Study demonstrates a new method to search for meV dark photons

Just as the sound of a guitar depends on its strings and the materials used for its body, the performance of a quantum computer depends on the composition of its building blocks. Arguably the most critical components are the devices that encode information in quantum computers.

One such device is the transmon qubit — a patterned chip made of metallic niobium layers on top of a substrate, such as silicon. Between the two materials resides an ultrathin layer that contains both niobium and silicon. The compounds of this layer are known as silicides (Nb_xSi_y). Their impact on the performance of transmon qubits has not been well understood — until now.

Silicides form when elemental niobium is deposited onto silicon during the fabrication process of a transmon qubit. They need to be well understood to make devices that reliably and efficiently store quantum information for as long as possible.

Researchers at the Superconducting Quantum Materials and Systems Center, hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory, have discovered how silicides impact the performance of transmon qubits. Their research has been published in APS Physical Review Materials.

An unexpected signal

Carlos Torres-Castanedo was analyzing the materials of a transmon qubit using x-rays, when he came across a peculiar signal.

“I thought the signal came from a surface oxide, because that’s just what usually happens,” said Torres-Castanedo, a doctoral candidate in materials science at Northwestern University. “After spending a day trying to fit the data to match an oxide, the only possibility was to introduce a niobium silicide layer. When the data beautifully fit the model, I showed the results to my co-workers, and we all became excited about what this could mean for transmon qubit performance.”

The SQMS Center researchers dug deeper. They identified the types of silicides present, the thickness of the layer — typically only a few nanometers thick — and its physical and chemical structure. After completing these measurements, they focused on figuring out how these compounds affect the performance of qubits.

The researchers simulated different types of silicides. Not only did they find that silicides are detrimental to the performance of transmon qubits, but they also found that some are more detrimental than others.

Impact on coherence time

Qubits are the basic and fragile units of information that a quantum computer uses to perform calculations. They are physically encoded through transmon qubits.

Similar to a street performer plucking an A note on a guitar string and allowing the tone to ring out before it becomes obscured by street noise, quantum information in a transmon qubit exists for a limited time before it dissipates or is obscured by environmental noise. This time span is known as the coherence time. The longer the coherence time, the better the performance of the transmon qubit.

“This interface will never be like silicon stop, niobium start,” said SQMS Center researcher James Rondinelli, Walter Dill Scott Professor of Materials Science and Engineering at Northwestern University. “The first observation was that there is not an atomically sharp interface, but rather a compositional gradient between the silicon substrate —which is the platform for the system — and the niobium.”

With that observation, Rondinelli and his group began a detailed computational study as part of a greater SQMS Center effort to improve qubit coherence times.

Simulations with a supercomputer

With a newfound curiosity about what the presence of silicides could mean for transmon qubits, the researchers used a supercomputer at the National Energy Research Scientific Computing Center, located at the DOE’s Lawrence Berkley National Laboratory.

Think of silicides as a thin material inside the street performer’s guitar that affects the sound of the guitar string. Researchers studying transmon qubits are essentially trying to isolate an A note and seeing to what extent the hidden material interferes.

Some silicides, for example, have magnetic properties that can interfere with the quantum information that rings out from the transmon qubit. The stronger the magnetism, the more the quantum information is obscured.

Through simulations, researchers found that the silicide compound Nb₆Si₅ does not have any magnetic properties, while Nb₅Si₃ introduces magnetic noise. If silicides will always be present in transmon qubits, whether researchers like it or not, Nb₆Si₅ is less detrimental, and scientists will have to make do.

“To really push the field forward, you have to embrace a little bit of an outsider perspective to make an advancement, and we’re optimistic our multidisciplinary approach will solve this challenge.” – James Rondinelli, SQMS Center researcher

“I find it interesting how the research on the properties of these silicides have been studied since the ’80s, but never have been understood in a nanometer-sized film,” said Torres-Castanedo. “I feel proud that I was able to work alongside my fellow researchers to conduct this important study.”

These findings by themselves are significant. In the greater context of the SQMS Center’s aim to develop a state-of-the-art quantum computer, however, the results have much further implications than just understanding the properties of materials.

“The community who’s worked on superconducting qubits has traditionally been quantum physicists and engineers. The reason the SQMS Center has been so successful is they’ve embraced material scientists,” said Rondinelli. “To really push the field forward, you have to embrace a little bit of an outsider perspective to make an advancement, and we’re optimistic our multidisciplinary approach will solve this challenge.”

The Superconducting Quantum Materials and Systems Center at Fermilab is supported by the DOE Office of Science.

The Superconducting Quantum Materials and Systems Center is one of the five U.S. Department of Energy National Quantum Information Science Research Centers. Led by Fermi National Accelerator Laboratory, SQMS is a collaboration of 24 partner institutions — national labs, academia and industry —working together to bring transformational advances in the field of quantum information science. The center leverages Fermilab’s expertise in building complex particle accelerators to engineer multiqubit quantum processor platforms based on state-of-the-art qubits and superconducting technologies. Working hand in hand with embedded industry partners, SQMS will build a quantum computer and new quantum sensors at Fermilab, which will open unprecedented computational opportunities. For more information, please visit sqmscenter.fnal.gov.

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.

Imagine running your fingers along a standing wave’s oscillating momentum and feeling a powerful sensation that your brain tries to interpret. As you touch “Cubic Entanglement” — a visual representation of a stationary wave’s harmonies — your eyes also marvel at how universal sounds could translate into a physical piece of art. The transfer of music to art is an example of Ricardo Mondragon’s life work, an opus that he will now continue at the U.S. Department of Energy’s Fermi National Laboratory as the lab’s 2023 artist-in-residence.

Mondragon is a visual artist who takes inspiration from harmonies in physics and transforms them into materials. He translates frequencies, vibrations and rhythms into visual works of art, such as a sculpture, paintings or installations. Born and raised in Mexico City with an interest in the arts, Mondragon eventually graduated with a Bachelor of Arts in Music Composition from Columbia College Chicago. He has exhibited his work at the Hyde Park Art Center and the Design Museum of Chicago. Mondragon has lectured at places like the Museum of Contemporary Art Chicago and the Chicago Symphony Center. He has also created a live projection for the Louisville Orchestra. For nearly 10 years, the 38-year-old artist has combined the scientific wonders of the world with art.

“Everything that I do involves physics, that’s just part of the process,” said Mondragon. “The main things my work focuses on are harmonies, which happen everywhere. It happens in physics when you align waveforms.”

These waveforms are the foundation Mondragon uses to build his visual masterpieces.

An intricate process

Each of his works displays harmonic interactions, including ratios, intervals, chords, standing waveforms, frequency entanglements, and Chladni forms in three dimensions — a technique discovered by physicist Ernest Chladni that visualizes acoustics when sand moves on a metal plate. In his opinion, one of his strongest pieces is his 2022 piece, called “Cubic Entanglement,” which captures a perfect 3D representation of the Chladni plate. While this piece has a curvilinear internal body with many different facets and intricacies within and outside of it, it serves as a prime example of his previous and future work.

“For me, music is aligned physics. So, when I do a three-dimensional chord with sine waves, they behave in ratios and tonalities that are aligned frequencies,” Mondragon said. “So that’s why music works. It can make harmonic structures look very beautiful to the eye, especially when they don’t have friction.”

“I hope my art helps convey the message that physics is the language of the universe, just like mathematics or music.” – Ricardo Mondragon

Once inspired by a fractal or waveform interactions, Mondragon takes up to a year to create a piece that follows a multi-step process. First, he starts with sculpting and shaping sounds with digital or analog signal processing techniques through waveform generators — a tool that creates electrical waveforms into various wave shapes: sine, pulse, triangular and squares. After this, he hones in on the details of the waveforms, looks for their harmonies, and then transfers them into a three-dimensional shape with various 3D software programs. After creating the digital models and reshaping the files, he will lastly pick the material that best suits how he wants to portray his work, which also dictates how it’s created, via manual tools, 3D printing, routers, cast or any other process that suits the form.

For example, Mondragon created “Harmonic Interactions” with an aluminum material that underwent a manual casting process and handcrafted “Cubic Entanglement” with a CNC machine–generally used for computerized manufacturing processes. Another piece he added to his unique collection was a 3D-printed sculpture called “The Fourth,” from the musical interval.

Art and physics intersect at Fermilab

Initiated by the Fermilab Art Gallery in 2014, the artist-in-residence program is now in its eighth year. The program aims to foster new artists through mentorship and allow them to build new connections to translate a sector of science into creative and inspiring work to share with the public. A committee of seven comprised of scientists, administrators and artists reviews the submitted applications. Initially, in the program’s first year, the committee received 20 submissions, but this year, the application pool grew to 75 submissions.

“This cross-disciplinary aspect of learning is beneficial because everyone comes from different points of view,” said Georgia Schwender, founder and director of the artist-in-residence program. “Professions can grow by just listening to one another and sharing such work with the community.”

Schwender is most excited to see how Mondragon visualizes particle physics because of the many parallels his past work has with higher-energy physics.

“I just loved his different use of materials and the imagery he does with paper,” she said. “It is so exciting, and I just loved his enthusiasm and his dedication to art and communication.”

From Jan. 1 to Dec. 31, Mondragon will embark on an interdisciplinary journey with the artist-in-residence program that will give him the freedom and creative space to create new work while also helping him to suit his needs.

Currently, the artist is sculpting a piece made of birch wood. He also is working on some large-scale screenprints, which is a common printing process of applying ink to a mesh material. Humbled and excited about his year-long residency, he hopes to follow his heart and speak to the hearts of others.

“I hope my art helps convey the message that physics is the language of the universe, just like mathematics or music,” said Mondragon. 

The Fermilab artist-in-residence program is funded by the Fermi Research Alliance.

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.

Deep below the surface in South Dakota, construction crews have been working tirelessly to carve out a network of caverns and tunnels that one day will house a huge neutrino experiment. Their efforts are paying off: With almost 400,000 tons of rock extracted from the earth, the excavation is now half complete.

Once finished, the Long-Baseline Neutrino Facility will be the site of the international Deep Underground Neutrino Experiment. DUNE will focus on studying neutrinos, elusive particles that may hold the answers to many of the universe’s mysteries, such as why our universe is made of matter and how black holes and neutron stars are born. More than 1,000 scientists and engineers from over 30 countries are a part of LBNF/DUNE.

LBNF will provide the space, infrastructure and particle beam for DUNE, hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory. It includes underground caverns for a near detector at Fermilab, about 40 miles west of Chicago, and a far detector located 800 miles away at the Sanford Underground Research Facility in South Dakota.

The new underground area at SURF will consist of three large caverns. Two will measure around 500 feet long, 65 feet wide and 90 feet high. These will provide space to house four detector modules — each filled with 17,000 tons of ultrapure liquid argon. The third will be around 625 feet long, 65 feet wide and 36 feet tall and contain cryogenic support systems, detector electronics and data acquisition equipment.

To create these caverns, a total of approximately 800,000 tons of rock will be excavated and moved to the surface. Once complete, the footprint of the underground area with the three caverns will cover about the size of eight soccer fields.

Thyssen Mining Inc., the company contracted to carry out the excavation, began the underground work at SURF in 2021. This January, construction crews reached a critical milestone: 50% completion.

“We have excavated roughly 395,000 tons,” said Ryan Moe, the U.S. general manager at Thyssen Mining. “It’s going well.”

Careful, painstaking work

The first half of the excavation involved several important steps: mobilizing large equipment underground; creating a ventilation shaft; carefully creating a network of tunnels known as drifts; and excavating enormous caverns.

Moving all the necessary equipment underground was no simple task. It involved taking the underground construction mining equipment apart, lowering the parts a mile below the ground through a narrow shaft, then reassembling the construction machinery underground. It was a process that “required a lot of time,” said Michael Gemelli, the LBNF Far-Site Conventional Facilities project manager.

“We have excavated roughly 395,000 tons. It’s going well.” – Ryan Moe, the U.S. general manager at Thyssen Mining

One of the pieces of equipment brought underground was the raise bore machine, which was used to create a ventilation shaft for cooling and airflow to the underground caverns. To create this shaft, workers used the raise bore machine to drill a 13 3/4-inch pilot hole. Then, they attached a 12-foot-diameter reamer head to the drill stem and back-reamed the pilot hole to form a raise bore hole that is 1,200 feet in height.

Once the equipment was underground, construction crews began excavating the drifts, an interconnecting highway of tunnels that connect the three caverns. To form these underground tunnels, the miners used the drill-and-blast technique, which involves drilling a series of holes, then filling those holes with explosives to blast away the rock.

Caverns begin to take shape

Construction crews now are in the process of excavating the caverns using the drill-and-blast method. An important milestone during this first half of the excavation was the completion of the caverns’ top headings: dome-shaped upper sections of each of the caverns.

When forming the cavern top headings, the contractor had to execute this work methodically, said Gemelli. It involves initially excavating a small pilot tunnel to assess the geology and ground water conditions, then enlarge the sides to create the full span of the caverns. “This type of intensive mining required a lot of different steps to support the ground during excavation,” Gemelli added.

Following the excavation, workers installed steel monorail beams in the caverns to accommodate the cranes that will later be used to erect scientific equipment. They also reinforced both the drifts and cavern top headings with ground support anchors, wire mesh and sprayed concrete.

Safety first

Currently, 145 people from Thyssen work on site at SURF. The operations team, which works underground each day, consists of roughly 115 people. The rest includes engineers and administrative staff working on the surface.

The Thyssen team has successfully reached the halfway point of the excavation while maintaining an excellent standard of safety.

“Our safety record underground has been very good, and we would like to continue to the end of the project with nobody getting hurt,” said Moe. “Second to safety is to deliver a high-quality project, and everybody’s been happy with the quality of the work that we’ve done.”

Accelerating forward

The completion of the top headings sets the stage for the next phase of the excavation, which will involve drilling and blasting downward from those headings to carve out the rest of the caverns. “The last half of the project is all about excavating these three caverns,” Gemelli said. “This will be the peak period of rock excavation.”

Teams will also pour concrete floors in the base of the caverns and in all the interconnecting drifts. Once that’s complete, they will move the construction machinery out of the caverns — a process that will require first breaking down the equipment into smaller pieces, then sending the components up through the shaft to the surface.

This last half of the excavation will move much faster than the first half, according to Gemelli. The second phase of the excavation is now in full swing and is expected to be complete in 2024. “The hardest part of this project is now over with,” Gemelli said. “But we’ve still got a lot to do.”

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.