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arXiv:1902.03698

Reliable quantum circuits have defects

Alexandru Paler, Austin G. Fowler, Robert Wille

Published Feb 11, 2019• 0 citations
arXivHTMLPDF

Abstract

State of the art quantum computing architectures are founded on the decision to use scalable but faulty quantum hardware in conjunction with an efficient error correcting code capable of tolerating high error rates. The promised effect of this decision is that the first large-scale practical quantum computer is within reach. Coming to grips with the strategy and the challenges of preparing reliable executions of an arbitrary quantum computation is not difficult. Moreover, the article explains why defects are good.

DOI
10.48550/arxiv.1902.03698

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State of the art quantum computing architectures are founded on the decision to use scalable but faulty quantum hardware in conjunction with an efficient error correcting code capable of tolerating high error rates. The promised effect of this decision is that the first large-scale practical quantum computer is within reach. Coming to grips with the strategy and the challenges of preparing reliable executions of an arbitrary quantum computation is not difficult. Moreover, the article explains why defects are good.
Quantum computing is a new and promising technology with the potential of exponentially powerful computation - if only a large-scale one can be built. There are several challenges in building a large-scale quantum computer - fabrication, verification, and architecture. The power of quantum computing comes from the ability to store a complex state in a single bit. This also what makes quantum systems difficult to build, verify, and design. Quantum states are fragile, so fabrication must be precise, and bits must often operate at very low temperatures. Unfortunately, the complete state may not be measured precisely, so verification is difficult. Imagine verifying an operation that is expected to not always get the same answer, but only an answer with a particular probability! Finally, errors occur much more often than with classical computing, making error correction the dominant task that quantum architectures need to perform well. We provide a basic tutorial of quantum computation for the system designer and examine the fundamental design and verification issues in constructing scalable quantum computers. We find the primary issues to be the verification of precise fabrication constraints, the design of quantum communication mechanisms, and the design of classical control circuitry for quantum operation.
This thesis presents an overview of my published work on streamlining and optimizing quantum processors. Complete, microscopic, quantum-gate–by–quantum-gate simulations of a variety of quantum circuits are implemented and executed on state-of-the-art multi-processor parallel computers. A host of analytical scaling formulas addressing the quality of quantum processors, e.g., in the presence of realistic gate errors and defects, are presented. These formulas reliably describe the practically interesting, large-number-of-qubit regime, inaccessible to simulations on classical computers. Much of the technical details may be found in my published articles, which appear in the Appendix. This thesis also presents the generic and specific directions into which my current investigations are headed. In addition, several preliminary results are presented and discussed.
Quantum computers are domain-specific accelerators that can solve commercially important problems that are beyond the capability of conventional computers. Quantum computing is at an inflection point where small systems with a few dozen qubits have been demonstrated and the number of qubits is expected to increase to several hundred over the coming years. However, the qubit quality is likely to remain quite low making it difficult to execute programs reliably. Furthermore, scaling the designs to a large number of qubits would require specialized hardware that operates at cryogenic conditions. This article describes the compiler and hardware support for enabling reliable and scalable quantum computers.
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