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The precise measurement of states in atomic and molecular systems can help to validate fundamental physics theories and their predictions. Among the various platforms that can help to validate theoretical predictions are so-called diatomic molecular hydrogen ions (MHI), molecular ions that consist of two hydrogen nuclei (i.e., protons or their isotopes) and a single electron.

Compared to atomic ions, these molecular ions have a more complex internal structure, as they contain two nuclei instead of one. Even when they are in their lowest possible electronic energy level (i.e., the electron's ground state), these two nuclei can still rotate and vibrate, producing a wide range of rovibrational states.

Researchers at the Max Planck Institute for Nuclear Â鶹ÒùÔºics recently introduced a new method to precisely control and non-destructively measure the rovibrational ground state of a single molecular hydrogen ion in a Penning trap (i.e., a device that confines charged particles using static electric and magnetic fields).

This method, outlined in a paper in Â鶹ÒùÔºical Review Letters, could open new possibilities for the manipulation and measurement of rich quantum states in individual molecular ions.

"The work for the paper was inspired by the goal of the fundamental physics research community to compare H₂⁺ and its antimatter counterpart H₂^- in the future," Charlotte König, first author of the paper, told Â鶹ÒùÔº. "An overview on this topic and measurement proposals can be found in a paper by Myers .

Therefore, we have now developed and demonstrated nondestructive state detection and measurement techniques on a single molecular hydrogen ion (HD⁺) in a Penning trap; applicable to other molecular ions with an unpaired electron spin, i.e. to H₂⁺ and H₂^-."

The new method developed by König and her colleagues relies on an effect known as the continuous Stern Gerlach effect, first . This is a physical phenomenon that can be leveraged to measure the orientation of the magnetic moment (e.g., the electron spin) of single trapped particles, including ions, without destroying them.

"In our experiments, the orientation of the electron spin in the external magnetic field (B) of the Penning trap is mapped onto the ion motion in a magnetic bottle (B=B₀+B₂ x²), which is an established technique for atomic ions in Penning traps," explained König.

"In the molecule, the energy splitting between electron spin up or down is unique to each rovibrational and hyperfine state. Therefore, resonantly driving an electron spin transition (detected by the continuous Stern Gerlach effect), gives us the information about which internal quantum state the ion is in."

Using their newly proposed method, König and her colleagues demonstrated the confinement of an externally produced molecular hydrogen ion (HD⁺) for more than a month. In addition, they were able to detect the internal quantum state of this ion and control its hyperfine state.

"These are necessary requirements to enable future measurements of the antimatter molecular hydrogen ion H₂^- for tests of the fundamental charge-parity-time reversal symmetry," said König. "The techniques could also be applied to other molecular ions, for which single particle control is envisioned."

The recent research by this team of researchers and the new techniques it introduced could be used in future studies probing the states of both matter and antimatter molecular systems. Ultimately, it could help to unveil deviations from the Standard Model, shedding light on the limitations of current physical predictions.

"Our future research plans will include applying the demonstrated techniques to high-precision spectroscopy of single molecular hydrogen ions in our Penning trap apparatus," added König. "This research will address either the hyperfine and Zeeman structure or the rovibrational level structure."

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