Coherently coupling distinct spin ensembles through a high-Tcsuperconducting resonator

Abstract

The problem of coupling multiple spin ensembles through cavity photons is revisited by using PyBTM organic radicals and a high-Tc 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.PACS numbers: 33.90.+h, 75.50.Xx, 76.30Rn, 07.57.Pt Controlling light-matter interaction at the quantum level is a central problem in modern physics and technology. The paradigmatic system for such investigation is represented by a two-level emitter coupled to a confined mode of the electromagnetic field [1]. The experimental benchmark of a coherent light-matter interaction is the creation of hybridized modes, which can be observed if the coupling between the field and the emitter is larger than that between these and environment. This strongcoupling regime has been achieved by employing a variety of emitters, ranging from Rydberg atoms to superconducting qubits, all characterized by large electric-dipole transition amplitudes [2]. Spin-photon coupling is much weaker, but can be dramatically enhanced by exploiting cooperative phenomena in N -spin ensembles (SEs) [3,4]. In this way, the strong coupling regime has been demonstrated with different spin systems in high quality factor microwave resonators [5][6][7][8]. Along the same lines, experimental evidence of the coherent coupling between 3D-cavity photons and magnons in ferro-and ferri-magnetic crystals has been provided [9-11].Molecular spin systems display features that are potentially exploitable in quantum-information processing [12], such as a wide tunability of the physical parameters and decoherence times exceeding 10 3 the gating times at liquid nitrogen temperature [13][14][15][16]. Organic radicals provide possibly the simplest spin systems, consisting of single unpaired electrons with isotropic g-factors.In addition, the presence of intermolecular exchange interactions in non-diluted ensembles gives rise to exchange narrowing, which averages out the intermolecular dipolar and hyperfine interactions [17]. 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 reachi...

Coherent coupling between Vanadyl Phthalocyanine spin ensemble and microwave photons: towards integration of molecular spin qubits into quantum circuits

“…2e,f and g), where only a single peak is visible at resonance. The presence of two anti-crossed branches is the experimental fingerprint of the high cooperativity regime 10,11,13,39,40 . Reproducibility of this behaviour was tested on a different sample and found in similar experimental conditions (Figure S9).…”

“…Moreover, concentrated spin ensembles of organic radicals have been efficiently coupled to planar resonators by exploiting the exchange narrowing of their ESR lines 31,38–40 . Diluted ensembles potentially show longer coherence times but the conditions for reaching the strong coupling regime with these samples still need to be found.…”

Molecular Spins in the Context of Quantum Technologies

“…Superconducting circuits are normally used as a bus to transfer quantum information between different quantum memories and registers. In a recent experiment, we have demonstrated that we can couple two or more distinguished spin ensembles through resonant microwaves photons by spectroscopic measurements [62]. Next steps along this line could be the integration of molecular spins in more complex superconducting circuits with the encoding of sequences of MW pulses.…”

“…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.…”

“…Experiments are currently testing different solutions since the detection of tiny signals requires extraordinary sensitivity. By using Josephson junctions [62,63], respectively. Data taken at different temperatures are indicated by the black arrows and by symbols of different sizes that range from 2 K (larger symbols) to 50 K (smaller symbols).…”