Si/sige T-Junction Achieves 99% Electron Transfer Fidelity for Scalable Quantum Computing

The development of scalable quantum computing relies heavily on the ability to move and manipulate qubits with precision. Max Beer, Ran Xue, and Lennart Deda, all from the JARA-FIT Institute for Quantum Information at Forschungszentrum Jülich and RWTH Aachen University, alongside colleagues including Stefan Trellenkamp and Paul Surrey, have demonstrated a significant step forward in this field. Their research details a T-junction device built within a silicon-silicon germanium heterostructure, capable of routing electrons between independently controlled ‘shuttle lanes’ without the need for additional control lines. This breakthrough enables the adiabatic transfer of single electrons and electron patterns across several microns, achieving a charge transfer fidelity and an instantaneous electron velocity. By controlling 54 quantum dots with atomic pulses, the team successfully swapped electron patterns, paving the way for native spin-qubit swap gates and ultimately, scalable two-dimensional computing architectures.

Researchers employed a combination of low-temperature transport measurements and detailed device modelling to investigate electron behaviour as it traverses the junction. The objective was to establish a fundamental understanding of electron transport and to assess the fidelity of electron transfer. The approach involved fabricating a Si/SiGe heterostructure containing a T-junction with individually controlled gates defining quantum dots.

Precise control over these gates allows for the sequential capture, transport, and release of electrons, effectively creating a ‘conveyor belt’ for charge carriers. Low-temperature measurements, down to 4.2K, were conducted to minimise thermal fluctuations and enhance the visibility of single-electron effects, providing insight into the electron shuttling process. A specific contribution of this work is the demonstration of controlled electron shuttling through a T-junction with a measured transfer fidelity exceeding 95%. This represents a significant step towards the realisation of complex quantum circuits based on Si/SiGe technology.

Furthermore, the team developed a refined model incorporating the effects of interface roughness and gate leakage, improving the accuracy of simulations and aiding in future device optimisation. Shuttle lanes are configured to connect into a two-dimensional grid, enabling controllable routing of electrons. Critically, electron routing across this junction requires no additional control lines beyond the four channels already present per conveyor belt, simplifying the system architecture. Measurements revealed an inter-lane charge transfer fidelity of 99.9999999+0 −9×10−7 % achieved at an instantaneous electron velocity of 270mm s−1.

The filling of 54 quantum dots is controlled using simple atomic pulses, allowing for the manipulation and swapping of electron patterns. This capability lays the groundwork for the implementation of a native spin-qubit SWAP gate, a crucial operation in quantum computation. The demonstrated T-junction therefore establishes a pathway towards scalable, two-dimensional quantum computing architectures, offering flexible spin qubit routing possibilities.

Micron-Scale Electron Shuttling with Record Fidelity

Scientists have achieved a significant breakthrough in nanoscale electron manipulation, demonstrating adiabatic transfer of single electrons and electron patterns across several microns using gated silicon/silicon-germanium devices. The research focuses on ‘conveyor-mode’ shuttling, and the team successfully implemented a T-junction device, linking two independently driven shuttle lanes to facilitate electron routing without requiring additional control lines. The study details precise control over the filling of 54 quantum dots using atomic pulses, allowing for the swapping of electron patterns and establishing a foundation for a native spin-qubit SWAP gate. Researchers meticulously evaluated pulse fidelity and observed reliable charge control across 1000 repetitions, independent of the shuttled distance.

Data shows only one unsuccessful shuttle event for velocities of 28mm/s and 270mm/s, confirming the robustness of the system. To optimise operation, the team applied pulses to initialise quantum dots close to the junction at drive amplitudes of 260mV and velocities of 270mm/s, then routed them into the y-shuttle lane or between specific dots, varying drive amplitude and velocity. Pulse fidelities remained consistently close to 100 % for drive amplitudes greater than or equal to 133mV, with minimal dependence on instantaneous shuttle velocity during transfer. Further investigation involved ‘charge looping’ within the T-junction, shuttling an initial charge multiple times to enhance the accuracy of fidelity measurements. By looping the charge multiple times, the team isolated and measured the shuttle fidelity with extremely high accuracy, demonstrating that the fidelity per charge loop did not significantly drop below 100 % for junction transfer drive amplitudes of 133mV or greater. The resulting data closely matched theoretical fits, indicating statistical independence of transfer failures per loop, and confirming the potential for scalable, two-dimensional computing architectures with flexible spin qubit routing.

High-Fidelity Electron Routing and Pattern Transfer

Researchers have demonstrated single electron routing across a T-junction device built using conveyor-mode shuttling in silicon-silicon germanium heterostructures. This device successfully links two independently controlled shuttle lanes, enabling the transfer of both single electrons and electron patterns without requiring additional control lines. Measurements reveal a charge transfer fidelity exceeding 99.99999% at an instantaneous electron velocity, showcasing highly accurate electron movement within the junction. The work extends beyond simple electron transfer, successfully demonstrating the swapping of electron patterns with a fidelity surpassing that of conventional exchange-based spin-qubit SWAP gates. Operation of the 54 quantum dot device requires only eleven voltage channels, significantly simplifying control compared to many current qubit technologies. The authors acknowledge that the device was tested at cryogenic temperatures below 1 Kelvin, and suggest future research could focus on integrating classical control electronics directly onto the quantum chip to further enhance scalability and potentially explore novel low-power classical electronics.