Computational Role of Multiqubit Tunneling in a Quantum Annealer (1502.05754v1)

Abstract: 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.

Computational Role of Multiqubit Tunneling in a Quantum Annealer

The paper presented in "Computational Role of Multiqubit Tunneling in a Quantum Annealer" explores the significance and computational capabilities offered by multiqubit tunneling in the domain of quantum annealing. Quantum tunneling, a known quantum phenomenon where a quantum state navigates through energy barriers without possessing the requisite energy, is examined for its potential to enhance optimization processes. This paper presents a non-perturbative model to predict rates of many-body dissipative quantum tunneling while reflecting on the practical implications of multiqubit tunneling in contemporary quantum annealers.

Key Findings and Computational Implications

This research details a computational primitive incorporating 16 qubits to show how quantum evolution facilitates tunneling towards a global minimum, overcoming classical paths caught in false minima. Experimental evidence demonstrates that quantum tunneling can surpass classical thermal hopping in problems involving up to 200 qubits. Such observations support the premise that many-body quantum effects could potentially offer superior solutions to complex optimization challenges.

Theoretical Framework and Experimental Insights

The paper elaborates on a sophisticated theory of open quantum dynamics, accounting for real-world noise characteristics. Using D-Wave’s quantum annealing chips as a test bed, the paper illustrates that multiqubit tunneling remains advantageous despite environmental perturbations. A remarkable aspect is the demonstration of 'thermal reduction,' where success probabilities diminish with increasing temperature, emphasizing the influence of quantum tunneling beyond thermal activation. Such findings counter predictions from models strictly employing classical paths, notably Spin Vector Monte Carlo (SVMC).

Analytical and Experimental Observations

Figures within the paper present substantial data on quantum energy gaps, success probabilities across varying temperatures, and the implications of complex qubit interactions. These visualizations underscore the distinctive behavior of quantum systems particularly regarding energy landscape traversal via quantum annealing.

A detailed exploration of the energy gap dynamics, using a multiqubit master equation, underlines the distinct advantage conferred by quantum tunneling over traditional thermal processes. Moreover, non-perturbative analyses akin to the Non-interacting Blip Approximation (NIBA) further cement the computational edge offered by multiqubit tunneling, validated through rigorous alignment with D-Wave’s experimental outcomes.

Speculations and Future Directions

Potential future directions encompass exploring maximal $K$-qubit tunneling achievable with contemporary technological setups and how enhancements in this capacity might translate into genuine computational speedups. There is room to paper whether classical simulations can emulate or replicate these multiqubit tunneling phenomena effectively. Furthermore, this research forms a foundational step in progressing towards a 'physical speedup' relative to hypothetically classical models of current hardware.

The paper contributes significant insights into the ongoing discourse concerning quantum advantage, particularly in optimization solutions using quantum annealing frameworks. While declarations of quantum speedup demand thorough validation against established classical algorithms, these results pave the way for improved understanding and future quantum computational methodologies.