The advanced atomic clock makes a better dark matter detector

JILA researchers used a state-of-the-art atomic clock to narrow the search for elusive dark matter, an example of how continuous improvements in watches have value beyond timekeeping.

Older atomic clocks operating at microwave frequencies have already been looking for dark matter, but this is the first time that a newer clock, operating at higher optical frequencies, and an ultra-stable oscillator to ensure stable light waves, have been exploited. to set more precise search limits. The research is described in Physical Review Letters .

Astrophysical observations show that dark matter makes up most of the “things” in the universe, but has so far escaped capture. Researchers around the world have searched for it in various forms. The JILA team focused on ultralight dark matter, which in theory has a tiny mass (much less than a single electron) and a huge wavelength – how far a particle spreads out in space – which could be as large as its size. of dwarf galaxies. This type of dark matter would be linked by gravity to galaxies and therefore to ordinary matter.

Ultralight dark matter is expected to create tiny fluctuations in two fundamental physical “constants”: the mass of the electron and the fine structure constant. The JILA team used a strontium lattice clock and a hydrogen maser (a microwave version of a laser) to compare their well-known optical and microwave frequencies, respectively, to the frequency of light resonating in an ultra-stable cavity. composed of a single crystal of pure silicon. The resulting frequency ratios are sensitive to changes over time in both constants. Relative fluctuations in ratios and constants can be used as sensors to link cosmological dark matter models to accepted physical theories.

The JILA team set new limits on a plan for “normal” fluctuations, beyond which any unusual signals discovered later could be due to dark matter. The researchers bound the coupling force of the ultralight dark matter to the electron mass and fine structure constant to be on the order of 10.^-5 (1 in 100,000) or less, the most accurate measurement of this value.

JILA is jointly managed by the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder.

“Nobody actually knows at what level of sensitivity you will start seeing dark matter in lab measurements,” said Jun Ye, a NIST / JILA colleague. “The problem is that physics as we know it isn’t quite complete at this point. We know something is missing but we don’t know how to fix it yet.”

“We know that dark matter exists from astrophysical observations, but we don’t know how dark matter relates to ordinary matter and the values ​​we measure,” Ye added. “Experiments like ours allow us to test various theoretical models that people have put together to try to explore the nature of dark matter. By setting ever better boundaries, we hope to rule out some flawed theoretical models and eventually make a discovery in the future.”

Scientists aren’t sure whether dark matter is made up of oscillating particles or fields that affect local environments, Ye noted. The JILA experiments are aimed at detecting the “pull” effect of dark matter on ordinary matter and electromagnetic fields, he said.

Atomic clocks are the first probes for dark matter because they can detect changes in fundamental constants and are rapidly improving in accuracy, stability and reliability. Cavity stability was also a crucial factor in the new measurements. The resonant frequency of light in the cavity depends on the length of the cavity, which can be traced back to the Bohr radius (a physical constant equal to the distance between the nucleus and the electron in a hydrogen atom). The Bohr radius is also related to the values ​​of the fine structure constant and the mass of the electron. Therefore, changes in resonant frequency relative to transition frequencies in atoms may indicate fluctuations in these constants caused by dark matter.

The researchers collected data on the strontium / cavity frequency ratio for 12 days with the clock running 30% of the time, resulting in a data set of 978.041 seconds. The hydrogen maser data lasted 33 days with the maser running 94% of the time, with a record 2,826,942 seconds. The cavity’s hydrogen / frequency ratio provided useful electron mass sensitivity although the maser was less stable and produced noisier signals than the strontium clock.

JILA researchers collected dark matter research data during their recent demonstration of an improved time scale, a system that incorporates data from multiple atomic clocks to produce a single, highly accurate timing signal for distribution. As the performance of atomic clocks, optical cavities, and time scales improve in the future, frequency ratios can be reexamined with ever higher resolution, further extending the scope of dark matter research.

“Whenever you run an optical atomic time scale, there’s a chance to set a new limit or make a dark matter discovery,” Ye said. “In the future, when we can put these new systems into orbit, it will be the largest ‘telescope’ ever built to search for dark matter.”

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Funding was provided by NIST, the Defense Advanced Research Projects Agency and the National Science Foundation.

Document: CJ Kennedy, E. Oelker, JM Robinson, T. Bothwell, D. Kedar, WR Milner, GE Marti, A. Derevianko and J. Ye. Precision Metrology Meets Cosmology: Enhanced Constraints on Ultralight Dark Matter by Atomic Cavity Frequency Comparisons. Physical Review Letters. Published online 12 November 2020 DOI: 10.1103 / PhysRevLett.125.201302