Advanced atomic clocks make it a better dark matter detector


PICTURE: A cartoon depicting a clock looking for a view of dark matter more

Credit: Hanacek / NIST

JILA scientists have used state-of-the-art atomic clocks to narrow the search for elusive dark matter, an example of how the continuous improvement of clocks beyond time data has value.

Older atomic clocks operating at microwave frequencies used to hunt dark matter, but this is the first time that new clocks have been used that operate at higher optical frequencies and an ultra-stable oscillator that provides stable light waves. to search. The research is described in Letters of physical control .

Astrophysical observations show that dark matter makes up most of the “things” in the universe, but has so far escaped capture. Scientists from all over the world have been looking for it in various forms. The JILA team focused on ultralight dark matter, which theoretically has a small mass (much smaller than one electron) and a huge wavelength – how far the particles propagate in space – which could be as large as the size of dwarf galaxies. This type of dark matter would be bound by gravity to galaxies, and therefore to ordinary matter.

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

The JILA team set new limits for “normal” fluctuations, beyond which any unusual signals discovered later could be caused by dark matter. Scientists have limited the binding power of ultralight dark matter to electron matter and the fine structure constant to the order of 10-5 (1 in 100,000) or less, the most accurate measurement of this value in history.

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

“No one really knows what level of sensitivity you will start to see laboratory measurements of dark matter,” said NIST / JILA member Jun Ye. “The problem is that physics as we know it is not completely complete at the moment. 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 connects with ordinary matter and the values ​​we measure,” Ye added. “Experiments like ours allow us to test different models of theory that people have put together to try to explore the nature of dark matter. By setting better and better boundaries, we hope to rule out some wrong models of theory and eventually discover them in the future. ”

Scientists are not sure whether dark matter consists of particles or oscillating fields that affect the local environment, Ye noted. JILA’s experiments are designed to detect the “tensile” effect of dark matter on ordinary matter and electromagnetic fields, he said.

Atomic clocks are the main probes of dark matter because they can detect changes in fundamental constants and improve rapidly in accuracy, stability, and reliability. Cavity stability was also a decisive factor in the new measurements. The resonant frequency of light in a 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 a nucleus and an electron in a hydrogen atom). Bohr’s radius is also related to the values ​​of the fine structure constant and the mass of the electrons. Therefore, changes in the resonant frequency compared to the transient frequencies in the atoms may indicate fluctuations in these constants due to dark matter.

The researchers collected data on the strontium / cavity frequency ratio for 12 days with a clock running 30% of the time, resulting in a 978,041 second data set. The hydrogen massager data lasted 33 days and the masseur ran 94% of the time, resulting in a record of 2,826,942 seconds. The hydrogen / cavity frequency ratio provided useful sensitivity to the electron mass, although the maser was less stable and produced noisier signals than strontium clocks.

JILA scientists collected data on the search for dark matter during their recent demonstration of an improved time scale – a system that includes data from multiple atomic clocks to create a single, highly accurate time signal for distribution. As the performance of atomic clocks, optical cavities, and time scales improves in the future, frequency ratios can be re-examined with ever-increasing resolution, further expanding the range of dark matter search.

“Every time someone uses an optical atomic time scale, there is a chance to set a new boundary or make a discovery of dark matter,” 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.”


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

Contribution: CJ Kennedy, E. Oelker, JM Robinson, T. Bothwell, D. Kedar, WR Milner, GE Marti, A. Derevianko and J. Ye. Precision Metrology Meets Cosmology: Improved Constraints on Ultralight Dark Matter from Atom-Cavity Frequency Comparisons. Letters of physical control. Published online November 12, 2020. DOI: 10.1103 / PhysRevLett.125.201302

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