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Nonlocal interactions between photonic waveguide and giant atoms have attracted extensive attentions. Researchers have studied how to optimize and control quantum states via giant atoms. We here study the dynamical scattering of a single-photon wave packet by a giant atom coupled to a two-dimensional photonic waveguide via multiple spatial points. We show that arbitrary target scattering single-photon wave packets can be generated by adjusting the coupling strength between the giant atom and different lattice sites of the waveguide. Furthermore, the dynamical scattering of the wave packets enables us to study the propagating properties of the target scattering wave packets and observe the excitation of giant atoms. Our study provides alternative way for photon state control based on giant atoms., Comment: 13 pages, 6 figure

The giant atom system is a new paradigm in quantum optics, in which the traditional dipole approximation is not available. In this paper, we construct an artificial giant atom model by coupling a superconducting circuits to a transmission line by two coupling points. In the ultrastrong coupling regime, we show that the Lamb shift of the giant atom, which is induced by the non-negligible counter-rotating atom-waveguide coupling term, will modify its dissipation process. Furthermore, we investigate quantum Zeno and anti-Zeno effect where the size of the giant atom serves as a sensitive controller. Specifically, by comparing the critical measurement interval and the life time of the giant atom, we clarify the condition for the occurring of quantum anti-Zeno effect. We hope our work is useful for the application of giant atom system in the investigation of fundamental problems of quantum mechanics., Comment: 7 Pages, 5 Figures, comments are welcomed

We theoretically propose a scheme to perform rotation sensing in a Whispering-gallery-mode resonator setup. With the assistance of a large detuned two-level atom, which induces the effective coupling between clockwise and counterclockwise propagating modes in the resonator, we realize an effective interferometry with SU(2) algebraic structure. By studying the quantum Fisher information of the system, we find that the estimate accuracy for the angular velocity of the rotation can achieve and even break the Heisenberg limit in linear and nonlinear setup, respectively. The high performance of quantum metrology is proved to be associated with the state compressibility during the time evolution. We hope that our investigation will be useful in the design of a quantum gyroscope based on spinning resonators., Comment: 8 pages, 5 figures, accepted by Phys. Rev. A

The interaction between the quantum emitter and topological photonic system makes both the emitter and the photon behave in exotic ways. We here study a system that a giant atom is coupled to two points of a one-dimensional topological waveguide formed by the Su-Schrieffer-Heeger (SSH) chain. The topological nature of the hybrid system is studied. We find that the giant atom can act as an effective boundary and induce the chiral zero energy modes for the waveguide under the periodical boundary. The properties of these modes are similar to those in the SSH model with open boundary. Meanwhile, the SSH waveguide, as a structured environment, induces the retarded effect and the non-Markovian dissipation of the giant atom. Our work may promote more studies on the interaction between matter and topological environment. Experimental demonstration for our study using superconducting quantum circuits is very possible within current technology., Comment: 21 Pages, 20 Figures

Recently, the tunable Weyl-semimetal bands and the associate topological phase transition have been successfully simulated in superconducting quantum circuits [X. Tan, \textit{et al.} Phys. Rev. Lett. {\bf 122}, 010501 (2019)]. Since the superconducting quantum circuits inevitably couple to the environment, we here focus on the steady state and decoherence process by taking the reservoir into consideration via quantum master equation. Our results show that the purity of the steady state can be used to indicate the topological phase transition and Weyl points. Furthermore, the coherence will exponentially decay to zero at the Weyl points, and decay to a nonzero value with oscillation at other points in the momentum space. Our work may have significant impact on the study of quantum open topological system., Comment: Some mistakes have been found

We propose a quantum metrology scheme in a cavity QED setup to achieve the Heisenberg limit. In our scheme, a series of identical two-level atoms randomly pass through and interact with a dissipative single-mode cavity. Different from the entanglement based Heisenberg limit metrology scheme, we do not need to prepare the atomic entangled states before they enter into the cavity. We show that the initial atomic coherence will induce an effective driving to the cavity field, whose steady state is an incoherent superposition of orthogonal states, with the superposition probabilities being dependent on the atom-cavity coupling strength. By measuring the average photon number of the cavity in the steady state, we demonstrate that the root-mean-square of the fluctuation of the atom-cavity coupling strength is proportional to $1/N_c^2$ ($N_c$ is the effective atom number interacting with the photon in the cavity during its lifetime). It implies that we have achieved the Heisenberg limit in our quantum metrology process. We also discuss the experimental feasibility of our theoretical proposal. Our findings may find potential applications in quantum metrology technology., Comment: 7 Pages, 2 figures, Accepted by Phys. Rev. A

In this paper, we study the dynamical behavior and quantum metrology in a rotating Nitrogen-Vacancy(NV) center system which is subject to an external magnetic field. Based on the recently realized rapid rotation of nano-rotor [J. Ahn, et. al., Phys. Rev. Lett. 121, 033603 (2018) and R. Reimann, et. al., Phys. Rev. Lett. 121, 033602 (2018)], the frequency of the rotation is close to that of the intrinsic frequency of the NV center system, we predict the quantum beats phenomenon in the time domain and show that the quantum metrology can be enhanced by the superposition effect in our system., Comment: 6 pages, 4 figures. Accepted by EPJ D