Quantum optics studies the interaction of light and matter at their most fundamental level (single or few quanta). Apart from its fundamental interest, such interaction is instrumental for several quantum technologies such as sending quantum information between different nodes, measuring systems beyond the limits imposed by classical physics, or inducing quantum gates for quantum computing arquitectures.

In free-space this interaction is generally weak, which is why in the last years there has been a lot of research of developing new platforms that surpass this limitation, e.g., integrating quantum emitters with nanophotonic platforms (quantum nanophotonics), exploiting collective effects in subwavelength atomic arrays, or mimicking this interaction with superconducting circuits (circuit QED).

In our group, we work at the frontier of this field by both developing novel light-matter interfaces that provide enhanced functionalities over state-of-the-art systems and exploiting them for generating novel states of light and matter that can be used for applications. Some of the research lines we are currently exploring:

### Topological Quantum Photonics and Amplifiers

Topological phases of matter are characterized by exotic boundary physics, namely by the existence of protected modes at the edge of the system that are immune to local perturbations such as disorder or defects. Such robustness triggered the interest of the photonics community to export these ideas into their platforms to obtain robust and unidirectional photon flows. These efforts have recently culminated in the experimental realization of these ideas in several platforms, but mostly focusing in the linear (classical) regime since the interactions between photons are generally very weak.

In our group, we aim to study the quantum optical consequences of such topological photonic systems beyond the regimes usually considered in the literature. For example, by considering the interaction of quantum emitters, a strongly interacting system, with such topological photonic environments, which results in unconventional quantum emitter dynamics and interactions [1]. Besides, we also consider driven dissipative situations that lead to topological quantum amplification [2] that can be potentially harnessed to amplify weak quantum signals.

### Generation of non-classical states of light & matter

Engineering and control of quantum states of light and matter are central in photonic quantum technologies, ranging from quantum communication and computation (single photon states) or quantum metrology (single and multi-mode Fock states).

In our group, we develop protocols aiming to surpass the limitations of current ones based on novel light-matter interfaces, such as waveguide QED [3] or circuit QED platforms [4].

### Quantum spectroscopy: multi-photon scattering and detection

Complex quantum optical systems can have an intrinsic complicated dynamics which is sometimes difficult to know a priori. One alternative to characterize them is to use their interaction with light to unravel their behaviour. When the light has a non-classical character, this is tipically labelled as quantum spectroscopy.

In our group, we explore several of these scenarios. For example, by sending multi-photon wave-packets and analyzing the associated reflection and transmission properties, we have shown how one can gain a lot of information of the system [5]. Besides, we also explore how to use a frequency-resolved version of Hanbury-Brown-Twiss interferometer to obtain also such information “passively”.

#### Selected Group Publications

- Bello, M., Platero, G., Cirac, J. I., & González-Tudela, A. (2019).
*Unconventional quantum optics in topological waveguide QED.**Science advances*,*5*(7), eaaw0297. - Ramos, T., García-Ripoll, J. J., & Porras, D. (2021).
*Topological input-output theory for directional amplification*.*Physical Review A*,*103*(3), 033513. - González-Tudela, A, Paulisch, V., Kimble, H. J., Cirac, J. I.
*Efficient multiphoton generation in waveguide quantum electrodynamics*. Physical Review Letters 118 (21), 213601 (2017). - Li, M., García-Ripoll, J. J., Ramos. T,
*Scalable multiphoton generation from cavity-synchronized single-photon sources*. arXiv: 2009.02382. - Ramos, T., & García-Ripoll, J. J. (2017).
*Multiphoton scattering tomography with coherent states*.*Physical Review Letters*,*119*(15), 153601. - Schmidt, M. K., Esteban, R., Giedke, G., Aizpurua, J. González-Tudela, A.
*Frequency-resolved photon correlations in cavity optomechanics.*Quantum Science and Technology 6 (3), 034005 (2021).