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AsianScientist (Jun. 12, 2020) In a study published in New Journal of Physics, researchers in Japan have devised a way to connect qubits that could make quantum computers more feasible.

Quantum computers use the fundamentals of quantum mechanics to process significantly greater amounts of information much faster than classical 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 that these are arranged in two-dimensional (2D) 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 three-dimensional (3D) wiring systems across multiple planes in which many wires intersect, making their construction a significant engineering challenge.

Now, a team of scientists from Tokyo University of Science, RIKEN Centre for Emergent Matter Science and University of Technology, Sydney, have developed a unique solution to this 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 3D external line technology and reverts to a completely planar design, the researchers said.

The scientists began with a qubit square lattice array and stretched out each column in the 2D plane. They then folded each successive column on top of each other, forming a dual one-dimensional array called a bi-linear array. This put all qubits on the edge and simplified the arrangement of the required wiring system.

In this new architecture, some of the inter-qubit wiringeach qubit is also connected to all adjacent qubits in an arraydoes overlap, but because these are the only overlaps in the wiring, simple local 3D systems such as airbridges at the point of overlap are enough and the system overall remains in 2D.

The team 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 that it is possible to build and run this system using existing technology and without any 3D arrangement.

The experiments also showed that their architecture solves several problems that plague the 3D structures, namely that 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-2D design reduces the number of times wires cross each other, thereby reducing the crosstalk and consequently increasing the efficiency of the system.

The quantum computer is an information device expected to far exceed the capabilities of modern computers, said study corresponding author Professor Tsai Jaw-Shen. We are planning to construct a small-scale circuit to further examine and explore the possibility.

The article can be found at: Mukai et al. (2020) Pseudo-2D Superconducting Quantum Computing Circuit for the Surface Code: Proposal and Preliminary Tests.

Source: Tokyo University of Science; Photo: Adapted from Tokyo University of Science video.Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

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Flattening The Complexity Of Quantum Circuits - Asian Scientist Magazine