Silicon Spin Qubits Enhance Logical Performance with Hybrid Encoding Schemes.

Research demonstrates superior performance of a hybrid encoding scheme, utilising both Zeeman and singlet-triplet qubits, when implementing surface and Bacon-Shor codes with silicon spin qubits. Logical qubit fidelity is limited not by qubit memory, but by the speed and accuracy of one and two qubit gate operations.

The pursuit of scalable quantum computation necessitates robust methods for mitigating the inherent fragility of quantum information, a challenge addressed through quantum error correction. Researchers are actively evaluating various qubit technologies and error-correcting codes to determine the most viable path towards fault-tolerant quantum computers. A new study, detailed in a forthcoming publication, compares the performance of two prominent error-correcting codes, the surface code and the Bacon-Shor code, when implemented using spin qubits in silicon. Mauricio Gutiérrez, from the Escuela de Química at the Universidad de Costa Rica, leads the investigation, collaborating with Juan S. Rojas-Arias of the RIKEN Center for Quantum Computing, David Obando from the Escuela de Física at the Universidad de Costa Rica, and Chien-Yuan Chang from the Institute of Electrical Engineering at National Tsing Hua University. Their work, entitled “Comparison of spin-qubit architectures for quantum error-correcting codes”, assesses both all-Zeeman qubit and hybrid Zeeman-singlet-triplet encoding schemes, revealing that the latter consistently achieves superior performance, with limitations stemming primarily from gate fidelity rather than memory coherence.

Silicon spin qubits represent a compelling platform for constructing scalable quantum computers, and ongoing research concentrates on refining error correction strategies to optimise their functionality. Recent investigations assess the viability of implementing two prominent quantum error correction codes, the surface code and the Bacon-Shor code, utilising spin qubits fabricated in silicon, and constructing logical qubits from planar arrays of quantum dots. A quantum dot is a semiconductor nanocrystal exhibiting quantum mechanical properties, and serves as the physical realisation of a qubit. The study directly compares the performance of single-electron Zeeman qubits, where information is encoded in the electron’s spin manipulated by a magnetic field, and a hybrid approach integrating Zeeman and singlet-triplet qubits. Singlet-triplet qubits leverage the combined spin states of two electrons, offering potentially enhanced coherence and control.

The research consistently demonstrates a performance advantage for the hybrid encoding scheme, exhibiting higher fidelity in logical state preparation and improved cycle-level performance, crucial metrics for evaluating the effectiveness of quantum error correction. Fidelity refers to the accuracy of a quantum operation. At the same time, cycle-level performance assesses the efficiency of error correction cycles, which are repeated sequences of operations designed to detect and correct errors. Detailed analysis reveals the logical error rate is not constrained by the qubit’s memory or coherence time, but rather by the fidelity of quantum gates, indicating both single-qubit and two-qubit gate operations contribute significantly to the overall error rate. Coherence time represents the duration for which a qubit maintains its quantum state before decoherence, the loss of quantum information, occurs.

The study identifies gate fidelity as the primary bottleneck for achieving scalable, fault-tolerant quantum computation with these silicon-based spin qubits, directing future development efforts toward improving the precision and reliability of quantum gate operations. Researchers actively explore optimising gate designs and control sequences to minimise error rates, a process involving careful consideration of various factors, including pulse shaping and timing. Pulse shaping involves precisely controlling the form of electromagnetic pulses used to manipulate qubits, while accurate timing is essential for coordinating operations between qubits.

Researchers highlight the importance of understanding the dominant error mechanisms within the system, pinpointing these limitations and offering a clear pathway towards enhancing spin qubit hardware and progressing towards practical, scalable fault-tolerant quantum architectures. This improvement will require significant advancements in materials science and device fabrication techniques, including the development of more precise control over the size and purity of quantum dots, and the creation of more robust interfaces between qubits and control circuitry.