est pulses of light-at the highest temperatures reported for the photon source used. This advance marks a step toward prac- tical, ultrasecure quantum communications. "Single photon turnstiles" are being hotly pursued for quantum communi- cations and cryptography, which involve the use of streams of individual photons in different quantum states to transmit encoded information. Due to the peculiarities of quantum mechanics, such transmissions could not be inter- cepted without being altered, thus ensuring that eavesdropping would be detected. The photon source used in the NIST study was a "quantum dot," 10 to 20nm wide, made of semiconductor materials. Quantum dots have special electronic properties that, when excited, cause the emission of light at a single wavelength that depends on dot size. An infrared laser tuned to a particular wavelength and intensity was used to excite the quantum dot, which produced photons one by one more than 91 percent of the time at temperatures close to absolute zero (5K or about minus 459 degrees F) and continued to work at 53 percent efficiency at 120K (minus 243 degrees F). Higher operating tem- peratures are preferable from a cost standpoint because the need for cooling is reduced. The NIST quantum dots are made of indium gallium arsenide, can be fab- ricated easily, and can be integrated with microcavities, which increase photon capture efficiency. According to NIST electrical engineer Richard Mirin, this design offers advantages over other single photon sources, many of which exhibit blinking, stop working under prolonged exposure to light, or are diffi- cult to fabricate. 7.8.2 Quantum Photon Detector Steps are also being made to transform quantum encryption from a theoreti- cal possibility into a practical technology. Researchers from NIST and Boston University have demonstrated a detector that counts single pulses of light, while simultaneously reducing false or "dark counts" to virtually zero. The advance provides a key technology needed for future development of secure qu