An international team of researchers, including scientists from Paderborn University, has achieved a significant milestone on the road to a quantum internet. For the first time, they successfully teleported the polarization state of a single photon from one quantum dot to another physically separated dot. In layman's terms, they made one photon's properties jump to another through quantum teleportation, which sounds like something from Star Trek but is actually just physics showing off.
The experiment used a 270-meter free-space optical link to connect the systems, and the results have been published in the journal Nature Communications. Doctoral and postdoctoral researchers at Paderborn spent about ten years on optical measurements, data analysis, and evaluation, working closely with Professor Rinaldo Trotta's team at Sapienza University of Rome.
"The experiment impressively demonstrates that quantum light sources based on semiconductor quantum dots could serve as a key technology for future quantum communication networks," explained Professor Klaus Jöns, head of the 'Hybrid Photonics Quantum Devices' research group at Paderborn. "Successful quantum teleportation between two independent quantum emitters represents a vital step towards scalable quantum relays and thus the practical implementation of a quantum internet."
The breakthrough relied on contributions from several European research centers. Quantum dots were engineered at Johannes Kepler University Linz, resonator nanofabrication was done at the University of Würzburg, and the teleportation experiments took place at Sapienza University of Rome, where scientists connected two buildings using that 270-meter free-space optical link. The system used GPS-assisted synchronization, ultra-fast single photon detectors, and stabilization methods to counter atmospheric turbulence. The achieved teleportation state fidelity reached up to 82 ± 1%, exceeding the classical limit by more than 10 standard deviations - which is science's way of saying "we're pretty sure this worked."
This accomplishment opens the door to 'entanglement swapping' between two quantum dots, which would create the first quantum relay using two deterministic sources of entangled photon pairs. Deterministic sources can reliably produce single photons almost on demand, though developing them has been a major challenge - because of course nothing in quantum mechanics is ever easy.
Coincidentally, another research team from Stuttgart and Saarbrücken reported a similar achievement using frequency conversion at nearly the same time. Together, these results mark an important milestone for quantum research in Europe and bring the vision of a functional quantum internet closer to reality - assuming reality doesn't get too entangled with itself.