18,202 research outputs found
Determinisitic Writing and Control of the Dark Exciton Spin using Short Single Optical Pulses
We demonstrate that the quantum dot-confined dark exciton forms a long-lived
integer spin solid state qubit which can be deterministically on-demand
initiated in a pure state by one optical pulse. Moreover, we show that this
qubit can be fully controlled using short optical pulses, which are several
orders of magnitude shorter than the life and coherence times of the qubit. Our
demonstrations do not require an externally applied magnetic field and they
establish that the quantum dot-confined dark exciton forms an excellent solid
state matter qubit with some advantages over the half-integer spin qubits such
as the confined electron and hole, separately. Since quantum dots are
semiconductor nanostructures that allow integration of electronic and photonic
components, the dark exciton may have important implications on implementations
of quantum technologies consisting of semiconductor qubits.Comment: Added two authors, minor edits to figure captions, expanded
discussion of dark exciton eigenstate
“The Map of the Mexican’s Genome”: overlapping national identity, and population genomics
This is the final version of the article. Available from Springer Verlag via the DOI in this record.This paper explores the intersections between national identity and the production of medical/population genomics in Mexico. The ongoing efforts to construct a Haplotype Map of Mexican genetic diversity offers a unique opportunity to illustrate and analyze the exchange between the historic-political narratives of nationalism, and the material culture of genomic science. Haplotypes are central actants in the search for medically significant SNP’s (single nucleotide polymorphisms), as well as powerful entities involved in the delimitation of ancestry, temporality and variability (www.hapmap.org). By following the circulation of Haplotypes, light is shed on the alignments and discordances between socio-historical and bio-molecular mappings. The analysis is centred on the comparison between the genomic construction of time and ethnicity in the laboratory (through participant observation), and on the public mobilisation of a “Mexican Genome” and its wider political implications. Even though both: the scientific practice and the public discourse on medical/population genomics are traversed by notions of “admixture”, there are important distinctions to be made. In the public realm, the nationalist post-revolutionary ideas of Jose Vasconcelos, as expressed in his Cosmic Race (1925), still hold sway in the social imaginary. In contrast, admixture is treated as a complex, relative and probabilistic notion in laboratory practices. I argue that the relation between medical/population genomics and national identity is better understood as a process of re-articulation (Fullwiley Social Studies of Science 38:695, 2008), rather than coproduction (Reardon 2005) of social and natural orders. The evolving process of re-articulation conceals the novelty of medical/population genomics, aligning scientific facts in order to fit the temporal and ethnic grids of “Mestizaje”. But it is precisely the social and political work, that matches the emerging field of population genomics to the pre-existing projects of national identity, what is most revealing in order to understand the multiple and even subtle ways in which population genomics challenges the historical and identitarian frames of a “Mestizo” nation.The Mexican Council of Science and Technology (CONACYT), has been generous to fund the studies and research interests of both of u
All-Optical Depletion of Dark Excitons from a Semiconductor Quantum Dot
Semiconductor quantum dots are considered to be the leading venue for
fabricating on-demand sources of single photons. However, the generation of
long-lived dark excitons imposes significant limits on the efficiency of these
sources. We demonstrate a technique that optically pumps the dark exciton
population and converts it to a bright exciton population, using intermediate
excited biexciton states. We show experimentally that our method considerably
reduces the DE population while doubling the triggered bright exciton emission,
approaching thereby near-unit fidelity of quantum dot depletion.Comment: 5 pages, 3 figure
Complete control of a matter qubit using a single picosecond laser pulse
We demonstrate for the first time that a matter physical two level system, a
qubit, can be fully controlled using one ultrafast step. We show that the spin
state of an optically excited electron, an exciton, confined in a quantum dot,
can be rotated by any desired angle, about any desired axis, during such a
step. For this we use a single, resonantly tuned, picosecond long, polarized
optical pulse. The polarization of the pulse defines the rotation axis, while
the pulse detuning from a non-degenerate absorption resonance, defines the
magnitude of the rotation angle. We thereby achieve a high fidelity, universal
gate operation, applicable to other spin systems, using only one short optical
pulse. The operation duration equals the pulse temporal width, orders of
magnitude shorter than the qubit evolution life and coherence times.Comment: main text: 4 pages, 3 figures Supplemental material: 3 pages, 1
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