"Engineering interactions in the Quantum Hall effect through vacuum field in metamaterial cavities"

• Jérôme Faist (ETH Zurich)

Room: Siemens Auditorium, HIT E 51 Jérôme Faist1, J. Enkner1, L. Graziotto1, M.Beck1, C. Reichl2, W. Wegscheider2, G.Scalari1 1 Institute of Quantum Electronics, ETH Zurich 2 Laboratory for Solid State Physics, ETH Zurich, Zurich 8093, Switzerland In a microcavity, the strength of the electric field caused by the vacuum fluctuations, to which the strength of the light-matter coupling WR is proportional, scales inversely with the cavity volume. One very interesting feature of the circuit-based metamaterials is the fact that this volume can be scaled down to deep subwavelength values in all three dimension of space.1 We have used transport to probe the ultra-strong light-matter coupling2 and shown that the latter can induce a breakdown of the integer quantum Hall effect3. The phenomenon is explained in terms of cavityassisted hopping, an anti-resonant process where a an electron can scatter from one edge of the sample to the other by “borrowing” a cavity photon from the vacuum4. Recently a proposal suggested that the value of the quantized Hall voltage can be renormalized by the cavity5, but later work demonstrated that such renormalization corresponds to a singular point in the parameter space. We have investigated this effect experimentally using a Wheatstone bridge geometry6 and found the quantization to be held7. We have also investigated a new experimental geometry where a hovering resonator is positioned with nanoactuators above the Hall bar, providing a way to continuously vary the light-matter coupling up to a value of WR/w=0.33 while the sample is maintained at millikelvin temperatures. Using this approach, we observe the effect of light-matter coupling on the effective electron g-factor as well as its effect on the gap of the Laughlin states. In particular, we observe a enhancement of the 5/3,4/3 and 5/7 fractional state gaps by up to a factor of two8. 1. Scalari, G. et al. Ultrastrong Coupling of the Cyclotron Transition of a 2D Electron Gas to a THz Metamaterial. Science 335, 1323–1326 (2012). 2. Paravicini-Bagliani, G. L. et al. Magneto-Transport Controlled by Landau Polariton States. Nat. Phys. 15, 186–190 (2019). 3. Appugliese, F. et al. Breakdown of topological protection by cavity vacuum fields in the integer quantum Hall effect. Science 375, 1030–1034 (2022). 4. Ciuti, C. Cavity-mediated electron hopping in disordered quantum Hall systems. Phys. Rev. B 104, 155307 (2021). 5. Rokaj, V., Penz, M., Sentef, M. A., Ruggenthaler, M. & Rubio, A. Polaritonic Hofstadter butterfly and cavity control of the quantized Hall conductance. Phys. Rev. B 105, 205424 (2022). 6. Schopfer, F. & Poirier, W. Testing universality of the quantum Hall effect by means of the Wheatstone bridge. J. Appl. Phys. 102, 054903 (2007). 7. Enkner, J. et al. Testing the Renormalization of the von Klitzing Constant by Cavity Vacuum Fields. Preprint at https://doi.org/10.48550/arXiv.2311.10462 (2023). 8. Enkner, J. et al. Enhanced fractional quantum Hall gaps in a two-dimensional electron gas coupled to a hovering split-ring resonator. Preprint at https://doi.org/10.48550/arXiv.2405.18362 (2024).

"Engineering of many-body states in a driven-dissipative cavity QED system"

• Tobias Donner (ETH Zurich)

Room: Siemens Auditorium, HIT E 51 Exposing a many-body system to external drives and losses can fundamentally transform the nature of its phases, and opens perspectives for engineering new properties of matter. How such characteristics are related to the underlying microscopic processes is a central question for our understanding of materials. A versatile platform to address it are quantum gases coupled to the dynamic light fields inside optical resonators. This setting allows to create synthetic many-body systems with tunable, well-controlled dissipation channels, and at the same time to induce cavity-mediated long-range atom-atom interactions. After an introduction to this platform, I will describe experiments in which the interplay between drive and dissipation induces limit cycles and transport of the atoms. In a second set of experiments, we make use of the cavity-mediated interaction to induce the formation of pairs of correlated atoms. We demonstrate that this process is based on the amplification of vacuum fluctuations.

"Ergodicity breaking and quantum trajectories in multimode cavities"

• Sarang Gopalakrishnan (Princeton University)

Room: Siemens Auditorium, HIT E 51 I will briefly review why a spin-glass phase can form in a multimode cavity. I will discuss how monitoring the light emitted from the cavity (or more generally, the radiation from an open quantum system) can yield signatures of ergodicity breaking. As a specific example I will discuss the distribution of overlaps between low-energy states in a spin glass. If time permits I will also discuss how cavities can be used to sequentially generate highly entangled states.

"Exploring Collective Physics in a Cold Atom Cavity-QED System"

• James K. Thompson (JILA, University of Colorado Boulder)

Room: Siemens Auditorium, HIT E 51 Laser-cooled atoms in a high-finesse optical cavity are a powerful tool for quantum simulation and quantum sensing. The optical-cavity enhances the light-matter interaction, mediating effective atom-atom interactions and probing of the quantum state below the mean-field level. In this talk, I will provide an overview of my group’s recent work in this area. We perform cavity-enhanced quantum non-demolition measurements to create highly-entangled states [1], with the first realization of a squeezed matter wave interferometer for inertial sensing [2] and a squeezing-enhanced differential strontium optical lattice clock comparison [3]. We have also realized cavity-mediated momentum-exchange interactions that give rise to a collective recoil mechanism with analogies to Mössbauer spectroscopy for suppressing Doppler dephasing [4] in matterwave interferometers and on optical transitions. We have realized arbitrary XYZ Hamiltonian engineering in a matter wave interferometer, including realizing two-axis counter twisting for the first time since its proposal more than 30 years ago [5]. We have utilized spin-exchange interactions [6] to explore several dynamical phase transitions [7] including an emulation of long-predicted dynamical phases of a BCS superconductor [8]. If time permits, I will lastly briefly touch on the development of a superradiant laser utilizing a mHz linewidth optical transition [9] with applications for ultranarrow linewidth lasers [10] and searches for new physics. [1] Cox et al, Phys. Rev. Lett. 116(9), 093602 (2016). [2] Greve, Luo et al, Nature, 610(7932), 472-477 (2022). [3] Robinson et al, Nature Physics 20, 208 (2024). [4] Luo et al, Science 384, 551 (2024). [5] Luo et al, arXiv:2402.19492 (2024). [6] Norcia et al, Science 361, 6399, 259 (2018). [7] Muniz et al, Nature 580, 602 (2020). [8] Young et al, Nature 625, 679-684, (2024). [9] Norcia et al, Science Advances, 2(10), e1601231 (2016). [10] Norcia et al, Phys. Rev. X 8(2) 021036 (2018).