Optimized Quantum Compilation for Near-Term Algorithms with OpenPulse (2004.11205v2)

Abstract: Quantum computers are traditionally operated by programmers at the granularity of a gate-based instruction set. However, the actual device-level control of a quantum computer is performed via analog pulses. We introduce a compiler that exploits direct control at this microarchitectural level to achieve significant improvements for quantum programs. Unlike quantum optimal control, our approach is bootstrapped from existing gate calibrations and the resulting pulses are simple. Our techniques are applicable to any quantum computer and realizable on current devices. We validate our techniques with millions of experimental shots on IBM quantum computers, controlled via the OpenPulse control interface. For representative benchmarks, our pulse control techniques achieve both 1.6x lower error rates and 2x faster execution time, relative to standard gate-based compilation. These improvements are critical in the near-term era of quantum computing, which is bottlenecked by error rates and qubit lifetimes.

• The paper introduces a pulse-level quantum compiler that directly controls hardware to substantially reduce error rates and execution times.
• It bypasses traditional gate-level methods by leveraging pre-calibrated analog pulses and novel cancellation techniques for efficient operation.
• Experimental benchmarks on IBM quantum computers demonstrate up to a 2x speed improvement and a 1.6x reduction in error rates for key operations.

Optimized Quantum Compilation for Near-Term Algorithms with OpenPulse

The paper "Optimized Quantum Compilation for Near-Term Algorithms with OpenPulse" introduces an approach that leverages pulse-level control to significantly enhance the performance of quantum programs. This approach is implemented through a compiler that operates at the microarchitectural level, thereby offering substantial improvements in execution time and error rates for quantum computing applications. This paper particularly emphasizes the direct control over quantum operations via analog pulses, which is distinct from traditional gate-based quantum compilers.

Summary of Techniques and Results

The key contribution of this research is the development of a quantum compiler that exploits the OpenPulse interface for directly controlling quantum hardware. This interface facilitates pulse-level compilations that are more akin to how quantum hardware operates, as opposed to standard gate-level communications. The authors argue that while Quantum Optimal Control (QOC) has shown potential in certain settings, its reliance on accurate machine modeling (Hamiltonians) and high calibration overhead make it less practical in noisy, real-world conditions. The techniques proposed in this work do not require extensive Hamiltonian knowledge and operate effectively using existing calibrations conducted for basic gates on current quantum devices.

The authors conducted extensive validation of their approach through experiments on IBM quantum computers equipped with OpenPulse. Across a series of benchmarks representative of near-term quantum algorithms, such as the Variational Quantum Eigensolver (VQE) and Quantum Approximate Optimization Algorithm (QAOA), the proposed techniques achieved notable performance improvements. Specifically, they report a 1.6x reduction in error rates and a 2x improvement in execution times compared to traditional gate-based compilations.

Detailed Analysis of Compiler Enhancements

The paper identifies and implements several pulse-level optimizations within the compiler framework which are achievable on existing quantum hardware:

1. Direct Rotations: By accessing direct control over pulses, any single-qubit operation can be implemented directly, bypassing inefficiencies inherent in standard compilation methods. For instance, the implementation of X gates is significantly accelerated by leveraging pre-calibrated pulses, resulting in faster and more accurate execution.
2. Cross-Gate Pulse Cancellation: The compiler takes advantage of non-atomic gate decomposition. By augmenting basis gates to expose true atomic units at the pulse level, new cancellation opportunities arise that can further speed up operations by approximately 24%.
3. Two-Qubit Operations: The compiler employs optimized decomposition of two-qubit gates, making use of parametrized hardware operations such as the Cross-Resonance pulse. This results in up to 60% lower error rates when implementing common quantum operations like the ZZ Interaction.
4. Qudit Operations: Pulse-level control allows addressing energy levels beyond the qubit subspace. The authors demonstrate this with a base-3 counter implementation using a single qutrit, showcasing the potential for qutrit and qudit applications.

Implications and Future Directions

This research underscores the importance of aligning quantum compilation efforts closely with the underlying hardware capabilities, particularly in the context of near-term quantum computers. By leveraging pulse-level controls, the compiler they propose can substantially enhance both speed and fidelity, which are critical for practical quantum applications constrained by error rates and qubit lifetimes.

In terms of practical implications, the presented techniques are immediately implementable on current hardware compatible with OpenPulse, encouraging widespread adoption. Theoretically, this work suggests a paradigm shift in quantum compilation strategies moving beyond gate-level abstractions to more hardware-tailored solutions.

Future directions could include the extension of these techniques to other forms of quantum hardware and further exploration of qudit operation benefits. As more quantum hardware becomes accessible with similar control functionalities, this approach may become a foundational methodology for quantum compilation in the near-term era. The proposed techniques also offer a potential bridge to more robust implementations of QOC as experimental barriers are overcome.