Mechanical Vibrations A "Vibrating" Memory for Quantum Computers

When it comes to storage, quantum computers are still reaching their limits. A research team at ETH Zurich has now developed a new approach: mechanical vibrations instead of electromagnetic storage. The innovative "vibrating" memory promises to store significantly more information in a smaller space.

"This computer works almost like a guitar," describes ETH Zurich regarding the new approach. Quantum physicist Yiwen Chu and her team at ETH Zurich use tiny vibrations to store and process information. These behave similarly to the vibrations of strings that produce tones on a guitar.

What sounds like music is actually quantum physics. The vibrations with which Chu and her team work cannot be heard: they occur deep inside a quantum chip and are used to store quantum information.

These vibrations are needed so that Chu's quantum computer can perform its calculations as efficiently as possible and flexibly access a memory. "The interplay between the processing unit and the memory creates a crucial foundation for establishing quantum computers as a powerful and reliable tool for calculations that are not possible with conventional computers," says Yiwen Chu.

The physics professor conducts research on quantum information and quantum computer architectures. Recently, her team and she presented a new approach in the scientific journal "Science," which separates computation much more clearly from memory compared to many existing quantum computer models that tightly link computation and storage.

Quantum Memory Modeled After Digital Design

To achieve this, Chu and her team have developed a new quantum computer architecture that deliberately draws inspiration from classical digital computers: in these, a central processing unit (CPU) processes the data, which is stored separately in a random-access memory (RAM). The computer architecture determines how the individual components of a computer are arranged to process data as efficiently as possible.

In Chu's approach, a so-called superconducting qubit takes on the role of the central processing and control unit, similar to the processor (CPU) in a digital computer. At the same time, the information to be processed is temporarily stored in a quantum memory, making it available during the calculation. "In our quantum memory, however, the information is not stored electromagnetically, as is usually the case today, but in the form of mechanical vibrations," says Chu.

To perform a calculation, the qubit accesses a piece of information—that is, a vibration!—in the quantum memory, processes and modifies it, and then stores it back there. "Specifically, our quantum chip contains so-called mechanical resonators. These are tiny components that begin to vibrate when storing," says Chu.

Each Vibration Stores information

Like the strings of a guitar, which produce different tones depending on their vibrations, the resonators can also vibrate in many different ways—in physics, this is referred to as vibrational modes. In the language of computer science, these modes correspond to the number of available memory slots. This means: each type of vibration stores different information.

Within the vibrational modes, different vibrational states can also be realized. This refers to the specific state of a vibration in which the information is stored in a way that allows it to be flexibly retrieved and stored again.

In terms of information theory, these states correspond to the respective content of the memory locations. However, in quantum physics, these states represent the crucial difference from a guitar—and also from a digital computer: the vibrations of a string follow the laws of classical physics, which describe our everyday world. In the quantum chip, however, the laws of quantum mechanics apply, explaining the behavior of the smallest particles. There, states can superpose and become entangled simultaneously—a both-and situation that classical physics does not know. Digital computers also work only with two clearly distinct states: 0 or 1.

This ability to superimpose or entangle states opens up additional computational pathways for quantum computing. The great promise of quantum computers is, therefore, that they will one day solve certain particularly complex tasks more efficiently than classical computers—or even solve problems that classical computers fail to address.

More Vibrations, More Memory

For quantum computers to compute and store reliably, researchers must precisely control and manipulate these states. This is achievable when the processing unit and memory are well-coupled.

With Chu, it works like this: The resonators store the respective information in a specific vibrational state. When the qubit retrieves information from the quantum memory, it processes and changes this vibrational state and then stores it back.

Until now, many quantum computer models have combined electromagnetic memory with superconducting qubits, as both—individually and in combination—are well-researched and have proven their effectiveness. Electromagnetic memory technologies allow quantum states to be read, modified, and controlled with high precision. Their drawback, however, is that they are comparatively large and require a lot of space—this could hinder the advancement of experimental laboratory devices into market-ready quantum computers for research and industry. This is where Chu steps in.

Mechanical resonators, on the other hand, are significantly smaller and more compact. They also offer greater storage capacity because they include many different vibrational modes, allowing them to store more information simultaneously than electromagnetic memory. Additionally, they keep quantum states stable for longer without the vibration decaying and information being lost. This extends the storage time.

Innovative Computer Architecture Passes Stress Test

In her study published in the journal Science, Chu has now experimentally demonstrated for the first time that mechanical resonators can effectively couple and combine with superconducting qubits to perform computations. She provides proof of feasibility: oscillating memory can be a promising alternative to electromagnetic approaches.

Whether her approach will prevail now depends on how well it can be scaled. The newly designed quantum chip must also function reliably in larger quantum computing systems with expanded computational capabilities. Chu's team is continuing their research on this. The team has already provided a fundamental proof in the published study: their approach of embedding qubits and resonators into a new computer architecture handles not only simple computational tasks but also more complex ones.

The research group tested the computational ability of their approach, among other things, on two key problems that are among the most important computational methods in quantum computing: the quantum Fourier transform and period finding.

"The quantum Fourier transform is a fundamental computational method required for many quantum algorithms. The algorithm we implemented for period finding demonstrates how this method can be used," explains Igor Kladaric, a doctoral student in Chu's team and co-author of the publication.

Both methods require a quantum computing system that can precisely control, store, and reliably interlink many quantum states simultaneously. If successful, a quantum computer is considered fundamentally capable of computing – and Chu's approach achieves exactly that.

Chu's quantum computing system masters all the fundamental computational steps needed to theoretically perform any quantum computation. This demonstrates that the team's approach is fundamentally suitable as a universally applicable and programmable quantum computer.

The path is still long to a sufficiently powerful and reliable quantum computer that can be used in research and industry. However, Chu's approach points in a promising direction along this path.