Incorporating news from O plus E magazine, Tokyo
TSUKUBA–A group at the Electrotechnical Laboratory (ETL; a part of the National Institute of Advanced Industrial Science and Technology) has developed a high-sensitivity single-photon detector for 1550-nm photons using an indium gallium arsenide avalanche photodiode (APD). The quantum efficiency is 32%, while the dark-count rate is approximately 10-3.
The ETL, which is engaged in research on data security, has focused on quantum cryptography. Quantum ciphers are distinguished by their reliance on the indivisibility of individual photons and the impossibility of replicating a quantum state. If the method can be successfully commercialized, extremely safe and reliable optical communications networks can be created. Various cryptographic schemes of this kind have been proposed before now, and various proof-of-principle experiments have been conducted as well. But the development of quantum cryptography technologies involving transmission of information over optical fibers has suffered extreme delays; the main hindrance has been the lack of a single-photon detector in the 1550 nm region—the low-loss region for optical fibers.
In the ETL apparatus, a voltage pulse is produced using a so-called "bias T," and a voltage larger than the yield voltage is applied to the APD (see figure). A single photon enters the APD from the optical fiber, causing the photon to be absorbed and generating an electron-hole pair. The hole is accelerated by the strong electric field in the APD and undergoes lattice collision, changing a valence electron into a free electron. More and more free electrons are created due to repeated lattice collisions, so the current in the APD increases dramatically in an avalanche effect. This current is then measured.
The APD is chilled using a two-level water-cooling Peltier component. To prevent condensation, the container is placed within a vacuum at a temperature of -35°C. The light source is synchronized with the voltage pulse applied to the APD. Light pulses are generated with a wavelength of 1552 nm, full-width at half-maximum of 23 ps, and a repetition frequency of 200 kHz. A tunable optical attenuator reduces the light pulses to the single-photon level. An optical fiber carries the resulting signal to the APD. A photon counter with both pulse-height distinguishing capability and counting capability measures the avalanche characteristics, allowing the probability of causing an avalanche per pulse to be determined. In addition, the dark-count rate is measured.
Since the electron cooling is accomplished using a Peltier component and not liquid nitrogen, the operation temperature is relatively high; however, because the APD has such a low dark current (less than 1% that of conventional devices) and the voltage pulse width is only one nanosecond, the temperature-dependent increase in the dark counting rate can be controlled. By lowering the voltage applied to the APD, the dark counting rate can be decreased from 10-3 to less than 10-4. Although the quantum efficiency then decreases to 24%, the voltage decrease is useful when low noise is required.
The group at ETL has been investigating quantum correlation photonics and aims to establish optical communications and measurement technologies that cannot be accomplished using coherent light. The newly developed detector will help to decrease the error rate and increase the transmission distance of quantum ciphers over optical fibers. The researchers are looking to increase performance of the detector, as well as to establish new optical interference measurement technologies or quantum cryptography communication technologies based on quantum correlation effects.
Courtesy O plus E magazine, Tokyo