A. F. Varón

Universität Siegen, Siegen, North Rhine-Westphalia, Germany

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Publications (5)56.51 Total impact

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    ABSTRACT: The addressing of a particular qubit within a quantum register is a key prerequisite for scalable quantum computing. We demonstrate addressing of individual qubits within a quantum byte (eight qubits) and measure the error induced in non-addressed qubits (cross-talk) associated with the application of single-qubit gates. This cross-talk is on the order of $10^{-5}$ breaching the threshold for fault-tolerant quantum computing. The quantum byte is implemented using $^{171}$Yb$^+$ ions confined in a Paul trap where a static magnetic gradient field is applied that lifts the hyperfine qubits' degeneracy. Hyperfine qubits are individually addressed using microwave radiation. In addition, we demonstrate a method for addressing individual qubits where an appropriate choice of addressing frequency and microwave pulse duration allows for further lowering cross-talk.
    03/2014;
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    ABSTRACT: Dephasing -- phase randomization of a quantum superposition state -- is a major obstacle for the realization of high fidelity quantum logic operations. Here, we implement a two-qubit Controlled-NOT gate using dynamical decoupling (DD), despite the gate time being more than one order of magnitude longer than the intrinsic coherence time of the system. For realizing this universal conditional quantum gate, we have devised a concatenated DD sequence that ensures robustness against imperfections of DD pulses that otherwise may destroy quantum information or interfere with gate dynamics. We compare its performance with three other types of DD sequences. These experiments are carried out using a well-controlled prototype quantum system -- trapped atomic ions coupled by an effective spin-spin interaction. The scheme for protecting conditional quantum gates demonstrated here is applicable to other physical systems, such as nitrogen vacancy centers, solid state nuclear magnetic resonance, and circuit quantum electrodynamics.
    Physical Review Letters 05/2013; 110(20):200501. · 7.94 Impact Factor
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    ABSTRACT: 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 towards scalable quantum information processing.
    Physical Review Letters 06/2012; 108(22):220502. · 7.94 Impact Factor
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    ABSTRACT: Trapped atomic ions have been used successfully to demonstrate basic elements of universal quantum information processing. Nevertheless, scaling up such methods to achieve large-scale, universal quantum information processing (or more specialized quantum simulations) remains challenging. The use of easily controllable and stable microwave sources, rather than complex laser systems, could remove obstacles to scalability. However, the microwave approach has drawbacks: it involves the use of magnetic-field-sensitive states, which shorten coherence times considerably, and requires large, stable magnetic field gradients. Here we show how to overcome both problems by using stationary atomic quantum states as qubits that are induced by microwave fields (that is, by dressing magnetic-field-sensitive states with microwave fields). This permits fast quantum logic, even in the presence of a small (effective) Lamb-Dicke parameter (and, therefore, moderate magnetic field gradients). We experimentally demonstrate the basic building blocks of this scheme, showing that the dressed states are long lived and that coherence times are increased by more than two orders of magnitude relative to those of bare magnetic-field-sensitive states. This improves the prospects of microwave-driven ion trap quantum information processing, and offers a route to extending coherence times in all systems that suffer from magnetic noise, such as neutral atoms, nitrogen-vacancy centres, quantum dots or circuit quantum electrodynamic systems.
    Nature 08/2011; 476(7359):185-8. · 38.60 Impact Factor
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    ABSTRACT: The control of internal and motional quantum degrees of freedom of laser-cooled trapped ions has been subject to intense theoretical and experimental research for about three decades. In the realm of quantum information science, the ability to deterministically prepare and measure quantum states of trapped ions is unprecedented. This expertise may be employed to investigate physical models conceived to describe systems that are not directly accessible for experimental investigations. Here, we give an overview of current theoretical proposals and experiments for such quantum simulations with trapped ions. This includes various spin models (e.g. the quantum transverse Ising model or a neural network), the Bose-Hubbard Hamiltonian, the Frenkel-Kontorova model, and quantum fields and relativistic effects.
    Journal of Physics B Atomic Molecular and Optical Physics 01/2009; 42(15). · 2.03 Impact Factor