In this type of experiment, a second-order nonlinear crystal placed inside an optical cavity, forming a so-called optical parametric oscillator, downconverts pump photons into signal and idler photons. The presence of squeezing is verified through homodyne detection: the generated light is mixed at a beam splitter with a laser mode at the same optical frequency, commonly referred to as a local oscillator, where fluctuations in the intensity difference across the two beam-splitter output modes can drop below shot-noise level as the phase of the local oscillator is varied. Typically, light propagating in an optical cavity oscillates only in the specific transverse mode that is resonant for a given cavity configuration. However, in the reported experiment the authors have employed a specific type of cavity, called a self-imaging cavity, which by design is degenerate for all transverse modes. Such a cavity is formed by a flat mirror and a concave mirror and contains an intracavity lens with specific lens-cavity reflector distances determined by the intracavity lens focal length and the curved mirror radius of curvature. Mode degeneracy for this cavity implies that multiple transverse modes may be simultaneously resonant. Having built an optical parametric oscillator based on this type of cavity, the authors have succeeded in demonstrating simultaneous squeezing for three different Hermite–Gauss transverse modes.
It is well known that squeezed light is highly vulnerable to optical losses, and indeed in the experimental work by Chalopin et al. losses associated with the intracavity lens limit the attainable degree of squeezing. Nevertheless, the observation of simultaneous squeezing for different transverse modes is a truly remarkable achievement. On the one hand this achievement is remarkable in terms of the technical challenge it represents, and on the other hand it is remarkable because it constitutes a possible route toward the generation of customizable multimode nonclassical light for quantum information processing applications. If this work could be extended so that simultaneous squeezing could be observed throughout all the elements of a specific basis, such as the basis formed by Hermite–Gauss modes, one could imagine the implementation of optical parametric oscillation of an arbitrary distribution of pump light. Because the squeezed light inherits the properties of the pump light distribution, this could lead to the exciting prospect of generating squeezed light with tailored properties suited to the specific needs of quantum information processing protocols.
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