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Physical Review A

Robust generation of a magnonic cat state via a superconducting flux qubit

Yu-Bo Hou, Xin‐Lei Hei, Xue‐Feng Pan, Ji-kun Xie, Ya-long Ren, Sheng-li Ma, Fuli Li, Fuli Li

Published Jul 22, 2024• 14 citations
Publisher

Abstract

We study the hybrid system of a high-quality yttrium-iron-garnet (YIG) sphere magnetically coupled to a superconducting flux qubit via the magnetic fields created by two persistent-current quantum states. When the flux qubit is operated away from its sweet point, a coherent nonlinear two-magnon interaction can be predicted; that is, the qubit is excited by absorbing a pair of magnons. We show that the spontaneous emission of the qubit can be exploited to steer the magnon mode of the YIG sphere into a Schr\"odinger cat state with high fidelity. Our scheme has practical advantages in that it eliminates the need for precise control of the evolution time and the projective measurement, and is also insensitive to the qubit's pure dephasing.

DOI
10.1103/physreva.110.013711

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We study the hybrid system of a high-quality yttrium-iron-garnet (YIG) sphere magnetically coupled to a superconducting flux qubit via the magnetic fields created by two persistent-current quantum states. When the flux qubit is operated away from its sweet point, a coherent nonlinear two-magnon interaction can be predicted; that is, the qubit is excited by absorbing a pair of magnons. We show that the spontaneous emission of the qubit can be exploited to steer the magnon mode of the YIG sphere into a Schr"odinger cat state with high fidelity. Our scheme has practical advantages in that it eliminates the need for precise control of the evolution time and the projective measurement, and is also insensitive to the qubit's pure dephasing.
The cavity-magnon-qubit system has recently been demonstrated as a new platform for preparing macroscopic quantum states in magnonic systems. Here, we propose to prepare a two-mode magnonic cat state, which is also a non-Gaussian entangled state, based on this practical system involving two yttrium-iron-garnet (YIG) spheres and a superconducting qubit coupled to a common microwave cavity. By adiabatically eliminating the cavity and resonantly driving the qubit, an effective magnon-qubit conditional-displacement interaction is achieved. Further working in the magnon-magnon strong-coupling regime and considering two identical magnon frequencies and coupling strengths to the cavity, two hybridized magnon modes are formed, of which the bright mode is prepared in a cat state after a projective measurement on the qubit, while the dark mode remains in its initial vacuum state. Such a state corresponds to a two-mode cat state of two original magnon modes, which share strong non-Gaussian entanglement. We also discuss practical dissipation and dephasing effects on the cat state. The results indicate that strong nonclassicality and non-Gaussian entanglement are present in the two-mode cat state using fully feasible parameters.
We propose a protocol for generating magnonic squeezed states (MSS) and their superpositions (SMSS) in a hybrid system comprising a superconducting flux qubit magnetically coupled to the Kittel mode of a yttrium iron garnet (YIG) sphere. The flux qubit provides an intrinsic longitudinal interaction with the magnon mode, which, under resonant microwave driving, gives rise to an effective qubit-state-dependent squeezing Hamiltonian. Numerical simulations incorporating realistic dissipation demonstrate that magnon quadrature noise reduction exceeding 8 dB8~\mathrm{dB}8 dB is achievable with experimentally accessible parameters.~By preparing the qubit in a superposition state followed by projective measurement, we further obtain symmetric and antisymmetric superpositions of orthogonally squeezed magnon states exhibiting clear phase-space interference fringes.~We discuss how the fourfold rotational symmetry of these states supports a bosonic logical encoding with potential for protecting against dominant error channels in magnonic platforms.
We propose an alternative scheme for implementing the antibunching effects of two-magnon bundle in a hybrid ferromagnet-superconductor system, where a magnon mode from the yttrium iron garnet (YIG) sphere interacts with a three-level superconducting qubit via photon virtual excitation in the microwave cavity. With the help of the qubit driving from the ground state to the excited state, the cascaded emission of magnon occurs and then the two-magnon bundle is formed. By analyzing the ordinary and generalized second-order correlation functions, it is found that the antibunched two-magnon bundle could be achieved via properly choosing the system parameters, which is originated from the anharmonicity of dressed energy levels induced by magnon-qubit couplings. The distinct feature is that high-proportion n-magnon emission could be obtained via relaxing the restriction on the strong qubit driving and magnon-qubit coupling, which may provide a feasible method to realize the high-quality multimagnon source for quantum metrology and quantum information processing.
The cavity-mediated coupling between magnons in an yttrium-iron-garnet (YIG) sphere and a superconducting qubit has recently been demonstrated as a new platform for preparing macroscopic quantum states. Here, based on this system, we propose to entangle two magnon modes in two YIG spheres by driving the qubit with a two-tone field and by appropriately choosing the frequencies and strengths of the two driving fields. We show that strong entanglement can be achieved with fully feasible parameters. We further provide a detection scheme for experimentally verifying the entanglement. Our results indicate that macroscopic entanglement between two magnon modes in two millimeter-sized YIG spheres, involving more than 101810^{18}1018 spins, can be realized using currently available parameters, which finds promising applications in fundamental studies, such as macroscopic quantum mechanics and the test of unconventional decoherence theories.
We experimentally demonstrate strong coupling between the ferromagnetic magnons in a small yttrium-iron-garnet (YIG) sphere and the drive-field-induced dressed states of a superconducting qubit, which gives rise to the double dressing of the superconducting qubit. The YIG sphere and the superconducting qubit are embedded in a microwave cavity and the effective coupling between them is mediated by the virtual cavity photons. The theoretical results fit the experimental observations well in a wide region of the drive-field power resonantly applied to the superconducting qubit and reveal that the driven qubit-magnon hybrid quantum system can be harnessed to emulate a particle-hole-symmetric pair coupled to a bosonic mode. This hybrid quantum system offers a novel platform for quantum simulation of the composite quasi-particles consisting of fermions and bosons.
We study the generation of a magnon blockade in a single-crystalline yttrium-iron-garnet (YIG) sphere coupled to a three-dimensional superconducting microwave resonator that interacts with a superconducting qubit. Based on the indirect coupling between the Kittel mode of YIG and the superconducting qubit mediated by the virtual photon excitation in the microwave cavity, we propose two different mechanisms for observing the magnon blockade effect in the ferromagnetic sphere. The first one is to use the strong anharmonicity of the dressed states of the coupled qubit-magnon system. However, the strong magnon antibunching can only be achieved under the strong-coupling regime. To relax this constraint, we propose another quantum interference method to produce and enhance the magnon blockade effect with the moderate-coupling strength. By optimizing the relative phase and the strength ratio of the external driving fields applied to the YIG and qubit, we show that the much stronger magnon antibunching with significantly increased steady-state mean magnon numbers can be realized by the destructive interference for the two-magnon excitation.