We present a scheme for the direct generation of four-photon states using photon pairs having minimal spectral and spatial entanglement. The method does not require polarization-entangled pairs, and can be generalized to larger states.
A growing number of experiments make use of multiple pairs of photons generated in the process of spontaneous parametric down-conversion. We show that entanglement in unwanted degrees of freedom can adversely affect the results of these experiments. We also discuss techniques to reduce or eliminate spectral and spatial entanglement, and we present results from two-photon polarization-entangled source with almost no entanglement in these degrees of freedom. Finally, we present two methods for the generation of four-photon polarization-entangled states. In one of these methods, four-photon can be generated without the need for intermediate two-photon entanglement.
Current photon entangling schemes require resources that grow with the photon number. We present a new approach that generates quantum entanglement between many photons, using only a single source of entangled photon pairs. The different spatial modes, one for each photon as required by other schemes, are replaced by different time slots of only two spatial modes. States of any number of photons are generated with the same setup, solving the scalability problem caused by the previous need for extra resources. Consequently, entangled photon states of larger numbers than before are practically realizable.
This article presents a proposal for preparing photon pairs in states that are entangled in their spatial modes. The method allows the encoding of any pair of spatial modes that is desired, without restrictions; a procedure that up to now has been elusive. This method consists of three steps: preparing photon pairs in polarization-entangled states, filtering the spatial mode, and use of polarization interferometers with diffractive mode-encoding elements to effect entanglement swapping between polarization and spatial modes. An extension of the method consists of entangling polarization and spatial mode, allowing the preparation of a 4-qubit cluster state of two photons.