5,268 research outputs found
Quantum Computing: From Bragg Reflections to Decoherence Estimates
We give an exposition of the principles of quantum computing (logic gates, exponential parallelism from polynomial hardware, fast quantum algorithms, quantum error correction, hardware requirements, and experimental milestones). A compact description of the quantum Fourier transform to find the period of a function-the key step in Shor\u27s factoring algorithm-illustrates how parallel state evolution along many classical computational paths produces fast algorithms by constructive interference similar to Bragg reflections in x-ray crystallography. On the hardware side, we present a new method to estimate critical time scales for the operation of a quantum computer. We derive a universal upper bound on the probability of a computation to fail due to decoherence (entanglement of the computer with the environment), as a function of time. The bound is parameter-free, requiring only the interaction between the computer and the environment, and the time-evolving state in the absence of any interaction. For a simple model we find that the bound performs well and decoherence is small when the energy of the computer state is large compared to the interaction energy. This supports a recent estimate of minimum energy requirements for quantum computation
Coronal condensations caused by magnetic reconnection between solar coronal loops
Employing Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA)
multi-wavelength images, we report the coronal condensation during the magnetic
reconnection (MR) between a system of open and closed coronal loops.
Higher-lying magnetically open structures, observed in AIA 171 A images above
the solar limb, move downward and interact with the lower-lying closed loops,
resulting in the formation of dips in the former. An X-type structure forms at
the interface. The interacting loops reconnect and disappear. Two sets of
newly-reconnected loops then form and recede from the MR region. During the MR
process, bright emission appears sequentially in the AIA 131 A and 304 A
channels repeatedly in the dips of higher-lying open structures. This indicates
the cooling and condensation process of hotter plasma from ~0.9 MK down to ~0.6
MK, and then to ~0.05 MK, also supported by the light curves of the AIA 171 A,
131 A, and 304 A channels. The part of higher-lying open structures supporting
the condensations participate in the successive MR. The condensations without
support by underlying loops then rain back to the solar surface along the
newly-reconnected loops. Our results suggest that the MR between coronal loops
leads to the condensation of hotter coronal plasma and its downflows. MR thus
plays an active role in the mass cycle of coronal plasma because it can
initiate the catastrophic cooling and condensation. This underlines that the
magnetic and thermal evolution has to be treated together and cannot be
separated, even in the case of catastrophic cooling.Comment: 10 pages, 6 figure
GeV detection of HESS J0632+057
HESS J0632+057 is the only gamma-ray binary that has been detected at TeV
energies, but not at GeV energies yet. Based on nearly nine years of Fermi
Large Area Telescope (LAT) Pass 8 data, we report here on a deep search for the
gamma-ray emission from HESS J0632+057 in the 0.1-300 GeV energy range. We find
a previously unknown gamma-ray source, Fermi J0632.6+0548, spatially coincident
with HESS J0632+057. The measured flux of Fermi J0632.6+0548 is consistent with
the previous flux upper limit on HESS J0632+057 and shows variability that can
be related to the HESS J0632+057 orbital phase. We propose that Fermi
J0632.6+0548 is the GeV counterpart of HESS J0632+057. Considering the Very
High Energy (VHE) spectrum of HESS J0632+057, a possible spectral turnover
above 10 GeV may exist in Fermi J0632.6+0548, as appears to be common in other
established gamma-ray binaries.Comment: 17 pages, 4 figures, 1 table; Accepted for publication in Ap
Nonlocal memory assisted entanglement distribution in optical fibers
Successful implementation of several quantum information and communication
protocols require distributing entangled pairs of quantum bits in reliable
manner. While there exists a substantial amount of recent theoretical and
experimental activities dealing with non-Markovian quantum dynamics,
experimental application and verification of the usefulness of memory-effects
for quantum information tasks is still missing. We combine these two aspects
and show experimentally that a recently introduced concept of nonlocal memory
effects allows to protect and distribute polarization entangled pairs of
photons in efficient manner within polarization-maintaining (PM) optical
fibers. The introduced scheme is based on correlating the environments, i.e.
frequencies of the polarization entangled photons, before their physical
distribution. When comparing to the case without nonlocal memory effects, we
demonstrate at least 12-fold improvement in the channel, or fiber length, for
preserving the highly-entangled initial polarization states of photons against
dephasing
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