Tunable Lanthanide Complexes for Quantum Information Processing

Quantum information processing and quantum computing are terms that, just 20 years ago, would have belonged to science fiction books and movies. Nonetheless, today, the race toward quantum supremacy is in full swing, with large firms such as Google and governments investing millions of dollars into pushing the boundaries of technology. Several potential qubit platforms are currently under investigation, including superconducting circuits, photons, trapped ions, and molecules. Each platform has its advantages, disadvantages, challenges, and potential.

The first part of this thesis sheds light on the previous work on the Ln(trensal) family as potential molecular qubits and the studies leading up to these investigations. Following this, a brief discussion of quantum computers and the five DiVincenzo criteria are presented, with a focus on molecules. Molecules are often highlighted for their chemical tunability in the context of quantum computing, herein, the concept of tunability is discussed and defined.

The subsequent two sections build upon these introductory topics as a foundation for further discussion. The themes of this thesis can be divided based on the properties attempted to be tuned, with the first section focused on tuning dynamic properties and the latter on physical properties. Using a collection of techniques, we examine the dynamics of Gd(trensal) via X-band pulsed EPR, revealing coherence up to 125 K compared to 30 K measured at 240 GHz.

This highlights the importance of external factors, including experimental frequency, in tuning the dynamics. Furthermore, we demonstrate the significance of removing magnetic nuclei from the matrix for future improvements in phase memory time, Tm. Systematic changes to the ligand scaffold were explored through the synthetic introduction of methoxy groups. This resulted in the formation of two Ln(trensal)-like compounds, Ln(trenpvan) and Ln(trenovan), for which the Yb and Gd versions were examined. The eigenvector compositions of Yb(trenovan) and Yb(trenpvan) were determined via a multi-technique approach, involving magnetometry, luminescence, and continuous wave (c.w.) EPR. This revealed that both compounds have eigenvectors and eigenspectra similar to that of Yb(trensal). However, their dynamics differ substantially, even though both relax via a combination of direct and Raman mechanisms. This suggests that the dynamics of the Yb(trensal) motif are quite sensitive to chemical modifications on the phenolate. In contrast, the eigenvectors and eigenspectra of Gd(trenovan) and Gd(trenpvan) were found to be very different based on single-crystal rotation X-band EPR. While their dynamics were fairly similar, with both compounds showing coherence times up to 100 K, beyond which spin-lattice relaxation limits coherence. The large difference in eigenvectors led to different Rabi frequencies between the two, demonstrating the potential for chemical control of Rabi frequencies in S > 1/2 systems through the eigenvector composition.

The second part of this thesis expands on the post-functionalization approach for the synthesis of lanthanide complexes with desired properties, developed by our group, with the goal of introducing new physical properties. By using a chiral amine in the functionalization, an enantiomerically pure Yb(trensal)-like compound was synthesized and structurally characterized by single-crystal diffraction. The compound exhibits a strong chiroptical response for the f-f transitions, due to the chiral environment of the Yb center. Furthermore, pulsed EPR on a single crystal revealed the ability to coherently manipulate the spin state, showing comparable coherence times to Yb(trensal), although spin-lattice relaxation limits coherence at 15 K. This work lays the foundation for further studies of magneto-electric coupling in these enantiomerically pure compounds.

The use of a long aliphatic amine (octadecylamine) for post-functionalization resulted in compounds that show clear van der Waals interactions in their crystal structures. The influence of the electronic structure was examined by a combination of magnetometry and luminescence, revealing very comparable eigenspectra via luminescence, though magnetometry indicated different compositions of the lowest-lying states, particularly for the non-Kramers ion. Through these studies, the scope of the post-functionalization approach has been expanded, and the influence of functionalization on both the dynamic and static properties has been examined.