Seminaria

"What was the last Nobel Prize in Physics given for?"

Maciej Zgirski (Institute of Physics, PAN,, Warsaw)

Usually effects of quantum tunneling and energy quantization are associated with behavior of single atoms or molecules. Electrons occupy orbitals of definite energy and can change their state in the process of absorbing or emitting well-defined energy quanta – photons. In the Sun, two protons fuse by tunneling over the Coulomb barrier to form deuterium. But can we extend the direct applicability of quantum mechanics to macroscopic objects, i.e. objects visible with a bare eye? Yes, we can. This year Nobel Prize has been awarded “for the discovery of macroscopic quantum tunneling and energy quantization in an electric circuit”. In their two seminal experiments (Phys.Rev.Lett. 55, 1908 (1985), Phys.Rev.Lett. 55, 1543 (1985)) laureates John Clarke, Michel Devoret and John Martinis showed that these two hallmarks of quantum mechanics can be observed also for macroscopic artificially-defined objects. They studied the escape rate from the superconducting state for current biased Nb-NbOx-PbIn tunnel Josephson junctions embedded in a properly defined electric circuit. The escape rate measured at the lowest temperatures (< 30 mK) appeared significantly larger than that expected from thermal activation, signaling appearance of a new escape mechanism. The laureates identified this mechanism as Macroscopic Quantum Tunneling (MQT) i.e. the process in which a collective state of many Cooper pairs switches between two macroscopic wavefunctions, although the two configurations are separated by a barrier which forbids the classical transition. Unlike familiar tunneling observed in real space, such MQT happens in a space of the superconducting phase and involves its abrupt change leading to the appearance of a non-zero voltage measured across the junction. Before the escape happens the electric current and superconducting phase across the junction oscillate around the local energy minimum. It is basically the behavior of a harmonic oscillator. At sufficiently low temperatures the oscillator becomes dominated by quantum effects and the energy of the macroscopic oscillatory current becomes quantized. The higher the energy of the quantum state the easier the escape. The laureates were able to capture this effect by resonantly activating the energy levels with microwaves and measured the enhancement of the escape rate at the expected photon frequencies. The ability to address the junction with microwave photons and force the transitions makes the designed circuit similar to an atom. Since the trapping potential is not ideally parabolic, the oscillator is anharmonic and the spacing between energy levels is not equal. This pioneering study initiated the field of quantum electronics, in which electric circuits are described fully quantum-mechanically and their components can be treated like artificial atoms. The best known example of such a circuit is a superconducting qubit, one of the current leaders in the race for a quantum computer. Maciej Zgirski Leader of the CoolPhon Group at MAGTOP at the Institute of Physics, Polish Academy of Sciences (IF PAN) - http://coolphongroup.ifpan.edu.pl/ zgirski@ifpan.edu.pl, +48730003175 Short CV: Maciej Zgirski (born 1979) graduated from the Faculty of Physics of Warsaw University of Technology in 2003. Defended his PhD in the field of the experimental low temperature mesoscopic physics at the University of Jyväskylä, Finland in 2008. In years 2008-2010 he joined Quantronics Group (CEA) in Saclay, France as a PostDoc. Since end of 2010 employed at the Institute of Physics, PAN in Warsaw. Since 06/2024 professor at the International Centre for Interfacing Magnetism and Superconductivity with Topological Matter (MAGTOP) at the Institute of Physics. Received habilitation in 2022 for the creation of the pioneering time-resolved low- temperature thermometry and its application for the experimental investigation of the dynamical thermal processes at the nanoscale. Specializing in thermodynamics of nanostructures with the emphasis on the fast thermometry based on the various types of superconducting Josephson junctions. Another his expertise involves building novel-concept superconducting devices based on manipulating just a single superconducting vortex. The experimental approaches, sample designs, and measuring protocols used in the CoolPhon Group are of pioneering character, and create a backbone for new applications in the field of applied mesoscopic superconductivity.

"X-ray variability properties of blazars"

Gopal Bhatta (Institute of Astronomy, University of Zielona Góra)

X-ray emission from blazars provides crucial insights into the underlying AGN processes, as it can arise from synchrotron and inverse-Compton mechanisms in leptonic models, or be part of particle cascades produced in hadronic models. Here we present multi-instrument, multi-epoch investigations of several prominent blazars (OJ 287, PKS 2155–304, 3C 273, and Mrk 421 and samples of other blazars) with a particular focus on their X-ray variability and spectral properties. We perform comprehensive timing and spectral analyses covering intraday to decadal timescales, using long-term observations from Swift, XMM-Newton, NICER, and NuSTAR spanning more than two decades. During the period, the sources consistently display strong variability, with X-ray flux changes reaching factors of ~10–200 during major flares. Timing analysis, including fractional variability, flux and rms distributions, PSD slopes, and rms–flux relations, reveal a combination of normal, lognormal, and bimodal flux states, indicating complex, multiplicative variability processes and transitions between distinct emission states. Spectral analyses using power-law, log-parabola, and broken power-law models demonstrate that log-parabolic or broken spectra best represent the majority of observations, pointing to curvature in the underlying electron energy distributions. Several sources show clear flux-dependent spectral evolution, including “harder-when-brighter’’ behavior and strong correlations between synchrotron peak energy, curvature parameters, and X-ray flux. In OJ 287, broad-band spectral energy distribution (SED) modeling highlights systematic transitions between inverse-Compton–dominated quiescent states and synchrotron-dominated flaring states. Collectively, the observed properties such as Gaussian and non-Gaussian flux distributions, variable PSD slopes, and evolving spectral curvature supports scenarios involving energy-dependent particle acceleration, turbulence, and/or magnetic reconnection within the jet.

"TBA"

Anirudh Nemmani (NCAC, Warsaw)