YBa2Cu3O7 microwave resonators for strong collective coupling with spin ensembles

Abstract

Coplanar microwave resonators made of 330 nm-thick superconducting YBa2Cu3O7 have been\ud realized and characterized in a wide temperature (T, 2–100 K) and magnetic field (B, 0–7 T) range.\ud The quality factor (QL) exceeds 104 below 55K and it slightly decreases for increasing fields,\ud remaining 90% of QLðB ¼ 0Þ for B¼7 T and T¼2K. These features allow the coherent coupling\ud of resonant photons with a spin ensemble at finite temperature and magnetic field. To demonstrate\ud this, collective strong coupling was achieved by using di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium\ud organic radical placed at the magnetic antinode of the fundamental mode: the in-plane magnetic\ud field is used to tune the spin frequency gap splitting across the single-mode cavity resonance\ud at 7.75 GHz, where clear anticrossings are observed with a splitting as large as 82 MHz at\ud T¼2K. The spin-cavity collective coupling rate is shown to scale as the square root of the number\ud of active spins in the ensemble

Molecular Spins in the Context of Quantum Technologies

“…The high coperativity regime was achieved by using coplanar superconducting resonators and NV centers [59] or Er spin impurities in inorganic matrix [60] at mK temperature. We are currently using high T c superconducting planar resonators [61] that show excellent performances at finite temperature (up to 50 K, at least) and in strong magnetic fields and, with these, we can achieve a strong coupling regime with 2,2-diphenyl-1-picrylhydrazyl (DPPH) [59] and (3,5-Dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl (PyBTM) [62] organic radicals at the temperature of liquid helium, and even above. It is worth noting that, in these latter cases, the strong coupling regime was obtained by using concentrate samples that present sharp resonance lines due to exchange narrowing.…”

Coherently coupling distinct spin ensembles through a high-Tcsuperconducting resonator

“…SEs of organic radicals can thus combine narrow magnetic transitions and high spin densities. For this reason, they are particularly suitable for reaching the strong coupling regime in a microwave cavity [18][19][20], while their versatility inspires the implementation of quantum gates [21].Here we exploit these features in order to demonstrate the coherent coupling between distinguishable SEs. By using (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radicals (PyBTM) [22], we first show that the strong-coupling regime is largely achieved in a broad temperature range, with values of the cooperativity reaching 4300 at 2 K. This allows us to provide an experimental evidence of the coherent coupling between up to three spatially separated SEs, mediated by the cavity mode of a high T c YBCO/sapphire coplanar resonator.…”

“…SEs of organic radicals can thus combine narrow magnetic transitions and high spin densities. For this reason, they are particularly suitable for reaching the strong coupling regime in a microwave cavity [18][19][20], while their versatility inspires the implementation of quantum gates [21].…”

Metallic coplanar resonators optimized for low-temperature measurements

“…Compared to the electroplated resonators, lower surface roughness and probable edge definition effects of the sputtered resonators could improve the quality factors up to 10%, although their film resistivity is slightly higher. We demonstrate the resonators working up to 7 T which is much further than the limits of similar conventional superconducting resonators (but reachable with cuprate devices) [34,43,46]. The values of the quality factor drop at 7 T by less than 3.5% which exert a very light background on the obtained spectra.…”

“…The observation can be explained by the magnetoresistance of copper at higher fields [35]. Planar structures (microstrip, stripline and coplanar) have already been employed for ESR experiments [15,[36][37][38][39][40][41][42][43][44][45][46]. Although microstrip and stripline resonators have also been successfully applied in ESR measurements [36-38, 41, 45, 47], coplanar configurations might be more appropriate candidates since they carry the signal non-dispersively in contrast to a microstrip configuration [45,48].…”