Selected topics on continuous variable qubit in 2010

Our recent research result on quantum superposition control of light wave has been selected as one of major achievements in Optics in 2010, and made the cover of the journal "Optics and Photonics News."

While conventional digital bit takes either "0" or "1", quantum mechanics allows a superposition state where the system can be in "0" and "1" simultaneously. This new kind of digital bit is called "qubit" and offers a new paradigm of super-parallel computation, i.e. quantum computation.

Qubit realizations in photonics so far have mainly based on polarization or "which-path" degrees of freedom of a single photon. Our team could have succeeded in realizing an optical qubit of two orthogonal states of light wave, which lie in the infinite dimensional space associated with continuous variables (quadrature amplitude and phase). This kind of qubit may be referred to as "continuous variable qubit." Such qubits would be useful, when used as local oscillators in optical coherent communication, to realize quantum decoder for the ultimate capacity communication.

This qubit state can be represented by a Wigner function distribution locating at a point in a Bloch sphere, symbolizing the continuous and discrete nature of this new state of light.