Johannes Feist

Department of Theoretical Condensed Matter Physics, Universidad Autonoma de Madrid, Spain

Using cavities to modify material properties

When: 12:00-13:00 CET, May 23th (Thursday), 2024

Where: Seminar Room (182), ICMM-CSIC, Campus de Cantoblanco, Madrid

The use of cavity quantum electrodynamical effects, i.e., of vacuum electromagnetic fields, to modify material properties has rapidly gained popularity and interest in the last decade. A canonical example of this is strong light-matter coupling, reached when the interaction of material excitations with confined light modes overcomes dissipation effects and the two parts hybridize to form mixed light-matter eigenstates, so-called polaritons. These polaritons inherit properties of both light and matter excitations and additionally display fundamentally new phenomena. The large range of possible material systems and cavity architectures opens a rich playground for novel functionalities. In the talk, I will discuss several topics related to this overall field, including the modification and photophysics and photochemistry in organic molecules, some fundamental results and pitfalls for the modification of low-energy excitations, and recent progress on few-mode field quantization in complex nanophotonic structures, i.e., strategies to obtain the construction of cavity-QED-like models for arbitrary cavity geometries and materials.

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Ricardo Lobo

ESPCI Paris – PSL University – CNRS – Sorbonne Université, France

Correlations and dispersive Dirac physics in the quantum material family BaCoS2-BaNiS2 – Reverse band-structure engineering of the optical conductivity

When: 12:00-13:00 CET, May 30th (Thursday), 2024

Where: Seminar Room (182), ICMM-CSIC, Campus de Cantoblanco, Madrid

BaCoS2 and BaNiS2 are the end members of a solid solution that shows a vast array of quantum properties. The Co material is close to a strongly correlated insulator with an antiferromagnetic transition, as well as a structural phase transition, around room temperature. At 28% Ni doping this material undergoes an electronic metal-insulator phase transition to a Drude metal. The metallic state persists all the way to the pure Ni compound. At this point, in addition to a Drude peak, we observe a strong contribution from bands with linear dispersion at the Fermi level, which give origin to dispersive Dirac nodal lines. We measured the optical conductivity of these materials and combined them with ab-initio calculations to reverse engineer the role of each band in the physical response of these materials. We explained uncommon features in their optical response such as a linear dispersion of the optical conductivity [1] and the existence of an isosbestic line separating a spectral-weight transfer across Dirac nodal states [2].

[1] D. Santos-Cottin, Y. Klein, P. Werner, T. Miyake, L. de’Medici, A. Gauzzi, R.P.S.M. Lobo, and M. Casula, Linear behavior of the optical conductivity and incoherent charge transport in BaCoS2, Phys. Rev. Materials 2, 105001 (2018). arXiv: 1712.01539.

[2] D. Santos-Cottin, M. Casula, L. de’Medici, F. Le Mardelé, J. Wyzula, M. Orlita, Y. Klein, A. Gauzzi, A. Akrap, and R.P.S.M. Lobo, Optical conductivity signatures of open Dirac nodal lines, Phys. Rev. B 104, L201115 (2021). arXiv: 2104.05521.

Nicolas Leconte

University of Seoul, Korea

Christopher W. Wächtler 

Department of Physics

University of California at Berkeley

Dissipation as versatile resource for collective quantum dynamics

When: 12:00-13:00 CET, April 16th (Tuesday), 2024

Where: Salon de Actos, ICMM-CSIC, Campus de Cantoblanco, Madrid

The widespread belief is that quantum systems need to be protected from the environment as well as possible for quantum technology to fulfill its promise of revolutionizing computing, communication, and sensing. However, the advent of the Noisy Intermediate-Scale Quantum (NISQ) era forces us to investigate dissipation and decoherence and to find ways to utilize them effectively. This talk aims to provide examples where interactions with the environment play a pivotal role in generating and detecting collective quantum phenomena, inspiring new perspectives on harnessing environment interactions for advancing quantum technologies. Firstly, we will delve into the emerging field of topological quantum synchronization, a novel form of synchronization where topology and dissipation intertwine to protect synchronized dynamics against perturbation. Next, we will explore how carefully tailored interactions with the environment induce energy migration within small quantum spin networks characterized by a superradiant speed-up, demonstrating the potential for utilizing dissipation as a resource rather than a hindrance. Finally, if time permits, I will introduce a novel methodology for probing quantum criticality in non-equilibrium systems. This method circumvents the shortcomings of standard perturbative expansions, enabling a comprehensive and thermodynamically consistent understanding of critical phenomena in systems coupled to non-Markovian reservoirs.

Georg Engelhardt

Southern University of Science and Technology in the Shenzhen Institute of Quantum Science and Engineering

Photon-Resolved Floquet Theory and its application to quantum sensing

When: 12:00-13:00 CET, April 11th (Thursday), 2024

Where: Seminar Room (182), ICMM-CSIC, Campus de Cantoblanco, Madrid

Quantum sensing uses quantum properties of matter to enhance the sensitivity in precision measurements. Besides others, it already finds important applications in atomic clocks, for medical purposes, and in the search for dark matter. Many currently employed quantum sensing protocols exhibit a simple setup, in which a laser probes the optical properties of an ensemble of atoms, molecules, or other quantum emitters, which are subject to the external stimulus to be measured. An important figure of merit to predict the sensitivity is the signal-to-noise ratio.  While it is easy to theoretically calculate the signal, the accurate prediction of the noise is challenging.

Motivated by this, we have developed the Photon-Resolved Floquet Theory (PRFT), which besides predicting the state of a driven quantum system (e.g., the atom or molecule), can also predict the number of photons exchanged with the coherent driving field [1,2]. To this end, the PRFT introduces counting fields into the semiclassical equations of motions, that track the photons in the driving field.  Interestingly, the PRFT predicts light-matter entanglement in the Floquet-state basis. This effect can be employed to devise a measurement-based quantum communication protocol, which has favorable scaling properties over long distances.  We apply the PRFT to spectroscopy, where it can predict the Fisher information of coherent spectroscopic signals [3]. The PRFT thus opens new paths to design and optimize quantum sensors based on AMO systems, which might assist in the discovery of new physics.

[1] G. Engelhardt, S. Choudhury, and W. V. Liu, Phys. Rev. Research 6, 013116 (2024)

[2] G. Engelhardt, JY. Luo, V. M. Bastidas, and G. Platero, arXiv: 2311.01509

[3] G. Engelhardt et al., in preparation

Paloma A. Huidobro 

Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid

Light and photons in time varying media

When: 12:00-13:00 CET, March 21st (Thursday), 2024

Where: Salon de Actos, ICMM-CSIC, Campus de Cantoblanco, Madrid

In this talk I will first introduce the basic concepts of wave interactions in time varying media. I will discuss how temporal modulations of the optical parameters offer new pathways in light control, as energy is not necessarily conserved in these time-dependent systems, and non-reciprocal effects can be realized [1]. I will concentrate on a class of space-time modulations where parameters are modulated in a travelling-wave form, such that there is an apparent motion of the optical properties, and discuss light propagation in these systems: from non-reciprocal effects even in the quasistatic limit, to synthetic motion and the link to the Fresnel drag effect of light in moving media, through an unconventional linear gain mechanism.

Next, I will discuss photon pair creation and squeezing in a photonic time crystal, the temporal counterpart of a conventional photonic crystal. I will show how modulating the refractive index in time causes dynamical Casimir processes, in which pairs of photons can be created. However, when there are multiple time interfaces, these photon pair creation phenomena depend dramatically on the photon’s wavelength, as well as on the contrast between the refractive indices, the duration of each temporal period and the number of periods that form the photonic time crystal [2].

 References:

[1] Galiffi, E., Tirole, R., Yin, S., Lia, H., Vezzoli, S., Huidobro, P.A., Silveirinha, M.G., Sapienza, R., Alu, A., and Pendry, J.B. “Photonics of time varying media” Advanced Photonics, 4(1), 014002 (2022)

[2] Echave, J., García-Vidal, F.J., &  Huidobro, P.A., “Photon squeezing in time varying media”, to be submitted (2024)