Astronomy/optics combination produces single-photon detector

A project that combined astronomy technology from Moscow State Pedagogical University (Moscow, Russia) with optical instrumentation from the University of Rochester (Rochester, NY) has resulted in the development of a picosecond superconducting single-photon detector capable of detecting a misfiring transistor from the billions residing on today's Pentium and PowerPC integrated circuits. The device is small and sensitive enough, according to the research team, to meet the demands of chip manufacturers that need to test billions of transistors on each chip as quickly and efficiently as possible.

"We were working on the photo response of superconductors and contacted the group at Moscow State Pedagogical University, led by physics professor Grigory Goltsman, using superconductors for radio astronomy," explains Roman Sobolewski, professor of electrical and computer engineering at the University of Rochester and cocreator of the device. "The upper radio bands are essentially far-infrared bands, so we got together and worked on putting their material into our detector."

Exploiting a quirk of physics is the key to the device. When transistors switch, they sometimes emit a very brief flash of light. This flash can reveal much about how the transistor is behaving, but only if a detector catches it. Conventional semiconductor detectors can't see in the infrared, or often report flashes when there aren't any.

The researchers built the detector by connecting 0.2-µm-wide and 1-µm-long microbridges—patterned from an ultrathin (5-nm-thick) niobium nitrite (NbN) film deposited on a sapphire substrate—to an external circuit via much thicker and larger gold-coated contact pads.

For the experiments, an NbN single-photon detector was mounted on a cold plate (4.2 K) inside an optical liquid-helium cryostat. Two cold glass filters were used to block thermal radiation longer than 2.5 µm from the sample. The sample was then dc-biased through a bias tee and mounted on a rigid 50-W coplanar transmission line with the ac output connected through a stainless steel co-axial cable to a cryogenic low-noise amplifier (placed inside a dewar), characterized by 30-dB gain and 1- to 2-GHz bandwidth. The noise temperature of the cryogenic amplifier was below 15 K, which yielded voltage fluctuations below 7 µV—several orders of magnitude below the signal levels. Outside the dewar, the signal passed through an isolator and a second broadband power amplifier (9-GHz; 20-dB gain) before going to a 6-GHz bandwidth single-shot oscilloscope for display, or to a 200-MHz voltage-level threshold counter for real-time event counting and statistical analysis.

The researchers used 100-fs-wide, 50-µm-diameter optical pulses with a 1-kHz repetition rate at 0.4-, 0.81-, 1.55-, and 2.1-µm wavelengths, with the bulk of the measurements performed using a 0.81-µm wavelength.

"Currently, this is by far the fastest (>10-GHz) single-photon detector, and the only device that counts both infrared and visible photons in the wide-wavelength range (from 3.0 to at least 0.4 µm) with virtually zero dark counts (<0.0011/s)," explains Sobolewski. "Already identified applications range from sensing ultraweak electroluminescence from submicron CMOS very-large-scale-integration circuits to high-data-rate free-space optical communication systems for planetary exploration and Earth-orbiting missions. Our devices are also currently the prime candidates for practical quantum communication systems, which allow for the implementation of unconditionally secret quantum key distribution for high-speed enciphering of large volumes of data."

REFERENCE

1. G.N. Gol'tsman et al., Appl. Phys. Let. 79, 705 (Aug. 6, 2001).