Protecting an optical qubit against photon loss - Konrad Banaszek

Quantum information technologies rely on the ability to prepare and maintain microscopic systems in coherent superposition states. Such states lose their unique properties due to uncontrolled interactions with the environment, which undermines the advantage of quantum technologies. However, these deleterious effects can be dealt with using the method of quantum error correction. In the first part, we present an elementary introduction to quantum error correction using the example of dephasing induced by an attempt to gain "which-way" information in a Mach-Zehnder interferometer. This specific scenario will lead us to general conditions for recoverability from environment-induced errors. In the second part, the general theory is applied to the process of photon loss, which a common deleterious effect in optical systems. We discuss simple codes for protecting one qubit against a single photon loss. We also show that the most natural way to manipulate such codes with the help of linear-optics networks has only a limited applicability.