We use superconducting circuits to study quantum optics with propagating microwaves.
Since the coupling is strong between a superconducting artificial atom and the propagating
microwaves in a one-dimensional (1D) open-ended transmission line, quantum
effects can be observed without using any cavity to confine the mirowave photons.
We developed several methods to characterize and distinguish different decay rates
of our superconducting qubit coupled to the end of an open transmission line. The
results across all methods are consistent. We observe the qubit emission without
external driving due to a hot bath with a temperature up to 135 mK. We also investigate
to load an exponential coherent pulse with the loading efficiency up to 96.5% due to
the nearly perfect symmetry (98%) between the input pulse and the qubit emission. We
also generate Wigner-negative states from two different approaches: by sending a short
π-pulse to excite the qubit, we demonstrate strong antibunching of the emitted photons
and determine its negative Wigner function to be for single photons; Compared to sending
a short pulse, in the second approach, we drive the qubit continuously, by applying
filters to the propagating emissions from the qubit, we create some nonclassical states
with negative Wigner functions which negativity obviously depends on the filter shape.
Keywords: quantum optics, microwave photons, superconducting qubit, decay
rates, hot bath, loading efficiency, Wigner-negative state