84 research outputs found
Temporal quantum correlations and Leggett-Garg inequalities in multi-level systems
We show that the quantum bound for temporal correlations in a Leggett-Garg
test, analogous to the Tsirelson bound for spatial correlations in a Bell test,
strongly depends on the number of levels that can be accessed by the
measurement apparatus via projective measurements. We provide exact bounds for
small , that exceed the known bound for the Leggett-Garg inequality, and
show that in the limit the Leggett-Garg inequality can be
violated up to its algebraic maximum.Comment: 6 pages, 2 figure
Maximum violations of the quantum-witness equality
We consider the quantum-witness test of macroscopic realism and derive an upper bound for possible violations of this equality due to quantum mechanics. The bound depends only on the number of possible outcomes for the blind measurement at the heart of the witness protocol. Mirroring recent results for the related Leggett-Garg inequality, we show that quantum mechanics can saturate the algebraic bound for large systems. We also establish a connection between the quantum witness and the trace distance between density matrices and discuss how the quantum witness can be used to obtain a bound on the Hilbert-space dimension of the system under study
Quantum dynamics in nonequilibrium environments
We present a formalism for studying the behaviour of quantum systems coupled
to nonequilibrium environments exhibiting nonGaussian fluctuations. We discuss
the role of a qubit as a detector of the statistics of environmental
fluctuations, as well as nonMarkovian effects in both weak and strong coupling
limits. We also discuss the differences between the influences of classical and
quantum environments. As examples of the application of this formalism we study
the dephasing and relaxation of a charge qubit coupled to nonequillibrium
electron transport through single and double quantum dots.Comment: 12 pages, 7 figures; significantly expanded versio
- …