The fragility of quantum states does not only allow for unconditionally secure communication, but also for sensing applications that go beyond classical precision. Our expertise in the construction of high-quality sources of photonic entanglement has enabled us to pursue this field of quantum technologies as well.
One of the milestones in experimental quantum physics was the observation of the HOM-dip in 1987: If one manages to create perfectly indistinguishable photons and lets them interfere on a beamsplitter, they will always end up in different outputs. Since this indistinguishability is very fragile, one can use its loss to sense photon interactions. The Ursin group has recently enhanced this scheme by using not only indistinguishable, but also frequency-entangled photons .
The Quantum Concrete Mixer
Optical gyroscopes are a well-established classical device. By letting two counterpropagating beams interfere in a spinning fiber spool, one can measure its rotation speed. Entangled photon number states, so-called NOON states, can be used to enhance this scheme. The Ursin group has managed to show this quantum sensing effect in the lab – by rotating a source of entanglement in a modified concrete mixer .
 Y. Chen, M. Fink, F. Steinlechner, J. P. Torres, and R. Ursin, “Hong-Ou-Mandel interferometry on a biphoton beat note,” npj Quantum Inf., Feb. 2019.
 M. Fink, F. Steinlechner, J. Handsteiner, J. P. Dowling, T. Scheidl, and R. Ursin, “Entanglement-enhanced optical gyroscope,” New J. Phys., Apr. 2019.