SPEAKER: Ziyuan Fu
Advisor: S. Anlage
TITLE: Experimental Study of Quantum Graph
ABSTRACT: An experimental setup consisting of a microwave network is used to simulate quantum graphs. The Random Coupling Model (RCM) is applied to describe the universal statistical properties of the system with and without time-reversal invariance. The networks, which are large compared to the wavelength, are constructed from coaxial cables connected by T junctions, and by making nodes with circulators time-reversal invariance for microwave propagation in the networks can be broken. The results of experimental study of microwave networks with and without time-reversal invariance are presented in the frequency domain. With the measured S-parameter data, the impedance statistics are examined and compared with the RCM prediction.
SPEAKER: Long NGuyen
Advisor: V. Manucharyan
TITLE: Protecting a multi-level superconducting artificial atom from energy relaxation.
ABSTRACT: The inter-well "fluxon" transition of a fluxonium circuit has a dipole matrix element that decreases drastically as the ratio EJ/EC becomes large due to a weak overlap of wavefunctions localized in the two Josephson wells. Thus naturally suppresses all linear energy relaxation mechanisms, such as dielectric loss. Despite the vanishing transition dipole of such a qubit, there is still a finite dispersive shift due to the presence of strongly-coupled intra-well "plasmon" transitions in the circuit. By tuning EJ/EC ratio with an external magnetic flux we observed a factor of 100 enhancement of qubit lifetime from about 20 microseconds to over 2 millisecond for a nearly the same transition frequency. T2 measurement is performed both at the spots maximally-sensitive to flux noise - where it never drops below a microsecond - and at the flux sweet spot where it is over 150us. Our experiment demonstrates that a highly-decoupled, long-lived qubit can still be coherently manipulated and read out in a multi-level superconducting circuit.