2011

N Timoney, I Baumgart, M Johanning, AF Varon, MB Plenio, A Retzker, and C Wunderlich. 8/10/2011. “Quantum gates and memory using microwave-dressed states.” Nature, 476, 7359, Pp. 185-U83. Publisher's Version Abstract
Trapped atomic ions have been used successfully to demonstrate1 basic elements of universal quantum information processing. Nevertheless, scaling up such methods to achieve large-scale, universal quantum information processing (or more specialized quantum simulations2,3,4,5) remains challenging. The use of easily controllable and stable microwave sources, rather than complex laser systems6,7, 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.
A del Campo, A Retzker, and MB Plenio. 8/16/2011. “The inhomogeneous Kibble-Zurek mechanism: vortex nucleation during Bose-Einstein condensation.” New Journal of Physics, 13. Publisher's Version Abstract
The Kibble–Zurek mechanism is applied to the spontaneous formation of vortices in a harmonically trapped thermal gas following a temperature quench through the critical value for Bose–Einstein condensation. Whereas in the homogeneous scenario, vortex nucleation is always expected, we show that it can be completely suppressed in the presence of the confinement potential whenever the speed of the spatial front undergoing condensation is lower than a threshold velocity. Otherwise, the interplay between the geometry and the causality leads to different scaling laws for the density of vortices as a function of the quench rate, as we also illustrate for the case of a toroidal trapping potential.
A Bermudez, J Almeida, F Schmidt-Kaler, A Retzker, and MB Plenio. 11/11/2011. “Frustrated Quantum Spin Models with Cold Coulomb Crystals.” Physical Review Letters, 107, 20. Publisher's Version Abstract
We exploit the geometry of a zigzag cold-ion crystal in a linear trap to propose the quantum simulation of a paradigmatic model of long-ranged magnetic frustration. Such a quantum simulation would clarify the complex features of a rich phase diagram that presents ferromagnetic, dimerized-antiferromagnetic, paramagnetic, and floating phases, together with previously unnoticed features that are hard to assess by numerics. We analyze in detail its experimental feasibility, and provide supporting numerical evidence on the basis of realistic parameters in current ion-trap technology.
A Bermudez, F Jelezko, MB Plenio, and A Retzker. 10/3/2011. “Electron-Mediated Nuclear-Spin Interactions between Distant Nitrogen-Vacancy Centers.” Physical Review Letters, 107, 15. Publisher's Version Abstract
We propose a scheme enabling controlled quantum coherent interactions between separated nitrogen-vacancy centers in diamond in the presence of strong magnetic fluctuations. The proposed scheme couples nuclear qubits employing the magnetic dipole-dipole interaction between the electron spins and, crucially, benefits from the suppression of the effect of environmental magnetic field fluctuations thanks to a strong microwave driving. This scheme provides a basic building block for a full-scale quantum-information processor or quantum simulator based on solid-state technology.
A Albrecht, A Retzker, C Wunderlich, and MB Plenio. 3/8/2011. “Enhancement of laser cooling by the use of magnetic gradients.” New Journal of Physics, 13. Publisher's Version Abstract
We present a laser cooling scheme for trapped ions and atoms using a combination of laser couplings and a magnetic gradient field. In a Schrieffer–Wolff transformed picture, this setup cancels the carrier and blue sideband terms completely (up to first order in the Lamb–Dicke parameter), resulting in an improved cooling behaviour compared to standard cooling schemes in the Lamb–Dicke regime (e.g. sideband cooling) and allowing cooling to the vibrational ground state. A condition for optimal cooling rates is presented and the cooling behaviour for different Lamb–Dicke parameters and spontaneous decay rates is discussed. Cooling rates of one order of magnitude less than the trapping frequency are achieved using the new cooling method. Furthermore, the scheme exhibits fast rates and low final populations, even for significant deviations from the optimal parameters, and provides good cooling rates also in the multi-particle case.