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

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

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

Spin topology from "Taylored" TOCSY

A new method is introduced which makes it possible to determine the number of successive couplings through which the magnetization giving rise to a particular TOCSY cross peak has been transferred. The method is based on two properties of the period of isotropic mixing used in TOCSY sequences: (i) the intensity variation of the in-phase part of a cross peak can be represented as an even-order power series in the mixing time, and (ii) if the cross peak is the result of magnetization having been transferred through n successive couplings, the lowest order term in this expansion is the one in which the mixing time is raised to the 2nth power. Proofs of both these theorems are described. A practical implementation of an algorithm which exploits these properties is given and its effectiveness in the presence of noise and experimental imperfections is assessed. An experimental example of the use of the procedure is described

Satellite transitions acquired in real time by magic angle spinning (STARTMAS): "Ultrafast" high-resolution MAS NMR spectroscopy of spin I=3/2 nuclei

The satellite transitions acquired in real time by magic angle spinning (STARTMAS) NMR experiment combines a train of pulses with sample rotation at the magic angle to refocus the first- and second-order quadrupolar broadening of spin I=3/2 nuclei in a series of echoes, while allowing the isotropic chemical and quadrupolar shifts to evolve. The result is real-time isotropic NMR spectra at high spinning rates using conventional MAS equipment. In this paper we describe in detail how STARTMAS data can be acquired and processed with ease on commercial equipment. We also discuss the advantages and limitations of the approach and illustrate the discussion with numerical simulations and experimental data from four different powdered solids.