[Show abstract][Hide abstract] 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 Wheeler-DeWitt 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.
[Show abstract][Hide abstract] ABSTRACT: The addressing of a particular qubit within a quantum register is a key pre-requisite 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 whole-quantum 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 non-addressed qubits (cross-talk) associated with the application of single-qubit gates. The quantum byte is implemented using microwave-driven hyperfine qubits of (171)Yb(+) ions confined in a Paul trap augmented with a magnetic gradient field. The measured cross-talk is on the order of 10(-5) and therefore below the threshold commonly agreed sufficient to efficiently realize fault-tolerant quantum computing. Hence, our results demonstrate how this threshold can be overcome with respect to cross-talk.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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