Transmission Spectroscopy of Molecular Spin Ensembles through Microwave Planar Resonators

*Claudio Bonizzoni (1,2), Alberto Ghirri (2), Filippo Troiani (2), and Marco Affronte (1,2)
(1) Università di Modena e Reggio Emilia, Dipartimento di Scienze Fisiche, Informatiche e Matematiche, via G. Campi 231/A, 41125 Modena, Italy, (2) Istituto Nanoscienze - CNR Nano S3, via G. Campi 231/A, 41125 Modena, Italy.

Over the last decade molecular spins have been object of intense research as potential candidates for quantum technologies [Ghirri et al., Magnetochem. 3, 12 (2017)]. The interest is given by the possibility of tailoring their magnetic properties at synthetic level as well as their relatively easy integration on surfaces. Another key aspect is the presence of both electronic and nuclear spin degrees of freedom which, in principle, might open wide possibilities for the implementation of protocols for quantum information processing. Here, two of the main challenges are addressing the transitions with external microwave and radiofrequency stimuli and developing suitable protocols for the manipulation of the spins.

In this work, we deal with the microwave excitation of molecular spin ensembles at low temperatures. To achieve this goal, we have first developed a set of planar superconducting resonators at microwave frequency to couple to the ensembles via magnetic dipolar interaction in an electron spin resonance-like transmission spectroscopy [Bonizzoni et al., Adv. in Phys. X 3, 1435305 (2018)]. The coupling conditions of several molecular ensembles are first studied under the continuous-wave excitation regime. Remarkably, experimental evidence of coherent spin-photon coupling between the ensembles and the resonators at low temperatures is found within two different approaches. In the first one the number of coupled spins of a diluted vanadyl phthalocyanine is maximized by using an optimized resonator [Bonizzoni et al., Sci. Rep. 7, 13096 (2017)], while in the second one the exchange narrowing effect of the spin linewidth in concentrated organic radicals is exploited [Ghirri et al., Phys. Rev. A 93, 063855 (2016)]. We then focus to experiments in the pulsed-wave regime. Here an arbitrary waveform generator is used for the synthesis of the microwave pulses which are then injected into the resonant geometry to drive the evolution of the spins. Our preliminary results concerning the measure of the phase memory time on an ensemble of vanadyl phthalocyanines are then given.