# Observing temperature fluctuations of a mesoscopic electron system

## TL;DR

This paper experimentally observes temperature fluctuations in a mesoscopic electron system, confirming theoretical predictions and revealing how nonequilibrium conditions affect these fluctuations, which are crucial for calorimeter sensitivity.

## Contribution

First direct measurement of temperature fluctuations in a nanoscale electron system under equilibrium and nonequilibrium conditions.

## Key findings

- Temperature fluctuations follow fluctuation dissipation theorem at equilibrium.
- Nonequilibrium conditions cause nontrivial temperature fluctuation dependence on chemical potential bias.
- Results establish fundamental limits for calorimeter energy resolution.

## Abstract

Almost a century ago, Johnson and Nyquist presented evidence of fluctuating electrical current and the governing fluctuation dissipation theorem (FDT). Whether, likewise, temperature T can fluctuate is a controversial topic and has led to scientific debates for several decades. To resolve this issue, there was an experiment initially in 1992 where the authors found good agreement between the FDT theory for heat and experiment on a macroscopic sample. A key question is what happens when we consider a nanoscale system with much fewer particles at 100 times lower temperatures. This challenge has not been addressed up to now, due to the demanding experimental requirement on fast and sensitive thermometry on a mesoscopic absorber. Here we observe equilibrium fluctuations of temperature in a canonical system of about 10^8 electrons exchanging energy with phonon bath at a fixed temperature. Moreover, temperature fluctuations under nonequilibrium conditions present a nontrivial dependence on the chemical potential bias of a hot electron source. These fundamental fluctuations of T set the ultimate lower bound of the energy resolution of a calorimeter.