Wavelength-multiplexed quantum networks with ultrafast frequency combs

Highly entangled quantum networks (cluster states) lie at the heart of recent approaches to quantum computing(1,2). Yet the current approach for constructing optical quantum networks does so one node at a time(3-5), which lacks scalability. Here, we demonstrate the single-step fabrication of a multimode quantum resource from the parametric downconversion of femtosecond-frequency combs. Ultrafast pulse shaping(6) is employed to characterize the comb's spectral entanglement(7,8). Each of the 511 possible bipartitions among ten spectral regions is shown to be entangled; furthermore, an eigenmode decomposition reveals that eight independent quantum channels(9) (qumodes) are subsumed within the comb. This multicolour entanglement imports the classical concept of wavelength-division multiplexing to the quantum domain by playing upon frequency entanglement to enhance the capacity of quantum-information processing. The quantum frequency comb is easily addressable, robust with respect to decoherence and scalable, which renders it a unique tool for quantum information.