Understanding magnetic phases in quantum mechanical systems is one of the essential goals in condensed matter physics, and the advent of prototype quantum simulation hardware has provided new tools for experimentally probing such systems. We report on the experimental realization of a quantum simulation of interacting Ising spins on three-dimensional cubic lattices up to dimensions 8 × 8 × 8 on a D-Wave processor (D-Wave Systems, Burnaby, Canada). The ability to control and read out the state of individual spins provides direct access to several order parameters, which we used to determine the lattice's magnetic phases as well as critical disorder and one of its universal exponents. By tuning the degree of disorder and effective transverse magnetic field, we observed phase transitions between a paramagnetic, an antiferromagnetic, and a spin-glass phase.

We present experiments on the driven dynamics of a two-level superconducting artificial atom. The driving strength reaches 4.78 GHz, significantly exceeding the transition frequency of 2.288 GHz. The observed dynamics is described in terms of quasienergies and quasienergy states, in agreement with Floquet theory. In addition, we observe the role of pulse shaping in the dynamics, as determined by nonadiabatic transitions between Floquet states, and we implement subnanosecond single-qubit operations. These results pave the way to quantum control using strong driving with applications in quantum technologies.

Hamiltonian-based quantum computation is a class of quantum algorithms in which the problem is encoded in a Hamiltonian and the evolution is performed by a continuous transformation of the Hamiltonian. Universal adiabatic quantum computing, quantum simulation, and quantum annealing are examples of such algorithms. Up to now, all implementations of this approach have been limited to qubits coupled via a single degree of freedom. This gives rise to a stoquastic Hamiltonian that has no sign problem in quantum Monte Carlo simulations. In this paper, we report implementation and measurements of two superconducting flux qubits coupled via two canonically conjugate degrees of freedom-charge and flux-to achieve a nonstoquastic Hamiltonian. We perform microwave spectroscopy to extract circuit parameters and show that the charge coupling manifests itself as a σ y σ y interaction in the computational basis. We observe destructive interference in quantum coherent oscillations between the computational basis states of the two-qubit system. Finally, we show that the extracted Hamiltonian is nonstoquastic over a wide range of parameters.

Efficient detection of magnetic fields is central to many areas of research and technology. High-sensitivity detectors are commonly built using direct-current superconducting quantum interference devices or atomic systems. Here we use a single artificial atom to implement an ultrasensitive magnetometer with micron range size. The artificial atom, a superconducting two-level system, is operated similarly to atom and diamond nitrogen-vacancy centre-based magnetometers. The high sensitivity results from quantum coherence combined with strong coupling to magnetic field. We obtain a sensitivity of 3.3 pT Hz À 1/2 for a frequency of 10 MHz. We discuss feasible improvements to increase sensitivity by one order of magnitude. The intrinsic sensitivity of this detector at frequencies in the 100 kHz-10 MHz range compares favourably with direct-current superconducting quantum interference devices and atomic magnetometers of equivalent spatial resolution. This result illustrates the potential of artificial quantum systems for sensitive detection and related applications.

We report experiments on superconducting flux qubits in a circuit quantum electrodynamics (cQED) setup. Two qubits, independently biased and controlled, are coupled to a coplanar waveguide resonator. Dispersive qubit state readout reaches a maximum contrast of 72 %. We find intrinsic energy relaxation times at the symmetry point of 7 µs and 20 µs and levels of flux noise of 2.6 µΦ0/ √ Hz and 2.7 µΦ0/ √ Hz at 1 Hz for the two qubits. We discuss the origin of decoherence in the measured devices. These results demonstrate the potential of cQED as a platform for fundamental investigations of decoherence and quantum dynamics of flux qubits.PACS numbers: 85.25. Cp, 42.50.Dv , 03.67.Lx, 74.78.Na Superconducting qubits are one of the main candidates for the implementation of quantum information processing [1] and a rich testbed for research in quantum optics, quantum measurement, and decoherence [2]. Among various types of superconducting qubits, flux-type superconducting qubits have unique features. Strong and tunable coupling to microwave fields enables fundamental investigations in quantum optics [3][4][5] and relativistic quantum mechanics [6]. The large magnetic dipole moment is a key ingredient in flux noise measurements [5], sensitive magnetic field measurements [8], microwave-optical interfaces [9], and hybrid systems formed with nanomechanical resonators [10]. Finally, flux qubits have a large degree of anharmonicity which is an advantage for fast quantum control [11]. Progress on these diverse research avenues has been hampered by relatively low and irreproducible coherence times compared to other types of superconducting qubits.In the last decade, circuit quantum electrodynamics (cQED) [12,13] has become increasingly popular. In cQED, resonators provide a controlled electromagnetic environment protecting qubits from energy relaxation. In addition, resonators are used for qubit state measurement [2] and as quantum buses for qubit-qubit coupling [15]. In this letter, we present an implementation of cQED with flux qubits strongly coupled to a superconducting coplanar waveguide resonator. The qubits and the resonator are made of aluminum. Local biasing and control lines provide a mean to implement fast single qubit gates as well as controlled two-qubit interactions. We measure energy relaxation times around 10 µs, an improvement over previous experiments with flux qubits coupled to coplanar waveguide resonators [16,17], and comparable with the longest measured to date on flux qubits [5,18]. We characterize in detail the decoherence of the flux qubits coupled to the resonator. Based on decoherence measurements, we extract levels of flux noise of 2.6 µΦ 0 / √ Hz and 2.7 µΦ 0 / √ Hz at 1 Hz for the two qubits. We also present a spectroscopic measurement of a resonator-mediated qubit-qubit coupling, which is relevant for implementation of two-qubit gates. These results demonstrate the versatility of cQED with flux qubits, and its potential for further understanding and improvements of decoherence of these qubits.T...