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Jellybeans – a sweet solution for overcrowded circuitry in quantum computer chips

Engineers at the University of New South Wales (UNSW) Sydney have made a significant stride towards solving a long-standing issue in quantum computing: overcrowding of qubits, the fundamental units of quantum information. This problem is the result of future quantum computers needing millions, possibly billions, of qubits which would necessitate millions of wires in the microchip circuitry, leading to severe overcrowding.

The researchers' ingenious solution revolves around the concept of 'jellybean quantum dots' - not actual jellybeans, but elongated spaces between pairs of qubits. These jellybean spaces allow for additional room for wiring without disrupting the necessary interaction between paired qubits.

Associate Professor Arne Laucht, the lead author of the study, explains that while the jellybean quantum dot is not new in quantum computing, the innovation here is that they have managed to show its viability in silicon. This is a breakthrough because silicon is a critical material in quantum computing - not only is the infrastructure to manufacture future quantum computing chips using silicon already in place due to its use in classical computers, but it also allows for a significant number of qubits to fit on a single chip. The challenge, however, has been placing wires between each pair of closely-positioned qubits, which is essential for them to share information.

In their study published in Advanced Materials, the team describes their successful demonstration in the lab that jellybean quantum dots were possible in silicon. This could pave the way for qubits to be spaced apart enough to ensure that the necessary wires can be fit in between.

To understand how this works, let's take a look at how qubits in a normal quantum dot work. Here, single electrons are drawn from a pool of electrons in silicon to sit under a 'quantum gate'. The spin of each electron represents the computational state - for instance, spin up could represent a 0 and spin down could represent a 1. Each qubit can then be controlled by an oscillating magnetic field of microwave frequency. But for a quantum algorithm to work, we also need two-qubit gates, where one qubit's control is conditional on the state of the other. This requires both quantum dots to be placed very closely, just a few tens of nanometres apart so their spins can interact.

However, spacing them further apart for wiring has been a challenging task because as the qubits move apart, they cease to interact. The jellybean solution mitigates this problem by ensuring that the spaced-out qubits continue to interact. To create a jellybean quantum dot, engineers trapped more electrons between the qubits. This forms a chain of electrons that acts as a quantum version of a string phone, enabling two paired qubit electrons at each end of the jellybean to continue to communicate. Only the electrons at each end participate in computations, while the electrons in the jellybean dot maintain their interaction while being spread apart.

The number of extra electrons pulled into the jellybean quantum dot is key to how they arrange themselves. Larger numbers of electrons (around 15 to 20) make the jellybean more continuous and homogeneous, creating well-defined spin and quantum states that can couple qubits to another.

Despite these promising results, there is still a lot of work to be done. The team's efforts for this study were focused on proving that the jellybean quantum dot is possible. The next step is to insert working qubits at each end of the jellybean quantum dot and make them communicate. This breakthrough enhances the potential for jellybean couplers to be utilized in silicon quantum computers, providing a significant step towards building more efficient and effective quantum computing systems.