QUANTUM-Seminar

Superradiance and subradiance are fundamental aspects of the open system dynamics of dense ensembles of quantum emitters exhibiting spontaneous emission rates well below or well above the rate for a single isolated system. At the purely theoretical level, superradiance has been first discussed by Dicke in 1954, in the context of accelerated decay of an ensemble of identical N initially inverted two-level quantum systems. In practice, such cooperative behavior associated with super- and subradiance at low excitation levels, has been observed in the 1930s by Jelley and Scheibe, in the context of molecular aggregates: unexpectedly large absorption cross-sections have been recorded for dye molecules. This has been later explained by Kasha in the 1960s as stemming from the alignment of the transition dipole moments of the many nanometer-spaced monomers forming the aggregate. We analytically tackle such issues with methods of open quantum system dynamics, in particular quantum Langevin equations and master equations. For the problem of Dicke superradiance we identify an exact analytical solution for the time evolution of the density operator, valid for any time t any number N of emitters. In the direction of quantum optics with molecules, we provide analytical models and solutions for the excitation migration between collective electronic levels in molecular aggregates and for processes involving non-radiative transitions due to non-adiabatic couplings of potential electronic landscapes in single large organic molecules. [1] R. Holzinger and C. Genes, Exact solution for Dicke superradiance, arXiv:2409.19040, (2024). [2] R. Holzinger, N. S. Bassler, H. Ritsch and C. Genes, Scaling law for Kasha's rule in photoexcited molecular aggregates, J. Phys. Chem. A 128, 19, 3910 (2024). [3] N. S. Bassler, M. Reitz, R. Holzinger, A. Vibók, G. J. Halász, B. Gurlek and C. Genes, Generalized energy gap law: An open system dynamics approach to non-adiabatic phenomena in molecules, arXiv:2405.08718 (2024).

Analytical and numerical tools of optimal control theory (1) have found widespread applications in NMR and EPR spectroscopy, imaging, and in quantum information processing (2). In the last decade, these tools not only provided pulse sequences of unprecedented performance and capabilities, but also new analytical and geometrical insight and a deeper understanding of pulse optimization problems. The definition of the figure of merit for a pulse sequence is crucial for the optimization for a desired range of applications. In addition to standard figures of merit for excitation, inversion and refocusing pulses, more general figures of merits have made it possible to significantly extend the range of applications. This will be illustrated for recent examples from the field of NMR and the control of trapped cold atoms (3). Furthermore, based on the DROPS (4) and BEADS (5) representations, novel intuitive visualization approaches have been developed to see the dynamics of multi-qubit systems in quantum information processing and beyond. (1) N. Khaneja, R. Brockett, S. J. Glaser, Phys. Rev. A 63, 032308/1-13 (2001); N. Khaneja, S. J. Glaser, R. Brockett, Phys. Rev. A 65, 032301 (2002); N. Khaneja, T. Reiss, C. Kehlet, T. Schulte-Herbrüggen, S. J. Glaser, J. Magn. Reson. 172, 296-305 (2005). (2) S. J. Glaser, U. Boscain, T. Calarco, C. P. Koch, W. Köckenberger, R. Kosloff, I. Kuprov, B. Luy, S. Schirmer, T. Schulte-Herbrüggen, D. Sugny, F. K. Wilhelm, Eur. Phys. J. D 69, 279/1-24 (2015); C. P. Koch, U. Boscain, T. Calarco, G. Dirr, S. Filipp, S. J. Glaser, R. Kosloff, S. Montangero, T. Schulte-Herbrüggen, D. Sugny, F. K. Wilhelm, Eur. Phys. J. Quantum Technology 9, 19/1-60 (2022). (3) Z. Zhang, L. Van Damme, M. Rossignolo, L. Festa, M. Melchner, R. Eberhard, D. Tsevas, K. Mours, E. Reches, J. Zeiher, S. Blatt, I. Bloch, S. J. Glaser, A. Alberti, arXiv:2410.02452 [quant-ph] (2024); L. Van Damme, Z. Zhang, A. Devra, S. J. Glaser, A. Alberti, arXiv:2410.02452 [quant-ph] (2024). (4) A. Garon, R. Zeier, S. J. Glaser, Phys. Rev. A 91, 042122 (2015); D. Leiner, R. Zeier, S. J. Glaser, J. Phys. A: Math. Theor. 53, 495301 (2020). (5) D. Huber, S. J. Glaser, arXiv:2410.01446 [quant-ph] (2024).

Laser spectroscopy techniques provide nuclear-model independent access to nuclear properties, such as the electromagnetic moments, spins and charge radii. Advances in radioactive ion beam instrumentation and laser technologies have enabled the study of a wide range of elements and isotopes, pushing out far from the valley of stability towards the drip lines. In this seminar, I will present experimental progress along two important frontiers. I will discuss the use of methods based on laser ionization spectroscopy and how they have allowed us to reach exotic nuclei, such as 94Ag 52K, which have long been out of reach. The role of these measurements in furthering our understanding of the atomic nucleus will also be put into context. Besides using efficient laser ionization and particle detection methods, another important frontier is the precision frontier. I will focus on ongoing research which aims to perform optical and radiofrequency spectroscopy of radioactive ions while they are trapped in a linear Paul trap. I will discuss the status and first commissioning results of a new setup currently under construction at the KU Leuven.

We apply innovative tools coming from quantum optimal control theory to improve theoretical and experimental techniques in quantum technologies [1,2]. This approach allows us to explore and to experimentally reach the physical limits of the corresponding dynamics in the presence of typical experimental imperfections and limitations. After a pedagogical introduction to these techniques [3], different applications in quantum technologies will be described. Recent theoretical and experimental results for the control of a Bose-Einstein Condensate in an optical lattice will be presented [4]. [1]- Quantum optimal control in quantum technologies. Strategic report on current status, visions ans goals for research in Europe C. P. Koch, U. Boscain, T. Calarco, G. Dirr, S. Filipp, S. Glaser, R. Kosloff, S. Montangero, T. Schulte-Herbruggen, D. Sugny and F. K. Wilhelm EPJ Quantum Technol. 9, 19 (2022) [2]- Introduction to the Pontryagin Maximum Principle for Quantum Optimal Control U. Boscain, M. Sigalotti, and D. Sugny PRX Quantum 2, 030203 (2021) [3]- Introduction to the theoretical and experimental aspects of quantum optimal control Q. Ansel, E. Dionis, F. Arrouas, B. Peaudecerf, S. Guérin, D. Guéry-Odelin and D. Sugny J. Phys. B 57, 133001 (2024) [4]- Quantum state control of a Bose Einstein condensate in an optical lattice N. Dupont, G. Chatelain, L. Gabardos, M. Arnal, J. Billy, B. Peaudecerf, D. Sugny, D. Guéry-Odelin PRX quantum 2, 040303 (2021)

Over the last twenty years, physicists have learned to manipulate individual quantum objects: atoms, ions, molecules, quantum circuits, electronic spins... It is now possible to build "atom by atom" a synthetic quantum matter. By controlling the interactions between atoms, one can study the properties of these elementary many-body systems: quantum magnetism, transport of excitations, superconductivity... and thus understand more deeply the N-body problem. More recently, it was realized that these quantum machines may find applications in the industry, such as finding the solution of combinatorial optimization problems. This seminar will present an example of a synthetic quantum system, based on laser-cooled ensembles of individual atoms trapped in microscopic optical tweezer arrays. By exciting the atoms into Rydberg states, we make them interact, even at distances of more than ten micrometers. In this way, we study the magnetic properties of an ensemble of more than a hundred interacting ½ spins, in a regime in which simulations by usual numerical methods are already very challenging. Some aspects of this research led to the creation of a startup, Pasqal.

Precision measurements of atomic transition frequencies have become a promising path for testing theories for new physics beyond the standard model. To achieve even higher precision more stable and narrow-linewidth laser sources are required. Superradiant lasers are a candidate for realising a narrow-linewidth, high-bandwidth active frequency reference. They shift the phase memory from the optical cavity, which is subject to technical and thermal vibration noise, to an ultra-narrow optical atomic transition of an ensemble of cold atoms trapped inside the cavity. Our previous demonstration of pulsed superradiance on the mHz transition in 87Sr achieved a fractional Allan deviation of 6.7*10−16 at 1s of averaging. Moving towards continuous-wave superradiance promises to further improve the short-term frequency stability by orders of magnitude. A key challenge is the continuous supply of cold atoms into a cavity, while staying in the collective strong coupling regime. We demonstrate continuous loading and transport of cold 88Sr atoms inside a ring cavity, after several stages of laser cooling and slowing. We further describe the emergence of distinct zones of collective continuous lasing of the atoms on the 7.5kHz transition, 7x narrower than the cavity linewidth, and pumped by the cooling lasers via inversion of the motional states. The lasing is supported by self-regulation of the number of atoms inside the cavity that pins the dressed cavity frequency to a fixed value over >3MHz of raw applied cavity frequency. In the process up to 80% of the original atoms are expelled from the cavity. I will also present a new project in Heidelberg aiming to use precision spectroscopy of highly charged ions to search for a variation of the fine-structure constant.

The field of ultra-cold chemistry of ions and atoms has been launching the fundamental quest for investigating its quantum regime for decades. A simplifying summary of the quest might be: How do interactions and chemical reactions proceed at extremely low temperatures? The classical picture predicts that all dynamics comes to a standstill as zero velocity is approached. However, deviations are expected since the classical model ceases to be appropriate at microscopic scales and at low temperatures, where particle-wave dualism of matter get’s important. In this regime, quantum effects dominate and reactions are predicted to obey fundamentally different rules. Examples are: (i) collisions of atoms, necessary for a reaction, cannot be described as a billiard-like impact between hard spheres anymore, but rather as interfering waves, interacting at long range, which can coherently amplify or even decoherently annihilate each other. (ii) energy barriers can exceed the available kinetic energy, but nevertheless be efficiently passed via quantum tunnelling, ruling the dynamics. Experimentally, we immerse a single barium (Ba+) ion in a bath of fermionic lithium (Li) atoms. We span temperatures from far above room temperature down deep into the s-wave regime of nano-Kelvin. We report our results on exploiting the collision energy dependence of magnetically tunable atom-ion scattering (Feshbach) resonances and explain how to assign their partial-wave-classification experimentally. In the first half, we will give a basic tutorial on quantum scattering of atom-ion ensembles and distill the substantial differences to atom-atom dynamics. We aim to discuss how to gain control and state-sensitive detection on the level of individual quanta within the merged ion-atom system and to study and establish optically trapping of ions and atoms in general - for example to reveal the quantum dynamics of ion-atom and ion-molecule reactions in absence of any detrimental radio-frequency fields.

In my talk, I will present a novel approach to magnetic resonance microscopy that exploits nitrogen-vacancy (NV) centers in diamond for optically detected magnetic resonance (ODMR). The fusion of optical microscopy and nuclear magnetic resonance (NMR) spectroscopy bypasses the conventional reliance on k-space sampling and magnetic field gradients for spatial encoding of NMR signals, enabling real-space magnetic resonance imaging (MRI). We demonstrate the capabilities of our widefield optical NMR microscopy technique by imaging NMR signals within a model microstructure, achieving a spatial resolution of approximately 10 μm over an area of ~235 × 150 μm². Each camera pixel captures a complete NMR spectrum, providing comprehensive information on signal amplitude, phase, local magnetic field strengths, and gradients. The integration of optical microscopy and NMR opens up new possibilities for a wide range of applications in the physical and life sciences, which I will discuss in the last part of my talk. These applications include imaging metabolic activity in single cells or tissue slices, analyzing battery materials, and facilitating high-throughput NMR analysis.

TBA