2,041 research outputs found
Measuring errors in single qubit rotations by pulsed electron paramagnetic resonance
The ability to measure and reduce systematic errors in single-qubit logic
gates is crucial when evaluating quantum computing implementations. We describe
pulsed electron paramagnetic resonance (EPR) sequences that can be used to
measure precisely even small systematic errors in rotations of
electron-spin-based qubits. Using these sequences we obtain values for errors
in rotation angle and axis for single-qubit rotations using a commercial EPR
spectrometer. We conclude that errors in qubit operations by pulsed EPR are not
limiting factors in the implementation of electron-spin based quantum
computers
High Fidelity Single Qubit Operations using Pulsed EPR
Systematic errors in spin rotation operations using simple RF pulses place
severe limitations on the usefulness of the pulsed magnetic resonance methods
in quantum computing applications. In particular, the fidelity of quantum logic
operations performed on electron spin qubits falls well below the threshold for
the application of quantum algorithms. Using three independent techniques, we
demonstrate the use of composite pulses to improve this fidelity by several
orders of magnitude. The observed high-fidelity operations are limited by pulse
phase errors, but nevertheless fall within the limits required for the
application of quantum error correction.Comment: 4 pages, 3 figures To appear in Phys. Rev. Let
Coherent state transfer between an electron- and nuclear spin in 15N@C60
Electron spin qubits in molecular systems offer high reproducibility and the
ability to self assemble into larger architectures. However, interactions
between neighbouring qubits are 'always-on' and although the electron spin
coherence times can be several hundred microseconds, these are still much
shorter than typical times for nuclear spins. Here we implement an
electron-nuclear hybrid scheme which uses coherent transfer between electron
and nuclear spin degrees of freedom in order to both controllably turn on/off
dipolar interactions between neighbouring spins and benefit from the long
nuclear spin decoherence times (T2n). We transfer qubit states between the
electron and 15N nuclear spin in 15N@C60 with a two-way process fidelity of
88%, using a series of tuned microwave and radiofrequency pulses and measure a
nuclear spin coherence lifetime of over 100 ms.Comment: 5 pages, 3 figures with supplementary material (8 pages
Changes in soil microbial community structure with tillage under long-term wheat-fallow management
Fatty acid methyl esters (FAMEs) were used to `fingerprint\u27 soil microbial communities that evolved during 25 years of wheat-fallow cropping following native mixed prairie sod at Sidney, Nebraska, USA. Total ester-linked FAMEs (EL-FAMEs) and phospholipid-linked FAMEs (PL-FAMEs) were compared for their ability to discriminate between plots remaining in sod and those cropped to wheat or left fallow under no-till, sub-till or plow management. Cropped plots were higher in microbial biomass than their fallowed counterparts, and did not differ significantly with tillage for the 0±15 cm depth. Under fallow, microbial biomass was greatest in no-till and least in plow. Both cluster and discriminant analysis of PL- and EL-FAMEs clearly separated the remaining native sod plots from the existing wheat-fallow plots. This separation was particularly pronounced for the EL-FAMEs and was largely driven by high amounts in sod of a single FAME, C16:1(cis11), which has been cited as a biomarker for arbuscular mycorrhizal (AM) fungi. Within wheat-fallow, C16:1(cis11) declined significantly from no-till to plow, which supports the origin of C16:1(cis11) from extraradical mycelium and spores of AM fungi known to be sensitive to soil disturbance. Although discriminant analysis of PL- and EL-FAMEs differentiated wheat and fallow systems by tillage, discrimination among tillage treatments was expressed most strongly during fallow. FAME profiles from fallow plow were most dissimilar from cropped soils which suggests a relationship between tillage management and the long-term resiliency of the microbial community developed under the wheat crop
Changes in soil microbial community structure with tillage under long-term wheat-fallow management
Fatty acid methyl esters (FAMEs) were used to `fingerprint\u27 soil microbial communities that evolved during 25 years of wheat-fallow cropping following native mixed prairie sod at Sidney, Nebraska, USA. Total ester-linked FAMEs (EL-FAMEs) and phospholipid-linked FAMEs (PL-FAMEs) were compared for their ability to discriminate between plots remaining in sod and those cropped to wheat or left fallow under no-till, sub-till or plow management. Cropped plots were higher in microbial biomass than their fallowed counterparts, and did not differ significantly with tillage for the 0±15 cm depth. Under fallow, microbial biomass was greatest in no-till and least in plow. Both cluster and discriminant analysis of PL- and EL-FAMEs clearly separated the remaining native sod plots from the existing wheat-fallow plots. This separation was particularly pronounced for the EL-FAMEs and was largely driven by high amounts in sod of a single FAME, C16:1(cis11), which has been cited as a biomarker for arbuscular mycorrhizal (AM) fungi. Within wheat-fallow, C16:1(cis11) declined significantly from no-till to plow, which supports the origin of C16:1(cis11) from extraradical mycelium and spores of AM fungi known to be sensitive to soil disturbance. Although discriminant analysis of PL- and EL-FAMEs differentiated wheat and fallow systems by tillage, discrimination among tillage treatments was expressed most strongly during fallow. FAME profiles from fallow plow were most dissimilar from cropped soils which suggests a relationship between tillage management and the long-term resiliency of the microbial community developed under the wheat crop
Electron spin relaxation of N@C60 in CS2
We examine the temperature dependence of the relaxation times of the
molecules N@C60 and N@C70 (which comprise atomic nitrogen trapped within a
carbon cage) in liquid CS2 solution. The results are inconsistent with the
fluctuating zero field splitting (ZFS) mechanism, which is commonly invoked to
explain electron spin relaxation for S > 1/2 spins in liquid solution, and is
the mechanism postulated in the literature for these systems. Instead, we find
a clear Arrhenius temperature dependence for N@C60, indicating the spin
relaxation is driven primarily by an Orbach process. For the asymmetric N@C70
molecule, which has a permanent non-zero ZFS, we resolve an additional
relaxation mechanism caused by the rapid reorientation of its ZFS. We also
report the longest coherence time (T2) ever observed for a molecular electron
spin, being 0.25 ms at 170K.Comment: 6 pages, 6 figures V2: Updated to published versio
Environmental effects on electron spin relaxation in N@C60
We examine environmental effects of surrounding nuclear spins on the electron
spin relaxation of the N@C60 molecule (which consists of a nitrogen atom at the
centre of a fullerene cage). Using dilute solutions of N@C60 in regular and
deuterated toluene, we observe and model the effect of translational diffusion
of nuclear spins of the solvent molecules on the N@C60 electron spin relaxation
times. We also study spin relaxation in frozen solutions of N@C60 in CS2, to
which small quantities of a glassing agent, S2Cl2 are added. At low
temperatures, spin relaxation is caused by spectral diffusion of surrounding
nuclear 35Cl and 37Cl spins in the S2Cl2, but nevertheless, at 20 K, T2 times
as long as 0.23 ms are observed.Comment: 7 pages, 6 figure
A new mechanism for electron spin echo envelope modulation
Electron spin echo envelope modulation (ESEEM) has been observed for the
first time from a coupled hetero-spin pair of electron and nucleus in liquid
solution. Previously, modulation effects in spin echo experiments have only
been described in liquid solutions for a coupled pair of homonuclear spins in
NMR or a pair of resonant electron spins in EPR. We observe low-frequency ESEEM
(26 and 52 kHz) due to a new mechanism present for any electron spin with S>1/2
that is hyperfine coupled to a nuclear spin. In our case these are electron
spin (S=3/2) and nuclear spin (I=1) in the endohedral fullerene N@C60. The
modulation is shown to arise from second order effects in the isotropic
hyperfine coupling of an electron and 14N nucleus.Comment: 15 pages, 4 figure
Towards a fullerene-based quantum computer
Molecular structures appear to be natural candidates for a quantum
technology: individual atoms can support quantum superpositions for long
periods, and such atoms can in principle be embedded in a permanent molecular
scaffolding to form an array. This would be true nanotechnology, with
dimensions of order of a nanometre. However, the challenges of realising such a
vision are immense. One must identify a suitable elementary unit and
demonstrate its merits for qubit storage and manipulation, including input /
output. These units must then be formed into large arrays corresponding to an
functional quantum architecture, including a mechanism for gate operations.
Here we report our efforts, both experimental and theoretical, to create such a
technology based on endohedral fullerenes or 'buckyballs'. We describe our
successes with respect to these criteria, along with the obstacles we are
currently facing and the questions that remain to be addressed.Comment: 20 pages, 13 figs, single column forma
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