Measuring coherent dynamics of a superconducting qubit in an open waveguide

Abstract

We measured the relaxation and decoherence rates of a superconducting transmon qubit in a resonator-free setting. In our experiments, the qubit is coupled to an open coplanar waveguide such that the transmission of microwaves through this line depends on the qubit's state. To determine the occupation of the first excited qubit energy level, we introduced a two-pulse technique. The first applied pulse, at a frequency close to the eigenfrequency of the qubit, serves to excite the qubit. A second pulse is then used for probing the transition between the first and second excited energy levels. Utilizing this measurement technique allowed for the reconstruction of the relaxation dynamics and Rabi oscillations. Furthermore, we demonstrate the consistency between the extracted parameters and the corresponding estimations from frequency-domain measurements.

Figures (6)

• Figure 1: Qubit's steady state responses. a) response at the sweet spot measured at $\Omega_p=2\pi\cdot 1.06$ MHz. Black solid and red dashed lines are the respective fits with the resonator \ref{['eq:circle']} and qubit model fits \ref{['eq:qubit']} b) qubit spectroscopy measured with $\Omega_p=2\pi \cdot 18.95$ MHz, showing both $\omega_{01} = 2\pi f_{01}$ and $\omega_{02}/2 = 2\pi \frac{f_{02}}{2}$ transitions fitted by second order polynomials as described in the main text.
• Figure 2: Rabi-chevron response obtained by sweeping the amplitude of the drive pulse with subsequent readout pulse probing the $\omega_{12}$ transition. For technical details see the text. The phase of the probing pulse is shown in the top colorbar.
• Figure 3: Qubit evolution. a) Damped Rabi oscillations, obtained by sweeping the drive pulse duration at a Rabi driving rate $\Omega_p=2\pi\cdot 22.47$ MHz; b) Relaxation of the first excited qubit state.
• Figure S1: SEM images of the investigated sample: a) the transmon qubit coupled to an open coplanar waveguide, b) zoomed area of the DC-SQUID loop and the DC-bias line, c) the air bridge above the central coplanar waveguide line and the DC-line.
• Figure S2: Rabi chevrons measured at different combinations of drive and readout pulse parameters.