1,588 research outputs found
Universal pulse sequence to minimize spin dephasing in the central spin decoherence problem
We present a remarkable finding that a recently discovered [G. S. Uhrig,
Phys. Rev. Lett. 98, 100504 (2007)] series of pulse sequences, designed to
optimally restore coherence to a qubit in the spin-boson model of decoherence,
is in fact completely model-independent and generically valid for arbitrary
dephasing Hamiltonians given sufficiently short delay times between pulses. The
series maximizes qubit fidelity versus number of applied pulses for
sufficiently short delay times because the series, with each additional pulse,
cancels successive orders of a time expansion for the fidelity decay. The
"magical" universality of this property, which was not appreciated earlier,
requires that a linearly growing set of "unknowns" (the delay times) must
simultaneously satisfy an exponentially growing set of nonlinear equations that
involve arbitrary dephasing Hamiltonian operators.Comment: Published in PRL, revise
Ks- and Lp-band polarimetry on stellar and bow-shock sources in the Galactic center
Infrared observations of the Galactic center (GC) provide a unique
opportunity to study stellar and bow-shock polarization effects in a dusty
environment. The goals of this work are to present new Ks- and Lp-band
polarimetry on an unprecedented number of sources in the central parsec of the
GC, thereby expanding our previous results in the H- and Ks-bands. We use
AO-assisted Ks- and Lp-band observations, obtained at the ESO VLT. High
precision photometry and the new polarimetric calibration method for NACO allow
us to map the polarization in a region of 8" x 25" (Ks) resp. 26" x 28" (Lp).
These are the first polarimetric observations of the GC in the Lp-band in 30
years, with vastly improved spatial resolution compared to previous results.
This allows resolved polarimetry on bright bow-shock sources in this area for
the first time at this wavelength. We find foreground polarization to be
largely parallel to the Galactic plane (Ks-band: 6.1% at 20 degrees, Lp-band:
4.5% at 20 degrees, in good agreement with our previous findings and with older
results. The previously described Lp-band excess in the foregound polarization
towards the GC could be confirmed here for a much larger number of sources. The
bow-shock sources contained in the FOV seem to show a different relation
between the polarization in the observed wavelength bands than what was
determined for the foreground. This points to the different relevant
polarization mechanisms. The resolved polarization patterns of IRS 5 and 10W
match the findings we presented earlier for IRS~1W. Additionally, intrinsic
Lp-band polarization was measured for IRS 1W and 21, as well as for other, less
prominent MIR-excess sources (IRS 2S, 2L, 5NE). The new data offer support for
the presumed bow-shock nature of several of these sources (1W, 5, 5NE, 10W, 21)
and for the model of bow-shock polarization presented in our last work.Comment: 19 pages, 18 figure
Electron Spin Dephasing due to Hyperfine Interactions with a Nuclear Spin Bath
We investigate pure dephasing decoherence (free induction decay and spin
echo) of a spin qubit interacting with a nuclear spin bath. While for infinite
magnetic field B the only decoherence mechanism is spectral diffusion due to
dipolar flip-flops of nuclear spins, with decreasing B the hyperfine-mediated
interactions between the nuclear spins become important. We give a theory of
decoherence due to these interactions which takes advantage of their long-range
nature. For a thermal uncorrelated bath we show that our theory is applicable
down to B~10 mT, allowing for comparison with recent experiments in GaAs
quantum dots.Comment: Published version, new title suggested by the PRL edito
A formal method for identifying distinct states of variability in time-varying sources: SgrA* as an example
Continuously time variable sources are often characterized by their power
spectral density and flux distribution. These quantities can undergo dramatic
changes over time if the underlying physical processes change. However, some
changes can be subtle and not distinguishable using standard statistical
approaches. Here, we report a methodology that aims to identify distinct but
similar states of time variability. We apply this method to the Galactic
supermassive black hole, where 2.2 um flux is observed from a source associated
with SgrA*, and where two distinct states have recently been suggested. Our
approach is taken from mathematical finance and works with conditional flux
density distributions that depend on the previous flux value. The discrete,
unobserved (hidden) state variable is modeled as a stochastic process and the
transition probabilities are inferred from the flux density time series. Using
the most comprehensive data set to date, in which all Keck and a majority of
the publicly available VLT data have been merged, we show that SgrA* is
sufficiently described by a single intrinsic state. However the observed flux
densities exhibit two states: a noise-dominated and a source-dominated one. Our
methodology reported here will prove extremely useful to assess the effects of
the putative gas cloud G2 that is on its way toward the black hole and might
create a new state of variability.Comment: Submitted to ApJ; 33 pages, 4 figures; comments welcom
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
Quantum theory of spectral diffusion induced electron spin decoherence
A quantum cluster expansion method is developed for the problem of localized
electron spin decoherence due to dipolar fluctuations of lattice nuclear spins.
At the lowest order it provides a microscopic explanation for the Lorentzian
diffusion of Hahn echoes without resorting to any phenomenological Markovian
assumption. Our numerical results show remarkable agreement with recent
electron spin echo experiments in phosphorus doped silicon.Comment: 5 pages, 1 figur
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