IC Design Enables a New 2D Structure for Quantum BitsApril 24, 2020 by Gary Elinoff
Using existing IC fabrication technologies, scientists have figured out how to eliminate crosstalk among qubits on a 2D-plane.
Quantum computing is hailed as the next quantum leap in computer science. Quantum events take place on a far smaller scale than the electrical interactions that animate present-day computers. This means not only can more function be packed into less space, but distances between functions are shortened, yielding unimaginably fast computational results.
Quantum bits or "qubits" must be connected to each other and the necessary external control lines and devices. Screenshot used courtesy of the Tokyo University of Science
The basic unit of a quantum computer is the quantum bit or “qubit” for short. These can be tiny, subatomic elements such as electrons. The quantum state of these elements conveys information, in a manner analogous to the far larger electrical registers of present computers.
Problems Associated with 3D Quantum Computing Structures
Ideally, the qubits would be arranged in two-dimensional (2D) arrays. In this manner, each qubit would communicate with its nearest neighbor and with I/O ports.
But when anticipating a practical device, the number of qubits has to be drastically increased, to the point where such simple structures won’t cut it because the qubits can only communicate so far on a long 2D plane. The qubits located toward the center or one far edge can’t communicate with the opposite edge, and as a result, individual qubits can’t communicate with enough of its fellows.
In the past, designers have worked around this issue by devising a 3D structure that spans multiple planes.
Three-dimensional quantum computing. Screenshot used courtesy of the Tokyo University of Science
Fabricating these structures can be difficult. Worse, there is an unacceptable level of cross-talk and interference in a 3D structure. This is one of the many reasons practical quantum computing hasn't come to fruition yet.
Pseudo-2D Quantum Computing
A solution has been developed by a consortium of scientists from the Tokyo University of Science, Japan, RIKEN Centre for Emergent Matter Science in Japan, and the University of Technology, Sydney. The team was led by led by Tokyo University’s Professor Jaw-Shen Tsai.
The results of the study is published in the New Journal of Physics. As the participants describe it, "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 solution involves placing all the qubits at the edge of the network and connecting them via air bridges, which are a well-used method employed in current IC fabrication. The idea is illustrated in the central portion of the image below.
Infographic of how the pseudo-2D structure may affect quantum computing. Video used courtesy of the Tokyo University of Science
The starting point was a qubit square lattice array. They then stretched out each column into the 2D plane. Next, they folded each successive column onto the top of each other. The result was a dual one-dimensional array called a “bi-linear” array.
Physical layout of the new pseudo-2D architecture. Image used courtesy of Tsai et. al
This method, completely 2D, put all qubits on the edge. Because each qubit is connected to its adjacent member, there is some overlap, but not so much as to cause crippling crosstalk. The other great plus is that not only will manufacturing be easier, but it can be accomplished largely through existing IC fabrication technologies.
Implications of this Finding for Quantum Computing
This research represents a significant step toward practical quantum computing. And, relevant to IC designers, it can be implemented using existing manufacturing methodologies. Professor Tsai states, "The quantum computer is an information device expected to far exceed the capabilities of modern computers."
Tsai's team is planning the next steps to build a small-scale circuit "to further examine and explore the possibility."
If you’re an engineer working with ultra-high-speed applications, can you imagine a situation where a small subsystem operating at astonishingly high speeds can make a dramatic difference? Share your thoughts in the comments below.