Abstract
Many quantum systems are being investigated in the hope of building a large-scale quantum computer. All of these systems suffer from decoherence, resulting in errors during the execution of quantum gates. Quantum error correction enables reliable quantum computation given unreliable hardware. Unoptimized topological quantum error correction (TQEC), while still effective, performs very suboptimally, especially at low error rates. Hand optimizing the classical processing associated with a TQEC scheme for a specific system to achieve better error tolerance can be extremely laborious. We describe a tool, Autotune, capable of performing this optimization automatically, and give two highly distinct examples of its use and extreme outperformance of unoptimized TQEC. Autotune is designed to facilitate the precise study of real hardware running TQEC, with every quantum gate having a realistic, physics-based error model.
- Received 28 February 2012
DOI:https://doi.org/10.1103/PhysRevX.2.041003
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Published by the American Physical Society
Popular Summary
In the context of quantum computers, quantum mechanical properties, such as the superposition principle and entanglement, work like a double-edged sword. On the one hand, they can give a quantum computer computational power unparalleled by classical computers; on the other, they make the quantum computer extremely sensitive to errors produced by the interaction of quantum bits (qubits) with their environment or by imperfect operations, whether the qubits are photons, superconducting devices, or trapped ions. Error correction is, therefore, a must in the design, engineering, and operation of a quantum computer. Exactly how hard is it to build a quantum computer that is robust against physically realistic error-occurrence rates? How high does the fidelity of the quantum components need to be? Finding answers to these questions for any specific system has been extremely laborious to date. In this paper, we present a powerful software-based tool, Autotune, that transforms this laborious task into a highly automated and efficient one for a very large range of hardware technologies.
One of the quantum error-correction approaches that is gaining dominance is topological error correction. In this approach, the logical qubits that do calculations are distributed over a lattice of physical qubits, with their boundaries cut into the lattice. The tolerance of the calculations to the unavoidable errors is high as physical errors must form chains between boundaries of the logical qubits. The formation of error chains can be made exponentially unlikely by increasing the distance between the boundaries. A proof-of-principle experimental demonstration of this approach with entangled photons has been reported recently. Optimization for specific hardware is now at the forefront of research on topological error correction. In approaching this central issue, Autotune tracks all possible errors through the potentially complex quantum circuits implementing topological error correction on specific hardware, and constructs a particular graph problem whose solution provides corrections that are highly likely to lead to reliable computation with that hardware.
We believe that Autotune is a major step toward a practical classical control system for a large-scale quantum computer and look forward to seeing its broad use in this development.
