An Ultrabright Tunable Photon-Pair Source

An ultrabright tunable photon-pair source created at MIT is the best generator so far of entangled photon pairs, a development which should help quantum communications systems to do their job more smoothly. Entangled photons possess a special correlation unlike anything in classical physics: if, say, we measure the spin (polarization) of one photon, then we automatically know the polarization of the other photon, even though it might be on the other side of the galaxy and even if, until the moment of measurement, the spins of both photons had been indeterminate. This weird property of quantum reality, it is hoped, will be a boon to encryption (perhaps in a "quantum teleportation" scheme - see Physics News Update 350) and future quantum computers. Indeed, for some time now quantum effects have been an important factor in communications engineering applications, especially insofar as quantum fluctuations (uncertainty in our knowledge of where an electron is or the value of its energy) can produce levels of electrical noise that can limit the effectiveness of practical devices. The use of entangled photons might be able to mitigate this problem. Quantum limitations are already a problem in such devices as optical amplifiers (whose amplified spontaneous emission noise limits communication performance) or soliton pulses (supposedly non-dispersing light pulses that are subject to quantum-induced timing jitter accumulation) used in fiber-optic communications. MIT's Research Laboratory of Electronics is a place where quantum aspects of electrical engineering are taken very seriously. The head of the lab, Jeffrey H. Shapiro (617-253-4179), will report on progress in a program aimed at developing a system for long-distance, high-fidelity teleportation of photon states at the upcoming Frontiers in Optics meeting of the Optical Society of America. As part of this work the MIT team has developed a source of entangled photons some ten times brighter than previous sources. The correlated photons are engendered by shooting a laser beam into a nonlinear optical crystal, where incoming photons are, in effect, split into two related photons of half the wavelength. This "down-conversion" process is even tunable over a certain wavelength range. Up to 12,000 photon pairs per second per milliwatt of input power have been produced. (Paper MI3, OSA meeting 5-9 October in Tucson, AZ; meeting website)