Dark matter will help explain not only how our universe was made, but also its components. It will help people understand fundamental physics and how additional particles and forces exist.

Quantum sensing drives particle detection innovation

The research team used two distributed sensors and leveraged two quantum resources for increasing detection sensitivity: squeezing, characterized by reduced quantum noise below the classical optical limit; and entanglement, characterized by quantum correlations between optical beams.

"We wouldn't be able to reach the sensitivity limits required to detect dark matter classically," said Marvinney. "We need a quantum advantage, so we're using these two resources to improve our sensor, demonstrating proof-of-principle improvements."

The team's findings successfully demonstrate the use of squeezing and entanglement as resources for improved quantum enhancement within a distributed network of quantum sensors.

By using light to detect these tiny mechanical movements, the distributed sensing scheme uses optomechanical systems to allow for the measurement of the average signal from multiple independent sensors, enabling a quantum-enhanced detection of their collective motion - thereby improving the sensitivity of the system to distributed signals that interact with all sensors.

The results will advance sensitive phase measurement efforts, such as the ongoing search for dark matter via direct detection, while paving the path toward these ultrasensitive measurements with proof-of-principle experiments that replicate dark matter interactions.

"With a two-mode, squeezed light source, the initial two modes are entangled," said Marvinney. "We can start with entanglement directly from the source and build from there, leveraging that source, so that's the novel new approach that we're using."

There may be different types of dark matter particles that may exist within a range of energy scales, and theoretical physicists don't always agree on post-Big Bang particle origin theories. As part of this work, Marvinney and Marino are developing the techniques that will be required to search for ultralight mass dark matter - one of the two dark matter candidates proposed to be sensitive - through a fifth force interaction with an array of optomechanical sensors, as in the 2022Snowmass Windchime white paper. This dark matter candidate could be as light as 10 billionths of a trillionth of the mass of an electron.

"Ultralight mass dark matter is like a wave, and if you have a lot of sensors, they will interact collectively with this dark matter wave and see the same signal, in the sense that they are all measuring the same signal, and the readout is an average measurement of all the sensors," said Marino, discussing the high density of bosonic dark matter with ultralight mass. "When we have conditions like these, we're looking at approaches where we can leverage quantum resources such as entanglement to make more accurate measurements."

Adding measurement precision with quantum light

This search for infinitesimal particles requires exacting instruments and considerable patience. Interferometers are precise measurement tools that can discern fluctuations in the interference between waves caused by the motion of one of the interferometer mirrors, enabling researchers to make conclusions about the particles or fields that interact with the mirror.

Quantum-enhanced sensing approaches based on squeezing and entanglement add precision to a measurement, making them more powerful than what is possible with classical measurement techniques. By showcasing the benefits of squeezing and the additional benefit of entanglement in the probing light of the sensor, scientists can improve the sensitivity of the joint measurement and detect faint signals that were previously unobservable for a better understanding of particle behavior and our physical world.

"The dark matter signal is expected to be so tiny that we need every advantage we can get," said Marvinney. "Not only do we want to take advantage of a giant array that's all going to detect this, but we want to utilize the types of quantum noise reduction and entanglement that we can add to the detection, so we can reduce the noise and enhance our measurement."

Dark matter searches are comparable to the mapping of a sea floor into grid patterns amid a search for a lost seafaring vessel, but each experiment is only searching within a single square on a vast grid. Internationally, many researchers are working within different squares on the same grid, which is gradually knit together as discoveries are unveiled. Over time, observed regions are eliminated as possible locations where a missing ship, or the elusive dark matter particles, could be found.

However, even finer dividing lines within a single examined grid square can enable observation of yet smaller features - akin to the broken-up pieces of a missing vessel - where before, each individual piece had been too small to observe. By improving the sensitivity of their systems using quantum noise reduction via the nonlinear interferometer, the team effectively "zoomed in" for a better view of the examined grid square.

This meticulous, reductive process clarifies even smaller features within the tiny set quadrant of "ocean floor" that the team is scanning, thus enabling an observation of features that was not previously possible within this once ruled-out region. Adding entanglement allows the researchers to further improve the signal-to-noise ratios of their measurements, which can reveal weaker signals, or as in the seafaring vessel analogy, even smaller components that have broken away from the missing ship.

"The two-mode squeezed state that we use in our experiments is comprised of two entangled optical beams," said Marino. "The entanglement, or quantum correlations, between them lead to squeezing, or reduced noise, when joint measurements are made. We can leverage the squeezing property to reduce the noise floor of the measurement - and make it more sensitive - by probing optomechanical sensors with the two-mode, squeezed states."

"For an array of sensors," Marino added, "there are approaches that probe each sensor separately with independent quantum states of light to obtain a quantum enhancement via reduced noise. It is also possible to leverage the entanglement, or quantum correlations between multiple - in our case, optical beams - to perform a collective measurement of multiple sensors and obtain a larger enhancement."

Whether it reveals the presence of unforeseen particles, corrects our understanding of gravity, or initiates a new understanding of black hole phenomena, the discovery of dark matter is sure to make an enormous impact. It is expected to unlock mysteries from astronomy and galaxies to the seemingly infinite particles that move among them, unveiling what governs their behavior, altering the standard physics model and rewriting our collective understanding of the universe.

"The very long-term goal is dark matter detection," said Marino. "What we're doing now is developing the necessary techniques - proof-of-principle, fundamental experiments to set the foundation."

This initial, "mapping the sea floor" approach is a tedious and time-consuming effort, but with each advancement of the measuring tools and techniques - like squeezing and entanglement - sensing capabilities are improved, and search sensitivities escalate in anticipation of a discovery with major implications for the study of physics.

"Dark matter will help explain not only how our universe was made, but also its components," said Marvinney. "It will help people understand fundamental physics and how additional particles and forces exist."

This study also involved contributions from researchers at the Korea Research Institute of Standards and Science (Daejeon), Yonsei University (Seoul) and the Korea Institute of Science and Technology (Seoul).

Research funding was provided in part by Phase I of Quantum Science Center at ORNL, the DOE Office of High Energy Physics' QuantISED program, and grants from the Korean government. ORNL continues to empower the pursuit of quantum innovation, advancing world-leading scientific discovery to enable a quantum revolution that promises to transform a vast range of technologies critical to American competitiveness. These traits are embodied by ORNL's celebration of the International Year of Quantum Science and Technology in 2025. Click here to learn more about quantum science at ORNL.

UT-Battelle manages ORNL for the Department of Energy's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science. - Chris Driver