Nanooptics

Hybrid quantum systems

haring entanglement between physically distinct systems is a relatively unexplored frontier in quantum physics [1]. We aim to connect alkali atoms in gas cells with semiconductor quantum dots or other solid-state emitters through photon-mediated interactions. By employing light-confining structures, such as optical fibers or light cages, that enhance these interactions [2], we can generate novel quantum states that may enable new approaches in quantum sensing and quantum information processing. Additional applications in quantum technologies, including quantum memories [3], photon synchronizers, and photon sources [4], can benefit from hybrid quantum elements that combine the superior properties of their constituent systems.

References
[1] Hybrid integrated quantum photonic circuits, A. W. Elshaari, W. Pernice, K. Srinivasan, O. Benson, V. Zwiller, Nature photonics 14, 285-298 (2020)
[2] Davidson-Marquis, E. Gómez-López, B. Jang, T. Kroh, C. Müller, M. Ziegler, S. A. Maier, H. Kübler, M. A. Schmidt, O. Benson, Light: Science & Applications 10:114 (2021)
[3] Light Storage in Light Cages: A Scalable Platform for Multiplexed Quantum Memories, E. Gómez-López, D. Ritter, J. Kim, H. Kübler, M. A. Schmidt, O. Benson, arXiv:2503.22423
[4] Slow and fast single photons from a quantum dot interacting with the excited state hyperfine structure of the Cesium D1-line, T. Kroh, J. Wolters, A. Ahlrichs, A. W. Schell, A. Thoma, S. Reitzenstein, J. S. Wildmann, E. Zallo, R. Trotta, A. Rastelli, O. G Schmidt, O. Benson, Scientific reports 9, 13728 (2019)

Team: Esteban Gomez Lopez, Hala Said
Partners: Univ. Würzburg; IPHT Jena

Non-Gaussian quantum states

Non-Gaussian photon states [1] are quantum states of light whose statistical properties cannot be described by a Gaussian Wigner function, often arising from processes such as photon subtraction, addition, or strong nonlinear interactions [2]. They are essential for tasks that cannot be achieved with Gaussian states alone, such as universal quantum computation with continuous variables, quantum error correction, and certain forms of entanglement distillation [3]. We generate such states combining Fock and squeezed states produced by spontaneous parametric conversion and subsequent photon-number resolved detection.

References
[1] Production and applications of non-Gaussian quantum states of light, A. I. Lvovsky, Philippe Grangier, Alexei Ourjoumtsev, Valentina Parigi, Masahide Sasaki, Rosa Tualle-Brouri, arXiv:2006.16985 (2020)
[2] Non-Gaussian Quantum States and Where to Find Them, Mattia Walschaers, PRX Quantum 2, 030204 (2021)
[3] Integrated photonic source of Gottesman-Kitaev-Preskill qubits, M. V. Larsen et al., Nature 642, 587-591 (2025)

Team: Elnaz Bazzazi, Sophie Bregadze, Gao Chao, Roger Alfredo Kögler, Marco Schmidt
Partners: FU Berlin; AIST, Japan; PTB

Quantum networks and photonic quantum gates

Photons are exceptionally robust carriers of quantum information: they retain their quantum properties even at room temperature and can transmit information across vast distances [1]. Using spontaneous parametric down-conversion in non-linear crystals, we generate heralded single photons and entangled photon pairs [2]. These sources are essential building blocks of quantum networks, the backbone of the future quantum internet and a bridge between quantum computers. In the Berlin Quantum Network, we are working to create and interconnect photonic quantum nodes, bringing this vision closer to reality. Quantum states can be fused into ever larger entangled networks as a vital resource for building measurement-based quantum computers [3]. Our research explores these cutting-edge concepts, investigating whether, and how, the first photonic quantum gates can be integrated and scaled into the architectures [4] required for large-scale photonic quantum computing.

References
[1] Solid-state single-photon emitters, I. Aharonovich, D. Englund, M. Toth, Nature Photon 10, 631–641 (2016)
[2] Bright source of indistinguishable photons based on cavity-enhanced parametric down-conversion utilizing the cluster effect, A. Ahlrichs, O. Benson, Applied Physics Letters 108, 02111 (2016)
[3] Quantum computing with photons: introduction to the circuit model, the one-way quantum computer, and the fundamental principles of photonic experiments, S. Barz, J. Phys. B: At. Mol. Opt. Phys. 48, 083001 (2015)
[4] Hybrid integrated quantum photonic circuits, A. W. Elshaari, W. Pernice, K. Srinivasan, O. Benson, V. Zwiller, Nature photonics 14, 285-298 (2020)

Team: Ralf-Peter Braun, Siavash Qodratipour, Thomas Häffner, William Staunton
Partners: Telekom T-Labs; Univ. Paderborn

Plasmonics on the nanoscale

Metal nanostructures support a special kind of optical excitation: surface plasmon polaritons (SPP). With SPPs, electromagnetic fields can be confined massively in the vicinity of the nanostructures to dimensions far below the wavelength of light [1]. Thus, metal nanostructures can be used for extreme light focusing and for modification of the optical density of states. In particular interesting are situations where a classical description breaks down, and quantum effects become significant: We explore new approaches to light generation under these extreme conditions, where both electronic and photonic properties enter the quantum domain [2]. As a specific example, we consider single-photon generation driven by inelastic electron tunneling [3].

References
[1] Special issue on quantum plasmonics, eds. Z., F. J. Garcia-Vidal, E. Potma, J. Optics 18 (2016)
[2] Optical spectra of plasmon–exciton core–shell nanoparticles: a heuristic quantum approach, F. Stete, W. Koopman, C. Henkel, O. Benson, G. Kewes, M. Bargheer,
ACS Photonics 10, 2511-2520 (2023)
[3] Proposal for a Tunable Room-Temperature Single-Photon Source Based on a Plasmonic Nanoantenna Driven by Inelastic Tunneling in the Coulomb Regime, G. Kewes and O. Benson, Phys. Status Solidi A 221, 2300366 (2023)

Team: Bendix Hartlaub, Günter Kewes
Partners: Okayama Univ., Japan