We report the preparation and verification of a genuine 12-qubit entanglement in a superconducting processor. The processor that we designed and fabricated has qubits lying on a 1D chain with relaxation times ranging from 29.6 to 54.6 µs. The fidelity of the 12-qubit entanglement was measured to be above 0.5544±0.0025, exceeding the genuine multipartite entanglement threshold by 21 statistical standard deviations. Our entangling circuit to generate linear cluster states is depth-invariant in the number of qubits and uses single-and double-qubit gates instead of collective interactions. Our results are a substantial step towards large-scale random circuit sampling and scalable measurement-based quantum computing.

Quantum walks are the quantum analogs of classical random walks, which allow for the simulation of large-scale quantum many-body systems and the realization of universal quantum computation without time-dependent control. We experimentally demonstrate quantum walks of one and two strongly correlated microwave photons in a one-dimensional array of 12 superconducting qubits with short-range interactions. First, in one-photon quantum walks, we observed the propagation of the density and correlation of the quasiparticle excitation of the superconducting qubit and quantum entanglement between qubit pairs. Second, when implementing two-photon quantum walks by exciting two superconducting qubits, we observed the fermionization of strongly interacting photons from the measured time-dependent long-range anticorrelations, representing the antibunching of photons with attractive interactions. The demonstration of quantum walks on a quantum processor, using superconducting qubits as artificial atoms and tomographic readout, paves the way to quantum simulation of many-body phenomena and universal quantum computation.

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Superconducting quantum circuits are a promising candidate for building scalable quantum computers. Here, we use a four-qubit superconducting quantum processor to solve a two-dimensional system of linear equations based on a quantum algorithm proposed by Harrow, Hassidim, and Lloyd [Phys. Rev. Lett. 103, 150502 (2009)PRLTAO0031-900710.1103/PhysRevLett.103.150502], which promises an exponential speedup over classical algorithms under certain circumstances. We benchmark the solver with quantum inputs and outputs, and characterize it by nontrace-preserving quantum process tomography, which yields a process fidelity of 0.837±0.006. Our results highlight the potential of superconducting quantum circuits for applications in solving large-scale linear systems, a ubiquitous task in science and engineering.

Entangling gates with error rates reaching the threshold for quantum error correction have been reported for CZ gates using adiabatic longitudinal control based on the interaction between the |11〉 and |20〉 states. Here, we design and implement nonadiabatic CZ gates, which outperform adiabatic gates in terms of speed and fidelity, with gate times reaching 1:25=ð2 ffiffi ffi 2 p g 01;10 Þ, and fidelities reaching 99.54 ± 0.08%. Nonadiabatic gates are found to have proportionally less incoherent error than adiabatic gates thanks to their fast gate times, which leave more room for further improvements in the design of the control pules to eliminate coherent errors. We also show that state leakage can be reduced to below 0.2% with optimisation. Furthermore, the gate optimisation process is highly feasible: experimental optimisation can be expected to take less than four hours. Finally, the gate design process can be extended to CCZ gates, and our preliminary results suggest that this process would be feasible as well, if we can measure the CCZ fidelity separate from the initialisation and readout errors in experimental optimisation.

The engineering of quantum devices has reached the stage where we now have small scale quantum processors containing multiple interacting qubits within them. Simple quantum circuits have been demonstrated and scaling up to larger numbers is underway [1,2]. However as the number of qubits in these processors increases, it becomes challenging to implement switchable or tunable coherent coupling among them. The typical approach has been to detune each qubit from others or the quantum bus it connected to [1, 2], but as the number of qubits increases this becomes problematic to achieve in practice due to frequency crowding issues. Here, we demonstrate that by applying a fast longitudinal control field to the target qubit, we can turn off its couplings to other qubits or buses (in principle on/off ratio higher than 100 dB). This has important implementations in superconducting circuits as it means we can keep the qubits at their optimal points, where the coherence properties are greatest, during coupling/decoupling processing. Our approach suggests a new way to control coupling among qubits and data buses that can be naturally scaled up to large quantum processors without the need for auxiliary circuits and yet be free of the frequency crowding problems.Superconducting quantum circuits [3,4] are promising candidates to realize quantum processors and simulators. They have been used to demonstrate various quantum algorithms and implement thousands of quantum operations within their coherence time [5], in which controllable couplings are inevitable. The typical way to couple/decouple two superconducting quantum elements with always-on coupling is to tune their frequencies in or out of resonance [6][7][8][9][10][11][12]. This method is widely adopted even for the most recent universal gate implementations [5,[13][14][15]. However, it suffers from several defects, namely, it is technically difficult to avoid frequency crowding problem in large scale circuits; the qubits cannot always work at the coherent optimal point and the fast tuning of the qubit frequency results in non-adiabatic information leakage. To overcome the above problems, significant * Electronic address: yuxiliu@mail.tsinghua.edu.cn † Electronic address: xbzhu16@ustc.edu.cn effort has been devoted both theoretically [16,17] and experimentally [18-23] to develop couplers for parametrically tuning the coupling strength between two elements. Recently high coherence and fast tunable coupling has been demonstrated [24], however, these quantum or classical couplers increase the complexity of the circuits and introduce new decoherence sources. Therefore, the implementation of quantum switch for coherent coupling between quantum elements is still a big challenge in scalable quantum circuits.In this letter, we demonstrate a simple yet reliable method to switch on/off the coupling between a quantum resonator and a superconducting flux qubit [25] via a control field, longitudinally applied to the qubit [26,27]. Our system is a gap tunable flux qubit [28,29], coupled to...

A resilient robot is a robot that can recover its function after the robot is partially damaged. In this paper, a study of an under-actuated resilient robot with closed loops and passive joints is presented. First, a prototype system was built, which serves as a study vehicle and is called R-Robot II for short. Second, the kinematics of the prototype robot R-Robot II, necessarily for the change of the robot structure in, was developed. Finally, the experimentation of the R-Robot II was carried out. The result shows that the desired resilient behavior of R-Robot II can be exhibited. The architecture of R-Robot II, along with the design of the mechanical modules and simulation, was reported elsewhere. This paper focuses on the physical realization of R-Robot II and on the experimentation. INDEX TERMSResilient robot, D-H parameter method, topology, kinematic analysis.

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