Semiconductor Spin Qubits Advance Quantum Computing, Leveraging Existing Transistor Technology

Semiconductor spin qubits, particularly silicon hole spin qubits, are a promising technology for large-scale quantum computers due to their fast, all-electrical control and resistance to charge and nuclear spin noise. A recent study demonstrated the electrical tunability of the exchange splitting in two hole-spin qubits in a silicon fin field-effect transistor (FinFET), a common device in the semiconductor industry. The research also showed that the exchange Hamiltonian can be engineered to enable two-qubit controlled rotation gates without a trade-off between speed and fidelity. This suggests that the technology could be a suitable approach for realizing a large-scale quantum computer.

What is the Potential of Semiconductor Spin Qubits?

Semiconductor spin qubits are a promising technology for the development of large-scale quantum computers. Silicon hole spin qubits, in particular, have the advantage of fast, all-electrical qubit control and sweet spots that counteract charge and nuclear spin noise. However, the demonstration of a two-qubit interaction has remained a challenge. One missing factor is an understanding of the exchange coupling in the presence of a strong spin-orbit interaction.

In a recent study, two hole-spin qubits in a silicon fin field-effect transistor, a common device in today’s semiconductor industry, were examined. The researchers demonstrated electrical tunability of the exchange splitting from above 500 MHz to close-to-off and performed a conditional spin-flip in 24 ns. The exchange is anisotropic because of the spin-orbit interaction. Upon tunnelling from one quantum dot to the other, the spin is rotated by almost 180 degrees.

The exchange Hamiltonian no longer has the Heisenberg form and can be engineered such that it enables two-qubit controlled rotation gates without a trade-off between speed and fidelity. This ideal behaviour applies over a wide range of magnetic field orientations, making the concept robust with respect to variations from qubit to qubit. This indicates that it is a suitable approach for realizing a large-scale quantum computer.

Why are Hole Spins Advantageous?

Compared to electron spins, hole spins can be controlled all-electrically without the added complexity of on-chip micromagnets or the need for orbital degeneracy, thanks to their intrinsic spin-orbit interaction. Moreover, holes benefit from a reduced hyperfine interaction and the absence of valleys.

Holes in quasi-one-dimensional nanostructures are highly attractive for implementing fast and coherent qubits. The mixing of heavy and light-hole states due to the 1D-confinement results in an unusually strong and electrically tunable direct Rashba spin-orbit interaction with sweet spots for charge and hyperfine noise. This enables ultrafast hole spin qubits with reduced sensitivity to noise.

Such a 1D-system can be realized using today’s industry-standard transistor design, known as the fin field-effect transistor (FinFET). Adapting FinFETs for quantum dot integration potentially facilitates quantum computer scale-up by leveraging decades of technology development in the semiconductor industry.

How Can FinFETs Contribute to Quantum Computing?

FinFETs are a promising platform for quantum computing due to their compatibility with existing semiconductor technology. Recent research has shown that individual hole spin qubits in a bulk-Si FinFET can be operated at temperatures above 4 K, paving the way for FinFET-based quantum integrated circuits that host both the qubit array and its classical control electronics on the same chip.

Universal quantum computation requires both single-qubit control and two-qubit interactions. Native two-qubit gates for spins, such as the SWAP, the controlled phase, or the controlled rotation, rely on the exchange interaction that arises from the wavefunction overlap between two adjacent quantum dots.

For electrons in silicon, two-qubit gate fidelities have recently surpassed 99%, but for holes in silicon or FinFETs, the demonstration of two-qubit logic is still missing due to the challenges in obtaining a controllable exchange interaction.

What is the Role of Exchange Interaction in Quantum Computing?

The exchange interaction is crucial for implementing high-fidelity two-qubit gates. However, for hole spins, it is still largely unexplored. The researchers measured the dependence of the exchange splitting on the magnetic field direction and found large values in some directions but close-to-zero values in other directions.

In addition, they developed a general theoretical framework applicable to a wide range of devices and identified the spin-orbit interaction as the main reason for the exchange anisotropy. From their measurements, they could extract the full exchange matrix and hence accurately determine the Hamiltonian of the two coupled spins, allowing them to predict the optimum operating points for the gates.

For holes, unlike electrons, the strong exchange anisotropy facilitates controlled rotations with both high fidelity and high speed for an experimental setting that is robust against device variation.

What are the Implications of this Research?

This research represents an important step towards a FinFET-based quantum processor by demonstrating control over the exchange of two holes in a silicon FinFET. The findings provide a deeper understanding of the exchange coupling in the presence of a strong spin-orbit interaction, a previously unexplored area in the field of hole spins.

The researchers’ ability to demonstrate electrical tunability of the exchange splitting and perform a conditional spin-flip in a short time frame is a significant achievement. Furthermore, the discovery that the exchange Hamiltonian can be engineered to enable two-qubit controlled rotation gates without a trade-off between speed and fidelity is a promising development for the future of quantum computing.

The robustness of this concept against variations from qubit to qubit suggests that it is a suitable approach for realizing a large-scale quantum computer. This research not only advances our understanding of semiconductor spin qubits but also paves the way for the development of more efficient and reliable quantum computing technologies.

What is the Future of Quantum Computing?

The future of quantum computing looks promising, with semiconductor spin qubits offering the potential to employ industrial transistor technology to produce large-scale quantum computers. The research conducted on silicon hole spin qubits and their interaction in a silicon FinFET provides valuable insights that could guide the development of future quantum computing technologies.

The ability to control the exchange of two holes in a silicon FinFET and the discovery of the anisotropic nature of the exchange due to the spin-orbit interaction are significant advancements in the field. These findings could lead to the development of more efficient and reliable quantum computing technologies.

Moreover, the potential to leverage decades of technology development in the semiconductor industry by adapting FinFETs for quantum dot integration could facilitate the scale-up of quantum computers. This research represents a significant step towards the realization of a large-scale quantum computer, bringing us closer to the dawn of a new era in computing.