As technological improvements enable the controlled manipulation of smaller elements, the prospects for quantum computing become more realistic. Now researchers have demonstrated a solid-state source of entangled photon pairs, a key component for quantum communication and computing, that may be used with silicon avalanche photodiodes.As currently envisioned, a quantum computer would operate on single particles, carrying information in their quantum states. Performing operations on such quantum bits irretrievably changes the information, however, so one must either compute with the data or access it. This implies that, although a computer could be built that would be incredibly powerful because of the speed and density of its information processing, it also could be useless because it could never accurately read the output.To overcome this difficulty, both in the final and intermediate states of a quantum calculation, scientists suggest the use of particles with entangled quantum states. In this scenario, one particle could be manipulated by a processing element while its entangled partner (or partners) is processed or displayed elsewhere.Compact sources of entangled photons exist, but they produce photons at wavelengths that are measured with relatively noisy germanium or InGaAs avalanche photodiodes. The hardware required to produce entangled states in the 600- to 900-nm wavelength region measurable with lower-noise silicon avalanche photodiodes is bulky and complex.Christian Kurtsiefer, Jürgen Volz and Harald Weinfurter of Ludwig Maximilian University in Munich, Germany, have developed a compact source of polarization-entangled photon pairs that is suited for use with these less noisy silicon detectors. The setup circularizes the beam from an SDL Inc. (now JDS Uniphase) single-mode, 856-nm continuous-wave laser diode and inserts it into a resonant frequency doubler. The resonant cavity of the frequency doubler lies between one curved face of a KNbO3 crystal and a coupling mirror.The coupling mirror is 95 percent reflective at 856 nm but is antireflection-coated at the doubled, 428-nm wavelength. This enhances the pump intensity by a factor of 33, yielding up to 12 mW of second-harmonic output from 125 mW of incident pump power.When tuned for maximum stability, the doubled source provides 6 mW of 428-nm light to a BBO crystal for spontaneous parametric down-conversion. To enhance the pump efficiency by a factor of 13, the BBO crystal is constructed in its own resonator.The net result is an entangled-polarization, 856-nm photon source with production rates that are comparable to those of ion laser systems, but with lower cost and less experimental complexity.