Fundamental limitations of time measurement precision in Hong-Ou-Mandel interferometry

In quantum mechanics, the precision achieved in parameter estimation using a quantum state as a probe is determined by the measurement strategy employed. The ultimate quantum limit of precision is bounded by a value set by the state and its dynamics. Theoretical results have revealed that in interference measurements with two possible outcomes, this limit can be reached under ideal conditions of perfect visibility and zero losses. However, in practice, this cannot be achieved, so precision {\it never} reaches the quantum limit. But how do experimental setups approach precision limits under realistic circumstances? In this work we provide a general model for precision limits in two-photon Hong-Ou-Mandel interferometry for non-perfect visibility. We show that the scaling of precision with visibility depends on the effective area in time-frequency phase space occupied by the state used as a probe, and we find that an optimal scaling exists. We demonstrate our results experimentally for different states in a set-up where the visibility can be controlled and reaches up to $99.5\%$. In the optimal scenario, a ratio of $0.97$ is observed between the experimental precision and the quantum limit, establishing a new benchmark in the field

Broadband Biphoton Generation and Polarization Splitting in a Monolithic AlGaAs Chip

The ability to combine various advanced functionalities on a single chip is a key issue for both classical and quantum photonic-based technologies. On-chip generation and handling of orthogonally polarized photon pairs, one of the most used resources in quantum information protocols, is a central challenge for the development of scalable quantum photonics circuits; in particular, the management of spectrally broadband biphoton states, an asset attracting growing attention for its capability to convey large-scale quantum information in a single spatial mode, is missing. Here, we demonstrate a monolithic AlGaAs chip, including the generation of broadband orthogonally polarized photon pairs and their polarization splitting; 85% of the pairs are deterministically separated by the chip over a 60 nm bandwidth. The quality of the two-photon interference at the chip output is assessed via a Hong–Ou–Mandel experiment displaying a raw visibility of 75.5% over the same bandwidth. These results, obtained for the first time at room temperature and telecom wavelength, in a platform combining strong confinement, high second-order nonlinearity, electro-optic effect, and direct bandgap, confirm the validity of our approach and represent a significant step toward miniaturized and easy-to-handle photonic devices working in the broadband regime for quantum information processing