17 research outputs found
Stable macroscopic quantum superpositions
We study the stability of superpositions of macroscopically distinct quantum
states under decoherence. We introduce a class of quantum states with
entanglement features similar to Greenberger-Horne-Zeilinger (GHZ) states, but
with an inherent stability against noise and decoherence. We show that in
contrast to GHZ states, these so-called concatenated GHZ states remain
multipartite entangled even for macroscopic numbers of particles and can be
used for quantum metrology in noisy environments. We also propose a scalable
experimental realization of these states using existing ion-trap set-ups.Comment: 4 pages, 1 figure; v2: minor changes due to referee report
Improved quantum metrology using quantum error-correction
We consider quantum metrology in noisy environments, where the effect of
noise and decoherence limits the achievable gain in precision by quantum
entanglement. We show that by using tools from quantum error-correction this
limitation can be overcome. This is demonstrated in two scenarios, including a
many-body Hamiltonian with single-qubit dephasing or depolarizing noise, and a
single-body Hamiltonian with transversal noise. In both cases we show that
Heisenberg scaling, and hence a quadratic improvement over the classical case,
can be retained. Moreover, for the case of frequency estimation we find that
the inclusion of error-correction allows, in certain instances, for a finite
optimal interrogation time even in the asymptotic limit.Comment: Version 2 is the published version. Appendices contain Supplemental
materia
Measures of macroscopicity for quantum spin systems
We investigate the notion of "macroscopicity" in the case of large quantum
spin systems and provide two main results. First, we motivate the Fisher
information as a measure for the macroscopicity of quantum states. Second, we
compare the existing literature of this topic. We report on a hierarchy among
the measures and we conclude that one should carefully distinguish between
"macroscopic quantum states" and "macroscopic superpositions", which is a
strict subclass of the former.Comment: Comments are welcome! v2: Minor improvements of the tex
A matrix product solution for a nonequilibrium steady state of an XX chain
A one dimensional XX spin chain of finite length coupled to reservoirs at
both ends is solved exactly in terms of a matrix product state ansatz. An
explicit representation of matrices of fixed dimension 4 independent of the
chain length is found. Expectations of all observables are evaluated, showing
that all connected correlations, apart from nearest neighbor z-z, are zero.Comment: 11 page
General measure for macroscopic quantum states beyond "dead and alive"
We consider the characterization of quantum superposition states beyond the pattern "dead and alive". We propose a measure that is applicable to superpositions of multiple macroscopically distinct states, superpositions with different weights as well as mixed states. The measure is based on the mutual information to characterize the distinguishability between the multiple branches of the superposition. This allows us to overcome limitations of previous proposals, and to bridge the gap between general measures for macroscopic quantumness and measures for Schrödinger-cat type superpositions. We discuss a number of relevant examples, provide an alternative definition using basis-dependent quantum discord and reveal connections to other proposals in the literature. Finally, we also show the connection between the size of quantum states as quantified by our measure and their vulnerability to noise