Putting Chaos to Work: Multi-Photon Entanglement in Complex Scattering Media

Long considered one of the big mysteries of quantum physics, entanglement has captured the imaginations of scientists and philosophers alike. Entangled quantum particles behave in a perfectly correlated way, regardless of how far apart they may be! First studied with polarised particles of light called photons, entanglement has today been been demonstrated with individual atoms, superconducting circuits, and even small diamonds. While entanglement certainly tells us something about the strange, counterintuitive ways in which nature behaves, it has recently emerged as a cornerstone of modern quantum technologies that promise unbreakable encryption, ultra-sensitive imaging, and yet-unheard-of computing power. From complex quantum simulators to large-scale quantum cryptographic networks, entangled photons will play a role in almost every future technology based on quantum physics.

Generating an entangled state of many photons remains an extremely demanding task in quantum optics. To create even the simplest such states, multiple pairs of entangled photons are intricately combined via a series of optical elements such as mirrors and beam splitters. As the complexity of these states is increased, the network of elements required to create them also becomes rather large. Each element in such a network must also be precisely controlled, which presents a difficult challenge to quantum physicists.

When a beam of light impinges on a layer of paint or a sugar cube, it undergoes millions of reflections and transformations. Such everyday objects are surprisingly analogous to a complex network of optical elements, with the difference being that any information transmitted through them is usually lost. In recent years however, advances in technology for shaping the wavefront of light, combined with fast computational algorithms have resulted in unprecedented control over how light propagates through such disordered media. Using these techniques, scientists have achieved remarkable feats such as sending an entire image down an optical fibre the thickness of a human hair!

In this project, I am proposing to harness the potential of disordered media as miniature "quantum optics laboratories" for generating, manipulating, and transporting large, complex entangled states of light. By carefully controlling the quantum states of photons entering such media, the millions of scattering events that would normally scramble their quantum information can be put to work for manipulating it instead! In this manner, complex scattering media can be made to serve the same function as large networks of quantum optical elements, while overcoming the problem of control and scalability that normally plague such networks.

In my research, I will focus on a specific type of scattering medium called a multi-mode fibre (MMF), which is commonly used in high-speed internet connections. MMFs have certain unique advantages. First, they are cheap, compact, and readily available. Second, due to their natural application in optical communications, they can be used not only for creating entanglement, but also for transporting it. This will allow me to vastly expand the information capacity of modern quantum cryptographic systems and develop practical techniques for supersensitive quantum imaging deep inside biological tissue! Finally, the generation of large, multi-photon entangled states will help me further push the limits of quantum mechanics and gain a better understanding of the complex dance of correlations that is entanglement.

This research programme will result in the development of quantum technology that will have a direct impact on our society and economy. The internet permeates our everyday lives, from messaging our loved ones to shopping for dog food. The need for ever-present data security is thus apparent, as we become more and more reliant on electronic means of communication and business. Quantum physics offers the promise of unhackable encryption techniques whose security is based on the laws of physics, as opposed to difficult codes that can be cracked. Modern quantum cryptographic systems (QKD) traditionally use a "qubit" encoding that allows them to send one bit of information per photon. My research programme is going to deliver QKD systems that can encode a large alphabet on a single photon, while simultaneously providing a higher degree of security than before. These quantum encryption systems will benefit the public in providing a means of communication that is faster and more secure than the current state-of-the-art.

The ability to image inside the body through bundles of fibres has revolutionised the field of medicine, allowing quick medical diagnoses and non-invasive surgical procedures. Quantum-entangled light allows us to create images of objects with a sensitivity below what is allowed classically. My research programme will develop ways to perform reduced-noise quantum-imaging techniques through fibre, enabling the design of a quantum-enhanced endoscope for supersensitive, minimally invasive imaging inside biological tissue. This will benefit the public as well as doctors through better methods for medical diagnostics and procedures. The same technology can be applied in industry for supersensitive imaging of hard-to-reach places in a complex machine or building, which will directly benefit the engineering diagnostics industry.

The development of a compact platform for multi-mode quantum photonics through my research will benefit the nascent quantum technology industry that is slowly emerging in the UK. The EU has recently confirmed the launch of a 1 billion euro flagship programme on quantum technologies which places an emphasis on the development of precisely such an industry, in order to remain competitive with the resources that US industry has already put in this direction (Google, IBM). My research programme endeavours to shrink complex laboratory setups to the size of a cheap, multi-mode fibre. This will benefit the economy in providing an affordable method for scaling up entanglement generation techniques, which are crucial for the future quantum communications and computing industry. Moreover, the specific advance in multi-photon entanglement generation will enable the development of multi-user quantum networks, and bring us closer to the realisation of a quantum internet.