Creating Entangled Pairs of Microwave Photons
Entanglement is the secret sauce of many quantum technologies, allowing correlations between the states of disconnected objects. A natural source of entangled particles is a superconductor, whose Cooper pairs consist of entangled electrons. These paired-up electrons can retain their entanglement when spatially separated. But researchers would like to create devices that exploit entangled particles other than electrons―specifically, microwave photons. Now Gianluca Rastelli of the National Institute of Optics, Italy, and his colleagues predict a way to do exactly that by creating entangled microwave photons from Cooper pairs [1]. The advance could allow researchers to build nanodevices that integrate low-temperature electronic components—such as superconducting contacts—with quantum microwave-photonic systems.
The proposed microwave-photon creator is based on quantum dots—islands of semiconductor embedded in a conducting material. The device consists of a superconducting material sandwiched between two double quantum dots (two sets of two quantum dots that are electrically and physically coupled). Under the right conditions, a Cooper pair from the superconductor interacts with these quantum dots and splits in half, with one electron entering one double quantum dot and the second entering the other. Once there, each electron jumps from a high energy level in the first dot to a lower level in the second dot, releasing a photon. These two photons form an entangled pair, and the quantum dots can be tuned so that the photons have microwave frequencies.
Rastelli notes that, as well as generating entangled photons, the proposed method could be used to verify the entanglement of single Cooper pairs, a challenging task that requires advanced charge-transport measurements. “No one has yet succeeded in directly observing or confirming entanglement in a single Cooper pair,” Rastelli says.
–Katherine Wright
Katherine Wright is the Deputy Editor of Physics Magazine.
References
- M. Governale et al., “Entangled photon-pair emission in waveguide circuit QED from a Cooper pair splitter,” PRX Quantum 6, 020339 (2025).