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arXiv:1605.02879 · Physical Review A

Coherently coupling distinct spin ensembles through a high- Tc superconducting resonator

Alberto Ghirri, Claudio Bonizzoni, Filippo Troiani, Nunzio Buccheri, Luca Beverina, A. Cassinese, M. Affronte

Published Jun 28, 2016• 51 citations
arXivHTMLPDF

Abstract

The problem of coupling multiple spin ensembles through cavity photons is revisited by using (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl (PyBTM) organic radicals and a high-${T}_{c}$ superconducting coplanar resonator. An exceptionally strong coupling is obtained and up to three spin ensembles are simultaneously coupled. The ensembles are made physically distinguishable by chemically varying the $g$ factor and by exploiting the inhomogeneities of the applied magnetic field. The coherent mixing of the spin and field modes is demonstrated by the observed multiple anticrossing, along with the simulations performed within the input-output formalism, and quantified by suitable entropic measures.

DOI
10.1103/physreva.93.063855

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We implement superconducting Yttrium barium copper oxide planar resonators with two fundamental modes for circuit quantum electrodynamics experiments. We first demonstrate good tunability in the resonant microwave frequencies and in their interplay, as emerges from the dependence of the transmission spectra on the device geometry. We then investigate the magnetic coupling of the resonant modes with bulk samples of 2,2-diphenyl-1-picrylhydrazyl organic radical spins. The transmission spectroscopy performed at low temperature shows that the coherent spin-photon coupling regime with the spin ensembles can be achieved by each of the resonator modes. The analysis of the results within the framework of the input-output formalism and by means of entropic measures demonstrates coherent mixing of the degrees of freedom corresponding to two remote spin ensembles and, with a suitable choice of the geometry, the approaching of a regime with spin-induced mixing of the two photon modes.
Within the quest for solid state quantum systems to be used for fundamental as well as applied research, molecular spins have recently emerged as a versatile platform with interesting performances in terms of quantum coherence and correlation. Molecular units provide well defined environment to electronic spins and they represent elementary bricks for complex nano-architectures and nano-devices. Here we review our recent efforts and results on their efficient integration in circuit Quantum ElectroDynamics and, more specifically, in reaching their coherent coupling with microwave photons in planar resonators. To monitor molecular spin performances over a wide temperature and magnetic field range we have first developed microwave planar resonators made of high T c superconductors, obtaining excellent performances up to liquid Nitrogen temperature and in magnetic fields up to 7 Tesla. Ensembles of different molecular spins systems are then systematically tested. The regime of high spin-photon cooperativity is achieved with molecular spins diluted in non-magnetic matrix at 0.5 K, while the strong coupling regime is observed with concentrated samples of organic radicals up to 50 K. The possibility to create coherent states among distinct spin ensembles is further explored in similar spectroscopic experiments. These results show that molecular spins can be efficiently integrated in quantum devices.
Hybrid quantum systems with high-Tc_cc​ superconducting resonators
Superconducting microwave resonators are crucial elements of microwave circuits, offering a wide range of potential applications in modern science and technology. While conventional low-T[Formula: see text] superconductors are mainly employed, high-T[Formula: see text] cuprates could offer enhanced temperature and magnetic field operating ranges. Here, we report the realization of [Formula: see text] superconducting coplanar waveguide resonators, and demonstrate a continuous evolution from a lossy undercoupled regime, to a lossless overcoupled regime by adjusting the device geometry, in good agreement with circuit model theory. A high-quality factor resonator was then used to perform electron spin resonance measurements on a molecular spin ensemble across a temperature range spanning two decades. We observe spin-cavity hybridization indicating coherent coupling between the microwave field and the spins in a highly cooperative regime. The temperature dependence of the Rabi splitting and the spin relaxation time point toward an antiferromagnetic coupling of the spins below 2 K. Our findings indicate that high-Tc superconducting resonators hold great promise for the development of functional circuits. Additionally, they suggest novel approaches for achieving hybrid quantum systems based on high-T[Formula: see text] superconductors and for conducting electron spin resonance measurements over a wide range of magnetic fields and temperatures.
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Spin ensembles are considered as potential candidates for quantum memory and quantum enhanced sensing applications. Here, we explore the controlled coupling of multiple superconducting microwave cavities to a spin ensemble, which shows signatures of strong coupling and, due to the multi-mode character, the formation of dark states. In particular, the latter are of interest, as they provide a potential pathway to enhance memory times and enable protected storage of non-classical states in spin ensembles due to the suppressed coupling to the circuit environment. We model the spin multi-cavity hybrid to reproduce the spectra and extract characteristic coupling strengths using the input-output formalism.