Quantum efficiency, purity and stability of a tunable, narrowband microwave single-photon source

TL;DR

This paper presents a tunable, narrowband microwave single-photon source with high quantum efficiency, demonstrating its stability, purity, and the effects of noise and defects on its performance.

Contribution

The work introduces a highly efficient, tunable microwave single-photon source with detailed analysis of its stability and noise influences, advancing quantum technology applications.

Findings

Quantum efficiency of 71-99% achieved

Narrowband emission of 300 kHz around 5.2 GHz

Device stability affected by two-level system defects

Abstract

We demonstrate an on-demand source of microwave single photons with 71--99\% intrinsic quantum efficiency. The source is narrowband (300\unite{kHz}) and tuneable over a 600 MHz range around 5.2 GHz. Such a device is an important element in numerous quantum technologies and applications. The device consists of a superconducting transmon qubit coupled to the open end of a transmission line. A $\pi$-pulse excites the qubit, which subsequently rapidly emits a single photon into the transmission line. A cancellation pulse then suppresses the reflected $\pi$-pulse by 33.5 dB, resulting in 0.005 photons leaking into the photon emission channel. We verify strong antibunching of the emitted photon field and determine its Wigner function. Non-radiative decay and $1/f$ flux noise both affect the quantum efficiency. We also study the device stability over time and identify uncorrelated discrete jumps of the pure dephasing rate at different qubit frequencies on a time scale of hours, which we attribute to independent two-level system defects in the device dielectrics, dispersively coupled to the qubit.