Quantum algorithms hold the promise of helping to solve a broad range of problems that are simply intractable with classical algorithms. The advantage of quantum calculations stems from exploiting the strange and nonintuitive properties of quantum systems: tunneling, superposition, quantum coherence, and entanglement. Building a general-purpose quantum computer, however, is extremely challenging; a more scalable and feasible approach may involve implementing a single, simpler quantum algorithm, such as quantum annealing. It is critical to demonstrate that such a scalable processor has access to quantum-mechanical resources such as coherence and entanglement. We build a processor based on quantum annealing and verify that specific two- and eight-qubit systems become entangled, a necessary and significant step in developing quantum annealing into a viable quantum-computing technology.
We run quantum annealing on a processor chip composed of magnetically coupled superconducting flux qubits. The chip is mounted on the mixing chamber of a dilution refrigerator held at 12.5 mK. We use qubit tunneling spectroscopy to infer nonclassical correlations in two- and eight-qubit systems based on eigenspectra and level occupations, effects that persist even at thermal equilibrium. Our measurements of spectral lines are dominated by the noise of the qubit tunneling spectroscopy probe, however, and we expect that follow-up experiments with improved probes will enable larger systems of qubits to be studied.
Our work provides promise that quantum annealing is a viable approach to realizing quantum-computing technologies. Moreover, our technique represents an effective way of studying quantum behavior in a practical processor, helping us to further understand the capabilities of quantum algorithms.
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 architecturally 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 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.
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