# A Parametric Oscillator for Phonons

Optical parametric oscillators (OPOs) are indispensable in quantum optics, generating entangled photons for quantum communication and creating “squeezed” photon states for precision sensing (see Focus: Squeezing More from Gravitational-Wave Detectors). Now, researchers have demonstrated a device that does the same with phonons [1]. As well as paving the way for producing entangled phonon pairs and squeezed phonon states, the new optomechanical parametric oscillator could be used for superresolution microscopy and for high-frequency modulation of light sources.

In an OPO, a photon interacts with a nonlinear optical crystal to produce two secondary photons at half the initial frequency. To engineer a parametric oscillator for phonons, Andrés Reynoso and colleagues from the Bariloche Atomic Center, Argentina, and the Paul Drude Institute for Solid State Electronics, Germany, replaced the optical crystal with a 2D array of optomechanical traps with embedded quantum wells. Such traps, or cavities, are resonant with both photon and phonon modes.

The team illuminated one of these traps with a laser, exciting in it a high-energy exciton-polariton condensate—a collective state of quasiparticles formed from the repeated absorption and reemission of a photon by the quantum well. Exciton-polaritons then tunneled to neighboring traps, populating them with condensates of lower-energy exciton-polaritons.

During this tunneling process, exciton-polariton pairs with twice the trap’s phonon-mode energy generated pairs of phonons. Feedback in the phonon-pair-producing cavities then led to a parametric-amplification effect equivalent to that of a two-phonon laser. The researchers hope to optimize this process to explore the quantum nature of the phonon pairs and to demonstrate mechanical modulation of quantum light sources.

–Marric Stephens

Marric Stephens is a Corresponding Editor for *Physics Magazine* based in Bristol, UK.

## References

- A. A. Reynoso
*et al.*, “Optomechanical parametric oscillation of a quantum light-fluid lattice,” Phys. Rev. B**105**, 195310 (2022).