We propose a single optical photon source for quantum cryptography based on the acoustoelectric effect. Surface acoustic waves ͑SAWs͒ propagating through a quasi-one-dimensional channel have been shown to produce packets of electrons that reside in the SAW minima and travel at the velocity of sound. In our scheme, the electron packets are injected into a p-type region, resulting in photon emission. Since the number of electrons in each packet can be controlled down to a single electron, a stream of single-͑or N-͒ photon states, with a creation time strongly correlated with the driving acoustic field, should be generated.PACS number͑s͒: 42.50. Dv, 72.50.ϩb, 78.60.Fi, 73.40.Kp Single-photon states at optical frequencies provide an ideal resource for quantum communication and information processing ͓1͔. The recent demonstration of teleportation ͓2͔ using entangled photon pairs generated by parametric downconversion is an example. In addition, there are proposals and experimental schemes to use single photons for quantum computation ͓3,4͔, but the most important and experimentally relevant application of single-photon sources is in secure optical communications using quantum cryptographic protocols ͓5͔. No true, unconditional, single-photon sources exist at optical frequencies, so other sources are used. Parametric down-conversion is used in conditional schemes where detection of one member of the pair determines the space-time location of the other. Highly attenuated coherent light approximates an unconditional source and is used for current optical implementations of quantum cryptography. However, weak coherent states may contain any number of photons, with an average of less than one, rendering present quantum cryptographic implementations insecure ͓6͔. At the single-photon level, the statistical fluctuations of an electromagnetic field can be described in terms of the variation and correlation in the detection times of individual photons ͓7͔. The distribution of detection events in a counting time for a coherent field is described by a Poissonian distribution about the mean. A thermal field has super-Poissonian fluctuations that correspond to the detection events tending to occur together or ''bunched.'' Light with sub-Poissonian noise is nonclassical, and has detection events with a more regular spacing-the photons are ''antibunched.'' Fluctuations cannot be reduced below the classical, or shot-noise, level using any conventional optical components, but can be reduced by a method known as squeezing ͓8͔. However, this has so far produced only small effects and seems to have limited application. The absolute limit for sub-Poissonian light is equally spaced detection events, corresponding to highly correlated emission events in the source. For such light, the intensity fluctuations are zero, although the phase fluctuations are maximal.The suppression of noise on a current of electrons is relatively easy to achieve since the strong Coulomb interaction between electrons assists in regulating electron flow and con...
