RIKEN’s breakthrough in silicon quantum dot lifetime

Figure 1: Illustration of a qubit in a silicon quantum dot. Optimizing the size, shape and geometry of the quantum dot leads to long-lived hole cycles. Credit: © Tony Mellow

Modeling long-lived holes in silicon quantum dots to improve the development of quantum computers

RIKEN physicists have developed a theoretical model to improve semiconductor nanodevices, demonstrating that carefully designed quantum dots can create robust silicon hole-spin qubits that resist electrical noise. This research is important for understanding and designing large-scale quantum computers.

A theoretical model developed by three RIKEN physicists to improve semiconductor nanodevices could help scale quantum hardware.

An electron trapped in a semiconductor device provides a promising building block for future quantum computers. Electrons have a property called spin that, when measured, exists in one of two states, like binary information or bits used in a conventional computer. But due to its quantum nature, the spin can be in a superposition of two states. These quantum bits or qubits are at the heart of quantum information processing.

Peter Stano

Peter Stano and two colleagues have developed a theoretical model to optimize the design of silicon quantum-dot-based spin qubits. Credit: © 2023 RIKEN

Their positively charged counterparts called electrons or holes can be isolated in tiny semiconductor bubbles called quantum dots.

But electron and hole spins maintain their quantum state only for a limited time. Disturbance or noise from the vortex’s environment can change the vortex’s position. „Once a quantum state is assigned to a qubit, it immediately begins to fade,” explains Peter Stano of the RIKEN Center for Emergent Matter Science (CEMS).

This inevitable decay, or decay, is a fundamental limitation and a major difference to traditional information, which can be perpetuated. Understanding dephasing is essential for developing methods to mitigate it, thus aiding the design of large-scale quantum computers.

Now, Stano, along with CEMS colleagues Ognjen Malkoc and Daniel Loss, has theoretically engineered a hole trapped in a silicon quantum dot. Using this model, they demonstrated that the length of time a hole spin maintains its quantum state depends on the size and shape of the quantum dot and the magnetic and electric fields applied to it.

The team identified strong configurations of quantum dots that go beyond the established theoretical model.

„Our results show that by carefully designing a quantum dot and placing the electric and magnetic fields in certain ways, we can find sweet spots where silicon hole-spin qubits are remarkably robust against electrical noise,” says Stano.

This highlights one of the main advantages of spin qubits – they are largely immune to electrical noise, the strongest type of noise present in every semiconductor device.

But dephasing is one of the design considerations when optimizing quantum dots for quantum information processing. The speed and reliability of reading, writing and processing quantum information are also important.

„All of these features have similar sensitivities in a quantum dot design,” says Stano. „Our goal is to use the sensitivity found here to improve spin-qubit design.”

Reference: “Charge-Noise-Induced Dephasing in Silicon Hole-Spin Qubits” by Ognjen Malkok, Peter Stano, and Daniel Laws, 8 December 2022. Physical review letters.
DOI: 10.1103/PhysRevLett.129.247701

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