208 research outputs found
Phonon-induced decoherence and dissipation in donor-based charge qubits
We investigate the phonon-induced decoherence and dissipation in a
donor-based charge quantum bit realized by the orbital states of an electron
shared by two dopant ions which are implanted in a silicon host crystal. The
dopant ions are taken from the group-V elements Bi, As, P, Sb. The excess
electron is coupled to deformation potential acoustic phonons which dominate in
the Si host. The particular geometry tailors a non-monotonous frequency
distribution of the phonon modes. We determine the exact qubit dynamics under
the influence of the phonons by employing the numerically exact quasi-adiabatic
propagator path integral scheme thereby taking into account all bath-induced
correlations. In particular, we have improved the scheme by completely
eliminating the Trotter discretization error by a Hirsch-Fye extrapolation. By
comparing the exact results to those of a Born-Markov approximation we find
that the latter yields appropriate estimates for the decoherence and relaxation
rates. However, noticeable quantitative corrections due to non-Markovian
contributions appear.Comment: 8 pages, 8 figures, published online in Eur.Phys.J.B, article in
press; the original publication is avaiable at www.eurphysj.or
Dynamical bistability in the driven circuit QED
We show that the nonlinear response of a driven circuit quantum
electrodynamics setup displays antiresonant multiphoton transitions, as
recently observed in a transmon qubit device. By including photon leaking, we
explain the lineshape by a perturbative and a semiclassical analysis. We derive
a bistable semiclassical quasienergy surface whose lowest quasienergy
eigenstate is squeezed, allowing for a squeezing-dependent local effective
temperature. We study the escape dynamics out of the metastable state and find
signatures of dynamical tunneling, similar as for the quantum Duffing
oscillator.Comment: submitted to PR
BCS theory of driven superconductivity
We study the impact of a time-dependent external driving of the lattice
phonons in a minimal model of a BCS superconductor. Upon evaluating the
driving-induced vertex corrections of the phonon-mediated electron-electron
interaction, we show that parametric phonon driving can be used to elevate the
critical temperature , while a dipolar phonon drive has no effect. We
provide simple analytic expressions for the enhancement factor of .
Furthermore, a mean-field analysis of a nonlinear phonon-phonon interaction
also shows that phonon anharmonicities further amplify . Our results hold
universally for the large class of normal BCS superconductors
Strong coupling theory for tunneling and vibrational relaxation in driven bistable systems
A study of the dynamics of a tunneling particle in a driven bistable
potential which is moderately-to-strongly coupled to a bath is presented. Upon
restricting the system dynamics to the Hilbert space spanned by the M lowest
energy eigenstates of the bare static potential, a set of coupled non-Markovian
master equations for the diagonal elements of the reduced density matrix,
within the discrete variale representation, is derived. The resulting dynamics
is in good agreement with predictions of ab-initio real-time path integral
simulations. Numerous results, analytical as well as numerical, for the quantum
relaxation rate and for the asymptotic populations are presented. Our method is
particularly convenient to investigate the case of shallow, time-dependent
potential barriers and moderate-to-strong damping, where both a semi-classical
and a Redfield-type approach are inappropriate.Comment: 37 pages, 23 figure
Quantum coherent biomolecular energy transfer with spatially correlated fluctuations
We show that the quantum coherent transfer of excitations between
biomolecular chromophores is strongly influenced by spatial correlations of the
environmental fluctuations. The latter are due either to propagating
environmental modes or to local fluctuations with a finite localization length.
A simple toy model of a single donor-acceptor pair with spatially separated
chromophore sites allows to investigate the influence of these spatial
correlations on the quantum coherent excitation transfer. The sound velocity of
the solvent determines the wave lengths of the environmental modes, which, in
turn, has to be compared to the spatial distance of the chromophore sites. When
the wave length exceeds the distance between donor and acceptor site, we find
strong suppression of decoherence. In addition, we consider two spatially
separated donor-acceptor pairs under the influence of propagating environmental
modes. Depending on their wave lengths fixed by the sound velocity of the
solvent material, the spatial range of correlations may extend over typical
interpair distances, which can lead to an increase of the decohering influence
of the solvent. Surprisingly, this effect is counteracted by increasing
temperature
Ultraslow quantum dynamics in a sub-Ohmic heat bath
We show that the low-frequency modes of a sub-Ohmic bosonic heat bath
generate an effective dynamical asymmetry for an intrinsically symmetric
quantum spin -1/2. An initially fully polarized spin first decays towards a
quasiequilibrium determined by the dynamical asymmetry, thereby showing
coherent damped oscillations on the (fast) time scale of the spin splitting. On
top of this, the dynamical asymmetry itself decays on an ultraslow time scale
and vanishes asymptotically since the global equilibrium phase is symmetric. We
quantitatively study the nature of the initial fast decay to the
quasiequilibrium and discuss the features of ultraslow dynamics of the
quasiequilibrium itself. The dynamical asymmetry is more pronounced for smaller
values of the sub-Ohmic exponent and for lower temperatures, which emphasizes
the quantum many-body nature of the effect. The symmetry breaking is related to
the dynamic crossover between coherent and overdamped relaxation of the spin
polarization and is not connected to the localization quantum phase transition.
In addition to this delocalized phase, we identify a novel phase which is
characterized by damped coherent oscillations in the localized phase. This
allows for a sketch of the zero-temperature phase diagram of the sub-Ohmic
spin-boson model with four distinct phases.Comment: published version (minor changes), 8 pages, 5 figure
Dynamics of the spin-boson model with a structured environment
We investigate the dynamics of the spin-boson model when the spectral density
of the boson bath shows a resonance at a characteristic frequency but
behaves Ohmically at small frequencies. The time evolution of an initial state
is determined by making use of the mapping onto a system composed of a quantum
mechanical two-state system (TSS) which is coupled to a harmonic oscillator
(HO) with frequency . The HO itself is coupled to an Ohmic environment.
The dynamics is calculated by employing the numerically exact quasiadiabatic
path-integral propagator technique. We find significant new properties compared
to the Ohmic spin-boson model. By reducing the TSS-HO system in the dressed
states picture to a three-level system for the special case at resonance, we
calculate the dephasing rates for the TSS analytically. Finally, we apply our
model to experimentally realized superconducting flux qubits coupled to an
underdamped dc-SQUID detector.Comment: 26 pages, 11 figures, Chemical Physics Special Issue on the
Spin-Boson Problem, ed. by H. Grabert and A. Nitzan, in pres
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