Carbon nanotube spin qubits

In our group we investigate carbon nanotube quantum dots for application in solid-state quantum information processing. Carbon nanotubes have unique potential as a host material for charge and spin-based qubits: carbon nanotubes provide natural spatial confinement, allow accurate control of electron charge, spin and valley degrees of freedom, are atomically perfect and can be made isotopically pure. In our group we already developed techniques for spin-to-charge conversion and fast readout of charge states [1]. Our current efforts focus on understanding and mitigating charge noise and developing techniques to create entanglement between spatially separated qubits using a measurement-based approach.

In our most recent work, we examined charge coherence in carbon nanotube quantum dots. The use of quantum dots to define charge and spin qubits is well established in the field of quantum information processing. Quantum dot charge qubits are attractive because of the possibility of fast qubit control but are sensitive to charge noise which can result in rapid decoherence. Spin qubits can have much longer coherence times but are susceptible to charge noise as well since fast manipulation, or the coupling of two or more spin qubits, often relies on mixing of charge and spin degrees of freedom. As a result, two-qubit quantum gate fidelities are still relatively low even for spin qubits. Scalability therefore depends critically on an understanding of charge noise and on developing techniques to lower the susceptibility of quantum dot devices to this source of decoherence.

To examine charge coherence, we used a carbon nanotube device consisting of two quantum dots in series: a charge qubit in which quantum information is encoded in the spatial position of an electron. To read out the quantum states, we developed a technique based on radio-frequency reflectometry measurements of the quantum capacitance of the device: a measure of how easy, or difficult, it is for an electron to move between the quantum dots, as illustrated in the figure above. In combination with microwave manipulation of the qubit, our group could measure the degree of charge noise experienced by the device and determine charge coherence [2]. Moreover, by operating the qubit at an optimal point (or sweet spot) coherence could be extended by more than an order of magnitude. This work is in collaboration with the Quantum and Nanoelectronics group Basel, Swizerland, the group of Prof. Takis Kontos, LPA, France, and C12 Quantum Electronics.