1,375 research outputs found
Strong Electron-Hole Exchange in Coherently Coupled Quantum Dots
We have investigated few-body states in vertically stacked quantum dots. Due
to small inter-dot tunneling rate, the coupling in our system is in a
previously unexplored regime where electron-hole exchange is the dominant spin
interaction. By tuning the gate bias, we are able to turn this coupling off and
study a complementary regime where total electron spin is a good quantum
number. The use of differential transmission allows us to obtain unambiguous
signatures of the interplay between electron and hole spin interactions. Small
tunnel coupling also enables us to demonstrate all-optical charge sensing,
where conditional exciton energy shift in one dot identifies the charging state
of the coupled partner.Comment: 10 pages, 3 figure
Enhancement of electron spin coherence by optical preparation of nuclear spins
We study a large ensemble of nuclear spins interacting with a single electron
spin in a quantum dot under optical excitation and photon detection. When a
pair of applied laser fields satisfy two-photon resonance between the two
ground electronic spin states, detection of light scattering from the
intermediate exciton state acts as a weak quantum measurement of the effective
magnetic (Overhauser) field due to the nuclear spins. If the spin were driven
into a coherent population trapping state where no light scattering takes
place, then the nuclear state would be projected into an eigenstate of the
Overhauser field operator and electron decoherence due to nuclear spins would
be suppressed: we show that this limit can be approached by adapting the laser
frequencies when a photon is detected. We use a Lindblad equation to describe
the time evolution of the driven system under photon emission and detection.
Numerically, we find an increase of the electron coherence time from 5 ns to
500 ns after a preparation time of 10 microseconds.Comment: 5 pages, 4 figure
Charge radii of the nucleon from its flavor dependent Dirac form factors
We have determined the proton and the neutron charge radii from a global
analysis of the proton and the neutron elastic form factors, after first
performing a flavor decomposition of these form factors under charge symmetry
in the light cone frame formulation. We then extracted the transverse
mean-square radii of the flavor dependent quark distributions. In turn, these
are related in a model-independent way to the proton and neutron charge radii
but allow us to take into account motion effects of the recoiling nucleon for
data at finite but high momentum transfer. In the proton case we find ,
consistent with the proton charge radius obtained from muonic hydrogen
spectroscopy \cite{pohl:2010,antog2013}. The current method improves on the
precision of the extraction based on the form factor
measurements. Furthermore, we find no discrepancy in the
determination among the different electron scattering measurements, all of
which, utilizing the current method of extraction, result in a value that is
consistent with the smallest extraction from the electron
scattering measurements \cite{Xiong:2019umf}. Concerning the neutron case, past
results relied solely on the neutron-electron scattering length measurements,
which suffer from an underestimation of underlying systematic uncertainties
inherent to the extraction technique. Utilizing the present method we have
performed the first extraction of the neutron charge radius based on nucleon
form factor data, and we find
Search for Short-Term Periodicities in the Sun's Surface Rotation: A Revisit
The power spectral analyses of the Sun's surface equatorial rotation rate
determined from the Mt. Wilson daily Doppler velocity measurements during the
period 3 December 1985 to 5 March 2007 suggests the existence of 7.6 year, 2.8
year, 1.47 year, 245 day, 182 day and 158 day periodicities in the surface
equatorial rotation rate during the period before 1996.
However, there is no variation of any kind in the more accurately measured
data during the period after 1995. That is, the aforementioned periodicities in
the data during the period before the year 1996 may be artifacts of the
uncertainties of those data due to the frequent changes in the instrumentation
of the Mt. Wilson spectrograph. On the other hand, the temporal behavior of
most of the activity phenomena during cycles 22 (1986-1996) and 23 (after 1997)
is considerably different. Therefore, the presence of the aforementioned
short-term periodicities during the last cycle and absence of them in the
current cycle may, in principle, be real temporal behavior of the solar
rotation during these cycles.Comment: 11 pages, 6 figures, accepted for publication in Solar Physic
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Wire bond vibration of forward pixel tracking detector of CMS
Wire bonds of the Forward Pixel (FPix) tracking detectors are oriented in the direction that maximizes Lorentz Forces relative to the 4 Tesla field of the Compact Muon Solenoid (CMS) Detector's magnet. The CMS Experiment is under construction at the Large Hadron Collider at CERN, Geneva, Switzerland. We were concerned about Lorentz Force oscillating the wires at their fundamental frequencies and possibly fracturing or breaking them at their heels, as happened with the CDF wire bonds. This paper reports a study to understand what conditions break such bonds
Quantum Computation with Quantum Dots and Terahertz Cavity Quantum Electrodynamics
A quantum computer is proposed in which information is stored in the two
lowest electronic states of doped quantum dots (QDs). Many QDs are located in a
microcavity. A pair of gates controls the energy levels in each QD. A
Controlled Not (CNOT) operation involving any pair of QDs can be effected by a
sequence of gate-voltage pulses which tune the QD energy levels into resonance
with frequencies of the cavity or a laser. The duration of a CNOT operation is
estimated to be much shorter than the time for an electron to decohere by
emitting an acoustic phonon.Comment: Revtex 6 pages, 3 postscript figures, minor typos correcte
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