Observation of scalable, highly multimode entanglement in frequency combs with ultrafast pulse shaping

Cluster states, in which entanglement is distributed amongst numerous parties, have attracted interest due to their role as a resource for measurement-based quantum computing. The traditional means for fabricating these highly-entangled states has been to sequentially combine the squeezed outputs from individual optical parametric oscillators (OPO) with a network of phase-shifters and beamsplitters. While successful, this approach demands an optical footprint commensurate with the dimensionality of the produced state. Within the continuous-variable regime, a promising alternative is to begin with a source that is intrinsically multimode. Toward that end, the frequency comb underlying femtosecond optical pulses contains upwards of ~ 10⁵ individual frequencies, which makes it an attractive starting point for the production of multipartite entanglement. This work demonstrates the ability of OPOs synchronously pumped by a femtosecond pulse train to create scalable, adaptable multipartite photonic states in a single beam while exploiting femtosecond pulse shaping to perform a complete state analysis.