KARL K. BERGGREN AND ANDREW J. KERMAN
As humans, we are all born equipped with a pair of single-photon detectors—our eyes. Actually, the story is a bit more complicated because, while the rod cells on the retina respond to the absorption of a single photon by generating a small current, the brain requires about ten photons to be absorbed before registering a flash of light.
For the applications in communications and metrology that we are interested in, however, the eye would not be a very good choice for a photodetector. First of all, the eye is insensitive to infrared (IR) light and responds to visible light only on millisecond timescales. In addition, high optical loss due to reflection and transmission at the ocular interfaces of the cells lowers the detection efficiency. Finally, the eye requires an integrated, highly sophisticated, and thermally stable package—the human body—to maintain its performance. These same challenges—sensitivity, speed, optical loss, and packaging—must be addressed by any photodetector technology for it to be of use.
Infrared detection
Though other IR single-photon detectors exist, they also have shortcomings. Avalanche photodetectors (APDs) exhibit impressively low jitter (the error in the time-of-arrival information of the photon) and good detection efficiencies, but are plagued by relatively long (microsecond timescale) reset times and high dark-count rates (the rate at which the detector fires even when a photon is not incident). Photomultiplier tubes (PMTs) can provide very fast reset times, but suffer from low detection efficiencies in the technically important—to the communications industry—IR region of the spectrum around 1550 nm.
In 2001, researchers at Moscow State Pedagogical University (Moscow, Russia) and the University of Rochester (Rochester, NY) used a superconducting nanowire to detect single photons.1 These detectors were very promising in that they appeared to be fast and have low jitter, but their detection efficiency was low and their reset speed was not well understood.
The quantum nanostructures and nanofabrication group at MIT and the optical communications group at MIT Lincoln Laboratory have recently developed and demonstrated a superconducting nanowire single-photon detector (SNSPD) that has solved these problems. With a variety of attractive performance characteristics (despite the complex packaging requirement of an integrated cryocooler to reduce its temperature to 3 K), the SNSPD achieves a detection efficiency of 57%, a 3 ns reset time at 1550 nm, and a jitter of only 30 ps.2,3 The device represents the fastest, highest efficiency approach to detecting single IR photons.
Operation of the nanowire detector
The currently theorized operating principle of the SNSPD is surprisingly straightforward. When a photon is absorbed by a superconducting wire it creates a hot spot—a small region where the superconductor becomes resistive (see Fig. 1). A current running through the wire will avoid that hot spot, preferring instead the zero-resistance regions outside the perimeter and on either side of the spot—regions we call the “sidewalks” of the device. The current density in these sidewalks therefore increases. But superconductors also have the interesting and useful property that they only superconduct up to a maximum current density, known as the critical current density. Above this current, they become resistive. If the biasing current is initially set very close to the critical current, the absorption of a photon will trigger the entire cross section of the wire to switch into the resistive state and a voltage will appear across the device terminals.
While this mechanism of operation may seem simplistic, great care must be taken to realize a device with the observed performance. The starting material for our work (and the majority of the experimental work on this kind of detector to date) is 4-nm-thick niobium nitride material grown epitaxially on a sapphire substrate by Gregory Gol’tsman’s group at Moscow State Pedagogical University.
After material growth, we pattern the substrates using electron-beam lithography and a series of custom-developed nanometer-length-scale etching, deposition, and optical-lithography steps. The electron-beam lithography is a particularly challenging aspect of the work, as the variation in the device linewidth must be kept below a few percent. This is accomplished using hydrogen silsesquioxane (HSQ), an electron-sensitive spin-on-glass resist with very low line-edge roughness and high resolution. The HSQ material has the very useful property that after exposure and development it is also a good optical material—similar to silicon dioxide (SiO2)—and so we use it throughout the process as part of the final integrated optical device, as well as an electron resist (see Fig. 2).
Device testing is as challenging as fabrication: we have screened several hundred devices in the course of this work at temperatures down to 1.7 K. The detectors must also be packaged carefully, to reduce electrical noise in the readout and biasing circuits, and to couple light efficiently onto the detector surface.
Other superconducting detectors
There exist a wide variety of other superconducting single-photon detectors with some remarkable capabilities. For example, transition-edge sensors are capable of photon-number resolution with exquisite detection efficiency (about 88%) at 1550 nm.4 Similar sensors are being integrated with superconducting electronics to form a large focal plane for astronomical observations—these sensors are somewhat slow—with reset times in the microsecond range.5 Superconducting-tunnel-junction detectors have also been developed that can sense not only the existence, but also the location, of an x-ray photon incident on a focal plane.6
There are a variety of tasks that the SNSPD cannot yet do or cannot do well. Rather than just sense its arrival, we would like to identify a photon’s wavelength and polarization state. Instead of simply counting photon pulses (where each pulse can contain one or more photons), we would like to develop a “photon-number-resolving” detector that can distinguish between the detection of one, two, or more photons. And, we would like to form an image with the detector by sensing the location of absorption of the photon across a focal plane.
Enabling interplanetary communication
These IR single-photon detectors are likely to find application in a variety of disciplines ranging from quantum information processing to biological imaging. But one tremendously exciting potential application would be as a receiver for interplanetary communications within the solar system.
Currently, interplanetary communication makes use of radio-frequency links that require large, heavy (and therefore expensive to launch) radio antennas on spacecraft to beam data back to Earth. The resulting data rates can sometimes make a dial-up modem seem speedy. But Don Boroson and his colleagues at MIT Lincoln Laboratory (Lexington, MA) and the NASA Jet Propulsion Laboratory (Pasadena, CA) have pointed out that free-space optical communication using photon-counting optical receivers could realize much higher data rates (or much smaller transmitters and receivers) over these long distances.7 Their proposed system was based on Geiger-mode APDs and would be capable of data transmission from Mars to a ground-based receiver at rates as high as 100 Mbit/s, fast enough to transmit live video streams. However, with timing jitter in the hundreds of picoseconds and a reset time measured in microseconds, it is difficult with Geiger-mode APD arrays to achieve higher transmission rates.
Our SNSPD detectors are ideal candidates to overcome these limitations. To prove this concept, a team at Lincoln Laboratory led by Bryan Robinson recently used one of our detectors to realize an error-free photon-counting communication link at a data rate of 781 Mbit/s.8 This result represents the fastest photon-counting data transmission ever demonstrated, and suggests that someday soon optical beams will send data from all around the solar system to scientists on Earth with unprecedented speed and efficiency.
ACKNOWLEDGMENT
This work was sponsored in part by the United States Air Force under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, recommendations and conclusions are those of the authors and are not necessarily endorsed by the United States Government.
REFERENCES
1. G.N. Gol’tsman et al., Appl. Phys. Lett. 79, 705 (2001).
2. A.J. Kerman et al., Appl. Phys. Lett. 88, 111116 (2006).
3. K.M. Rosfjord et al., Optics Express 14, 527 (2006).
4. D. Rosenberg et al., Phys. Rev. A 71, 61803-1 (2005).
5. C.D. Reintsema et al., Rev. of Scientific Instruments 74, 4500 (2003).
6. S. Friedrich et al., Nucl. Instrum. Meth. A 370, 44 (1996).
7. D.M. Boroson, R.S. Bondurant, and J.J. Scozzafava, Proc. SPIE 5338, 37 (2004).
8. B.S. Robinson et al., Optics Lett. 31, 444 (2005).
Karl K. Berggren is group leader and assistant professor in the Quantum Nanostructures and Nanofabrication Group within the Department of Electrical Engineering and Computer Science (EECS) at the Massachusetts Institute of Technology (MIT), 77 Massachusetts Ave., MS 36-219, Cambridge, MA 02139; e-mail: [email protected]. Andrew J. Kerman is a staff member at MIT Lincoln Laboratory, 244 Wood St., Lexington, MA 02420; e-mail: [email protected].