14. June 2021

Peter Maurer (University of Chicago)
Quantum Sensing: Probing biological systems in a new light

Quantum optics has had a profound impact on precision measurements, and recently enabled probing various physical quantities, such as magnetic fields and temperature, with nanoscale spatial resolution. In my talk, I will discuss the development and application of novel quantum metrological technologies that enable the study of biological systems in a new regime. I will start with a general introduction to quantum sensing, with a focus on the measurement of magnetic fields at a nanoscale. I will then show how we utilize such sensing techniques to control the temperature profile in living systems with subcellular resolution. Finally, I will provide an outlook on how quantum sensing and single-molecule biophysics can be utilized to perform NMR spectroscopy with unprecedented sensitivity, possibly down to the level of individual biomolecules.

07. June 2021

Astrid Eichhorn (University of Southern Denmark)
Status and frontiers of asymptotically safe gravity

The asymptotic safety paradigm is a promising contender to provide us with an improved understanding of the fundamental building blocks of nature. In this talk, I will review how asymptotic safety is based on a symmetry principle that may provide us with a predictive quantum field theory of gravity and matter. I will highlight the most pressing open questions of the approach, most importantly the link between theory and observations.

03. Mai 2021

Nicolas Treps (Sorbonne University)
Multimode quantum light fields for Quantum Metrology and Quantum Computing 

Light offers a vast potential in the development of modern quantum technologies due to its intrinsic resilience to decoherence effects and its capacity to convey a huge amount of information. The many modes of light, would they be spatial modes or spectral modes, are as many quantum harmonic oscillators, leading to a largely unexplored Hilbert space[1]. One avenue for employing light to process quantum information focuses on the continuous variable regime, where the observables of interest are the quadratures of the electric field. They have proven their worth as a platform for creating huge entangled states in a deterministic fashion, easily manipulatable with standard techniques in optics. In this presentation we will first review the basic principles of multimode quantum light in the continuous variable regime, and illustrate them in quantum metrology experiments. We will show how it allows for an intuitive understanding of the sensitivity limits in high precision measurement, and experimentally reach the fundamental limits impose by the vacuum fluctuations in simple problems, as for instance estimating the separation between two incoherent sources[2]. We will then consider quantum information processing with multimode light. We will first demonstrate how to generate large entangled states using time/frequency modes[3]. However to reach a quantum advantage, and perform a task that cannot be efficiently simulated with a classical device, we require more than just entanglement. The additional ingredient is non-Gaussian statistics in the outcomes of the quadrature measurements. We will demonstrate how photon subtraction, a well know non-gaussian operation, can be rendered mode-dependent and allow for the generation of non-Gaussian multimode state of lights, required for quantum information processing[4].

References:

[1] C. Fabre and N. Treps, Modes and States in Quantum Optics, Rev. Mod. Phys. 92, 035005 (2020).
[2] P. Boucher, C. Fabre, G. Labroille, and N. Treps, Spatial Optical Mode Demultiplexing as a Practical Tool for Optimal Transverse Distance Estimation, Optica, 7, 1621 (2020).
[3] J. Roslund, R. M. de Araújo, S. Jiang, C. Fabre, and N. Treps, Wavelength-Multiplexed Quantum Networks with Ultrafast Frequency Combs, Nature Photonics 8, 109 (2014).
[4] Y.-S. Ra, A. Dufour, M. Walschaers, C. Jacquard, T. Michel, C. Fabre, and N. Treps, Non-Gaussian Quantum States of a Multimode Light Field, Nature Physics 11, 1 (2019).

19. April 2021

Prineha Narang (Harvard University)
Controlling Correlations: Linear-, Nonlinear-, and Hydrodynamics in Quantum Materials

The physics of quantum materials hosts spectacular excited-state and nonequilibrium effects, but many of these phenomena remain challenging to control and, consequently, technologically under-explored. My group’s research, therefore, focuses on how quantum systems behave, particularly away from equilibrium, and how we can harness these effects. By creating predictive theoretical and computational approaches to study dynamics, decoherence and correlations in materials, our work could enable technologies that are inherently more powerful than their classical counterparts ranging from scalable quantum information processing and networks, to ultra-high efficiency optoelectronic and energy conversion systems. In this talk, I will present work from my research group on describing, from first principles, the microscopic dynamics, decoherence and optically-excited collective phenomena in quantum matter at finite temperature to quantitatively link predictions with 3D atomic-scale imaging, quantum spectroscopy, and macroscopic behavior. Capturing these dynamics poses unique theoretical and computational challenges. The simultaneous contribution of processes that occur on many time and length-scales have remained elusive for state-of-the-art calculations and model Hamiltonian approaches alike, necessitating the development of new methods in computational physics^2–4. I will show selected examples of our approach in ab initio design of active defects in quantum materials^5–7, and control of collective phenomena to link these active defects^8–10. Building on this, in the second part of my seminar, I will show our predictions of linear and nonlinear dynamics and transport in Weyl semimetals^11–14. I will discuss the anomalous landscape for electron hydrodynamics in systems beyond graphene, highlighting that previously-thought exotic fluid phenomena can exist in both two-dimensional and anisotropic three-dimensional materials¹⁵. Our work identifies phonon-mediated electron-electron interactions^16–18 as critical in a microscopic understanding of hydrodynamics. Non-diffusive electron flow, and in particular electron hydrodynamics, has far-reaching implications in quantum materials science, as I will show in this talk. Finally, I will present an outlook on driving topological quantum materials far out-ofequilibrium to control the coupled degrees-of-freedom^19,20.

References:

1. Head-Marsden, K., Flick, J., Ciccarino, C. J. & Narang, P. Quantum Information and Algorithms for Correlated Quantum Matter. Chem. Rev. (2020) doi:10.1021/acs.chemrev.0c00620.
2. Rivera, N., Flick, J. & Narang, P. Variational Theory of Nonrelativistic Quantum Electrodynamics. Phys. Rev. Lett. 122, 193603 (2019).
3. Flick, J., Rivera, N. & Narang, P. Strong light-matter coupling in quantum chemistry and quantum photonics. Nanophotonics 7, 1479–1501 (2018).
4. Flick, J. & Narang, P. Cavity-Correlated Electron-Nuclear Dynamics from First Principles. Physical Review Letters vol. 121 (2018).
5. Narang, P., Ciccarino, C. J., Flick, J. & Englund, D. Quantum Materials with Atomic Precision: Artificial Atoms in Solids: Ab Initio Design, Control, and Integration of Single Photon Emitters in Artificial Quantum Materials. Adv. Funct. Mater. 29, 1904557 (2019).
6. Hayee, F. et al. Revealing multiple classes of stable quantum emitters in hexagonal boron nitride with correlated optical and electron microscopy. Nat. Mater. 19, 534–539 (2020).
7. Ciccarino, C. J. et al. Strong spin–orbit quenching via the product Jahn–Teller effect in neutral group IV qubits in diamond. npj Quantum Materials 5, 75 (2020).
8. Neuman, T., Wang, D. S. & Narang, P. Nanomagnonic Cavities for Strong Spin-Magnon Coupling and Magnon-Mediated Spin-Spin Interactions. Phys. Rev. Lett. 125, 247702 (2020).
9. Wang, D. S., Neuman, T. & Narang, P. Dipole-coupled emitters as deterministic entangled photon-pair sources. Phys. Rev. Research 2, 043328 (2020).
10. Neuman, T. et al. A Phononic Bus for Coherent Interfaces Between a Superconducting Quantum Processor, Spin Memory, and Photonic Quantum Networks. arXiv [quant-ph] (2020).
11. Narang, P., Garcia, C. A. C. & Felser, C. The topology of electronic band structures. Nat. Mater. (2020) doi:10.1038/s41563-020-00820-4.
12. Nenno, D. M., Garcia, C. A. C., Gooth, J., Felser, C. & Narang, P. Axion physics in condensed-matter systems. Nature Reviews Physics 2, 682–696 (2020).
13. Coulter, J., Sundararaman, R. & Narang, P. Microscopic origins of hydrodynamic transport in the type-II Weyl semimetal WP2. Phys. Rev. B Condens. Matter 98, (2018).
14. Coulter, J. et al. Uncovering electron-phonon scattering and phonon dynamics in type-I Weyl semimetals. Phys. Rev. B Condens. Matter 100, 220301 (2019).
15. Varnavides, G., Jermyn, A. S., Anikeeva, P., Felser, C. & Narang, P. Electron hydrodynamics in anisotropic materials. Nat. Commun. 11, 1–6 (2020).
16. Vool, U. et al. Imaging phonon-mediated hydrodynamic flow in WTe2 with cryogenic quantum magnetometry. arXiv [cond-mat.mes-hall] (2020).
17. Garcia, C. A. C., Nenno, D. M., Varnavides, G. & Narang, P. Anisotropic phonon-mediated electronic transport in chiral Weyl semimetals. arXiv [cond-mat.supr-con] (2020).
18. Osterhoudt, G. B. et al. Evidence for Dominant Phonon-Electron Scattering in Weyl Semimetal WP2. Physical Review X vol. 11 (2021).
19. Juraschek, D. M. & Narang, P. Shaken not strained. Nat. Phys. 16, 900–901 (2020).
20. Juraschek, D. M., Meier, Q. N. & Narang, P. Parametric Excitation of an Optically Silent Goldstone-Like Phonon Mode. Physical Review Letters vol. 124 (2020).

12. April 2021

Adán Cabello (Universidad de Sevilla)
Bell nonlocality and Kochen-Specker contextuality: How are they connected?

Bell nonlocality and Kochen-Specker (KS) contextuality are logically independent concepts, fuel different protocols with quantum advantage, and have distinct classical simulation costs. A natural question is what are the relations between these concepts, advantages, and costs. To address this question, it would be useful to have a map that captures all the connections between Bell nonlocality and KS contextuality in quantum theory. Here, we introduce such a map. After defining the theory-independent notions of Bell nonlocality and KS contextuality for ideal measurements, we point out that, in quantum theory, due to Neumark's dilation theorem, every matrix of quantum Bell nonlocal correlations can be mapped to an identical matrix of KS contextual correlations. A more difficult problem is identifying connections from KS contextuality into Bell nonlocality. We show that there are "one-to-one'' and partial connections for some KS contextuality scenarios, but not for all of them. However, we also present a method that transforms any matrix of KS contextual correlations into a matrix of Bell nonlocal correlations.

22. March 2021

Silke Weinfurtner (University of Nottingham)
Quantum simulators for fundamental physics

Analogue gravity summarises an effort to mimic physical processes that occur in the interplay between general relativity and field theory in a controlled laboratory environment. The aim is to provide insights in phenomena that would otherwise elude observation: when gravitational interactions are strong, when quantum effects are important, and/or on length scales that stretch far beyond the observable Universe. The most promising analogue gravity systems up to date are fluids, superfluids, superconducting circuits, ultra-cold atoms, and optical systems. While deepening our understanding of the laboratory systems at hand, the long-term vision of analogue gravity studies is to advance fundamental physics through interdisciplinary research, by establishing and nurturing a new culture of collaboration between the various communities involved. I will discuss recent efforts to explore the quantum origin of the Universe, accelerated observer radiation, and rotating black hole physics in the laboratory.