Light is widely used for communications, carrying phone conversations and video signals through fiber-optic cables around the world in pulses composed of many photons. Light is also being used in optical wireless communication, a form of free space communications consisting of a LASER at source and detector at the destination. Both military and civilian users have started planning Laser communication systems from terrestrial short-range systems, to high data rate Aircraft and Satellite communications, unmanned aerial vehicles (UAVs) to high altitude platforms (HAPs), near-space communications for relaying high data rates from moon, and deep space communications from mars.
The detectors which detect these signals are most critical elements that determine the performance of wide range of civilian and military systems. These include systems such as light or laser detection and ranging (LIDAR or LADAR), photography, astronomy, quantum information processing, advanced metrology, quantum optics, medical imaging, microscopy, quantum and classical optical communications including underwater Blue-Green communications, and environmental sensing.
As the state of the art in these fields has advanced, so have the performance requirements of the constituent detectors. A single photon is the indivisible minimum energy unit of light, and therefore, detectors with the capability of single-photon detection are the ultimate tools for weak light detection. Single photon detectors have found application in various research fields such as quantum information, quantum optics, optical communication, and deep space communications.
There has been concerted effort to advance single-photon detection technologies to achieve higher efficiency, lower noise, higher speed and timing resolution, as well as to improve other properties, such as photon number resolution, imaging, and sensitivity to lower energy photons. High-bandwidth, high-sensitivity, compact and readily available photon-counting detector is a key technology for many future scientific developments and improved DoD application capabilities, according to DARPA.
Engineers have shown that a widely used method of detecting single photons can also count the presence of at least four photons at a time. The researchers say this discovery will unlock new capabilities in physics labs working in quantum information science around the world, while providing easier paths to developing quantum-based technologies.
Depending on the wavelength regime of interest, different technologies have been utilized, such as silicon avalanche photodiodes (APDs) for visible wavelengths, photomultiplier tubes, or InGaAs-based APDs for the telecommunication range. In recent years, superconducting nanowire single-photon detectors (SNSPDs) have been shown to be promising alternatives, particularly when they are integrated directly onto waveguides and into photonic circuits. Apart from these, there are also some new technologies like hybrid photodetectors, visible light photon counters, frequency up-conversion, quantum dots & defects and carbon nanotubes.
Semiconductor Single-Photon Avalanche Photodiodes (SPAD)
SPADs are currently the mainstream solution for single-photon detection in practical applications. SPAD device is operated in Geiger mode, for which biasing above the breakdown voltage results in a self-sustaining avalanche in response to the absorption of just a single photon. This electron cascade and multiplication effect, significantly amplifies the response and allows for an easy measurement of the response pulses.
In the visible light range, the best known and most widely used are Si avalanche photodiodes (APD’s). Detection of single-photon infrared (IR) radiation remains a major technological challenge because IR photons carry significantly less energy than those of visible light, making it difficult to engineer an efficient electron cascade. The most successful Si APD’s have their sensitivity restricted by the bandgap, while APD’s based on narrow-gap semiconductors exhibit unacceptably large dark counts.
The best quantum efficiency (QE) reported for InGaAs APD’s is 16% at 1.2 µm, but the large, 0.5-ns jitter and high, 10 -per-second dark counts make them not attractive for several important applications, including practical quantum communication systems.
The typical structure of an InGaAs/InP single-photon detector is made by a separate absorption and multiplication (SAM) region where a low-bandgap material (InGaAs) is used to absorb NIR photons and a compatible highbandgap material (InP) is used for avalanche multiplication through a high electric field.
Some tasks require free-running operation of the detector because the arrival time of the photons is unknown or they are spread over a long time slot (tens of microsecond). Free-running operation of InGaAs/InP detectors is challenging due to afterpulsing effects, where spontaneous dark detections can occur shortly after previous photon detections, due to trapping phenomena.
To minimize the afterpulsing effect, the avalanche current must be reduced since this reduces the probability that a trap gets filled in the first place. An appropriate circuit, referred to as quenching electronics, is necessary to rapidly suppress the avalanche by lowering the reverse bias down and to restore the SPAD to its armed state to detect the next incoming photon. The rapid quenching also reduces afterpulsing, therefore the quenching electronics plays a key role in a SPAD system. The after-pulsing effects in InGaAs APDs, make them ill-suited for applications requiring high duty-cycle and high-rate detection.
Usually, InGaAs APDs are operated in gated mode in which a periodic shot duration bias, synchronized to input photon timing, is applied. In the gated mode, however, InGaAs APDs cannot detect photons with a random input timing.
The InGaAs APDs can be operated at temperatures accessible via thermoelectric cooling, making them ideal for applications requiring compact photon-counting solutions.
NIST Patents Single-Photon Detector for Potential Encryption and Sensing Apps
Individual photons of light now can be detected far more efficiently using a device patented by a team including the National Institute of Standards and Technology (NIST), whose scientists have overcome longstanding limitations with one of the most commonly used type of single-photon detectors. Their invention could allow higher rates of transmission of encrypted electronic information and improved detection of greenhouse gases in the atmosphere.
Semiconductor Single-Photon Avalanche Photodiodes (SPAD) based on indium-gallium-arsenide semiconductors is widely used in quantum cryptography research because it can detect photons at the particular wavelengths (colors of light) that travel through fiber. Unfortunately, when the detector receives a photon and outputs a signal, sometimes an echo of electronic noise is induced within the detector. Traditionally, to reduce the chances of this happening, the detector must be disabled for some time after each detection, limiting how often it can detect photons.
Usually, InGaAs APDs are operated in gated mode in which a periodic shot duration bias, synchronized to input photon timing, is applied. In the gated mode, however, InGaAs APDs cannot detect photons with a random input timing.
The team, which also includes scientists working at the California Institute of Technology and the University of Maryland, has patented a method to detect the photons that arrive when the gates are either open or closed. The NIST team had developed a highly sensitive way to read tiny signals from the detector, a method that is based on electronic interferometry, or the combining of waves such that they cancel each other out.
The approach allows readout of tiny signals even when the voltage pulses that open the gate are large, and the team found that these large pulses allow the detector to be operated in a new way. The pulses turn on the detector during the gate as usual. But in between gate openings the pulses turn the detector off so well that signals produced by absorbing a photon can linger for a while in the device. Then the next time the gate opens, these lingering signals can be amplified and read out.
The added ability to detect photons that arrive when the gate is closed increases the detector’s efficiency, an improvement that would be particularly beneficial in applications in which photons could arrive at any moment, such as atmospheric scanning and topographic mapping.
The new detector can count individual photons at a very high maximum rate—several hundred million per second—and at higher than normal efficiency, while maintaining low noise. Its efficiency is at least 50 percent for photons in the near infrared, the standard wavelength range used in telecommunications. Commercial detectors operate with only 20 to 30 percent efficiency.
Superconductor single photon detectors
Superconducting SPDs include superconducting nanowire singlephoton detectors (SNSPD), transition edge sensors and superconducting tunnel junctions.
Superconducting nanowire single-photon detector (SNSPD) has emerged as the fastest single-photon detector (SPD) for photon counting. The SNSPD consists of a thin (≈ 5 nm) and narrow (≈ 100 nm) superconducting nanowire. The nanowire is cooled well below its superconducting critical temperature and biased with a DC current that is close to but less than the superconducting critical current of the nanowire.
The absorption of a single photon in superconducting nanowires results in creation of hotspot, and subsequently, the superconducting current density increases due to the size expansion of the hotspot. Once the superconducting current density in the nanowires reaches the critical value, the nanowires are changed from the superconducting state to the normal resistance state. This transition generates a voltage signal of single-photon detection.
The primary advantages of SNSPDs are low dark count rate, high photon count rate and very accurate time resolution. The detection efficiency was low (at the level of a few percent) for early generation devices, but recently, this parameter has been significantly improved through the efforts of the SNSPD community.
SNSPDs tend to be expensive because they need very low temperatures to operate while photomultiplier tubes do not have high detection efficiency and are costly too. SNSPDs have wide spectral range from visible to mid IR , far beyond that of the Si single-photon avalanche photodiode (SPAD) and the SNSPD is superior to the InGaAs SPAD in terms of signal-to-noise ratio.
Cooling technology challenges
Most SNSPDs are made of niobium nitride (NbN), which offers a relatively high superconducting critical temperature (≈ 10 K) and a very fast cooling time (<100 picoseconds). NbN devices have demonstrated device detection efficiencies as high as 67% at 1064 nm wavelength with count rates in the hundreds of MHz. NbN devices have also demonstrated jitter – the uncertainty in the photon arrival time – of less than 50 picoseconds, as well as very low rates of dark counts, i.e. the occurrence of voltage pulses in the absence of a detected photon.
This detector operates at the boiling point of liquid helium (4.2 K), this temperature can be reached by by immersing it in liquid helium (He) or mounting the device in a cryogenic probe station. Liquid He is expensive, hazardous and demands trained personnel for correct use. This technique is satisfactory for testing superconducting devices in a low temperature physics laboratory; however if the ultimate goal is to provide a working device for users in other scientific fields or in military applications, alternative cooling methods must be sought.
Operating SNSPDs in a closed-cycle refrigerator offers a solution to this problem. The circulating fluid is high pressure, high purity He gas which is enclosed inside the refrigerator allowing continuous operation and eliminating repeated cryogenic handling.
The requirement of very low temperatures limit the operation of these devices only on the ground, which limits the use of SNSPDs to ground-based applications. For example, in the Lunar Laser Communication Demonstration project of the National Aeronautics and Space Administration, G-M cryocooler-based SNSPD systems were adopted at the employed ground station. Meanwhile, semiconducting single photon detectors without complicated cryocoolers were used for the satellite.
Researchers from Chinese Academy of Sciences (CAS), have developed a hybrid cryocooler that is compatible with space applications, which incorporates a two-stage high-frequency pulse tube (PT) cryocooler and a 4He Joule–Thomson (JT) cooler.
“To make a practical SNSPD system for space applications, we chose a superconducting NbTiN ultrathin film, which can operate sufficiently well above 2 K, to fabricate the SNSPDs, instead of using WSi, which usually requires sub-1-K temperatures. The hybrid cryocooler successfully cooled an NbTiN SNSPD down to a minimum temperature of 2.8 K. The NbTiN SNSPD showed a maximum SDE of over 50% at a wavelength of 1550 nm and a SDE of 47% at a DCR of 100 Hz. Therefore, these results experimentally demonstrate the feasibility of space applications for this SNSPD system,” write the authors.
Single-photon detector can count to four
Duke University, the Ohio State University and industry partner Quantum Opus, have discovered a new method for using a photon detector called a superconducting nanowire single-photon detector (SNSPD). In the new setup, the researchers pay special attention to the specific shape of the initial spike in the electrical signal, and show that they can get enough detail to correctly count at least four photons traveling together in a packet.
“Here, we report multi-photon detection using a conventional single-pixel SNSPD, where photon-number resolution arises from a time- and photon-number-dependent resistance 𝑅hsRhs of the nanowire during an optical wavepacket detection event. The different resistances give rise to different rise times of the generated electrical signal, which can be measured using a low-noise read-out circuit.”
“Photon-number-resolution is very useful for a lot of quantum information/communication and quantum optics experiments, but it’s not an easy task,” said Clinton Cahall, an electrical engineering doctoral student at Duke and first author of the paper. “None of the commercial options are based on superconductors, which provide the best performance. And while other laboratories have built superconducting detectors with this ability, they’re rare and lack the ease of our setup as well as its sensitivity in important areas such as counting speed or timing resolution.”
Chinese Superconducting Nanowire Single-Photon Detector Sets Efficiency Record
Researchers have demonstrated the fabrication and operation of a superconducting nanowire single-photon detector (SNSPD) with detection efficiency that they believe is the highest on record. The photodetector is made of polycrystalline NbN with system detection efficiency of 90.2 percent for 1550-nm-wavelength photons at 2.1 K. In experiments, the system detection efficiency saturated at 92.1 percent when the temperature was lowered to 1.8 K. The research team believes that such results could pave the way for the practical application of SNSPD for quantum information and other high-end applications.
For their SNSPD device, researchers from the Shanghai Institute of Microsystem and Information Technology and the Chinese Academy of Sciences used an integrated distributed Bragg reflector (DBR) cavity offering near unity reflection at the interface while performing systematic optimization of the NbN nanowire’s meandered geometry. This approach enabled researchers to simultaneously achieve the stringent requirements for coupling, absorption and intrinsic quantum efficiency.
The device exhibited timing jitters down to 79 picoseconds (ps), almost half that of previously reported WSi-SNSPD, promising additional advantages in applications requiring high timing precision. Extensive efforts have been made to develop SNSPDs based on NbN, targeted at operating temperatures above 2 K, which are accessible with a compact, user-friendly cryocooler. Achieving a detection efficiency of more than 90 percent has required the simultaneous optimization of many different factors, including near perfect optical coupling, near perfect absorption and near unity intrinsic quantum efficiency.
The device has been applied to the quantum information frontier experiments at the University of Science and Technology of China
Graphene single photon detectors
Current detectors are efficient at detecting incoming photons that have relatively high energies, but their sensitivity drastically decreases for low frequency, low energy photons. In recent years, graphene has shown to be an exceptionally efficient photo-detector for a wide range of the electromagnetic spectrum, enabling new types of applications for this field.
Thus, in a recent paper published in the journal Physical Review Applied, and highlighted in APS Physics, ICFO researcher and group leader Prof. Dmitri Efetov, in collaboration with researchers from Harvard University, MIT, Raytheon BBN Technologies and Pohang University of Science and Technology, have proposed the use of graphene-based Josephson junctions (GJJs) to detect single photons in a wide electromagnetic spectrum, ranging from the visible down to the low end of radio frequencies, in the gigahertz range.
In their study, the scientists envisioned a sheet of graphene that is placed in between two superconducting layers. The so created Josephson junction allows a supercurrent to flow across the graphene when it is cooled down to 25 mK. Under these conditions, the heat capacity of the graphene is so low, that when a single photon hits the graphene layer, it is capable of heating up the electron bath so significantly, that the supercurrent becomes resistive – overall giving rise to an easily detectable voltage spike across the device. In addition, they also found that this effect would occur almost instantaneously, thus enabling the ultrafast conversion of absorbed light into electrical signals, allowing for a rapid reset and readout.
The results of the study confirm that we can expect a rapid progress in integrating graphene and other 2-D materials with conventional electronics platforms, such as in CMOS-chips, and shows a promising path towards single-photon-resolving imaging arrays, quantum information processing applications of optical and microwave photons, and other applications that would benefit from the quantum-limited detection of low-energy photons.
DARPA’s Fundamental Limits of Photon Detection—or Detect—program
Current photon detectors, such as semiconductor detectors, superconductor detectors, and biological detectors have various strengths and weaknesses as measured against eight technical metrics, including what physicists refer to as timing jitter; dark count; maximum rate; bandwidth; efficiency; photon-number resolution; operating temperature; and array size. There is currently no single detector that simultaneously excels at all eight characteristics. The fully quantum model developed and tested in Detect will help determine the potential for creating such a device.
“We want to know whether the basic physics of photon detection allows us, at least theoretically, to have all of the attributes we want simultaneously, or whether there are inherent tradeoffs,” Kumar said. “And if tradeoffs are necessary, what combination of these attributes can I maximize at the same time?”
“The goal of the Detect program is to determine how precisely we can spot individual photons and whether we can maximize key characteristics of photon detectors simultaneously in a single system,” said Prem Kumar, DARPA program manager. “This is a fundamental research effort, but answers to these questions could radically change light detection as we know it and vastly improve the many tools and avenues of discovery that today rely on light detection.”
Photons in the visible range fill at the minimum a cubic micron of space, which might seem to make them easy to distinguish and to count. The difficulty arises when light interacts with matter. A cubic micron of conventional photon-detection material has more than a trillion atoms, and the incoming light will interact with many of those atoms simultaneously. That cloud of atoms has to be modeled quantum mechanically to conclude with precision that a photon was actually there. And modeling at that massive scale hasn’t been possible—until recently.
“For decades we saw few significant advances in photon detection theory, but recent progress in the field of quantum information science has allowed us to model very large and complicated systems,” Kumar said. Advances in nano-science have also been critical, he added. “Nano-fabrication techniques have come a long way. Now not only can we model, but we can fabricate devices to test those models.”
The Fundamental Limits of Photon Detection (Detect) Program will establish the first-principles limits of photon detector performance by developing new models of photon detection in a variety of technology platforms, and by testing those models in proof-of-concept experiments.
DARPA SBIR to improve upon nanowire single-photon detector performance
DARPA issued SBIR project in 2014 to further improve upon the current state-of-the-art in nanowire single-photon detector performance while advancing the supporting technologies to allow for a compact, turn-key commercial system.
New results in superconducting nanowire devices have shown that high detection rates, low dark-count rates (DCRs), and high efficiency are all possible simultaneously with operating temperatures between 1 and 4 K.
Despite these results, further performance improvements are needed. For example, detection efficiency (DE) above 90% and bandwidth (BW) approaching 1 GHz has yet to be achieved simultaneously. In addition, innovations leading to a reduction in the system footprint and improved operability will provide better accessibility of such technologies to the relevant scientific and engineering communities.
The final system should provide multiple (>2), independent single-pixel detectors with performance superior to all current commercially available options (DE>90%, BW~1GHz, DCR< 1 Hz) in a ~5U 19™ rack-mount package. To achieve these goals, work under this SBIR may include the following: efforts to increase fabrication yields through the use of new materials or fabrication techniques, new device designs to improve bandwidth and sensitivity, efforts to reduce system SWaP through compact, application-specific cooling systems, electronics, and packaging.
The detectors developed under this SBIR will have applications for the DoD which include secure communications and active stand-off imaging systems. The improved availability and SWaP will allow the use of these detectors in all relevant government labs and open the door to new fieldable systems. For example, low power, portable optical communication links exceeding RF system bandwidths by 10-100x may be possible using the technology developed under this SBIR.
Photomultiplier (PMT) Tubes
A PMT consists of a photocathode and a series of dynodes in an evacuated glass enclosure. When a photon of sufficient energy strikes the photocathode, it ejects a photoelectron due to the photoelectric effect. The photocathode material is usually a mixture of alkali metals, which make the PMT sensitive to photons throughout the visible region of the electromagnetic spectrum. The photocathode is at a high negative voltage, typically -500 to -1500 volts.
The photoelectron is accelerated towards a series of additional electrodes called dynodes. These electrodes are each maintained at successively less negative potentials. Additional electrons are generated at each dynode. This cascading effect creates 105 to 107 electrons for each photoelectron that is ejected from the photocathode. The amplification depends on the number of dynodes and the accelerating voltage. This amplified electrical signal is collected at an anode at ground potential, which can be measured.
PMTs can have large active areas, but they suffer from low efficiency (~10%), high jitter (~150-ps) and high dark count rate. They are fragile, bulky, and sensitive to magnetic fields, require very high operating voltages, and are not conducive to making large format detector arrays. Moreover, their sensitivity in the SWIR spectral band is poor. Although PMT still plays an important role in some applications today, as with many vacuum tube-based devices, this 80-year-old technology is gradually being replaced by newer solid-state devices.