Towards Real Applications in Broadband Quantum Memories

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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.