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Photonic Integrated Circuits technologies promise Quantum computers and sensors, as system-on-chip solutions integrated into laptops and cell phones.

Photonics is a breakthrough technology as it uses photons (smallest unit of light) as the data carrier instead of electrons (smallest unit of electricity) used in electronic ICs. As light trav very high speeds, photonics is widely used to transfer huge amounts of data at a very high speed.Thus photnics based products are primarily deployed in the field of optical fiber & optical free space communications.


Just as Integrated circuit (IC) is a microelectronic device that houses multiple electric circuits on a chip, a photonic integrated circuit (PIC) or Integrated Photonic circuits (IPC) are devices that integrate multiple photonic functions on a chip. A typical IPC may consist of single photon sources, nonlinear photon processing circuits and photon detectors all integrated onto a solid-state chip. Photonic integrated circuits (PICs) have attracted considerable attention owing to their small footprint, scalability, reduced power consumption and enhanced processing stability.


Recently Photonic integrated circuits are also being used for Quantum Information Sciences like Quantum sensors, computing, Quantum key cryptography and simulation. Moreover, with the advancements in quantum computing, the adoption of photonic ICs is increasing as they allow multitasking that quantum computing readily requires.


Now, a Stanford University team led by Professor Jelena Vuckovic has built an integrated circuit to control the flow of light through a diamond chip, helping pave the way for quantum processors that, in theory, could perform some tasks, such as code-breaking, far faster than the fastest electronic computers today.


“Quantum technology is roughly where electronic technology was in the early 1970s,” Vuckovic said, who led the research team. “Researchers have figured out how to make very basic integrated circuits, but now they have to be scaled and made much better.”

Photonic Quantum computers

Photonic quantum computing is one of the leading approaches to universal quantum computation. Photonic systems have several unique and advantageous features. First, quantum states of photons are maintained without vacuum or cooling systems due to their extremely weak interaction with the external environment. In other words, photonic quantum computers can work in an atmospheric environment at room temperature. Second, photons are an optimal information carrier for quantum communication since they propagate at the speed of light and offer large bandwidth for a high data transmission capacity. Therefore, photonic quantum computers are completely compatible with quantum communication.


The large bandwidth of photons also provides high-speed (high clock frequency) operation in photonic quantum computers. These advantageous features, together with mature technologies to prepare and manipulate photonic quantum states with linear optical elements and nonlinear crystals, have made photonic systems one of the leading approaches to building quantum computers.


Miniaturising quantum technologies using photonic integration and making them available to users as system-on-chip solutions

Despite their success, photonic quantum technologies currently face a significant roadblock to widespread application: the circuits are severely limited in complexity. One measure of circuit complexity is the product of its breadth (number of photons), and its depth (number of interactions between the photons).


According to “The Australian Centre of Excellence for Quantum Computation & Communication Technology “, Increasing circuit complexity will require solutions to the following inherent difficulties:

1) It is difficult to store photons, since they interact weakly with other particles and move at the speed of light. This limits circuit breadth, since many protocols require holding information in one part of the circuit while waiting for information to be processed in parallel.

2) It is difficult to efficiently produce and detect single photons. The current best photon sources are spontaneous, i.e. the photons are produced at random times with probability, p<1. This quickly limits circuit breadth, since the probability of producing 1 photon per mode decreases exponentially (for N input modes it is p^N << 1).

3) Current photonic entangling gates are inherently random—with success rates varying between 9% and 25% which means they cannot be scaled.


The project UNIQORN (Affordable Quantum Communication for Everyone: Revolutionizing the Quantum Ecosystem from Fabrication to Application) has set itself the goal of miniaturising quantum technologies using photonic integration and making them available to users as system-on-chip solutions. The project will develop the key components for the quantum communications systems of the future and an important focus of the research is on integrated system-on-chip solutions. They are the basis for highly miniaturised optical systems that can fully exploit quantum mechanical properties such as entanglement and squeezed light.


The core of this integration is the micro-optical bench technology of the PolyBoard platform, which makes it possible to combine large, millimeter size, optical components such as crystals for generating entangled photons with typically sub-millimeter sized integrated optical components and functionalities on a PolyBoard chip. It is based on the generation of free-space optical areas inside photonic integrated chips with the help of specially adapted lenses. As a result, known material systems for quantum technology can be combined directly with photonic integrated circuits, without having to compromise on the performance of the micro-optical components. So far, this technology facilitated the development of miniaturized optical components for telecom and datacom applications as well as micro-optical chips for analytics and sensor technology.


This toolbox will be further developed in the coming years as part of the “Quantum Flagship” of the European Union to meet the specific requirements of quantum technologies.


Physicists at TUM have succeeded in accurately placing light sources in nano-thin material layers.

An international team headed up by Alexander Holleitner and Jonathan Finley, physicists at the Technical University of Munich (TUM), has succeeded in placing light sources in atomically thin material layers with an accuracy of just a few nanometers. The new method allows for a multitude of applications in quantum technologies, from quantum sensors and transistors in smartphones through to new encryption technologies for data transmission.


“This constitutes a first key step towards optical quantum computers,” says Julian Klein, lead author of the study. “Because for future applications the light sources must be coupled with photon circuits, waveguides for example, in order to make light-based quantum calculations possible.”


The critical point here is the exact and precisely controllable placement of the light sources. It is possible to create quantum light sources in conventional three-dimensional materials such as diamond or silicon, but they cannot be precisely placed in these materials.


The physicists then used a layer of the semiconductor molybdenum disulfide (MoS2) as the starting material, just three atoms thick. They irradiated this with a helium ion beam which they focused on a surface area of less than one nanometer.


In order to generate optically active defects, the desired quantum light sources, molybdenum or sulfur atoms are precisely hammered out of the layer. The imperfections are traps for so-called excitons, electron-hole pairs, which then emit the desired photons.


Technically, the new helium ion microscope at the Walter Schottky Institute’s Center for Nanotechnology and Nanomaterials, which can be used to irradiate such material with an unparalleled lateral resolution, was of central importance for this.


“It is possible to integrate our quantum light sources very elegantly into photon circuits,” says Klein. “Owing to the high sensitivity, for example, it is possible to build quantum sensors for smartphones and develop extremely secure encryption technologies for data transmission.”


Stanford University researchers have has built an integrated circuit to control the flow of light through a diamond chip

Building an optical integrated circuit in diamond is a practical step toward making quantum technologies useful. Engineers have long known how to design ordinary electronic circuits and control the electrons that help perform computational tasks. But they are still struggling to build quantum circuits with all the necessary pathways to control photons, the basic particles in light.


To meet this design challenge, Vuckovic’s lab used diamond, a crystal that can have atomic impurities that trap electrons. A laser beam can be pointed at one of these trapped electrons, causing it to spin. These spinning electrons — called qubits, or quantum bits — are the basis for performing quantum calculations just as transistors are the basis for performing electronic calculations. In essence, a quantum processor would have an array of qubits connected by light flowing through an optical integrated circuit, just as an electronic computer has transistors connected by current flowing through wires.


To create their optical integrated circuit, Vuckovic’s team, led by graduate student Constantin Dory, developed algorithms that considered the positions of the impurities that form qubits, and the ways that lasers could manipulate these qubits to perform calculations. The algorithms also took into account the capabilities of the equipment that engineers use to make chips. After considering all these variables together using a process called inverse design, the algorithms generated a schematic that the researchers used to fabricate their optical integrated circuit.


To date, the researchers were able to fabricate circuits consisting of six building blocks, potentially enabling interaction of only a few qubits. To build a useful quantum processor, the researchers say they’ll have to design and build a chip with hundreds of interacting qubits, all interconnected with optical pathways, which is extremely challenging, but possible. The Stanford team is also experimenting with other crystals that may prove useful for controlling light and qubits.


Vuckovic foresees one application of their diamond optical chip in the near term — spy-proofing fiber optic networks. Today, all sorts of sensitive data from bank transfers to government secrets flow as a stream of ordinary light through fiber optic cables. Spies or criminals can tap into this stream without leaving any trace. However, if quantum light, such as a single photon, is used for communication, eavesdropping can be detected because any intrusion would leave behind subtle fingerprints.


Challenges remain. For starters, it is difficult to transmit quantum light over large distances. Vuckovic is working with several other research teams to build quantum repeaters, in which optical chips with a few qubits each, positioned at regular intervals, would be used to transmit tamper-proof quantum signals over long distances, even across continents. “We think a long-distance quantum network is achievable within a five-year time frame,” she said


Single-photon emission enhancement’ seen as step toward quantum technologies

Researchers from Purdue, the Russian Quantum Center, Moscow Institute of Physics and Technology, Lebedev Physical Institute, and Photonic Nano-Meta Technologies Inc. have demonstrated a new way to enhance the emission of single photons by using “hyperbolic metamaterials,” a step toward creating devices in work aimed at developing quantum computers and communications technologies.


Optical metamaterials harness clouds of electrons called surface plasmons to manipulate and control light. Purdue University researchers had previously created “superlattices” from layers of the metal titanium nitride and the dielectric, or insulator, aluminum scandium nitride. Unlike some of the plasmonic components under development, which rely on the use of noble metals such as gold and silver, the new metamaterial is compatible with the complementary metal–oxide–semiconductor manufacturing process used to construct integrated circuits.


The metamaterial is said to be hyperbolic, meaning it possesses unique properties leading to the increased output of light. In new findings the researchers have demonstrated how attaching nanodiamonds containing “nitrogen-vacancy centers” to the new metamaterial further enhances the production of single photons, workhorses of quantum information processing, which could bring superior computers, cryptography and communications technologies. The single-photon emitters could be used to build highly efficient room temperature CMOS-compatible single-photon sources.


A nitrogen-vacancy center is an atomic-scale defect formed in the diamond lattice by substituting a nitrogen atom for a carbon atom and creating a neighboring void in the lattice. Placing a nanodiamond containing an NV center on the surface of hyperbolic metamaterials not only enhances the emission of photons, but also changes the pattern of light emitted, a trait that could be important for the development of quantum devices, said graduate student Mikhail Y. Shalaginov, the paper’s lead author.


The nitrogen vacancy also makes it possible to potentially record information based on the nuclear or electron “spin” state of the center, which is promising for quantum computing. The spin can be either “up” or “down” – forming the quantum superposition of the up and down states – representing a new technology for processing information.


Entanglement on a chip: Breakthrough promises secure communications and faster computers

Entanglement – the instantaneous connection between two particles no matter their distance apart – is one of the most intriguing and promising phenomena in all of physics. Properly harnessed, entangled photons could revolutionize computing, communications, and cyber security. Though readily created in the lab and by comparatively large-scale optoelectronic components, a practical source of entangled photons that can fit onto an ordinary computer chip has been elusive.


New research, reported today in The Optical Society’s (OSA) new high-impact journal Optica, describes how a team of scientists has developed, for the first time, a microscopic component that is small enough to fit onto a standard silicon chip that can generate a continuous supply of entangled photons. To date, entangled photon emitters – which are principally made from specially designed crystals — could be scaled down to only a few millimeters in size, which is still many orders of magnitude too large for on-chip applications. In addition, these emitters require a great deal of power, which is a valuable commodity in telecommunications and computing.


To overcome these challenges, the researchers explored the potential of ring resonators as a new source for entangled photons. These well-established optoelectronic components can be easily etched onto a silicon wafer in the same manner that other components on semiconductor chips are fashioned. To “pump,” or power, the resonator, a laser beam is directed along an optical fiber to the input side of the sample, and then coupled to the resonator where the photons race around the ring. This creates an ideal environment for the photons to mingle and become entangled.


The new design is based on an established silicon technology known as a micro-ring resonator. These resonators are actually loops that are etched onto silicon wafers that can corral and then reemit particles of light. By tailoring the design of this resonator, the researchers created a novel source of entangled photons that is incredibly small and highly efficient, making it an ideal on-chip component.


“The main advantage of our new source is that it is at the same time small, bright, and silicon based,” said Daniele Bajoni, a researcher at the Università degli Studi di Pavia in Italy and co-author on the paper. “The diameter of the ring resonator is a mere 20 microns, which is about one-tenth of the width of a human hair. Previous sources were hundreds of times larger than the one we developed.”


“In the last few years, silicon integrated devices have been developed to filter and route light, mainly for telecommunication applications,” observed Bajoni. “Our micro-ring resonators can be readily used alongside these devices, moving us toward the ability to fully harness entanglement on a chip.” As a result, this research could facilitate the adoption of quantum information technologies, particularly quantum cryptography protocols, which would ensure secure communications in ways that classical cryptography protocols cannot.


Photonic Circuits and Processing

Nanophotonic waveguides can take on the role of electrical wires to connect functional elements into nanophotonic integrated circuits (NPICs). By embedding superconducting nanowire single-photon detectors (SNSPDs) in nanophotonic circuits, these waveguide-integrated detectors are a key building block for future on-chip quantum computing applications. Nonlinear photon processing can be achieved either by colliding the photons in strongly nonlinear PhC cavities, or by using integrated interferometers and detectors.


Researchers at Münster University develop an easy-to-produce interface between quantum emitters and nanophotonic networks.

All over the world, researchers are engaged in intensive work on the individual components of quantum technologies – these include circuits that process information using single photons instead of electricity, as well as light sources producing such individual quanta of light. Coupling these two components to produce integrated quantum optical circuits on chips presents a particular challenge.


Researchers at the University of Münster have now developed an interface that couples light sources for single photons with nanophotonic networks. This interface consists of so-called photonic crystals, i.e. nanostructured dielectric materials that can enhance a certain wavelength range when light passes through. Such photonic crystals are used in many areas of research, but they had not previously been optimized for this type of interface. The researchers took particular care to achieve this feat in a way that allows for replicating the photonic crystals straightforwardly by using established nanofabrication processes.


As single photons obey the laws of quantum physics, researchers talk of quantum emitters with respect to the light sources involved. For their study, the researchers considered quantum emitters which are embedded in nanodiamonds and emit photons when they are stimulated by means of electromagnetic fields. In order to produce the interfaces desired, the researchers’ aim was to develop optical structures tailored to the wavelength of the quantum emitters.


Cavities or holes in photonic crystals are well suited for trapping light in minute volumes and getting it to interact with matter such as, in this case, nanodiamonds. Jan Olthaus, a PhD student in physics in Doris Reiter’s junior research group, developed theoretical concepts and special computer-assisted simulation techniques in order to compute the designs for these photonic crystals.


The theoretically developed designs were produced by physicists in the junior research group headed by Carsten Schuck at the Center for NanoTechnology and the Center for Soft Nanoscience at Münster University. PhD student Philipp Schrinner manufactured the crystals from a thin film of silicon nitride. For this purpose, he used modern electron beam lithography and special etching methods on the equipment at the Münster Nanofabrication Facility and succeeded in producing high-quality crystals directly on the base material of silicon dioxide.


The next steps for the researchers involve trying to position the quantum emitters, embedded in the nanodiamonds, at certain spots on the photonic crystals – with the aim of putting the results of the study into practice. To this end, the team headed by Carsten Schuck is already developing a special nanofabrication technique which is able, for example, to place a diamond just 100-nanometres in size with an accuracy of less than 50 nanometres.


“Our work shows that it is not only in highly specialized laboratories and unique experiments that complex quantum technologies can be produced,” says physicist Dr. Carsten Schuck, an assistant professor at Münster University who headed the study together with Dr. Doris Reiter, likewise an assistant professor, who works in the field of solid state theory. The results could help to make quantum technologies scalable. The study was published in the journal Advanced Quantum Technologies on October 1, 2019.


Integrating photons sources, detectors and circuits on a chip into useful subsystems

Single-photon detectors, sources and circuits have all been developed separately in silicon but putting them all together and integrating them on a chip is a huge challenge.


Scientists and engineers from an international collaboration of University of Bristol , Toshiba Corporation (Japan), Stanford University (US), University of Glasgow (UK) and TU Delft (The Netherlands), led by Mark Thompson have, for the first time, generated and manipulated high-quality identical photons in a reproducible way on a single silicon chip. The Previous attempts have required external light sources to generate the photons; this new chip integrates components that can generate photons inside the chip.


The core circuits of quantum teleportation, which generate and detect quantum entanglement, have been successfully integrated into a photonic chip by an international team of scientists from the universities of Bristol, Tokyo, Southampton and NTT Device Technology Laboratories. These results pave the way to developing ultra-high-speed quantum computers and strengthening the security of communication. The optical circuits have been implemented on to a silicon microchip measuring just a few millimetres (0.0001 square metres) using state-of-the-art nano-fabrication methods.


Researchers are currently working to integrate these components into useful subsystems and addressing issues such as feedback and control of large-scale PICs, electronic-photonic integration, electrical and optical power requirements and limitations, and packaging



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