56 research outputs found
Photoassociation adiabatic passage of ultracold Rb atoms to form ultracold Rb_2 molecules
We theoretically explore photoassociation by Adiabatic Passage of two
colliding cold ^{85}Rb atoms in an atomic trap to form an ultracold Rb_2
molecule. We consider the incoherent thermal nature of the scattering process
in a trap and show that coherent manipulations of the atomic ensemble, such as
adiabatic passage, are feasible if performed within the coherence time window
dictated by the temperature, which is relatively long for cold atoms. We show
that a sequence of ~2*10^7 pulses of moderate intensities, each lasting ~750
ns, can photoassociate a large fraction of the atomic ensemble at temperature
of 100 microkelvin and density of 10^{11} atoms/cm^3. Use of multiple pulse
sequences makes it possible to populate the ground vibrational state. Employing
spontaneous decay from a selected excited state, one can accumulate the
molecules in a narrow distribution of vibrational states in the ground
electronic potential. Alternatively, by removing the created molecules from the
beam path between pulse sets, one can create a low-density ensemble of
molecules in their ground ro-vibrational state.Comment: RevTex, 23 pages, 9 figure
Piecewise adiabatic population transfer in a molecule via a wave packet
We propose a class of schemes for robust population transfer between quantum
states that utilize trains of coherent pulses and represent a generalized
adiabatic passage via a wave packet. We study piecewise Stimulated Raman
Adiabatic Passage with pulse-to-pulse amplitude variation, and piecewise
chirped Raman passage with pulse-to-pulse phase variation, implemented with an
optical frequency comb. In the context of production of ultracold ground-state
molecules, we show that with almost no knowledge of the excited potential,
robust high-efficiency transfer is possibleComment: 4 pages, 5 figures. Submitted to Phys. Rev. Let
Pulsed Adiabatic Photoassociation via Scattering Resonances
We develop the theory for the Adiabatic Raman Photoassociation (ARPA) of
ultracold atoms to form ultracold molecules in the presence of scattering
resonances. Based on a computational method in which we replace the continuum
with a discrete set of "effective modes", we show that the existence of
resonances greatly aids in the formation of deeply bound molecular states. We
illustrate our general theory by computationally studying the formation of
Rb molecules from pairs of colliding ultracold Rb atoms. The
single-event transfer yield is shown to have a near-unity value for wide
resonances, while the ensemble-averaged transfer yield is shown to be higher
for narrow resonances. The ARPA yields are compared with that of (the
experimentally measured) "Feshbach molecule" magneto-association. Our findings
suggest that an experimental investigation of ARPA at sub-K temperatures
is warranted.Comment: 20 pages, 11 figure
Complete transfer of populations from a single state to a pre-selected superposition of states using Piecewise Adiabatic Passage
We develop a method for executing robust and selective transfer of
populations between a single level and pre-selected superpositions of energy
eigenstates. Viewed in the frequency domain, our method amounts to executing a
series of simultaneous adiabatic passages into each component of the target
superposition state. Viewed in {the} time domain, the method works by
accumulating the wavefunction of the target wave packet as it revisits the
Franck Condon region, in what amounts to an extension of the Piecewise
Adiabatic Passage technique [ Shapiro et.al., Phys. Rev. Lett. 99, 033002
(2007)] to the multi-state regime. The viability of the method is verified by
performing numerical tests for the Na_2 molecule.Comment: 8 pages, 4 figure
Biomolecular Filters for Improved Separation of Output Signals in Enzyme Logic Systems Applied to Biomedical Analysis
Biomolecular logic systems processing biochemical input signals and producing
"digital" outputs in the form of YES/NO were developed for analysis of
physiological conditions characteristic of liver injury, soft tissue injury and
abdominal trauma. Injury biomarkers were used as input signals for activating
the logic systems. Their normal physiological concentrations were defined as
logic-0 level, while their pathologically elevated concentrations were defined
as logic-1 values. Since the input concentrations applied as logic 0 and 1
values were not sufficiently different, the output signals being at low and
high values (0, 1 outputs) were separated with a short gap making their
discrimination difficult. Coupled enzymatic reactions functioning as a
biomolecular signal processing system with a built-in filter property were
developed. The filter process involves a partial back-conversion of the
optical-output-signal-yielding product, but only at its low concentrations,
thus allowing the proper discrimination between 0 and 1 output values
Integrated genome-wide chromatin occupancy and expression analyses identify key myeloid pro-differentiation transcription factors repressed by Myb
To gain insight into the mechanisms by which the Myb transcription factor controls normal hematopoiesis and particularly, how it contributes to leukemogenesis, we mapped the genome-wide occupancy of Myb by chromatin immunoprecipitation followed by massively parallel sequencing (ChIP-Seq) in ERMYB myeloid progenitor cells. By integrating the genome occupancy data with whole genome expression profiling data, we identified a Myb-regulated transcriptional program. Gene signatures for leukemia stem cells, normal hematopoietic stem/progenitor cells and myeloid development were overrepresented in 2368 Myb regulated genes. Of these, Myb bound directly near or within 793 genes. Myb directly activates some genes known critical in maintaining hematopoietic stem cells, such as Gfi1 and Cited2. Importantly, we also show that, despite being usually considered as a transactivator, Myb also functions to repress approximately half of its direct targets, including several key regulators of myeloid differentiation, such as Sfpi1 (also known as Pu.1), Runx1, Junb and Cebpb. Furthermore, our results demonstrate that interaction with p300, an established coactivator for Myb, is unexpectedly required for Myb-mediated transcriptional repression. We propose that the repression of the above mentioned key pro-differentiation factors may contribute essentially to Mybās ability to suppress differentiation and promote self-renewal, thus maintaining progenitor cells in an undifferentiated state and promoting leukemic transformation
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