Chemical control of spin-lattice relaxation to discover a room temperature molecular qubit

Abstract

The second quantum revolution harnesses exquisite quantum control for a slate of diverse applications including sensing, communication, and computation. Of the many candidates for building quantum systems, molecules offer both tunability and specificity, but the principles to enable high temperature operation are not well established. Spin-lattice relaxation, represented by the time constant T1, is the primary factor dictating the high temperature performance of quantum bits (qubits), and serves as the upper limit on qubit coherence times (T2). For molecular qubits at elevated temperatures (>100 K), molecular vibrations facilitate rapid spin-lattice relaxation which limits T2 to well below operational minimums for certain quantum technologies. Here we identify the effects of controlling orbital angular momentum through metal coordination geometry and ligand rigidity via π-conjugation on T1 relaxation in three four-coordinate Cu2+S = ½ qubit candidates: bis(N,N′-dimethyl-4-amino-3-penten-2-imine) copper(ii) (Me2Nac)2 (1), bis(acetylacetone)ethylenediamine copper(ii) Cu(acacen) (2), and tetramethyltetraazaannulene copper(ii) Cu(tmtaa) (3). We obtain significant T1 improvement upon changing from tetrahedral to square planar geometries through changes in orbital angular momentum. T1 is further improved with greater π-conjugation in the ligand framework. Our electronic structure calculations reveal that the reduced motion of low energy vibrations in the primary coordination sphere slows relaxation and increases T1. These principles enable us to report a new molecular qubit candidate with room temperature T2 = 0.43 μs, and establishes guidelines for designing novel qubit candidates operating above 100 K.