434 research outputs found
Concatenated dynamical decoupling in a solid-state spin bath
Concatenated dynamical decoupling (CDD) pulse sequences hold much promise as
a strategy to mitigate decoherence in quantum information processing. It is
important to investigate the actual performance of these dynamical decoupling
strategies in real systems that are promising qubit candidates. In this Rapid
Communication, we compute the echo decay of concatenations of the Hahn echo
sequence for a solid-state electronic spin qubit in a nuclear spin bath using a
cluster expansion technique. We find that each level of concatenation reverses
the effect of successive levels of intrabath fluctuations. On the one hand,
this advances CDD as a versatile and realistic decoupling strategy. On the
other hand, this invalidates, as overly optimistic, results of the simple pair
approximation used previously to study restoration, through CDD, of coherence
lost to a mesoscopic spin bath
Numerical calculations of the phase diagram of cubic blue phases in cholesteric liquid crystals
We study the static properties of cubic blue phases by numerically minimising
the three-dimensional, Landau-de Gennes free energy for a cholesteric liquid
crystal close to the isotropic-cholesteric phase transition. Thus we are able
to refine the powerful but approximate, semi-analytic frameworks that have been
used previously. We obtain the equilibrium phase diagram and discuss it in
relation to previous results. We find that the value of the chirality above
which blue phases appear is shifted by 20% (towards experimentally more
accessible regions) with respect to previous estimates. We also find that the
region of stability of the O5 structure -- which has not been observed
experimentally -- shrinks, while that of BP I (O8-) increases thus giving the
correct order of appearance of blue phases at small chirality. We also study
the approach to equilibrium starting from the infinite chirality solutions and
we find that in some cases the disclination network has to assemble during the
equilibration. In these situations disclinations are formed via the merging of
isolated aligned defects.Comment: 16 pages, 5 figures. Accepted for publication in Phys. Rev.
Nuclear Spins as Quantum Memory in Semiconductor Nanostructures
We theoretically consider solid state nuclear spins in a semiconductor
nanostructure environment as long-lived, high-fidelity quantum memory. In
particular, we calculate, in the limit of a strong applied magnetic field, the
fidelity versus time of P donor nuclear spins in random bath environments of Si
and GaAs, and the lifetime of excited intrinsic spins in polarized Si and GaAs
environments. In the former situation, the nuclear spin dephases due to
spectral diffusion induced by the dipolar interaction among nuclei in the bath.
We calculate the decay of nuclear spin quantum memory in the context of Hahn
and Carr-Purcell-Meiboom-Gill (CPMG) refocused spin echoes using a formally
exact cluster expansion technique which has previously been successful in
dealing with electron spin dephasing in a solid state nuclear spin bath. With
decoherence dominated by transverse dephasing (T2), we find it feasible to
maintain high fidelity (losses of less than 10^{-6}) quantum memory on nuclear
spins for times of the order of 100 microseconds (GaAs:P) and 1 to 2
milliseconds (natural Si:P) using CPMG pulse sequences of just a few (~2-4)
applied pulses. We also consider the complementary situation of a central
flipped intrinsic nuclear spin in a bath of completely polarized nuclear spins
where decoherence is caused by the direct flip-flop of the central spin with
spins in the bath. Exact numerical calculations that include a sufficiently
large neighborhood of surrounding nuclei show lifetimes on the order of 1-5 ms
for both GaAs and natural Si. Our calculated nuclear spin coherence times may
have significance for solid state quantum computer architectures using
localized electron spins in semiconductors where nuclear spins have been
proposed for quantum memory storage
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}
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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
Transverse NMR relaxation as a probe of mesoscopic structure
Transverse NMR relaxation in a macroscopic sample is shown to be extremely
sensitive to the structure of mesoscopic magnetic susceptibility variations.
Such a sensitivity is proposed as a novel kind of contrast in the NMR
measurements. For suspensions of arbitrary shaped paramagnetic objects, the
transverse relaxation is found in the case of a small dephasing effect of an
individual object. Strong relaxation rate dependence on the objects' shape
agrees with experiments on whole blood. Demonstrated structure sensitivity is a
generic effect that arises in NMR relaxation in porous media, biological
systems, as well as in kinetics of diffusion limited reactions.Comment: 4 pages, 3 figure
The structure of the distortion free-energy density in nematics: second-order elasticity and surface terms
The Cone Phase of Liquid Crystals: Triangular Lattice of Double-Tilt Cylinders
We predict the existence of a new defect-lattice phase near the nematic -
smectic-C (NC) transition. This tilt- analogue of the blue phase is a lattice
of double-tilt cylinders. We discuss the structure and stability of the cone
phase. We suggest that many `nematics' exhibiting short range layering and tilt
order may in fact be in the molten cone phase, which is a line liquid.Comment: 4 Pages, 3 Figure
Composite-pulse magnetometry with a solid-state quantum sensor
The sensitivity of quantum magnetometers is challenged by control errors and,
especially in the solid-state, by their short coherence times. Refocusing
techniques can overcome these limitations and improve the sensitivity to
periodic fields, but they come at the cost of reduced bandwidth and cannot be
applied to sense static (DC) or aperiodic fields. Here we experimentally
demonstrate that continuous driving of the sensor spin by a composite pulse
known as rotary-echo (RE) yields a flexible magnetometry scheme, mitigating
both driving power imperfections and decoherence. A suitable choice of RE
parameters compensates for different scenarios of noise strength and origin.
The method can be applied to nanoscale sensing in variable environments or to
realize noise spectroscopy. In a room-temperature implementation based on a
single electronic spin in diamond, composite-pulse magnetometry provides a
tunable trade-off between sensitivities in the microT/sqrt(Hz) range,
comparable to those obtained with Ramsey spectroscopy, and coherence times
approaching T1
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