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

当用灵敏的探测器记录时，光的电场在其相位和幅度上显示出不可避免的量子波动。这是海森堡测不准原理的一种表现。虽然不可能创建一个同时具有已知相位和幅度的光场，但可以降低相位或幅度的量子噪声，以换取另一个变量上增加的量子噪声。这种现象被称为压缩。到目前为止，强压缩光一次只在一种光学模式(通常是高斯光束)中产生。这意味着当场在整个空间范围内被考虑时，光束的涨落是被压缩的，但只测量光束横截面的一部分将产生很小的压缩。例如，振幅压缩光束的总功率涨落将减少，但其强度轮廓仍将呈现一些粗糙(局部涨落)。这一提议的目的是产生在其横向轮廓的任意点上振幅被压缩的光场。因此，即使在量子水平上，强度分布也应该是平滑的。从不同的角度来看，光束以多个横向光学模式被压缩，因此被称为多空间模式(MSM)压缩光。起点将是原子蒸气中的四波混合，这是一个非线性过程，在强激光的泵浦下，介质将一对入射光子转换为具有相关位置和方向的两个光子。四波混频过程的输出包含光子水平上的强关联，可以通过简单的线性光学进一步转化为MSM压缩光束，其中光子在光束内规则分布。我们将把这种非经典光态应用于两个实验，这两个实验概括了现代量子光学的两个方面：量子测量和量子信息处理。首先，我们将证明MSM压缩光可以提高量子对光学分辨率的极限。在显微镜装置中，MSM对被观测物体的压缩照明允许形成更平滑的图像，同时减少了空间量子噪声。使用已知的超分辨率技术，可以比使用经典照明更详细地重建对象。第二，我们将演示MSM压缩光产生与一类量子计算相关的量子态的潜力。这些态被称为团簇态，由一组纠缠模组成，可以由压缩光束和分束器网络组成。由于MSM压缩光对应于一组压缩的光学模式，因此它实际上构成了创建团簇态的适当资源。作为示范，我们将寻求产生一个小的簇状状态。

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.