A singlet triplet hole spin qubit in planar Ge (2011.13755v4)

Abstract: Spin qubits are considered to be among the most promising candidates for building a quantum processor. GroupIV hole spin qubits have moved into the focus of interest due to the ease of operation and compatibility with Si technology. In addition, Ge offers the option for monolithic superconductor-semiconductor integration. Here we demonstrate a hole spin qubit operating at fields below 10 mT, the critical field of Al, by exploiting the large out-of-plane hole g-factors in planar Ge and by encoding the qubit into the singlet-triplet states of a double quantum dot. We observe electrically controlled g-factor-difference-driven and exchange-driven rotations with tunable frequencies exceeding 100 MHz and dephasing times of 1 $\mu$s which we extend beyond 150 $\mu$s with echo techniques. These results demonstrate that Ge hole singlet-triplet qubits are competing with state-of-the art GaAs and Si singlet-triplet qubits. In addition, their rotation frequencies and coherence are on par with Ge single spin qubits, but they can be operated at much lower fields underlining their potential for on chip integration with superconducting technologies.

A Singlet-Triplet Hole Spin Qubit in Planar Ge

The research conducted explores the development and operation of a singlet-triplet hole spin qubit utilizing planar germanium (Ge). Such advancements are vital due to the compatibility of group IV hole spin qubits with existing silicon (Si) technology, alongside Ge's potential for integration with superconductor-semiconductor hybrid structures.

The paper presents a breakthrough in achieving a hole spin qubit function at magnetic fields below 10 mT, which is particularly notable as these fields are below the typical critical field of aluminum (Al). This is accomplished by leveraging the large out-of-plane g-factors inherent in planar Ge and encoding the qubit into the singlet-triplet states within a double quantum dot structure. The research indicates the ability to perform electrically controlled g-factor-difference-driven and exchange-driven rotations with tunable frequencies exceeding 100 MHz. Furthermore, the qubit's dephasing times are revealed to be approximately 1 μs, extendable beyond 150 μs with the application of echo techniques.

The experimental setup involves a meticulously grown strained Ge quantum well using low-energy plasma-enhanced chemical vapor deposition (LEPECVD). This technique results in a high hole mobility of 1.0 × 10^5 cm²/Vs at a density of 9.7 × 10^11 cm⁻². The device architecture includes an elaborate gate layout on a Ge quantum well to support the formation of a double quantum dot. The hole states confined in the Ge quantum well are predominantly heavy-hole types, providing a robust effective spin-1/2 system with strong spin-orbit coupling (SOC).

Significantly, the study demonstrates the capability to achieve two-axis control of the qubit by utilizing both electrically driven g-factor difference-driven rotations and exchange rotations. The $\Delta g$-driven rotations reach frequencies of 150 MHz at magnetic fields as low as 5 mT. The tunability of the g-factors through electrical gating accentuates the potential for fast and efficient qubit manipulation at low magnetic fields, which is a crucial requirement for the integration of quantum processing elements with existing computing architectures.

The implications of this research are profound, potentially paving the way for scalable quantum information processes using Ge-based hole spin qubits. The low magnetic field operation allows for easier integration with superconducting technologies, facilitating advancements in quantum processors. Additionally, the results align with existing technologies in GaAs and Si singlet-triplet qubits, thus demonstrating the competitive nature of Ge in the field of quantum computing.

Future directions might involve exploring the integration of this technology with superconducting circuits and further refinement in qubit coherence and manipulation speeds. Further investigation could provide insights into optimizing device fabrication and operational parameters for enhanced performance, with a view toward achieving higher gate fidelities necessary for fault-tolerant quantum computing. The promising results of this study underscore the potential of Ge as a pivotal material in the development of next-generation quantum processing technologies.