Spatially multimode squeezed light for quantum imaging and one-way quantum computing

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When recorded with sensitive detectors, the electric field of the light reveals unavoidable quantum fluctuations in its phase and amplitude. It is a manifestation of the Heisenberg uncertainty principle. While it is impossible to create a light field with simultaneously known phase and amplitude, it is possible to reduce the quantum noise of either the phase or the amplitude in exchange of increased quantum noise on the other variable. This phenomenon is known as squeezing.Up till now, strongly squeezed light has only been produced in one single optical mode at a time (typically a Gaussian beam). This means that the fluctuations of the beam are squeezed when the field is considered in its full spatial extent but that measuring only a portion of the cross-section of the beam will yield little squeezing. For instance, an amplitude-squeezed beam will have reduced fluctuations in its total power, but its intensity profile will still present some roughness (local fluctuations). This proposal aims to produce light fields that are squeezed in amplitude at any point of their transverse profile. As a consequence, the intensity profile should be smooth, even at the quantum level. From a different point of view, the beam is squeezed in multiple transverse optical modes, hence the name multi-spatial-mode (MSM) squeezed light .The starting point will be four-wave mixing in an atomic vapour, a non-linear process in which the medium, when pumped by an intense laser beam, converts a pair of incoming photons into two photons with correlated positions and directions. The output of the four-wave mixing process contains strong correlations at the photon level and can be further transformed through simple linear optics into a MSM squeezed beam of light, where the photons are regularly distributed inside the beam.We will apply this non-classical state of light to two experiment which epitomise two aspects of modern quantum optics: quantum measurements and quantum information processing.First, we will show that MSM squeezed light can improve the quantum limit on optical resolution. In a microscopy set-up, MSM squeezed illumination of the observed object allows for the formation of a smoother image, with reduced spatial quantum noise. Using known techniques of super-resolution, it is then possible to reconstruct the objects in greater details than what would be possible with classical illumination.Second, we will demonstrate the potential of MSM squeezed light to produce quantum states of light that are relevant to a class of quantum computing. These states, called cluster states, are made of a collection of entangled modes and can be built from an ensemble of squeezed beams with a network of beam-splitters. Since MSM squeezed light corresponds to a collection of squeezed optical modes, it actually constitutes the proper resource for the creation of a cluster state. As a demonstration, we will seek to produce a small cluster state.