Building a quantum capacitance microscope for probing coherence in molecular qubits

Prof Charles Smith and Dr Malcolm Connolly (Semiconductor Physics)

Quantum technologies are expected to have far-reaching consequences on our ability to use natural resources more sustainably. In information processing, for instance, a quantum computer would improve the efficiency of large data centers used for cloud computing. In materials science, the evolution of chemical reactions is also more efficiently simulated by a quantum computer so they are thus likely to aid developments in synthetic chemistry, where understanding the behaviour of large molecules will lead to smarter power-saving materials. Quantum simulators will also elucidate the role of quantum effects in biological processes related to energy harvesting such as photosynthesis, and inform the design of materials with exotic many-body ground states such as a high-temperature superconductivity.

At the heart of most quantum computers are building blocks known as quantum bits, or “qubits”. The spin of an electron trapped in a semiconducting quantum dot is a natural qubit capable of realising a universal quantum computer. A major challenge with this implementation at the moment is miminising disturbances from the atomic lattice on fragile quantum superpositions. Using a previously developed cryogenic microwave reflectometry technique [Petersson et al. Nano Lett. 10, 2789 (2010)], our lab recently found that electrons confined at low temperature in large molecules such as carbon nanotubes leave such states untouched for longer than conventional semiconductors. This suggests that rationally designed molecules could have lifetimes long enough to execute quantum algorithms on a self-assembled molecular quantum computer. For this to become a reality we first of all need an instrument which can rapidly measure the coherence properties of electrons in the wide variety of candidate molecules that are too small for measuring using conventional ohmic contacts. The aim of this Winton Pump Prime project is to build such a microscope and use it to measure the coherence time of electronic excitations in nanoparticles and single molecules on arbitrary substrates.

Project Completed June 2014

In summary, by building and operating the new microscope we have reached one of the main milestones of our Pump Prime project proposal. The next step will involve cooling nanoparticles and molecules to low temperature in order to detect single-electron motion along with topography. The experiments have provided an excellent opportunity for training students in scanning probe techniques and stimulated collaborations with the Microelectronics group and Materials Science department. Once we have obtained proof-of-principle results the microscope will be used to explore quantum effects in nanostructured materials, superconducting circuits, and a vast number of molecules where the role of quantum effects is not well understood. Plans to develop the next version of the microscope were also incorporated into a £1M EPSRC Early Career Fellowship. The latter was successfully awarded to Dr Malcolm Connolly and started in July 2014. The results are already suitable for publication but it is likely these preliminary data will form part of a larger set obtained at low temperature on nanoparticles.