The goal is ambitious; develop a novel device that can reliably measure special quantum states known as “Bell-States”. A total of seven research institutes have joined forces to pursue the goal of an optical "Bell-State Analyzer". The coordinator of the new project at Atominstitut of TU Wien is Prof. Arno Rauschenbeutel, and the European Union is providing a total of 3 million euros of funding.

Quantum Entanglement
An astonishing phenomenon plays a central role in practically all modern quantum technology; quantum entanglement, in which individual particles can no longer be viewed as separate from another. When the state of one particle is measured, this inevitably changes the state of the other particle, meaning the pair can only be described as a single entity.

The degree of entanglement of particles can vary. Two-particle states with the maximum possible degree of entanglement are known as “Bell-States”, named after the Quantum Theoretician, John Bell. In modern quantum technology, Bell-states play an important role and often consist of pairs of photons. Their use spans the fields of quantum teleportation, quantum cryptology, and even quantum sensor technology.

Unfortunately, as of yet, it has not been possible to reliably detect optical Bell-states. Only the state of individual photons can be measured, which however is not the same as measuring entangled quantum states, which simultaneously describe both photons.

New Capabilities through Novel Ideas
New discoveries in recent years, including some made at TU Wien, have shown that the detection of Bell-states should indeed be possible. With help of a single atom, photon pairs can be manipulated such that their shared state becomes measurable. Thanks to novel methods of nano- fabrication, optical chips may as well allow for the miniaturization of this novel technology.

Still, there remain important scientific and technical problems that need solving, but the path is clear: “We are confident that in the next few years our combined efforts will bring an optical Bell-state analyzer to fruition,” says project coordinator Arno Rauschenbeutel. In addition to TU Wien, the network comprises the Universität Rostock, the University of Nottingham, the Universität Wien, Syddansk Universitet, Aarhaus Universitet, and the Ferdinand-Braun-Institut in Berlin. The project officially started on the July 1st, 2018 and will continue for the next 3 years.

[1] Photo: Sebastian Pucher, TU Wien