Challenges and perspectives in resonator-mediated quantum many-body physics: From atoms to solid state

17 Jun 2024, 08:30 → 18 Jun 2024, 18:00 Europe/Zurich

Siemens Auditorium, HIT E 51 (ETH Zürich)

Markus Müller (PSI)

The interaction between light and matter can be significantly enhanced by confining photons and matter within a small volume. This allows to control quantum matter by coupling it to driven cavities, so as to generate collective states of matter and light, to couple and entangle matter degrees of freedom at large distances mediated by a cavity, as well as to read out the state of the quantum matter in real-time via the out-coupled light field. At the same time, this approach gives rise to a driven-dissipative many-body system, the understanding and control of which represents a current challenge.  These characteristics are fundamental to quantum information processing, quantum communication, quantum simulations, and precision sensing. Understanding and controlling light-matter coupling in cavities is pivotal for leveraging quantum phenomena on the nanoscale, promising significant breakthroughs in a variety of quantum technologies.

This workshop aims to bring together world-leading experts in both experimental and theoretical many-body physics, focusing on interactions facilitated by resonators across microwave and optical domains. The objective is to delve into uncharted territories, perspectives, and potential of cavity-mediated physics, specifically emphasising the coupling of solid-state materials (such as doped ions in magnetic insulators or semiconductors) and optically trapped systems with coherent electromagnetic fields for quantum information processing, with ample time to discuss concepts, perspectives and open challenges.

 

 

Monday, 17 June

Registration

Opening remarks

Convener: Markus Müller (Paul Scherrer Institut)

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

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).

Speaker: Jérôme Faist (ETH Zurich)

2 Solid state systems coupled to resonators

Speaker: Gabriel Aeppli (ETH Zurich / PSI / EPFL)

3 "Cavity control of phase transition in complex quantum materials"

D.Fausti ¹,²,3
¹ Department of Physics, University of Trieste, Trieste
² Elettra Sincrotrone Trieste S.c.p.a., Trieste
3 Department of Physics, University of Erlangen Nuremberg

The physical properties of many complex Quantum Materials (QM), such as transition metal oxides, arise from intricate interactions among electrons, phonons, and magnons. This complexity renders QM highly responsive to external factors like pressure, doping, magnetic fields, or temperature. Consequently, many compound families exhibit intricate phase diagrams, enabling the switching between entirely different macroscopic functionalities through precise adjustments of "control parameters" like temperature or pressure. This responsiveness also positions QM as an excellent platform for designing experiments where tailored electromagnetic fields interacting with matter can give rise to novel, sometimes exotic, physical properties. This avenue has been extensively explored in time-domain studies [1,2,3,4], demonstrating that ultrashort mid-IR light pulses can induce the formation of quantum coherent states in matter.

In this presentation, I will discuss the potential of controlling macroscopic properties of quantum materials not only by subjecting them to ultrashort pulses but also by integrating the materials into resonant optical cavities. I will explore the effects of vacuum fluctuations, as well as the notion that embedding materials into optical cavities alters the exchange of energy between the materials and their thermal electromagnetic environment [6]. Additionally, I will dig into the impacts of strong and weak coupling to cavity modes in the archetypal material for Charge Density Wave systems, 1T-TaS2. Furthermore, I will present our recent discovery of cavity thermal control over the metal-insulator transition [5] and discuss the significant sensitivity of vibrational coupling to the cavity mode structure. Finally, I will examine the evidence of cavity-enhanced non-linearities in quantum paraelectric SrTiO3 and explore the potential of utilizing cavity electrodynamics to sustain non-equilibrium stationary states in complex matter.

References:
[1] Advances in physics 65, 58-238, 2016
[2] Science 331, 189-191 (2011)
[3] Phys. Rev. Lett. 122, 067002 (2019)
[4] Nature Physics 17, 368–373 (2021)
[5] Nature 622, 487–492 (2023)
[6] https://arxiv.org/abs/2403.00851

Speaker: Daniele Fausti (FAU Erlangen)

Coffee break

4 "Quantum Matter in Strong Coupling Cavities"

In this talk I will mainly focus on the effects a strongly coupling cavity mode has on the physics of fermionic systems

Speaker: Dieter Jaksch (Universität Hamburg / Oxford University)

Lunch

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

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.

Speaker: Tobias Donner (ETH Zurich)

6 Cold atoms

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

7 "Microscopy of and for fermions with cavity-mediated interactions"

The ability to address atoms or emitters locally is among the most important capabilities of quantum gases experiments. Cavity QED methods for detection and manipulation, by singling out one or a set of modes determined by the geometry, usually sacrifice spatial resolution in favor of enhanced sensitivity or strong non-linearities. I will describe two experiments in which we demonstrate local measurements and addressing of atoms in a high-finesse cavity. In the first one, I will show the direct, in-situ observation of the self-organization transition in a unitary Fermi gas under side-pumping. In the second, I will describe a new device combining a high-finesse cavity and a microscope in a single optical element, that allows for the local control over light-matter interaction. I will then discuss some of the perspectives that these systems open for analogue quantum simulation.

Speaker: Jean-Philippe Brantut (EPFL Lausanne)

8 "Semilocalization of disordered spins in cavity QED"

Light-matter interactions are playing an increasingly crucial role in the understanding and engineering of new states of matter with relevance to the fields of quantum optics, solid state physics, chemistry and materials science. Experiments have shown that significant modifications of material properties and transport can occur in a cavity in the regime of collective strong light-matter coupling even without external irradiation – “in the dark”. In this colloquium-style talk we focus on disorder -- a key feature of many materials --, in particular on general models for disordered spins coupled to the photon field of a cavity. We show that collective light matter interactions can dramatically alter the many-particle spin wavefunctions even in the limit of vanishingly small photon numbers: Subtle, permanent changes in the wavefunctions result from the combined effects of vacuum hybridization and long-range cavity-mediated couplings between the spins. A surprising, general, result is the realization of “semilocalization”, a famous and elusive effect in quantum physics, usually associated to critical states of Anderson-like transitions. We discuss implications for energy transport and novel quantum phases mediated by long-range couplings in molecular physics and quantum optical systems.

Speaker: Guido Pupillo (CNRS, Université de Strasbourg)

Coffee break

9 Discussion 1: Solid state systems in cavities; equilibrium vs driven light-matter states

Possible topics:

What are promising engineering principles for solid state systems (electronic motion, nuclear spins)? What are the interesting many-body states of matter to create, and how to probe them?

Is driving crucial to obtain interesting stationary states in resonators, or can equilibrium properties be modified substantially by the presence of cavities?

What are the open / non-understood issues in solid state? Could quantum simulation with cold atoms help to elucidate these questions, and how to translate concepts from one field to the other?

Speaker: Markus Müller (Paul Scherrer Institut)

Posters

Posters Workshop.pdf

Tuesday, 18 June

10 "Ergodicity breaking and quantum trajectories in multimode cavities"

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.

Speaker: Sarang Gopalakrishnan (Princeton University)

11 Multimode cavities

Speaker: Vladimir Manucharian (EPFL)

12 "Replica symmetry breaking in a quantum-optical vector spin glass"

Spin glasses are canonical examples of complex matter. Although much about their structure remains uncertain, they inform the description of a wide array of complex phenomena, ranging from magnetic ordering in metals with impurities to aspects of evolution, protein folding, climate models, combinatorial optimization, and artificial intelligence. Advancing experimental insight into their structure requires repeatable control over microscopic degrees of freedom. I will present how we achieved this at the atomic level using a quantum optical system comprised of ultracold gases of atoms coupled via photons resonating within a confocal cavity. This realizes an unusual form of transverse-field vector spin glass with all-to-all connectivity. The controllability provided by this new spin-glass system may enable the study of spin glass physics in novel regimes, with application to quantum associative memory.

Speaker: Ben Lev (Stanford University)

Coffee break

13 "Recent advances and perspectives in multi-mode circuit QED"

In this talk, we will begin with a general introduction on multi-mode cavity quantum electrodynamics (QED). Following this, we will delve into three pioneering frontiers. First, we will explore the phenomenon of strong photon-photon interactions and many-body localization in multi-mode circuit QED systems consisting of a single superconducting qubit coupled to a transmission line resonator [1,2]. Next, we will examine the emergence of unconventional phase transitions in high-impedance circuit QED systems [3]. Finally, we will explore the emergence of direct and dual Shapiro steps using multi-mode circuit QED and their robustness, crucial to close the quantum metrological triangle [4]. We will discuss general perspectives in this emerging field.

1. N. Mehta, R. Kuzmin, C. Ciuti, V. E. Manucharyan, Nature 613, 650-655 (2023). "Down-conversion of a single photon as a probe of many-body localization."
2. N. Mehta, C. Ciuti, R. Kuzmin, V. E. Manucharyan, arXiv:2210.14681. "Theory of strong down-conversion in multi-mode cavity and circuit QED."
3. L. Giacomelli, C. Ciuti, arXiv:2307.06383, Nature Comm. in press. "Emergent quantum phase transition of a Josephson junction coupled to a high-impedance multi-mode resonator."
4. F. Borletto, L. Giacomelli, C. Ciuti, arXiv:2405.12935. "Circuit QED theory of direct and dual Shapiro steps with finite-size transmission line resonators."

Speaker: Cristiano Ciuti (Université Paris Cité)

Lunch

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

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).

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

15 Collective states of light and matter

Speaker: Markus Müller (Paul Scherrer Institut)

16 "Realizing and probing strongly correlated states of the quantum fluid of light"

In this talk I will review the state-of-the-art of the theory and experiments aiming at the stabilization and the manipulation of strongly correlated fluids of light. A special attention will be paid to Mott insulator and fractional quantum Hall states, for which pioneering experiments have appeared. Perspectives towards the observation of novel many-body effects in these systems will be outlined, such as non-equilibrium Goldstone modes, fractional statistics, and entangled states of collective excitations.

Pitaevskii BEC Center, INO-CNR, I-38123 Trento, Italy

Speaker: Iacopo Carusotto (INO-CNR BEC Center, Trento)

Coffee break

17 Discussion 2: Multimode cavities; non-classical states of light; coupled light matter many body systems

Prospects, limits, challenges of multimode cavities:
Are they more than an “inhomogeneous continuum”
Are the couplings quasi-random, can they be shaped in interesting ways?

How to numerically (and efficiently) simulate light-matter systems (gauge choice, truncation problems) ?

What kind of non-classical light states can be created from cavity-coupled systems?
Effects on matter by driving with squeezed / non-classical light?

Use of light-matter states as strong THz sources?
Use of optical combs? Of acoustic resonators?

Speaker: Gabriel Aeppli (ETH Zurich / PSI / EPFL)

Closing