Waterloo physicists prove a hard limit on Gaussian quantum repeaters

[Ava_75]

A new proof from the University of Waterloo puts a firm limit on one popular idea for building long distance quantum networks. Quantum Zeitgeist reports that Rabsan Galib Ahmed and Graeme Smith of the Institute for Quantum Computing have proved a no-go theorem for Gaussian quantum repeaters

Quantum repeaters are meant to carry quantum information over long distances, since photons get lost in optical fibre. Gaussian operations are attractive because they are relatively easy to do with standard optics. The proof shows that a repeater chain built only from Gaussian operations, homodyne measurements and classical communication cannot beat simply sending the quantum information straight down the lossy channel

That holds even with unlimited classical communication and adaptive strategies, which makes the result very strong. To get there, the authors introduced a new tool they call fractional extendibility for Gaussian states

The practical message is that long distance quantum networks will need non-Gaussian operations, such as photon counting or other nonlinear tricks. Those are much harder to build reliably. The work was funded jointly by NSERC and the NSF and appears in an APS journal

No-go theorems are not glamorous, but they save the field a lot of wasted effort. Knowing what cannot work points researchers towards what might. It also explains why so many repeater proposals already lean on non-Gaussian ingredients

Are results like this more useful than new experiments? And how far away is a working quantum repeater?

University Of Waterloo Finds A Fundamental Limit To Quantum Repeatersquantumzeitgeist.com