A key requirement for scalable quantum computing is that elementary quantum gates can be implemented with sufficiently low error. One method for determining the error behavior of a gate implementation is to perform process tomography. However, standard process tomography is limited by errors in state preparation, measurement and one-qubit gates. It suffers from inefficient scaling with number of qubits and does not detect adverse error-compounding when gates are composed in long sequences. An additional problem is due to the fact that desirable error probabilities for scalable quantum computing are of the order of 0.0001 or lower. Experimentally proving such low errors is challenging. We describe a randomized benchmarking method that yields estimates of the computationally relevant errors without relying on accurate state preparation and measurement. Since it involves long sequences of randomly chosen gates, it also verifies that error behavior is stable when used in long computations. We implemented randomized benchmarking on trapped atomic ion qubits, establishing a one-qubit error probability per randomized / 2 pulse of 0.00482͑17͒ in a particular experiment. We expect this error probability to be readily improved with straightforward technical modifications.

Among the classes of highly entangled states of multiple quantum systems, the so-called 'Schrödinger cat' states are particularly useful. Cat states are equal superpositions of two maximally different quantum states. They are a fundamental resource in fault-tolerant quantum computing and quantum communication, where they can enable protocols such as open-destination teleportation and secret sharing. They play a role in fundamental tests of quantum mechanics and enable improved signal-to-noise ratios in interferometry. Cat states are very sensitive to decoherence, and as a result their preparation is challenging and can serve as a demonstration of good quantum control. Here we report the creation of cat states of up to six atomic qubits. Each qubit's state space is defined by two hyperfine ground states of a beryllium ion; the cat state corresponds to an entangled equal superposition of all the atoms in one hyperfine state and all atoms in the other hyperfine state. In our experiments, the cat states are prepared in a three-step process, irrespective of the number of entangled atoms. Together with entangled states of a different class created in Innsbruck, this work represents the current state-of-the-art for large entangled states in any qubit system.

A single Nitrogen Vacancy (NV) center hosted in a diamond nanocrystal is positioned at the extremity of a SiC nanowire. This novel hybrid system couples the degrees of freedom of two radically different systems, i.e. a nanomechanical oscillator and a single quantum object. The dynamics of the nano-resonator is probed through time resolved nanocrystal fluorescence and photon correlation measurements, conveying the influence of a mechanical degree of freedom given to a non-classical photon emitter. Moreover, by immersing the system in a strong magnetic field gradient, we induce a magnetic coupling between the nanomechanical oscillator and the NV electronic spin, providing nanomotion readout through a single electronic spin. Spin-dependent forces inherent to this coupling scheme are essential in a variety of active cooling and entanglement protocols used in atomic physics, and should now be within the reach of nanomechanical hybrid systems.Owing to recent developments in cavity opto-and electro-mechanics [1-3], it is now realistic to envision the observation of macroscopic mechanical oscillators cooled by active or traditional cryogenic techniques close to their ground state of motion. This conceptually elegant accomplishment would give access to a vast playground for physicists if the resonator wavefunction could be coherently manipulated such as to create, maintain and probe Fock or other non-classical states. It would provide a remarkable opportunity to extend the pioneering experiments with trapped ions [4] to encompass macroscopic objects. However, standard continuous measurements techniques used to actively cool and probe the resonator [5], when utilized to manipulate its quantum state, tend to blur its non-classical nature. An attractive alternative consists in interfacing the mechanical degrees of freedom with a single quantum object such as a 2-level system whose quantum state can be externally controlled [6][7][8][9][10][11]. Successful realization of this type of coupling between a nanomechanical oscillator in the quantum regime and a phase qubit was recently reported [12] and motivates the development of similar hybrid quantum systems presenting extended coherence times at room temperature and compatible with continuous measurement approaches.Here we report a first step in this direction by coupling a nanomechanical oscillator to a single negatively-charged Nitrogen Vacancy (NV) defect hosted in a diamond nanocrystal attached to its extremity (fig 1a). In that context, the NV defect appears as an attractive quantum system, both for its optical and electronic spin properties. Indeed, perfect photostability at room temperature makes the NV defect a robust and practical single-photon source [13,14]. Moreover, the NV defect ground state is a spin triplet (fig 1b) which can be initialized and read-out by optical means, and manipulated by resonant microwave excitation with an unprecedented coherence time for a solid-state system under ambient conditions [15,16]. Such properties are at the heart of diamond-base...