# Correlations and entanglement of microwave photons emitted in a cascade decay

## TL;DR

This paper demonstrates how a superconducting artificial atom can be coherently driven to produce correlated and entangled microwave photons through cascade decay, revealing new ways to generate entanglement in quantum microwave systems.

## Contribution

It introduces a novel protocol using a superconducting atom to generate entangled microwave photons via cascade decay with phase-sensitive detection.

## Key findings

- Successfully demonstrated phase correlation of emitted microwave photons
- Generated entanglement between itinerant microwave modes
- Highlighted the coherent nature of cascade decay in superconducting systems

## Abstract

An excited emitter decays by radiating a photon into a quantized mode of the electromagnetic field, a process known as spontaneous emission. If the emitter is driven to a higher excited state, it radiates multiple photons in a cascade decay. Atomic and biexciton cascades have been exploited as sources of polarization-entangled photon pairs. Because the photons are emitted sequentially, their intensities are strongly correlated in time, as measured in a double-beam coincidence experiment. Perhaps less intuitively, their phases can also be correlated, provided a single emitter is deterministically prepared into a superposition state, and the emitted radiation is detected in a phase-sensitive manner and with high efficiency. Here we have met these requirements by using a superconducting artificial atom, coherently driven to its second-excited state and decaying into a well-defined microwave mode. Our results highlight the coherent nature of cascade decay and demonstrate a novel protocol to generate entanglement between itinerant field modes.