Unlocking the potential of fibre-based single-photon sources

Huge computational power. 100% secure communications. Ultra-precise measurements. All this (and more) can be yours - and all you require is the ability to make photons one-by-one! What could be simpler?

Photons are fundamental particles of light, and there are lots of them about. Billions are entering your eyes every second as you read this. However, in recent years, we have begun to develop the tools and techniques required to generate and manipulate single photons one at a time. Not only has this enabled experiments that demonstrate the counter-intuitive nature of the hidden quantum world underlying the one we inhabit, but also it has heralded a revolution in the way in which we process information and communicate with one another. Unlike the bits of information that are processed by a normal computer or transmitted through the fibre optic networks that make up the internet, single photons have the capability to carry quantum information: a single photon can exist in a superposition state that is both 1 and 0 simultaneously, and two single photons in remote locations can be closely linked through quantum entanglement. These additional capabilities can be applied in three critical areas: computation can be sped up so that tasks that are intractable even for a supercomputer, for example the simulation of complex systems such as new pharmaceuticals, could be carried out easily by a sufficient number of single photons; communications using single photons allows provably secure information transmission and the detection of any attempted eavesdropping; and measurements made with entangled states of light composed of single photons can increase precision beyond that possible with conventional light sources such as lasers.

Significant progress has been made towards achieving these objectives in research laboratories worldwide. However, generating the single photons needed for quantum information processing is a difficult task in itself. Current implementations of small-scale photonic quantum processors are based on single photon sources that are very unreliable - typically these sources function correctly only approximately 1% of the time. This is a significant problem when trying to scale experiments up, something that must be done now in order to run useful algorithms or transmit quantum information at high data rates. These tasks require many single-photon sources each to produce one photon at the same time - this is not possible with the current generation of single-photon sources as the probability of at least one source failing increases exponentially with the number of sources. Hence current state-of-the-art experiments using just four sources must run for days at a time to capture only a few hundred occasions when all four have fired together.

The aim of this project is to demonstrate a method for dramatically improving the performance of single-photon sources and go some way towards solving this scalability problem. We will build an optical fibre network of individual single-photon sources and combine their outputs using a fast optical switch to create one "multiplexed" single-photon source. The multiplexed source will have significantly enhanced performance relative to an individual source while retaining all its favourable characteristics such as room-temperature operation and ease of use. Furthermore, the multiplexed source will form the building-blocks of a future ultra-high-performance source: theoretical studies have shown that a few of these blocks together provide the required resource for almost perfect single-photon source operation.

Quantum technologies have the potential to transform completely information and communications technologies during the next 50 years, developing a new industrial sector with significant economic impact. The UK is well-placed to lead this effort but in order to realise the full potential of quantum technologies we must develop the tools required. This project addresses the urgent need for higher-performance single-photon sources. The lack of deterministic single photon sources is a major challenge that stands in the way of scalable photonic quantum information processing. Surmounting this challenge would radically alter how all end users (industrial, commercial, government, military, general public) transmit and process information. Therefore, in the broadest sense, the outcomes of this research could have far-reaching impacts on the whole of society.

Three disruptive photonic quantum technologies stand to benefit from improved single photon sources:
1. Quantum computation using single photons can yield exponential speedup relative to a classical computer for tasks including factorization, database searches, simulation of complex systems, and matrix inversion. Algorithms using these processes are ubiquitous in modern computing, and the improvement in performance from a quantum computer of sufficient capability would enable calculations to be performed that are not possible with conventional computing resources. Such capability requires many single photons to be delivered from separate sources simultaneously and would have impact in areas as diverse as data security, drug discovery, internet searching, and advanced computer simulations.
2. Quantum key distribution allows a cryptographic key to be shared that is provably secure against eavesdropping, either using single photons or entangled pairs of photons. This results in communications protocols whose security is guaranteed by the laws of quantum mechanics. Higher key-generation rates require improved single-photon sources; these systems are of interest in telecoms wherever information security is paramount, for example banking, the security services, and commerce.
3. Quantum metrology uses multi-photon entangled states of light to measure optical phase changes (and thereby any related quantity such as changes in distance, time delay, refractive index, thickness, etc) with a precision greater than that which can be obtained with classical light. Again, the entangled states required to implement these measurement protocols can be formed from large numbers of single photons delivered simultaneously by separate sources, and can be applied to a wide range of sensing applications from biological imaging to gravitational wave detection.

In addition to these wide-ranging technological impacts, quantum mechanics generates significant enthusiasm and interest in the general public: everyone has heard of Einstein, even though only those with a scientific education will appreciate his misgivings about quantum theory! The counter-intuitive features of quantum entanglement, the measurement paradox, and Heisenberg's Uncertainty Principle naturally generate excitement and discussion. This project will maximise the impact of the proposed research on the public as detailed in the Pathways to Impact document.

During the course of this project a PhD student (fully funded by the University of Bath on a specific studentship linked to the student and the PI, beginning in October 2012) will be working in the same laboratory on tasks closely aligned with the goals of the proposal. This project will aid the development of the student into a highly skilled researcher, crucial to the scientific future of the UK. In addition, there will be associated undergraduate research projects run alongside this project that will contribute to the skills development of the next generation of scientists.