Abstract

Color, or frequency, is one of the most familiar degrees of freedom (DOFs) of light and has been routinely analyzed in spectroscopy for centuries. However, it has been used relatively little for quantum technologies. Entangled qubits are a key ingredient in many such technologies. For other optical degrees of freedom discretely entangled states have been extensively investigated, e.g. [1–5]. In contrast, discrete frequency entanglement has not yet been unambiguously demonstrated, despite potentially interesting applications such as enhanced clock synchronization beyond the classical limit [6,7] and improved quantum communication in noisy channels [8]. Moreover, quantum information encoded in discrete, tunable frequency bins of photons would be an ideal mediator between stationary qubits with different energy levels. For many of those applications it is essential, that the color-entangled stated can be directly created – in contrast to spectrally post-selecting them from broadband continuous spectral entanglement [9,10]. So far, no experiment was able to achieve this.

© 2009 IEEE