Â鶹ÒùÔº

Modular network offers fault-tolerant scaling of superconducting qubit devices

Quantum computers, devices that can perform computations relying on the principles of quantum mechanics, are expected to outperform classical computers on some types of optimization and processing tasks. While physicists and engineers have introduced various quantum computing systems over the past decades, reliably scaling these systems so that they can tackle real-world problems while correcting errors arising during computations has so far proved challenging.

Researchers at the University of Illinois at Urbana-Champaign recently introduced a new, modular quantum architecture for scaling superconducting quantum processors in a fault-tolerant, scalable and reconfigurable way. Scaling in a fault-tolerant way is required to maintain the quantum effects and conditions necessary to perform long-term quantum computations.

Their proposed system, outlined in a paper in Nature Electronics, is comprised of several modules (i.e., superconducting qubit devices) that can operate independently and be connected to others via a low-loss interconnect, forming a larger quantum network.

"The starting point for this study was current insight in the field of superconducting quantum computing that we will need to break out processors into multiple independent devices—an approach we call 'modular quantum computing,'" Wolfgang Pfaff, senior author of the paper, told Â鶹ÒùÔº.

"This has, in the last years, become a widespread belief, and even companies like IBM are pursuing it. We wanted to know if we can realize an engineering-friendly interconnect for this approach."

Essentially, Pfaff and his colleagues wanted to devise a strategy to connect quantum devices while minimizing signal degradation or energy dissipation when quantum information is transmitted between them. Moreover, they wanted to be able to easily connect, disconnect and reconfigure the devices.

"Very simply speaking, our approach entails the use of a high-quality superconducting coaxial cable called a bus-resonator," explained Pfaff. "We connect a qubit capacitively to a cable through a custom connector that places the cable very close (sub-mm precision) to the qubit. This allows us then to effectively perform gates between qubit and cable, and then multiple qubits if they are connected to the same cable.

"The key in what we've shown is the ability to combine a very low-loss connection between the cable and the qubit with a fast and high-efficiency gate; that gate is also a new development by us, by exploiting a fast frequency-conversion process that our style of qubits (transom qubits) allows us to do."

The researchers' new approach for creating modular quantum networks has notable advantages over previous methods to scale quantum systems. In initial tests, they found that it allowed them to robustly connect superconductor-based quantum devices and disconnect them later without damaging them; all without introducing significant signal loss in quantum gates.

"Using our approach, I think that we have opportunities to build reconfigurable quantum systems from the bottom up, with, for example, the option to over time 'plug in' more processor modules to a network of quantum devices," added Pfaff.

"We are currently working on a design in which we want to see if we can increase the number of elements that we are connecting, making our networks larger. We are also exploring how we can better overcome losses in the system and make the architecture compatible with quantum error correction."