Electrically driven spin resonance in a bent disordered carbon nanotube

Resonant manipulation of carbon nanotube valley-spin qubits by an electric field is investigated theoretically. We develop a new analysis of electrically driven spin resonance exploiting fixed physical characteristics of the nanotube: a bend and inhomogeneous disorder. The spectrum is simulated for an electron valley-spin qubit coupled to a hole valley-spin qubit and an impurity electron spin, and features that coincide with a recent measurement are identified. We show that the same mechanism allows resonant control of the full four-dimensional spin-valley space.Comment: 11 pages, 7 figure

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We introduce the `displacemon' electromechanical architecture that comprises a vibrating nanobeam, e.g. a carbon nanotube, flux coupled to a superconducting qubit. This platform can achieve strong and even ultrastrong coupling enabling a variety of quantum protocols. We use this system to describe a protocol for generating and measuring quantum interference between two trajectories of a nanomechanical resonator. The scheme uses a sequence of qubit manipulations and measurements to cool the resonator, apply an effective diffraction grating, and measure the resulting interference pattern. We simulate the protocol for a realistic system consisting of a vibrating carbon nanotube acting as a junction in a superconducting qubit, and we demonstrate the feasibility of generating a spatially distinct quantum superposition state of motion containing more than $10^6$ nucleons.Comment: 12 pages, 7 figure

Keeping perfect time with caged atoms

For Fridtjof Nansen, 13 April 1895 started well. Six days earlier, the Norwegian explorer had set a new record for the closest approach to the North Pole, and now he was moving quickly over unbroken sea ice toward Cape Fligely and home. But then came a sickening realization: In his eagerness to break camp, he had forgotten to wind the chronometers. He had lost track of precise time, and thus the ability to track his longitude. Although Nansen couldn't have lost his position by more than a few minutes, it forced him to take a circuitously conservative route to avoid being swept into the North Atlantic. His expedition thus had to endure a hungry winter, camped on an unknown shore. Not until June the following year did he encounter other explorers and learn his true position-on Cape Felder, in Franz Josef Land

Valley-spin blockade and spin resonance in carbon nanotubes

Manipulation and readout of spin qubits in quantum dots made in III-V materials successfully rely on Pauli blockade that forbids transitions between spin-triplet and spin-singlet states. Quantum dots in group IV materials have the advantage of avoiding decoherence from the hyperfine interaction by purifying them with only zero-spin nuclei. Complications of group IV materials arise from the valley degeneracies in the electronic bandstructure. These lead to complicated multiplet states even for two-electron quantum dots thereby significantly weakening the selection rules for Pauli blockade. Only recently have spin qubits been realized in silicon devices where the valley degeneracy is lifted by strain and spatial confinement. In carbon nanotubes Pauli blockade can be observed by lifting valley degeneracy through disorder. In clean nanotubes, quantum dots have to be made ultra-small to obtain a large energy difference between the relevant multiplet states. Here we report on low-disorder nanotubes and demonstrate Pauli blockade based on both valley and spin selection rules. We exploit the bandgap of the nanotube to obtain a large level spacing and thereby a robust blockade. Single-electron spin resonance is detected using the blockade.Comment: 31 pages including supplementary informatio

Can the displacemon device test objective collapse models?

Testing the limits of the applicability of quantum mechanics will deepen our understanding of the universe and may shed light on the interplay between quantum mechanics and gravity. At present there is a wide range of approaches for such macroscopic tests spanning from matter-wave interferometry of large molecules to precision measurements of heating rates in the motion of micro-scale cantilevers. The “displacemon” is a proposed electromechanical device consisting of a mechanical resonator flux-coupled to a superconducting qubit enabling generation and readout of mechanical quantum states. In the original proposal, the mechanical resonator was a carbon nanotube, containing 106 nucleons. Here, in order to probe quantum mechanics at a more macroscopic scale, we propose using an aluminum mechanical resonator on two larger mass scales, one inspired by the Marshall–Simon–Penrose–Bouwmeester moving-mirror proposal, and one set by the Planck mass. For such a device, we examine the experimental requirements needed to perform a more macroscopic quantum test and thus feasibly detect the decoherence effects predicted by two objective collapse models: Diósi–Penrose and continuous spontaneous localization. Our protocol for testing these two theories takes advantage of the displacemon architecture to create non-Gaussian mechanical states out of equilibrium with their environment and then analyzes the measurement statistics of a superconducting qubit. We find that with improvements to the fabrication and vibration sensitivities of these electromechanical devices, the displacemon device provides a new route to feasibly test decoherence mechanisms beyond standard quantum theory

Conditioned spin and charge dynamics of a single electron quantum dot

In this article we describe the incoherent and coherent spin and charge dynamics of a single electron quantum dot. We use a stochastic master equation to model the state of the system, as inferred by an observer with access to only the measurement signal. Measurements obtained during an interval of time contribute, by a past quantum state analysis, to our knowledge about the system at any time t within that interval. Such analysis permits precise estimation of physical parameters, and we propose and test a modification of the classical Baum-Welch parameter re-estimation method to systems driven by both coherent and incoherent processes

A New Spectroscopic and Photometric Analysis of the Transiting Planet Systems TrES-3 and TrES-4

We report new spectroscopic and photometric observations of the parent stars of the recently discovered transiting planets TrES-3 and TrES-4. A detailed abundance analysis based on high-resolution spectra yields [Fe/H] = –0.19 ± 0.08, T_(eff) = 5650 ± 75 K, and log g = 4.4 ± 0.1 for TrES-3, and [Fe/H] = +0.14 ± 0.09, T_(eff) = 6200 ± 75 K, and log g = 4.0 ± 0.1 for TrES-4. The accuracy of the effective temperatures is supported by a number of independent consistency checks. The spectroscopic orbital solution for TrES-3 is improved with our new radial velocity measurements of that system, as are the light-curve parameters for both systems based on newly acquired photometry for TrES-3 and a reanalysis of existing photometry for TrES-4. We have redetermined the stellar parameters taking advantage of the strong constraint provided by the light curves in the form of the normalized separation a/R_* (related to the stellar density) in conjunction with our new temperatures and metallicities. The masses and radii we derive are M_* = 0.928^(+0.028)_(–0.048) M_⊙, R_* = 0.829^(+0.015)_(–0.022) R_⊙, and M_* = 1.404^(+0.066)_(–0.134) M_⊙, R_* = 1.846^(+0.096)_(–0.087) R_⊙ for TrES-3 and TrES-4, respectively. With these revised stellar parameters, we obtain improved values for the planetary masses and radii. We find M_p = 1.910^(+0.075)_(–0.080) M_(Jup), R_p = 1.336^(+0.031)_(–0.036) R_(Jup) for TrES-3, and M_p = 0.925 ± 0.082 M_(Jup), R_p = 1.783^(+0.093)_(–0.086) R_(Jup) for TrES-4. We confirm TrES-4 as the planet with the largest radius among the currently known transiting hot Jupiters

Measuring the thermodynamic cost of timekeeping

All clocks, in some form or another, use the evolution of nature toward higher entropy states to quantify the passage of time. Because of the statistical nature of the second law and corresponding entropy flows, fluctuations fundamentally limit the performance of any clock. This suggests a deep relation between the increase in entropy and the quality of clock ticks. Indeed, minimal models for autonomous clocks in the quantum realm revealed that a linear relation can be derived, where for a limited regime every bit of entropy linearly increases the accuracy of quantum clocks. But can such a linear relation persist as we move toward a more classical system? We answer this in the affirmative by presenting the first experimental investigation of this thermodynamic relation in a nanoscale clock. We stochastically drive a nanometer-thick membrane and read out its displacement with a radio-frequency cavity, allowing us to identify the ticks of a clock. We show theoretically that the maximum possible accuracy for this classical clock is proportional to the entropy created per tick, similar to the known limit for a weakly coupled quantum clock but with a different proportionality constant. We measure both the accuracy and the entropy. Once nonthermal noise is accounted for, we find that there is a linear relation between accuracy and entropy and that the clock operates within an order of magnitude of the theoretical bound

Assessing agonistic potential of a candidate therapeutic anti-IL21R antibody

<p>Abstract</p> <p>Background</p> <p>Selective neutralization of the IL21/IL21R signaling pathway is a promising approach for the treatment of a variety of autoimmune diseases. Ab-01 is a human neutralizing anti-IL21R antibody. In order to ensure that the activities of Ab-01 are restricted to neutralization even under <it>in vitro </it>cross-linking and <it>in vivo </it>conditions, a comprehensive assessment of agonistic potential of Ab-01 was undertaken.</p> <p>Methods</p> <p><it>In vitro </it>antibody cross-linking and cell culture protocols reported for studies with a human agonistic antibody, TGN1412, were followed for Ab-01. rhIL21, the agonist ligand of the targeted receptor, and cross-linked anti-CD28 were used as positive controls for signal transduction. <it>In vivo </it>agonistic potential of Ab-01 was assessed by measuring expression levels of cytokine storm-associated and IL21 pathway genes in blood of cynomolgus monkeys before and after IV administration of Ab-01.</p> <p>Results</p> <p>Using a comprehensive set of assays that detected multiple activation signals in the presence of the positive control agonists, <it>in vitro </it>Ab-01-dependent activation was not detected in either PBMCs or the rhIL21-responsive cell line Daudi. Furthermore, no difference in gene expression levels was detected in blood before and after <it>in vivo </it>Ab-01 dosing of cynomolgus monkeys.</p> <p>Conclusions</p> <p>Despite efforts to intentionally force an agonistic signal from Ab-01, none could be detected.</p