Engineers at UNSW Sydney have made a significant advancement in quantum computing by introducing a solution known as “jellybean quantum dots.” These elongated areas between qubit pairs create additional space for wiring, allowing for a greater number of qubits to be packed onto a silicon microchip without disrupting their interaction.

While the concept of jellybean quantum dots has been explored in other material systems such as gallium arsenide, this is the first demonstration of its feasibility in silicon. Silicon holds immense importance in quantum computing due to its existing infrastructure for producing quantum computing chips, as well as its ability to accommodate a large number of qubits.

Traditionally, the challenge in quantum computing has been the close proximity required between qubits for effective information sharing. This made it difficult to incorporate the necessary wiring between each qubit pair. However, the recent study by the UNSW team, published in Advanced Materials, proves that jellybean quantum dots can be implemented in silicon, thereby allowing for increased spacing between qubits and facilitating the integration of wiring required to connect and control the qubits.

This breakthrough holds great promise for the future of quantum computers, as it enables the packing of millions, if not billions, of qubits onto microchips. With this increased capacity for quantum information, researchers hope to address some of the most complex challenges facing humanity.

## How it works

In a conventional quantum dot utilizing spin qubits, individual electrons are selected from a pool of electrons in silicon and placed beneath a “quantum gate.” The spin state of each electron represents a computational state, with spin up indicating 0 and spin down indicating 1. These qubits can be controlled through a microwave-frequency oscillating magnetic field.

However, to execute a quantum algorithm, two-qubit gates are necessary, where the state of one qubit depends on the state of the other. To achieve this, the quantum dots hosting the qubits need to be positioned very close to each other, just a few tens of nanometers apart, to enable interaction between their spins. (To put this into perspective, a single human hair is about 100,000 nanometers thick.)

The challenge for scientists and engineers has always been how to create more space for wiring while maintaining the required proximity between the paired qubits. If the qubits are moved further apart, their interaction ceases.

The introduction of jellybean quantum dots offers a solution by allowing qubits to be spaced apart while still influencing each other. To create the jellybean structure, engineers discovered a method to trap additional electrons between the qubits, forming a chain of electrons. This arrangement functions like a quantum version of a string phone, enabling the two qubit electrons at each end of the jellybean to maintain communication. The electrons within the jellybean dot do not directly participate in computations but serve to sustain interaction between the spread-apart qubits.

Zeheng Wang, the paper’s lead author and former Ph.D. student, explains that the number of extra electrons within the jellybean quantum dot determines their arrangement. When only a few electrons are loaded, they segregate into smaller puddles, resulting in a non-continuous jellybean quantum dot. However, with a higher number of electrons, around 15 to 20, the jellybean becomes more continuous and uniform, allowing for well-defined spin and quantum states that can be used to couple qubits together.

This innovative approach provides a pathway to overcoming the space constraints in quantum computing by enabling the placement of wiring between qubits while maintaining their essential interactions.

## Post-jellybean quantum world

A/Prof. Laucht emphasizes the importance of continued efforts, highlighting that there is still a significant amount of work remaining. The team’s primary objective for this paper was to provide evidence supporting the feasibility of the jellybean quantum dot. Moving forward, their next task is to integrate functioning qubits at both ends of the jellybean quantum dot and establish communication between them.

“The realization of this work is truly gratifying. It significantly enhances our confidence in the applicability of jellybean couplers in silicon-based quantum computers. We are eagerly looking forward to the next phase, where we aim to incorporate qubits and explore their interaction within this system.”

Source: University of New South Wales