Via a reconfigurable photonic frequency circuit, we show spectral bosonic and fermionic Houng-Ou-Mandel interference between independently created pure single photons, demonstrating photon number scalability and versatility of the frequency processing approach.
The frequency degree-of-freedom, being compatible with well-developed omnipresent optical telecommunication infrastructure, allows for stable, controllable and scalable encoding of quantum information [1] . The HOM effect between independent photons, as a central building block to, e.g., quantum key distribution (QKD), quantum teleportation and entanglement swapping, has been mainly employed within the spatial domain [2] . A key element yet missing to enable scalable and thus real-world frequency-based implementation of quantum computation and non-classical communication, is the demonstration of Hong-Ou-Mandel (HOM) interference between independently created photons of different frequencies. To date, spectral bosonic HOM interference between single photons, produced out of the same continuous-wave (cw)-excited spontaneous parametric down conversion (SPDC) process has been demonstrated [3] , which however does not allow scalability due to the spectral impurity inherent to the proposed approach.
Abstract The photon's frequency degree of freedom, being compatible with mature telecom infrastructure, offers large potential for the stable and controllable realization of photonic quantum processing applications such as the quantum internet. The Hong–Ou–Mandel effect, as a two‐photon interference phenomenon, serves as a central building block for such frameworks. A key element yet missing to enable meaningful frequency‐based implementations as well as scalability in the number of processed photons, is the demonstration of the Hong–Ou–Mandel effect between independently created photons of different frequencies. The experimental implementation of bosonic and fermionic frequency domain Hong–Ou–Mandel interference between independently generated single photons is reported here, with measured visibilities of 74.31% ± 3.56% and 86.44% ± 8.27%, respectively. This is achieved through a scalable photonic frequency circuit that creates two post‐selected pure single photons, which undergo frequency mixing at an electro‐optic phase modulator. The system is on‐the‐fly reconfigurable allowing to probe bosonic and fermionic Hong–Ou–Mandel interference in the same experimental setup. The work demonstrates the versatility of frequency domain processing and its scalability toward higher photon numbers, which enables new quantum gate concepts as well as the establishment of frequency‐based large‐scale quantum networks.