Yong Lu, Microtechnology and Nanoscience - MC2

​Mid term seminar with the title: Qubit characterization and generating single microwave photons
​Abstract: The first part of the talk will show how we use different methods to extract the decay rates of a superconducting qubit in waveguide QED*. After characterizing the sample well, we use it to generate a tunable on-demand single photon source**.

Typically, a qubit has pure dephasing〖 Γ〗_ϕ, radiative Γ_r and non-radiative Γ_n decay rates, and the total relaxation rate and decoherence rate are given by Γ_1=Γ_r+Γ_n and Γ_2=Γ_1/2+Γ_ϕ respectively.  Firstly, we use a reflection-coefficient measurement to obtain Γ_r and Γ_2 of the qubit. Then, combined with the atomic fluorescence, the non-radiative decay and pure dephasing rate can be distinguished. When the pump is strong, the qubit states are dressed, and the emission spectrum reveals the Mollow triplet. We find that if the pure dephasing rate is finite, the sidebands of the off-resonant Mollow triplet will be asymmetric around the central peak. We also explain in detail how the asymmetry occurs. Another straight-forward way to measure these decay rates is to study the photons scattered by the qubit. Interestingly, the power difference between the input and output can determine〖 Γ〗_n. Finally, we also study the qubit short-time dynamics. This technology allows us not only to extract the decay rates but also to investigate the fluctuation of the qubit frequency and coherence.

We demonstrate an on-demand tunable single-photon source based on a transmon qubit coupled to the end of an open transmission line. First, π –pulse is sent to the qubit.  In order to cancel the reflection from the input pulse, we send another pulse with a π-phase shift to interfere with the input destructively, resulting in 35 dB suppression. The cancellation is robust over 600 MHz. The purity of a single photon after the cancellation can be up to 99.2%, verified by the excellent antibunching of the emitted field. In addition, we find that both 1/f noise and the non-radiative decay can reduce the quantum efficiency. We also reconstruct the Wigner function for the output field showing that it is indeed a single photon.

* Y. Lu, A. Bengtsson, J. J. Burnett, E. Wiegand, B. Suri, P. Krantz, A. F. Roudsari, A. F. Kockum, S. Gasparinetti, G. Johansson, P. Delsing, Characterizing decoherence rates of a superconducting qubit by direct microwave scattering , arXiv:1912.02124, submitted to npj Quantum Information.
** Y. Lu, A. Bengtsson, S. Gasparinetti, J. J. Burnett, S. R. Sathyamoorthy, B. Suri, H. R. Nilsson, J. Bylander, G. Johansson, P. Delsing, Tunable Microwave single-photon source based on the pulse cancellation with high purity , in preparation.
Category Seminar
Location: Kollektorn, lecture room, Kemivägen 9, MC2-huset
Starts: 20 February, 2020, 10:00
Ends: 20 February, 2020, 11:00

Published: Tue 04 Feb 2020.