Introduction

Abstract

In the classical realm the best known type of noise is thermal noise. Due to thermal agitation, free electrons in a metallic conductor are moving around continuously causing collisions with the atoms and a continuous exchange of energy between the modes. This was first investigated experimentally by J.B. Johnson and H. Nyquist in 1928. Nyquist’s theorem states that the power spectral density (psd) of voltage fluctuations through a resistor R at temperature T is S _V (f) = 4k RT, with k the Boltzman’s constant. The quantum approach is also a very efficient way to understand the limitation in the accuracy of measurements performed in thermal equilibrium. The paper by J.M. Courty et al. examines the fluctuations which are expected in an operational amplifier from a quantum network approach. The ultimate sensitivity of a cold damped accelerometer designed for space applications is calculated as well.