Sources of Entanglement
Entanglement is the quintessential characteristic of quantum mechanics, and a fundamental resource for quantum information. We have built and continue to develop one of the world's purest and brightest sources of entangled photons, using a process known as downconversion.
To realize our downconversion source, we use two back-to-back nonlinear crystals: the first transforms single horizontally polarized ultraviolet photons into two vertically polarized infrared daughter photons; the second transforms single vertically polarized ultraviolet photons into two horizontally polarized infrared daughter photons. By sending a superposition of horizontal and vertical light (light at 45 degrees) into these crystals, we obtain a superposition of two horizontal and two vertical downconverted photons: entanglement. We have also expanded the scope of how photons can be entangled. By focusing the pump on our nonlinear crystals, we experimentally demonstrated the first quantum system entangled in every degree of freedom, a phenomenon called hyperentanglement. The generated photons are entangled in polarization, orbital angular momentum and emission time.
Although we have already demonstrated bright and pure downconversion sources that utilize gas or solid state lasers, many applications of entangled photon pairs such as quantum key distribution systems, quantum repeaters, and in the future, quantum computing networks, require more compact and inexpensive sources. To achieve this goal, we have been developing a downconversion source in which the nonlinear crystals are pumped by a blue semiconductor laser diode. However, because laser diodes produce pump photons having a wide range of wavelengths and directions, the downconverted photon pairs are all distinguishable from each other, resulting in a source that is less pure. To correct this effect, we have developed techniques for compensating the wavelength- and direction-dependent phases of the photons, creating a brighter and higher fidelity entanglement source based on laser diodes.