Quantum simulator to unlock new electronic materials

This approach, tested on a processor with 16 qubits, allows scientists to explore material properties by simulating the behaviour of electrons in an adjustable electromagnetic field. By precisely controlling the coupling between qubits, the team successfully replicated the way electrons move within materials when influenced by electromagnetic forces.

The ability to simulate electromagnetic fields is pivotal in studying a range of material characteristics that are otherwise difficult to replicate on quantum hardware. With this new technique, researchers can investigate properties like conductivity, magnetization, and polarization, providing insights into the underlying physics of materials that could lead to innovations in electronics, such as enhanced semiconductors, insulators, and superconductors. These advancements could contribute to the development of faster, more powerful, and energy-efficient devices.

Ilan Rosen, an MIT postdoctoral researcher and lead author, explained: “Quantum computers are powerful tools for studying the physics of materials and other quantum mechanical systems. Our work enables us to simulate much more of the rich physics that has captivated materials scientists.”

The study was led by senior author William D. Oliver, the Henry Ellis Warren Professor of Electrical Engineering and Computer Science and of Physics, Director of the Center for Quantum Engineering, leader of the Engineering Quantum Systems group, and Associate Director of the Research Laboratory of Electronics. The research, which included collaborators from MIT’s departments of Electrical Engineering, Computer Science, Physics, and the MIT Lincoln Laboratory, has been published in Nature Physics.

Quantum emulation

IBM and Google have been advancing the development of large-scale digital quantum computers, which promise to surpass classical computers by executing certain algorithms with greater efficiency. However, the potential of quantum computers extends beyond general computation: by carefully engineering the dynamics of qubits and their interactions, scientists can emulate the behaviour of electrons moving between atoms in solid materials.

Jeffrey Grover, an MIT Research Scientist and Co-Author of the Study, noted: “That leads to an obvious application, which is to use these superconducting quantum computers as emulators of materials.”

Rather than constructing vast digital quantum systems to tackle immensely complex problems, researchers can harness smaller quantum processors in an analog mode, using qubits to simulate a material system within a controlled setting. According to Ilan Rosen, another researcher involved in the study:

“General-purpose digital quantum simulators hold tremendous promise, but they are still a long way off. Analog emulation is another approach that may yield useful results in the near term, particularly for studying materials. It is a straightforward and powerful application of quantum hardware.”

With an analog quantum emulator, researchers can set an initial configuration and observe the material’s simulated dynamics over time. However, some characteristics inherent to materials are challenging to replicate on quantum hardware. A significant factor is the effect of magnetic fields on electron behaviour. In real materials, electrons exist in atomic orbitals, and when two atoms are close, their orbitals can overlap, allowing electrons to "hop" between atoms. This hopping behaviour becomes more intricate when a magnetic field is present.

On a superconducting quantum processor, microwave photons move between qubits in a manner that resembles electron hopping between atoms. However, unlike electrons, photons lack an electric charge, so their hopping behaviour does not change in a real magnetic field. Unable to introduce a physical magnetic field in their simulation, the MIT team developed techniques to synthetically mimic the effects of one, thereby capturing the nuanced interactions between particles in a magnetic environment.

Qubit processing

To replicate the intricate hopping behaviour that electromagnetic fields induce in electrons, the researchers adjusted the coupling between adjacent qubits in their processor. They achieved this by applying carefully varied microwave signals to each qubit, altering their energy levels. Normally, qubits are set to the same energy to enable straightforward photon hopping. However, for this approach, the team dynamically modified each qubit’s energy to control how they interact with one another.

Through precise modulation of these energy levels, photons were made to hop between qubits in a complex pattern, closely resembling the movement of electrons in a magnetic field. The ability to fine-tune the microwave signals also allowed the team to emulate a wide range of electromagnetic fields, each with unique strengths and distributions.

The process involved multiple rounds of experimentation to determine the optimal energy settings, modulation strength, and microwave frequency for each qubit. Ilan Rosen explained: “The most challenging part was finding modulation settings for each qubit so that all 16 qubits work at once.”

After establishing the correct parameters, the team confirmed that the photons’ dynamics followed key equations foundational to electromagnetism, even demonstrating the Hall effect—a phenomenon of electrical conduction that arises in the presence of an electromagnetic field. These results showed that the synthetic electromagnetic field accurately mirrored the behaviour of a real field.

Looking ahead, this method could enable precise studies of complex phenomena in condensed matter physics, such as phase transitions that occur when a material shifts from conducting to insulating states. William Oliver noted: “A nice feature of our emulator is that we need only change the modulation amplitude or frequency to mimic a different material system. In this way, we can scan over many material properties or model parameters without having to physically fabricate a new device each time.”

While this work served as an initial demonstration, it opens up numerous avenues for discovery, according to Rosen: “The beauty of quantum computers is that we can look at exactly what is happening at every moment in time on every qubit, so we have all this information at our disposal. We are in a very exciting place for the future.”

The project received support from multiple institutions, including the U.S. Department of Energy, DARPA, the U.S. Army Research Office, the Oak Ridge Institute for Science and Education, the Office of the Director of National Intelligence, NASA, and the National Science Foundation.