a sweet solution to overcrowded circuits in a quantum computer chip

May 11, 2023

(Nanowerk News) The silicon microchips of future quantum computers will be packed with millions, if not billions of qubits – the basic units of quantum information – to solve the biggest problems facing humanity. And with millions of qubits needing millions of wires in microchip circuits, it’s always going to be tight there.

But now engineers at UNSW Sydney have made an important step toward solving a longstanding problem of giving qubits more breathing room — and everything revolves around jellybeans.

Not the kind we rely on for sugar to get us through a 3pm slump. But jellybean quantum dots – elongated areas between pairs of qubits that create more room for wiring without disrupting the way pairs of qubits interact with each other. An artist’s impression of how qubits can be physically separated without breaking the lines of communication between them that are essential for quantum processing. (Image: Tony Melov/UNSW)

As lead author Associate Professor Arne Laucht explains, the jellybean quantum dot is not a new concept in quantum computing, and has been discussed as a solution for some of the many pathways to building the world’s first working quantum computer.

“It has been shown in different material systems such as gallium arsenide. But it had never been shown on silicon before,” he said.

Silicon is arguably one of the most important materials in quantum computing, A/Prof. Laucht said, because the infrastructure to manufacture future quantum computing chips is already in place, considering that we use silicon chips in classical computers. Another benefit is that you can fit a large number of qubits (in the form of electrons) on a single chip.

“But because the qubits have to be very close together to share information with each other, placing cables between each pair will always be a challenge.”

In a study published in Advanced Materials (“Jellybean Quantum Dots in Silicon for Qubit Coupling and On-Chip Quantum Chemistry”), a team of UNSW engineers explain how they demonstrated in the laboratory that jellyfish quantum dots are possible in silicon. This now paved the way for qubit spacing to ensure that the cables needed to connect and control the qubits could be installed between them.

How it works

In a normal quantum dot that uses spin qubits, single electrons are pulled from the electron pool in silicon to sit below a ‘quantum gate’ – where each electron’s spin represents a computational state. For example, an up spin could represent 0 and a down spin could represent 1. Each qubit could then be controlled by a magnetic field of oscillating microwave frequency.

But to implement quantum algorithms, we also need two-qubit gates, where the control of one qubit depends on the state of the other. For this to work, the two quantum dots have to be placed very close together, only a few 10 nanometers apart so their spins can interact with each other. (For illustration, a human hair is about 100,000 nanometers thick.)

But separating them further to create more real estate for wiring has always been a challenge scientists and engineers face. The problem is that as the pair of qubits move apart, they stop interacting.

Jellybean’s solution represents a way to have both: well-spaced qubits that continuously influence each other. To make jellybeans, engineers devised a way to create electron chains by trapping more electrons between the qubits. It acts as a quantum version of a string telephone so that the two qubit electrons attached to each end of the jellybean can continuously talk to each other. Only the electrons at each end are involved in any calculations, while the electrons at the jellyfish point are there to keep them interacting as they spread out.

The paper’s lead author, former PhD student Zeheng Wang said the amount of extra electrons pulled into the jellybean’s quantum dots is key to how they organize themselves.

“We showed in the paper that if you just put a few electrons into the pool of electrons that you have underneath, they will break up into smaller pools. So it’s not one continuous jellybean quantum dot, it’s smaller here, and it’s bigger in the middle and smaller there. We’re talking about a total of three to maybe ten electrons.

“It’s only when you use a larger number of electrons, say 15 or 20 electrons, that the jellybean becomes more continuous and homogeneous. And that’s where you have well-defined spins and quantums that you can use to couple qubits to others.

The post-jellybean quantum world

A/Prof. Laucht emphasized that there is still a lot of work to be done. The team’s efforts for this paper focused on proving that jellybean quantum dots are possible. The next step is to insert functioning qubits at each end of the jellybean quantum dot and make them talk to each other.

“Glad to see this work materialized. This increases our confidence that jellybean coupler can be used in silicon quantum computers, and we are excited to try to implement it with qubits next.”

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