6,017 research outputs found
Quantum trajectories of interacting pseudo-spin-networks
We consider quantum trajectories of composite systems as generated by the
stochastic unraveling of the respective Lindblad-master-equation. Their
classical limit is taken to correspond to local jumps between orthogonal
states. Based on statistical distributions of jump- and inter-jump-distances we
are able to quantify the non-classicality of quantum trajectories. To account
for the operational effect of entanglement we introduce the novel concept of
"co-jumps".Comment: 15 pages, 12 figure
Gaussian quantum fluctuations in interacting many particle systems
We consider a many particle quantum system, in which each particle interacts
only with its nearest neighbours. Provided that the energy per particle has an
upper bound, we show, that the energy distribution of almost every product
state becomes a Gaussian normal distribution in the limit of infinite number of
particles. We indicate some possible applications.Comment: 10 pages, formulation made mathematically more precise, two examples
added, accepted for publication in Letters in Mathematical Physic
Scaling behavior of interactions in a modular quantum system and the existence of local temperature
We consider a quantum system of fixed size consisting of a regular chain of
-level subsystems, where is finite. Forming groups of subsystems
each, we show that the strength of interaction between the groups scales with
. As a consequence, if the total system is in a thermal state with
inverse temperature , a sufficient condition for subgroups of size
to be approximately in a thermal state with the same temperature is , where is the width of the occupied
level spectrum of the total system. These scaling properties indicate on what
scale local temperatures may be meaningfully defined as intensive variables.
This question is particularly relevant for non-equilibrium scenarios such as
heat conduction etc.Comment: 7 pages, accepted for publication in Europhysics Letter
Local effective dynamics of quantum systems: A generalized approach to work and heat
By computing the local energy expectation values with respect to some local
measurement basis we show that for any quantum system there are two
fundamentally different contributions: changes in energy that do not alter the
local von Neumann entropy and changes that do. We identify the former as work
and the latter as heat. Since our derivation makes no assumptions on the system
Hamiltonian or its state, the result is valid even for states arbitrarily far
from equilibrium. Examples are discussed ranging from the classical limit to
purely quantum mechanical scenarios, i.e. where the Hamiltonian and the density
operator do not commute.Comment: 5 pages, 1 figure, published versio
On the Optimal Choice of Spin-Squeezed States for Detecting and Characterizing a Quantum Process
Quantum metrology uses quantum states with no classical counterpart to
measure a physical quantity with extraordinary sensitivity or precision. Most
metrology schemes measure a single parameter of a dynamical process by probing
it with a specially designed quantum state. The success of such a scheme
usually relies on the process belonging to a particular one-parameter family.
If this assumption is violated, or if the goal is to measure more than one
parameter, a different quantum state may perform better. In the most extreme
case, we know nothing about the process and wish to learn everything. This
requires quantum process tomography, which demands an informationally-complete
set of probe states. It is very convenient if this set is group-covariant --
i.e., each element is generated by applying an element of the quantum system's
natural symmetry group to a single fixed fiducial state. In this paper, we
consider metrology with 2-photon ("biphoton") states, and report experimental
studies of different states' sensitivity to small, unknown collective SU(2)
rotations ("SU(2) jitter"). Maximally entangled N00N states are the most
sensitive detectors of such a rotation, yet they are also among the worst at
fully characterizing an a-priori unknown process. We identify (and confirm
experimentally) the best SU(2)-covariant set for process tomography; these
states are all less entangled than the N00N state, and are characterized by the
fact that they form a 2-design.Comment: 10 pages, 5 figure
Identification of Decoherence-Free Subspaces Without Quantum Process Tomography
Characterizing a quantum process is the critical first step towards applying
such a process in a quantum information protocol. Full process characterization
is known to be extremely resource-intensive, motivating the search for more
efficient ways to extract salient information about the process. An example is
the identification of "decoherence-free subspaces", in which computation or
communications may be carried out, immune to the principal sources of
decoherence in the system. Here we propose and demonstrate a protocol which
enables one to directly identify a DFS without carrying out a full
reconstruction. Our protocol offers an up-to-quadratic speedup over standard
process tomography. In this paper, we experimentally identify the DFS of a
two-qubit process with 32 measurements rather than the usual 256, characterize
the robustness and efficiency of the protocol, and discuss its extension to
higher-dimensional systems.Comment: 6 pages, 5 figure
Measurable Consequences of the Local Breakdown of the Concept of Temperature
Local temperature defined by a local canonical state of the respective
subsystem, does not always exist in quantum many body systems. Here, we give
some examples of how this breakdown of the temperature concept on small length
scales might be observed in experiments: Measurements of magnetic properties of
an anti-ferromagnetic spin-1 chain. We show that those magnetic properties are
in fact strictly local. As a consequence their measurement reveals whether the
local (reduced) state can be thermal. If it is, a temperature may be associated
to the measurement results, while this would lead to inconsistencies otherwise.Comment: some comments added, results remain unchange
Local Versus Global Thermal States: Correlations and the Existence of Local Temperatures
We consider a quantum system consisting of a regular chain of elementary
subsystems with nearest neighbor interactions and assume that the total system
is in a canonical state with temperature . We analyze under what condition
the state factors into a product of canonical density matrices with respect to
groups of subsystems each, and when these groups have the same temperature
. While in classical mechanics the validity of this procedure only depends
on the size of the groups , in quantum mechanics the minimum group size
also depends on the temperature ! As examples, we apply our
analysis to a harmonic chain and different types of Ising spin chains. We
discuss various features that show up due to the characteristics of the models
considered. For the harmonic chain, which successfully describes thermal
properties of insulating solids, our approach gives a first quantitative
estimate of the minimal length scale on which temperature can exist: This
length scale is found to be constant for temperatures above the Debye
temperature and proportional to below.Comment: 12 pages, 5 figures, discussion of results extended, accepted for
publication in Phys. Rev.
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