Quantum Networks & Control

Quantum communication networks consist of a distributed arrangement of nodes (quantum devices such as repeaters and other processors) linked by quantum channels (through which quantum information flows).

Quantum devices and networks are described by open quantum models (systems that interact with external channels such as optical fields that may carry quantum information). These open models are characterized by parameters describing the internal energy of the system as well as how energy (and information) flows into and out of the system. Quantum networks may be represented in terms of device parameters by simple algebraic rules. For example, the series product may be used to describe the cascade connection of two devices, where the output of one device is fed into the input of another. Tools such as these allow for the development of systematic methods for analysis and design of quantum communication networks that integrate memories, repeaters and transmission channels.

Feedback control of a system (often called the ‘plant’) is achieved by connecting the plant to a device, called the controller, which is designed to achieve desired behaviour (such as keeping the plant in a desired state). When the controller is a classical system, which can only process classical information, some form of measurement of the quantum plant is needed, and so this is called measurement feedback. It is also possible to use another quantum system as the controller. This second type of feedback does not use measurements, and the information exchanged between the plant and the controller is fully quantum. This is called coherent or quantum feedback control. Feedback control systems may be considered as examples of quantum networks. The QNC project will develop feedback control systems for quantum information systems.