Spin qubits and radio-frequency optomechanics with carbon nanotubes

Carbon nanotubes are versatile materials in which many aspects of condensed matter physics come together. The chemical vapour deposition process through which nanotubes are synthesized leaves very low disorder; by incorporating these pristine nanotubes we can fabricate clean electrical devices in which delicate effects can be studied. Recently, we developed the technology of stamping, which allows single nanotubes to be positioned in quantum dot devices, and is compatible with many other fabrication techniques. I will present two sets of experiments that make use of the unique properties of clean nanotube devices.

I will begin by describing the first qubit in this material, the spin-valley qubit. This makes use of both the electron spin and the valley magnetic moment, which are coupled by the spin-orbit interaction. To realize this qubit, we first used gate potentials to define a double quantum dot. Then, we made use of the Pauli exclusion principle to configure the device as an electrical spin filter. Finally, we exploited a bend in the nanotube to manipulate the qubit electrically, reading it out and characterizing its coherence properties via the current through the device.

In the second part of the talk, I will present electromechanical measurements of vibrating nanotubes. Nanotubes combine light mass (leading to large zero-point motion) and high stiffness (leading to large mode spacing), making them potentially interesting for studying the quantum limit of mechanical motion. We have measured nanotube mechanical modes with frequencies as high as 39 GHz, implying efficient cooling to the ground state in a standard laboratory cryostat. We have also shown how to measure nanotube mechanics without passing a current, by coupling optomechanically to a radio-frequency tank circuit. I will discuss how we may build on these experiments to study quantum effects in a vibrating nanotube.

References
“A valley-spin qubit in a carbon nanotube”
EA Laird, F Pei, LP Kouwenhoven
Nature Nanotechnology 8, 565-568 (2013)
“A high quality factor carbon nanotube mechanical resonator at 39 GHz”
EA Laird, F Pei, W Tang, GA Steele, LP Kouwenhoven
Nano Letters 12 193-197 (2011)
“Quantum transport in carbon nanotubes”
E.A. Laird, F. Kuemmeth, G. Steele, K. Grove-Rasmussen, J. Nygård, K. Flensberg, L.P. Kouwenhoven
Reviews of Modern Physics 87, 703 (2015)