The aim of the project is to fully exploit the potential of photon counting measurements combined with linear optics, homodyne detection, and electro-optic feedforward. We use these tools to generate non-classical non-Gaussian states of light and to implement various non-linear quantum operations on light modes. The project should result in core tools for quantum computing with continuous variables in the long term.

The state of light can be fully characterized by its Wigner function W(x,p) which is a two dimensional quasi-probability distribution of the quadrature values x and p connected with noncommuting quadrature operators X and P, respectively. Amplitude and phase (angle) are often used instead of Cartesian coordinates x and p. The former reflects a power of the optical field and the latter is connected with a number of its periods.

Simple light fields such as the thermal states generated by an incandescent lamp or coherent states generated by a laser are described by Gaussian and positive Wigner functions (left). The Wigner functions for non-classical states, such as the single photon state (middle) or the Schrodinger Cat state (right), show oscillations and even negative values revealing pure quantum interference. These states are therefore referred to as non-classical and non-Gaussian states and serve as resources for quantum metrology and quantum information processing.

                         

Schrodinger cat state (SCS) which is of fundamental interest in quantum optics is given as a superposition of two or more classical coherent states which are indistinguishable and interfere with each other. For small amplitude of constituent coherent states the Schrodinger cat state can be prepared by subtracting a single photon from a squeezed vacuum state. Such small Schrodinger kittens can be combined into one larger cat which yields stronger non-classical properties or they can be used directly for various quantum information processing tasks.