828 research outputs found
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
Switchable ErSc2N rotor within a C80 fullerene cage: An EPR and photoluminescence excitation study
Systems exhibiting both spin and orbital degrees of freedom, of which Er3+ is
one, can offer mechanisms for manipulating and measuring spin states via
optical excitations. Motivated by the possibility of observing
photoluminescence and electron paramagnetic resonance from the same species
located within a fullerene molecule, we initiated an EPR study of Er3+ in
ErSc2N@C80. Two orientations of the ErSc2N rotor within the C80 fullerene are
observed in EPR, consistent with earlier studies using photoluminescence
excitation (PLE) spectroscopy. For some crystal field orientations, electron
spin relaxation is driven by an Orbach process via the first excited electronic
state of the 4I_15/2 multiplet. We observe a change in the relative populations
of the two ErSc2N configurations upon the application of 532 nm illuminations,
and are thus able to switch the majority cage symmetry. This
photoisomerisation, observable by both EPR and PLE, is metastable, lasting many
hours at 20 K.Comment: 4 pages, 4 figure
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
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
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Immature HIV-1 assembles from Gag dimers leaving partial hexamers at lattice edges as potential substrates for proteolytic maturation
The CA (capsid) domain of immature HIV-1 Gag and the adjacent spacer peptide 1 (SP1) play a key role in viral assembly by forming a lattice of CA hexamers, which adapts to viral envelope curvature by incorporating small lattice defects and a large gap at the site of budding. This lattice is stabilized by intrahexameric and interhexameric CA-CA interactions, which are important in regulating viral assembly and maturation. We applied subtomogram averaging and classification to determine the oligomerization state of CA at lattice edges and found that CA forms partial hexamers. These structures reveal the network of interactions formed by CA-SP1 at the lattice edge. We also performed atomistic molecular dynamics simulations of CA-CA interactions stabilizing the immature lattice and partial CA-SP1 helical bundles. Free energy calculations reveal increased propensity for helix-to-coil transitions in partial hexamers compared to complete six-helix bundles. Taken together, these results suggest that the CA dimer is the basic unit of lattice assembly, partial hexamers exist at lattice edges, these are in a helix-coil dynamic equilibrium, and partial helical bundles are more likely to unfold, representing potential sites for HIV-1 maturation initiation
Electron spin coherence in metallofullerenes: Y, Sc and La@C82
Endohedral fullerenes encapsulating a spin-active atom or ion within a carbon
cage offer a route to self-assembled arrays such as spin chains. In the case of
metallofullerenes the charge transfer between the atom and the fullerene cage
has been thought to limit the electron spin phase coherence time (T2) to the
order of a few microseconds. We study electron spin relaxation in several
species of metallofullerene as a function of temperature and solvent
environment, yielding a maximum T2 in deuterated o-terphenyl greater than 200
microseconds for Y, Sc and La@C82. The mechanisms governing relaxation (T1, T2)
arise from metal-cage vibrational modes, spin-orbit coupling and the nuclear
spin environment. The T2 times are over 2 orders of magnitude longer than
previously reported and consequently make metallofullerenes of interest in
areas such as spin-labelling, spintronics and quantum computing.Comment: 5 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
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
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
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