Non-linear Interferometry Beyond the Standard Quantum Limit

P. Hyllus, K. Eckert

Franck Pereira dos Santos

C. Fruy, A. Théry, B. Hue +6 more

Interferometry is a powerful method for studying many fundamental phenomena ranging from gravitational waves to anyon excitations. Besides loss of coherence in the interferometer paths, quantum mechanics sets an incompressible bound for linear interferometry arising from the quantum noise of the light beam, called the shot noise limit. Here, we implement a non-linear quantum interferometer that uses a microwave cavity and a magnetic field resilient anharmonic superconducting quantum circuit made of granular aluminum. The circuit enables interferometry beyond the shot noise limit. A direct application of our findings is the detection of dark matter (axions or dark photons), high-frequency gravitational waves or astronomical masers.

Alfredo Luis, Luis Lorenzo Sánchez-Soto

Liam Paul McGuinness

In their paper "Entanglement-enhanced matter-wave interferometry in a high-finesse cavity" Nature (2022), Greve et. al. claim to use entanglement in a matter-wave interferometer to achieve a sensitivity beyond that achievable with the same number of independent particles -- a limit known as the standard quantum limit (SQL). In particular, using squeezed momentum states of 700 atoms, the authors claim to directly observe a sensitivity 3.4 dB (a factor of 1.5) below the SQL. This claim is incorrect. The authors do not measure anything beyond the SQL, nor do they achieve a sensitivity beyond what one could obtain with a single atom. The achieved sensitivity is at least a factor of 39 worse than the claimed value.

Rafał Demkowicz-Dobrzański, Marcin Jarzyna, Jan Kołodyński