Robust CNOT gates from almost any interaction

There are many cases where the interaction between two qubits is not precisely known, but single qubit operations are available. In this paper we show how, regardless of an incomplete knowledge of the strength or form of the interaction between two qubits, it is often possible to construct a CNOT gate which has arbitrarily high fidelity. In particular, we show that oscillations in the strength of the exchange interaction in solid state Si and Ge structures are correctable.Comment: 5 pages, 2 figure

Highly efficient broadband conversion of light polarization by composite retarders

Driving on an analogy with the technique of composite pulses in quantum physics, we propose highly efficient broadband polarization converters composed of sequences of ordinary retarders rotated at specific angles with respect to their fast-polarization axes.Comment: 5 pages, 3 figures; check JOSA A 201

High-fidelity local addressing of trapped ions and atoms by composite sequences of laser pulses

A vital requirement for a quantum computer is the ability to locally address, with high fidelity, any of its qubits without affecting their neighbors. We propose an addressing method using composite sequences of laser pulses, which reduces dramatically the addressing error in a lattice of closely spaced atoms or ions, and at the same time significantly enhances the robustness of qubit manipulations. To this end, we design novel high-fidelity composite pulses for the most important single-qubit operations. In principle, this method allows one to beat the diffraction limit, for only atoms situated in a small spatial region around the center of the laser beam are excited, well within the laser beam waist.Comment: Optics Letters Vol. 36, No. 7 (2011

Robust Ising Gates for Practical Quantum Computation

I describe the use of techniques based on composite rotations to combat systematic errors in controlled phase gates, which form the basis of two qubit quantum logic gates. Although developed and described within the context of Nuclear Magnetic Resonanace (NMR) quantum computing these sequences should be applicable to any implementation of quantum computation based on Ising couplings. In combination with existing single qubit gates this provides a universal set of robust quantum logic gates.Comment: 3 Pages RevTex4 including 2 figures. Will submit to PR

Tackling Systematic Errors in Quantum Logic Gates with Composite Rotations

We describe the use of composite rotations to combat systematic errors in single qubit quantum logic gates and discuss three families of composite rotations which can be used to correct off-resonance and pulse length errors. Although developed and described within the context of NMR quantum computing these sequences should be applicable to any implementation of quantum computation.Comment: 6 pages RevTex4 including 4 figures. Will submit to Phys. Rev.

A multinuclear NMR study of six forms of AlPO-34:structure and motional broadening

We report a study of the CHA-type aluminophosphate AlPO-34, prepared with six different structure-directing agents (SDAs): piperidine (pip), morpholine (mor), pyridine (pyr), 1,4,8,11-tetraazacyclotetradecane (cyclam), 1,3 dimethylimidazolium (dmim) chloride and 1-ethyl-3-methylimidazolium (emim) bromide. Using a combination of solid-state NMR spectroscopy, periodic density functional theory (DFT) calculations and synchrotron X-ray diffraction, we show that, even in crystallographically well-ordered materials such as AlPO-34 with dmim as the SDA, local disorder may be present. For such disordered structures, where it is challenging to use DFT to assign NMR spectra, we show that the 31P isotropic chemical shift can be predicted accurately using the mean P-O bond length and P-O-Al bond angle, in an extension of previous work. Variable-temperature 27Al NMR reveals the presence of microsecond-timescale dynamics in all forms of AlPO- 34, with two different motional regimes observed, depending on whether structural H2O is also present. H2O is detected in AlPO-34 prepared with mor as the SDA, although this material was previously reported as anhydrous, suggesting that this form of AlPO-34 may be hygroscopic despite the presence of the SDAs within the pores

Feasibility and preliminary efficacy of visual cue training to improve adaptability of walking after stroke : multi-centre, single-blind randomised control pilot trial

Objectives: Given the importance of vision in the control of walking and evidence indicating varied practice of walking improves mobility outcomes, this study sought to examine the feasibility and preliminary efficacy of varied walking practice in response to visual cues, for the rehabilitation of walking following stroke. Design: This 3 arm parallel, multi-centre, assessor blind, randomised control trial was conducted within outpatient neurorehabilitation services Participants Community dwelling stroke survivors with walking speed <0.8m/s, lower limb paresis and no severe visual impairments. Intervention: Over-ground visual cue training (O-VCT), Treadmill based visual cue training (T-VCT), and Usual care (UC) delivered by physiotherapists twice weekly for 8 weeks. Main outcome measures: Participants were randomised using computer generated random permutated balanced blocks of randomly varying size. Recruitment, retention, adherence, adverse events and mobility and balance were measured before randomisation, postintervention and at four weeks follow-up. Results: Fifty-six participants participated (18 T-VCT, 19 O-VCT, 19 UC). Thirty-four completed treatment and follow-up assessments. Of the participants that completed, adherence was good with 16 treatments provided over (median of) 8.4, 7.5 and 9 weeks for T-VCT, O-VCT and UC respectively. No adverse events were reported. Post-treatment improvements in walking speed, symmetry, balance and functional mobility were seen in all treatment arms. Conclusions: Outpatient based treadmill and over-ground walking adaptability practice using visual cues are feasible and may improve mobility and balance. Future studies should continue a carefully phased approach using identified methods to improve retention. Trial Registration: Clinicaltrials.gov NCT0160039

Solid-State Dynamics in the closo-Carboranes:A (11)B MAS NMR and Molecular Dynamics Study

This work explores the dynamic behaviour of the three closo-carborane isomers (formula C2B10H12) using modern solid-state magic angle spinning (MAS) NMR techniques and relates the experimental measurements to theoretical results obtained using molecular dynamics simulations. At high temperatures and at B0 = 9.4 T, the 11B MAS linewidths are narrow (40-90 Hz) for the three isomers. The rotational correlation times (tauc) calculated by molecular dynamics are on the picosecond timescale, showing a quasi-isotropic rotation at these temperatures, typical for liquid systems. For all three isomers, the values of the 11B spin-lattice relaxation times (T1) show discontinuities as the temperature is decreased, confirming the phase changes reported in the literature. At low temperatures, the 11B MAS spectra of all three isomers exhibit much broader lines. The simulations showed that the molecular reorientation was anisotropic around different symmetry axes for each isomer, and this was supported by the values of the reduced quadrupolar parameter PQeff derived from “dynamic shift” measurements using 11B MQMAS NMR spectroscopy. The behaviour of PQeff as a function of temperature for para-carborane suggests that molecular reorientation is about the C5 symmetry axis of the molecule at low temperatures and this was supported by the molecular dynamics simulations