1 May 2024 | Martín Larocca, Supanut Thanaisilp, Samson Wang, Kunal Sharma, Jacob Biamonte, Patrick J. Coles, Łukasz Cincio, Jarrod R. McClean, Zoë Holmes, and M. Cerezo
Variational quantum computing (VQC) is a flexible computational paradigm with applications in various fields, but it faces a significant challenge known as the Barren Plateau (BP) phenomenon. The BP occurs when the parameter optimization landscape becomes exponentially flat and featureless as the problem size increases, making it difficult to find optimal parameters. This review aims to provide a comprehensive understanding of BPs and their impact on VQC.
The introduction highlights the importance of VQC and the challenges it faces, particularly in training large-scale quantum algorithms. The BP is identified as a key barrier to the scalability and trainability of VQC. The review discusses the different types of BPs, including probabilistic and deterministic concentration, and methods to analyze and mitigate them.
The origins of BPs are explored, emphasizing the curse of dimensionality, where the exponentially large Hilbert space can lead to exponentially small inner products and concentration of loss functions. The expressive power of the parameterized quantum circuit (PQC) is a critical factor, with highly expressive circuits being more prone to BPs. The review also examines the role of initial states, measurements, and hardware noise in contributing to BPs.
Several strategies to avoid or mitigate BPs are discussed, including the use of shallow circuits, alternative initialization methods, and problem-encoding schemes. Shallow circuits, such as logarithmic-depth hardware-efficient ansatzes and quantum convolutional neural networks, can reduce the impact of BPs by constraining the exploration space of the measurement operator. Alternative initialization methods, such as problem-encoding schemes, can help jump-start training in regions with large gradients.
The review concludes by discussing the broader implications of BPs, including their connection to classical simulability and the potential for alternative variational paradigms. It emphasizes the ongoing research efforts to understand and overcome the challenges posed by BPs, highlighting the potential for VQC to become a robust and scalable computational tool.Variational quantum computing (VQC) is a flexible computational paradigm with applications in various fields, but it faces a significant challenge known as the Barren Plateau (BP) phenomenon. The BP occurs when the parameter optimization landscape becomes exponentially flat and featureless as the problem size increases, making it difficult to find optimal parameters. This review aims to provide a comprehensive understanding of BPs and their impact on VQC.
The introduction highlights the importance of VQC and the challenges it faces, particularly in training large-scale quantum algorithms. The BP is identified as a key barrier to the scalability and trainability of VQC. The review discusses the different types of BPs, including probabilistic and deterministic concentration, and methods to analyze and mitigate them.
The origins of BPs are explored, emphasizing the curse of dimensionality, where the exponentially large Hilbert space can lead to exponentially small inner products and concentration of loss functions. The expressive power of the parameterized quantum circuit (PQC) is a critical factor, with highly expressive circuits being more prone to BPs. The review also examines the role of initial states, measurements, and hardware noise in contributing to BPs.
Several strategies to avoid or mitigate BPs are discussed, including the use of shallow circuits, alternative initialization methods, and problem-encoding schemes. Shallow circuits, such as logarithmic-depth hardware-efficient ansatzes and quantum convolutional neural networks, can reduce the impact of BPs by constraining the exploration space of the measurement operator. Alternative initialization methods, such as problem-encoding schemes, can help jump-start training in regions with large gradients.
The review concludes by discussing the broader implications of BPs, including their connection to classical simulability and the potential for alternative variational paradigms. It emphasizes the ongoing research efforts to understand and overcome the challenges posed by BPs, highlighting the potential for VQC to become a robust and scalable computational tool.