Operating state-of-the-art quantum circuits is typically limited by noise, especially if they work in the microwave domain like superconducting quantum bits, qubits. Instead of trying to avoid noise, this project enables a circuit architecture that profits from it. The concept of transforming noise from an omnipresent obstacle into a useful resource in quantum control experiments will renew the field of circuit quantum electrodynamics (QED). The project implements noise-enhanced quantum control by integrating photon-assisted single-electron tunneling (SET) devices into superconducting qubits. This integration allows for quantum control because we can measure the qubit state and in return manipulate the qubit dynamics and its degrees of freedom. Such quantum control experiments that utilize noise will impact research in quantum computing, nano-electronics, and quantum simulations of chemical compounds. Hence, my proposed circuit realization will expand the range of possible applications of state-of-the-art quantum circuits. Furthermore, qubits with tunable decoherence rates will extend the current knowledge of dynamics in dissipative open quantum systems.
The project outcomes help in future applications beyond basic science. The expected societal impact of the project is based on the fact that quantum effects in superconducting nano-electronics occur in both large-scale quantum computers and in novel sensing applications. Both topics belong to long-term scientific goals and can significantly reduce industrial production costs and generate new jobs in modern industry on a global scale. Promising examples are in quantum computing: One requirement in quantum computing is the fast initialization of qubitstates. As coherence times are approaching the millisecond regime, the conventional passive initialization protocol by waiting takes a too large fraction of the overall computing time. This project contributes a versatile tool for in-situ initialization of qubits in large-scale quantum computers. Due to its exponential speedup, the quantum computer itself may revolutionize pharmaceuticals, telecommunication, and financial services.
The overall objectives of this proposal are to realize noise-enhanced quantum control using transmon qubits with tunable decoherence rates in circuit QED setups. The ideas are based on two important objectives, which are implemented in QCD Labs at Aalto University, having strong experience in SET: The first objective is to realize a single qubit with tunable decoherence rates. This objective enables new quantum computing applications by realizing a fast qubit reset and explores new physics by investigating non-Markovian qubit dynamics. The second objective is to use the tunable coupling between two of these qubits to build the unit cell of a fully controllable Ising model. This objective is used to study remote-cooling of one qubit via the other and to simulate multi-dimensional master equations.
In conclusion, the proposed actions have been achieved to an extent possible considering the fact that the project was ended after 13 months instead of 24 months. We have published 7 peer-reviewed scientific articles, organized a research stay at ETH Zurich, instructed 8 students, created 3 videos for dissemination, participated in conferences, were mentioned in 10 newspaper articles, became member of 3 physical societies, referee for 2 scientific journals, and participated in 4 vocational trainings. We have applied for a docentship a Aalto University, which was successfully granted to the fellow after the end of this project. The reason for the termination of the project was that the fellow and the supervisor of this project have spin-out a company from Aalto University, which meanwhile is the leading European company for quantum computing: IQM Finland Oy.
