Session G25: Cold Chemical Physics: Complexity Frontier

Wes Campbell Optical cycling of aromatic molecules for quantum state detection

An optical cycling transition in a molecule is an electronic transition in which the upper state preferentially decays back to the original rovibrational state (or states) from which it was excited upon spontaneous emission.  Because the laser-induced fluorescence can be repeated many times with a nearly deterministic final internal state, these transitions are useful for laser-driven applications such as Doppler cooling and quantum state preparation and detection of qubits.  I will discuss recent progress toward endowing molecules as large as polycyclic arenes with optical cycling centers.  Frequency selectivity allows us to use the laser-induced fluorescence to detect conformation changes, a prerequisite for using those degrees of freedom for quantum information applications the single molecule level.   

Laser-Coolable Molecules Built from Coinage Metal and Carbon-Group Atoms

The broad scientific opportunities promised by ultracold molecules have spurred recent efforts to apply direct laser cooling to diverse sets of molecules. In recent years, increasing attention has been attracted by molecules that offer new resources to the areas of quantum science and precision measurements. This includes molecules with complex structure (often polyatomic molecules), where long-lived states arising from internal angular momenta are suspected to provide rich qubit platforms and/or internal co-magnetometers that provide robust systematic error rejection. Due to the difficulty associated with laser cooling polyatomic molecules, it is of interest to identify diatomic molecules that share these properties.

In this talk, after reviewing the recent progress achieved with complex structure in polyatomic molecules, we will focus on a new class of diatomic molecules that we have identified as potentially laser coolable: coinage metals bonded to carbon-group atoms. We will describe theoretical and experimental efforts focused on molecules such as CuX, AgX, and AuX (X=C and Pb). We will present theoretical results* predicting that these molecules combine many of the properties desirable for next-generation experiments using ultracold molecules. These include large relativistic enhancements, long-lived and highly polarizable X²Π_1/2 ground states with negligible magnetic-field sensitivity, and diagonal Franck-Condon factors that may enable optical cycling and laser cooling. We will also describe ongoing progress at Williams College to produce these molecules and study their optical spectra.

* Conducted in collaboration with Lan Cheng and the Johns Hopkins University   

Emergence, symmetry, and ergodicity breaking in C60 fullerenesLee Liu

An interacting many-body system can possess qualitatively different properties than those of its microscopic constituents. Examples of such "emergent phenomena" include hydrodynamics, quasi-particles, and phase transitions.

 

Polyatomic molecules comprise a unique class of composite quantum system that hosts emergent phenomena. Molecules feature perfect geometric symmetries and rotate freely in three dimensions. Additionally, they possess a hierarchy of energy scales spanning six orders of magnitude–(nuclear spin interactions), rotational, vibrational, electronic–that enable a spectroscopist to peel back successive layers of emergent behaviour and their underlying physics. This motivates broadband, frequency-domain spectroscopy of gas-phase molecules at the quantum state-resolved limit.

 

Large polyatomic molecules are particularly attractive due to their complexity. However, they typically exhibit intrinsic spectral broadening that precludes quantum state resolution [1]. C_60 is a notable exception: its rigidity and nuclear spin statistics suppresses density of states and allows excited vibrational states to be well-resolved [2].

 

In this talk, I will discuss this perspective on emergence as well as our recent work on C_60 [3]. Infrared spectra showed that C_60 repeatedly breaks and restores rotational ergodicity as it rotates faster and faster. Moreover, the ergodicity breaking occurs without breaking symmetry. This is enabled by the molecule's high degree of symmetry and rich spectrum of vibrations. These insights may be relevant to protecting quantum coherence in complex systems, engineering useful quantum materials, and probing matter out of equilibrium, inspired by the dynamics of non-rigid rotors.

1. Ben Spaun, P. Bryan Changala, David Patterson, Bryce J. Bjork, Oliver H. Heckl, John M. Doyle, and Jun Ye. Nature 533, 517–520 (2016)

2. P. Bryan Changala , Marissa L. Weichman, Kevin F. Lee, Martin E. Fermann, and Jun Ye. Science 363, 49-54 (2019)

3. Lee R. Liu, Dina Rosenberg, P. Bryan Changala, Philip J. D. Crowley, David J. Nesbitt, Norman Y. Yao, Timur V. Tscherbul, and Jun Ye. Science 381, 778-783 (2023)   

Molecular Color Centers

Molecules combine atomic precision, tunability, and scalability. This unique combination poises molecules to impact "quantum" in areas where designer systems are important. One such area is in quantum sensing. Progress towards designing molecules for quantum sensing will be described with a focus on molecular color centers, or molecules that feature optical readout of spin informaiton (ODMR).   

Harnessing chemical reactions for quantum science

Chemical reactions through collisions are often seen as violent processes, where chemical bonds break and form. Can quantum coherence be preserved throughout the entire reaction? Can chemical reactions be harnessed to produce entangled product pairs as quantum resources?  Here, we investigate these questions by studying a ‘model’ gas-phase reaction 2KRb → K₂ + Rb₂ at ultracold temperatures, focusing on the nuclear spin degrees of freedom. In particular, we prepare the initial nuclear spins in KRb in an entangled state and characterize the preserved coherence in the nuclear spin wavefunction after the reaction by measuring the product state distribution. We find that the coherence in reactants is almost fully preserved, suggesting that entanglement can be prepared within the reactants, followed by a chemical reaction that produces separate, entangled molecules.   

Accurate oscillator strengths for 2P - 2S transitions in neutral boron

We report benchmark calculations for the ground state and 18 lowest bound excited ²S and ²P states of the boron atom performed in the framework of the variational method. The wave functions were expanded in terms of all-particle explicitly correlated gaussian basis sets, while the Hamiltonian included a finite nuclear mass. These highly accurate, isotope-dependent, nonrelativistic wave functions were used to compute the transition dipole moments and the corresponding oscillator strengths in both length and velocity gauge for all allowed transitions between the considered states. Some of these quantities have not been reported in the literature before, while for others we improved the accuracy by several orders of magnitude. Moreover, the explicit mass dependence in our wave functions allowed us to identify several transitions where, in relative terms, the isotopic shift between ¹⁰B and ¹¹B in the oscillator strength values should be most significant.   

2023 APS Fellow. Talk Title: Spectroscopy and Structure of the Simplest Actinide Bonds

Processing and remediation of nuclear waste requires the development of efficient chemical separation processes. The exploration of suitable chelating agents for such separations processes is challenging as actinide chemistry is not well-understood. This is, in part, due to the technical difficulties and expense of working with radiologically hazardous elements.  Theoretical modeling of actinide chemistry is highly desirable, but this task poses a severe challenge for computational quantum chemistry models due to the large numbers of metal-centered electrons and the presence of strong relativistic effects. For the development of computational models with truly predictive capabilities it is essential that they can be tested and validated by comparisons with accurate experimental data.  

We are using multiple resonance spectroscopy and jet cooling techniques to unravel the complex electronic spectra of Th and U compounds.  Recent results for the oxides, sulfides, nitrides and halides will be discussed.  The patterns of electronic states for these molecules provide information concerning the occupation of the 5f orbitals and their participation in bond formation.