We report on the experimental investigation of an individual pseudomolecule using trapped ions with adjustable magnetically induced J-type coupling between spin states. Resonances of individual spins are well separated and are addressed with high fidelity. Quantum gates are carried out using microwave radiation in the presence of thermal excitation of the pseudomolecule's vibrations. Demonstrating controlled-NOT gates between non-nearest neighbors serves as a proof-of-principle of a quantum bus employing a spin chain. Combining advantageous features of nuclear magnetic resonance experiments and trapped ions, respectively, opens up a new avenue toward scalable quantum information processing.

A promising scheme for building scalable quantum simulators and computers is the synthesis of a scalable system using interconnected subsystems. A prerequisite for this approach is the ability to faithfully transfer quantum information between subsystems. With trapped atomic ions, this can be realized by transporting ions with quantum information encoded into their internal states. Here, we measure with high precision the fidelity of quantum information encoded into hyperfine states of a 171 Yb + ion during ion transport in a microstructured Paul trap. Ramsey spectroscopy of the ion's internal state is interleaved with up to 4000 transport operations over a distance of 280 µm each taking 12.8 µs. We obtain a state fidelity of 99.9994 +6 −7 % per ion transport.PACS numbers: 03.67. Lx,37.10.Ty Ion traps have been a workhorse in demonstrating many proof-of-principle experiments in quantum information processing using small ion samples [1]. A major challenge to transform this ansatz into a powerful quantum computing machine that can handle problems beyond the capabilities of classical super computers remains its scalability [2][3][4][5]. Error correction schemes allow us to fight the ever sooner death of fragile quantum information stored in larger and larger quantum systems, but their economic implementation requires computational building blocks to be executed with sufficient fidelity [6,7]. Essential computational steps have been demonstrated with fidelities beyond a threshold of 99.99% that is often considered as allowing for economic error correction [8], and, thus for fault-tolerant scalable quantum information processing (QIP). These building blocks include single qubit rotation [9,10], individual addressing of interacting ions [11], and internal state detection [12]. In addition, high fidelity twoqubit quantum gates [10,[13][14][15][16] and coherent three-qubit conditional quantum gates [17,18] have been implemented.Straightforward scaling up to an arbitrary size of a single ion trap quantum register, at present, appears unlikely to be successful because the growing size of a single register usually introduces additional constraints imposed by the confining potential and by the Coulomb interaction of ion strings [19]. Even though, for instance, transverse modes and anharmonic trapping [20] may be employed for conditional quantum logic, a general claim might be that, at some point it is useful to divide a single ion register into subsystems and to exchange quantum information between these subsystems [2][3][4][5]. One might do that by transferring quantum information from ions to photons (and vice versa) and by then exchanging photons between subsystems [4,21].Alternatively, when exchanging quantum information between spatially separated individual registers within an ion trap-based quantum information processor, the transport of ions carrying this information is an attractive approach [2,3,5]. Methods to transport ions in segmented Paul traps have been developed and demonstrated [22][23][24][25], and opti...

For experiments with ions confined in a Paul trap, minimization of micromotion is often essential. In order to diagnose and compensate micromotion we have implemented a method that allows for finding the position of the radio-frequency (rf) null reliably and efficiently, in principle, without any variation of direct current (dc) voltages. We apply a trap modulation technique and focus-scanning imaging to extract three-dimensional ion positions for various rf drive powers and analyze the power dependence of the equilibrium position of the trapped ion. In contrast to commonly used methods, the search algorithm directly makes use of a physical effect as opposed to efficient numerical minimization in a high-dimensional parameter space. Using this method we achieve a compensation of the residual electric field that causes excess micromotion in the radial plane of a linear Paul trap down to 0.09 V/m. Additionally, the precise position determination of a single harmonically trapped ion employed here can also be utilized for the detection of small forces. This is demonstrated by determining light pressure forces with a precision of 135 yN. As the method is based on imaging only, it can be applied to several ions simultaneously and is independent of laser direction and thus well-suited to be used with, for example, surface-electrode traps.