QUANTUM-Seminar

The weak force is predicted to break the parity symmetry between left and right-handed chiral molecules, but so far the effect has eluded detection. We are developing a trapped chiral molecular ion version of the search for parity violation (PV). Our candidate molecule, CHDBrI+ is predicted to be preparable via state-selective ionization and to exhibit a large PV shift of a few Hz for the C-H bend vibrational transition, where the transition’s natural linewidth is narrower than the shift. Other transitions such as the C-H stretch are also predicted to have Hz level PV shifts between enantiomers. We plan to probe the PV signature in a racemic, mixed-handedness ensemble of trapped CHDBrI+, using vibrational Ramsey spectroscopy. Our newly developed ion trap is integrated with a pulsed velocity map imaging detector to probe multiple internal state populations of the molecules by separating photo-fragment velocities. This technology will assist in overcoming the molecular complexity and help develop quantum control schemes for our molecule. We will also discuss the advantages chiral molecules have in searches for new physics as well as the status of the experiment.

Imagine two light fields propagating in opposite directions along a ring-shaped contour. When set into rotation, the constancy of the speed of light will induce a phase shift between the two light fields. This observation, known as the Sagnac effect, is a very powerful yet simple approach to measure rotation. The first successful measurement of Earth's rotation via the Sagnac effect was performed by Albert A. Michelson and co-workers in late 1924, exactly 100 years ago. I will give a brief review of the historic work, present the current state of the art, and elaborate on potential applications and future developments.

Quantum computing is moving beyond its traditional mainframe infrastructure with the realization of room-temperature technologies powered by the nitrogen-vacancy (NV) centers in diamond. At Quantum Brilliance, we are at the forefront of this innovation, developing compact quantum accelerators based on NV centers—artificial atoms that enable fully functional qubits in a solid-state environment. This innovation holds the potential to make quantum computing not only more accessible but also more practical. Our mission is to deliver room-temperature quantum processors that can be deployed across a variety of environments, from centralized data centers to the network edge. To achieve this, we are overcoming key technological challenges, such as the precise arrangement of NV centers at nanometer scales to enable magnetic coupling for multi-qubit operations across nodes. Quantum Brilliance addresses this using a breakthrough 'bottom-up' fabrication technique, leveraging atomically precise surface chemistry and lithography to build scalable diamond devices. Beyond scalability, we are also focused on advancing the performance, miniaturization, and manufacturability of these devices—crucial for achieving high-speed, high-fidelity spin control and efficient qubit readout in low-power, compact systems. These technological advancements are positioning diamond quantum technologies as a leading force in the transition to compact, high-performance quantum computing and quantum information processing. With pre-production prototypes underway, Quantum Brilliance is on track to develop quantum accelerators with over 50 qubits, poised to outperform classical CPUs and GPUs in critical applications within the next five years. In this presentation, I will explore the key innovations driving the performance, miniaturization, and scalability of diamond-based quantum technologies, and how these breakthroughs are set to transform the quantum computing landscape, enabling scalable, mass deployable quantum compute systems.

Optically addressable spins in the solid state are promising candidates for realizations of quantum networks and quantum computing nodes. We study NV centers in diamond coupled to an optical microcavity to enhance the optical emission and get efficient access to the spin degree of freedom. Studying small ensembles, we observe collectively enhanced emission and non-trivial photon statistics, despite the presence of inhomogeneities and spatial separation between emitters [1]. As an alternative color center, we study SnV centers in diamond, which can possess superior optical coherence properties. We observe hour-long spectral stability and Fourier-limited emission linewidths of individual emitters. We leverage their spin degree of freedom by studying a strained diamond at mK temperature. To avoid Ohmic losses in the microwave line, we fabricate a superconducting coplanar waveguide on a diamond membrane. We demonstrate coherent manipulation of the electron spin and evaluate the decoherence properties for different magnetic field orientations at mK temperature [2]. We furthermore identify strongly coupled nuclear spins and achieve nuclear spin state preparation and coherent control. Prospects for integration into a microcavity for efficient spin-photon interfacing are discussed [3]. A complementary platform is rare earth ion-based materials. I will report investigations of molecular rare-earth-complexes with promising coherence properties for quantum applications [4] and efforts to study single ions coupled to a cavity as qubits [5]. References [1] Pallmann et al., arxiv:2311.12723 [2] Karapatzakis et al., Phys Rev X 14, 031036 (2024) [3] Körber et al., Phys Rev Appl. 19, 064057 (2023) [4] Serrano et al., Nature 603, 241 (2022) [5] Deshmukh et al., Optica 10, 1339 (2023)

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)