Recent progress in quantum information processing highlights the fundamental necessity for a reliable single photon source that exhibits a well-defined single-mode character and well-defined photon numbers. A very high degree of photon-number correlation is vital for linear optical quantum computation [Knill2001] and quantum communication [Browne2003] including networking via quantum repeaters; lack thereof implies the need for post-selection, which precludes scaling. Moreover, both aspects of quantum information processing can be enhanced by exploiting the "non-Gaussian" photon statistics associated with states containing higher photon numbers.
The generation of single photons on demand can be accomplished by several means. Singleemitter sources typically provide emission on demand, but into modes that are not well-matched to detectors, and not in pure states. Consequently the photons are detected randomly. On the other hand, heralded sources based, say, on parametric downconversion, have random emission events, but the heralded photon can be detected with high probability.
We have developed a novel source of conditionally-prepared single photons based on PDC that overcomes and number of barriers to pure single photon wavepacket generation.[Uren2004] First, waveguiding leads to emission in well-defined modes, which significantly enhances photon collection efficiencies, leads to higher generation rates due to mode confinement, removes space-time coupling and allows high-visibility interference between single photons from distinct sources. The photons are easily separable, and have precise timing, paving the road towards the concatenation of multiple waveguides for quantum networking. We have demonstrated a conditional detection efficiency of 51.5% at a brightness of 0.85 million coincidences/(s mW).
Another important technology for LOQC and state preparation is a photon number resolving detector. We have implemented a time multiplexed detector (TMD) capable of photon number resolution [Achilles2003], built from standard optical elements, and which allows both source verification and higher-order conditional state preparation. We have used this device to prove the nonclassical nature of the heralded single photon source.
[Uren2004] A.B. U'Ren et al., quant-ph/0312118, to appear in Phys. Rev. Lett. [Knill2001] E. Knill, et al., Nature 409, 46 (2001) [Browne2003] D.E. Browne, et al., Phys. Rev. A 67, 062320 (2003) [Achilles2003] D. Achilles et al., Opt. Lett. 28, 2387 (2003)