Speaker
Description
Two-photon interference at a beam splitter is a foundational primitive in quantum optics, underpinning applications from precision metrology to photonic quantum computing. When two indistinguishable photons enter the input arms of a 50:50 (reflection:transmission) beam splitter, bosonic statistics dictate that they bunch and exit the same port, producing the Hong–Ou–Mandel (HOM) effect and creating the path-entangled $N 0 0N$ state (|2,0> + |0,2>)$/\sqrt{ 2 }$. However, theoretical work by Alsing et al. predicts that this balanced configuration does not maximise the entanglement between the output modes: a beam splitter offset from 50:50, slightly favouring either transmission or reflection, yields a greater von Neumann entropy.
We interrogate this prediction by introducing a tomographic method based on frame reconstruction, in which a frame of measurement operators is constructed to directly reconstruct the density operator of the two-photon output state. We implement the corresponding photonic circuit on a cloud-based photonic quantum processing unit, reconstruct the output density operator across a range of beam splitter angles, and evaluate the von Neumann entropy at each.
Our results confirm that maximal two-photon entanglement is achieved not at 50:50, but at 21:79 and 79:21, where the output becomes an equal superposition of the |2,0>, |1,1>, and |0,2> number states. By exploiting the full three-dimensional output space, that is, by treating the interference as producing a qutrit rather than a qubit, one can access a peak entanglement entropy of $\ln 3$, exceeding the $\ln 2$ of the conventional $N 00N$ state.
| I am the presenting author | Yes |
|---|