Quantum stochastic equation for the low density limit

A new derivation of quantum stochastic differential equation for the evolution operator in the low density limit is presented. We use the distribution approach and derive a new algebra for quadratic master fields in the low density limit by using the energy representation. We formulate the stochastic golden rule in the low density limit case for a system coupling with Bose field via quadratic interaction. In particular the vacuum expectation value of the evolution operator is computed and its exponential decay is shown.Comment: Replaced with version published in J. Phys. A. References are adde

Laser and Diffusion Driven Optimal Discrimination of Similar Quantum Systems in Resonator

A method for solving the problem of efficient population transfer from the ground to some excited state by available technical means (varying resonator length) is proposed. We consider a mixture of similar quantum systems distributed in a stationary gas flow in the resonator with variable resonator length, which implements tailored laser field close to the optimal one. In difference from previous works, in this work piezoelectric transducer (PZT; or actuator) is used as mean of control to manipulate variable resonator length. The external actions are optimized to selectively prepare different stationary states of different species which are then separated using their diffusion with rates which differ due to different masses. This system provides an example of the general problem of optimizing states of distributed systems with stationary gas flow with diffusion.Comment: 18 pages, 1 figur

Quantum feedback control in quantum photosynthesis

A model of charge separation in quantum photosynthesis as a model of quantum feedback control in a system of interacting excitons and vibrons is introduced. Quantum feedback in this approach describes the Landau--Zener transition with decoherence. The model explains irreversibility in the process of charge separation for quantum photosynthesis -- direct transitions for this quantum control model will have probabilities close to one and reverse transitions will have probabilities close to zero. This can be considered as a model of quantum ratchet. Also this model explains coincidence of energy of the vibron paired to the transition and Bohr frequency of the transition.Comment: 10 pages, commentaries adde

A stochastic golden rule and quantum Langevin equation for the low density limit

A rigorous derivation of quantum Langevin equation from microscopic dynamics in the low density limit is given. We consider a quantum model of a microscopic system (test particle) coupled with a reservoir (gas of light Bose particles) via interaction of scattering type. We formulate a mathematical procedure (the so-called stochastic golden rule) which allows us to determine the quantum Langevin equation in the limit of large time and small density of particles of the reservoir. The quantum Langevin equation describes not only dynamics of the system but also the reservoir. We show that the generator of the corresponding master equation has the Lindblad form of most general generators of completely positive semigroups

Coherent quantum control of Λ\Lambda-atoms through the stochastic limit

We investigate, using the stochastic limit method, the coherent quantum control of a 3-level atom in $\Lambda$-configuration interacting with two laser fields. We prove that, in the generic situation, this interaction entangles the two lower energy levels of the atom into a single qubit, i.e. it drives at an exponentially fast rate the atom to a stationary state which is a coherent superposition of the two lower levels. By applying to the atom two laser fields with appropriately chosen intensities, one can create, in principle, any superposition of the two levels. Thus {\it relaxation} is not necessarily synonymous of {\it decoherence}.Comment: RevTex, 7 page