A flying Schr\"odinger cat in multipartite entangled states

TL;DR

This paper demonstrates a scalable method to create and verify multipartite Schr"odinger cat states in the microwave domain, enabling advances in quantum information and metrology with macroscopic superpositions.

Contribution

The authors present a deterministic, scalable approach to generate and fully characterize flying multipartite cat states using superconducting circuits and quantum state tomography.

Findings

Successful creation of up to four photonic mode cat states

Verification of quantum entanglement among multiple modes

Observation of hybrid entanglement between qubit and cat states

Abstract

Schr\"odinger's cat originates from the famous thought experiment querying the counterintuitive quantum superposition of macroscopic objects. As a natural extension, several "cats" (quasi-classical objects) can be prepared into coherent quantum superposition states, which is known as multipartite cat states demonstrating quantum entanglement among macroscopically distinct objects. Here we present a highly scalable approach to deterministically create flying multipartite Schr\"odinger cat states, by reflecting coherent state photons from a microwave cavity containing a superconducting qubit. We perform full quantum state tomography on the cat states with up to four photonic modes and confirm the existence of quantum entanglement among them. We also witness the hybrid entanglement between discrete-variable states (the qubit) and continuous-variable states (the flying multipartite cat) through a joint quantum state tomography. Our work demonstrates an important experimental control method in the microwave region and provides an enabling step for implementing a series of quantum metrology and quantum information processing protocols based on cat states.