Multipartite entanglement in a microwave frequency comb*

The entanglement of multiple, classically distinct states lies at the heart of quantum communication and information processing. In the microwave regime with superconducting circuits, significant progress has been made with multipartite entanglement of discrete qubits, but continuous variable (CV) systems may provide another scalable path toward generation and control of entanglement in large ensembles. For instance, large CV entangled states have been generated in optics by pumping optical parametric oscillators (OPO).  Working with microwave and superconducting circuits may provide additional advantages, such as digital electronics and a strong tunable non-linearity in the Josephson junction. In this work, we measure up to 64 correlated modes propagating in a transmission line connected to a Josephson parametric amplifier (JPA) using a multi-frequency digital signal processing methodology, verifying full inseparability between 7 modes. The correlations are achieved by applying a bi-chromatic pump to the JPA at roughly double its resonance frequency. Our method provides a clear path for scaling to more modes and maybe eventual generation of CV cluster states.