Cavity-enhanced nonlinearities, in particular optical parametric oscillation (OPOs), are among the most widely used systems for the generation of non-classical states of light, of interest for several quantum applications, such as imaging [1], communication [2], and computing [3]. However, OPOs remain bulky and complex, especially when one accounts for the addition of the noise suppression required for typical laser sources. Vertical-external-cavity surface-emitting lasers (VECSELs), on the other hand, are capable of very low noise operation and are intrinsically shot-noise limited over a broad frequency range. Further they provide the means to embed the nonlinear process directly inside the laser cavity. Recently, our group demonstrated the use of a VECSEL as the pump source for a single-frequency OPO for the first time, achieving broadly tuneable, narrow linewidth at optical communications wavelengths [4], taking advantage of low noise and relaxation-oscillation-free laser dynamics. To-date, this intra-VECSEL cavity singly resonant OPO (VECSEL ICSRO), has undergone ‘standard classical’ characterisation only. In this project, we will investigate the quantum properties of the system for the first time, in a variety of operating regimes:

• Below threshold, OPOs are a source of squeezed vacuum, a quantum state typically exploited for achieving sub-shot noise sensitivity in optical interferometry and narrowband heralded single photons [5].
• Above threshold the OPO generates squeezed coherent states and intense twin beams, characterised by an intensity difference noise that is theoretically zero. These beams can then be exploited for sensing and metrology e.g. for high-sensitivity spectroscopy.
• Far above threshold (>4x threshold) can lead to regimes in which signal and idler beams are simultaneously individually squeezed in intensity, i.e. not only would their intensity be quantum correlated but the individual beams could also exhibit sub-shot-noise statistics. However, this is typically spoiled by the classical noise of the pump laser and by cavity relaxation oscillations. This has previously been experimentally realised in microcavity OPOs [6], owing to their low threshold of a few μW, where low noise pump lasers are easily available. A relaxation-oscillation-free VECSEL ICSRO could potentially target the realisation of sub-shot-noise laser beams at relatively high power (100s mW).

Demonstration of quantum states of light from a compact, highly stable, ultra-low noise VECSEL-based OPO platform will allow for the creation of transportable lasers for tests in quantum imaging and sensing, such as spectroscopy and sensitive trace gas-detection.