Climatic forcing of evolution in Amazonia during the Cenozoic: On the refuge theory of biotic differentiation

Climatic-vegetational fluctuations due to astronomical Milankovitch cycles caused global changes in the distribution of tropical forests and nonforest vegetation during the Cenozoic (Tertiary-Quaternary) and before. Forest and nonforest biomes on the continents changed continuously in distribution during the geological past, breaking up into isolated blocks and again expanding and coalescing under the varying dry to humid climatic conditions. However, plant and animal communities disrupted and species changed their distributions individualistically during the various climatic phases. Field data indicating vegetational changes in Amazonia exist for the Quaternary; The refuge theory postulates that extensive patches of humid rainforests persisted during dry periods of the Tertiary and Quaternary, especially near areas of surface relief in peripheral portions of Amazonia, where many extant species and subspecies of plants and animals probably originated. The humid 'refugia' may have been separated by various types of savanna and dry forests as well as other intermediate vegetation types of seasonally dry climates. The number and size of refugia during different dry periods remain unknown. Biogeographic evidence for the former existence of forest refugia include areas of endemism and sharply defined contact zones between species and subspecies of Amazonian forest birds and other animals which represent zones of conspicuous biogeographic discontinuity in a continuous forest environment. Alternative models of barrier formation in Amazonia leading to allopatric speciation include the river hypothesis, river-refuge hypothesis; canopy-density hypothesis, disturbance-vicariance hypothesis, museum hypothesis and various paleogeography hypotheses, some aspects of which may be applicable to certain periods in the evolution of the biota

Identifying single electron charge sensor events using wavelet edge detection

The operation of solid-state qubits often relies on single-shot readout using a nanoelectronic charge sensor, and the detection of events in a noisy sensor signal is crucial for high fidelity readout of such qubits. The most common detection scheme, comparing the signal to a threshold value, is accurate at low noise levels but is not robust to low-frequency noise and signal drift. We describe an alternative method for identifying charge sensor events using wavelet edge detection. The technique is convenient to use and we show that, with realistic signals and a single tunable parameter, wavelet detection can outperform thresholding and is significantly more tolerant to 1/f and low-frequency noise.Comment: 11 pages, 4 figure

Impulsos climáticos da evolução na Amazônia durante o Cenozóico: sobre a teoria dos Refúgios da diferenciação biótica

AS FLUTUAÇÕES climático-vegetacionais causadas pelos ciclos astronômicos de Milan-kovitch provocaram mudanças globais na distribuição de florestas tropicais e demais vegetações não-florestais antes e durante o Cenozóico (Terciário-Quaternário). Os biomas continentais de florestas e vegetações não-florestais mudaram continuamente sua distribuição durante o seu passado geológico, fragmentando-se em blocos isolados, expandindo-se e juntando-se novamente sob condições climáticas alternadas entre secas e úmidas. Entretanto, durante as diversas fases climáticas, comunidades de plantas e animais fragmentaram-se e as espécies mudaram suas distribuições de maneira individual. Existem, para o Quaternário, dados de campo indicando mudanças na vegetação da Amazônia. A teoria dos Refúgios postula a persistência de grandes manchas de florestas tropicais úmidas durante os períodos secos do Terciário e do Quaternário, especialmente aquelas localizadas próximo de superfícies rebaixadas, sobretudo nas porções periféricas da Amazônia. Essas áreas são, provavelmente, a origem de muitas espécies e subespécies de plantas e animais existentes hoje em dia. Os "refúgios" úmidos podem ter sido separados por vários tipos de savana e florestas secas, como também por outros tipos de vegetação intermediária de climas sazonalmente secos. A quantidade e o tamanho dos refúgios durante os diferentes períodos de seca continuam desconhecidos. Indícios biogeográficos da existência de refúgios florestais anteriores incluem áreas de endemismo e zonas de contato entre espécies e subespécies de pássaros e outros animais da floresta amazônica nitidamente definidos. Essas áreas representam zonas de distinta descontinuidade biogeográfica num ambiente florestal contínuo. Modelos alternativos para a formação de barreiras na Amazônia que conduzem à especiação alopátrica incluem as seguintes hipóteses: do Rio, dos Refúgios do Rio, da Densidade do Dossel, da Perturbação da Vicariânia, do Museu e várias hipóteses paleogeográficas, das quais alguns aspectos poderiam ser aplicáveis a certos períodos na evolução da biota

Tunable singlet-triplet splitting in a few-electron Si/SiGe quantum dot

We measure the excited-state spectrum of a Si/SiGe quantum dot as a function of in-plane magnetic field, and we identify the spin of the lowest three eigenstates in an effective two-electron regime. The singlet-triplet splitting is an essential parameter describing spin qubits, and we extract this splitting from the data. We find it to be tunable by lateral displacement of the dot, which is realized by changing two gate voltages on opposite sides of the device. We present calculations showing the data are consistent with a spectrum in which the first excited state of the dot is a valley-orbit state.Comment: 4 pages with 3 figure

Single-shot measurement of triplet-singlet relaxation in a Si/SiGe double quantum dot

We investigate the lifetime of two-electron spin states in a few-electron Si/SiGe double dot. At the transition between the (1,1) and (0,2) charge occupations, Pauli spin blockade provides a readout mechanism for the spin state. We use the statistics of repeated single-shot measurements to extract the lifetimes of multiple states simultaneously. At zero magnetic field, we find that all three triplet states have equal lifetimes, as expected, and this time is ~10 ms. At non-zero field, the T0 lifetime is unchanged, whereas the T- lifetime increases monotonically with field, reaching 3 seconds at 1 T.Comment: 4 pages, 3 figures, supplemental information. Typos fixed; updated to submitted versio

Two-axis control of a singlet-triplet qubit with an integrated micromagnet

The qubit is the fundamental building block of a quantum computer. We fabricate a qubit in a silicon double quantum dot with an integrated micromagnet in which the qubit basis states are the singlet state and the spin-zero triplet state of two electrons. Because of the micro magnet, the magnetic field difference $\Delta B$ between the two sides of the double dot is large enough to enable the achievement of coherent rotation of the qubit's Bloch vector about two different axes of the Bloch sphere. By measuring the decay of the quantum oscillations, the inhomogeneous spin coherence time $T_{2}^{*}$ is determined. By measuring $T_{2}^{*}$ at many different values of the exchange coupling $J$ and at two different values of $\Delta B$, we provide evidence that the micromagnet does not limit decoherence, with the dominant limits on $T_{2}^{*}$ arising from charge noise and from coupling to nuclear spins.Comment: 10 pages, 9 figure

Tunable spin-selective loading of a silicon spin qubit

The remarkable properties of silicon have made it the central material for the fabrication of current microelectronic devices. Silicon's fundamental properties also make it an attractive option for the development of devices for spintronics and quantum information processing. The ability to manipulate and measure spins of single electrons is crucial for these applications. Here we report the manipulation and measurement of a single spin in a quantum dot fabricated in a silicon/silicon-germanium heterostructure. We demonstrate that the rate of loading of electrons into the device can be tuned over an order of magnitude using a gate voltage, that the spin state of the loaded electron depends systematically on the loading voltage level, and that this tunability arises because electron spins can be loaded through excited orbital states of the quantum dot. The longitudinal spin relaxation time T1 is measured using single-shot pulsed techniques and found to be ~3 seconds at a field of 1.85 Tesla. The demonstration of single spin measurement as well as a long spin relaxation time and tunability of the loading are all favorable properties for spintronics and quantum information processing applications.Comment: 4 pages, 3 figures, Supplemental Informatio

Quantum control and process tomography of a semiconductor quantum dot hybrid qubit

The similarities between gated quantum dots and the transistors in modern microelectronics - in fabrication methods, physical structure, and voltage scales for manipulation - have led to great interest in the development of quantum bits (qubits) in semiconductor quantum dots. While quantum dot spin qubits have demonstrated long coherence times, their manipulation is often slower than desired for important future applications, such as factoring. Further, scalability and manufacturability are enhanced when qubits are as simple as possible. Previous work has increased the speed of spin qubit rotations by making use of integrated micromagnets, dynamic pumping of nuclear spins, or the addition of a third quantum dot. Here we demonstrate a new qubit that offers both simplicity - it requires no special preparation and lives in a double quantum dot with no added complexity - and is very fast: we demonstrate full control on the Bloch sphere with $\pi$-rotation times less than 100 ps in two orthogonal directions. We report full process tomography, extracting high fidelities equal to or greater than 85% for X-rotations and 94% for Z-rotations. We discuss a path forward to fidelities better than the threshold for quantum error correction.Comment: 6 pages, excluding Appendi

A Fully Quantum Mechanical Model of a SQUID Ring Coupled to an Electromagnetic Field

A quantum system comprising of a monochromatic electromagnetic field coupled to a SQUID ring with sinusoidal non-linearity, is studied. A magnetostatic flux $\Phi_{x}$ is also threading the SQUID ring, and is used to control the coupling between the two systems. It is shown that for special values of $\Phi_{x}$ the system is strongly coupled. The time evolution of the system is studied. It is shown that exchange of energy takes place between the two modes and that the system becomes entangled. A second quasi-classical model that treats the electromagnetic field classically is also studied. A comparison between the fully quantum mechanical model with the electromagnetic field initially in a coherent state and the quasi-classical model, is made.Comment: 7 pages, 9 figures. Uploaded as implementing a policy of arXiving old paper