In the search for dark matter — the mysterious, invisible substance that makes up more than 80% of matter in our universe — scientists and engineers are turning to a new ultra-sensitive tool: the optical atomic clock.
These clocks, which measure time by using an ultra-stable laser to monitor the resonant frequency of atoms, are now precise enough that if they ran for the age of the universe, they would lose less than one second. That stability also enables these devices to act as extremely sensitive quantum sensors that could be deployed into space to search for dark matter.
The challenge: The equipment required to operate such ultra-precise clocks — including lasers, electronics and coolers — can fill a large table or even a room. It would make launching them into space very expensive if not impossible.
Scientists working on a joint U.S. Department of Energy and Department of Defense project aim to miniaturize these elements to the size of a shoebox. After more than two years of work, the researchers — from the DOE’s Fermi National Accelerator Laboratory and the Massachusetts Institute of Technology Lincoln Laboratory — have reported initial promising results.
Fermilab researchers Hongzhi Sun and Pamela Klabbers test the chip at the test stand. Photo: Ryan Postel, Fermilab
Fermilab researchers designed and developed the compact electronics needed to control the voltages within the device, while MIT LL researchers are developing the tiny ion traps and corresponding photonics needed to build the clock. The chip designed by the Fermilab team is currently under testing at MIT LL.
“This is the first step toward a high-accuracy, small footprint atomic clock,” said Fermilab Microelectronics Division Director Farah Fahim, who leads the project for the lab.
A new way to detect dark matter
MIT LL’s optical atomic clock uses an ion trap as a sensor — in this case, a Strontium ion that is confined by an electrical field. A laser acts as the clock’s oscillator, measuring the oscillation frequency of the ion’s transition between two quantized energy levels.
This sort of compact atomic clock could be ideal for deployment to space to search for ultralight dark matter, which is theorized to cause oscillations in the masses of electrons. If several atomic clocks traveled through a clump of dark matter in space, the dark matter could increase or decrease the photon energy measured by each clock, changing how it “ticks.” The clocks would desynchronize as the dark matter passes and resynchronize thereafter.
Researchers conducted these experiments with GPS satellites, which each contain multiple atomic clocks based on a different technology. But they found no evidence for dark matter in these experiments. Perhaps, the researchers considered, dark matter could be detected with a more sensitive clock.
Graphic rendering of the chip. Image: Samantha Koch, Fermilab
Funded by the DOD, MIT LL researchers have miniaturized the trapped-ion atomic clock, integrating laser delivery and detection all onto one chip. But to complete the system, MIT LL researchers needed more than just miniaturized atomic and photonic components. They needed help designing a miniature electronic control system. That’s where Fermilab came in; DOE’s high-energy physics QuantISED program funded the electronics development and integration.
“We have more than 30 years of experience developing compact electronics for collider physics, and we have developed chips for extreme environments,” Fahim said. “That’s not unlike the electronics needed for controlling atoms and reading out their state.”
“It’s a project that really leverages the unique capabilities of different government laboratories,” said Robert McConnell, staff scientist at MIT LL who led development of the photonic ion trap chip for the project.
Integrating compact electronics with the ion trap
The difficulty lies in creating a small chip that can control the high voltages needed for the system — at least 20 volts — while both retaining high speed and utilizing low power. Working with a semiconductor manufacturer, the Fermilab team recently created a chip that could control up to nine volts. “It also has low voltage noise, so it won’t perturb the quantum state of the ion,” said Hongzhi Sun, the lead chip designer on the project.
Ready for testing: The chip’s custom test board is connected the test equipment. The chip is wire-bonded to the board and protected by the square white plastic cap. Photo: Ryan Postel, Fermilab
MIT LL researchers now look to integrate the chip with the ion trap through a technique that allows them to stack the two chips on top of each other and connect them through vias, or electrical connections between layers. Fermilab researchers will then continue to hone the electronics design to increase the voltage up to 20 volts. The goal is to create a compact atomic clock with frequency uncertainty of 10-18.
Other applications
“This collaboration allows us the benefits of both worlds,” said McConnell. “By having Fermilab design circuits and integrating them with our ion traps, we can create well-controllable quantum sensors.”
The clocks could be used beyond high-energy physics research, including in space defense or even as extremely sensitive sensors that could predict tsunamis or earthquakes. These ion traps could also form the basis for future quantum computers.
“There is a great disparity in the application goals for DOD and DOE but an equally compelling synergy in the underlying technology development; we simply need to find ways to work together,” Fahim said.
Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.
Scientists working on the Dark SRF experiment at the U.S. Department of Energy’s Fermi National Accelerator Laboratory have demonstrated unprecedented sensitivity in an experimental setup used to search for theorized particles called dark photons.
Researchers trapped ordinary, massless photons in devices called superconducting radio frequency cavities to look for the transition of those photons into their hypothesized dark sector counterparts. The experiment has put the world’s best constraint on the dark photon existence in a specific mass range, as recently published in Physical Review Letters.
“The dark photon is a copy similar to the photon we know and love, but with a few variations,” said Roni Harnik, a researcher at the Fermilab-hosted Superconducting Quantum Materials and Systems Center and co-author of this study.
The Dark SRF experiment demonstrated unprecedented sensitivity by using two SRF cavities as the key components for experiment. Photo: Reidar Hahn, Fermilab
Light that allows us to see the ordinary matter in our world is made of particles called photons. But ordinary matter only accounts for a small fraction of all matter. Our universe is filled with an unknown substance called dark matter, which comprises 85% of all matter. The Standard Model that describes the known particles and forces is incomplete.
In theorists’ simplest version, one undiscovered type of dark matter particle could account for all the dark matter in the universe. But many scientists suspect that the dark sector in the universe has many different particles and forces; some of them might have hidden interactions with ordinary matter particles and forces.
Just as the electron has copies that differ in some ways, including the muon and tau, the dark photon would be different from the regular photon and would have mass. Theoretically, once produced, photons and dark photons could transform into each other at a specific rate set by the dark photon’s properties.
Innovative use of SRF cavities
To look for dark photons, researchers perform a type of experiment called a light-shining-through-wall experiment. This approach uses two hollow, metallic cavities to detect the transformation of an ordinary photon into a dark matter photon. Scientists store ordinary photons in one cavity while leaving the other cavity empty. They then look for the emergence of photons in the empty cavity.
Fermilab researchers in the SQMS Center have years of expertise working with SRF cavities, which are used primarily in particle accelerators. SQMS Center researchers have now employed SRF cavities for other purposes, such as quantum computing and dark matter searches, due to their ability to store and harness electromagnetic energy with high efficiency.
“We were looking for other applications with superconducting radio frequency cavities, and I learned about these experiments where they use two copper cavities side-by-side to test for light shining through the wall,” said Alexander Romanenko, SQMS Center quantum technology thrust leader. “It was immediately clear to me that we could demonstrate greater sensitivity with SRF cavities than cavities used in previous experiments.”
This experiment marks the first demonstration of using SRF cavities to perform a light-shining-through-wall experiment.
Standing around the Dark SRF experiment from left to right are SQMS Center Director Anna Grassellino, SQMS Science Thrust Leader Roni Harnik and SQMS Technology Thrust Leader Alexander Romanenko. Photo: Reidar Hahn, Fermilab
The SRF cavities used by Romanenko and his collaborators are hollow chunks of niobium. When cooled to ultralow temperature, these cavities store photons, or packets of electromagnetic energy, very well. For the Dark SRF experiment, scientists cooled the SRF cavities in a bath of liquid helium to around 2 K, close to absolute zero.
At this temperature, electromagnetic energy flows effortlessly through niobium, which makes these cavities efficient at storing photons.
“We have been developing various schemes trying to handle the new opportunities and challenges brought in by this ultra-high-quality superconducting cavities for this light-shining-through-wall experiment,” said study co-author Zhen Liu, an SQMS Center physics and sensing team member from the University of Minnesota.
Researchers now can use SRF cavities with different resonance frequencies to cover various parts of the potential mass range for dark photons. This is because the peak sensitivity on the mass of the dark photon is directly related to the frequency of the regular photons stored in one of the SRF cavities.
“The team has done many follow-ups and cross-checks of the experiment,” said Liu, who worked on the data analysis and the verification design. “SRF cavities open many new search possibilities. The fact we covered new parameter regions for the dark photon’s mass shows their successfulness, competitiveness and great promise for the future.”
“The Dark SRF experiment has paved the way for a new class of experiments under exploration at the SQMS Center, where these very high Q cavities are employed as extremely sensitive detectors.” said Anna Grassellino, director of the SQMS Center and co-PI of the experiment. “From dark matter to gravitational waves searches, to fundamental tests of quantum mechanics, these world’s-highest-efficiency cavities will help us uncover hints of new physics.”
Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.
The creative ideas of Fermilab research fellow Marco Del Tutto have brought neutrino detection in the Short Baseline Near Detector to the next level with innovations, such as removing unnecessary shielding; exploiting the connection between neutrino energy and neutrino direction when the neutrinos are produced in a beam; and researching an idea to add magnetic fields to the detector.
Del Tutto is this year’s recipient of the Universities Research Association Tollestrup Award for Postdoctoral Research. He was chosen for his “innovative contributions to searches for new physics using liquid argon detectors.” This annual award, named after Fermilab scientist Alvin Tollestrup, recognizes outstanding work by postdoctoral scientists that is either conducted at Fermilab or with Fermilab scientists.
“I often observe that the best Tollestrup nomination packets read like a particularly strong tenure case, and Marco’s is no exception,” said Michael Mulhearn, chair of the URA Tollestrup Award Committee.
Looking at neutrinos from a new angle
Del Tutto’s history with Fermilab goes back nearly a decade. He participated in a summer program in 2014 during his master’s degree education at the University of Rome. That summer, he was assigned to work on the NOvA experiment.
Fermilab research fellow Marco Del Tutto received this year’s URA Tollestrup Award for Postdoctoral Research. Photo: Ryan Postel, Fermilab
“That’s how I started neutrino physics, and I loved it from that moment,” said Del Tutto. “I love the collaboration of the people, I love the physics, and so I decided to continue in this field.”
He then earned his doctoral degree at the University of Oxford with work on the MicroBooNE experiment, where he made the first measurements of the experiment’s muon neutrino interactions.
After six months as a postdoctoral fellow at Harvard University, Del Tutto came back to Fermilab in 2019 as a Lederman postdoctoral fellow. In the years since, his focus shifted from MicroBooNE to the upcoming SBND. This detector, one of three that make up Fermilab’s Short-Baseline Neutrino program, will be the closest to the neutrino source, the Booster Neutrino Beam. The three detectors will measure how neutrinos change flavor as they travel through space.
“SBND is going to have unprecedented statistics, lots of neutrino events to look at, a lot of physics to do,” Del Tutto said.
Del Tutto applied a technique, called Precision Reaction Independent Spectrum Measurement, or PRISM, to SBND, using the same detector to sample neutrinos at many different energy distributions at the same time. It can be done by observing neutrinos that approach the detector at different angles from the neutrino beam’s axis.
“Marco Del Tutto has pioneered the Short-Baseline Neutrino Detector program through his contributions to the SBND-PRISM technique,” said URA president John Mester. “His creativity, ingenuity and innovation have also helped advance the cosmic ray background analysis.”
The neutrino beam isn’t perfectly focused, and the neutrinos can hit the detector at different angles, with the energy distribution of the neutrino depending on its angle. As a result, the detector can simultaneously measure a variety of neutrinos with different energy levels at different locations on the detector.
“You have this for free because you’re not moving the detector, you’re not changing the detector, you’re just looking at different regions of the detector itself,” Del Tutto said.
Innovative techniques and experiments, such as PRISM, are crucial to the search for signs of new information beyond the Standard Model, the current best understanding of particle physics, according to Del Tutto.
“We study neutrino interactions, we study neutrino oscillations, but on top of that, we need to find what is missing in the Standard Model,” he said. “Now, we have a lot of new theories that we can probe, and it turns out that we can probe many of those with the detectors that we already have. We need to be creative and come up with new analysis techniques that will allow us to explore these new physics models.”
To make the most out of PRISM, Del Tutto founded a collaboration between experimentalists and theorists to come up with ideas of what to look for with this technique. These objectives include looking for possible dark matter particles and sterile neutrinos, a hypothetical new type of neutrino.
“It’s very easy for this type of communication between experimentalists and theorists to be lacking or not there at all,” Del Tutto said. “Starting this from the beginning makes everything easier: We can provide to the theorists what we can do with the experiments, and they can identify theories that we are actually able to test with the experiment.”
Sorting the positive from the negative
Another idea Del Tutto has developed to improve detectors’ capabilities is incorporating a magnetic field into liquid argon time projection chambers, or LArTPCs.
LArTPCs are full of highly pure argon that is kept cold enough to be a liquid. Neutrinos interact with other matter very rarely because they have no charge, making them impossible to see themselves. However, a LArTPC can detect the charged particles that are the product of a collision between a neutrino and an argon atom, such as a proton and a muon in the case of muon neutrinos. When that happens, the charged particles unleash electrons along their trajectories, which are then forced via an electric field to drift sideways to a series of wires on one side of the detector. When the electrons interact with the wires, they send signals that come together into a 3-D reconstruction of the neutrino-argon collision.
LArTPC detectors can reveal almost everything about a neutrino collision except what charge each particle has, such as the difference between an electron and a positron. But if a magnetic field is added to the detector, these oppositely charged particles will curve in opposite directions. The curved path of the particles would also depend on the energy of the particle, making it possible to measure the particle’s momentum. To take advantage of this phenomenon, Del Tutto looked for a way to add a magnetic field to a LArTPC.
“What will really make LArTPCs the ultimate detector, not only for neutrino physics but also for many other analyses, is the addition of a magnetic field,” he said.
Adding a magnetic field would also make it easier to detect antimuons that would be produced by switching the neutrino beam to an antineutrino beam. The magnetic field would help distinguish antimuons from the background noise of muons. This technique can also separate pairs of electrons and positrons that overlap, which is one predicted sign of some new particles.
Del Tutto’s creative work with neutrino detectors hasn’t just benefited the science itself. In 2020, he figured out a way to streamline the construction of SBND.
Detectors like SBND are prone to background noise due to the constant flow of cosmic rays, such as muons and neutrons, that rocket through Earth’s atmosphere from space. Instead of putting the detector far below the Earth’s surface — an expensive solution — the plan was to keep the detector on the surface and cover it in three meters of concrete.
However, when Del Tutto joined the experiment in 2019, how effective those three meters of concrete would be with cosmic interference. He ran simulations to see how cosmic rays would interact with the materials in the detector. His study confirmed that muons from cosmic rays pass through the concrete anyways, and neutrons are so low energy that they produce identifiable signals that can be ignored.
“Three meters of concrete will not stop muons coming from cosmic radiation: you will need hundreds of meters to stop a muon,” Del Tutto said.
A benefit of not using the concrete when it’s not needed, apart from the lower cost, is to ensure that the detector is more easily accessible for repairs.
Del Tutto brought his results to the Short-Baseline Neutrino program’s oversight board, and the decision was made to remove the concrete shielding from the SBND’s design.
“Marco has made remarkably impactful contributions to his experiments, through widespread contributions that range from the highly practical, such as showing that expensive concrete shielding would be ineffective, to the highly ambitious, such as magnetizing a LArTPC,” Mulhearn said. “Marco’s pioneering efforts with the SBND-PRISM technique have made the SBND physics program dramatically more ambitious.”
As of early June, Del Tutto started a new position at Fermilab as a Wilson fellow. He continues to work on the magnetized LArTPC project, with plans to use a magnet on a small detector to test the concept on a small scale. Beyond that, further work with SBND is on the horizon for Del Tutto when construction on the detector is completed.
“We’re going to have data from SBND pretty soon, so I’m looking forward to analyzing it,” he said. “I’m mainly interested in neutrino physics, but my main focus will be what kind of new models and new physics we can explore with this data. It’s going to be really exciting.”
Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.