23 research outputs found
Adiabatically steered open quantum systems: Master equation and optimal phase
We introduce an alternative way to derive the generalized form of the master
equation recently presented by J. P. Pekola et al. [Phys. Rev. Lett. 105,
030401 (2010)] for an adiabatically steered two-level quantum system
interacting with a Markovian environment. The original derivation employed the
effective Hamiltonian in the adiabatic basis with the standard interaction
picture approach but without the usual secular approximation. Our approach is
based on utilizing a master equation for a non-steered system in the first
super-adiabatic basis. It is potentially efficient in obtaining higher-order
equations. Furthermore, we show how to select the phases of the adiabatic
eigenstates to minimize the local adiabatic parameter and how this selection
leads to states which are invariant under a local gauge change. We also discuss
the effects of the adiabatic noncyclic geometric phase on the master equation.Comment: 8 pages, no figures, final versio
Decoherence of adiabatically steered quantum systems
We study the effect of Markovian environmental noise on the dynamics of a
two-level quantum system which is steered adiabatically by an external driving
field. We express the master equation taking consistently into account all the
contributions to the lowest non-vanishing order in the coupling to the
Markovian environment. We study the master equation numerically and
analytically and we find that, in the adiabatic limit, a zero-temperature
environment does not affect the ground state evolution. As a physical
application, we discuss extensively how the environment affects Cooper pair
pumping. The adiabatic ground state pumping appears to be robust against
environmental noise. In fact, the relaxation due to the environment is required
to avoid the accumulation of small errors from each pumping cycle. We show that
neglecting the non-secular terms in the master equation leads to unphysical
results, such as charge non-conservation. We discuss also a possible way to
control the environmental noise in a realistic physical setup and its influence
on the pumping process.Comment: 13 pages, 11 figures. Final versio
Conservation law of operator current in open quantum systems
We derive a fundamental conservation law of operator current for master
equations describing reduced quantum systems. If this law is broken, the
temporal integral of the current operator of an arbitrary system observable
does not yield in general the change of that observable in the evolution. We
study Lindblad-type master equations as examples and prove that the application
of the secular approximation during their derivation results in a violation of
the conservation law. We show that generally any violation of the law leads to
artificial corrections to the complete quantum dynamics, thus questioning the
accuracy of the particular master equation.Comment: 5 pages, final versio
Shot Noise in Ballistic Graphene
We have investigated shot noise in graphene field effect devices in the
temperature range of 4.2--30 K at low frequency ( = 600--850 MHz). We find
that for our graphene samples with large width over length ratio , the
Fano factor reaches a maximum 1/3 at the
Dirac point and that it decreases strongly with increasing charge density. For
smaller , the Fano factor at Dirac point is significantly lower. Our
results are in good agreement with the theory describing that transport at the
Dirac point in clean graphene arises from evanescent electronic states.Comment: Phys. Rev. Lett. 100, 196802 (2008
Fluctuations of work in nearly adiabatically driven open quantum systems
We extend the quantum jump method to nearly adiabatically driven open quantum
systems in a way that allows for an accurate account of the external driving in
the system-environment interaction. Using this framework, we construct the
corresponding trajectory-dependent work performed on the system and derive the
integral fluctuation theorem and the Jarzynski equality for nearly adiabatic
driving. We show that such identities hold as long as the stochastic dynamics
and work variable are consistently defined. We numerically study the emerging
work statistics for a two-level quantum system and find that the conventional
diabatic approximation is unable to capture some prominent features arising
from driving such as the continuity of the probability density of work. Our
results reveal the necessity of using accurate expressions for the
drive-dressed heat exchange in future experiments probing jump time
distributions.Comment: 15 pages, 5 figure
Cooper pair current in the presence of flux noise
We study the effect of the flux noise on the Cooper pair current and pumping.
We generalize the definition of the current in order to take into account the
contribution induced by the environment. It turns out that this dissipative
current vanishes for charge noise but it is finite in general for noise
operators that do not commute with the charge operator. We discuss in a generic
framework the effect of flux noise and present a way to engineer it by coupling
the system to an additional external circuit. We calculate numerically the
pumped charge through the device by solving the master equation for the reduced
density matrix of the system and show how it depends on the coupling to the
artificial environment.Comment: 10 pages, 6 figure
Evanescent wave transport and shot noise in graphene: ballistic regime and effect of disorder
We have investigated electrical transport and shot noise in graphene field
effect devices. In large width over length ratio graphene strips, we have
measured shot noise at low frequency ( = 600--850 MHz) in the temperature
range of 4.2--30 K. We observe a minimum conductivity of
and a finite and gate dependent Fano factor reaching the universal value of 1/3
at the Dirac point, i.e. where the density of states vanishes. These findings
are in good agreement with the theory describing that transport at the Dirac
point should occur via evanescent waves in perfect graphene samples with large
. Moreover, we show and discuss how disorder and non-parallel leads affect
both conductivity and shot noise.Comment: Extended version (19 pages, 10 figures) of Phys. Rev. Lett. 100,
196802 (2008). Additional data on the effect of disorder and non-parallel
leads. Submitted for publication in Journal of Low Temperature Physics for
the Proceedings of the International Symposium on Quantum Phenomena and
Devices at Low Temperatures (ULTI 2008), Espoo, Finlan