251 research outputs found
Qubit coherence control in a nuclear spin bath
Coherent dynamics of localized spins in semiconductors is limited by spectral
diffusion arising from dipolar fluctuation of lattice nuclear spins. Here we
extend the semiclassical theory of spectral diffusion for nuclear spins I=1/2
to the high nuclear spins relevant to the III-V materials and show that
applying successive qubit pi-rotations at a rate approximately proportional to
the nuclear spin quantum number squared (I^2) provides an efficient method for
coherence enhancement. Hence robust coherent manipulation in the large spin
environments characteristic of the III-V compounds is possible without
resorting to nuclear spin polarization, provided that the pi-pulses can be
generated at intervals scaling as I^{-2}
Cluster Model DFT Study of the Intermediates of Benzene to Phenol Oxidation by N2O on FeZSM-5 Zeolites
An Fe(II) ion at an a-cation exchange position of ZSM-5 zeolite (Fe/Z) was taken as a model for the active site in the nitrous oxide decompn. and in the selective oxidn. of benzene to phenol with nitrous oxide. The oxygen deposited by decompn. of N2O is commonly referred to as a-oxygen (OFe/Z). Cluster model DFT calcns. show that the interaction of the OFe/Z center with benzene resulted easily in arene oxide formation. The results indicate a rather low activation energy for this step. Possible transformations of the adsorbed arene oxide are considered and the exptl. evidence for the absence of the kinetic H/D isotope effect in phenol formation is discussed. It is concluded that the rate-limiting step for the in situ oxidn. of benzene to phenol is the desorption of the product. [on SciFinder (R)
Quantum theory for electron spin decoherence induced by nuclear spin dynamics in semiconductor quantum computer architectures: Spectral diffusion of localized electron spins in the nuclear solid-state environment
We consider the decoherence of a single localized electron spin due to its
coupling to the lattice nuclear spin bath in a semiconductor quantum computer
architecture. In the presence of an external magnetic field and at low
temperatures, the dominant decoherence mechanism is the spectral diffusion of
the electron spin resonance frequency due to the temporally fluctuating random
magnetic field associated with the dipolar interaction induced flip-flops of
nuclear spin pairs. The electron spin dephasing due to this random magnetic
field depends intricately on the quantum dynamics of the nuclear spin bath,
making the coupled decoherence problem difficult to solve. We provide a
formally exact solution of this non-Markovian quantum decoherence problem which
numerically calculates accurate spin decoherence at short times, which is of
particular relevance in solid-state spin quantum computer architectures. A
quantum cluster expansion method is developed, motivated, and tested for the
problem of localized electron spin decoherence due to dipolar fluctuations of
lattice nuclear spins. The method is presented with enough generality for
possible application to other types of spin decoherence problems. We present
numerical results which are in quantitative agreement with electron spin echo
measurements in phosphorus doped silicon. We also present spin echo decay
results for quantum dots in GaAs which differ qualitatively from that of the
phosphorus doped silicon system. Our theoretical results provide the ultimate
limit on the spin coherence (at least, as characterized by Hahn spin echo
measurements) of localized electrons in semiconductors in the low temperature
and the moderate to high magnetic field regime of interest in scalable
semiconductor quantum computer architectures.Comment: 23 pages, 15 figure
Wavefunction considerations for the central spin decoherence problem in a nuclear spin bath
Decoherence of a localized electron spin in a solid state material (the
``central spin'' problem) at low temperature is believed to be dominated by
interactions with nuclear spins in the lattice. This decoherence is partially
suppressed through the application of a large magnetic field that splits the
energy levels of the electron spin and prevents depolarization. However,
dephasing decoherence resulting from a dynamical nuclear spin bath cannot be
removed in this way. Fluctuations of the nuclear field lead to uncertainty of
the electron's precessional frequency in a process known as spectral diffusion.
This article considers the effect of the electron's wavefunction shape upon
spectral diffusion and provides wavefunction dependent decoherence time
formulas for free induction decay as well as spin echoes and concatenated
dynamical decoupling schemes for enhancing coherence. We also discuss dephasing
of a qubit encoded in singlet-triplet states of a double quantum dot. A central
theoretical result of this work is the development of a continuum approximation
for the spectral diffusion problem which we have applied to GaAs and InAs
materials specifically
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
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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