Modern computer applications usually consist of a variety of components that often require quite different computational co-processors. Some examples of such co-processors are TPUs, GPUs or FPGAs. A more recent and promising technology that is being investigated is quantum co-processors. In this paper, we present a modern computer architecture where a quantum co-processor is included as an additional accelerator. In such an environment, the idea is to execute the application on a heterogeneous architecture where the classic processor will execute the host part, but certain components will be mapped, in our case, on the quantum accelerator. To this purpose, we define the distinct layers for the quantum computer architecture where there is a clear boundary between the host program and quantum kernel(s). We also discuss the opportunities and challenges of mapping hybrid algorithms to such a heterogeneous quantum computer architecture.

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A widely-used quantum programming paradigm comprises of both the data flow and control flow. Existing quantum hardware cannot well support the control flow, significantly limiting the range of quantum software executable on the hardware. By analyzing the constraints in the control microarchitecture, we found that existing quantum assembly languages are either too high-level or too restricted to support comprehensive flow control on the hardware. Also, as observed with the quantum microinstruction set QuMIS [1], the quantum instruction set architecture (QISA) design may suffer from limited scalability and flexibility because of microarchitectural constraints. It is an open challenge to design a scalable and flexible QISA which provides a comprehensive abstraction of the quantum hardware. In this paper, we propose an executable QISA, called eQASM, that can be translated from quantum assembly language (QASM), supports comprehensive quantum program flow control, and is executed on a quantum control microarchitecture. With efficient timing specification, single-operationmultiple-qubit execution, and a very-long-instruction-word architecture, eQASM presents better scalability than QuMIS. The definition of eQASM focuses on the assembly level to be expressive. Quantum operations are configured at compile time instead of being defined at QISA design time. We instantiate eQASM into a 32-bit instruction set targeting a seven-qubit superconducting quantum processor. We validate our design by performing several experiments on a two-qubit quantum processor.

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The Pauli frame mechanism allows Pauli gates to be tracked in classical electronics and can relax the timing constraints for error syndrome measurement and error decoding. When building a quantum computer, such a mechanism may be beneficial, and the goal of this paper is not only to study the working principles of a Pauli frame but also to quantify its potential effect on the logical error rate. To this purpose, we implemented and simulated the Pauli frame module which, in principle, can be directly mapped into a hardware implementation. Simulation of a surface code 17 logical qubit has shown that a Pauli frame can reduce the error rate of a logical qubit up to 70% compared to the same logical qubit without Pauli frame when the decoding time equals the error correction time, and maximum parallelism can be obtained.

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In this paper, we present a high level view of the heterogeneous quantum computer architecture as any future quantum computer will consist of both a classical and quantum computing part. The classical part is needed for error correction as well as for the execution of algorithms that contain both classical and quantum logic. We present a complete system stack describing the different layers when building a quantum computer. We also present the control logic andcorresponding data path that needs to be implemented when executing quantum instructions and conclude by discussing design choices in the quantum plane.@en