1,700 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
Coherent caloritronics in Josephson-based nanocircuits
We describe here the first experimental realization of a heat interferometer,
thermal counterpart of the well-known superconducting quantum interference
device (SQUID). These findings demonstrate, on the first place, the existence
of phase-dependent heat transport in Josephson-based superconducting circuits
and, on the second place, open the way to novel ways of mastering heat at the
nanoscale. Combining the use of external magnetic fields for phase biasing and
different Josephson junction architectures we show here that a number of heat
interference patterns can be obtained. The experimental realization of these
architectures, besides being relevant from a fundamental physics point of view,
might find important technological application as building blocks of
phase-coherent quantum thermal circuits. In particular, the performance of two
different heat rectifying devices is analyzed.Comment: 34 pages, 15 figures, review article for Ultra-low temperatures and
nanophysics ULTN2013. Microkelvin Proceeding
Coherent diffraction of thermal currents in Josephson tunnel junctions
We theoretically investigate heat transport in temperature-biased Josephson
tunnel junctions in the presence of an in-plane magnetic field. In full analogy
with the Josephson critical current, the phase-dependent component of the heat
flux through the junction displays coherent diffraction. Thermal transport is
analyzed in three prototypical junction geometries highlighting their main
differences. Notably, minimization of the Josephson coupling energy requires
the quantum phase difference across the junction to undergo \pi-slips in
suitable intervals of magnetic flux. An experimental setup suited to detect
thermal diffraction is proposed and analyzed.Comment: 6.5 pages, 4 color figures, updated versio
Non-Abelian geometric phases in ground state Josephson devices
We present a superconducting circuit in which non-Abelian geometric
transformations can be realized using an adiabatic parameter cycle. In contrast
to previous proposals, we employ quantum evolution in the ground state. We
propose an experiment in which the transition from non-Abelian to Abelian
cycles can be observed by measuring the pumped charge as a function of the
period of the cycle. Alternatively, the non-Abelian phase can be detected using
a single-electron transistor working as a charge sensor.Comment: 5 pages, 3 figures; added references and clarified discussion about
earlier research on the fiel
Ground-state geometric quantum computing in superconducting systems
We present a theoretical proposal for the implementation of geometric quantum
computing based on a Hamiltonian which has a doubly degenerate ground state.
Thus the system which is steered adiabatically, remains in the ground-state.
The proposed physical implementation relies on a superconducting circuit
composed of three SQUIDs and two superconducting islands with the charge states
encoding the logical states. We obtain a universal set of single-qubit gates
and implement a non-trivial two-qubit gate exploiting the mutual inductance
between two neighboring circuits, allowing us to realize a fully geometric
ground-state quantum computing. The introduced paradigm for the implementation
of geometric quantum computing is expected to be robust against environmental
effects.Comment: 9 pages, 5 figures. Final version with notation and typos correcte
Geometric quantum gates with superconducting qubits
We suggest a scheme to implement a universal set of non-Abelian geometric
transformations for a single logical qubit composed of three superconducting
transmon qubits coupled to a single cavity. The scheme utilizes an adiabatic
evolution in a rotating frame induced by the effective tripod Hamiltonian which
is achieved by longitudinal driving of the transmons. The proposal is
experimentally feasible with the current state of the art and could serve as a
first proof of principle for geometric quantum computing.Comment: 7 pages, 5 figure
Decoherence in adiabatic quantum evolution - application to Cooper pair pumping
One of the challenges of adiabatic control theory is the proper inclusion of
the effects of dissipation. Here, we study the adiabatic dynamics of an open
two-level quantum system deriving a generalized master equation to consistently
account for the combined action of the driving and dissipation. We demonstrate
that in the zero temperature limit the ground state dynamics is not affected by
environment. As an example, we apply our theory to Cooper pair pumping which
demonstrates the robustness of ground state adiabatic evolution.Comment: 7 pages, derivation of the master equation in the appendi
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
The Coherent Crooks Equality
This chapter reviews an information theoretic approach to deriving quantum
fluctuation theorems. When a thermal system is driven from equilibrium, random
quantities of work are required or produced: the Crooks equality is a classical
fluctuation theorem that quantifies the probabilities of these work
fluctuations. The framework summarised here generalises the Crooks equality to
the quantum regime by modeling not only the driven system but also the control
system and energy supply that enables the system to be driven. As is reasonably
common within the information theoretic approach but high unusual for
fluctuation theorems, this framework explicitly accounts for the energy
conservation using only time independent Hamiltonians. We focus on explicating
a key result derived by Johan {\AA}berg: a Crooks-like equality for when the
energy supply is allowed to exist in a superposition of energy eigenstates
states.Comment: 11 pages, 3 figures; Chapter for the book "Thermodynamics in the
Quantum Regime - Recent Progress and Outlook", eds. F. Binder, L. A. Correa,
C. Gogolin, J. Anders and G. Adess
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