Spin Readout and Initialization in a Semiconductor Quantum Dot

Electron spin qubits in semiconductors are attractive from the viewpoint of long coherence times. However, single spin measurement is challenging. Several promising schemes incorporate ancillary tunnel couplings that may provide unwanted channels for decoherence. Here, we propose a novel spin-charge transduction scheme, converting spin information to orbital information within a single quantum dot by microwave excitation. The same quantum dot can be used for rapid initialization, gating, and readout. We present detailed modeling of such a device in silicon to confirm its feasibility.Comment: Published versio

Can we always get the entanglement entropy from the Kadanoff-Baym equations? The case of the T-matrix approximation

We study the time-dependent transmission of entanglement entropy through an out-of-equilibrium model interacting device in a quantum transport set-up. The dynamics is performed via the Kadanoff-Baym equations within many-body perturbation theory. The double occupancy $< \hat{n}_{R \uparrow} \hat{n}_{R \downarrow} >$, needed to determine the entanglement entropy, is obtained from the equations of motion of the single-particle Green's function. A remarkable result of our calculations is that $< \hat{n}_{R \uparrow} \hat{n}_{R \downarrow} >$ can become negative, thus not permitting to evaluate the entanglement entropy. This is a shortcoming of approximate, and yet conserving, many-body self-energies. Among the tested perturbation schemes, the $T$-matrix approximation stands out for two reasons: it compares well to exact results in the low density regime and it always provides a non-negative $< \hat{n}_{R \uparrow} \hat{n}_{R \downarrow} >$. For the second part of this statement, we give an analytical proof. Finally, the transmission of entanglement across the device is diminished by interactions but can be amplified by a current flowing through the system.Comment: 6 pages, 6 figure

The Initial Conditions of Clustered Star Formation III. The Deuterium Fractionation of the Ophiuchus B2 Core

We present N2D+ 3-2 (IRAM) and H2D+ 1_11 - 1_10 and N2H+ 4-3 (JCMT) maps of the small cluster-forming Ophiuchus B2 core in the nearby Ophiuchus molecular cloud. In conjunction with previously published N2H+ 1-0 observations, the N2D+ data reveal the deuterium fractionation in the high density gas across Oph B2. The average deuterium fractionation R_D = N(N2D+)/N(N2H+) ~ 0.03 over Oph B2, with several small scale R_D peaks and a maximum R_D = 0.1. The mean R_D is consistent with previous results in isolated starless and protostellar cores. The column density distributions of both H2D+ and N2D+ show no correlation with total H2 column density. We find, however, an anticorrelation in deuterium fractionation with proximity to the embedded protostars in Oph B2 to distances >= 0.04 pc. Destruction mechanisms for deuterated molecules require gas temperatures greater than those previously determined through NH3 observations of Oph B2 to proceed. We present temperatures calculated for the dense core gas through the equating of non-thermal line widths for molecules (i.e., N2D+ and H2D+) expected to trace the same core regions, but the observed complex line structures in B2 preclude finding a reasonable result in many locations. This method may, however, work well in isolated cores with less complicated velocity structures. Finally, we use R_D and the H2D+ column density across Oph B2 to set a lower limit on the ionization fraction across the core, finding a mean x_e, lim >= few x 10^{-8}. Our results show that care must be taken when using deuterated species as a probe of the physical conditions of dense gas in star-forming regions.Comment: ApJ accepte

Looking back while moving forward:How past responses to climate change can inform future adaptation and mitigation strategies in the Arctic

Modern Arctic Indigenous peoples face many interconnected pressures, not the least of which is anthropogenic climate change, which is emerging as one of the most dramatic drivers of social and economic change in recent memory. In this paper, we investigate whether or not insights into premodern strategies for coping with climate change—and especially the “deeper histories” of traditional ways-of-knowing—can play a useful role in future planning, management and mitigation efforts. We do this in two ways. First, we assess this special issue's 17 archaeological case studies, in order to determine whether they are conducted within a framework that is consistent with approaches to resilience in studies of modern Arctic communities. Second, we focus on three climate-driven challenges faced by Canadian Arctic Inuit: safe travel, food security and food safety. For each, we identify specific ways in which studies of past social-ecological systems intersect with modern climate adaptation. We conclude that since archaeological insights highlight the operation of decision-making processes within long-term culture-adaptive trajectories, they can offer unique insights into the much shorter-term processes currently underway. While we highlight many potential directions for productive collaboration, much more work is required in local and regional settings to demonstrate the full potential of archaeology for future-oriented planning and mitigation efforts

Global control and fast solid-state donor electron spin quantum computing

We propose a scheme for quantum information processing based on donor electron spins in semiconductors, with an architecture complementary to the original Kane proposal. We show that a naive implementation of electron spin qubits provides only modest improvement over the Kane scheme, however through the introduction of global gate control we are able to take full advantage of the fast electron evolution timescales. We estimate that the latent clock speed is 100-1000 times that of the nuclear spin quantum computer with the ratio $T_{2}/T_{ops}$ approaching the $10^{6}$ level.Comment: 9 pages, 9 figure

Learning from the early adopters: developing the digital practitioner

This paper explores how Sharpe and Beetham’s Digital Literacies Framework which was derived to model students’ digital literacies, can be applied to lecturers’ digital literacy practices. Data from a small-scale phenomenological study of higher education lecturers who used Web 2.0 in their teaching and learning practices are used to examine if this pyramid model represents their motivations for adopting technology-enhanced learning in their pedagogic practices. The paper argues that whilst Sharpe and Beetham’s model has utility in many regards, these lecturers were mainly motivated by the desire to achieve their pedagogic goals rather than by a desire to become a digital practitioner

VLA Survey of Dense Gas in Extended Green Objects: Prevalence of 25 GHz Methanol Masers

We present $\sim1-4"$ resolution Very Large Array (VLA) observations of four CH$_3$OH $J_2-J_1$-$E$ 25~GHz transitions ($J$=3, 5, 8, 10) along with 1.3~cm continuum toward 20 regions of active massive star formation containing Extended Green Objects (EGOs), 14 of which we have previously studied with the VLA in the Class~I 44~GHz and Class~II 6.7~GHz maser lines (Cyganowski et al. 2009). Sixteen regions are detected in at least one 25~GHz line ($J$=5), with 13 of 16 exhibiting maser emission. In total, we report 34 new sites of CH$_3$OH maser emission and ten new sites of thermal CH$_3$OH emission, significantly increasing the number of 25~GHz Class I CH$_3$OH masers observed at high angular resolution. We identify probable or likely maser counterparts at 44~GHz for all 15 of the 25~GHz masers for which we have complementary data, providing further evidence that these masers trace similar physical conditions despite uncorrelated flux densities. The sites of thermal and maser emission of CH$_3$OH are both predominantly associated with the 4.5 $\mu$m emission from the EGO, and the presence of thermal CH$_3$OH emission is accompanied by 1.3~cm continuum emission in 9 out of 10 cases. Of the 19 regions that exhibit 1.3~cm continuum emission, it is associated with the EGO in 16 cases (out of a total of 20 sites), 13 of which are new detections at 1.3~cm. Twelve of the 1.3~cm continuum sources are associated with 6.7~GHz maser emission and likely trace deeply-embedded massive protostars

Concerted evolution of duplicated mitochondrial control regions in three related seabird species

<p>Abstract</p> <p>Background</p> <p>Many population genetic and phylogenetic analyses of mitochondrial DNA (mtDNA) assume that mitochondrial genomes do not undergo recombination. Recently, concerted evolution of duplicated mitochondrial control regions has been documented in a range of taxa. Although the molecular mechanism that facilitates concerted evolution is unknown, all proposed mechanisms involve mtDNA recombination.</p> <p>Results</p> <p>Here, we document a duplication of a large region (cytochrome <it>b</it>, tRNA^Thr, tRNA^Pro, ND6, tRNA^Gluand the control region) in the mitochondrial genome of three related seabird species. To investigate the evolution of duplicate control regions, we sequenced both control region copies (CR1 and CR2) from 21 brown (<it>Sula leucogaster</it>), 21 red-footed (<it>S. sula</it>) and 21 blue-footed boobies (<it>S. nebouxii</it>). Phylogenetic analysis suggested that the duplicated control regions are predominantly evolving in concert; however, approximately 51 base pairs at the 5' end of CR1 and CR2 exhibited a discordant phylogenetic signal and appeared to be evolving independently.</p> <p>Conclusions</p> <p>Both the structure of the duplicated region and the conflicting phylogenetic signals are remarkably similar to a pattern found in <it>Thalassarche </it>albatrosses, which are united with boobies in a large clade that includes all procellariiform and most pelecaniform seabirds. Therefore we suggest that concerted evolution of duplicated control regions either is taxonomically widespread within seabirds, or that it has evolved many times.</p

Second Harmonic Coherent Driving of a Spin Qubit in a Si/SiGe Quantum Dot

We demonstrate coherent driving of a single electron spin using second harmonic excitation in a Si/SiGe quantum dot. Our estimates suggest that the anharmonic dot confining potential combined with a gradient in the transverse magnetic field dominates the second harmonic response. As expected, the Rabi frequency depends quadratically on the driving amplitude and the periodicity with respect to the phase of the drive is twice that of the fundamental harmonic. The maximum Rabi frequency observed for the second harmonic is just a factor of two lower than that achieved for the first harmonic when driving at the same power. Combined with the lower demands on microwave circuitry when operating at half the qubit frequency, these observations indicate that second harmonic driving can be a useful technique for future quantum computation architectures.Comment: 9 pages, 9 figure