10,330 research outputs found
Violation of the Leggett-Garg inequality with weak measurements of photons
By weakly measuring the polarization of a photon between two strong
polarization measurements, we experimentally investigate the correlation
between the appearance of anomalous values in quantum weak measurements, and
the violation of realism and non-intrusiveness of measurements. A quantitative
formulation of the latter concept is expressed in terms of a Leggett-Garg
inequality for the outcomes of subsequent measurements of an individual quantum
system. We experimentally violate the Leggett-Garg inequality for several
measurement strengths. Furthermore, we experimentally demonstrate that there is
a one-to-one correlation between achieving strange weak values and violating
the Leggett-Garg inequality.Comment: 5 pages, 4 figure
Experimental feedback control of quantum systems using weak measurements
A goal of the emerging field of quantum control is to develop methods for
quantum technologies to function robustly in the presence of noise. Central
issues are the fundamental limitations on the available information about
quantum systems and the disturbance they suffer in the process of measurement.
In the context of a simple quantum control scenario--the stabilization of
non-orthogonal states of a qubit against dephasing--we experimentally explore
the use of weak measurements in feedback control. We find that, despite the
intrinsic difficultly of implementing them, weak measurements allow us to
control the qubit better in practice than is even theoretically possible
without them. Our work shows that these more general quantum measurements can
play an important role for feedback control of quantum systems.Comment: 4 pages, 3 figures. v2 Added extra citation, journal reference and
DOI. Minor typographic correction
Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: II. 1D hydrodynamical models of wind driven shocks
Following the early Swift X-ray observations of the latest outburst of the
recurrent nova RS Ophiuchi in February 2006 (Paper I), we present new 1D
hydrodynamical models of the system which take into account all three phases of
the remnant evolution. The models suggest a novel way of modelling the system
by treating the outburst as a sudden increase then decrease in wind mass-loss
rate and velocity. The differences between this wind model and previous
Primakoff-type simulations are described. A more complex structure, even in 1D,
is revealed through the presence of both forward and reverse shocks, with a
separating contact discontinuity. The effects of radiative cooling are
investigated and key outburst parameters such as mass-loss rate, ejecta
velocity and mass are varied. The shock velocities as a function of time are
compared to the ones derived in Paper I. We show how the manner in which the
matter is ejected controls the evolution of the shock and that for a
well-cooled remnant, the shock deceleration rate depends on the amount of
energy that is radiated away.Comment: 9 pages, 5 figure
Ancilla-assisted quantum process tomography
Complete and precise characterization of a quantum dynamical process can be
achieved via the method of quantum process tomography. Using a source of
correlated photons, we have implemented several methods investigating a wide
range of processes, e.g., unitary, decohering, and polarizing. One of these
methods, ancilla-assisted process tomography (AAPT), makes use of an additional
``ancilla system,'' and we have theoretically determined the conditions when
AAPT is possible. All prior schemes for AAPT make use of entangled states. Our
results show that, surprisingly, entanglement is not required for AAPT, and we
present process tomography data obtained using an input state that has no
entanglement. However, the use of entanglement yields superior results.Comment: To appear in Physical Review Letter
Double radio peak and non-thermal collimated ejecta in RS Ophiuchi following the 2006 outburst
We report Multi-Element Radio-Linked Interferometer Network, Very Large Array, One-Centimetre Radio Array, Very Long Baseline Array (VLBA), Effelsberg and Giant Metrewave Radio Telescope observations beginning 4.5 days after the discovery of RS Ophiuchi undergoing its 2006 recurrent nova outburst. Observations over the first 9 weeks are included, enabling us to follow spectral development throughout the three phases of the remnant development. We see dramatic brightening on days 4 to 7 at 6 GHz and an accompanying increase in other bands, particularly 1.46 GHz, consistent with transition from the initial ‘free expansion’ phase to the adiabatic expansion phase. This is complete by day 13 when the flux density at 5 GHz is apparently declining from an unexpectedly early maximum (compared with expectations from observations of the 1985 outburst). The flux density recovered to a second peak by approximately day 40, consistent with behaviour observed in 1985. At all times the spectral index is consistent with mixed non-thermal and thermal emission. The spectral indices are consistent with a non-thermal component at lower frequencies on all dates, and the spectral index changes show that the two components are clearly variable. The estimated extent of the emission at 22 GHz on day 59 is consistent with the extended east and west features seen at 1.7 GHz with the VLBA on day 63 being entirely non-thermal. We suggest a two-component model, consisting of a decelerating shell seen in mixed thermal and non-thermal emission plus faster bipolar ejecta generating the non-thermal emission, as seen in contemporaneous VLBA observations. Our estimated ejecta mass of 4 ± 2 × 10−7 M⊙ is consistent with a white dwarf (WD) mass of 1.4 M⊙. It may be that this ejecta mass estimate is a lower limit, in which case a lower WD mass would be consistent with the data
Correlated photon-pair generation in a periodically poled MgO doped stoichiometric lithium tantalate reverse proton exchanged waveguide
We demonstrate photon-pair generation in a reverse proton exchanged waveguide
fabricated on a periodically poled magnesium doped stoichiometric lithium
tantalate substrate. Detected pairs are generated via a cascaded second order
nonlinear process where a pump laser at wavelength of 1.55 m is first
doubled in frequency by second harmonic generation and subsequently
downconverted around the same spectral region. Pairs are detected at a rate of
42 per second with a coincidence to accidental ratio of 0.7. This cascaded pair
generation process is similar to four-wave-mixing where two pump photons
annihilate and create a correlated photon pair
Identification and characterization of an imidazolium by-product formed during the synthesis of 4-methylmethcathinone (mephedrone)
4-Methylmethcathinone (2-methylamino-1-(4-methylphenyl)propan-1-one, mephedrone) is a psychoactive substance that has been associated with recreational use worldwide. Analytical data related to mephedrone are abundantly available but the characterization of by-products obtained during organic synthesis remains to be explored. This study presents the identification of a 1,2,3,5-tetramethyl-4-(4-methylphenyl)-1H-imidazol-3-ium salt (TMMPI), which was formed during the synthesis of mephedrone. When diethyl ether was added to the crude reaction product, solid material precipitated from the solution. Analytical characterization of TMMPI employed a range of analytical techniques including chromatographic analysis in combination with various mass spectrometric detection methods, nuclear magnetic resonance spectroscopy, and crystal structure analysis. Additional confirmation was obtained from organic synthesis of the imidazolium by-product. When TMMPI was subjected to analysis by gas chromatography-mass spectrometry (GC-MS), isomerization and degradation into two distinct compounds were observed, which pointed towards thermal instability under GC conditions. A liquid chromatography-mass spectrometry (LC-MS) based investigation into a micro-scale synthesis of mephedrone and three additional analogues revealed that the corresponding TMMPI analogue was formed. Interestingly, storage of mephedrone freebase in a number of organic solvents also gave rise to TMMPI and it appeared that its formation during storage was significantly reduced in the absence of air. The present study aimed to support clandestine forensic investigations by employing analytical strategies that are applicable to manufacturing sites. The imidazolium salts will most likely be found amongst the waste products of any clandestine lab site under investigation rather than with the desired product
Excited Boundary TBA in the Tricritical Ising Model
By considering the continuum scaling limit of the RSOS lattice model
of Andrews-Baxter-Forrester with integrable boundaries, we derive excited state
TBA equations describing the boundary flows of the tricritical Ising model.
Fixing the bulk weights to their critical values, the integrable boundary
weights admit a parameter which plays the role of the perturbing
boundary field and induces the renormalization group flow between
boundary fixed points. The boundary TBA equations determining the RG flows are
derived in the example. The
induced map between distinct Virasoro characters of the theory are specified in
terms of distribution of zeros of the double row transfer matrix.Comment: Latex, 14 pages - Talk given at the Landau meeting "CFT and
Integrable Models", Sept. 2002 - v2: some statements about
perturbations correcte
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