New measurements point to silicon as a major contributor to performance limitations in superconducting quantum processors

Silicon is a material widely used in computing: It is used in computer chips, circuits, displays and other modern computing devices. Silicon is also used as the substrate, or the foundation of quantum computing chips.

Researchers at the Superconducting Quantum Materials and Systems Center, hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory, demonstrated that silicon substrates could be detrimental to the performance of quantum processors. SQMS Center scientists have measured silicon’s effect on the lifespan of qubits with parts-per-billion precision. These findings have been published in Physical Review Applied.

New approaches to computing

Calculations once performed on pen and paper have since been handed to computers. Classical computers rely on bits, 1 or 0, which have limitations. Quantum computers offer a new approach to computing that relies on quantum mechanics. These novel devices could perform calculations that would take years or be practically impossible for a classical computer to perform.

Using the power of quantum mechanics, qubits—the basic unit of quantum information held within a quantum computing chip—can be both a 1 and a 0 at the same time. Processing and storing information in qubits is challenging and requires a well-controlled environment. Small environmental disturbances or flaws in the qubit’s materials can destroy the information.

Qubits require near-perfect conditions to maintain the integrity of their quantum state, and certain material properties can decrease the qubit lifespan. This phenomenon, called quantum decoherence, is a critical obstacle to overcome to operate quantum processors.

Disentangling the architecture

The first step to reduce or eliminate quantum decoherence is to understand its root causes. SQMS Center scientists are studying a broadly used type of qubit called the transmon qubit. It is made of several layers of different materials with unique properties. Each layer, and each interface between these layers, play an important role in contributing to quantum decoherence. They create “traps” where microwave photons—key in storing and processing quantum information—can be absorbed and disappear.

Researchers cannot unequivocally distinguish where the traps are located or which of the various materials or interfaces are driving decoherence based on the measurement of the qubit alone. Scientists at the SQMS Center use uniquely sensitive tools to study these effects from the materials that make up the transmon qubits.

“We are disentangling the system to see how individual sub-components contribute to the decoherence of the qubits,” said Alexander Romanenko, Fermilab’s chief technology officer, head of the Applied Physics and Superconducting Technology Division and SQMS Center quantum technology thrust leader. “A few years ago, we realized that our [superconducting radio frequency] cavities could be tools to assess microwave losses of these materials with a preciseness of parts-per-billion and above.”

Measurements at cold temperatures

SQMS Center researchers have directly measured the loss tangent—a material’s ability to absorb electromagnetic energy—of high-resistivity silicon. These measurements were performed at temperatures only hundreds of a degree above absolute zero. These cold temperatures offer the right conditions for superconducting transmon qubits to operate.

“The main motivation for why we did this experiment was that there were no direct measurements on this loss tangent at such low temperatures,” said Mattia Checchin, SQMS Center scientist and the lead researcher on this project.

No material is perfect. Through rigorous testing and studies, researchers are building a more comprehensive understanding of the materials and properties best suited for quantum computing.

Checchin cooled a metallic niobium SRF cavity in a dilution refrigerator and filled it with a standing electromagnetic wave. After placing a sample of silicon inside the cavity, Checchin compared the time the wave dissipated without the silicon present to the time with it present. He found that the waves dissipated more than 100 times faster with the silicon present—from 100 milliseconds without silicon to less than a millisecond with it.

“The silicon dissipation we measured was an order of magnitude worse than the number widely reported in the [quantum information science] field,” said Anna Grassellino, director of the SQMS Center. “Our approach of disentangling the problem by studying each qubit sub-component with uniquely sensitive tools has shown that the contribution of the silicon substrate to decoherence of the transmon qubit is substantial.”

Re-evaluating silicon

Companies developing quantum computers based on quantum computing chips often use silicon as a substrate. SQMS Center studies highlight the importance of understanding which of silicon’s properties have negative effects. This research also helps define specifications for silicon that would ensure that substrates are useful. Another option is to substitute the silicon with sapphire or another less lossy material.

“Sapphire, in principle, is like a perfect insulator—so much better than silicon,” said Checchin. “Even sapphire has some losses at really low temperatures. In general, you would like to have a substrate that is lossless.”

Researchers often use the same techniques for fabricating silicon-based microelectronic devices to place qubits on silicon substrate. So sapphire has rarely been used for quantum computing.

“It has taken years of material science and device physics studies to develop the niobium material specifications that would ensure consistently high-performances in SRF cavities,” said Romanenko. “Similar studies need to be done for materials that comprise superconducting qubits. This effort includes researchers working together with the material industry vendors.”

Regardless of which material is used for qubits, eliminating losses and increasing coherence time is crucial to the success of quantum computing. No material is perfect. Through rigorous testing and studies, researchers are building a more comprehensive understanding of the materials and properties best suited for quantum computing.

This loss tangent measurement is a substantial step forward in the search for the best materials for quantum computing. SQMS Center scientists have isolated a problem and can now explore whether a more refined version of silicon or sapphire will harness the computational power of a qubit.

 

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 23 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 sqms.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.

 

The U.S. Department of Energy’s Fermi National Accelerator Laboratory welcomed Consul General Alan Gogbashian of the British Consulate in Chicago and leaders from United Kingdom Research and Innovation and the Science and Technology Facilities Council at a visit to Fermilab on Aug. 24. The visitors discussed with Fermilab management ongoing collaborative projects in particle accelerator and neutrino science and emerging collaborations in quantum information science. They also toured some of the lab’s research facilities.

The visitors were greeted by Fermilab Director Lia Merminga and members from the PIP-II and Long-Baseline Neutrino Facility projects, as well as Fermilab’s neutrino and quantum programs. The visit marked the first in-person meeting between STFC Executive Chair Mark Thomson and Merminga since her appointment to the position in April.

“The ongoing support and collaboration of UKRI and STFC with Fermilab is so important to advancing accelerator technologies and neutrino research for the benefit of the worldwide community,” said Merminga. “By working together and utilizing the strengths and expertise of all organizations, we are better positioned for science discoveries and applications in the future.”

STFC—one of UKRI’s research councils—funds U.K. research in areas including particle physics, nuclear physics, space science and astronomy. The visit presented an opportunity for the visitors to see the progress made by the Fermilab-led LBNF/DUNE neutrino project and learn more about the work on the construction of the PIP-II particle accelerator. STFC is a major contributor to both projects.

Stops on the tour included Fermilab’s work on superconducting radio frequency cavities for PIP-II, the high-power target experiment for LBNF/DUNE, the assembly of the near detector for the Short Baseline Neutrino Program and a visit to the Superconducting Quantum Materials and Systems Center at Fermilab. During the tours, the delegation also met with students from the U.K. institutions who enthusiastically shared their experience and their research in the neutrino experiment.

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 Large Hadron Collider at CERN is back in action after a three-year scheduled technical shutdown period. Experts circulated beam in the powerful particle accelerator at the end of April, and Run 3 physics started in early July at the highest collision energy ever achieved.

The LHC experiments expect to collect petabytes of data to better understand nature at the smallest scale. Thousands of collaborators are testing the Standard Model of particle physics and hunting for new physics—things like supersymmetry, dark matter or undiscovered particles.

At the same time, researchers continue to prepare for the next iteration of the LHC.

Later this decade, scientists will begin operating with an upgraded accelerator for the High-Luminosity LHC, which will collide more protons with more luminosity than ever before. With it, scientists expect to see at least five to seven times as many collisions as they do now. Researchers are building technology to improve their detectors so that they can handle the increased luminosity. The detectors are running until the end of the 2030 and will cumulate a factor of 20 more data.

The CMS experiment, which co-discovered the Higgs boson in 2012, along with the ATLAS experiment, is upgrading several systems. Hundreds of people from universities and labs around the world, including U.S. institutions funded by the U.S. Department of Energy and the National Science Foundation, are designing, building and installing the new detector components. These technologies aim to improve the existing experiment, which as of today has been in operation for more than a decade.

Experts are making upgrades in six key areas: the tracker system, timing detector, trigger and data acquisition system, endcap calorimeter, barrel calorimeter and muon system. These upgrades mean CMS scientists can precisely measure and better reconstruct how particles interact in the detector. Studying their behavior may lead to new insights and potential discoveries about how our universe works.

The tracker

The CMS tracker charts a particle’s path through a magnetic field. It has two components: an inner pixel detector and an outer strip detector, both of which will be completely replaced. The tracker is the innermost area to be upgraded, closest to where the LHC’s protons collide. Because the HL-LHC will collide protons more quickly, particle paths will rapidly begin piling up.

“The new pixel detector has a finer granularity,” said Anders Ryd, the principal investigator for the National Science Foundation-funded upgrades and a professor at Cornell University. “We need higher rates and higher granularity so that they can actually detect each particle. Otherwise, you have so many particles going through that you just see a smear.”

Collaborators will add eight discs in the forward region of the inner tracker, extending the pixel detector’s coverage. To handle the rapid-fire data, the team will assemble and add thousands of small modules to the outer tracker. They’ll be equipped with sensors and application-specific integrated circuit chips that can start filtering and reducing the data immediately, enabling the outer tracker to process information at a staggering rate of 40 million times per second.

Timing detector

CMS researchers are building a brand-new layer outside of the tracker called the Minimum Ionizing Particles, or MIP, timing detector. The timing detector mitigates pile-up, or a tangled mess of particle paths, by giving researchers information on when a particle entered the detector. Using unprecedented precision in measuring the time of arrival of particles will allow researchers to distinguish individual paths and reconstruct them in 4D.

“We are adding a detector layer that will give us a precision timing measurement of individual charged particles from LHC collisions along their path,” said Patricia McBride, a scientist at the DOE’s Fermi National Accelerator Laboratory who, elected by 3,000 physicists in the international CMS Collaboration to the role, will become head of the collaboration early this autumn. “This will give us information about the type of particle it is and which primary collision it came from. We will be able to use space and time information to identify the interesting tracks in the event.”

The timing detector is shaped like a barrel with two endcaps, and its airtight seal will prevent energy loss and keep out dust. The upgrade team is now designing and building modules, electronics and software for this timing detector.

Trigger and data acquisition

The CMS trigger selects potentially interesting collision events and captures relevant data, discarding more scientifically benign events to keep the amount of data manageable. When operational, one of the new triggers will take in information from the upgraded outer tracker. Importantly, the new trigger will employ artificial intelligence and machine learning in its data acquisition of the large volume of data expected from LHC collisions.

The CMS experiment, which co-discovered the Higgs boson with the ATLAS experiment in 2012, is upgrading several systems. [Researchers] are designing and building the new detector components, and once installed, these technologies will improve the existing experiment, which has been in operation for more than a decade.

“We need to introduce some smartness into the event selection early on,” said Vaia Papadimitriou, who is the deputy manager of the upgrade project and a scientist at Fermilab, the host laboratory for the US-CMS collaboration. “This lets us reduce the amount of data we need to process and helps us eliminate background signals that would get in the way of what we’re actually trying to study.”

Upgrades to the data acquisition system will allow the team to collect data more quickly to keep up with the increased LHC collision rates.

Calorimeters

CMS is equipped with barrel and endcap calorimeters, detectors that measure particles’ energies.

The endcap calorimeter flanks the inner detectors and analyzes the particle showers from collisions. The current endcap calorimeter will be completely replaced by a new, high-granularity calorimeter, or HGCal, the first of its kind to be used at a collider experiment.

The detector will have excellent time resolution and incredibly fine spatial resolution, which allow precise reconstruction of the many particles produced. To build it, collaborators will assemble tens of thousands of modules with small silicon or scintillator sensors. The modules will form hundreds of cassettes, which incorporate the integrated circuits and electronics that can handle data directly on the detector and transmit it to the data acquisition system.

The team is also upgrading part of the barrel electromagnetic calorimeter. “We’ll replace what we call the ‘front-end electronics,’ the electronic system installed right there on the detector,” said Paolo Rumerio, the deputy upgrade coordinator and a physicist at the University of Alabama. The new system will be able to handle the increased flow of data.

“These calorimeters will provide a wealth of information that will enable CMS to reconstruct energy deposits, or showers, that come from different particles,” Rumerio said. “The energy and precise timing of each particle can be measured and used in the data analysis.”

Muons

Collecting information on muons is essential for CMS, as one would expect from its name: the Compact Muon Solenoid. The muons from particle collisions can travel fairly far without interacting, so this layer of the detector sits outside the calorimeters.

The new muon system will have upgraded electronics, better time resolution and an increased ability to detect muons coming off the beam at a wider range of angles. Several new electronic boards will handle data processing and readout. Collaborators are also improving the firmware and software used to control the electronics on these boards.

“The MREFC [Major Research Equipment and Facility Construction project] supported upgrades to the forward muon detectors include new electronics to support the higher data rates at the HL-LHC, as well as readout of new Gas Electron Multiplier detectors that will extend the muon detector coverage closer to the beam line,” Ryd said. “These upgrades will provide a significant enhancement of the CMS muon detection capabilities.”

Moving forward

Today, upgrades for the CMS detector are at different stages, but all will follow a similar path. After years of development and prototyping, the collaboration now moves to build or acquire the parts, begin fabricating system components at different U.S. laboratories, vet them with rigorous testing, and then deliver them to the experiments at CERN. Scientists will install the upgrade components during the LHC’s third long shutdown, currently scheduled to take place from 2026 to 2028.

Once the HL-LHC starts up, the increased data volume will help researchers search for rare physics processes and further investigate the Higgs boson. Researchers believe the Higgs provides the mechanism by which all other particles get their mass, but scientists still have a lot to learn about the universe by studying the particle to greater precision.

“The Higgs boson is such a fundamental particle that discovering it is not good enough,” Papadimitriou said. “We need to have a lot of complementary information in order to study all the properties of the Higgs boson. And because the Higgs boson is predicted by the Standard Model, if we find any properties to be different from what the Standard Model predicts, it’s a major breakthrough.”

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.