Multiple-post reentrant 3D lumped cavity modes have been realized to design the concept of a discrete Whispering Gallery and Fabry-Pérot-like Modes for multimode microwave Quantum Electrodynamics experiments. Using the magnon spin-wave resonance of a submillimeter-sized Yttrium-Iron-Garnet sphere at millikelvin temperatures and a four-post cavity, we demonstrate the ultra-strong coupling regime between discrete Whispering Gallery Modes and a magnon resonance with a strength of 1.84 GHz. By increasing the number of posts to eight and arranging them in a D4 symmetry pattern, we expand the mode structure to that of a discrete Fabry-Pérot cavity and modify the Free Spectral Range (FSR). We reach the superstrong coupling regime, where spin-photon coupling strength is larger than FSR, with coupling strength in the 1.1 to 1.5 GHz range.
Multiple-post reentrant 3D lumped cavity modes have been realized to design the concept of a discrete Whispering Gallery and Fabry-Pérot-like Modes for multimode microwave Quantum Electrodynamics experiments. Using the magnon spin-wave resonance of a submillimeter-sized Yttrium-Iron-Garnet sphere at millikelvin temperatures and a four-post cavity, we demonstrate the ultra-strong coupling regime between discrete Whispering Gallery Modes and a magnon resonance with a strength of 1.84 GHz. By increasing the number of posts to eight and arranging them in a D4 symmetry pattern, we expand the mode structure to that of a discrete Fabry-Pérot cavity and modify the Free Spectral Range (FSR). We reach the superstrong coupling regime, where spin-photon coupling strength is larger than FSR, with coupling strength in the 1.1 to 1.5 GHz range.
Progress in the emerging field of engineered quantum systems requires the development of devices that can act as quantum memories. In this thesis, a novel spectroscopic technique using electron spin resonance (ESR) is presented. Microwave whispering gallery modes (WGM) in cylindrical dielectric resonators are used to probe paramagnetic impurities at millikelvin temperatures under the influence of an applied dc magnetic field. This technique couples the naturally high quality factor and the relatively high magnetic filling factor of WGMs in crystalline hosts. Precision spectroscopy has been performed on a number of dielectric samples including, sapphire, ruby, quartz, yttrium orthosilicate (YSO) and yttrium aluminium garnet (YAG). Spectroscopic characterisation of sapphire reveals trace ion impurities at parts per billion, which have been used to control strong coupling between the WGM doublets. Moreover, time-reversal symmetry breaking has been observed when a WGM interacts with an ESR, a result of selection rules being created as a consequence of conservation of angular momentum. Following this, interactions between WGM microwave photons and spin resonances are shown in a range of systems. Results achieved include coupling between WGM photons and Cr3+ ions in ruby with normal mode splitting on the order of 600 MHz at 13.9 GHz, the coupling of iron group ion impurities to WGM photons in YSO, which also revealed a strong coupling regime with very low magnetic field, and coupling of Er3+ to WGMs in YAG with evidence of dilute ferromagnetism - the first observation of such an effect outside of iron group doped semiconductors. In addition a novel double-post re-entrant cavity is presented, which is able to focus the rf magnetic field into a small sample, in this case a sub-millimetre sized Yttrium Iron Garnet (YIG) ferrite microsphere that supports magnon resonances. Ultra-high cooperatively of 105 between photon and magnon modes is demonstrated at millikelvin temperatures and microwave frequencies. A coupling strength (normal mode splitting) of 2 GHz at 20 GHz, equivalent to 76 cavity linewidths, is achieved for the bright cavity mode. This constitutes about 10% of the photon energy and shows that ultra-strong coupling is possible in spin systems at microwave frequencies.
Abstract Hybridizing collective spin excitations and a cavity with high cooperativity provides a new research subject in the field of cavity quantum electrodynamics and can also have potential applications to quantum information. Here we report an experimental study of cavity quantum electrodynamics with ferromagnetic magnons in a small yttrium-iron-garnet (YIG) sphere at both cryogenic and room temperatures. We observe for the first time a strong coupling of the same cavity mode to both a ferromagnetic-resonance (FMR) mode and a magnetostatic (MS) mode near FMR in the quantum limit. This is achieved at a temperature ~22 mK, where the average microwave photon number in the cavity is less than one. At room temperature, we also observe strong coupling of the cavity mode to the FMR mode in the same YIG sphere and find a slight increase of the damping rate of the FMR mode. These observations reveal the extraordinary robustness of the FMR mode against temperature. However, the MS mode becomes unobservable at room temperature in the measured transmission spectrum of the microwave cavity containing the YIG sphere. Our numerical simulations show that this is due to a drastic increase of the damping rate of the MS mode.
We present a scheme to generate distant bipartite and tripartite entanglement between an atomic ensemble and an yttrium iron garnet (YIG) sphere in coupled microwave cavities. We consider an atomic ensemble in a single-mode microwave cavity which is coupled with a second single-mode cavity having a YIG sphere. Our system, therefore, has five excitation modes, namely, cavity-1 photons, an atomic ensemble, cavity-2 photons, a magnon and a phonon mode in the YIG sphere. We show that significant bipartite entanglement exists between indirectly coupled subsystems in the cavities which is robust against temperature. Moreover, we present suitable parameters for a significant tripartite entanglement of the ensemble, magnon, and phonon modes. We also demonstrate the existence of tripartite entanglement between the magnon and phonon modes of the YIG sphere with indirectly coupled cavity photons. Interestingly, this distant tripartite entanglement is of the same order as that previously found for a single-cavity system. We show that cavity-cavity coupling strength affects both the degree and transfer of quantum entanglement between various subsystems. Therefore, an appropriate cavity-cavity coupling optimizes the distant entanglement by increasing the entanglement strength and its robustness against temperature.
Realizing a strong light-matter interaction via magnetic dipoles is of fundamental importance in physics, and a cavity magnonics system has provided a unique platform to achieve strong and even ultrastrong light-matter interaction. In this work, transparency induced by magnon-photon interactions in cavity quantum electrodynamics (QED) system coupling a yttrium-iron-garnet (YIG,Y3Fe5O12) (YIG) sphere has been investigated. The spectral separation reveals a linear relationship with the coupling strength, which might find applications in magnon dark state and magnon-based quantum information processing.
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.
We propose a scheme to engineer the superradiant phase transition (SPT) in cavity magnonics by periodically modulating the frequency of the magnon mode. The studied system is composed of a yttrium iron garnet (YIG) sphere positioned inside a microwave cavity, where magnons in the YIG sphere are strongly coupled to microwave photons. Under the Floquet drive, the effective frequencies of both the cavity and magnon modes can be readily controlled via the frequency and strength of Floquet field. This tunability allows the cavity magnonic system to support a rich steady-state phase diagram, featuring parity-symmetric, parity-symmetry-broken, bistable, and unstable phases. With the increase of Floquet-field strength, the system exhibit a discontinuous phase transition from the parity-symmetric phase to the parity-symmetry-broken phase at a critical threshold, accompanied by an abrupt jump of the magnon occupation from zero to a finite value. Upon further increase of Floquet-field strength, the magnon occupation declines continuously from a nonzero value back to zero, corresponding to a second-order phase transition that restores the parity-symmetric phase. Additionally, fluctuations in magnon number during the SPT process are examined. Our work establishes an alternative route to engineer the cavity-magnon SPT without relying on microwave parametric drive.