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Computational Role of Multiqubit Tunneling in a Quantum Annealer
Sergio Boixo, Vadim N. Smelyanskiy, Alireza Shabani, Sergei V. Isakov, Mark Dykman, Vasil S. Denchev, Mohammad Amin, Anatoly Smirnov, Masoud Mohseni, Hartmut Neven
TL;DR
The paper asks whether multiqubit tunneling provides a computational advantage over classical thermal hopping in noisy quantum annealers. It develops an open-system theory and tests a 16-qubit tunneling primitive, finding contrasting temperature dependence and higher success probabilities on instances up to 200 qubits.
Problem
The paper examines whether quantum tunneling can help optimization when classical product-state paths are trapped by multiqubit energy barriers.
Method
The authors combine a non-perturbative open-quantum-system model using experimentally characterized noise with a 16-qubit primitive and larger instances containing repeated tunneling motifs.
Results
For instances up to 200 qubits, D-Wave Two achieved consistently higher success probabilities than product-state models, with fitted exponent α=(1.1±0.05)·10^-2 versus (2.8±0.17)·10^-2 for SVMC.
Takeaways & Limitations
The results indicate that multiqubit quantum phenomena can play a computational role in finding lower-energy solutions for optimization problems containing the tunneling primitive.
Takeaways & Limitations
For sufficiently small minimum gaps, multiqubit freezing can begin before the avoided crossing, changing the temperature dependence of success probability.
Abstract
from arXiv · showhide
Quantum tunneling, a phenomenon in which a quantum state traverses energy barriers above the energy of the state itself, has been hypothesized as an advantageous physical resource for optimization. Here we show that multiqubit tunneling plays a computational role in a currently available, albeit noisy, programmable quantum annealer. We develop a non-perturbative theory of open quantum dynamics under realistic noise characteristics predicting the rate of many-body dissipative quantum tunneling. We devise a computational primitive with 16 qubits where quantum evolutions enable tunneling to the global minimum while the corresponding classical paths are trapped in a false minimum. Furthermore, we experimentally demonstrate that quantum tunneling can outperform thermal hopping along classical paths for problems with up to 200 qubits containing the computational primitive. Our results indicate that many-body quantum phenomena could be used for finding better solutions to hard optimization problems.