Towards Real Applications in Broadband Quantum Memories

Imagine a banknote that cannot be forged, because the serial number is scrambled every time someone tries to read it. But if you are the banker, you can read it. Sounds like Harry Potter? Imagine a computer that predicts how drugs will behave by simulating all possible chemical reactions at once! This is not an idea from Phillip Pullman's fantasy of parallel universes. Real technologies like this are just around the corner.

This is the fascinating, counter-intuitive world of quantum physics. Huge advances in communications and computing technology over the last several decades have made this the information age and changed the way people live and interact even more drastically than did the industrial revolution. These advances have piggy-backed on the development of devices such as semiconductor transistors and lasers, devices which wouldn't be possible without the weird properties of quantum physics.

But although modern computers have far-outstripped the early technology of punch cards and vacuum tube valves, at an underlying conceptual level, they still use exactly the same type of information - strings of 0s and 1s called bits. Quantum physics will allow us go far beyond this into the strange world of quantum information, where the "quantum bits" can be both 0 and 1 simultaneously! Computers that could work with this sort of information would be exponentially faster at performing difficult simulations or cracking codes. And communicating using quantum information can be made "eavesdropper proof" - perfectly secure.

Over the past ten years, an enormous research effort has brought these extraordinary technologies from abstract ideas to small-scale experiments. One of the most promising ways to build a quantum computer is based on single particles of light, called photons, which can be sent over long distances in optical fibres and manipulated with ordinary lenses and mirrors. But like normal computers, quantum computers need memories to be able to synchronise different parts of a computation by storing the quantum information until it is needed. So to build a photonic quantum computer, we also need to have a quantum memory that can store single photons. What makes this difficult is that these special memories need to be able to store the fragile quantum information without destroying or even "looking" at it (measuring it).

In this project, we will develop a quantum memory for photons which can store short pulses for long times with high efficiency and very low noise. To do this, we will use a "Raman memory", an approach pioneered in our group which uses a strong laser pulse to cause the photon to be absorbed by a sample of atoms which is normally transparent. Because the absorption is created by the strong laser (which is not absorbed), there is no noise from excited atoms, and the atoms don't need to be specially prepared by cooling them or trapping them.

The simplicity of our design will allow us to build the first practically feasible memory, which would even potentially be capable of operating in isolated, harsh environments, such as on the ocean floor. This will also allow us to perform novel photonics experiments which are too complex to operate without the memory. We will also develop a miniaturized memory that could be mass-produced and integrated with existing telecoms fibres. Such a device will do for quantum photonics what the transistor did for conventional electronics.

Quantum memories will open the way to a new era of quantum enabled devices, with super-fast computers, perfectly secure communications and ultra-precise measurements. Our research is the key to bringing these truly magical technologies to life.

Quantum memories are vital to quantum networks. If quantum communications make up just 3% of the UK telecoms industry alone in the future, quantum memories will be the enabler of a market worth over a billion pounds annually. Quantum memories are also crucial if LOQC is to become viable. Key to the development of quantum computers, good quantum memories would pave the way for a novel and disruptive industry with global impact. Quantum computers offer exponential speed-ups over classical devices, perhaps most notably in the solution of systems of linear equations. This operation is so ubiquitous in numerics that nearly all computations would enjoy exponential acceleration. The practical and commercial benefits of realising such a device justify research on quantum memories as a matter of urgency.

These possible applications illustrate that quantum memories could underpin a technology revolution which will have a transformative impact on our ever-more-information-based society, both economically and in changing the way we live. The key road-block to the realization of this potential is practicality: to date, the most successful quantum memories rely on prohibitively complex apparatus such as atom traps and cryogenics. By contrast, the proposed project will develop a technically simple quantum memory requiring no cryogenics or complex alignment which is compatible with integrated optics using state-of-the-art solid-state or hollow-core waveguide technologies. Even now our memory provides competitive or world-leading performance by comparison with other memories. But technical simplicity will be crucial for any memory to support a scalable QIP architecture, and our memory is by far the simplest protocol. We are currently the only research group worldwide pursuing this approach, which places us in a prime position to capitalize on intellectual property and industrial development possibilities. The economic and societal benefits to the UK of being in a leading position for developing the key enabling device for quantum computation and communication will be profound.

The first-stage beneficiaries will be those in academia, but this will have some indirect non-academic benefits. For example, the question of building an industry-compatible memory has to date mostly been overlooked. Addressing this issue is a key aim of our research and our publications will stimulate the research community in moving towards a more operational approach. This will build momentum towards realising a global quantum network, with important impacts outside the academic community.

The second-stage beneficiaries include non-academic industrial and commercial sectors, and indeed society as a whole. Firstly, global quantum networks would provide unconditionally secure communication based on quantum key distribution. Since most transactions are processed over the internet (eg, banking, retail, stock-market, etc), this technology will revolutionise security for nearly all monetary transactions and would have an enormous impact on the global economy. With cybercrime and identity theft now threatening everyone, this would have a tangible impact on people's quality of life. Further, unconditionally secure communication would have a major impact on the effectiveness of intelligence services in combating terrorism.

Secondly, quantum computers enabled by our quantum memories would have a great impact in many fields. For example, quantum simulations would transform methods of research and commercialization in the health, pharmaceuticals and green energy energy sectors. It would assist epidemiology and genetic research, cut costs in drug design, and help improve alternative energy sources, for example by permitting simulations of efficient photosynthesis. Indeed, it will have an impact in any area requiring innovation at the molecular scale, where classical computers are ineffectual and time-consuming, expensive empirical testing is currently the only option.