In circuit quantum electrodynamics the coherence of Cooper pairs in superconductors is employed to create macroscopic electric circuits with quantized energy levels. Such circuits can be coupled with each other and exploited as building blocks of a quantum computer. Accurate and robust control of the quantum state in the circuits is a central condition for the operation of the quantum computer and one of the prerequisites for the implementation of complex algorithms used in quantum information processing, such as quantum error correction. In this thesis adiabatic control in circuit quantum electrodynamics is investigated with the focus on manipulating three-level systems. In adiabatic control the eigenstates of the system are slowly modified by changing the external control parameters, which govern the evolution of the system. If the changes in the parameters are slow enough, the state of the system follows the eigenstates in the adiabatic basis, thereby realizing the intended operation. The advantage of adiabatic control is its inherent robustness to small errors or noise in the control parameters; the result of the state manipulation only depends on the asymptotic values of the control parameters, but not on their exact values during the process. Shortcuts to adiabaticity can be used to speed up the otherwise slow adiabatic control by introducing a correction pulse that compensates the diabatic losses during the state manipulation. This allows one to overcome the limitation on the speed of the protocol but simultaneously reduces the method's robustness to the variations in the control parameters. If the level of noise in the control parameters is known, using the shortcut it is possible to find the optimal level of robustness which is required to mitigate the noise. Circuit quantum electrodynamics offers a perfect experimental platform for investigating quantum control due to the possibility of realizing complicated control schemes using microwave electronics. With commercially available digital-to-analog converters the control signals can be digitally created, which enables accurate and coherent control of the quantum circuit. In this thesis both theoretical and experimental results on adiabatic control applied to superconducting transmon circuits are presented. It is shown that stimulated Raman adiabatic passage can be used for population transfer in a three-level transmon, which can be further improved using shortcuts to adiabaticity. Furthermore, a scheme for implementing robust superadiabatic rotation gates in transmon is proposed. Finally, it is demonstrated that superconducting qubit can be used as an ultra-sensitive detector of magnetic flux.
|Translated title of the contribution||Adiabaattinen hallinta piirikvanttisähködynamiikassa|
|Publication status||Published - 2018|
|MoE publication type||G5 Doctoral dissertation (article)|
- adiabatic control
- superadiabatic control
- circuit QED
- quantum metrology