Dynamic Nuclear Polarization in the Fractional Quantum Hall Regime

We investigate dynamic nuclear polarization in quantum point contacts (QPCs) in the integer and fractional quantum Hall regimes. Following the application of a dc bias, fractional plateaus in the QPC shift symmetrically about half filling of the lowest Landau level, $\nu = 1/2$, suggesting an interpretation in terms of composite fermions. Polarizing and detecting at different filling factors indicates that Zeeman energy is reduced by the induced nuclear polarization. Mapping effects from integer to fractional regimes extends the composite fermion picture to include hyperfine coupling.Physic

Fast Sensing of Double-Dot Charge Arrangement and Spin State with a Radio-Frequency Sensor Quantum Dot

Single-shot measurement of the charge arrangement and spin state of a double quantum dot are reported with measurement times down to 100 ns. Sensing uses radio-frequency reflectometry of a proximal quantum dot in the Coulomb blockade regime. The sensor quantum dot is up to 30 times more sensitive than a comparable quantum point-contact sensor and yields three times greater signal to noise in rf single-shot measurements. Numerical modeling is qualitatively consistent with experiment and shows that the improved sensitivity of the sensor quantum dot results from reduced lifetime broadening and screening.PhysicsOther Research Uni

Precision Cutting and Patterning of Graphene with Helium Ions

We report nanoscale patterning of graphene using a helium ion microscope configured for lithography. Helium ion lithography is a direct-write lithography process, comparable to conventional focused ion beam patterning, with no resist or other material contacting the sample surface. In the present application, graphene samples on $Si/SiO_2$ substrates are cut using helium ions, with computer controlled alignment, patterning, and exposure. Once suitable beam doses are determined, sharp edge profiles and clean etching are obtained, with little evident damage or doping to the sample. This technique provides fast lithography compatible with graphene, with ~15 nm feature sizes.Engineering and Applied SciencesPhysicsOther Research Uni

Electron–Nuclear Interaction in 13C^{13}C Nanotube Double Quantum Dots

For coherent electron spins, hyperfine coupling to nuclei in the host material can either be a dominant source of unwanted spin decoherence or, if controlled effectively, a resource enabling storage and retrieval of quantum information. To investigate the effect of a controllable nuclear environment on the evolution of confined electron spins, we have fabricated and measured gate-defined double quantum dots with integrated charge sensors made from single-walled carbon nanotubes with a variable concentration of $^{13}C$ (nuclear spin $(I=\frac{1}{2})$ among the majority zero-nuclear-spin $^{12}C$ atoms. We observe strong isotope effects in spin-blockaded transport, and from the magnetic field dependence estimate the hyperfine coupling in $^{13}C$ nanotubes to be of the order of $100 \mu eV$, two orders of magnitude larger than anticipated. $^{13}C$-enhanced nanotubes are an interesting system for spin-based quantum information processing and memory: the $^{13}C$ nuclei differ from those in the substrate, are naturally confined to one dimension, lack quadrupolar coupling and have a readily controllable concentration from less than one to $10^5$ per electron.Physic

Relaxation and Dephasing in a Two-Electron 13C^{13}C Nanotube Double Quantum Dot

We use charge sensing of Pauli blockade (including spin and isospin) in a two-electron $^{13}C$ nanotube double quantum dot to measure relaxation and dephasing times. The relaxation time $T_1$ first decreases with a parallel magnetic field and then goes through a minimum in a field of $1.4 T$. We attribute both results to the spin-orbit-modified electronic spectrum of carbon nanotubes, which at high field enhances relaxation due to bending-mode phonons. The inhomogeneous dephasing time $T_2^*$ is consistent with previous data on hyperfine coupling strength in $^{13}C$ nanotubes.PhysicsOther Research Uni