While spin-based quantum computers have been developed on small scales, to this point they’ve only been fabricated in controlled, clean, academic environments. This is currently one of the hurdles that needs to be overcome in order to make quantum computers feasible and scalable. 

The IBM Q System One in its own sterile, isolated chamber. Image used courtesy of IBM and Forbes

Looking to address this problem, researchers from the University of Copenhagen, along with partners from the French group CEA-Leti, have been working together on a new method to fabricate electron-spin based quantum computers in an industrial setting. 

Single-Electron Control in Silicon 

A qubit, which is the quantum mechanical equivalent of a classical computer bit, can be achieved in a variety of different ways, including the polarization of a photon, the discrete energy level of an ion, and the spin of an electron. In recent years, there has been heavy interest in spin-based qubits, where the spin polarization of an electron encodes the “1” or “0” bit value of the qubit. 

Spin-based qubits encode data based on electron (or hole) spin. Image used courtesy of Baldovi et al.

One of the main reasons that spin-based qubits are of such interest is because they couple very weakly with their environment, meaning they aren’t easily affected by external noise. 

Achieving Spin-Based Qubits 

In classical electronics, you cannot simply control the spin of a single electron. A solution to this problem is called the quantum dot, which are man-made nanoscale crystals that can transport electrons. 

Quantum dots in this context can be thought of as a means of confining electronics into an area so small that they begin to behave as single atoms. Unfortunately, achieving this technology in silicon has been hindered greatly by the large effective mass associated with electrons in silicon, 1.08 as opposed to 0.57 for holes. 

Quantum Dot-Enabled Spin-Based Qubits 

In their article published in Nature, the researchers have shown a way to achieve quantum dot-enabled spin-based qubits in silicon. The researchers were able to use traditional silicon transistor-based fabrication techniques to demonstrate “single-electron occupations in all four quantum dots of a 2 x 2 split-gate silicon device fabricated entirely by 300-mm-wafer foundry processes.”

a) SEM image of the device fabrication. b) Device schematic. Image used courtesy of Ansaloni et al.

Their technique requires an architecture that consists of an undoped silicon channel, connected to a highly-doped source and drain. Metallic polysilicon gates partially overlap the channel, inducing quantum dots with a controllable number of electrons. The technique allows for each dot to control a single electron, using gate-voltage pulsed techniques to make measurements and “single-shot charge” techniques to perform readouts using RF reflectometry. 

A significant aspect of this research was the ability to achieve a 2D array of qubits. Bringing qubits to the second dimension, meaning that each qubit can interact with one another, is important for error correction in quantum computing. 

Opening Doors for Scalable Quantum Computing

This research may represent some significant achievements in the world of quantum computing. 

First, by proving a silicon-based scheme, the research has introduced the possibility of foundry-manufacturable quantum computers, a big improvement from controlled academic environments. This opens the possibility for these computers to be manufactured easily and at a great scale. Beyond this, the research was able to bring 2D arrays of qubits to life, another necessity for the future of quantum computing. 

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