Final Report Summary - QUEST (Quantum Entanglement in Electronic Solid State Devices)
Entanglement plays a central role in the emerging quantum technology. With original experiments the nanoelectronics group at the University of Basel (www.nanoelectronics.ch) created a new source of spin-entangled electron pairs based on a superconductor. In this device, known as the Cooper-pair splitter (CPS), the two electrons of a Cooper-pair are forced to tunnel into two different quantum dots (QDs) by Coulomb interaction. While the static properties of the CPS device have already been explored to some extend, QUEST aims at dynamical properties of entanglement generation in these devices. We have done do this by coupling CPS devices to radio-frequency (RF) circuits.
Within QUEST we have engineered and studied advanced Cooper-pair splitter devices, in which multiple bottom gates allow to tune key parameters of the device. In order to address charge and spin-correlations at the intrinsic time-scale of the device, i.e. at the nanosecond scale, we have developed RF circuits based on superconducting transmission lines and lumped LC resonators and build a dedicated cryogenic measurement station dedicated to measurements in the GHz regime. These circuits are coupled to CPS devices, or part of a CPS device, and allow to for impedance matching by design. We have demonstrated impedance matching to QDs, which are typically of very high impedance, in the 1-5 GHz regime and used the same circuit to measure the RF admittance by reflectometry. Due to the excellent matching, one can measure the noise (current-current correlations) emitted from the device in a very effective way. This we have demonstrated in a detailed study of shot-noise in QDs. The new measurement system has proven to be so versatile, that we have applied it to various quantum systems, for example to suspended and encapsulated graphene. Along the line of the project we have further explored spin-physics in QDs with novel ferromagnetic side-gates, entanglement detection using ferromagnetic contacts, GHz charge pumping, electron-beam splitters in graphene and various new hybrid device structures that are derived from the CPS concept.
Within QUEST we have engineered and studied advanced Cooper-pair splitter devices, in which multiple bottom gates allow to tune key parameters of the device. In order to address charge and spin-correlations at the intrinsic time-scale of the device, i.e. at the nanosecond scale, we have developed RF circuits based on superconducting transmission lines and lumped LC resonators and build a dedicated cryogenic measurement station dedicated to measurements in the GHz regime. These circuits are coupled to CPS devices, or part of a CPS device, and allow to for impedance matching by design. We have demonstrated impedance matching to QDs, which are typically of very high impedance, in the 1-5 GHz regime and used the same circuit to measure the RF admittance by reflectometry. Due to the excellent matching, one can measure the noise (current-current correlations) emitted from the device in a very effective way. This we have demonstrated in a detailed study of shot-noise in QDs. The new measurement system has proven to be so versatile, that we have applied it to various quantum systems, for example to suspended and encapsulated graphene. Along the line of the project we have further explored spin-physics in QDs with novel ferromagnetic side-gates, entanglement detection using ferromagnetic contacts, GHz charge pumping, electron-beam splitters in graphene and various new hybrid device structures that are derived from the CPS concept.