Harvard-MIT Team Advances Quantum Science with Polar Molecules and Fluorescence Detection

A team from the Department of Physics, Department of Chemistry and Chemical Biology, and the Harvard-MIT Center for Ultracold Atoms has made significant strides in quantum science by demonstrating a method for defect removal in a molecular array and sequential state-selective detection. The research, which focuses on the role of polar molecules in quantum science, also showed the ability to initialize an array of molecules in an arbitrary pattern of computational basis states. This work is expected to contribute significantly to the study of many-body interactions, the execution of large quantum circuits, and further advances in state preparation and measurement.

What is the Significance of Polar Molecules in Quantum Science?

Polar molecules are a quantum resource with a rich internal structure that can be coherently controlled. This structure, however, also makes the state preparation and measurement (SPAM) of molecules challenging. Polar molecules feature a tunable long-range dipolar interaction, making them a resource for a wide range of quantum science applications. Coherent control of internal states is key to many recent advances. The dipole-dipole interaction has been used to realize a spin Hamiltonian, the site-resolved correlations of which have been measured in an optical lattice, and to produce Bell states in optical tweezers for dipolar quantum gates.

Control over nuclear spins has transferred their entanglement from reactants to products in chemical reactions. In the search for the electron electric dipole moment, internal states have been polarized to generate large laboratory-frame electric fields and used to evaluate systematic errors. Many applications require further advances in state preparation and measurement (SPAM), which remains an outstanding challenge due to the dense electronic and vibrational structure that complicates molecule production and detection.

How is Nondestructive Detection Using Fluorescence Beneficial?

Nondestructive detection using fluorescence from optical-cycling transitions has become an indispensable technique in atomic physics. Real-time detection of individual atoms enables the rearrangement of occupied traps to produce a densely filled array, as well as the selective readout of their internal states. Both capabilities are crucial for studying many-body interactions and executing large quantum circuits.

For molecules, a subset does possess optical-cycling transitions, enabling advances in their laser cooling, trapping, and rearrangement. Other molecules can be produced by the coherent association of their constituent atoms, with the advantage that cold-atom temperatures are preserved in the molecule creation process. However, optical-cycling transitions for fluorescence or absorption imaging are generally not available.

What are the Desirable Capabilities for the Molecule SPAM Toolbox?

To make use of the inherent multilevel structure of molecules, three capabilities for the molecule SPAM toolbox are highly desirable: rearrangement while maintaining low temperature, site-resolved state preparation, and resolved readout of multiple internal states. The first capability would allow for the production of deterministically filled arrays of molecules. The latter two would facilitate experiments using the many long-lived rotational states as a qudit system or multilevel quantum simulator.

Various proposals have been put forward for molecule readout via a controlled interaction between a ground-state molecule and an ancilla atom or optical cavity. However, these methods require the introduction of a new coherently controlled interaction between the molecule and the detection ancilla, which requires more experimental overhead to implement.

How Does the Proposed Scheme for Defect Removal and Sequential State-Selective Detection Work?

The proposed scheme uses fluorescence imaging of the constituent atoms of NaCs. Unassociated Cs atoms are detected immediately following molecule formation to infer which sites contain molecules. Those sites are rearranged and increase the local filling probability at the edge of the array threefold. The atom signal is also used for the detection of the two molecular states in the ground and first excited rotational levels that form the computational basis.

In a single experimental cycle, molecules are sequentially dissociated from each state and the resulting Cs atoms are detected. The ability to initialize an array of molecules in an arbitrary pattern of the computational basis states using global microwave pulses in combination with an auxiliary rotational state outside the computational basis for site-selective shelving of molecules is also demonstrated.

What is the Future Scope of this Research?

This research, conducted by a team from the Department of Physics, Department of Chemistry and Chemical Biology, and the Harvard–MIT Center for Ultracold Atoms, has demonstrated an immediately applicable method for the removal of defects in a molecular array and sequential state-selective detection. The team has also shown the ability to initialize an array of molecules in an arbitrary pattern of the computational basis states.

This work opens up new possibilities for the study of many-body interactions and the execution of large quantum circuits. It also provides a foundation for further advances in state preparation and measurement, which is a critical aspect of quantum science applications. The team’s findings have been published by the American Physical Society and are expected to contribute significantly to the field of quantum science.