TOKYO – Efficient quantum computing is expected to enable advancements that are impossible with classic computers. Scientists from Japan and Sydney have collaborated and proposed a novel 2-D design that can be constructed using existing IC technology. This design solves typical problems facing the current 3-D packaging for scaled-up quantum computers, according to a new study, led by Prof. Jaw-Shen Tsai from Tokyo University of Science.

Quantum computers use the fundamentals of quantum mechanics to process significantly greater amounts of information much faster than classic computers. It is expected that when error-corrected and fault-tolerant quantum computation is achieved, scientific and technological advancement will occur at an unprecedented scale.

But building quantum computers for large-scale computation is proving to be a challenge in terms of their architecture. The basic units of a quantum computer are the “quantum bits” or “qubits.” These are typically atoms, ions, photons, subatomic particles such as electrons, or even larger elements that simultaneously exist in multiple states, making it possible to obtain several potential outcomes rapidly for large volumes of data. The theoretical requirement for quantum computers is these are arranged in 2-D arrays, where each qubit is both coupled with its nearest neighbor and connected to the necessary external control lines and devices. When the number of qubits in an array is increased, it becomes difficult to reach qubits in the interior of the array from the edge. The need to solve this problem has so far resulted in complex 3-D wiring systems across multiple planes in which many wires intersect, making their construction a significant engineering challenge.

In the latest issue of New Journal of Physics, a group of scientists from Tokyo University of Science, Japan, Riken Centre for Emergent Matter Science, Japan, and University of Technology, Sydney, proposes a unique solution to the qubit accessibility problem by modifying the architecture of the qubit array.

“Here, we solve this problem and present a modified superconducting micro-architecture that does not require any 3-D external line technology and reverts to a completely planar design,” they say.

The scientists began with a qubit square lattice array and stretched out each column in the 2-D plane. They then folded each successive column on top of each other, forming a dual 1-D array called a bi-linear array. This put all qubits on the edge and simplified the arrangement of the required wiring system. The system is also completely in 2-D. In this new architecture, some of the inter-qubit wiring – each qubit is also connected to all adjacent qubits in an array – overlaps, but because these are the only overlaps in the wiring, simple local 3-D systems such as airbridges at the point of overlap are enough, and the system overall remains in 2-D.

The scientists evaluated the feasibility of this new arrangement through numerical and experimental evaluation in which they tested how much of a signal was retained before and after it passed through an airbridge. Results of both evaluations showed it is possible to build and run this system using existing technology and without any 3-D arrangement.

The scientists’ experiments also showed their architecture solves several problems that plague the 3-D structures: They are difficult to construct; there is crosstalk or signal interference between waves transmitted across two wires; and the fragile quantum states of the qubits can degrade. The novel pseudo-2-D design reduces the number of times wires cross each other, thereby reducing the crosstalk and consequently increasing the efficiency of the system.

At a time when large labs worldwide are attempting to find ways to build large-scale fault-tolerant quantum computers, the findings of this new study indicate such computers can be built using existing 2-D IC technology.

“The quantum computer is an information device expected to far exceed the capabilities of modern computers,” Prof. Tsai said. The research journey in this direction has only begun with this study, and “we are planning to construct a small-scale circuit to further examine and explore the possibility.”

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