Showing papers on "Quantum channel published in 1989"

Existence of quantum diffusions

Abstract: A quantum diffusion (A, A′, j) comprises of unital *-algebras A and A′ and a family of identity preserving *-homomorphisms j=(j t : t≧0) from A into A′. Also j satisfies a system of quantum stochastic differential equations dj t (x 0=j t(μ (x 0))dM , j 0(x 0)=x 0⊗I for all x 0∈A where μ , 1≦i, j≦N are maps from A to itself and are known as the structure maps. In this paper an existence proof is given for a class of quantum diffusions, for which the structure maps are bounded in the operator norm sense. A solution to the system of quantum stochastic differential equations is first produced using a variation of the Picard iteration method. Another application of this method shows that the solution is a quantum diffusion.

Quantum limited detectors for weak classical signals

Abstract: When a classical signal is very weak, the quantum features of its detector cannot be ignored. There appears then to be a conflict between the continuous nature of the classical signal and the discrete spectrum of a quantum device. Moreover, the final output cannot be read directly from a quantum system: The latter has to be ``measured'' by another device (the ``meter'') which then yields another classical signal---a real number. This paper examines the amount of distortion caused by the presence of a quantum interface between two classical signals. It is shown that the meter should have a moderate resolution, so as to lump together numerous levels of the detector. A finer resolution deteriorates the correspondence between the input and output signals. A perfect resolution, down to isolated eigenvalues, may completely lock the output signal (this is the quantum Zeno effect).