Quantum clock interferometry, as introduced by Zych et al. [1], offers a striking conceptual approach to probing the interplay between general relativity and quantum mechanics, asking how differences in relativistic proper time manifest themselves in genuinely quantum systems. In particular, gravitational time dilation can become directly observable through the visibility and phase of quantum interference. While photonic implementations provide a natural route towards such experiments, the required proper-time difference typically translates into very large interferometric path lengths [2].
In this talk, I will present a method in which optical quantum memories overcome this limitation by effectively compactifying the interferometer [2,3]. Instead of accumulating phase over long propagation distances, photons can be stored at spatially separated locations and allowed to accumulate different proper times while remaining confined to compact devices. Using frequency-nondegenerate entangled photon pairs, the resulting proper-time difference is mapped onto a measurable two-photon phase. Large optical frequency separations and long storage times can substantially enhance this signal [2]. I will also discuss ways to achieve multiphoton enhancements [3] and schemes for generating the required photonic input states [4].
Finally, I will present ongoing work extending this framework beyond gravitational redshift towards gravitational gradients and spacetime curvature, using distributed quantum memories as sensors of gravitational geometry [5].
[1] M. Zych, F. Costa, I. Pikovski and Č. Brukner, “Quantum interferometric visibility as a witness of general relativistic proper time,” Nature Communications 2, 505 (2011).
[2] R. Barzel, M. Gündoğan, M. Krutzik, D. Rätzel and C. Lämmerzahl, “Entanglement dynamics of photon pairs and quantum memories in the gravitational field of the Earth,” Quantum 8, 1273 (2024).
[3] M. Gündoğan, R. Barzel and D. Rätzel, “Gravitational time dilation in quantum clock interferometry with entangled multi-photon states and quantum memories,” Quantum Science and Technology 11, 045047 (2026).
[4] M. Gündoğan and D. Rätzel, “Programmable photonic state fusion via heralded storage of asynchronously generated resources,” arXiv:2609.10312 (2026).
[5] M. Pisaturo, M. Gündoğan and D. Rätzel, “Gravitational curvature sensing with multiport quantum memory clock interferometry,” in preparation (2026).