Previous work had measured radium monofluoride (RaF) using large accelerator facilities like CERN's ISOLDE and MIT's super collider techniques, but those studies worked with fast-moving, room-temperature molecules . The Caltech experiment is the first to produce radium molecules that are cold and slow enough to be laser-cooled and trapped — essential prerequisites for the ultra-precise measurements needed to search for new physics.

Why radium is special: a pear-shaped nucleus

Radium's nucleus is pear-shaped (octupole-deformed) . While most atomic nuclei are spherical or slightly elongated, a pear-shaped nucleus is inherently asymmetric. This deformation dramatically amplifies sensitivity to symmetry-violating effects — on the order of 1,000 times greater than spherical nuclei — that could explain why matter dominated over antimatter in the early universe .

When combined with the enhancement that molecules provide over single atoms (another factor of roughly 1,000), radium molecules become extraordinarily powerful probes of physics beyond the Standard Model . They are uniquely sensitive to subtle violations of fundamental symmetries — such as time-reversal symmetry — that would manifest as measurable electric dipole moments (EDMs) or nuclear Schiff moments.

How researchers overcame the handling challenges

Radium-226 presents a triple challenge: it is highly radioactive (about one million times more radioactive than the same mass of uranium ), chemically reactive, and available in only tiny quantities. The team started with approximately 1 mg of ²²⁶Ra nitrate salt — an amount that represents about 1 mCi of activity .

The researchers developed a multi-step, tabletop-friendly solution:

• Novel target production: They dissolved the radium salt, drop-cast microgram-sized aliquots onto gold foil, and paired them with solid reagent targets (e.g., calcium hydroxide for RaOH) inside a copper cell .
• Laser ablation in cryogenic buffer gas: A pulsed laser vaporized the reagents, and the resulting radicals reacted in the gas phase inside a cell held at roughly 4 K by a helium buffer gas . The cold buffer gas immediately thermalized the molecules, slowing them down.
• Optically driven chemistry: For RaOH, the reaction Ra + H₂O → RaOH + H was driven by laser ablation, with molecules detected via laser-induced fluorescence at extremely low background levels .
• Safety and containment: The entire apparatus is compact (under 1 m tall) and sealed, keeping the radioactivity contained while allowing laser access for spectroscopy .

The process has been likened to making candy: the target plate acts like a lollipop stick, the radium salt like sugar, and the cold buffer gas like the cooling step that solidifies the final product . The methods enable rapid prototyping in a university laboratory setting using only microgram quantities of radium .

Next steps: from cold molecules to quantum sensors

The measured molecular spectra confirm that RaOH and RaF have electronic structures compatible with laser cooling, enabling the next phase of research . The planned experimental steps include:

• Laser cooling and optical trapping: Producing ultracold, trapped samples of radium molecules, which would dramatically extend observation times and experimental sensitivity .
• Quantum precision measurements: Using these trapped molecules to perform high-precision searches for electric dipole moments (EDMs) and nuclear Schiff moments — signals that would indicate the existence of new particles and forces .
• Extension to other exotic nuclei: The methodology is designed to be generalizable to other rare, octupole-deformed nuclei such as radon and protactinium, opening a broader program of molecular quantum sensing .

The bigger picture

The ability to create and study cold radium molecules in a tabletop experiment represents a significant shift in how physicists can search for physics beyond the Standard Model. Previously, such investigations required large accelerator facilities. Now, many of the same questions can be pursued in a university lab — safely, rapidly, and with extraordinary sensitivity.

As Nick Hutzler, the project lead, has noted, these molecules combine nearly every known experimental enhancement for symmetry-violation searches in a single system: molecular enhancement, an octupole-deformed nucleus, high polarizability, and compatibility with laser cooling . If any detectable violation exists at the levels predicted by many beyond-Standard-Model theories, these cold radium molecules should find it.

Previous work had measured radium monofluoride (RaF) using large accelerator facilities like CERN's ISOLDE and MIT's super collider techniques, but those studies worked with fast-moving, room-temperature molecules . The Caltech experiment is the first to produce radium molecules that are cold and slow enough to be laser-cooled and trapped — essential prerequisites for the ultra-precise measurements needed to search for new physics.

Why radium is special: a pear-shaped nucleus

Radium's nucleus is pear-shaped (octupole-deformed) . While most atomic nuclei are spherical or slightly elongated, a pear-shaped nucleus is inherently asymmetric. This deformation dramatically amplifies sensitivity to symmetry-violating effects — on the order of 1,000 times greater than spherical nuclei — that could explain why matter dominated over antimatter in the early universe .

When combined with the enhancement that molecules provide over single atoms (another factor of roughly 1,000), radium molecules become extraordinarily powerful probes of physics beyond the Standard Model . They are uniquely sensitive to subtle violations of fundamental symmetries — such as time-reversal symmetry — that would manifest as measurable electric dipole moments (EDMs) or nuclear Schiff moments.

How researchers overcame the handling challenges

Radium-226 presents a triple challenge: it is highly radioactive (about one million times more radioactive than the same mass of uranium ), chemically reactive, and available in only tiny quantities. The team started with approximately 1 mg of ²²⁶Ra nitrate salt — an amount that represents about 1 mCi of activity .

The researchers developed a multi-step, tabletop-friendly solution:

• Novel target production: They dissolved the radium salt, drop-cast microgram-sized aliquots onto gold foil, and paired them with solid reagent targets (e.g., calcium hydroxide for RaOH) inside a copper cell .
• Laser ablation in cryogenic buffer gas: A pulsed laser vaporized the reagents, and the resulting radicals reacted in the gas phase inside a cell held at roughly 4 K by a helium buffer gas . The cold buffer gas immediately thermalized the molecules, slowing them down.
• Optically driven chemistry: For RaOH, the reaction Ra + H₂O → RaOH + H was driven by laser ablation, with molecules detected via laser-induced fluorescence at extremely low background levels .
• Safety and containment: The entire apparatus is compact (under 1 m tall) and sealed, keeping the radioactivity contained while allowing laser access for spectroscopy .

The process has been likened to making candy: the target plate acts like a lollipop stick, the radium salt like sugar, and the cold buffer gas like the cooling step that solidifies the final product . The methods enable rapid prototyping in a university laboratory setting using only microgram quantities of radium .

Next steps: from cold molecules to quantum sensors

The measured molecular spectra confirm that RaOH and RaF have electronic structures compatible with laser cooling, enabling the next phase of research . The planned experimental steps include:

• Laser cooling and optical trapping: Producing ultracold, trapped samples of radium molecules, which would dramatically extend observation times and experimental sensitivity .
• Quantum precision measurements: Using these trapped molecules to perform high-precision searches for electric dipole moments (EDMs) and nuclear Schiff moments — signals that would indicate the existence of new particles and forces .
• Extension to other exotic nuclei: The methodology is designed to be generalizable to other rare, octupole-deformed nuclei such as radon and protactinium, opening a broader program of molecular quantum sensing .

The bigger picture

The ability to create and study cold radium molecules in a tabletop experiment represents a significant shift in how physicists can search for physics beyond the Standard Model. Previously, such investigations required large accelerator facilities. Now, many of the same questions can be pursued in a university lab — safely, rapidly, and with extraordinary sensitivity.

As Nick Hutzler, the project lead, has noted, these molecules combine nearly every known experimental enhancement for symmetry-violation searches in a single system: molecular enhancement, an octupole-deformed nucleus, high polarizability, and compatibility with laser cooling . If any detectable violation exists at the levels predicted by many beyond-Standard-Model theories, these cold radium molecules should find it.