Keynote Lectures

Abstract
Integrated photonics plays a key role in enabling compact systems for light routing and conditioning with increasingly complex optical functions. By offering scalability in both ensemble size and system complexity, it enables robust quantum technologies based on trapped atoms and ions. In addition to photonic systems for trapping, cooling, and addressing atoms and ions, ultra-stable lasers (USLs) are essential for providing the stable frequencies needed to interrogate narrow atomic transitions with high precision. USLs are also at the heart of precision measurements, such as gravitational wave detection and fundamental physics tests. Current USLs achieve a relative instability of 4×10⁻¹⁷, currently limited by resonator thermal noise. However, current setups are bulky and would significantly benefit from photonic integration, for example, of input/output optics, sensing elements, and devices needed for laser stabilization. In this talk, I will present an overview of integrated photonic devices for quantum sensing and computing with trapped atoms and ions. I will discuss the physical requirements and key material considerations for these applications and introduce a novel resonator concept based on multifunctional mirrors designed to meet stringent requirements on size, weight, and noise in next-generation quantum systems.

Abstract
Metamaterials have revolutionised the way we control light transport and generation. Yet, to date, they rely on static and passive architectures, only redistributing incident wave energy - for example a metalens that focuses light or a cloak that makes an object invisible. The next frontier is to control metamaterials in space and time, and make waves from the past and future to interact.
I will discuss our first steps towards temporal control and experiments on double-slit time diffraction at optical frequencies in time-varying metamaterials [1]. Intertwining space and time, I will discuss the observation of scattering of light from optical modulations traveling faster than the speed of light [2], and coherent perfect absorption [3] and how this will enable us to simulate more complex spatio-temporal optical and relativistic phenomena.

References
[1] Double-slit time diffraction at optical frequencies, Romain Tirole, Stefano Vezzoli, Emanuele Galiffi, Iain Robertson, Dries Maurice, Benjamin Tilmann, Stefan A Maier, John B Pendry, Riccardo Sapienza, Nature Physics 19, 999 (2023).
[2] Super-luminal Synthetic Motion with a Space-Time Optical Metasurface A. C. Harwood, S. Vezzoli, T. V. Raziman, C. Hooper, R. Tirole, F. Wu, S. A. Maier, J. B. Pendry, S. A. R. Horsley, R. Sapienza, Nature Communications 16, 5147 (2025)
[3] Optical coherent perfect absorption and amplification in a time-varying medium, Emanuele Galiffi, Anthony C. Harwood, Stefano Vezzoli, Romain Tirole, Andrea Alù, Riccardo Sapienza, Nature Photonics in press and ArXiV 2410.16426 (2025)

Abstract
The thermalization of light and its ground state condensation has been extensively explored in recent years [1], with the link between laser action and Bose Einstein condensation of a thermalized photon gas in an open microcavity [2, 3] opening new ways to understand laser system. In this talk we report thermalization and condensation of light in a semiconductor quantum well weakly coupled to an open microcavity system [4]. This system consists of half a vertical external cavity surface emitting laser, constructed on GaAs with an InGaAs quantum well emitting near 925 nm, with a piezo controlled external spherical dielectric mirror positioned to achieve low cavity mode orders with well-defined transverse modes. We present evidence of cavity photon thermalization and since we have used a single quantum well with minimized absorption, α, to match the cavity loss, κ, we explore the influence of thermalization coefficient, 0.1<γ=α/κ<10. This level of control allows us to compare our data to recent theory on photon condensation in semiconductor systems [5]. In the condensation regime, we identify a region of ground state mode stability with good thermalization γ>1. Meanwhile regions of poor thermalization γ<1, and at high operation power, show multi-mode or higher order spatial mode lasing, which is consistent with the theory of dye-based condensates [6, 7]. We also assess the strength of photon-photon interactions and find a normalized interaction parameter, g̃ = 2.7 × 10^-3. Since this value increases with quantum well number, this system is promising for the possibility of observing rich interaction physics.

References
J. Bloch, et al, Nature Reviews Physics 4, 470 (2022)
J. Klaers et al, Nature Physics 6, 512 (2010)
J. Klaers et al., Nature 468, 545 (2010)
R. C. Schofield et al., arXiv:2306.15314 (2023)
A. Loirette-Pelous & J-J.Greffet, Laser & Photonics Rev.,17, 2300366 (2023)
H. Heston et al, Phys. Rev. Lett., 120, 040601 (2018)
J.D. Rodrigues, et al, Phys. Rev. Lett. 126, 150602 (2021)