# Non-Markovian noise that cannot be dynamically decoupled by periodic spin echo pulses

## TL;DR

This paper demonstrates that certain non-Markovian quantum noise cannot be mitigated by periodic spin echo pulses, challenging assumptions about the effectiveness of dynamical decoupling based solely on decay profiles.

## Contribution

It provides a counterexample showing that non-Markovian noise with quadratic decay cannot be suppressed by periodic spin echo pulses, regardless of pulse speed.

## Key findings

- Quadratic decay does not guarantee decoupling effectiveness.
- Non-Markovian noise can be inherently resistant to dynamical decoupling.
- Decoupling strategies must consider noise type beyond decay profiles.

## Abstract

Dynamical decoupling is the leading technique to remove unwanted interactions in a vast range of quantum systems through fast rotations. But what determines the time-scale of such rotations in order to achieve good decoupling? By providing an explicit counterexample of a qubit coupled to a charged particle and magnetic monopole, we show that such time-scales cannot be decided by the decay profile induced by the noise: even though the system shows a quadratic decay (a Zeno region revealing non-Markovian noise), it cannot be decoupled by periodic spin echo pulses, no matter how fast the rotations.