Quantum annealing is useful for tackling optimization, machine learning, and quantum simulation problems by finding the low energy configurations of a Hamiltonian. The currently available quantum annealers provide solutions to the transverse Ising model up to thousands of qubits. These problems fall into the class of stoquastic Hamiltonian problem, where all the off-diagonal matrix elements of the Hamiltonian are nonpositive. In this case, the annealing results of some problems can be generated classically using stochastic methods such as quantum Monte Carlo. However, systems with nonstoquastic Hamiltonians, which exhibit a "sign problem" with positive off-diagonal matrix elements, cannot be simulated with stochastic methods and has been shown to enhance the efficiency of quantum annealing theoretically. Here, we demonstrate a nonstoquastic Hamiltonian in a two superconducting-qubit system where the qubits are coupled both magnetically and electrostatically. We observe clear signatures of destructive interference between different tunneling paths through microwave spectroscopy as well as coherent oscillations, which are signatures of the "sign problem". Moreover, we observe a broken gauge invariance, which provides samples to the Ising problems of a greater variety. Our implementation is based on the current scalable superconducting technologies which is ready to be expanded to a large number of qubits and serves as a testbed of nonstoquastic quantum annealing on practical problems.
Chunqing Deng is a Quantum Scientist at Alibaba Quantum Lab (AQL). He received his BSc from Peking University in 2008 and his PhD from University of Waterloo in 2015. His PhD work involved the development of new control scheme and the investigation of decoherence mechanism in superconducting qubits. After that, he joined D-Wave Systems Inc. and became a lead scientist there. At D-Wave Systems, Chunqing worked on improving the coherence of superconducting qubits and developing novel qubit-qubit interactions for future-generation adiabatic quantum processors. He is currently leading the quantum hardware effort at Alibaba to build a quantum computer for real-world computational problems.