Yu-Xi Liu

We study two-photon scattering by a two-level giant atom in a waveguide. We first study the case that the giant atom is coupled to the waveguide via two coupling points, and obtain Bethe ansatz (BA) eigenstates and eigenvalues in the Hilbert space of two-excitation. Then we derive bound states by subtracting the states corresponding to BA solutions from the entire two-excitation Hilbert space, and construct the two-photon scattering matrix ( S -matrix) by using BA eigenstates and bound states. We further study the properties of output states, which include both the scattering and bound states, for arbitrarily incident two-photon states by using a concrete example. We find that the oscillation period of the scattering states and decay rates of the bound states strongly depend on the distance between two coupling points. Moreover, we find that the two-photon correlation in the bound states can be enhanced by changing such distance when the total energy of two incident photons equals to two times of single photon resonance energy. We also generalize our study to the case that the giant atom is coupled to the waveguide via N coupling points. We obtain all the eigenstates and eigenvalues of the S -matrix and construct the S -matrix. Comparing with the case of the two coupling points, we find the photon correlation can be further enhanced by increasing the number of the coupling points for the same incident states when the distance of any two nearest neighbor coupling points is half of the wavelength.

Single-photon sources are crucial for quantum information technologies. Here, we demonstrate a microwave single-photon source based on tantalum thin films, whose favorable material properties enable high-quality, stable photon emission. The antibunching behavior of the emitted radiation is revealed by second-order correlation measurements. Furthermore, traveling-wave parametric amplifiers are used as low-noise pre-amplifiers in the detection chains, substantially improving the signal-to-noise ratio and thereby greatly reducing the acquisition time required for second-order correlation measurements. These results demonstrate the viability of tantalum-based superconducting devices as reliable platforms for microwave quantum photonics.

• It is the first time the decoherence-free interactions between three-level giant atoms are systematically studied. • We propose feasible simulation schemes for several prototypical photonic lattice models, which are highly protected from decoherence and can be programmed in situ. • Our proposals can be readily implemented in a lab with state-of-the-art circuit QED techniques and can outperform other conventional schemes in terms of many figures of merit.