Like any classical computer, a quantum computer requires a set of basic gates that perform the elementary logic operations.  Many of such gates can then be combined to perform the complex tasks of the full computation.  Physicists working on a quantum computer -- a computer that exploits inherently quantum mechanical properties of nature that makes it much faster at certain tasks -- have known for years what gates are required to make up this 'universal set'.  The problem lies in the implementation in a physical system.  One of the most promising candidates for such a system is combining photons, which are very good at carrying quantum information from one place to another, with atomic-like particles, which are very good for switching.  This approach is described by cavity quantum electrodynamics, or CQED.  First explained by Richard Feynman in the nineteen fifties and sixties, the theory explains the interaction of matter with light.

In their paper [1], B. Wang and L.M. Duan propose using qubits encoded in photons.  Two photon states are incident on the two sides of a double-sided cavity. As was shown previously, the coupled dipole transmits and reflects depending on its state - whether it is in the ground or another uncoupled state.  By placing the dipole an in equal superposition of these two states, the resulting flying qubits are swapped depending on the dipole-state.  In this way, a SWAP gate is created.  Such a gate is interesting for quantum fingerprinting and quantum signatures, where the encoded message is condensed into a smaller measurement that serves as a check -- analogous to hash functions in classical communication. 

Interestingly, four instances of the proposed SWAP gate can be combined with a set of single-qubit gates to make a set of universal gates.  This proposal puts forth a relatively simple physical system for making quantum gates with high fidelity, reaching beyond 99%.  The proposal could be implemented in the near future by groups in atomic physics as well as nanophotonics (see for example CVitae group cvitae.org/jvgroup/)