Verónica Fernández Mármol

Inés Díaz, Spanish National Research Council (CSIC) C. de Serrano, 144, Madrid, Spain T: +34 636514928, ines.diaz@csic.es Daniel Cano, Spanish National Research Council (CSIC) Verónica Fernández, Spanish National Research Council (CSIC) Quantum Key Distribution (QKD) encompasses a combination of protocols that allow for the exchange of a secret key between two communicating parties (Alice and Bob) under unconditional security by exploiting quantum mechanics principles. However, not all QKD setups fulfill the ideal model requirements, since imperfections in real devices have been proved to make the secret exchange vulnerable to certain attacks. These types of vulnerabilities are overcome by new Measurement Device Independent (MDI) QKD protocols, which protect the security against detector side-channel attacks and enable secure communication over longer distances. As such, the MDI-QKD protocol involves a third-party relay (Charlie) who might be under the control of an eavesdropper and to which Alice and Bob send their states for the Hong-Ou-Mandel effect to take place. The Hong-Ou-Mandel effect is a quantum interference phenomenon between two indistinguishable photons at a beam splitter that bears significant weight within the context of Bell state measurements and is crucial for the security of MDI-QKD and its realistic implementation. In this work, we put together an experimental setup to observe and monitor the Hong-Ou-Mandel interference. By synchronizing two indistinguishable weak coherent pulses (WCP) to arrive simultaneously at a 50:50 beam splitter (following (2)), it has been possible to verify that the two photons exit together through the same output only if the quantum interference takes place. Consequently, detection event occurs at just one detector. The most significant experimental evidence of such effect is the Hong-Ou-Mandel dip (3), a plot representing the number of coincidence detections against the temporal delay induced in one of the pulses. It should be noted that, since we are using WCPs instead of photon number states, the interference contrast is 50%. We successfully attested to the Hong-Ou-Mandel effect by obtaining a clear and pronounced dip. To enhance its visibility, we varied and tested several experimental components. These results thus obtained, along with the discussion of the experimental techniques relevant to test such a phenomenon, provide valuable insights for future advancements in quantum interference studies in the context of quantum communications. References 1. Lo, H. K., Curty, M., & Qi, B. (2012). Measurement-device-independent quantum key distribution. Physical review letters, 108(13), 130503. 2. Chen, H., An, X. B., Wu, J., Yin, Z. Q., Wang, S., Chen, W., & Han, Z. F. (2016). Hong–Ou–Mandel interference with two independent weak coherent states. Chinese Physics B, 25(2), 020305 3. Ge, H., Tomita, A., Okamoto, A., & Ogawa, K. (2023). Analysis of the effects of the two-photon temporal distinguishability on measurement-device-independent quantum key distribution. IEEE Transactions on Quantum Engineering, 4, 1-8. Figure 1 – Experimental Hong-Ou-Mandel dip Acknowledgements This work had the support of Grant PID2020-118178RB-C22 funded by AEI/10.13039/501100011033 and QURSA project (TED2021- 130369BC33, MCIN/AEI/10.13039/501100011033).