Unveiling the potential of hole-rotating qubits in quantum computing

Scientists from the University of Basel and NCCR SPIN have achieved the first controllable interaction between two hole spin qubits in a typical silicon transistor. The discovery makes it possible to use established manufacturing techniques to combine millions of these qubits on a single chip. The research was published in the journal Physics of nature.

Unveiling the potential of hole-rotating qubits in quantum computing

Two interacting hole-spin qubits: As a hole (magenta/yellow) tunnels from one place to another, its spin rotates due to spin-orbit coupling, resulting in anisotropic interactions represented by the surrounding bubbles. Image credit: NCCR SPIN

The race to build a practical quantum computer is on, with researchers around the world working on a wide variety of qubit technologies. Until now, there has been no consensus on which type of qubit is best suited to maximize the potential of quantum information science.

A quantum computer’s qubits, which handle data processing, transport and storage, are its fundamental building blocks. They must process information quickly and store it accurately to function properly. Stable and fast interactions between multiple qubits whose states are reliably externally controllable form the basis for fast information processing.

Millions of qubits need to fit on a single chip for a quantum computer to be useful. With only a few hundred qubits, the most sophisticated quantum computers available today are limited to performing tasks that can already be completed (and often faster) by conventional computers.

Electrons and holes

Researchers at the University of Basel and NCCR SPIN are tackling the challenge of organizing and linking thousands of qubits using a type of qubit that exploits the spin (intrinsic angular momentum) of an electron or hole.

A hole is essentially a missing electron in a semiconductor. Both holes and electrons have spin, which can adopt one of two states: up or down, analogous to 0 and 1 in classical bits. An advantage of a hole spin over an electron spin is that it can be controlled entirely by electrical means, eliminating the need for additional components such as on-chip micromagnets.

As early as 2022, Basel physicists showed that hole spins could be trapped and used as qubits in existing electronic devices. These devices, known as “FinFETs” (fin field-effect transistors), are integral components of modern smartphones and are manufactured using widespread industrial processes.

Recently, a team led by Dr. Andreas Kuhlmann made a breakthrough by successfully facilitating a controllable interaction between two qubits within this configuration for the first time.

Fast and precise controlled Spin-Flip

Quantum computers require “quantum gates” to perform calculations; these gates are operations that manipulate qubits and bind them together. As detailed in the journal Nature Physics, researchers have successfully coupled two qubits and achieved a controlled spin of one qubit’s spin based on the spin state of the other, a process known as controlled spin.

Hole spins allow us to create fast, high-fidelity two-qubit gates. This principle now also allows a larger number of qubit pairs to be coupled.

Dr. Andreas Kuhlmann, Department of Physics, University of Basel

The exchange interaction between two electrostatically interacting indistinguishable particles provides the basis for the coupling of two spin qubits.

Surprisingly, the hole exchange energy is not only electrically controllable but strongly anisotropic. This is due to spin-orbit coupling, which means that the spin state of a hole is influenced by its motion through space.

Experimental and theoretical physicists from NCCR SPIN and the University of Basel joined forces to describe this observation in a model.

Anisotropy makes two-qubit gates possible without the usual trade-off between speed and fidelity, hole-spin-based qubits not only take advantage of tried and tested silicon chip fabrication, but are also highly scalable and have proven be fast and robust in experiments.

Dr. Andreas Kuhlmann, Department of Physics, University of Basel

The study highlights how promising this strategy is for building a large-scale quantum computer.

Journal reference:‌

Geyer, S., et al. (2024) Anisotropic exchange interaction of two hole-spin qubits. Physics of nature. doi.org/10.1038/s41567-024-02481-5.

Source: https://www.unibas.ch/en.htm

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Image Source : www.azoquantum.com

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