Chemical Engineering of Molecular Qubits

We show that the electron spin phase memory time, the most important property of a molecular nanomagnet from the perspective of quantum information processing, can be improved dramatically by chemically engineering the molecular structure to optimize the environment of the spin. We vary systematically each structural component of the class of antiferromagnetic ${\mathrm{Cr}}_7\mathrm{Ni}$ rings to identify the sources of decoherence. The optimal structure exhibits a phase memory time exceeding $15\mu \mathrm{s}$.

Figure 1

Structures of the ${\mathrm{Cr}}_7\mathrm{Ni}$ molecules. (a) Crystal structure of $[{}^n\mathrm{Pr}_2{\mathrm{NH}}_2][{\mathrm{Cr}}_7{\mathrm{NiF}}_8{\mathrm{Ac}}_{16}]$. The colored balls represent either different atom types: Cr (light green), Ni (dark green), F (yellow), or different interchangeable substituents: central templating cation (blue), carboxylate bridging ligand (red). (b) Appropriately color coded chemical structures of some possible variants with abbreviated names. The abundance of hydrogen atoms in the molecular building blocks is tabulated in the Supplemental Material [34].

Figure 2

Temperature dependence of phase decoherence parameters for representative alkyl-ammonium templated compounds with carboxylate bridging ligands Piv ($\mathbf{1}$), $d$-Piv ($\mathbf{2}$) and $\mathrm{Ad}1{\mathrm{CO}}_2$ ($\mathbf{3}$) in frozen $h$-toluene solution.

Figure 3

Temperature dependence of the phase memory time for three ${\mathrm{Cs}}^{+}$ templated ${\mathrm{Cr}}_7\mathrm{Ni}$ compounds ($\mathbf{1}\mathbf{b}$, $\mathbf{2}\mathbf{b}$ and $\mathbf{3}\mathbf{b}$), in protonated and deuterated frozen toluene solutions.

Figure 4

Two pulse electron spin-echo decay for $\mathbf{2}\mathbf{b}$ in $d$-toluene at 1.5 K. Electron spin-echo envelope modulations [16] at early time occur at the ${}^2\mathrm{H}$ Larmor frequency.