Quantum noise minimization in transistor amplifiers.

General quantum restrictions on the noise performance of linear transistor amplifiers are used to identify the region in parameter space where the quantum-limited performance is achievable and to construct a practical procedure for approaching it experimentally using only the knowledge of directly measurable quantities: the gain, (differential) conductance, and the output noise. A specific example of resonant barrier transistors is discussed.

Generalized constraints on quantum amplification.

We derive quantum constraints on the minimal amount of noise added in linear amplification involving input or output signals whose component operators do not necessarily have c-number commutators, as is the case for fermion currents. This is a generalization of constraints derived for the amplification of bosonic fields whose components possess c-number commutators.

Shot-noise in transport and beam experiments.

Consider two Fermi gases with the same average currents: a transport gas, as in solid-state experiments where the chemical potentials of terminal 1 is mu+eV and of terminal 2 and 3 is mu, and a beam, i.e., electrons entering only from terminal 1 having energies between mu and mu+eV. By expressing the current noise as a sum over single-particle transitions we show that the temporal current fluctuations are very different: The beam is noisier due to allowed single-particle transitions into empty states below mu. Surprisingly, the correlations between terminals 2 and 3 are the same.