Quantum entanglement in the microwave regime

A recent theoretical study by an international team, which also involves the National Institute of Optics of the National Research Council (Cnr-Ino), proposes an innovative method to generate entangled photon pairs in the microwave domain. Published in PRX QUANTUM and reported by the Physics Magazine of the American Physical Society, the work opens up new scenarios for quantum technologies.

Quantum entanglement — a deep bond between particles, inexplicable according to classical physics — is at the basis of the next technological revolution in the fields of communication, computation and metrology. Until now, the controlled production of entangled photon pairs was mainly confined to the optical field. The new study instead proposes to exploit the so-called Cooper pairs — pairs of entangled electrons in superconductors — to generate entanglement between microwave photons.

The heart of the proposal is a nanometric device that combines superconductors, quantum dots and photonic circuits: a true hybrid architecture integrable on a chip. Using Cooper pair splitters based on double quantum dots, the system separates a pair of electrons into two distinct channels, inducing the simultaneous emission of two entangled photons in frequency.

"So far, it has not been possible to directly observe the entanglement of a single Cooper pair of a BCS condensate. Our scheme aims to overcome this challenge, offering a way to access this fundamental aspect of superconductivity. Compared to past theoretical schemes, based on complex measurements of charge or spin transport, our scheme transfers the entanglement from electrons to photons, for which reliable detection techniques exist. Furthermore, the proposal could offer a new tool to explore more complex quantum states, such as those present in topological or unconventional superconductivity, where electronic entanglement takes different forms.", explains Gianluca Rastelli, a researcher at Cnr-Ino and one of the authors of the work. "Our approach, in fact, integrates advanced components of nanometric quantum systems, combining superconducting nanocontacts, semiconductor quantum dots and microwave devices. We have identified possible critical issues and risks, and shown that overcoming them requires cutting-edge nanofabrication techniques. We believe that this work paves the way for future experimental studies on this hybrid architecture".

This approach could make it possible, for the first time, to experimentally verify entanglement in a single Cooper pair, a goal so far inaccessible to the scientific community. The work develops a theoretical proposal born from the close collaboration between two theorists – Gianluca Rastelli (Cnr-Ino) and Michele Governale (Victoria University of Wellington, New Zealand) - and two experimentalists – Pasquale Scarlino (EPFL, Lausanne, CH) and Christian Schönenberger (University of Basel, CH). Its importance also lies in the long-term vision: to bring quantum photonics in the microwave regime to the level of maturity reached by optical photonics: this would allow the development of new quantum computing architectures, ultra-sensitive sensors and secure communication networks fully integrable on chips.