We propose how to realize nonreciprocity for a weak input optical field via nonlinearity and synthetic magnetism. We show that the photons transmitted from a linear cavity to a nonlinear cavity (i.e., a linear-nonlinear optical molecule) exhibit nonreciprocal photon blockade but no clear nonreciprocal transmission. Both nonreciprocal transmission and nonreciprocal photon blockade can be observed, when one or two auxiliary modes are coupled to the linear-nonlinear optical molecule to generate an artificial magnetic field. A similar method can be used to create and manipulate nonreciprocal transmission and nonreciprocal photon blockade for photons bidirectionally transport in a nonlinear-nonlinear optical molecule. Additionally, a photon circulator with nonreciprocal photon blockade is designed in cyclic three-nonlinear-mode system. The combination of nonlinearity and synthetic magnetism provides us an effective way towards the realization of quantum nonreciprocal devices, e.g., nonreciprocal single-photon sources and single-photon diodes.

We study the multimagnon blockade and magnon-induced tunneling in the hybrid ferromagnet-superconductor system, where a magnon mode representing the collective motion of spins in the yttrium iron garnet (YIG) sphere is coupled with a three-level $\mathrm{\Delta }$-type fluxonium qubit via the virtual-photon excitation from the microwave cavity. By appropriately choosing the parameters, the single magnon blockade, multimagnon blockade including two- and three-magnon blockades, and magnon-induced tunneling could be achieved. Fortunately, the switch from magnon blockade to magnon-induced tunneling could be obtained via controlling the overall phase of the loop transition determined by the driving fields. Furthermore, the single-magnon blockade could be transformed into a multimagnon blockade via the phase modulation. The scheme we present may provide an alternative method to manipulate the few-magnon states and have potential applications in quantum communication and quantum information processing.

We propose to achieve nonreciprocal quantum control of photons in a quadratic optomechanical (QOM) system based on directional nonlinear interactions. We show that by optically pumping the QOM system in one side, the effective QOM coupling can be enhanced significantly in that side, but not for the other side. This, contrary to the intuitive picture, allows the emergence of a nonreciprocal photon blockade in such optomechanical devices with weak single-photon QOM coupling. Our proposal opens up the prospect of exploring and utilizing quantum nonreciprocal optomechanics, with applications ranging from single-photon nonreciprocal devices to on-chip chiral quantum engineering.

We investigate theoretically macroscopic quantum coherence and tripartite entanglement in a three-mode magnomechanical system with a yttrium iron garnet sphere, which is simultaneously driven by a magnetic pump and a microwave field pump to establish an effective magnomechanical coupling. We discuss mainly the dependence of quantum properties on the driving power, the cavity field detuning and decay, the magnomechanical coupling strength, the bias magnetic field, and the temperature of an environment. It is found that the quantum coherence of the system can be attained easily in a large range of parameters. In contrast, tripartite photon-magnon-phonon entanglement can only be realized in a relatively small range of optical and magnetic parameters. In particular, the quantum coherence of the mechanical mode can be larger than that of the photon and magnon modes, which means that the environmental incoherence of the mechanical mode in the cavity magnomechanical system can be suppressed significantly. These results may help manipulate simultaneously the macroscopic quantum coherence and the tripartite entanglement of massive objects in coupled magnomechanical system, which may have potential applications for realizing highly tunable information processing through the flexible control of quantum properties.

We explore a scheme to investigate the tunable magnon antibunching characteristics of the conventional magnon blockade (CMB) and the unconventional magnon blockade (UMB) in a hybrid ferromagnet-superconductor system. The magnon excited in the yttrium iron garnet is indirectly coupled to two qubits by eliminating two mutually perpendicular microwave cavity modes. In our paper, we relax the magnon-qubits detuning constraint. In two cases with and without magnon-qubits detuning, it is found that in the strong coupling regime CMB and UMB coexist in our system, and they can be regulated simultaneously by varying the magnon-qubits coupling strength ratio and the qubits detuning ratio. In the weak coupling regime, there exists only the UMB in the system. Without magnon-qubits detuning, the UMB shows a nonmonotonic dependence on the magnon-qubits coupling strength ratio. With magnon-qubits detuning, for the UMB, the magnon bunching effect and antibunching effect can be converted mutually by adjusting the magnon-qubits coupling strength ratio and the qubits detuning ratio. Moreover, changing the magnon-qubits coupling strength ratio and the qubits detuning ratio has a significant impact on the antibunching effect against the thermal magnon excitation. Our research results provide a theoretical scheme for the manipulation of magnon antibunching characteristics, and have important practical significance for the construction of a single magnon source.

The ground-state cooling of a macroscopic mechanical oscillator is a crucial prerequisite for implementing the quantum manipulation of the mechanical oscillator. Here we show that a degenerate optical parametric amplifier (OPA) can significantly improve the cooling of the mechanical membrane in the purely dissipative coupling system in the unresolved sideband regime. In the presence of the OPA with a parametric phase $\pi /2$, the mechanical membrane can be cooled from room temperature 293 K to 10 mK, a factor of 112 lower than that in the absence of the OPA. If the mechanical oscillator is precooled to 0.1 K, the mechanical membrane can be cooled into the microkelvin regime.