Self‐Cooling Molecular Spin Qudits

Abstract The need of operating molecular spin qubits at very low temperatures constitutes a technological limitation. This challenge is addressed by integrating, in the same material and at the molecular scale, quantum processing and magnetic refrigeration capabilities. The molecular unit is a GdEr heterolanthanide coordination complex, where Er(III) encodes a qubit while Gd(III) provides a large magnetocaloric effect. The properties of each component are separately studied in isostructural LaEr and GdLu complexes, where each functional ion lies next to a diamagnetic metal. All complexes are characterized by magnetic, heat capacity, and EPR measurements. The results show that the presence of both ions in the same molecule has a synergic effect on both functionalities. Thus, the coupling between Er(III) and Gd(III) spins lifts any level degeneracies even close to zero magnetic field, leading to a d = 16 set of spin states that, as revealed by pulse EPR measurements, can be coherently manipulated. In turn, Er(III) enhances the magnetocaloric effect compared to GdLu, extending it to lower temperatures. This is corroborated by direct magnetocaloric measurements, which show the ability of this material to cool itself, and a device, down to temperatures as low as 0.4 K.