Programme: Swiss National Science Foundation (SNSF)
The quest for building quantum information processors is currently pursued along two largely orthogonal paths, one based on optical frequency excitations in atoms or ions in vacuum or embedded in a solid and the other based on microwave frequency excitations in superconducting or semiconducting micro- and nano-structures. While optical approaches drastically differ in their implementation from microwave implementations, solid state approaches based on superconducting electronic circuits and semiconductor quantum dots have very similar requirements for their successful implementation. Both superconducting and semiconducting qubits with computational states used to encode quantum information separated by microwave frequency energy scales require refrigeration for initialization and coherence, dc and microwave control pulses with GHz bandwidths for realizing gates, high fidelity read-out and versatile non-local coupling schemes.
The similarities in the requirements present the potential to address challenges for both superconducting and semiconducting qubits in hybrid systems. In the project proposed here we plan combine on-chip superconducting control and read-out electronics with semiconductor quantum dot (QD) based qubits. We will use superconducting resonators with controlled impedance to allow for the conversion of quantum information stored in the charge degree of freedom of a QD qubit to microwave frequency photons in an approach know in superconducting electronics as circuit quantum electrodynamics (QED). Circuit QED in the strong coupling limit will allow us to interconnect electronic excitations localized in QDs to mobile microwave frequency photons guided in waveguide structures for quantum non-demolition (QND) read-out of QD qubits, for remote coupling between QD qubits, and for communication between on-chip quantum device elements.
In this project we will initially investigate the use of the recently realized strong coupling GaAs based double QD (DQD) charge qubits to high impedance microwave resonators based on superconducting quantum interference device (SQUID) arrays to further test the relevant concepts of semiconductor circuit QED elaborated on below. We will then transition to GeSi nanowire based DQDs in which the hole degree of freedom and the strong spin-orbit coupling are expected to allow for making use of the much more coherent spin degree of freedom of electrons in DQDs. To operate spin qubits at finite magnetic fields while employing superconducting circuit elements, we will explore the use of high impedance thin film resonators.
In both material systems, we will investigate the strong coupling of charges and spins to microwave photons. We will use the dispersive coupling of QD qubits to microwave photons for qubit read-out, also in combination with parametric amplifiers. We will perform microwave controlled single qubit operations and characterize their performance with randomized benchmarking and gate set tomography. We will realize microwave mediated coupling between QD qubits, which we will characterize using spectroscopy and time resolved measurements.
As a proof of concept, we will integrate QD based qubits on the same device with superconducting transmon type qubits. We will realize information transfer between these two types of qubits. This capability will enable the use of both charge/spin and charge/flux degrees of freedom in the same device. One may imagine to transfer information form electronic degrees of freedom of the circuit to long lived electron or even nuclear spin degrees of freedom in future devices. The methods and techniques to be developed here are expected to be transferable to QD devices based on material systems such as SiGe heterostructures and Si charge accumulation devices.
New regimes of hybrid systems will be investigated using super- and semiconductor qubits in a single device. Strong coupling will enable coherent control of multi-qubit devices containing both transmons and DQDs in regimes where mutual benefits can be created for qubit control, quantum coherence and scalability. Our circuit QED-based approach is expected to enable time-domain manipulation and measurements with high fidelity single shot read-out and long-range coupling and thus make important contributions to quantum information processing with hybrid quantum systems.