Quantum information science promises un-hackable communication links, computing systems with unprecedented power, and ultra-precise measurement devices. However, without the right materials, quantum technology will never meet its envisioned potential.

"All technologies begin with the materials that make them up, and quantum information technology demands material properties that have never been optimized before. Quantum information science has deep roots in fundamental physics research, but developing practical technology requires a shift to an engineering mindset. That’s where materials science and engineering can come in," said Chris Anderson, a professor of materials science and engineering and a quantum information technology researcher.

Researchers in The Grainger College of Engineering at the University of Illinois Urbana-Champaign are bridging the gap between fascinating science and powerful technology, applying the principles of materials science to engineer material systems that operate under stringent conditions and can perform the functions needed to support quantum technology.

Anderson is one of those researchers. A physicist by training, he now works in materials science to engineer materials optimized for quantum applications. His group focuses on the development of both materials for quantum transduction — the conversion of quantum information from one form to another — and electron spin qubits that can store and control quantum information in the solid state.

Another one of those researchers is physics professor Angela Kou, who is leading an initiative sponsored by the U.S. Air Force to investigate two-level defects, a ubiquitous phenomenon that hinders the performance of the superconducting qubit technologies used across academia and by companies such as Amazon, Google and IBM. The collaboration is performing systematic investigations into these materials defects with the goal of identifying their origin and managing their impact.

Other exciting work in this area is being pursued by physics professor Elizabeth Goldschmidt with materials science and engineering professor Daniel Shoemaker, who are collaborating to create quantum memory technology capable of storing quantum information for extended periods of time. They are designing materials that can hold europium, a metal that shows promise for use in quantum memory, while maintaining its quantum properties.

Chris Anderson: A physicist doing materials science

Anderson’s laboratory in Grainger Engineering is dedicated to solving the problems facing quantum technology by developing materials that create and manipulate light. He sees his work as taking a vital step towards transforming fundamental science into useful technology.

"Quantum physics is old. The key experiments and theoretical developments took place a century ago. What’s new is harnessing it and controlling it in a way that makes for powerful technology. To give an analogy, it’s like we’re at the stage after the transistor was invented but before computers were widely adopted. The science principles have been proven, but there’s still a lot of hard engineering ahead to build something useful," Anderson said.

Photo Credit: Fred Zwicky / University of Illinois Urbana-Champaign

His group works on two main research themes. The first is development of new materials for quantum transduction, the conversion of quantum information from one medium to another. Technologies for doing so will be crucial because quantum technology will hinge on the ability to network different devices together, much as large data centers are based on the ability to link smaller computers together to make web searches faster and cloud storage larger. The main challenge is the temperature requirement for operation, which typically is less than a degree above absolute zero.

Anderson’s research group is addressing this problem by developing low-temperature electro-optic crystals, nonlinear materials capable of transforming electric fields into optical signals. Their work often involves revisiting materials that have previously been studied but whose properties were not considered useful at the time.

"One example of this is strontium titanate. At room temperature, it’s not that interesting. Back in the day, it was used for fake diamonds in jewelry. However, we figured out that based on their fundamental physics properties, they should have exceptional optical nonlinearity at low temperatures. We then went and measured them, and they did! We breathed new life into a material," Anderson said.

Anderson and his group are also investigating the use of crystals with material defects, such as missing atoms, to host "electron spin qubits": units of quantum information contained in the magnetic orientations of electrons. They recently set a record using silicon carbide — a material commonly used in power electronics and battery technology. When it was cooled to ultra-low temperatures, the researchers found that the quantum state it hosted could be maintained for over 1 minute, a giant leap beyond the millisecond lifetimes of most other quantum systems. These imperfections in semiconductors may be a powerful, wafer-scale platform for quantum technology.

Industrial collaborations are important to Anderson’s work and have helped motivate his research directions.

"Just like we are, industry is looking for new materials that will help them realize their five- to 10-year roadmaps for quantum technology. Companies are recognizing the importance of having materials designed specifically for applications, and they're looking to academia to come up with solutions," Anderson said.

Angela Kou: A team attack on a mysterious qubit problem

Currently, the most popular hardware platform for quantum computing is superconducting qubits. Their appeal is that, of all existing qubit technologies, supercomputing qubits most easily integrate with standard electronics. They are made using fabrication techniques similar to those for semiconductor devices, and there are straightforward ways to scale their manufacturing and integrate them with other components.

The technology has been demonstrated at an intermediate scale — thousands of qubits — but many challenges remain before it reaches its full potential. One of these challenges is the need to reduce noise and unwanted influences as much as possible. Superconducting qubit users have identified defects known as two-level systems (TLS) that closely mimic qubits, allowing them to draw energy and information from the system. Their origin is unknown, making it difficult to mitigate them effectively.

Physics professor Angela Kou is the head of a four-year, $4.8 million initiative to identify the cause of TLS defects and offset their effects. Sponsored by the Air Force Office of Scientific Research, the initiative brings together six faculty members from three academic departments in Grainger Engineering to address the problem.

Photo Credit: Heather Coit/Grainger Engineering

"TLS defects are especially problematic because they essentially pull quantum information away from the system under a user’s control. They’re a major limiting factor for superconducting quantum hardware, and they’re uncontrollable because we don’t know what causes them. Identifying their origin is a massive undertaking, one that Grainger Engineering is uniquely positioned to tackle with its expertise across multiple science and engineering disciplines," Kou said.

A significant part of the initiative is close collaboration with materials scientists. Superconducting qubits require the integration of specific metals with electronic components, and the unit must be reliable at the low temperatures required for the superconducting state to exist. The TLS defect problem involves a large materials science component and will require a materials-based solution.

"A superconducting qubit is just a nonlinear circuit. You need a Josephson junction — two layers of superconducting metal joined with an insulating junction — to form the actual qubit, but you also need inductors, capacitors and other circuit elements. It's not just quantum physics, but also materials science, electrical engineering and microelectronic fabrication," Kou said.

Each co-investigator is bringing unique capabilities to the problem.

Electrical and computer engineering professor Minjoo (Lawrence) Lee is a leading expert in materials growth. His group will apply molecular beam epitaxy to grow superconducting qubit devices atom by atom, allowing for precise, systematic control over their structures.

Materials science and engineering professor Pinshane Huang specializes in transmission electron microscopy, allowing the behaviors of individual atoms in the devices to be captured in real time.

Materials science and engineering professor David Cahill will measure the devices’ thermal reflectance and conductance, providing insight into the structures of junctions and interfaces and whether they host defects.

Kou’s research group will perform microwave characterization, which allows information loss and the number of TLS defects coupled to the qubit to be measured.

Physics professor Wolfgang Pfaff can directly probe the energy structure and time dynamics of the devices.

Materials science and engineering professor André Schleife is a theorist specializing in atomic-scale simulation who will calculate the properties of different kinds of defects and compare them to observed data.

"It's going to be a loop: growth, characterization, theory, repeat," Kou said. "By the end, we should have a recipe for reducing the occurrence of TLS defects, and hopefully we'll understand what causes them. We hope that we'll come out the other side with knowledge that will make superconducting quantum processors even better than they are now."

Elizabeth Goldschmidt and Daniel Shoemaker: Engineering solid memory

To link quantum processing modules to perform meaningful quantum computations, it is necessary to share quantum information. This sharing is achieved by encoding the information into photons and transmitting them between modules. However, the module receiving the information may not be ready to use it when it arrives. A way to store quantum information without destroying its quantum characteristics is needed.

The development of such a quantum memory technology is proving to involve significant technical challenges. Quantum information can only exist in isolation from external influences, so it cannot be “read” and “copied” in the usual sense. It must be transferred from the transmitting photon to another quantum entity without being directly observed or measured, and then it must be held in suspension until the processor is ready to use it.

"Photons are light, so the simplest way to suspend them is to have them bounce between mirrors. It borders on silly, but it's the most effective quantum memory in existence. But as you can imagine, it becomes difficult to scale this up. That's why we're looking for materials that can function as quantum memories," said physics professor Elizabeth Goldschmidt.

With materials science and engineering professor Daniel Shoemaker, Goldschmidt is investigating the use of europium ions embedded in a crystalline solid for quantum memory. Europium, a rare earth element, has an electron arrangement that makes it an ideal candidate for absorbing photons while preserving the quantum information they carry. The difficulty is that an atom’s electron cloud is altered whenever it is incorporated into a solid material.

"The main advantage of having a solid-state quantum memory is compactness. If we can make a 'material-level' device, then we can very easily integrate it with traditional electronic hardware. The problem is that once we put europium in a solid, it interacts with all the other atoms in the material and significantly complicates the picture. This is why we need to talk to materials scientists and really dig into some hard materials science problems," Goldschmidt said.

Rare earth elements are already being incorporated into solid materials that exploit their quantum properties for devices such as lasers and LEDs. Since the 1960s, hundreds of such materials have been shown to be useful for those classical applications. The challenge is that the requirements for a quantum memory’s optical properties are much more stringent.

Photo Credit: Courtesy of Daniel Shoemaker

The two researchers are combining their laboratories and resources to identify materials containing europium that have optimized quantum properties. Goldschmidt's group provides a set of design criteria for quantum memory applications, and Shoemaker’s group translates them into requirements for the chemistry of the material. His group then grows candidate materials, and Goldschmidt's group tests the quantum properties.

"Ideally, everything that is not the rare earth atom should be invisible. There should be no complicated optical spectrum, no communication between different atoms, no magnetic features that scramble the information. We try to achieve this with material chemistry. My lab looks like any other chemistry lab, and we're equipped to do crystal growth. We try to make the best and cleanest materials we can, and professor Goldschmidt takes it to study its optical properties," Shoemaker said.

Goldschmidt and Shoemaker's collaboration has produced three scientific publications on their work to create europium-based solid state quantum memory. The two also co-advise graduate students who gain expertise in the methods of both quantum optics and materials science. As a result, the collaboration is producing a cohort of researchers with the multidisciplinary expertise needed to implement quantum technology in material systems.

Illinois Grainger Engineering Affiliations

Chris Anderson is an Illinois Grainger Engineering assistant professor in the Department of Materials Science and Engineering and is affiliated with both the Department of Physics and the Department of Electrical and Computer Engineering. He is a member of the Illinois Quantum Information Science and Technology Center, the Materials Research Laboratory and the Holonyak Micro and Nanotechnology Lab.

Angela Kou is an Illinois Grainger Engineering assistant professor of physics in the Department of Physics. She is a member of the Illinois Quantum Information Science and Technology Center in the Materials Research Laboratory.

Elizabeth Goldschmidt is an Illinois Grainger Engineering associate professor in the Department of Physics and the Department of Electrical and Computer Engineering. She is also affiliated with the Materials Research Lab, the Holonyak Micro and Nanotechnology Lab, and the NSF Quantum Leap Challenge Institute Hybrid Quantum Architectures and Networks. She is the Director of the Illinois Quantum Information Science and Technology Center.

Daniel Shoemaker is an Illinois Grainger Engineering associate professor of materials science and engineering in the Department of Materials Science and Engineering. He is affiliated with the Illinois Quantum Information Science and Technology Center in the Materials Research Laboratory. He holds the Racheff Faculty Fellow appointment.

Minjoo Lawrence Lee is an Illinois Grainger Engineering professor of electrical and computer engineering in the Department of Electrical and Computer Engineering. He is also affiliated with the Department of Materials Science and Engineering. He is the director of the Holonyak Micro and Nanotechnology Laboratory and a member of the Illinois Quantum Information Science and Technology Center in the Materials Research Laboratory. He holds an Intel Alumni Faculty Scholar appointment.

Pinshane Huang is an Illinois Grainger Engineering professor of materials science and engineering in the Department of Materials Science and Engineering. She is the associate director of the Materials Research Laboratory. She holds a Racheff Faculty Scholar appointment.

David Cahill is an Illinois Grainger Engineering professor of materials science and engineering in the Department of Materials Science and Engineering. He is also affiliated with the Department of Mechanical Science and Engineering and the Department of Physics. He holds a Grainger Distinguished Chair in Engineering appointment.

Wolfgang Pfaff is an Illinois Grainger Engineering assistant professor of physics in the Department of Physics. He is a member of the Illinois Quantum Information Science and Technology Center in the Materials Research Laboratory.

André Schleife is an Illinois Grainger Engineering professor of materials science and engineering in the Department of Materials Science and Engineering. He is a member of the Materials Research Laboratory.