Quantum State Control in Electrical Circuits: Artificial Superconducting `Atoms'

P. Forn-Díaz

K. S. Kumar

Yu-xi Liu, J. Q. You, L. F. Wei +2 more

We analyze the optical selection rules of the microwave-assisted transitions in a flux qubit superconducting quantum circuit (SQC). We show that the parities of the states relevant to the superconducting phase in the SQC are well defined when the external magnetic flux phi(e) = phi(0)/2; then the selection rules are the same as the ones for the electric-dipole transitions in usual atoms. When phi(e) does not = phi(0)/2, the symmetry of the potential of the artificial "atom" is broken, a so-called delta-type "cyclic" three-level atom is formed, where one- and two-photon processes can coexist. We study how the population of these three states can be selectively transferred by adiabatically controlling the electromagnetic field pulses. Different from lambda-type atoms, the adiabatic population transfer in our three-level delta atom can be controlled not only by the amplitudes but also by the phases of the pluses.

Kosuke Kakuyanagi, Shiro Saito, Hayato Nakano +1 more

We are investigating, from both the theoretical and experimental standpoints, how to perform measurements on quantum systems by using an electric circuit that has quantum characteristics.A superconducting flux qubit, which is constructed with a superconducting loop circuit, has discrete energy, so it can be regarded as an artificial atom.* Josephson bifurcation readout method: A method for detecting small displacements by using a bistable state caused by the nonlinearity of a Josephson junction.

R. Bianchetti

Blake Johnson

Controlling Photons in Superconducting Electrical Circuits Blake Robert Johnson 2011 Circuit quantum electrodynamics (circuit QED) is a system that allows for strong coupling between microwave photons in transmission line cavities and superconducting qubits or artificial atoms. While circuit QED is often studied in the context of quantum information processing, it also provides an attractive platform for performing quantum optics experiments on-a-chip, because the level of control and coupling strengths available in circuit QED opens a vast array of possibilities for the creation, manipulation, and detection of quantum states of light. In this thesis, the extension of circuit QED to two cavities is examined, including design issues for cavities with very different Q-factors, and a new qubit design is proposed that couples to both cavities. The qubit-cavity interaction, while providing much of the utility of circuit QED, also introduces additional qubit relaxation. A powerful formalism for calculating this energy decay due to the classical admittance of the electromagnetic environment is presented in the context of circuit QED. Measurements of a wide range of samples validate this theory as providing an effective model for relaxation. A new circuit element, called the ‘Purcell filter’, is introduced and demonstrated to decouple the relationship between cavity Q and qubit relaxation. Finally, a new method for performing quantum nondemolition measurements of microwave photons is demonstrated. Controlling Photons in Superconducting Electrical Circuits A Dissertation Presented to the Faculty of the Graduate School of Yale University in Candidacy for the Degree of Doctor of Philosophy by Blake Robert Johnson Dissertation Director: Professor Robert J. Schoelkopf