Publications (6)70.47 Total impact

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ABSTRACT: Even though quantum systems in energy eigenstates do not evolve in time, they can exhibit correlations between internal degrees of freedom in such a way that one of the internal degrees of freedom behaves like a clock variable, and thereby defines an internal time, that parametrises the evolution of the other degrees of freedom. This situation is of great interest in quantum cosmology where the invariance under reparametrisation of time implies that the temporal coordinate dissapears and is replaced by the WheelerDeWitt constraint. Here we show that this paradox can be investigated experimentally using the exquisite control now available on moderate size quantum systems. We describe in detail how to implement such an experimental demonstration using the spin and motional degrees of freedom of a single trapped ion. 
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ABSTRACT: The addressing of a particular qubit within a quantum register is a key prerequisite for scalable quantum computing. In general, executing a quantum gate with a single qubit, or a subset of qubits, affects the quantum states of all other qubits. This reduced fidelity of the wholequantum register could prevent the application of quantum error correction protocols and thus preclude scalability. Here we demonstrate addressing of individual qubits within a quantum byte (eight qubits) and measure the error induced in all nonaddressed qubits (crosstalk) associated with the application of singlequbit gates. The quantum byte is implemented using microwavedriven hyperfine qubits of (171)Yb(+) ions confined in a Paul trap augmented with a magnetic gradient field. The measured crosstalk is on the order of 10(5) and therefore below the threshold commonly agreed sufficient to efficiently realize faulttolerant quantum computing. Hence, our results demonstrate how this threshold can be overcome with respect to crosstalk.Nature Communications 08/2014; 5:4679. DOI:10.1038/ncomms5679 · 10.74 Impact Factor 
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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 twoqubit ControlledNOT 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 wellcontrolled prototype quantum system  trapped atomic ions coupled by an effective spinspin 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. DOI:10.1103/PhysRevLett.110.200501 · 7.73 Impact Factor 
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ABSTRACT: We report on the experimental investigation of an individual pseudomolecule using trapped ions with adjustable magnetically induced Jtype 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 ControlledNOT gates between nonnearest neighbors serves as a proofofprinciple 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. DOI:10.1103/PhysRevLett.108.220502 · 7.73 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 largescale, 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 magneticfieldsensitive 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 magneticfieldsensitive states with microwave fields). This permits fast quantum logic, even in the presence of a small (effective) LambDicke 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 magneticfieldsensitive states. This improves the prospects of microwavedriven 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, nitrogenvacancy centres, quantum dots or circuit quantum electrodynamic systems.Nature 08/2011; 476(7359):1858. DOI:10.1038/nature10319 · 42.35 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 BoseHubbard Hamiltonian, the FrenkelKontorova model, and quantum fields and relativistic effects.Journal of Physics B Atomic Molecular and Optical Physics 08/2009; 42(15). DOI:10.1088/09534075/42/15/154009 · 1.92 Impact Factor
Publication Stats
138  Citations  
70.47  Total Impact Points  
Top Journals
Institutions

2009–2014

Universität Siegen
 Department of Physics
Siegen, North RhineWestphalia, Germany
