Jörgen Rammer

Charge transfer counting statistics revisited

Charge transfer statistics of quantum particles is obtained by analysing the time evolution of the many-body wave function. Exploiting properly chosen gauge transformations, we construct the probabilities for transfers of a discrete number of particles. Generally, the derived formula for counting statistics differs from the one previously obtained by Levitov et al. (J. Math. Phys., 37 (1996) 4845). The two formulae agree only if the initial state is prohibited from being a superposition of different charge states. Their difference is illustrated for cases of a single particle and a tunnel junction, and the role of charge coherence is demonstrated.

Quantum measurement in charge representation

Counting statistics of charge transfers in a point contact interacting with an arbitrary quantum system is studied. The theory for the charge specific density matrix is developed, allowing the evaluation of the probability of the outcome of any joint measurement of the state of the quantum system and the transferred charge. Applying the method of charge projectors, the master equation for the charge specific density matrix is derived in the tunneling Hamiltonian model of the point contact. As an example, the theory is applied to a quantum measurement of a two-state system: The evolution of the charge specific density matrix in the presence of Nyquist or Schottky noise is studied and the conditions for the realization of a projective measurement are established.

Noise spectrum of a tunnel junction coupled to a nanomechanical oscillator

A nanomechanical resonator coupled to a tunnel junction is studied. The oscillator modulates the transmission of the junction, changing the current and the noise spectrum. The influence of the osc ...

Level crossing in a dissipative environment

We present results of an analytical study of the dissipative Landau-Zener model, H ( t ) = υtσ z + Δσ x + σ z Σ α λ α ( a α † + a α ) + Σ α . ħω α ( a α † a α + 1/2). Analytical results for the excitation transition from the ground state of the two-state system at large negative times to the excited state at large positive times (as well as the opposite, decay, transition) are obtained as function of the speed at which the two levels approach each other, the energy gap between the adiabatic energies, the strength of the coupling to the environment and its temperature. The analytical treatment is sufficiently exhaustive that a complete description of the dissipative quantum dynamics can be given and, in particular, the quantitative criterion for adiabaticity is established.

Effective-action approach to a trapped Bose gas

The effective-action formalism is applied to a gas of bosons. The equations describing the condensate and the excitations are obtained using the loop expansion for the effective action. For a homo ...

Influence of typical environments on quantum processes.

We present the results of studying the influence of different environmental states on the coherence of quantum processes. We choose to discuss a simple model which describe two electronic reservoirs connected through tunneling via a resonant state. The model could, e.g., serve as an idealization of inelastic resonant tunneling through a double barrier structure. We develop Schwingers closed time path formulation of non-equilibrium quantum statistical mechanics, and show that the influence of the environment on a coherent quantum process can be described by the value of a generating functional at a specific force value, thereby allowing for a unified discussion of destruction of phase coherence by various environmental states: thermal state, classical noise, time dependent classical field, and a coherent state. The model allows an extensive discussion of the influence of dissipation on the coherent quantum process, and expressions for the transmission coefficient are obtained in the possible limits.

WEAK AND STRONG LOCALIZATION IN LOW-DIMENSIONAL SEMICONDUCTOR STRUCTURES

The dependence of the localization length on the number of occupied subbands

Self‐consistent theory of dynamic melting of a vortex lattice

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INFLUENCE OF THE HALL FORCE ON THE VORTEX DYNAMICS IN TYPE-II SUPERCONDUCTORS

in low-dimensional semiconductors is investigated. The localization length is shown to be proportional to the number of occupied subbands in quasi-one-dimensional quantum wires while it grows exponentially with

Destruction of phase coherence by electron-phonon interactions in disordered conductors.

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