Generation of two-mode bi-chromatic squeezing and entanglement

A step towards quantum network and quantum communication

Creating entangled optical fields are important for quantum information processing. Traditionally, there are two distinct classes: the discrete variable(DV) approach encodes information in single photons while the continuous variable(CV) approach encodes information in bright optical fields. The discrete method is marked with high fidelity,but it not only requires single-photon detector but also suffers probabilistic generation of photons. On the other hand, the continuous variable alternative is more robust, but optical loss and limited fidelity lead to serious problem too. My project is part of a more grand design of combining DV and CV, and the goal is to build a reliable CV source of paired photons.

One reliable source generating time-correlated photons is the four-wave mixing process. This is a third order nonlinearity process in which two strong optical fields have a pair of photons annihilated while a pair of photons at different wavelength being generated simoutenously.

We choose to work with the double ladder, or as we call the diamond scheme, because the upper transitions do have our desired wavelengths. We are working with a diamond scheme formed by 5S1/2,5D1/2,5D3/2 and 6S1/2 levels,with 5S1/2 to 5D3/2(780nm) and 5S1/2 to 5D1/2(1367nm) as pump, 5S1/2 to 5D1/2(795nm) as probe seed. The 795nm field is amplified at output while a 4th field(conjugate field) at 1324nm is generated, as shown in the picture above.

Our main parameter to major is the "squeezing", a measure of the quantum correlation of the output optical fields. The following picture shows some preliminary results. The y axis shows the "noise power", a measure of how much intensity fluctuation of one beam relative to the other, while the x-axis is the frequency of detection. We can see that the relative fluctuations(red) is smaller than the fluctuation of only one field. This implies that the fluctuation, and from that the photon creations, are timely correlated.