Person:

Kuemmeth, Ferdinand

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.

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.