When a periodic waveform is subjected to small perturbations to its amplitude and phase, any nonlinearity in the system can lead to its fragmentation — a so-called modulation instability. The phenomenon often results in the formation of solitons — self-reinforcing solitary wave packets — aligned in a train. This has been observed in both optical systems and atomic Bose–Einstein condensates. In the latter, it has long been debated whether the noise or the self-interference of the condensate order parameter serves to seed the modulation instability.
Jason Nguyen and colleagues have now addressed the problem with ultracold 7Li atoms. In their system, the modulation instability was boosted by suddenly switching the interactions from repulsive to attractive. A nearly non-destructive imaging technique allowed them to monitor the subsequent dynamical formation of the soliton train. Nguyen et al. found that solitons developed first in the centre of the condensate rather than at the edges, suggesting that the seed for the modulation instability is dominated by noise, which may be technical, thermal or quantum in origin.
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Li, Y. Search for the seed. Nature Phys 13, 529 (2017). https://doi.org/10.1038/nphys4172