# Conditional Hybrid Nonclassicality

## TL;DR

This paper introduces a new method to characterize and verify nonclassicality in hybrid quantum systems, demonstrating its application through experimental generation and analysis of a hybrid Schr"odinger cat state.

## Contribution

It develops a general framework for conditional hybrid nonclassicality, including a sampling approach for its verification, and experimentally demonstrates this with a hybrid entangled state.

## Key findings

- Successfully generated a hybrid Schr"odinger cat state.
- Verified nonclassicality with high statistical significance.
- Provided a new operational method for hybrid quantum state characterization.

## Abstract

We derive and implement a general method to characterize the nonclassicality in compound discrete- and continuous-variable systems. For this purpose, we introduce the operational notion of conditional hybrid nonclassicality which relates to the ability to produce a nonclassical continuous-variable state by projecting onto a general superposition of discrete-variable subsystem. We discuss the importance of this form of quantumness in connection with interfaces for quantum communication. To verify the conditional hybrid nonclassicality, a matrix version of a nonclassicality quasiprobability is derived and its sampling approach is formulated. We experimentally generate an entangled, hybrid Schr\"odinger cat state, using a coherent photon-addition process acting on two temporal modes, and we directly sample its nonclassicality quasiprobability matrix. The introduced conditional quantum effects are certified with high statistical significance.