Multimode cavity optomechanical systems allow light to couple otherwise non-interacting mechanical resonators, enabling non-Hermitian phenomena such as exceptional points, where eigenfrequencies and eigenvectors of coupled modes coalesce. Accessing an exceptional point and its nearby parameter space is a first step towards chiral mode dynamics and topological state transfer. Diamond optomechanical devices support strong coherent optomechanical coupling required to tune resonances to an exceptional point, as well as strain-coupling to spin-defects for hybrid quantum technologies, but have not yet been used for multimode non-Hermitian physics. Here we tune to an exceptional point in a diamond optomechanical crystal, which uses structural symmetry breaking to produce two high-frequency mechanical resonances coupled to an optical cavity. The exceptional point is reached within a stable operating window below the phonon-lasing threshold, and we observe asymmetric redistribution of optomechanical damping and anti-damping between hybridized modes. These results establish diamond optomechanical crystals as a platform for non-Hermitian optomechanics, opening routes to topological mechanical dynamics in hybrid spin-phonon interfaces.
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Main text: 7 pages, 4 figures. Supplementary information included
Multimode cavity optomechanical systems allow light to couple otherwise non-interacting mechanical resonators, enabling non-Hermitian phenomena such as exceptional points, where eigenfrequencies and eigenvectors of coupled modes coalesce. Accessing an exceptional point and its nearby parameter space is a first step towards chiral mode dynamics and topological state transfer. Diamond optomechanical devices support strong coherent optomechanical coupling required to tune resonances to an exceptional point, as well as strain-coupling to spin-defects for hybrid quantum technologies, but have not yet been used for multimode non-Hermitian physics. Here we tune to an exceptional point in a diamond optomechanical crystal, which uses structural symmetry breaking to produce two high-frequency mechanical resonances coupled to an optical cavity. The exceptional point is reached within a stable operating window below the phonon-lasing threshold, and we observe asymmetric redistribution of optomechanical damping and anti-damping between hybridized modes. These results establish diamond optomechanical crystals as a platform for non-Hermitian optomechanics, opening routes to topological mechanical dynamics in hybrid spin-phonon interfaces.
Quantum optomechanical technologies benefit from mechanical oscillators that are high-frequency, can be coherently driven, and are capable of coupling to other quantum systems. Diamond supports all of these criteria: its large elastic modulus increases mechanical resonance frequency, its low nonlinear optical absorption increases the allowed intensity of fields used for coherent optomechanics, and it hosts spin qubits that interact with mechanical modes. Here we demonstrate a diamond optomechanical crystal cavity that supports multiple mechanical resonances with $\sim$12 GHz frequency and high Qm×fm product that can be coherently coupled to multiple optical modes. By exciting this sideband resolved system into mechanical self-sustained oscillations, we generate a frequency comb spanning 143 GHz. Analysis of the comb spectrum, combined with systematic characterization of the system's optomechanical coupling, allows us to quantitatively show that its mechanical oscillation amplitude reaches 130 pm. This corresponds to a maximum total dynamic strain of 1.1×10−3 that is sufficiently high for future demonstrations of optomechanical control of diamond spin qubits.
Exceptional points, where two or more eigenstates of a non-Hermitian system coalesce, are now of interest across many fields of physics, from the perspective of open-system dynamics, sensing, nonreciprocal transport, and topological phase transitions. In this work, we investigate exceptional points in cavity optomechanics, a platform of interest to diverse communities working on gravitational-wave detection, macroscopic quantum mechanics, quantum transduction, etc. Specifically, we clarify the role of quantum jumps in making a clear distinction between Liouvillian and Hamiltonian exceptional points in optomechanical systems. While the Liouvillian exceptional point arises from the unconditional Lindblad dynamics and is independent of the phonon-bath temperature, the Hamiltonian exceptional point emerges from the conditional no-jump evolution and acquires a thermal shift due to an enhanced conditional damping. Employing the thermofield formalism, we derive a unified spectral framework that interpolates between these regimes via an analytical hybrid-Liouvillian description. Remarkably, in the weak-quantum-jump regime, the exceptional point is perturbed only at the second order, highlighting the robustness of the Hamiltonian exceptional point under small hybrid perturbations. Our work reveals a continuous family of hybrid exceptional points, clarifies the operational and physical differences between the conditional and unconditional dissipative dynamics in optomechanical systems, and provides a probe for thermal baths.
Higher-order exceptional points (EPs) in non-Hermitian systems have attracted great interest due to their advantages in sensitive enhancement and distinct topological features. However, realization of such EPs is still a challenge because more fine-tuning parameters are generically required in quantum systems, compared to the second-order EP (EP2). Here, we propose a non-Hermitian three-mode optomechanical system in the blue-sideband regime for predicting the third-order EP (EP3). By deriving the pseudo-Hermitian condition for the proposed system, one cavity with loss and the other with gain must be required. Then we show that EP3 or EP2 can be observed when the mechanical resonator (MR) is neutral, loss, or gain. For the neutral MR, we find that two degenerate or two nondegenerate EP3s can be predicted by tuning system parameters in the parameter space, while four nondegenerate EP2s can be observed when the system parameters deviate from EP3s, which is distinguished from the previous study in the red-detuned optomechanical system. For the gain (loss) MR, we find that only two degenerate EP3s or EP2s can be predicted by tuning enhanced coupling strength. Our proposal provides a potential way to predict higher-order EPs or multiple EP2s and study multimode quantum squeezing around EPs using blue-detuned non-Hermitian optomechanical systems.
Cooling a mechanical mode in finite time is a routing problem: excitation must reach a lossy mode before thermal noise rebuilds the population. Exceptional points are non-Hermitian degeneracies at which two hybrid modes and their eigenvectors merge, so a loop around them can exchange connected spectral branches. The main obstacle in testing whether this topology improves cooling is causal. Independently optimized enclosing and non-enclosing protocols normally have different waveforms, so topology and waveform geometry change together. We remove this ambiguity in a three-mode optomechanical system with a strongly damped main cavity and an exactly lossless auxiliary cavity. We keep one power modulation and one detuning shape fixed and change only their relative phase. The shift preserves the matched local control resources but changes the loop from non-enclosing to a braid around both exceptional points. Certified dynamics show that the two-exceptional-point protocol lowers the final mechanical occupation by more than 55 percent. The advantage survives the complete admissible phase family, deliberate waveform changes, a finite neighborhood of nearby smooth controls, and the restoration of counter-rotating heating processes. The auxiliary cavity acts as a coherent buffer, while the main cavity remains the only optical loss channel.
We investigate how the exceptional-point structure of a dissipative cavity optomechanical system is inherited by experimentally accessible dynamical, spectral, and quantum-statistical observables. At optical-mechanical resonance, the effective non-Hermitian first-moment dynamics exhibits a second-order exceptional point, where two eigenvalues and their eigenvectors coalesce and the eigenvalue splitting follows the characteristic square-root dependence on perturbations. We show that the same square-root feature also governs transient photon and phonon populations, first-order coherence, spectral poles, and second-order intensity correlations. Below the exceptional point, the dynamics is non-oscillatory, whereas above it damped oscillations emerge together with frequency splitting of the spectral poles. At the exceptional point, the Jordan-block structure produces polynomial-exponential relaxation and a second-order spectral pole. Using the quantum regression theorem and Gaussian moment factorization, we further show that the stationary fluctuations satisfy the Siegert relation linking first- and second-order correlations. Although the zero-delay autocorrelations retain their thermal value, the finite-delay intensity correlations exhibit clear exceptional point signatures. This finding connects non-Hermitian mode coalescence with measurable dynamical and correlation observables in cavity optomechanics.
Cavity magnomechanical systems, which coherently couple magnons, photons, and phonons, offer a powerful platform for exploring quantum phenomena and developing hybrid quantum technologies. We study a non-Hermitian cavity magnomechanical system featuring a yttrium iron garnet (YIG) sphere driven by an external magnetic field, where magnons interact with cavity photons via magnetic dipole coupling and with phonons via magnetostrictive forces. Non-Hermiticity is introduced through a traveling optical field that directly excites the YIG sphere, enabling precise control over parity-time (PT) symmetry. Analysis of the eigenfrequency spectrum reveals a third-order exceptional point (EP) that demarcates distinct PT-symmetric phases: a unique configuration with coexisting broken and protected PT-symmetry regimes, tunable via the interplay of the traveling field strength and magnon-photon coupling. We also verify this by measuring the divergence around EP with Petermann Factor. Crucially, the system exhibits stable PT-symmetry only at specific field incidence angles, with instability or broken symmetry dominating elsewhere. Beyond PT-symmetry, we demonstrate robust quantum entanglement among the magnon, photon, and phonon subsystems. The exceptional point serves as a critical boundary, with unbroken and broken PT symmetry phases enabling dynamic entanglement swapping between subsystem pairs. By modulating the non-Hermitian parameters, we achieve controlled entanglement transfer and suppression of fluctuations, highlighting the system’s potential for quantum information processing. These results establish a direct connection between non-Hermitian topology, dynamical stability, and quantum correlations, providing a framework for leveraging PT symmetry in cavity magnomechanics for quantum technologies.