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Chiral and nonreciprocal quantum devices are crucial for signal routing and processing in a quantum network. In this work, we study the chirality and nonreciprocity of a giant atom coupled to a one-dimensional waveguide. We clarify that the chiral emission of the giant atom is not directly related to the time-reversal symmetry breaking but to the mirror-symmetry breaking. We propose a passive scheme to realize the chiral emission of a giant atom without breaking time-reversal symmetry by extending the legs of the giant atom. We find the time-reversal symmetry breaking via nonuniform coupling phases is artificial and thus cannot result in nonreciprocal single-photon scattering for the giant atom. The nonreciprocity of the giant atom can be obtained by the external dissipation of the giant atom that truly breaks the time-reversal symmetry. Our work clarifies the roles of symmetries in the chirality and nonreciprocity of giant-atom systems and paves the way for the design of on-chip functional devices with superconducting giant atoms., Comment: 14 pages, 4 figures

We perform simulations for transport and nontrivial fusion of Majorana zero modes in topological superconducting quantum wires. We uncover interesting behaviors of nonadiabatic transition associated with the transport through mini-gate-controlled multiple-segments modulations. Owing to breaking of the initial fermion parity induced by nonadiabatic transitions, a deviation from the statistics of outcomes of nontrivial fusion arises and is analyzed. Moreover, we develop a measurement scheme to infer the amount of fermion parity breaking and nonadiabatic transition probability to excited states, based on the characteristic spectrum of measurement current by a quantum-point-contact detector, by measurement of the charge occupation dynamics in a fusion-outcome-probing quantum dot., Comment: 9 pages, 6 figures

The non-Markovianity of physical systems is considered to be a valuable resource that has potential applications to quantum information processing. The control of traveling quantum fields encoded with information (flying qubit) is crucial for quantum networks. In this work, we propose to catch and release the propagating photon/phonon with a non-Markovian giant atom, which is coupled to the environment via multiple coupling points. Based on the Heisenberg equation of motion for the giant atom and field operators, we calculate the time-dependent scattering coefficients from the linear response theory and define the criteria for the non-Markovian giant atom. We analyze and numerically verify that the field bound states due to non-Markovianity can be harnessed to catch and release the propagating bosonic field on demand by tuning the parameters of giant atom., Comment: 26 pages, 7 figures

Materials with kagome lattice have attracted significant research attention due to their nontrivial features in energy bands. In this work, we theoretically investigate the evolution of electronic band structures of kagome lattice in response to uniaxial strain using both a tight-binding model and an antidot model based on a periodic muffin-tin potential. It is found that the Dirac points move with applied strain. Furthermore, the flat band of unstrained kagome lattice is found to develop into a highly anisotropic shape under a stretching strain along y direction, forming a partially flat band with a region dispersionless along ky direction while dispersive along kx direction. Our results shed light on the possibility of engineering the electronic band structures of kagome materials by mechanical strain., Comment: 12 pages, 9 figures

In this work, we perform real time simulations for probing the non-Abelian fusion of a pair of Majorana zero modes (MZMs). The nontrivial fusion outcomes can be either a vacuum, or an unpaired fermion, which reflect the underlying non-Abelian statistics. The two possible outcomes can cause different charge variations in the nearby probing quantum dot (QD), while the charge occupation in the dot is detected by a quantum point contact. In particular, we find that gradual fusion and gradual coupling of the MZMs to the QD (in nearly adiabatic switching-on limit) provide a simpler detection scheme than sudden coupling after fusion to infer the coexistence of two fusion outcomes, by measuring the occupation probability of the QD. For the scheme of sudden coupling (after fusion), we propose and analyze continuous weak measurement for the quantum oscillations of the QD occupancy. From the power spectrum of the measurement currents, one can identify the characteristic frequencies and infer thus the coexistence of the fusion outcomes., Comment: 7 pages, 5 figures

In this work we perform real time simulations for the dynamics of braiding a pair of Majorana zero modes (MZMs) through a quantum dot in a minimal setup of pure 1D realization. We reveal the strong nonadiabatic effect when the dot energy level approaches to zero in order to achieve a geometric phase $\pi/4$ which is required for a full exchange between the MZMs. Rather than the strategies of nonuniformly manipulating the system according to adiabatic condition and shortcuts-to-adiabaticity, we propose and illustrate a more feasible scheme to suppress the nonadiabatic transition, meanwhile which allows for a full exchange between the Majorana modes., Comment: 6 pages, 5 figures

We present a theoretical analysis for the metrology quality of joint weak measurements (JWM), in close comparison with the weak-value-amplification (WVA) technique. We point out that the difference probability function employed in the JWM scheme cannot be used to calculate the uncertainty variance and Fisher information (FI). In order to carry out the metrological precision, we reformulate the problem in terms of difference-combined stochastic variables, which makes all calculations well defined. We reveal that, in general, the metrological precision of the JWM scheme cannot reach that indicated by the total FI, despite that all the readouts are collected without discarding. We also analyze the effect of technical noise, showing that the technical noise cannot be removed by the subtracting procedure, which yet can be utilized to outperform the conventional measurement, when considering the imaginary WV measurement., Comment: 10 pages, 6 figures