HYWIRE · Hybrid Nanowire Devices for Quantum Information Processing

Hybrid Nanowire Devices for Quantum Information Processing

The primary objective of this project was to develop next generation hybrid superconductor semiconductor nanowire quantum devices, using superconducting resonator circuits to demonstrate spin-photon coupling and probe Majorana bound states. The project was to focus on molecular beam epitaxy-grown InSb nanowires and use deterministic assembly techniques to precisely place nanowires. The project began by developing spin qubit devices based on Ge/Si core/shell nanowires (NWs). Ge/Si NWs were chosen over InSb NWs as these materials have potentially much longer coherence times (as nuclear spins can be eliminated) while still possessing strong spin-orbit interaction for fast electrical control. Shortly after the project began however we decided to pursue a new – and so far very promising – research direction developing a different type of nanowire-based hybrid qubit. This change in research direction occurred for two reasons. First, it became clear early on that Ge/Si NW quantum dot devices, although they have great potential, are challenging to work with and presently suffer from substantial charge noise. Second, the very recent development at the Center for Quantum Devices, University of Copenhagen (QDev) of semiconductor InAs nanowires with epitaxial superconducting Al contacts presented the opportunity to develop high quality superconducting qubits based around these novel nanowire materials. We have subsequently made and demonstrated a superconducting qubit where the Josephson junction is a semiconductor nanowire with superconducting Al contacts. This qubit, which we call the ‘gatemon’, is controlled using voltages on high impedance gate electrodes, in contrast to most conventional superconducting qubits which are controlled using large (~ milliampere) on chip currents. Voltage control may offer significant advantages for scaling superconducting qubit devices as it allows for very low power dissipation operation; this is an important consideration given that superconducting qubit devices require millikelvin temperatures. In the first year of this project, we demonstrated the gatemon qubit and showed promising coherence times (~1 microsecond). Our work was published in Physical Review Letters as an Editor’s choice and highlighted by Science Magazine and the American Physical Society’s journal Physics. In the second year of the project we further explored this gatemon qubit. We have demonstrated improved coherence times (up to 10 microseconds) and performed randomized benchmarking experiments to show single qubit gate errors better than 0.7% (limited by qubit relaxation times). We have also shown two qubit controlled-phase gate operation using two capacitively coupled gatemon devices. Just prior to the completion of the project our paper on this work was accepted to Physical Review Letters and has since been published. Along with developing this new type of qubit we have also implemented a setup for picking up and precisely placing individual nanowires. This setup allows for deterministic assembly of nanowire devices and is currently being used by many researchers at QDev. All transfer of knowledge objectives for the project have been achieved with two dilution refrigerator systems presently configured for measurements of high frequency superconducting circuits and qubits. Students and a postdoc have been trained in the fabrication and measurement of these newly developed gatemon qubit devices. In summary, while the project has evolved somewhat from its original objectives, we believe it has opened up a promising new research direction that is very much in the spirit of the original proposal. Going beyond this project we plan to continue development of semiconductor-based superconducting qubit devices. We are optimistic these qubits will present a highly viable alternative to conventional superconducting qubit technology, enabling novel qubit designs as well as new means of controlling and coupling superconducting qubits.

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