A silicon photonic chip that emits entangled photon pairs in a controlled fashion could one day enable quantum transceivers. Developed at the University of California, San Diego, the chip is pumped by a telecommunications-grade low-power diode laser. Through a filter, the chip produces two daughter photons for each pair of photons it absorbs. One of the daughter photons will have a higher frequency than the input photons, and the other will have a lower frequency; their total frequency will be the same as that of the parent photons. During the experiments, the silicon photonic chip was placed on a temperature-controlled stage. Fibers were used to couple the optical pump beam to one side of the chip and extract the generated photon pair from the other side. The silicon photonic chip is about 3 × 15 mm in size and features planar waveguides. The horizontal stripes of the chip correspond to different types of silicon photonics structures used to generate and control light; the features used in this experiment were located near the middle of the chip. Courtesy of the Jacobs School of Engineering/UC San Diego. “Silicon is known to be a poor light-emitting material — there is no silicon diode laser, for example, despite many decades of research,” said professor Dr. Shayan Mookherjea. “However, if you want to make a chip that emits quantum light such as pairs of single photons which are entangled in some quantum mechanical properties, and you want to do it at room temperature so that the chip can be widely used, then it turns out that silicon is actually quite a good material for generating photons.” The chip’s light-changing process, called spontaneous optical nonlinear mixing (SONM), has been demonstrated in a number of materials including glass fibers, crystals and semiconductors like silicon. “One thing you have to do, though, is to pattern the silicon into waveguides and microresonators which enhance the optical intensity at specific wavelengths,” said postdoctoral researcher Marc Savanier. “A piece of silicon by itself is not going to have a very high SONM coefficient and it won’t generate a measurable number of entangled photon pairs.” Using CMOS-compatible lithography, the researchers gave the chip a patterned structure that allows the joint spectral intensity and Schmidt number of emitted photon pairs to be tuned simply by varying the pump frequency or the temperature of the chip. “A low Schmidt number represents a device generating photon pairs that has been tuned for a particular quantum optic property called heralding, whereas a high Schmidt number shows that the device generates photons which can be used to encode more than a single quantum bit of information per photon,” said graduate student Ranjeet Kumar. “Such control can benefit high-dimensional communications where detector-timing constraints can be relaxed by realizing a large Schmidt number in a small frequency range,” the researchers wrote in a study published in Nature Communications (doi:10.1038/ncomms6489). Funding for the research came from the National Science Foundation and DARPA. For more information, visit www.jacobsschool.ucsd.edu.