945 research outputs found
Collision model approach to steering of an open driven qubit
We investigate quantum steering of an open quantum system by measurements on
its environment in the framework of collision models. As an example we consider
a coherently driven qubit dissipatively coupled to a bath. We construct local
non-adaptive and adaptive as well as nonlocal measurement scenarios specifying
explicitly the measured observable on the environment. Our approach shows
transparently how the conditional evolution of the open system depends on the
type of the measurement scenario and the measured observables. These can then
be optimized for steering. The nonlocal measurement scenario leads to maximal
violation of the used steering inequality at zero temperature. Further, we
investigate the robustness of the constructed scenarios against thermal noise.
We find generally that steering becomes harder at higher temperatures.
Surprisingly, the system can be steered even when bipartite entanglement
between the system and individual subenvironments vanishes
Collision model for non-Markovian quantum dynamics
We study the applicability of collisional models for non-Markovian dynamics
of open quantum systems. By allowing interactions between the separate
environmental degrees of freedom in between collisions we are able to construct
a collision model that allows to study quantum memory effects in open system
dynamics. We also discuss the possibility to embed non-Markovian collision
model dynamics into Markovian collision model dynamics in an extended state
space. As a concrete example we show how using the proposed class of collision
models we can discretely model non- Markovian amplitude damping of a qubit. In
the time-continuous limit, we obtain the well-known results for spontaneous
decay of a two level system in to a structured zero-temperature reservoirComment: 10 pages, 6 figure, comments welcome
Revealing the nature of non-equilibrium phase transitions with quantum trajectories
A damped and driven collective spin system is analyzed by using quantum state
diffusion. This approach allows for a mostly analytical treatment of the
investigated non-equilibrium quantum many body dynamics, which features a phase
transition in the thermodynamical limit. The exact results obtained in this
work, which are free of any finite size defects, provide a complete
understanding of the model. Moreover, the trajectory framework gives an
intuitive picture of the two phases occurring, revealing a spontaneously broken
symmetry and allowing for a qualitative and quantitative characterization of
the phases. We determine exact critical exponents, investigate finite size
scaling, and explain a remarkable non-algebraic behaviour at the transition in
terms of torus hopping.Comment: 5 pages, 5 figure
Geometric Characterization of True Quantum Decoherence
Surprisingly often decoherence is due to classical fluctuations of ambient
fields and may thus be described in terms of random unitary (RU) dynamics.
However, there are decoherence channels where such a representation cannot
exist. Based on a simple and intuitive geometric measure for the distance of an
extremal channel to the convex set of RU channels we are able to characterize
the set of true quantum phase-damping channels. Remarkably, using the
Caley-Menger determinant, our measure may be assessed directly from the matrix
representation of the channel. We find that the channel of maximum quantumness
is closely related to a symmetric, informationally-complete positive
operator-valued measure (SIC-POVM) on the environment. Our findings are in line
with numerical results based on the entanglement of assistance.Comment: 5 pages, 3 figure
Parametrization and optimization of Gaussian non-Markovian unravelings for open quantum dynamics
We derive a family of Gaussian non-Markovian stochastic Schr\"odinger
equations for the dynamics of open quantum systems. The different unravelings
correspond to different choices of squeezed coherent states, reflecting
different measurement schemes on the environment. Consequently, we are able to
give a single shot measurement interpretation for the stochastic states and
microscopic expressions for the noise correlations of the Gaussian process. By
construction, the reduced dynamics of the open system does not depend on the
squeezing parameters. They determine the non-hermitian Gaussian correlation, a
wide range of which are compatible with the Markov limit. We demonstrate the
versatility of our results for quantum information tasks in the non-Markovian
regime. In particular, by optimizing the squeezing parameters, we can tailor
unravelings for optimal entanglement bounds or for environment-assisted
entanglement protection.Comment: 5 pages, 1 figur
Typical Gaussian Quantum Information
We investigate different geometries and invariant measures on the space of
mixed Gaussian quan- tum states. We show that when the global purity of the
state is held fixed, these measures coincide and it is possible, within this
constraint, to define a unique notion of volume on the space of mixed Gaussian
states. We then use the so defined measure to study typical non-classical
correlations of two mode mixed Gaussian quantum states, in particular
entanglement and steerability. We show that under the purity constraint alone,
typical values for symplectic invariants can be computed very elegantly,
irrespectively of the non-compactness of the underlying state space. Then we
consider finite volumes by constraining the purity and energy of the Gaussian
state and compute typical values of quantum correlations numerically.Comment: 22 pages, 4 figure
Work as an external quantum observable and an operational quantum work fluctuation theorem
We propose a definition of externally measurable quantum work in driven
systems. Work is given as a quantum observable on a control device which is
forcing the system and can be determined without knowledge of the system
Hamiltonian . We argue that quantum work fluctuation theorems
which rely on the knowledge of are of little practical
relevance, contrary to their classical counterparts. Using our framework, we
derive a fluctuation theorem which is operationally accessible and could in
principle be implemented in experiments to determine bounds on free energy
differences of unknown systems
Steering heat engines: a truly quantum Maxwell demon
We address the question of verifying the quantumness of thermal machines. A
Szil\'ard engine is truly quantum if its work output cannot be described by a
local hidden state (LHS) model, i. e. an objective local statistical ensemble.
Quantumness in this scenario is revealed by a steering-type inequality which
bounds the classically extractable work. A quantum Maxwell demon can violate
that inequality by exploiting quantum correlations between the work medium and
the thermal environment. While for a classical Szil\'ard engine an objective
description of the medium always exists, any such description can be ruled out
by a steering task in a truly quantum case
Continuous quantum measurement for general Gaussian unravelings can exist
Quantum measurements and the associated state changes are properly described
in the language of instruments. We investigate the properties of a time
continuous family of instruments associated with the recently introduced family
of general Gaussian non-Markovian stochastic Schr\"odinger equations. In this
Letter we find that when the covariance matrix for the Gaussian noise satisfies
a particular -function constraint, the measurement interpretation is
possible for a class of models with self-adjoint coupling operator. This class
contains, for example the spin-boson and quantum Brownian motion models with
colored bath correlation functions. Remarkably, due to quantum memory effects
the reduced state, in general, does not have a closed form master equation
while the unraveling has a time continuous measurement interpretation.Comment: 5 page
Diffusive limit of non-Markovian quantum jumps
We solve two long standing problems for stochastic descriptions of open
quantum system dynamics. First, we find the classical stochastic processes
corresponding to non-Markovian quantum state diffusion and non-Markovian
quantum jumps in projective Hilbert space. Second, we show that the diffusive
limit of non-Markovian quantum jumps can be taken on the projective Hilbert
space in such a way that it coincides with non-Markovian quantum state
diffusion. However, the very same limit taken on the Hilbert space leads to a
completely new diffusive unraveling, which we call non-Markovian quantum
diffusion. Further, we expand the applicability of non-Markovian quantum jumps
and non-Markovian quantum diffusion by using a kernel smoothing technique
allowing a significant simplification in their use. Lastly, we demonstrate the
applicability of our results by studying a driven dissipative two level atom in
a non-Markovian regime using all of the three methods.Comment: 6 pages, 2 figures. Comments are welcome
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