Electron–nuclear interaction in 13C 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 decoherence1–3 or, if controlled effectively, a resource enabling storage and retrieval of quantum information4–7. 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 13C (nuclear spin I = 1/2) among the majority zero-nuclear-spin 12C atoms. We observe strong isotope effects in spin-blockaded transport, and from the magnetic field dependence estimate the hyperfine coupling in 13C nanotubes to be of the order of 100μeV, two orders of magnitude larger than anticipated8,9. 13C-enhanced nanotubes are an interesting system for spin-based quantum information processing and memory: the 13C 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 105 per electron. Techniques to prepare, manipulate and measure few-electron spin states in quantum dots have advanced considerably in recent years, with the leading progress in III–V semiconductor systems2,3,10,11. All stable isotopes of III–V semiconductors, such as GaAs, have non-zero nuclear spin, and the hyperfine coupling of electron spins to host nuclei is a dominant source of spin decoherence in these materials1,2,12,13. To eliminate this source of decoherence, group-IV semiconductors—various forms of carbon, silicon and silicon–germanium—which have predominantly zero nuclear spin, are being vigorously pursued as the basis of coherent spin electronic devices. Double quantum dots have recently been demonstrated in carbon nanotubes14–16, including the investigation of spin effects17,18. The devices reported are based on single-walled carbon nanotubes grown by chemical vapour deposition using methane feedstock containing either 99% 13C (denoted 13C devices) or 99% 12C (denoted 12C devices; see the Methods section)19. The device design (Fig. 1a) uses two pairs of Pd contacts on the same nanotube; depletion by top-gates (blue, green and grey in Fig. 1a) forms a double dot between one pair of contacts and a single dot between the other. Devices are highly tunable, as demonstrated in Fig. 1, which shows that tuning the voltage on gate M (Fig. 1a) adjusts the tunnel rate between dots, enabling a crossover from large single-dot behaviour (Fig. 1b) to double-dot behaviour (Fig. 1c). Left and right tunnel barriers can be similarly tuned using the other gates shown in blue in Fig. 1a.