No abstract

Berry curvature is an imaginary component of the quantum geometric tensor (QGT) and is well studied in many branches of modern physics; however, the quantum metric as a real component of the QGT is less explored. Here, by using tunable superconducting circuits, we experimentally demonstrate two methods to directly measure the quantum metric tensor for characterizing the geometry and topology of underlying quantum states in parameter space. The first method is to probe the transition probability after a sudden quench, and the second one is to detect the excitation rate under weak periodic driving. Furthermore, based on quantum-metric and Berry-curvature measurements, we explore a topological phase transition in a simulated time-reversal-symmetric system, which is characterized by the Euler characteristic number instead of the Chern number. The work opens up a unique approach to explore the topology of quantum states with the QGT.

No abstract

We propose a protocol to realize parametric control of two-qubit coupling, where the amplitude and phase are tuned by a longitudinal field. Based on the tunable Hamiltonian, we demonstrate the superadiabatic two-qubit quantum gate using superconducting quantum circuits. Our experimental results show that the state of qubits evolves adiabatically during the gate operation even though the processing time is close to the quantum limit. In addition, the quantum state transition is insensitive to the variation of control parameters, and the fidelity of a SWAP gate achieved 98.5%. This robust parametric two-qubit gate can alleviate the tension of frequency crowding for quantum computation with multiple qubits.