Coulomb-Modified Fano Resonance in a One-Lead Quantum Dot

We investigate a tunable Fano interferometer consisting of a quantum dot coupled via tunneling to a one-dimensional channel. In addition to Fano resonance, the channel shows strong Coulomb response to the dot, with a single electron modulating channel conductance by factors of up to 100. Where these effects coexist, lineshapes with up to four extrema are found. A model of Coulomb-modified Fano resonance is developed and gives excellent agreement with experiment.Comment: related papers available at http://marcuslab.harvard.ed

Fast Single-Charge Sensing with an rf Quantum Point Contact

We report high-bandwidth charge sensing measurements using a GaAs quantum point contact embedded in a radio frequency impedance matching circuit (rf-QPC). With the rf-QPC biased near pinch-off where it is most sensitive to charge, we demonstrate a conductance sensitivity of 5x10^(-6) e^(2)/h Hz^(-1/2) with a bandwidth of 8 MHz. Single-shot readout of a proximal few-electron double quantum dot is investigated in a mode where the rf-QPC back-action is rapidly switched.Comment: related papers available at http://marcuslab.harvard.ed

Lie algebraic noncommuting structures from reparametrisation symmetry

We extend our earlier work of revealing both space-space and space-time noncommuting structures in various models in particle mechanics exhibiting reparametrisation symmetry. We show explicitly (in contrast to the earlier results in our paper \cite{sg}) that for some special choices of the reparametrisation parameter $\epsilon$, one can obtain space-space noncommuting structures which are Lie-algebraic in form even in the case of the relativistic free particle. The connection of these structures with the existing models in the literature is also briefly discussed. Further, there exists some values of $\epsilon$ for which the noncommutativity in the space-space sector can be made to vanish. As a matter of internal consistency of our approach, we also study the angular momentum algebra in details.Comment: 9 pages Latex, some references adde

Two-stage Kondo effect in a four-electron artificial atom

An artificial atom with four electrons is driven through a singlet-triplet transition by varying the confining potential. In the triplet, a Kondo peak with a narrow dip at drain-source voltage V_ds=0 is observed. The low energy scale V_ds* characterizing the dip is consistent with predictions for the two-stage Kondo effect. The phenomenon is studied as a function of temperature T and magnetic field B, parallel to the two-dimensional electron gas. The low energy scales T* and B* are extracted from the behavior of the zero-bias conductance and are compared to the low energy scale V_ds* obtained from the differential conductance. Good agreement is found between kT* and |g|muB*, but eV_ds* is larger, perhaps because of nonequilibrium effects.Comment: 7 pages, 7 figures. Added labels on Fig. 3f and one referenc

The de Sitter Relativistic Top Theory

We discuss the relativistic top theory from the point of view of the de Sitter (or anti de Sitter) group. Our treatment rests on Hanson-Regge's spherical relativistic top lagrangian formulation. We propose an alternative method for studying spinning objects via Kaluza-Klein theory. In particular, we derive the relativistic top equations of motion starting with the geodesic equation for a point particle in 4+N dimensions. We compare our approach with the Fukuyama's formulation of spinning objects, which is also based on Kaluza-Klein theory. We also report a generalization of our approach to a 4+N+D dimensional theory.Comment: 25 pages, Latex,commnets and references adde

Cotunneling Spectroscopy in Few-Electron Quantum Dots

Few-electron quantum dots are investigated in the regime of strong tunneling to the leads. Inelastic cotunneling is used to measure the two-electron singlet-triplet splitting above and below a magnetic field driven singlet-triplet transition. Evidence for a non-equilibrium two-electron singlet-triplet Kondo effect is presented. Cotunneling allows orbital correlations and parameters characterizing entanglement of the two-electron singlet ground state to be extracted from dc transport.Comment: related papers available at http://marcuslab.harvard.ed

Minimizing Strong Telluric Absorption in Near Infra-red Stellar Spectra

We have obtained high resolution spectra (R = 25000) of an A star over varying airmass to determine the effectiveness of telluric removal in the limit of high signal to noise. The near infra-red line HeI at 2.058 microns, which is a sensitive indicator of physical conditions in massive stars, supergiants, HII regions and YSOs, resides among pressure broadened telluric absorption from carbon dioxide and water vapor that varies both in time and with observed airmass. Our study shows that in the limit of bright stars at high resolution, accuracies of 5% are typical for high airmass observations (greater than 1.9), improving to a photon-limited accuracy of 2% at smaller airmasses (less than 1.15). We find that by using the continuum between telluric absorption lines of a ro-vibrational fan a photon-limited 1% accuracy is achievable.Comment: 14 pages, 7 figures. Accepted for publication in PAS

Interlaced Dynamical Decoupling and Coherent Operation of a Singlet-Triplet Qubit

We experimentally demonstrate coherence recovery of singlet-triplet superpositions by interlacing qubit rotations between Carr-Purcell (CP) echo sequences. We then compare performance of Hahn, CP, concatenated dynamical decoupling (CDD) and Uhrig dynamical decoupling (UDD) for singlet recovery. In the present case, where gate noise and drift combined with spatially varying hyperfine coupling contribute significantly to dephasing, and pulses have limited bandwidth, CP and CDD yield comparable results, with T2 ~ 80 microseconds.Comment: related papers at http://marcuslab.harvard.ed