Quantum technology (QT) applies quantum mechanical properties such as quantum entanglement, quantum superposition, and No-cloning theorem to quantum systems such as atoms, ions, electrons, photons, or molecules.
Quantum technology has many Quantum applications that can be classified in three major classes. Quantum computers shall bring power of massive parallel processing, equivalent of supercomputer to a single chip. Quantum communication refers to a quantum information exchange that uses photons as quantum information carriers over optical fibre or free-space channels.
Quantum Sensing exploit high sensitivity of quantum systems to external disturbances to develop highly sensitive sensors. They can measure Quantities such as time, magnetic and electrical fields, inertial forces, temperature, and many others. They employ quantum systems such as NV centers, atomic vapors, Rydberg atoms, and trapped ions.
Quantum computing and quantum cryptography are expected to give much higher capabilities than their classical counterparts. For example, the computation power in a quantum system may grow at a double exponential rate instead of a classical linear rate due to the different nature of the basic unit, the qubit (quantum bit). Entangled particles enable the unbreakable codes for secure communications.
Quantum Key Distribution, or QKD, enables two remote parties, “Alice” and “Bob”, who are connected by a passive optical link to securely generate secret key material. Single-photon sources (SPSs) and single-photon detectors (SPDs) are key devices for enabling practical quantum key distributions (QKDs). Single photon generation is necessary for secure quantum transmission; otherwise, an eavesdropping party might intercept one of the transmitted photons and thus get a copy of the message. Moreover, Single-photon sources (SPSs) at Telecom-band are of special interest because the existing telecom backbone networks exhibit a minimal transmission loss around 1.55 μm.
Other applications to exploit sources of single photons could include quantum computing, where photons would play the role of quantum bits. It has also been shown that the availability of a single-photon source enables implementation of quantum computation using only linear optical elements and photodetectors.
Quantum metrology, meanwhile, would benefit from a true single-photon source because its signal-to-noise ratio would not be restricted by lasers’ “shot-noise” limit (which is equal to the square root of the laser intensity). They would greatly support the emergence of applications of quantum technology such as entanglement assisted measurement techniques, i.e. sub-shot noise metrology, microscopy and spectroscopy.
“Without having a source of coherent single photons, you can’t use any of these quantum effects that are the foundation of optical quantum information manipulation,” says Bawendi, who is the Lester Wolfe Professor of Chemistry. Another important quantum effect that can be harnessed by having coherent photons, he says, is entanglement, in which two photons essentially behave as if they were one, sharing all their properties.
Light-emitting silicon for photonic computing
Today, the electronics industry is geared up to use silicon in computer chips because of its advantageous electronic properties and availability. It is a good semiconductor, an abundant element, and—as silicon oxide—a constituent of glass and sand. However, silicon is not very good at dealing with light because of its crystalline structure. For example, it cannot generate photons or control their flux for data processing. Researchers have investigated light-emitting materials such as gallium arsenide and indium phosphine, but their application in computers remains limited because they don’t integrate well with current silicon technology.
Recently, European researchers reported in the journal Nature an innovative alloy of silicon and germanium that is optically active. It is a first step, says Jos Haverkort, a physicist at the Eindhoven University of Technology in the Netherlands: “We showed that this material is very suitable for light emission, and that it is compatible with silicon.” The next step is to develop a silicon-compatible laser that will be integrated into the electronic circuitry as the light source of photonics chips. This is the ultimate aim of the project SiLAS, supported by the EU program FET. The team, led by Erik Bakkers from the Eindhoven University, also includes researchers from the universities of Jena and Munich in Germany, Linz in Austria, Oxford in the UK and from IBM in Switzerland.
To create the laser, the scientists combined silicon and germanium in a hexagonal structure that is able to emit light, overcoming the drawbacks of silicon, in which the atoms are arranged in a pattern of cubes. It was a difficult project. An initial attempt to coax silicon into adopting a hexagonal structure by depositing silicon atoms on a layer of hexagonal germanium failed. Silicon stubbornly refuses to change its cubic structure when grown on planar hexagonal germanium, explains Jonathan Finley of the Technical University of Munich, who took part in the research by measuring the optical properties of the created silicon samples. “You have to convince nature to allow the growth of this unusual form of silicon germanium. It likes to grow cubic, that is what it does,” he says.
However, over the years, the research group at Eindhoven has developed expertise in growing nanotubes, and reasoned that what does not work on a planar surface of germanium might work on a curved surface of a nanotube. And this time things worked out. “What we did was to use a nanowire of gallium arsenide, which has a hexagonal structure. So we had a hexagonal stem, and we created a silicon shell around the core, which also had a hexagonal structure,” says Haverkort. By varying the amount of silicon and germanium deposited on the nanotubes, the researchers found that the hexagonal alloy was capable of emitting light when the concentration of germanium was above 65 percent.
The next step is a demonstration of lasing, in other words, determining how the silicon-germanium alloy can amplify and emit light as a laser, and measure it. There are several open questions to resolve before silicon germanium can become fully integrated with silicon-based electronics, remarks Haverkort: “First, these devices have to be integrated with existing technologies and that is still a hurdle.” He expects that future quantum computers will use applications such as low-cost silicon-based LEDs, optical fibre lasers, light sensors, and light-emitting quantum dots. In general, the shift from electrical to optical communication will boost innovation in many sectors, from laser-based radars for autonomous driving to sensors for medical diagnosis or air pollution detection in real-time.
Carbon Nanotube based Single photon emitter
Most recently, room temperature SPEs in carbon nanotubes have equally been demonstrated to have emission in the telecom range. Researchers at Los Alamos and partners in France and Germany are exploring the enhanced potential of carbon nanotubes as single-photon emitters for quantum information processing.
Nanotubes integrated into electroluminescent devices can provide greater control over timing of light emission and they can be feasibly integrated into photonic structures. They are highlighting the development and photophysical probing of carbon nanotube defect states as routes to room-temperature single photon emitters at telecom wavelengths.
“It has been long known that semiconducting single-wall carbon nanotubes (SWCNTs) display strong excitonic binding and emit light over a broad range of wavelengths, but their use has been hampered by a low quantum yield and a high sensitivity to spectral diffusion and blinking. In this Perspective, we discuss recent advances in the mastering of SWCNT optical properties by chemistry, electrical contacting and resonator coupling towards advancing their use as quantum light sources. We describe the latest results in terms of single-photon purity, generation efficiency and indistinguishability. Finally, we consider the main fundamental challenges stemming from the unique properties of SWCNTs and the most promising roads for SWCNT-based chip integrated quantum photonic sources.”
MIT and RICE researchers develop Carbon Nanotube based single photon sources
To achieve practical usage, the single photons should be in the telecom wavelengths, which range from 1,260-1,675 nanometers, and the device should be functional at room temperature. To date, only a single fluorescent quantum defect in carbon nanotubes possesses both features simultaneously. However, the precise creation of these single defects has been hampered by preparation methods that require special reactants, are difficult to control, proceed slowly, generate non-emissive defects, or are challenging to scale.
Now, research from Angela Belcher, head of the MIT Department of Biologicial Engineering, Koch Institute member, and the James Crafts Professor of Biological Engineering, and postdoc Ching-Wei Lin, published online in Nature Communications, describes a simple solution to create carbon-nanotube based single-photon emitters, which are known as fluorescent quantum defects. “We can now quickly synthesize these fluorescent quantum defects within a minute, simply using household bleach and light,” Lin says. “And we can produce them at large scale easily.”
Belcher’s lab has demonstrated this amazingly simple method with minimum non-fluorescent defects generated. Carbon nanotubes were submerged in bleach and then irradiated with ultraviolet light for less than a minute to create the fluorescent quantum defects.
The availability of fluorescent quantum defects from this method has greatly reduced the barrier for translating fundamental studies to practical applications. Meanwhile, the nanotubes become even brighter after the creation of these fluorescent defects. In addition, the excitation/emission of these defect carbon nanotubes is shifted to the so-called shortwave infrared region (900-1,600 nm), which is an invisible optical window that has slightly longer wavelengths than the regular near-infrared. What’s more, operations at longer wavelengths with brighter defect emitters allow researchers to see through the tissue more clearly and deeply for optical imaging. As a result, the defect carbon nanotube-based optical probes (usually to conjugate the targeting materials to these defect carbon nanotubes) will greatly improve the imaging performance, enabling cancer detection and treatments such as early detection and image-guided surgery.
Cancers were the second-leading cause of death in the United States in 2017. Extrapolated, this comes out to around 500,000 people who die from cancer every year. The goal in the Belcher Lab is to develop very bright probes that work at the optimal optical window for looking at very small tumors, primarily on ovarian and brain cancers. If doctors can detect the disease earlier, the survival rate can be significantly increased, according to statistics. And now the new bright fluorescent quantum defect can be the right tool to upgrade the current imaging systems, looking at even smaller tumors through the defect emission.
“We have demonstrated a clear visualization of vasculature structure and lymphatic systems using 150 times less amount of probes compared to previous generation of imaging systems,” Belcher says, “This indicates that we have moved a step forward closer to cancer early detection. ” In collaboration with contributors from Rice University, researchers can identify for the first time the distribution of quantum defects in carbon nanotubes using a novel spectroscopy method called variance spectroscopy. This method helped the researchers monitor the quality of the quantum defect contained-carbon nanotubes and find the correct synthetic parameters easier.
Other co-authors at MIT include biological engineering graduate student Uyanga Tsedev, materials science and engineering graduate student Shengnan Huang, as well as Professor R. Bruce Weisman, Sergei Bachilo, and Zheng Yu of Rice University.
Carbon nanotubes based Single-photon emitter has promise for quantum info-processing
By shining laser light at carbon nanotubes containing special defects, scientists in the US and Japan have taken a step forward in the quest to deliver single photons at room temperature and at wavelengths suited to the telecommunications industry. The technique, which would be a boon for developers of quantum technology, allows the researchers to tune the light emitted by the nanotubes across a range of infrared wavelengths, at some of which they showed room-temperature, single-photon emission.
The disadvantage of using carbon nanotubes is that these rolled-up sheets of carbon atoms are one-dimensional and so do not naturally have the energy-level structure of a (zero-dimensional) atom. In the latest work, Stephen Doorn at the Los Alamos National Laboratory in New Mexico and colleagues generate electron–hole pairs known as excitons on the surface of a nanotube using a laser beam, and then trap a single exciton within a defect on the nanotube surface such that the electron drops back to its ground state and emits a single photon.
Such a scheme was first reported by Atac Imamoǧlu of ETH Zürich and colleagues in 2008. That work exploited very slight dips in electric potential on the surface of carbon nanotubes caused by variations in the nanotubes’ environment when they are stored in a liquid suspension and then dispersed on a substrate. Unfortunately, the dips are just a few milli-electronvolts (meV) deep, which is far less than the thermal energy of an exciton at room temperature. As such, the scheme relied on cooling the nanotubes down to 4 K.
To operate at room temperature, Doorn and co-workers add defects that create much deeper potential wells. They did so first two years ago when they introduced oxygen defects with depths of up to 300 meV. But although they managed to generate single photons at room temperature, they found that the emission was unstable – blinking on and off rather than remaining constant – and were unable to tune the emission to telecom wavelengths.
Now the researchers have turned to molecules similar to benzene, a ring of six carbon-hydrogen pairs that form a 130–300 meV-deep pit when attached covalently to a nanotube. By varying a nanotube’s diameter, the nanotube plus ring can be made to emit from 1.15 to 1.6 μm at three wavelengths within the telecom band. They have also shown that the modified nanotubes can generate single photons stably at room temperature at wavelengths less than 1.5 μm. Unfortunately, however, at the wavelength most useful for telecoms applications – 1.55 μm – the tubes had to be held at 220 K.
Beyond raising efficiencies, Doorn and colleagues will have to show that they can drive the nanotube emitters electrically so as to make chip-mountable devices, with their existing laboratory experiment being driven by a laser that occupies half of an optical bench. “We expect that we can demonstrate an electrically driven photon source within a year or two,” says Htoon. “But integration into a photonic network will take a bit longer.”
SiC based Single photon emitter
SiC is a wide band gap semiconductor widely used in LED industry. It is also a prominent material in the application of advanced high power, high temperature electronics. In recent years, defects in SiC have attracted increasing attention owing to their magneto-optical properties and the convenience for fabrication and scalability. Different types of SPEs have been discovered in SiC, such as carbon antisite–vacancy pair, silicon vacancies, and divacancies. However, those SPEs have emission either in the visible range or being weak in the near-infrared range.
Russian Researchers produce ultra high speed single photon emitter based on SiC
A research team from Moscow Institute of Physics and Technology (MIPT) showed how a single-photon emitting diode based on silicon carbide (SiC), a semiconductor material used in optoelectronics, could be used to emit up to several billion photons per second. Researchers further showed that the electroluminescence of color centers in SiC could be used to increase the data transfer rate in unconditionally secure quantum communication lines to more than 1 Gb/s.
The MIPT team focused on SiC’s color center — a point defect in the lattice structure of SiC that can emit or absorb a photon at a wavelength to which the material is transparent in the absence of defects. The team investigated the physics behind the process of single-photon emission from color centers of SiC under electrical pumping. Researchers showed that color centers in SiC could be superior to any other quantum light emitter under electrical control at room temperature.
Using a theoretical approach and numerical simulations, researchers demonstrated that at room temperature, the photon emission rate from a PIN SiC single-photon emitting diode could exceed 5 Gcounts per second, which is higher than what can be achieved with electrically driven color centers in diamond or epitaxial quantum dots.
Researchers point out that new materials are likely to be found that will rival SiC in terms of brightness of single-photon emission. However, unlike SiC, the new materials will require new technological processes to be used in mass production of devices. By contrast, SiC-based single-photon sources are compatible with CMOS technology, which is a standard for manufacturing electronic integrated circuits. This makes SiC a promising material for building ultrawide-bandwidth, unconditionally secure data communication lines for quantum communications.
According to researchers, these findings could lay the foundation for the development of practical photonic quantum devices, which could be produced in a well-developed CMOS compatible process flow.
Bright room temperature single photon source at telecom range in cubic silicon carbide
Researchers led by Junfeng Wang from Singapore, in their paper, presented a type of bright ( ~ MHz) single emitters in 3C-SiC, which work at room temperature and emit in the telecom range. “The sample we use is high-purity 3C-SiC epitaxy layer grown on a silicon substrate. First, we measured the photoluminescence (PL) spectrum of different SPEs and find that their fluorescence wavelengths lie in the telecom region. Then we investigated their optical properties: photostability and saturation behavior.
Our results show that they have stable count rates of ~ MHz at room temperature. Finally, we investigate their polarization properties for both excitation and emission, which demonstrate that these emitters can be treated as almost perfect single dipole. The polarization degree of both excitation and emission can reach up to around 97%. All these properties are highly desired in the QKD protocols with polarization coding scheme. Altogether with the fact that SiC is a growth and fabrication-friendly material, our result may be relevant for future applications in quantum communication technology.
Honeycomb material is single photon source at room temperature
A compound called hexagonal boron nitride (hBN) is about to make many quantum dreams a reality, according to a pair of Australian physicists writing in Science. The material has a honeycomb crystalline structure, like graphite, and can be created in thin sheets comprising just a single layer of atoms. What’s more, it can be made to emit single photons on demand without any fancy refrigeration.
That property was discovered in 2016, by a team of researchers at the University of Technology Sydney in Australia. The two authors of this new Science article, Igor Aharonovich and Milos Toth, were part of that team. “Scientific applications are here already,” says Aharonovich. “It’s so far the brightest single photon source. Many people use it for calibration of their setups, and confocal microscope alignment.”
Commercial applications may only be 3–5 years away, he adds, if everything “works as we think it should”. Even so, it won’t all be plain sailing. Making single-layer hBN itself in any large quantity “will require dramatic advances in fabrication methods”.
Thin-film breakthrough promise 2D materials based single photon source
Efforts to create reliable light-based quantum computing, quantum key distribution for cybersecurity, and other technologies got a boost from a new study demonstrating an innovative method for creating thin films to control the emission of single photons.
“Efficiently controlling certain thin-film materials so they emit single photons at precise locations—what’s known as deterministic quantum emission—paves the way for beyond-lab-scale quantum materials,” said Michael Pettes, a Los Alamos National Laboratory materials scientist and leader of the multi-institution research team.
The scalability of these two-dimensional, tungsten/selenium thin films makes them potentially useful in processes to manufacture quantum technologies. Single-photon generation is a requirement for all-optical quantum computing and key distribution in quantum communications, and it is crucial for advancing quantum information technologies.
The project, documented as a Featured Article in the journal Applied Physics Letters this week, exploits strain at highly spatially localized and well-separated emission sites, or tips, in a tungsten/selenium film. The team synthesized the film through chemical vapor deposition using a multi-step, diffusion-mediated gas source.
Because the material is very thin, it conforms to the radius of the tips and the material bends towards the substrate by more than a few percent, like someone lying on a bed of nails. The resulting strain is enough to change the electronic structure, but only at the tips. The affected area emits light of a different color and nature than light from the rest of the film.
“While more research is needed to fully understand the role of mechanical deformation in creating these quantum emission sites, we may enable a route to control quantum optical properties by using strain,” Pettes said. “These single-photon sources form the basis for photonics-based, all-optical quantum computing schemes.”
Engineering of quantum emission in 2D materials is still in a very early stage, the authors note. While studies have observed single photons originating from defect structures in these materials, previous work has suggested that non-uniform strain fields might govern the effect. However, the mechanism responsible for this emergent phenomenon remains unclear and is the focus of ongoing work at Los Alamos.
Quantum dot single-photon source
These light sources are nano-sized semiconductor “quantum dots”–tiny manufactured collections of tens of thousands to a million atoms packed within a volume of linear size less than a thousandth of the thickness of typical human hair buried in a matrix of another suitable semiconductor. They have so far been proven to be the most versatile on-demand single photon generators. The optical circuit requires these single photon sources to be arranged on a semiconductor chip in a regular pattern. Photons with nearly identical wavelength from the sources must then be released in a guided direction. This allows them to be manipulated to form interactions with other photons and particles to transmit and process information.
A quantum dot single-photon source is based on a single quantum dot placed in an optical cavity. It is an on-demand single-photon source. A laser pulse can excite a pair of carriers known as an exciton in the quantum dot. The decay of a single exciton due to spontaneous emission leads to the emission of a single photon. Due to interactions between excitons, the emission when the quantum dot contains a single exciton is energetically distinct from that when the quantum dot contains more than one exciton.The spontaneous emission rate of the emitted photons can be enhanced by integrating the quantum dot in an optical cavity. Additionally, the cavity leads to emission in a well-defined optical mode increasing the efficiency of the photon source.
Until now, there has been a significant barrier to the development of such circuits. For example, in current manufacturing techniques quantum dots have different sizes and shapes and assemble on the chip in random locations. The fact that the dots have different sizes and shapes mean that the photons they release do not have uniform wavelengths. This and the lack of positional order make them unsuitable for use in the development of optical circuits.
In recently published work in Feb 2021, Scientists in USC’s Mork Family Department of Chemical Engineering and Materials Science have shown that single photons can indeed be emitted in a uniform way from quantum dots arranged in a precise pattern. It should be noted that the method of aligning quantum dots was first developed at USC by the lead PI, Professor Anupam Madhukar, and his team nearly thirty years ago, well before the current explosive research activity in quantum information and interest in on-chip single-photon sources. In this latest work, the USC team has used such methods to create single-quantum dots, with their remarkable single-photon emission characteristics. It is expected that the ability to precisely align uniformly-emitting quantum dots will enable the production of optical circuits, potentially leading to novel advancements in quantum computing and communications technologies.
To create the precise layout of quantum dots for the circuits, the team used a method called SESRE (substrate-encoded size-reducing epitaxy) developed in the Madhukar group in the early 1990s. In the current work, the team fabricated regular arrays of nanometer-sized mesas (Fig. 1(a)) with a defined edge orientation, shape (sidewalls) and depth on a flat semiconductor substrate, composed of gallium arsenide (GaAs). Quantum dots are then created on top of the mesas by adding appropriate atoms using the following technique.
“The breakthrough paves the way to the next steps required to move from lab demonstration of single photon physics to chip-scale fabrication of quantum photonic circuits,” Zhang said. “This has potential applications in quantum (secure) communication, imaging, sensing and quantum simulations and computation.” Madhukar said that it is essential that quantum dots be ordered in a precise way so that photons released from any two or more dots can be manipulated to connect with each other on the chip. This will form the basis of building unit for quantum optical circuits.
“This advance is an important example of how solving fundamental materials science challenges, like how to create quantum dots with precise position and composition, can have big downstream implications for technologies like quantum computing,” said Evan Runnerstrom, program manager, Army Research Office, an element of the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory. “This shows how ARO’s targeted investments in basic research support the Army’s enduring modernization efforts in areas like networking.”
Quantum dot emitters provide on-demand production of single photons, but the photons are emitted in all directions, which results in low collection efficiency. Abudayyeh et al. increased the directionality of photons emitted from quantum dots operating at room temperature by placing them in nanoantennas. They achieved a record-high collection efficiency of 85% of the emitted single photons. The nanoantennas directed the photons in a single direction, increasing the number of collected photons. The authors optimized the coupling between the antennas and quantum dots with improved antenna fabrication techniques and enhanced quantum dots known as CdSe/CdS core/thick-shell or giant quantum dots, which are non-blinking and non-photobleaching at room temperature. The authors plan couple these on-chip, room temperature nanoantenna-emitter devices with a fiber to produce plug-and-play single photon sources with high collection efficiencies.
Perovskite quantum dots
Many researchers have tried to produce sources that could emit such coherent single photons, but all have had limitations. Random fluctuations in the materials surrounding these emitters tend to change the properties of the photons in unpredictable ways, destroying their coherence. Finding emitter materials that maintain coherence and are also bright and stable is “fundamentally challenging,” Utzat says. That’s because not only the surroundings but even the materials themselves “essentially provide a fluctuating bath that randomly interacts with the electronically excited quantum state and washes out the coherence,” he says.
Previous chemically-made colloidal quantum dot materials had impractically short coherence times, but this team found that making the quantum dots from perovskites, a family of materials defined by their crystal structure, produced coherence levels that were more than a thousand times better than previous versions. The coherence properties of these colloidal perovskite quantum dots are now approaching the levels of established emitters, such as atom-like defects in diamond or quantum dots grown by physicists using gas-phase beam epitaxy.
One of the big advantages of perovskites, they found, was that they emit photons very quickly after being stimulated by a laser beam. This high speed could be a crucial characteristic for potential quantum computing applications. They also have very little interaction with their surroundings, greatly improving their coherence properties and stability. Such coherent photons could also be used for quantum-encrypted communications applications, Bawendi says. A particular kind of entanglement, called polarization entanglement, can be the basis for secure quantum communications that defies attempts at interception.
Now that the team has found these promising properties, the next step is to work on optimizing and improving their performance in order to make them scalable and practical. For one thing, they need to achieve 100 percent indistinguishability in the photons produced. So far, they have reached 20 percent, “which is already very remarkable,” Utzat says, already comparable to the coherences reached by other materials, such as atom-like fluorescent defects in diamond, that are already established systems and have been worked on much longer.
“Perovskite quantum dots still have a long way to go until they become applicable in real applications,” he says, “but this is a new materials system available for quantum photonics that can now be optimized and potentially integrated with devices.” It’s a new phenomenon and will require much work to develop to a practical level, the researchers say. “Our study is very fundamental,” Bawendi notes. “However, it’s a big step toward developing a new material platform that is promising.”
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