Building Large Quantum States out of Light

We aim to build the world's biggest quantum photonic network, in which up to twenty photons, elementary particles of light, are connected to produce a large, controllable quantum system. This new tool will open up important realms of physics that have been too complex to study conventionally, such as biological energy transport and high-temperature superconductivity. Since photons are used to transport information, the network will also form a platform for revolutionary new quantum technologies like ultra-precise sensing and guaranteed-secure communication across the globe. To achieve such a large quantum system, we will introduce new techniques that fundamentally change the scalability of photonics. This will lay the ground for even larger networks in the future, establishing the UK as a leader in the nascent quantum technology industry.

We have known for over a hundred years that atoms and molecules don't move according to Newton's laws. Instead, they obey the laws of quantum mechanics. These laws are strange but they explain how chemical bonds form and why silicon chips can make computers. These insights drove a profound technological revolution in the 20th century, spanning extraordinary advances in medicine, telecoms, and computing. It is now clear that our current knowledge of quantum systems is just the tip of the iceberg. While we can understand quantum effects between just two particles exactly, or between many atoms in an approximate way, as is the case for a semiconductor transistor, large objects composed of many particles cannot be analysed in detail. They are too complicated, and in fact beyond a few atoms, they cannot even be simulated with a supercomputer. The problem is that quantum systems are fuzzy, in a sense, so each particle is a distribution, not a single point. To describe many particles requires distributions of distributions of distributions and so on. This explosion in complexity means that many interesting systems in nature - in biology and medicine, particle physics and materials science - have so far been largely closed to analysis. The only way to study complex quantum systems in detail is to build a machine that can create them in a tailored, controllable way, so that we can build models of the real systems we want to study.

Over the past two decades, a new science of quantum information has developed. In addition to their application to problems in the natural sciences, it has been shown that large controllable quantum systems can underpin a host of transformative new technologies, including the possibility of quantum computers that are exponentially faster than today's best computers. Perhaps surprisingly, one of the most advanced approaches to quantum computation involves photons instead of atoms. Photons can easily be transported by optical fibres, which are a mature technology used for telecoms and the internet, and they experience almost no noise. Because of these advantages, optical quantum cryptography over short distances is already commercially available.

To go further and realise the most ambitious goals of quantum information science, and to open up the investigation of complex quantum systems, many photons must be connected and precisely manipulated. We aim to meet this challenge by leveraging advanced fabrication methods developed for the modern telecoms industry to build a large-scale controllable quantum photonic network, at the level of around twenty photons. In particular, we will use silica integrated optics -- circuits for light written on small glass chips -- to connect photons with minimal losses. These will be joined to superconducting detectors that count photons with high efficiency, and novel quantum memories that can store photons and synchronise the network. Combining quantum memories with these highly efficient technologies will enable the network to operate with at an unprecedented scale, giving access to new physics and new technologies.

Information is at the heart of modern society. From talking with friends across the world, shopping on to the internet, accessing an ATM, and even using a checkout at the grocery store, information is critical to everyday life.
All current information technologies - communications, sensors, computers, imaging devices - are designed using principles of classical physics. Yet the world is at its core described by quantum physics. The emergence of quantum information science has shown how new design principles for IT based on this theory can deliver radically new and different solutions for information processing: superfast computers, able to break the strongest known secret codes; completely secure communications systems; ultra-precise measurement and imaging. The current project will provide a demonstration of a first generation quantum communications and sensing network.
The project and its outcomes will therefore impact on:

(a) Society, by providing a template for new information technologies to enhance our quality of life by providing powerful new computers and secure data handling

(b)The Economy, by demonstrating a new kind of information processing, that requires new machines. The IT sector is an immensely successful and important part of the UK economy, with turnover in excess of £140bn annually;

(c)Knowledge: both academic and commercial, as the new regimes of physics afforded by the research become accessible for discovery;

(d) People, through the new understanding, technical expertise and skills developed by the researchers during the project, including training in engagement with the media, the public and policy makers.

In addition to academic benefits, the outcomes of the research will be of value to UK and global commerce, the general public and government. This will happen in the following ways:

(i) Commerce: The development of new information technologies has always led to economic impact. From Babbage's Difference Engine, Flowers' Colossus and Mauchly and Eckert's ENIAC grew Apple - the world's largest company. Although those computer pioneers could not foresee this impact, it is fair to argue that the breakthrough in digital computation they instigated has radically transformed the economy.

(ii) The Public: Everyone, whether they live in highly industrialized countries, or in the developing world, is dependent on access to information that is increasingly delivered by new technologies. Radical changes in the technology itself will foment equally radical changes in how it is used. Consider, for instance, how cellular telephony has transformed society in the last 20 years. Similar changes will happen as quantum technologies become embedded. Beyond this, however, the science associated with large-scale quantum states is something that captures the imagination. Schrödinger's Cat is not simply a metaphor in physics, but a re-conceiving of the natural world. This is something for which there is a great appetite.

(iii) Government: The security of society is the responsibility of governments and their agents. This responsibility ranges from the security of patient data in the NHS and national defense to secure communications for the smart grid. Regulation must be informed by the capabilities of current and future technology, so that legislation can be effective. Therefore it is critical to explore the boundaries of what is possible so that what it may, or may not, deliver is evident. For example, the US National Security Agency expects its currently encrypted data to be secure for 20 years. What if a quantum computer that can crack the RSA code appears in 15 years time? How will the security of back data be guaranteed, and what steps should be taken now to ensure this?

Even a quantum network that is in modern terms as nascent as Babbage's and Flowers' clunky mechanical computers will open the way to demonstration of a radical information technology of transformative power.