Spin-exchanging interactions govern the properties of strongly correlated electron systems such as many magnetic materials. When orbital degrees of freedom are present, spin exchange between different orbitals often dominates, leading to the Kondo effect, heavy fermion behaviour or magnetic ordering. Ultracold ytterbium or alkaline-earth ensembles have attracted much recent interest as model systems for these effects, with two (meta-) stable electronic configurations representing independent orbitals. We report the observation of spin-exchanging contact interactions in a two-orbital SU(N )-symmetric quantum gas realized with fermionic 173 Yb. We find strong inter-orbital spinexchange by spectroscopic characterization of all interaction channels and demonstrate SU(N = 6) symmetry within our measurement precision. The spin-exchange process is also directly observed through the dynamic equilibration of spin imbalances between ensembles in separate orbitals. The realization of an SU(N )-symmetric two-orbital Hubbard Hamiltonian opens the route to quantum simulations with extended symmetries and with orbital magnetic interactions, such as the Kondo lattice model. arXiv:1403.4761v3 [cond-mat.quant-gas] 21 May 2015

Quantum computation requires quantum logic gates that use the interaction within pairs of quantum bits (qubits) to perform conditional operations. Superconducting qubits may offer an attractive route towards scalable quantum computing. In previous experiments on coupled superconducting qubits, conditional gate behaviour and entanglement were demonstrated. Here we demonstrate selective execution of the complete set of four different controlled-NOT (CNOT) quantum logic gates, by applying microwave pulses of appropriate frequency to a single pair of coupled flux qubits. All two-qubit computational basis states and their superpositions are used as input, while two independent single-shot SQUID detectors measure the output state, including qubit-qubit correlations. We determined the gate's truth table by directly measuring the state transfer amplitudes and by acquiring the relevant quantum phase shift using a Ramsey-like interference experiment. The four conditional gates result from the symmetry of the qubits in the pair: either qubit can assume the role of control or target, and the gate action can be conditioned on either the 0-state or the 1-state. These gates are now sufficiently characterized to be used in quantum algorithms, and together form an efficient set of versatile building blocks.

In quantum mechanics, the process of measurement is a subtle interplay between extraction of information and disturbance of the state of the quantum system. A

We realize indirect partial measurement of a transmon qubit in circuit quantum electrodynamics by interaction with an ancilla qubit and projective ancilla measurement with a dedicated readout resonator. Accurate control of the interaction and ancilla measurement basis allows tailoring the measurement strength and operator. The tradeoff between measurement strength and qubit backaction is characterized through the distortion of a qubit Rabi oscillation imposed by ancilla measurement in different bases. Combining partial and projective qubit measurements, we provide the solid-state demonstration of the correspondence between a nonclassical weak value and the violation of a Leggett-Garg inequality. DOI: 10.1103/PhysRevLett.111.090506 PACS numbers: 03.67.Lx, 42.50.Dv, 42.50.Pq, 85.25.Àj Quantum measurement involves a fundamental tradeoff between information gain and disturbance of the measured system that is traceable to uncertainty relations [1]. The backaction, or kickback, is a nonunitary process that depends on the measurement result and premeasurement system state. Thought experiments in the 1980s unveiled paradoxes [2][3][4] where the backaction of multiple measurements of one system puts quantum mechanics at odds with macrorealism (MAR) [2], a set of postulates distilling our common assumptions about the macroscopic world. Steady developments in the control of single quantum systems have opened the road to testing these paradoxes with photons [5-9], superconducting circuits [10], and semiconductor spins [11][12][13].The Leggett-Garg inequality (LGI), for example, investigates the impact of backaction on the correlations between sequential measurements of one system [2,14]. A violation of the inequality certifies the failure of MAR to describe the system behavior. Although the original test called for multiple configurations of pairs of strong measurements, a generalization of the LGI using partial measurements requires only one configuration [15,16]. The first demonstration of LGI violations, by PalaciosLaloy et al.[10], used continuous weak measurement of a superconducting qubit. Further demonstrations followed using discrete measurements in photonic [7,8] and semiconductor-spin [12] systems. A second paradox is the nonclassicality of weak values, i.e., averages of a partial measurement conditioned on the result of a subsequent projective measurement [3]. These values are termed nonclassical when they lie outside the eigenspectrum of the weak measurement observable. Williams and Jordan [17] predicted an intriguing correspondence between nonclassical weak values (NCWVs) and the violation of generalized LGIs, first observed by Goggin et al.[7] using a photonic system. Moving beyond fundamental investigation, the emergent field of quantum feedback control [18] balances the tradeoff between information gain and backaction. Applications requiring controllable measurement strength can be found in quantum error correction [19], qubit stabilization [20,21], and state discrimination [22]. A variable-strength meas...