Vibronic Engineering for Quantum Functional Groups.

ConspectusChemists have a firm understanding of the concept of a functional group: a small molecular moiety that confers properties (reactivity, solubility, and chemical recognition) onto a larger scaffold. Analogously, a quantum functional group (QFG) would act as an isolated "quantum handle" that could attach onto an extended molecule and enable quantum state preparation and measurement (SPAM). However, the complexity associated with molecular chemistry is often at odds with the requirements of nonthermal state preparation. The rest of the molecule acts as a local bath that leads to dephasing and loss of quantum information upon excitation and relaxation. Yet, there exists an enormous chemical space of potential chemical bonding motifs to design isolated QFGs. The goal of this Account is to explore the underlying chemical design principles for the optimization of QFG performance.For typical state preparation, an applied field is used to put the qubit into a specific known state (via optical cycling and laser cooling), where it can be manipulated or entangled with other species. That same field (or another) can be used to read out or report on the qubit state at the end of the operation. For example, in trapped ions/neutral atoms, state preparation is accomplished by pumping a specific transition using a narrowband laser. From there, further operations can be performed on the qubit via selective RF or laser excitation, and the state can be read out via fluorescence. However, extending this paradigm to molecular systems is highly challenging: molecules have many more degrees of freedom that can couple to the absorbed or emitted field. Overcoming this requires greatly limiting the number of these "off-diagonal" decay pathways through the judicious selection of the QFG and vibronic engineering of the molecular substrate.Our work has demonstrated that alkaline-earth (I) alkoxides (MOR) may meet the necessary requirements for efficient SPAM. In particular, we capitalize on the -OM (M = Ca, Sr) motif, which acts as a quantum handle that has been attached to a variety of aliphatic and aromatic hydrocarbons. The precise breakdown of the optical cycling property depends on familiar chemical concepts, including conjugation, conformer formation, electron-withdrawing abilities, and symmetry. In this Account, we review the recent efforts in the field to construct QFGs and codesign molecular scaffolds that can host them without destruction of their desired quantum properties. QFGs are explored as attachments to photoswitching scaffolds and mounted in pairs to larger hosts. A variety of physical phenomena relevant to the ability of these QFGs to function as qubits, from Fermi resonances to super radiance, have been explored. We thus began deriving the first set of rules for vibronic engineering toward the QFG functionality. Prospects toward increasing the number densities of these QFGs through molecular and material design are also presented.