QND Detectors
We are working towards realizing the first quantum non-demolition (QND) measurement of the photon number of an optical pulse, which will be a revolutionary and enabling technology in various areas in quantum information processing with photons. These areas include: building controlled quantum logic gates, testing eavesdropping strategies in quantum cryptography, implementing quantum repeaters, making optical pulses with a well-determined number of photons, etc. One of the obstacles in optical quantum information processing is the lack of an appreciable direct photon-photon interaction with which to perform controlled logic operations. Recently, however, it was shown that it is possible to make deterministic two-photon logic gates even with weak nonlinearities. The basic principle is the intensity-dependent cross-phase modulation between two optical beams in a medium possessing third-order (Chi^(3)) optical nonlinearity (Kerr nonlinearity). The key feature is the use of an auxiliary intense classical light beam to interact with the single photons, essentially using the same principle as QND measurements. Unfortunately, even these weak nonlinearities cannot be achieved with conventional methods. A while ago (1996), a method to achieve giant Kerr nonlinearities between two optical signals using a four-level atomic system was suggested, employing the phenomenon of electromagnetically induced transparency (EIT). Ultra cold Alkali atoms such as Rubidium were used experimentally to confirm this giant nonlinearity about a decade later, but not at the single-photon level. In order to achieve large interaction strengths, the light has to be confined. It was recently shown that Rubidium atoms can be filled into a holey photonic bandgap (PBG) fiber, which basically acts as a waveguide where light travels in the empty central region. EIT was also demonstrated in the atoms confined to the holey PBG fiber, at a very low light level. Our efforts are towards identifying the most promising methods and pursuing them in order to realize the first optical QND measurement.