# Measuring mutual friction in superfluids: the role of initial vortex configuration fluctuations

## TL;DR

This paper investigates how initial vortex configuration fluctuations affect the measurement of mutual friction parameters in superfluids, revealing potential biases and improving the accuracy of experimental inferences.

## Contribution

It introduces an analysis of vortex fluctuations' impact on mutual friction coefficient measurements using the dissipative point vortex model.

## Key findings

- Fluctuations can significantly bias the inferred mutual friction coefficients.
- Accounting for fluctuations improves the accuracy of experimental parameter estimation.
- Comparison with experiments validates the importance of initial condition considerations.

## Abstract

The physical origin of mutual friction in quantum fluids is deeply connected to the fundamental nature of superfluidity. It stems from the interaction between the superfluid and normal components, mediated by the dynamics of quantized vortices that induce the exchange of momentum and energy. Despite the complexity of these interactions, their essential features can be effectively described by the dissipative point vortex model, an extension of classical vortex dynamics that incorporates finite-temperature dissipation. Mutual friction is parametrized by the longitudinal (dissipative) coefficient $\alpha$ and the transverse (reactive) coefficient $\alpha'$. Accurate measurement of these parameters provides critical insights into the microscopic mechanisms governing vortex motion and dissipation in quantum fluids, serving as a key benchmark for theoretical models. In this work, we employ the dissipative point vortex model to study how fluctuations in the initial conditions influence the inference of $\alpha$ and $\alpha'$ from the time evolution of the vortex trajectories. Using experimentally realistic parameters, we show that fluctuations can introduce significant biases in the extracted values of the mutual friction coefficients. We compare our findings with recent experimental measurements in strongly interacting atomic superfluids. Applying this analysis to our recent experimental results allowed us to account for fluctuations in the correct determination of $\alpha$ and $\alpha'$.