technology
platforms.Comment: 13 pages, 5 figures, 2 pages supplementary, 1 supplementary figur
Current-induced nonequilibrium vibrations in single-molecule devices
Finite-bias electron transport through single molecules generally induces
nonequilibrium molecular vibrations (phonons). By a mapping to a Fokker-Planck
equation, we obtain analytical scaling forms for the nonequilibrium phonon
distribution in the limit of weak electron-phonon coupling within a
minimal model. Remarkably, the width of the phonon distribution diverges as
when the coupling decreases, with voltage-dependent,
non-integer exponents . This implies a breakdown of perturbation theory
in the electron-phonon coupling for fully developed nonequilibrium. We also
discuss possible experimental implications of this result such as
current-induced dissociation of molecules.Comment: 7 pages, 4 figures; revised and extended version published in Phys.
Rev.
Fractional Shot Noise in the Kondo Regime
Low temperature transport through a quantum dot in the Kondo regime proceeds
by a universal combination of elastic and inelastic processes, as dictated by
the low-energy Fermi-liquid fixed point. We show that as a result of inelastic
processes, the charge detected by a shot-noise experiment is enhanced relative
to the noninteracting situation to a universal fractional value, .
Thus, shot noise reveals that the Kondo effect involves many-body features even
at low energies, despite its Fermi-liquid nature. We discuss the influence of
symmetry breaking perturbations.Comment: 4 pages, 2 figure
Nonequilibrium charge-Kondo transport through negative-U molecules
Low-temperature transport through molecules with effectively negative
charging energy U exhibits a charge-Kondo effect. We explore this regime
analytically by establishing an exact mapping between the negative-U and the
positive-U Anderson models, which is suitable for the description of
nonequilibrium transport. We employ this mapping to demonstrate the intimate
relation between nonequilibrium tranport in the spin-Kondo and charge-Kondo
regimes, and derive analytical expressions for the nonlinear current-voltage
chracteristics as well as the shot noise in the latter regime. Applying the
mapping in the opposite direction, we elucidate the finding of super-Poissonian
noise in the positive-U Anderson model at high temperatures, by relating the
correlations between spin flips to pair-tunneling processes at negative U.Comment: 11 pages, 5 figure
Avalanche of Bifurcations and Hysteresis in a Model of Cellular Differentiation
Cellular differentiation in a developping organism is studied via a discrete
bistable reaction-diffusion model. A system of undifferentiated cells is
allowed to receive an inductive signal emenating from its environment.
Depending on the form of the nonlinear reaction kinetics, this signal can
trigger a series of bifurcations in the system. Differentiation starts at the
surface where the signal is received, and cells change type up to a given
distance, or under other conditions, the differentiation process propagates
through the whole domain. When the signal diminishes hysteresis is observed
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