Photonic cluster states are fabricated within the internal structure of a multimode frequency comb. Projective measurements combined with ultrafast pulse shaping allow the creation of arbitrary cluster states with no change in the optical footprint.
Entanglement is an essential resource at the heart of quantum information processing and quantum metrology. Moreover, multimode entanglement is necessary for the fabrication of complex quantum networks, in which entanglement is distributed among several quantum modes. The nodes of the network are made from quantum modes, which are the fundamental information carriers, and these nodes are interconnected by a series of channels through which the information flows. Multimode entanglement is traditionally viewed as many-body correlations, in which the nodes are spatially separated and can be measured individually. In our multimode resource, all of the modes travel together within a single laser beam [1]. This co-propagation of modes enables us to construct on-demand quantum networks via arbitrary shaping within the measurement process.
Cluster states, in which entanglement is distributed amongst numerous parties, have attracted interest due to their role as a resource for measurement-based quantum computing. The traditional means for fabricating these highly-entangled states has been to sequentially combine the squeezed outputs from individual optical parametric oscillators (OPO) with a network of phase-shifters and beamsplitters. While successful, this approach demands an optical footprint commensurate with the dimensionality of the produced state. Within the continuous-variable regime, a promising alternative is to begin with a source that is intrinsically multimode. Toward that end, the frequency comb underlying femtosecond optical pulses contains upwards of ~ 105individual frequencies, which makes it an attractive starting point for the production of multipartite entanglement. This work demonstrates the ability of OPOs synchronously pumped by a femtosecond pulse train to create scalable, adaptable multipartite photonic states in a single beam while exploiting femtosecond pulse shaping to perform a complete state analysis.
We present a detailed analysis of the multimode quantum state embedded in an optical frequency comb generated by a synchronously pumped optical parametric oscillator (SPOPO) [J.Roslund, R. Medeiros de Araújo, S. Jiang, C. Fabre, and N. Treps, Nat. Photon. 8, 109 (2014)].The full covariance matrix of the state is obtained with homodyne detection where the local oscillator is spectrally controlled with pulse-shaping techniques.The resulting matrix reveals genuine multipartite entanglement.Additionally, the beam is comprised of several independent eigenmodes that correspond to specific pulse shapes.The experimental data is confirmed with numerical simulations.Finally, the potential to create continuous-variable cluster states from the quantum comb is analyzed.Multiple cluster states are shown to be simultaneously embedded in the SPOPO state, and these states can be revealed by a suitable basis change applied to the measured covariance matrix.