Strong long-range coupling between distant spins is crucial for spin-based quantum information processing. However, achieving such a strong spin-spin coupling remains challenging. Here we propose to realize a strong coupling between two distant spins via the Kerr effect of magnons in a yttrium-iron-garnet nanosphere. By applying a microwave field on this nanosphere, the Kerr effect of magnons can induce the magnon squeezing, so that the coupling between the spin and the squeezed magnons can be exponentially enhanced. This in turn allows the spin-magnon distance increased from nano- to micrometer scale. By considering the virtual excitation of the squeezd magnons in the dispersive regime, strong spin-spin coupling mediated by the squeezed magnons can be achieved, and a remote quantum-state transfer, as well as the nonlocal two-qubit iSWAP gate with high fidelity becomes implementable. Our approach offers a feasible scheme to perform quantum information processing among distant spins. 

Strong long-range spin-spin coupling via a Kerr magnon interface

Very recently, dissipative coupling was discovered, which develops and broadens methods for controlling and utilizing light-matter interactions. Here, we propose a scheme to generate the tripartite $W$ state in a dissipatively coupled system, where one qubit and two resonators simultaneously interact with a common reservoir. With appropriate parameters, we find the $W$ state is a dark state of the system. By driving the qubit, the dissipatively coupled system will evolve from the ground state to the tripartite $W$ state. Because the initial state is the ground state of the system and no measurement is required, our scheme is easy to implement in experiments. Moreover, the $W$ state decouples from the common reservoir and thus has a very long lifetime. This scheme is applicable to a wide class of dissipatively coupled systems and we specifically illustrate how to prepare the $W$ state in a hybrid qubit-photon-magnon system by using this scheme. 

/pdf/generation-of-long-lived-mml-math-xmlns-mml-http-www-w3-org-jhsbsm0f.pdf

Generation of long-lived <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>W</mml:mi></mml:math> states via reservoir engineering in dissipatively coupled systems

Abstract In this work, we propose a novel protocol for cyclic controlled remote state preparation (CCRSP) with single-qutrit equatorial states. In this protocol, three distant parties, Alleyn, Breden and Carden, are not only senders but also receivers. Alleyn wants to remotely prepare a single-qutrit equatorial state at Breden’s site, while at the same time, Breden (Carden) wishes to help Carden (Alleyn) remotely preparing another single-qutrit equatorial state. It is shown that, only if the three senders and the controller collaborate with each other, the CCRSP can be completed. To present this protocol more clearly, only maximally entangled channel is considered. 

Cyclic controlled remote state preparation in the three-dimensional system

Strong long-distance spin-magnon coupling is essential for solid-state quantum information processing and single-qubit manipulation. Here, we propose an approach to realize strong spin-magnon coupling in a hybrid optomechanical cavity-spin-magnon system, where the optomechanical system, consisting of two cavities coupled to a common high-frequency mechanical resonator, acts as quantum interface. By eliminating the mechanical mode, position-position coupling and two-mode squeezing of two cavities are induced. In the squeezing representation, the spin-photon, magnon-photon, and photon-photon coupling strengths are exponentially amplified, thus lower- and upper-branch polaritons (LBP and UBP) are generated by strongly coupled squeezed modes of two cavities. Utilizing the critical property of the LBP, the coupling between the spin qubit (magnon) and LBP is greatly enhanced, while the coupling between the spin qubit (magnon) and UBP is fully suppressed. In the dispersive regime, strong and tunable spin-magnon coupling is induced by the virtual LBP, allowing quantum state exchange between them. Our proposal provides a promising platform to construct magnon-based hybrid systems and realize solid-state quantum information processing with optomechanical interfaces. 

/pdf/optomechanical-interface-induced-strong-spin-magnon-coupling-1p2dcskp.pdf

Optomechanical-interface-induced strong spin-magnon coupling

Multichannel synchronous amplifications are an inevitable key problem in quantum communication process, which can broaden the bandwidth of transmitted signals and establish correlation among different optical channels. Here we study a nonreciprocal system with four concurrent amplification channels using hot cesium atoms, both theoretically and experimentally. For the forward probe field, the double--electromagnetically induced transparency structure is formed and the phase-matching condition of the multiwave mixing process is satisfied, which are both destroyed when the probe field is reversed. In addition, the four-channel nonreciprocal amplifications are formed in the Zeeman sublevels of the system with special selection of light field polarization, which will also dramatically enhance the signal-to-noise ratio by suppressing the spontaneous emission noise of the system. In our experiment, the quadruple nonreciprocal amplifications are achieved with the maximum forward gain reaching 30 dB and the reverse suppression reaching--23 dB. The gain adjustability allows the construction of a gain-loss balanced system, providing a scheme for an atomic system to engineer a parity-time-symmetric (or -antisymmetric) structure. 

Multichannel nonreciprocal amplifications using cesium vapor

We propose an experimentally realizable nonreciprocal magnonic device at the single-magnon level by exploiting magnon blockade in a magnon-based hybrid system. The coherent qubit-magnon coupling, mediated by virtual photons in a microwave cavity, leads to the energy-level anharmonicity of the composite modes. In contrast, the corresponding dissipative counterpart, induced by traveling microwaves in a waveguide, yields inhomogeneous broadenings of the energy levels. As a result, the cooperative effects of these two kinds of interactions give rise to the emergence of the direction-dependent magnon blockade. We show that this can be demonstrated by studying the equal-time second-order correlation function of the magnon mode. Our study opens an avenue to engineer nonreciprocal magnonic devices in the quantum regime involving only a small number of magnons. 

/pdf/dissipation-induced-nonreciprocal-magnon-blockade-in-a-1k4n0l07.pdf

Tobias Riedl

Papers

Dissipation-induced nonreciprocal magnon blockade in a magnon-based hybrid system