The process of detecting light—whether with our eyes, cameras or other devices—is at the heart of a wide range of civilian and military applications, including light or laser detection and ranging (LIDAR or LADAR), photography, astronomy, quantum information processing, medical imaging, microscopy and communications.
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.
Detection of photons—the fundamental particles of light—is ubiquitous, but performance limitations of existing photon detectors hinders the effectiveness of the aforementioned applications. The detectors which detect these signals are the most critical elements that determine the performance of wide range of civilian and military systems.
Photon detectors count photons of light. A photon detector has some surface that absorbs photons and produces some effect (current, voltage) proportional to the number of photons absorbed.
A photovoltaic cell consists of a layer of semiconductor (like selenium, Hg-Cd-Te, Cu2O, etc.) sandwiched between two metallic electrodes, with the exposed electrode thin enough to be transparent. Photons of light are absorbed by the semiconductor, forming electrons and holes that create a current proportional to the number of photons absorbed.
A phototube uses the photoelectric effect to generate a current from absorbed light. Light is absorbed by a metallic surface with a low work function. Electrons are emitted and attracted to a positively biased anode. Electronics measure the current, which is proportional to the number of photons absorbed.
Single Photon detectors
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.
Single-photon detection has become a vital tool in many applications, from single-molecule fluorescence, particle characterization through scattering, and quantum cryptography to astronomy, lidar, and more.
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. In all of these applications, performance could be improved by replacing classical, analog light detectors with high-performance photon counting detectors.
The development of single-photon counting stems from the discovery of the photoelectric effect in the late 19th century. Later, it was achieved in silicon avalanche photodiodes (APDs) in the 1960s. Today, turnkey modules provide simple, plug-and-run photon detection in OEM instruments and research laboratories alike. Single-photon detection is now enabling quantum communications, as it’s being deployed in space and underground in fiber networks, and in new single-molecule and
small-particle characterization applications, with future development and uses continue to emerge.
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.
A photon is the quantum of an electromagnetic field, including electromagnetic radiation, such as light and radio waves, which is an elementary particle with a certain energy E = hυ = hc/λ, where h = 6.626 × 10−34 J s is the Planck’s constant, c = 2.998 × 108 m/s is the speed of light in vacuum, υ and λ are the frequency and wavelength of the photons, respectively. Further, microwave photons, terahertz photons, visible/near-infrared (NIR) photons, and high-energy photons/particles exist
Indeed, single-photon photodetectors have been developed based on a wide range of physical processes that span from the photoelectric effect in semiconductors to superconductivity, and moreover, these photodetectors have achieved impressive performance in terms of efficiency, dark count rate, and jitter.
Superconductor Detectors: Devices operated below the superconducting critical temperature that can detect and/or resolve the energy of absorbed photons.
Semiconductor Detectors: The most common electronic photon detectors in use today; in a semiconductor material absorbed photons generate electron-hole pairs which are then amplified.
Biological Detectors: The most common photon detectors are biological (e.g., human eyes); it is possible that either biology-based detectors, bio-inspired detectors, or both will play a role in Detect.
Hybrid / Other Detectors: Detect will investigate new designs with the potential for transformative progress. Teams are encouraged to investigate hybrid designs (combining two or more of the categories above, or combining existing and new designs) or completely new technology platforms.
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.
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. Since the photocathode is far from 100% efficient, not every photon will
generate an electron. Different photocathode materials respond to different wavelength ranges and have different photon energies below which they will not emit electrons. Electrons can also
be emitted by other means (e.g., thermionic emission, where thermal energy boosts the electron enough to escape the electrode), giving rise to “dark electrons,” which, when operated in Geiger
mode, leads to “dark counts” with no photons present.
Because the PMT can detect low levels of light, even down to a single photon, it has become an important tool in many applications including astronomy, nuclear particle physics, and
biomedical instrumentation. However, PMTs are very sensitive to overstimulation and are easily damaged by exposure to ambient light.
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.
Semiconductor Single-Photon Avalanche Photodiodes (SPAD)
A photodiode in its simplest form is a p-n junction whose materials enable the right radiation frequency or wavelength to release electrons and create a photocurrent within the junction. When reverse-biased with the cathode voltage raised positive compared to the anode, this photocurrent
can be quickly extracted to produce a current proportional to the light level on the photodiode.
Adding an undoped region (“intrinsic” semiconductor) between the p- and n-type regions allows doping levels to be increased, causing higher levels of charge carriers and therefore greater
operation speed. A PIN junction, invented by Jun-ichi Nishizawa et al. in 1950, is also ideal for a photodiode.
Adding an internal current gain region within a PIN photodiode turns it into an APD. Invented
by Nishizawa in 1952, the APD uses a careful doping structure to allow high-voltage application, creating high fields within the junction region. These high fields accelerate the photoelectrons, causing them to release other electrons through impact ionization and create a typical internal current gain of 100. Thus, a single photon can create 100 photoelectrons—however, this is still not enough to enable a simple single-photon detector.
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.
Launched by RCA in 1987, the SPCM-100 was a self-contained, user-friendly device with built-in temperature control, stabilized high-voltage supply, and a Geiger-mode APD passive-quenching circuit. An on-board logic circuit detected the avalanche current pulse and generated a simple TTL pulse of 35 ns, and the passive quench circuit readied the APD for the next photon after only
60 ns. With its low dark-count, low timing jitter, low after-pulse, and a photon-detection efficiency (PDE) of over 50%, this first-generation module enabled single-photon studies to move deeper into red and near-infrared regions of the electromagnetic spectrum that were difficult to reach with PMTs. APDs’ much lower bias voltages provide immunity to magnetic fields, eliminating the need for complex shielding, and APDs are also less vulnerable to accidental ambient light exposure.
In 2011, improved control electronics allowed PDEs as high as 70% while retaining industry-leading specifications for dark count and after-pulsing. The pulse width and dead time were both further reduced, affording 10 ns and 24 ns, respectively, to reliably detect over 37 million single-photon events per second.
SPADs are currently the mainstream solution for single-photon detection in practical applications. The market and applications for single-photon detection continue to expand. Different APD technologies for detecting single photons optimize key performance parameters to suit different applications.
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.
For automotive lidar at 905 nm, where cost is important for volume production, single-photon silicon avalanche detectors use micro-APDs on top of a CMOS structure.
New indium gallium arsenide (InGaAs) Geiger-mode APDs can detect single photons at 1.5 µm, with higher-power lasers allowing significantly greater range to detect photons that are reflected back by objects around the vehicle
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.
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.
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.
In 2020 Draper patents high-speed single-photon detector to advance lidar
The patented single-photon detector can absorb and detect a single photon, and refresh for the next one within nanoseconds. A new single-photon detector developed by engineers at Draper (Cambridge, MA) can outperform existing technologies and promises significant improvements in detection range and resolution–a boon for self-driving cars and other applications–according to Draper. The detector uses a silicon-germanium photodiode, has ultralow dark-counting rate, and timing resolution of better than one nanosecond.
“A sensor needs to be very efficient at detecting light. In applications like LiDAR, you are often limited in how much laser power you can use, but you want to be able to get a lot information from the objects in the scene. The most efficient detector you can have is one that can measure every single photon coming in to identify specific objects of interest,” Spector said.
The next-generation SPD designed at Draper is so fast and efficient that it can absorb and detect a single particle of light, called a photon, and refresh for the next one within nanoseconds. The engineers designed the system as an array of photodiodes and coupled them with single electron bipolar avalanche transistors (SEBAT) that turn an incoming photon into a large electric current that can be detected.
“A broad range of industries and research fields will benefit from a single photon detector with these capabilities,” says Spector. Other applications include quantum communications, surveillance, bioscience, imaging and nighttime operation, he added. The patent lists the inventors as Steven Spector, Robin Dawson, Michael Moebius and Ben Lane.
The new offering adds to Draper’s growing portfolio of autonomous system and self-driving car capabilities. The portfolio includes the Draper APEX Gyroscope–a MEMS gyroscope that provides centimeter-level localization accuracy; Draper’s all-weather LiDAR technology, named Hemera, a detection capability designed to see through dense fog and is compatible with most LiDAR systems; and Draper’s LiDAR-on-a-Chip with MEMS beam-steering technology that creates a three-dimensional point cloud of a car’s surroundings.
InGaAs APDs for single-photon detection will also play a vital role in infrastructure and banking security as quantum key distribution (QKD) systems are deployed on fiber-optic links at telecom wavelengths. QKD allows two parties to set up a secure cryptographic key remotely in real time,
without needing physical contact to share the key. QKD has already been demonstrated in free-space links on the ground and in satellite-ground and satellite-satellite links, using both silicon and InGaAs APDs.
As bandwidths increase, not only is photon detection important, but jitter in the time delay from the photon reaching the surface of the APD and the system output pulse being registered also becomes a key performance parameter. QKD relies on comparing a sequence of photons arriving at various detectors—if the internal delay time varies too much, it will be impossible
to know which photons are being compared. APDs with small surface areas and thin structures can create the avalanche with much less variability in time delay, so their reduced detection
performance compared to larger APDs is tolerated for applications where timing resolution is important.
NIST Patents Single-Photon Detector for Potential Encryption and Sensing Apps in 2016
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.
Credit: Bienfang/NIST: In a single-photon detector, individual photons from a light source produce detectable electronic signals (large multicolored pulse at left), as well as pulses of electronic noise (subsequent, smaller signals) that are correlated with the original signal. NIST’s newly patented detection system reduces this noise and increases the detector’s efficiency, improving the ability to detect single photons. The image above shows 4000 output signals from the detection system, some of which show the signal produced by single-photon detection.
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 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.
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 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.
Many applications leveraging the quantum nature of single atoms or electrons also need to be able to detect single photons. A single electron trapped in a crystal defect can only emit one photon at a time, so a single-photon detector is needed to understand this electron’s interactions. Creating an entangled photon pair starts with a single photon—again, the single-photon detector is a vital tool
for characterization and monitoring of single-photon sources.
With development of ever-smaller features on electronic wafers, air and water cleanliness in semiconductor wafer fabs becomes more critical to their operation. Single-photon counters with high detection efficiency and low dark-counts are often key parts of monitoring systems in ultra-clean workspaces.
High detection efficiency minimizes estimations for translating detector response to contamination levels, which reduces the likelihood of missing small increases in contamination levels. On the other hand, a low, stable dark count reduces false counts that may lead to false alarms and cause unnecessary and expensive shutdowns.
Recently, detection of small particles and single molecules has become an important application for single-photon detection. This technique provides ultimate sensitivity for environmental monitoring or diagnostic measurements.
SPCMs continue to support applications including astronomy, flow cytometry, fluorescence lifetime, particle sizing, and wind lidar, as well as recent and developing applications including
but not limited to quantum computing, QKD, and single-molecule analysis.
Superconducting nanowire single-photon detectors (SNSPD)
Superconductors can be used to achieve sensing and detection because of their many extraordinary properties, including zero resistance, the Josephson effect, and the Cooper-pair. . Superconductors can be engineered for detecting photons. Various superconducting sensors and detectors have demonstrated an unparalleled performance for almost the whole electromagnetic spectrum from a low-frequency microwave to a high-energy particle.
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 primary advantages of SNSPDs are low dark count rate, high photon count rate and very accurate time resolution. Superconducting nanowire detectors are able to count nearly a billion photons per second, and they operate over a large range of wavelengths, have low dark (false) counts, and produce strong signals, especially at telecom wavelengths. Conventional SSPDs usually are made up of a simple cavity structure, consisting of dielectric resonant layers with a mirror layer, and can achieve high absorptance at the target wavelength.
Ultrafast, high quantum efficiency single photon detectors are essential for scalable quantum computers and quantum key distribution. Recently, superconducting nanowire single-photon detectors (SNSPDs) with >90% detection efficiency in the 1.5 μm band have been realized using superconducting nanowires made of amorphous tungsten silicide (WSi).
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.
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.
High quantum efficiency High Temperature superconducting single photon detectors
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.
However the Researchers employed molybdenum silicide (MoSi), superconducting single photon detectors, to perform photonic quantum teleportation over fiber. The choice of MoSi instead of WSi allowed operation at a higher temperature with less jitter. “Only about 1 percent of photons make it all the way through 100 km of fiber,” NIST’s Marty Stevens says. “We never could have done this experiment without these new detectors, which can measure this incredibly weak signal.”
However, such structures are not ideal in building detectors with high efficiency over a carefully controlled spectral range, with rejection at other wavelengths, which are functionalities desirable for emerging applications in the life sciences and atmospheric remote sensing. Japan’s National Institute of Information and Communications Technology (NICT) has devised a flexible optical design method for superconducting nanowire single-photon detectors (SSPDs or SNSPDs) that is more efficient at counting individual photons than conventional SSPDs. In order to improve high detection efficiency, Japanese scientists have developed a new device structure with a non-periodic dielectric multilayer (DML) structure to achieve a flexible design for the visible to near infrared spectrum.
For the dielectric multilayer (DML), silicon dioxide (SiO2) and titanium oxide (TiO2) were used, and the niobium nitride (NbN) superconducting nanowire was put on the dielectric multilayer. “By optimizing the thicknesses of each dielectric layer in the DML, one can design the required wavelength dependence of the optical absorptance in the superconducting nanowire. Advantage of the SSPD with non-periodic DML is that one can achieve the various wavelength dependences such as wider or narrower bandwidth and/or an intrinsic bandpass filter to minimize the effect of blackbody radiation, pump- or stray light without changing the basic device structure,” the scientists wrote in the paper, published in Scientific Reports.
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.
In 2017, Single-photon superconducting nanowire single-photon detector (SNSPD) detector was discovered which 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.
The energy carried by each photon varies inversely with its wavelength. Hence, the photon flux in the UV is much lower than in the IR for the same radiant power. Consequently, the responsivity of the photon detector, in terms of V/W or A/W, is much lower in the UV region than in the IR region of the spectrum.
Besides the responsivity curves, the quantum efficiency (QE) curves are equally important to be analyzed for evaluating the absolute performance a detector. The QE curve demonstrates the efficiency of the detector, providing the data about the amount of incident photon flux that is being transformed into electrical signals. Figure 1 illustrates a relative responsivity curve for a 100% efficient photon detector.
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.
Furthermore, advances in materials science and nanoscale engineering open up possibilities for not only tuning the microscopic properties and dynamics of photodetectors, but also to develop entirely new classes of photodetectors.
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.