Entanglement in a quantum annealing processor (1401.3500v1)

Abstract: Entanglement lies at the core of quantum algorithms designed to solve problems that are intractable by classical approaches. One such algorithm, quantum annealing (QA), provides a promising path to a practical quantum processor. We have built a series of scalable QA processors consisting of networks of manufactured interacting spins (qubits). Here, we use qubit tunneling spectroscopy to measure the energy eigenspectrum of two- and eight-qubit systems within one such processor, demonstrating quantum coherence in these systems. We present experimental evidence that, during a critical portion of QA, the qubits become entangled and that entanglement persists even as these systems reach equilibrium with a thermal environment. Our results provide an encouraging sign that QA is a viable technology for large-scale quantum computing.

• The paper presents experimental evidence of entanglement in a superconducting flux qubit quantum annealing processor using qubit tunneling spectroscopy.
• The study employs energy gap measurements and entanglement witnesses such as concurrence and negativity to verify non-classical correlations.
• The findings underscore quantum annealing’s viability for scalable quantum computing and its robustness against thermal noise in practical settings.

Entanglement in a Quantum Annealing Processor

The paper "Entanglement in a Quantum Annealing Processor" explores the observation and quantification of entanglement within a quantum annealing (QA) processor, created using superconducting flux qubits, to verify the viability of QA in quantum computing. The documented research presents methods and experimental evidence for entanglement during QA, which is a notable indication of quantum coherence among qubits in the context of quantum annealing.

Quantum Annealing and Entanglement

Quantum annealing is an optimization algorithm used to find low-energy states of a system, which are solutions to computationally complex problems. It employs an annealing schedule governed by a time-dependent Hamiltonian, balancing between quantum fluctuations (transverse field) and problem-specific interactions (longitudinal field). A critical aspect of such systems is entanglement, serving both as a core quantum resource and as an indicator of quantum advantage over classical approaches. The paper addresses the persistent challenge of detecting entanglement due to architectural constraints, including environmental couplings and control limitations.

Experimental Demonstration and Methodology

The processor in question contains a network of superconducting flux qubits. Through careful incorporation of qubit tunneling spectroscopy (QTS), the research examines a two-qubit system and an eight-qubit unit cell, navigating around limitations of typical characterization methods. QTS allows the mapping of the eigenspectrum, revealing energy gaps that suggest the presence of entanglement.

The experiment involves incorporating individual probe qubits to detect the energy differences in a degeneracy region, revealing avoided crossings—as predicted by QA theory—between the ground and first excited states. Measurements at intermediate annealing parameters $(s)$ showcase an energy gap that exceeds the thermal energy of the environment $(g \gg k_BT)$, suggesting robustness to thermal influences, which is crucial for effective operation.

Entanglement Measures

Several entanglement measures and witnesses, such as concurrence, negativity, and susceptibility-based witnesses, are employed. These metrics provide quantitative verification of bipartite and global entanglement across different qubit configurations, enabled by analyzing thermally driven occupations across energy levels. Importantly, indications of global entanglement in the eight-qubit system imply all possible partitions achieve non-classical correlations, lending credence to the architecture's scalability.

Implications and Future Directions

The experimental data fosters optimism about the potential role of QA systems in large-scale quantum computing, providing tangible evidence of entanglement—a milestone for any quantum computing paradigm. The resilience of entangled states even in contact with a thermal environment amplifies their applicability in practical settings, where complete isolation from environmental noise is infeasible.

Future developments could focus on improving control fidelity and expanding the scale of qubit networks, in addition to fine-tuning interactions with probing systems to achieve higher resolution spectroscopy. These advances may pave the way for utilizing QA in diverse domains, fundamentally transforming problem-solving strategies across scientific and engineering fields. In essence, the paper is pivotal for delineating the power of quantum annealing and reinforcing its place among quantum disciplinary paradigms.