Detection of a plasmon-polariton quantum wave packet

Plasmon polaritons (plasmons, for simplicity) have become paramount for tailored nanoscale light–matter interaction, and extensive research has been conducted to monitor 1 , 2 and manipulate their spatial 3 and spatio-temporal dynamics 4 . These dynamics result from the superposition of various plasmon modes, which are classical wave packets. Beyond this classical picture, plasmon modes are treated as quasiparticles 5 , 6 and they are considered essential for the realization of future nanoscale quantum functionality 7 – 10 . Implementing and demonstrating such functionality requires access to the quasiparticle’s quantum state to monitor and manipulate its corresponding quantum wave packet dynamics in Hilbert space. Here we report the local detection of nanoscale plasmon quantum wave packets using plasmon-assisted electron emission as a signal in coherent two-dimensional nanoscopy 11 . The observation of a quantum coherence oscillating at the third harmonic of the plasmon frequency is traced back to the superposition of energetically non-adjacent plasmon occupation number states and is therefore a direct fingerprint of the quantum wave packet. Beyond demonstrating the existence of a plasmon quantum wave packet via the coherence between certain occupation number states and providing an improved model for plasmon-assisted electron emission processes, the results may enable time-dependent probing and manipulation of coupled quantum states and dynamics on the nanoscale.