Non-Markovian effects are often significant when the system-environment coupling is not weak. Indeed, we find that the non-Markovianity is negligible for a single two-level system undergoing pure dephasing via a weak interaction with a harmonic-oscillator environment. In this paper we show that, within the framework of pure dephasing, the non-Markovianity displayed by a two-level system can in fact be far more pronounced. To demonstrate that this is indeed the case, we consider a pure dephasing model where a collection of two-level systems interacts with a common environment. We obtain analytically the dynamics of the collection of the two-level systems and then take a partial trace over all the two-level systems except one. This remaining single two-level system exhibits markedly non-Markovian dynamics, even when the system-environment coupling is weak. This is due to the environment-mediated indirect interaction between the two-level systems. In fact, this indirect interaction can not only increase the non-Markovianity substantially, but also qualitatively change the characteristics of the non-Markovian behavior. For instance, for a single two-level system undergoing pure dephasing, the dynamics is Markovian for both Ohmic and sub-Ohmic environments. This is markedly not the case when we consider multiple two-level systems. These findings provide insights into controlling decoherence in multiqubit quantum systems and have implications for quantum technologies where non-Markovianity can be a resource rather than a limitation.

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• Open quantum systems & decoherence
• Quantum control
• Quantum correlations in quantum information
• Quantum correlations, foundations & formalism