A room-temperature cavity-magnonic source of correlated microwave pairs

Correlated microwave photon sources are key enablers for technologies in quantum-limited sensing, signal amplification and communication, but the reliance on millikelvin operating temperature limits their scalability for broader applications. Here, at room temperature, we demonstrate strong correlated microwave signals emitted from a hybrid magnon-photon platform. Different from traditional parametrically induced magnons with degenerate frequencies, we achieve non-degenerate excitations by coupling magnon modes simultaneously with two cavity photon modes. Through the magnon-photon interactions in the corresponding linear and nonlinear regimes, one input microwave photon splits into a pair of magnon polaritons that possess distinct frequencies but maintain strong inter-mode correlations. The nonlinear magnon polariton dynamics empowered by this new parametric platform brings both verified true randomness and robust multi-channel correlations, from which we construct a microwave communication experiment for noise resilient signal transmission with added security. This work establishes cavity magnonics as a versatile and compact platform for generating correlated multi-mode microwave signals, opening new avenues for applications in classical and quantum domains.

💡 Research Summary

This paper reports a room‑temperature source of correlated microwave photon pairs based on a hybrid magnon‑photon platform. By embedding a thin Yttrium Iron Garnet (YIG) film onto a microstrip resonator that supports two distinct cavity modes—one at 3.354 GHz (λ/2) and another at 6.708 GHz (λ)—the authors achieve simultaneous strong coupling of the magnon mode to both photon modes. The resonator is engineered so that the higher‑frequency mode is exactly twice the lower‑frequency mode, a condition that enables non‑degenerate parametric down‑conversion: a single pump photon at the higher frequency splits into two magnon‑polariton excitations (signal and idler) with different frequencies but strong inter‑mode correlations.

Experimentally, a continuous‑wave pump at 6.708 GHz with 12 dBm power is applied while a static magnetic field H₀ is swept. At low H₀ the system exhibits the usual degenerate parametric process (identical‑frequency magnon pairs). As H₀ increases, two sidebands emerge symmetrically around the pump frequency, their separation growing with H₀. Power‑dependence measurements reveal a clear threshold and abrupt increase of output power, confirming the nonlinear parametric nature. Importantly, the frequency splitting and linewidth of the non‑degenerate modes remain essentially unchanged when the pump power is varied, indicating that the splitting originates from intrinsic magnon‑photon hybridization rather than extrinsic effects such as microwave‑dressed states.

A minimal Hamiltonian including the Kittel magnon mode (±k), the two cavity modes, a Kerr‑type four‑magnon term, and linear damping reproduces the observed spectra. The parametric term reduces to a two‑mode squeezing operator when the pump is treated classically, directly analogous to optical spontaneous parametric down‑conversion (SPDC) or a non‑degenerate Josephson parametric amplifier. The eigenfrequencies of the resulting magnon‑polariton pairs are ω₀ ± √(4g_k² − γ²), where g_k is the k‑dependent magnon‑photon coupling and γ the weighted average loss. This explains the transition from degenerate to non‑degenerate behavior as H₀ shifts the magnon band and modifies g_k.

Correlation measurements are performed by simultaneously detecting the quadratures (X, P) of the two output modes using homodyne detection after down‑conversion. Time‑domain traces show a short‑term coherent beat (fixed phase relationship) superimposed on long‑term stochastic phase wandering. Phase histograms form a uniform circle, demonstrating true randomness, while the in‑phase components (X₁, X₂) are positively correlated and the out‑of‑phase components (P₁, P₂) are anti‑correlated, yielding a cross‑correlation coefficient of ≈ ±0.98. Autocorrelation and cross‑correlation decay with a coherence time of ~0.18 ms, consistent with the measured linewidth (~6.7 kHz) and theoretical predictions based on a thermally driven diffusive oscillator model. NIST statistical tests confirm that sampled phase data constitute high‑quality random numbers when the sampling interval exceeds the coherence time.

To showcase a practical application, the authors implement a microwave communication scheme. The signal (3.3532 GHz) is phase‑modulated with QPSK at 500 Hz, broadcast publicly, while the idler (3.3548 GHz) is retained locally or transmitted via a private channel. At the receiver, mixing the received signal with the stored idler recovers the data, exploiting the strong inter‑mode correlation to suppress environmental noise and provide inherent security. This demonstration illustrates that the non‑degenerate magnon‑polariton pair offers both true randomness and deterministic phase correlation, features valuable for quantum‑enhanced communication, quantum radar, and adaptive imaging, while operating at room temperature in a compact footprint.

In summary, the work establishes (i) a room‑temperature, non‑degenerate parametric down‑conversion mechanism in a cavity‑magnonic system, (ii) robust inter‑mode correlations and thermally seeded true randomness despite ambient thermal noise, and (iii) a proof‑of‑concept multi‑channel microwave communication protocol that leverages these properties. The platform bridges the gap between optical SPDC (room‑temperature but bulk) and superconducting Josephson devices (cryogenic but highly tunable), opening a versatile pathway for both classical and quantum microwave technologies.