Perfect Spatial Mode Measurements in Optics Express

Measuring spatial modes (almost) perfectly with a single phase screen

The polarisation of a photon is a quantum photonics workhorse for many reasons—optical devices such as waveplates and polarising beam splitters make it possible to manipulate and measure polarisation states with ease, allowing their integration into quantum information technologies.

While photonic spatial modes promise high-dimensional quantum information systems with massive information capacities and an increased robustness to noise, tools for manipulating and measuring them are far from perfect.

In our paper published today in Optics Express, we demonstrate a new technique that allows us to measure the azimuthal and radial modes of a photon using a single phase screen with greater than 99% accuracy! We expect our method will enable quantum and classical communication systems that exploit the full information-carrying potential of light.

Entanglement Record in Nature Physics!

Artist’s depiction of photonic high-dimensional entanglement
Image credit: Harald Ritsch/IQOQI Vienna

While being incredibly useful for high-capacity quantum communication, high-dimensional entanglement is notoriously hard to measure. Quantum state tomography of a high-dimensional system of even modest dimension can take up to days, and the best entanglement witnesses aren’t much better.

In a recent paper published in Nature Physics, we developed and demonstrated a new method for measuring high-dimensional entanglement that requires only two measurement settings to work, independent of how large your system may be! We used this technique (which we fondly call the Mubby Witness) to certify a record entanglement dimensionality, without making any assumptions on the state.

This work came out of an intense theory-experiment collaboration with our collaborators in Vienna and Brno. From outdoor debates in Sunny Benasque to rapid back-and-forths between the lab and the blackboard, this project couldn’t have been more fun. Stay tuned for Mubby 2.0!

For coverage of this work in the popular media, please check out our media coverage page.

Marcus and Nico having “heated” discussions on entanglement fidelities in Benasque