Practical creation and detection of polarization Bell states using parametric down-conversion

The generation and detection of maximally-entangled two-particle states, `Bell states,' are crucial tasks in many quantum information protocols such as cryptography and teleportation. Unfortunately, they require strong inter-particle interactions lacking in optics. For this reason, it has not previously been possible to perform complete Bell state determination in optical systems. In this work, we show how a recently developed quantum interference technique for enhancing optical nonlinearities can make efficient Bell-state measurement possible. We also discuss weaknesses of the scheme including why it cannot be used for unconditional quantum teleportation.