Speaker: Sigmund Kohler
Affiliation: ICMM-CSIC
Date: Tuesday, 7 February 2023 at 12:00
Location: Seminar Room, Serrano 121 (CFMAC)
The dynamics of a qubit that is swept repeatedly through an
avoided crossing is known as Landau-Zener-Stückelberg-Majorana
(LZSM) interference. Lately it is used for demonstrating quantum
coherence as well as for determining qubit parameters such as the
T2 time. One method for recording these interference patterns is
dispersive readout performed by measuring the transmission of a
cavity coupled to the qubit. I will present a universal
theory for dispersive readout of quantum systems in and out of
equilibrium. It is based on the backaction of the measured
system to the cavity obtained with non-equilibrium linear
response theory, which provides the signal in terms of a system
susceptibility [1] as well as resonance conditions that relate
the cavity transmission to spectral properties and Berry phases.
Examples are the readout of detuned qubits and thermally
excited multi-level systems. For ac-driven quantum systems, we
identify the relevant Fourier component of the susceptibility and
introduce a computational scheme based on Floquet theory. The
theory is applied to LZSM interference in Si/SiGe double quantum
dots, where the interference patterns exhibit a harp-like
structure stemming from the valley degree of freedom [2].
Moreover, the sub-structure of the LZSM pattern allows one to
draw conclusions about the steady-state populations of the
Floquet states [3,4].