Quantum key distribution systems promise to enable unbreakable codes for the transmission of sensitive data. A group at Toshiba Research Europe Ltd. and Cambridge University, both in Cambridge, UK, has reported the development of a crucial element for such systems, an electrically driven single-photon emitter.By introducing quantum dots into the intrinsic region of a GaAs PIN photodiode, researchers have developed an electrically driven source of single photons for quantum cryptography. The InAs quantum dots (right) displayed an areal density of approximately 5 x 108/cm2. An aperture in the ohmic contact on the device ensures that only the emission of a single dot escapes the structure. Courtesy of Andrew J. Shields.Up to now, most experimental quantum cryptography systems have employed an attenuated pulsed laser as a light source. Improved systems use parametric down-conversion of injected laser pulses in a nonlinear crystal to generate a gated source of single photons. The problem with such setups is that the production of two-photon pulses by the laser can be reduced but never eliminated -- yielding extra photons that may be interrogated by an eavesdropper to intercept the code key."Even a very low rate of two-photon pulses, similar to that used in previous demonstrations of quantum key distribution, renders the technique completely insecure," said Andrew J. Shields, quantum information group leader at Toshiba. "Thus, developing a true single-photon source is a vital step towards realization of a secure communication system."For their source, the research team employed quantum dots in a conventional semiconductor structure because the dots confine electron-hole pairs in three dimensions, enabling single-exciton recombination (and single-photon emission) at a low injection current. Essentially a GaAs PIN photodiode grown by molecular beam epitaxy, the emitter incorporates InAs quantum dots in the intrinsic region. An aperture in the ohmic contact ensures that only the emission from a single dot can escape.Although the emission wavelength of the dots in the experiment was selected to fit the response of the silicon avalanche photodiodes used to test the device, InAs dots can produce 1300-nm photons, suggesting that key distribution systems could be deployed over long-haul fiber links. In May 2000, the team reported the development of an InAs quantum dot single-photon detector for such an application.A challenge will be developing an emitter that operates at room temperature or with thermoelectric cooling. The device used in the experiment functioned at 4 K, Shields noted.True single-photon production had been realized in optically excited quantized systems, but an electrically driven emitter carries significant benefits.Mass production possibleFirst, it avoids the alignment issues associated with optical pumping. Second, the new emitter is based on established LED technology, which makes economical mass production possible."If we are successful, this could make single-photon sources and their applications commonplace," Shields said.