Condensed Matter Physics, Atomic and Molecular Physics, and Optics
21
Scopus Publications
Scopus Publications
Observation of perfect absorption in hyperfine levels of molecular spins with hermitian subspaces Claudio Bonizzoni, Daniele Lamberto, Samuel Napoli, Simon Günzler, Dennis Rieger, Fabio Santanni, Alberto Ghirri, Wolfgang Wernsdorfer, Salvatore Savasta, Marco Affronte Nature Communications, 2026 We investigate Perfect Absorption (PA) of radiation, in which incoming energy is entirely dissipated, in a system consisting of molecular spin centers coherently coupled to a planar microwave resonator operated at milliKelvin temperature and in the single photon regime. This platform allows us to fine tune the spin-photon coupling and to control the effective dissipation of the two subsystems towards the environment, thus giving us the opportunity to span over a wide space of parameters. Our system can be effectively described by a non-Hermitian Hamiltonian exhibiting distinct Hermitian subspaces. We experimentally show that these subspaces, linked to the presence of PA, can be engineered through the resonator-spin detuning, which controls the composition of the polaritons in terms of photon and spin content. In such a way, the required balance between the feeding and the loss rates is effectively recovered even in the absence of PT-symmetry. We show that Hermitian subspaces influence the overall aspect of coherent spectra of cavity QED systems and enlarge the possibility to explore non-Hermitian effects in open quantum systems. We finally discuss how our results can be potentially exploited for applications, in particular as single-photon switches and modulators.
Coherent coupling between YBCO superconducting resonators and sub-micrometer-thick YIG films Alberto Ghirri, Mattia Cavani, Claudio Bonizzoni, Marco Affronte Journal of Magnetism and Magnetic Materials, 2026 In cavity magnonics, magnon-photon hybridization has been widely investigated for both fundamental studies and applications. Planar superconducting resonators operating at microwave frequencies have demonstrated the possibility to achieve high couplings with magnons by exploiting the confinement of the microwave field in a reduced volume. Here we report a study of the coupling of high- T c YBCO superconducting waveguides with 104-nm-thick YIG magnetic films. We study the evolution of mode frequencies as a function of temperature and extract the coupling strength of hybrid magnon-photon modes. We show that the experimental results can be reproduced using a simple model in which the temperature dependence of the penetration depth accounts for the evolution of the polaritonic spectrum.
Quantum sensing of time-dependent magnetic signals with molecular spins Matteo Lanza, Claudio Bonizzoni, Olga Mironova, Fabio Santanni, Alessio Nicolini, Alberto Ghirri, Andrea Cornia, Marco Affronte Physical Review Applied, 2026 Molecular spins offer a promising platform for quantum sensing, particularly in organic, supramolecular, and biological environments. Recognition of signals by these systems is of particular interest given their possible integration into more complex structures and their possible use as sensors in close proximity to analytes. In this work, we develop two quantum sensing protocols that enable discrimination between different time-dependent magnetic fields without requiring their periodicity to match the microwave manipulating sequence. These are based on the Hahn echo sequence and have been tested on VO(TPP) and VOPt ( SOCPh ) 4 molecular spins embedded in a superconducting yttrium barium copper oxide (YBCO) microwave planar resonator. We report a magnetic field sensitivity up to 2.57 × 10 − 7 T Hz − 1 2 (with lower bounds approaching 2.87 × 10 − 8 T Hz − 1 2 ) for signals with a duration of a few microseconds. Under the given conditions, the minimum signal area that can be measured is in the 10 − 10 T s range, suggesting a potential trade-off between the minimum measurable field, and the required signal duration and the memory time.
Quantum sensing of magnetic fields with molecular spins Claudio Bonizzoni, Alberto Ghirri, Fabio Santanni, Marco Affronte Npj Quantum Information, 2024 Spins are prototypical systems with the potential to probe magnetic fields down to the atomic scale limit. Exploiting their quantum nature through appropriate sensing protocols allows to enlarge their applicability to fields not always accessible by classical sensors. Here we first show that quantum sensing protocols for AC magnetic fields can be implemented with molecular spin ensembles embedded into hybrid quantum circuits. We then show that, using only echo detection at microwave frequency and no optical readout, Dynamical Decoupling protocols synchronized with the AC magnetic fields can enhance sensitivity up to S ≈ 10−10 − 10−9 T Hz−1/2 with a low (4-5) number of applied pulses. These results paves the way for the development of strategies to exploit molecular spins as quantum sensors.
Interplay between magnetism and superconductivity in a hybrid magnon-photon bilayer system Alberto Ghirri, Claudio Bonizzoni, Maksut Maksutoglu, Marco Affronte Physical Review Applied, 2024 Spin waves in magnetic films are affected by the vicinity to a superconductor. Here we focus on a bilayer stack made of an insulating Yttrium Iron Garnet (YIG) film and a high-$T_c$ YBCO superconducting planar resonator and report microwave transmission spectra to monitor the temperature evolution of magnon-photon polaritons. We show that the observed temperature dependence of normal mode splitting and frequency shift with respect to the unperturbed magnon mode can be ultimately related to the penetration depth of YBCO, as an effect of the interplay between spin waves and Meissner currents.
Ultrastrong Magnon-Photon Coupling Achieved by Magnetic Films in Contact with Superconducting Resonators Alberto Ghirri, Claudio Bonizzoni, Maksut Maksutoglu, Alberto Mercurio, Omar Di Stefano, Salvatore Savasta, Marco Affronte Physical Review Applied, 2023 Coherent coupling between spin wave excitations (magnons) and microwave photons in a cavity may disclose new paths to unconventional phenomena as well as for novel applications. Here, we present a systematic investigation on YIG (Yttrium Iron Garnet) films on top of coplanar waveguide resonators made of superconducting YBCO. We first show that spin wave excitations with frequency higher than the Kittel mode can be excited by putting in direct contact a 5~$\\mu$m thick YIG film with the YBCO coplanar resonator (cavity frequency $\\omega_c/2 \\pi = 8.65$~GHz). With this configuration, we obtain very large values of the collective coupling strength $\\lambda/2 \\pi \\approx 2$~GHz and cooperativity $C=5 \\times 10^4$. Transmission spectra are analyzed by a modified Hopfield model for which we provide an exact solution that allows us to well reproduce spectra by introducing a limited number of free parameters. It turns out that the coupling of the dominant magnon mode with photons exceeds 0.2 times the cavity frequency, thus demonstrating the achievement of the ultrastrong coupling regime with this architecture. Our analysis also shows a vanishing contribution of the diamagnetic term which is a peculiarity of pure spin systems.
Coherent Quantum Network of Superconducting Qubits as a Highly Sensitive Detector of Microwave Photons for Searching of Galactic Axions C. Gatti, M. Affronte, A. Balanov, C. Bonizzoni, Giorgio Brida, F. Chiariello, N. Chikhi, G. Coda, A. D'Elia, D. Di Gioacchino, E. Enrico, I. Eremin, M. Ejrnaes, E. Il'ichev, L. Fasolo, M. Fistul, A. Ghirri, A. Greco, C. Ligi, G. Maccarone, A. Meda, P. Navez, G. Oelsner, M. Rajteri, A. Rettaroli, B. Ruggiero, S. Savel'ev, P. Silvestrini, S. Tocci, A. Ustinov, P. Vanacore, A. Zagoskin, M. Lisitskiy IEEE Transactions on Applied Superconductivity, 2023 We propose a novel approach to detect a low power microwave signal with a frequency of the order of several GHz based on a coherent collective response of quantum states occurring in a superconducting qubits network (SQN). An SQN composes of a large number of superconducting qubits embedded in a low-dissipative superconducting resonator. Our theory predicts that an SQN interacting with the off-resonance microwave radiation, demonstrates the collective alternating current Stark effect that can be measured even in the limit of single photon counting. A design of the layout of three terminals SQN detectors containing 10 flux qubits weakly coupled to a low-dissipative R-resonator and T-transmission line was developed. The samples were fabricated by Al-based technology with Nb resonator. The SQN detector was tested in terms of microwave measurements of scattering parameters and two-tone spectroscopy. A substantial shift of the frequency position of the transmission coefficient drop induced by a second tone pump signal was observed, and this effect clearly manifests a nonlinear multiphoton interaction between the second-tone microwave pump signal and an array of qubits.
Machine-Learning-Assisted Manipulation and Readout of Molecular Spin Qubits Claudio Bonizzoni, Mirco Tincani, Fabio Santanni, Marco Affronte Physical Review Applied, 2022 Machine Learning finds application in the quantum control and readout of qubits. In this work we apply Artificial Neural Networks to assist the manipulation and the readout of a prototypical molecular spin qubit - an Oxovanadium(IV) moiety - in two experiments designed to test the amplitude and the phase recognition, respectively. We first successfully use an artificial network to analyze the output of a Storage/Retrieval protocol with four input pulses to recognize the echo positions and, with further post selection on the results, to infer the initial input pulse sequence. We then apply an Artificial Neural Network to ascertain the phase of the experimentally measured Hahn echo, showing that it is possible to correctly detect its phase and to recognize additional single-pulse phase shifts added during manipulation.
Ultra-strong coupling in the hybrid quantum system consisting of planar MW resonator and YIG material Optics Infobase Conference Papers, 2022
