Tanya Zelevinsky at Columbia University

Molecular Lattice Clock for Fundamental Measurements

Summary

There is a close connection between AMO science and fundamental physics. The techniques of quantum state control are leading to a new generation of table-top experiments that can offer a glimpse of new physics over an extensive range of length scales, energies, and types of fundamental interactions. The unprecedented reach of these experiments arises from an immaculate control of atomic and molecular quantum states and their environmental influences. In particular, atomic clocks play a special role as extremely precise scientific measurement tools, contributing to diverse scientific questions such as dark energy, gravitational waves, and high-precision many-body physics. While microwave and optical atomic clocks are based on different types of electronic transitions, molecules possess a significantly more extensive set of internal degrees of freedom than atoms. A clock based on molecular physics such as vibrations can access new fundamental measurements that are out of reach for atomic clocks. Significant progress has recently taken place in molecular quantum state control. This makes possible state-of-the-art clocks that utilize ultracold molecules. In this project, we developed a prototype vibrational molecular lattice clock based on strontium dimers, with upcoming scientific applications that include tests of ultrashort-range gravity. Tightly trapping ultracold neutral molecules in an optical lattice affords a large signal-to-noise ratio while eliminating motional effects that lead to rapid decoherence. A key challenge to holding molecules in optical traps is to overcome frequency shifts and decoherence arising from the bright trapping light. This challenge is common to the development of both clock states and future qubits. By testing a series of ”magic” or state-insensitive optical lattices, we suppressed vibrational decoherence by several orders of magnitude, leading to the spectroscopic quality factors of several trillion. Moreover, we characterized the full statistical and systematic uncertainty of a 32 THz molecular clock at the 14th decimal place, and compared its frequency to the Cs time standard at the 13th decimal place. The primary scientific application is for the most precise measurement of an interatomic force. Combined with state-of-the-art quantum chemistry theory that is being developed concurrently, this clock-based measurement could lead to the best limit on non-Newtonian gravity at the nanometer scale, while providing a test of QED for heavy molecules.

Big scientific questions addressed

The research was mainly trying to address the following big question: "Can we probe massdependent forces at the nanoscale by using fully quantum systems (such as molecular clocks) that are decoupled from the environment as well as possible?"

Researchers Whose Research Profited from these Funds

1. Tanya Zelevinsky (PI, faculty)


2. Kon Leung (student, main direct beneficiary)


3. Emily Tiberi (student)


4. Brandon Iritani (student)


5. Qi Sun (student)


6. Jinyu Dai (student)


7. Debayan Mitra (research scientist)

Notable scientific progress was enabled by the CFP Templeton grant?

The grant enabled the following scientific progress:

1. For the first time we explored the landscape of "magic" or state-insensitive optical-lattice traps for vibrational transitions in molecules. In order to make educated choices of good magic lattice wavelengths, we carried out careful experimental and theoretical spectroscopy of diatomic strontium molecules in pure vibrational states. This guided us to choose lattice wavelengths in vicinities of relatively strong lattice-driven transitions. We performed measurements of vibrational state coherence at three clock state / magic wavelength combinations, each resulting in respectable coherence times of nearly 100 ms for roughly 30 THz of energy separation between the clock states.


2. In the current configuration, reactive molecular collisions limit the clock coherence to approximately 100 ms, but we found that lattice light scattering imposes a slightly more stringent limitation that so far has not been overcome. This ¡100 ms coherence limitation is not consistent with what is known about the optical transition nearest to the lattice wavelength. We also discovered that this lattice-induced decoherence scales quadratically with the lattice light intensity rather than linearly, likely implying a two-photon process. Such a process could be explained by lattice-induced two-photon dissociation, but theory is not very precise for two-photon molecular processes. This fundamental issue deserves further investigation. It can also be minimized in clock configurations that allow using less trapping light intensity.


3. With the 100 ms coherence that we achieved, we implemented the most precise molecular vibrational clock. The combined statistical and systematic uncertainty is below 5 parts in 10¹⁴. We investigated effects due to the optical lattice light, the two-color probe light, molecular density, and blackbody radiation. This first generation of the molecular clock was limited by shifts and decoherence from the optical lattice.


4. We found that molecular vibrational states experience strong lattice-induced hyperpolarizability, i.e. frequency shifts that are quadratic in lattice intensity. We characterized these at the hertz level. These shifts are likely to be important for other optically trapped molecules as well.


5. We compared the 32 THz vibrational molecular clock to the Cs frequency standard at 1 part in 10¹³.


6. We implemented a STIRAP scheme to transfer weakly-bound strontium dimers to their absolute ground state with a nearly 90% efficiency. This allowed us to explore collisional losses of these ground-state van der Waals molecules for the first time. It also provides an excellent starting point for a molecular clock that only uses deeply bound states; such a clock could be highly insensitive to blackbody radiation shifts.

Notable scientific progress that was Triggered, Launched or Enabled that Can be Reasonably Expected in the Next Couple of Years?

The following scientific progress could be expected in reasonably near future:

1. Longer lattice-limited molecular-clock coherence times could be achieved by utilizing different trapping geometries that do not require high intensities of trapping light.


2. Potentially, much longer lattice-limited molecular-clock coherence times could be achieved by utilizing lattices with longer wavelengths, as theoretical calculations predict that infrared magic wavelengths could be possible far from any resonances and thus leading to much less decoherence. This possibility is currently under investigation.


3. With improvements to lattice-induced vibrational clock state decoherence, the molecular clock precision could improve by an order of magnitude in the next generation.


4. With some reconfiguration of the laser systems, it will be possible to run the molecular clock on a large range of vibrational states, thus scanning the interatomic force as a function of separation, between approximately 0.5 and 5 nm.


5. The groundwork will be laid to create strontium dimers using other isotopes and isotopic combinations, ultimately enabling the scanning of the spin-independent interatomic force as a function of the mass.

Full citations and whether support was properly acknowledged

The publications are attached when possible, and described here:

1. K. H. Leung, I. Majewska, H. Bekker, C.-H. Lee, E. Tiberi, S. S Kondov, R. Moszynski, and T. Zelevinsky, Phys. Rev. Lett. 125, 153001 (2020), "Transition strength measurements to guide magic wavelength selection in optically trapped molecules" [the grant is unfortunately not acknowledged due to an omission]


2. K. H. Leung, E. Tiberi, B. Iritani, I. Majewska, R. Moszynski, and T. Zelevinsky, New J. Phys. 23, 115002 (2021), "Ultracold 88Sr2 molecules in the absolute ground state" [the grant is acknowledged]


3. K. H. Leung, B. Iritani, E. Tiberi, I. Majewska, M. Borkowski, R. Moszynski, and T. Zelevinsky, arXiv 2209.10864, "A terahertz vibrational molecular clock with systematic uncertainty at the 10⁻¹⁴ level", under review in PRX [the grant is acknowledged]


4. K. H. Leung, PhD thesis, defended on 11/11/2022, to be deposited in early 2022 [the grant will be acknowledged]

Resulting publications reasonably expected in the next couple of years

We expect that the submitted paper (item (3) above) will be accepted and published. Otherwise, we can expect at least two novel research papers to follow in the next couple of years, likely along the lines of 1) improving the coherence and therefore the precision of the vibrational molecular clock and 2) utilizing the molecular clock to scan the interatomic force over a range of internuclear separations with high precision.