Demonstration of tripartite cat states in two distinct classes of entanglement

Abstract

Entanglement is a cornerstone of quantum mechanics and an essential resource for quantum computation, communication, and metrology. While bipartite entanglement is extensively studied, genuine multipartite entangled states remain largely inaccessible in the macroscopic continuous-variable domain. Here, we experimentally realize macroscopic tripartite entangled cat states of the two distinct classes with fundamentally inequivalent properties, namely GHZ-cat and W-cat states, encoded across three microwave resonators coupled to an ancillary superconducting transmon. We develop one-to-all conditional controls on the three oscillators by fully utilizing three energy levels of the transmon to generate these states in a single piece of hardware and employ an efficient subspace tomography protocol for state reconstruction. We obtain GHZ-cat and W-cat states with fidelities $0.83\pm0.02$ and $0.70\pm0.02$, respectively, certifying genuine multipartite entanglement among the three oscillators. We further validate the distinct entanglement classes by experimentally demonstrating that pairwise entanglement is present in the W-cat state but absent in the GHZ-cat state. Our technique for the on-demand generation and characterization of inequivalent classes of tripartite continuous-variable entanglement provides a valuable testbed for future studies of macroscopic many-body entanglement and fault-tolerant quantum information processing with multipartite bosonic codes.