5 research outputs found
The Global versus Local Hamiltonian Description of Quantum Input-Output Theory
The aim of this paper is to derive the global Hamiltonian form for a quantum system and bath, or more generally a quantum network with multiple quantum input field connections, based on the local descriptions. We give a new simple argument which shows that the global Hamiltonian for a single Markov component arises as the singular perturbation of the free translation operator. We show that the Fermi analogue takes an equivalent form provided the parity of the coefficients is correctly specified. This allows us to immediately extend the theory of quantum feedback networks to Fermi systems. </jats:p
The Stratonovich formulation of quantum feedback network rules
We express the rules for forming quantum feedback networks using the
Stratonovich form of quantum stochastic calculus rather than the Ito, or SLH
form. Remarkably the feedback reduction rule implies that we obtain the Schur
complement of the matrix of Stratonovich coupling operators where we short out
the internal input/output coefficients.Comment: 14 pages, 6 figures (The Stratonovich form of the Series Product
added in the revision.
Control of quantum noise: on the role of dilations
We show that every finite-dimensional quantum system with Markovian time
evolution has an autonomous unitary dilation which can be dynamically
decoupled. Since there is also always an autonomous unitary dilation which
cannot be dynamically decoupled, this highlights the role of dilations in the
control of quantum noise. We construct our dilation via a time-dependent
version of Stinespring in combination with Howland's clock Hamiltonian and
certain point-localised states, which may be regarded as a C*-algebraic
analogue of improper bra-ket position eigenstates and which are hence of
independent mathematical and physical interest.Comment: 17