Quantum communication with coherent states of light
Professor Gerd Leuchs, Max Planck Institute for the Science of Light, Germany
Quantum communication offers long-term security especially relevant for government and industry users. On a very fundamental level the security of quantum key distribution relies on the non-orthogonality of the quantum states used. So even coherent states are well suited for this task, the quantum states that describe the light generated by laser systems. Continuous variable quantum key distribution with coherent states uses a technology that is very similar to the one employed in classical coherent communication systems, the backbone of today's internet connections. Here we review recent developments in this field in two regimes: (1) improving QKD equipment by implementing front-end telecom devices and (2) research into satellite QKD for bridging long distances by building upon existing optical satellite links.
In optical fibre systems continuous variable quantum cryptography reaches GHz speed and offers efficient integration with commercially available telecommunication techniques, especially in short links within an inner-city or a data centre. Compact and efficient sending and receiving devices can be made of integrated optical components including quantum random number generators.
Optical free space communication is a reliable means to transmit classical and quantum information. Free space links offer ad-hoc deployment in intra-city communication, air-to-ground or satellite-to-ground scenarios. In quantum communication the experimental effort has so far been devoted mostly to discrete variables such as the polarisation state of single photons. We present experiments investigating free space transmission of quantum continuous variable states using homodyne measurement, rendering the quantum states immune to stray light and enabling daylight operation. Quantum communication with satellites offers a viable solution to bridge long distances. We will discuss our current joint project with Tesat Spacecom / Airbus in the development of quantum key distribution with coherent optical communication in satellite systems.
Noise: the fundamental obstacle for scalable quantum information systems
Professor Kae Nemoto, Japan National Institute for Informatics, Japan
One of the fundamental challenges in the development of quantum technologies is to control noise in these quantum systems. A quantum system always suffers from noise due to unwanted interactions with the environment and imperfect controls. The noise effects on each component of our quantum system tend to accumulate, and it is impossible without noise control for the system to maintain its quantum coherence required for quantum information processing. The control of noise is hence essential for one to realise scalable quantum information systems. In this presentation, we show how we can overcome noise to create a scalable quantum information system both in terms of its size and running time. To achieve this, we consider an approach in which both of the basic device and system architecture are designed together. Such a device can be realised using a single NV centre embedded within an optical cavity. We show how this device as a fundamental module can be used to construct a scalable quantum information system.
Quantum communications in telecom networks
Dr Andrew Shields FREng, Toshiba Cambridge, UK
Secure communications based on quantum cryptography is already employed to transmit highly sensitive medical or financial data. Expanding the range of applications to more widespread use will require a dramatic reduction in the cost of deploying the technology. Central to this challenge is the requirement to avoid expensive dedicated dark fibre for the quantum channel and rather to send the qubits in the ordinary data-carrying network.
Here I review progress on integrating quantum communications into conventional telecom infrastructures. Recent work demonstrates it is possible to send qubits along optical fibres which are simultaneously carrying the high data bandwidths found in modern communication systems. This allows a high bandwidth quantum encryption system to be realised, in which multiple 100 Gb/s data channels are secured by quantum keys transmitted along the same fibre.
Deployment costs can be greatly reduced in conventional fibre optic communications using point to multi-point links to connect many customers to a common point in the network through shared fibre. This concept can also be applied to quantum communications, allowing multiple users to share the fibre and equipment costs. Indeed recent studies show that quantum key distribution can be combined with data transmission in GPON access networks able to connect up to 128 users.
Finally, I will discuss plans to implement these technologies in a large-scale quantum network in the UK.