Person:

Kuemmeth, Ferdinand

Carbon nanotubes bridge the molecular and crystalline quantum worlds, and their extraordinary electronic, mechanical and optical properties have attracted enormous attention from a broad scientific community. We review the basic principles of fabricating spin-electronic devices based on individual, electrically-gated carbon nanotubes, and present experimental efforts to understand their electronic and nuclear spin degrees of freedom, which in the future may enable quantum applications.

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.