Quantum control of a single $\mathrm{H}_2^+$ molecular ion

TL;DR

This paper demonstrates a novel method combining buffer gas cooling and quantum logic to achieve high-precision control and spectroscopy of the H2+ molecular ion, overcoming previous experimental challenges.

Contribution

It introduces a new approach for controlling and performing high-resolution spectroscopy on H2+ using buffer gas cooling and quantum logic, enabling precise measurements of its hyperfine structure.

Findings

Achieved 2 Hz precision in microwave spectroscopy of H2+ hyperfine structure.

Demonstrated quantum state preparation and non-destructive readout of H2+.

Established a general method applicable to other molecular ions.

Abstract

Science is founded on the benchmarking of theoretical models against experimental measurements, with the challenge that for all but the simplest systems, the calculations required for high precision become extremely challenging. $\mathrm{H}_2^+$ is the simplest stable molecule, and its structure is calculable to high precision. However, studying $\mathrm{H}_2^+$ experimentally presents significant challenges: Standard control methods such as laser cooling are not applicable due to the long lifetimes of its rotational and vibrational states. Here we solve this issue by combining buffer gas cooling to quench the $\mathrm{H}_2^+$ rovibrational excitation with quantum logic operations between $\mathrm{H}_2^+$ and a co-trapped 'helper' ion to control the molecule's hyperfine structure. This enables us to perform pure quantum state preparation, coherent control, and non-destructive readout, which we use to demonstrate high-resolution microwave spectroscopy in the hyperfine structure of $\mathrm{H}_2^+$ with a precision of 2 Hz. Our results pave the way for high precision spectroscopy of $\mathrm{H}_2^+$ in both the microwave and optical domains. Due to the wide applicability of buffer gas cooling, our method provides a general tool for molecular ion species that are hard to control with quantum logic tools alone.