Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences最新文献

Quantum retrodiction, in which the state of a quantum system prior to a measurement is assigned based on the results of that measurement, has had a long history and has been used in quantum optics research for decades. Here we summarize the theory and point out some of the more interesting results, before applying the theory to state identification from multiple shots of an experiment. One surprising result is that we show that a photodetector with low quantum efficiency can discriminate between photonic states better than a detector with a higher efficiency.This article is part of the theme issue 'The quantum theory of light'.

We derive a compact expression for the second-order correlation function [Formula: see text] of a quantum state in terms of its Wigner function, thereby establishing a direct link between [Formula: see text] and the state's shape in phase space. We conduct an experiment that simultaneously measures [Formula: see text] through direct photocounting and reconstructs the Wigner function via homodyne tomography. The results confirm our theoretical predictions.This article is part of the theme issue 'The quantum theory of light'.

Towards the end of his life, Rudolf Peierls became interested in photon momentum and radiation pressure. He subsequently published a number of theoretical papers on the subject. Although the work of Peierls has neither been widely adopted nor developed, it did nevertheless have a provoking and fruitful effect on the radiation pressure research undertaken by Alan Gibson and Rodney Loudon at the University of Essex.This article is part of the theme issue, 'The quantum theory of light'.

The quantum theory of light in real media requires attention to a number of physical features. Even in near-transparent dielectrics, we have to incorporate dispersion, losses and the effects of interfaces. Here, we review the quantization of light in a dielectric and see how this affects radiative processes and light propagation. In the second half of the paper, we turn to optical forces and momentum. There we show why there are two rival force densities in a medium and also why there must be two distinct optical momenta. In the process, this resolves the century-old Abraham-Minkowski dilemma.This article is part of the theme issue 'The quantum theory of light'.

We present a brief introduction to the quantum theory of light as it is understood in the field of quantum optics. Our aim is not to review the topic, which would require a very extensive article (or even a book of several volumes) but rather to provide sufficient background to set the ideas in the following papers in their correct context.This article is part of the theme issue 'The quantum theory of light'.

Quantum optical networks are instrumental in addressing the fundamental questions and enable applications ranging from communication to computation and, more recently, machine learning (ML). In particular, photonic artificial neural networks (ANNs) offer the opportunity to exploit the advantages of both classical and quantum optics. Photonic neuro-inspired computation and ML have been successfully demonstrated in classical settings, while quantum optical networks have triggered breakthrough applications such as teleportation, quantum key distribution and quantum computing. We present a perspective on the state of the art in quantum optical ML and the potential advantages of ANNs in circuit designs and beyond, in more general, analogue settings characterized by recurrent and coherent complex interactions. We consider two analogue neuro-inspired applications, namely quantum reservoir computing and quantum associative memories, and discuss the enhanced capabilities offered by quantum substrates, highlighting the specific role of light squeezing in this context.This article is part of the theme issue 'The quantum theory of light'.

In 1992, Allen et al. (Allen L, Beijersbergen MW, Spreeuw RJC, Woerdman JP. 1992 Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes. Phys. Rev. A 45, 8185-8189. (doi:10.1103/physreva.45.8185)) published their seminal paper on the orbital angular momentum of light, drawing together seemingly unrelated themes and ideas in optics. This breakthrough initiated a new area of optical science concerning the physics and applications of structured light beams. This orbital angular momentum is an important concept for both classical and quantum science, especially where the framing in terms of angular momentum demystifies some of the quantum properties of light. Loudon's own work (Loudon R. 2003 Theory of the forces exerted by Laguerre-Gaussian light beams on dielectrics. Phys. Rev. A 68, 013806. (doi:10.1103/PhysRevA.68.013806)) in this area focused on the interactions between light and matter where the orbital angular momentum extended his studies from linear impulses to rotational torques.This article is part of the theme issue 'The quantum theory of light'.

This work provides a mathematical derivation of a quasi-stationary (QS) model for multimode parametric down-conversion (PDC), which was presented in Gatti et al. (Gatti et al., Sci. Rep. 13, 16786) with heuristic arguments. Here, the model is derived from the 3D + 1 propagation equation of the quantum fields in a nonlinear crystal, and its approximations are discussed thoroughly. Thanks to its relative simplicity, and to the fact that it is valid in any gain regime, both at a quantum and classical level, it allows a unified description of disparate experimental observations conducted over the last 20 years, often described in the past by means of limited ad hoc models.This article is part of the theme issue 'The quantum theory of light'.

Quantum entanglement describes superposition states in multi-dimensional systems-at least two partite-which cannot be factorized and are thus non-separable. Non-separable states also exist in classical theories involving vector spaces. In both cases, it is possible to violate a Bell-like inequality. This has led to controversial discussions, which we resolve by identifying an operational distinction between the classical and quantum cases.This article is part of the theme issue 'The quantum theory of light'.

It has been shown that measurements involving indefinite causal order can be superior to those in which a sequence of operations occurs in a specified order. In optics, such measurements are realized naturally in a Sagnac interferometer. We show that such an arrangement can measure the solid angle (on the Poincaré sphere) enclosed by a sequence of unitary transformations of the polarization. This is the Pancharatnam-Berry phase. Extension from the classical or single-photon treatment to a fully quantized treatment allows the analysis of the interferometer for arbitrary quantum states of light.This article is part of the theme issue 'The quantum theory of light'.