The second quantum revolution five quantum technology applications for integrated photonics

In quantum computing, optical quantum processors still lack the number of quantum bits compared to other types of quantum bits. However, they hold promise because of their potential to scale up optical quantum processors.

For a fully functional optical quantum computer, photons must be generated, processed and detected. The generation is achieved by a quantum light source that produces single photons that can be used to encrypt information. The combination of linear optics forms the necessary quantum gates where these quantum bits of information are processed. Finally, an efficient single-photon detector detects these photons and reads the information.

The amazing thing about optical quantum computing is that all these building blocks can be implemented in photonic integrated circuits (PICs). Different promising platforms can serve the above processes, such as silicon on insulator (SiO2), silicon on insulator (Si), silicon nitride (Si3Na4), lithium niobate (LN), gallium arsenide (GaAs), indium phosphide (InP) ...... Inevitably, the properties of each platform are different; each platform also offers a different set of desirable features. Innovative solutions are needed that allow the combination of these platforms into hybrid PICs.

In the near future, we will see the rise of hybrid platforms that combine different photonic technologies into a single functional unit and that can manage to overcome the limitations of monolithic photonic circuits.

01Integrated single-photon sources

The generation of single photons and their states on a chip is essential for all photon-based quantum applications. Notably, by actively demultiplexing (demultiplexing) single photon flows into different spatial modes, the door can be opened to the possibilities of quantum computing and advanced quantum sensing.

Quandela (Paris, France), a developer of quantum light sources and provider of photonic quantum computing solutions, is working in this direction with their quantum dots, which are highly efficient sources of undifferentiated single photons. Quantum dots are "artificial atoms" fully integrated in a semiconductor material, following standard semiconductor technology.

Prometheus, Quandela's stand-alone single-photon source

The company takes this goal one step further with Prometheus, its first stand-alone quantum light source. The patented fiber braid technology allows for high integration in a plug-and-play 19-inch rack, providing high-quality photonic quantum bits for quantum information processing.

Sparrow Quantum (Copenhagen, Denmark) offers a single photon source chip as a scalable solution for efficient photon generation; the chip is based on a GaAs platform with InAs quantum dots embedded in a photonic crystal nanostructure. Precise control of the quantum dot surroundings allows for the world's highest fidelity in single photon generation and enables multiphoton interactions. the next step for Sparrow Quantum is to develop a user-friendly single photon source for a plug-and-play experience.

02Integrated single-photon detector

On-chip detection of single photons allows for the final readout of quantum information. Several single-photon detector technologies are available, such as avalanche photodiodes and superconducting nanowire single photon detectors (SNSPD).

Pixel Photonics (Germany) has developed a method to integrate SNSPDs onto nanophotonic waveguides. The technique is material independent of the waveguide and allows the integration of highly parallel, efficient and ultrafast single photon detection with the preferred quantum photonic platform for the respective application. The detector geometry can be easily optimized for any desired wavelength, from ultraviolet (UV) to approximately 2 µm. By combining passive (e.g. silicon nitride) or active (e.g. lithium niobate) quantum photonic elements with the superior capabilities of SNSPD, Pixel Photonics' technology is inherently suited for wafer-level fabrication, enabling a new level of integration for quantum photonic applications requiring highly efficient single-photon detection.

03Integrated photonics for quantum communications

KETS Quantum's (Bristol, UK) quantum security uses integrated photonics chips to enable flexible, cost-effective and robust solutions that can be deployed at scale, even in the most challenging environments. Over the past five years, KETS has engaged with a number of silicon organizations in diverse sectors including telecommunications, government, defense and finance to help them protect their systems and data with quantum-secure cryptographic solutions. Their technology miniaturizes quantum-secure hardware in a size, form factor and price point that is ready for the quantum age.

Specifically, KETS has worked with Airbus and other partners to successfully demonstrate quantum-secure communications between unmanned aerial vehicles (UAVs) and ground stations - an important step toward satellite-based quantum key distribution (QKD). Their chip-based platform provided a prototype solution under very challenging size, weight and power (SWaP) constraints and demonstrated secure key generation with a gigahertz-operated transmitter under daylight conditions over a high-loss free-space link (~25 dB).

In the telecommunications sector, KETS is the sole supplier of quantum encryption hardware to ParisQCI, one of the first regional steps toward a larger, EU-wide secure quantum communications infrastructure. In order to roll out this technology globally, the project "Building a Standardized Architecture for Quantum Secure Networks" (BaSQuaNa) is a collaboration between the UK and Canada with the overall goal of developing the first transatlantic QKD communications network.

Data centers are an important aspect of any data-driven world, and KETS is co-leading a newly announced £11.6 million ISCF project for future quantum data centers; it will eventually develop prototype integrations using KETS' development kit as the core building block, and test and refine their full applications with end users.

04Programmable quantum chips and photonics-based quantum processors

Photonic computing and processing occupies a leading position in the range of so-called beyond Moore methods. The controlled manipulation of the amplitude, phase, and wavelength of light waves, enabled by integrated photonics, is a fundamental advance on the established electronic digital computing paradigm using only electrons.

iPronics' photonic processor operates in quantum computing applications

iPronics (Valencia, Spain) has introduced the first programmable photonic processor with a wide range of applications, including 5/6G, communications, sensing and computing. In photonic computing, this technology explores optical interference to perform matrix vector multiplication in an ultra-fast and parallel manner. By integrating single photon lasers and photodetectors, these capabilities can be directly applied to quantum computing. This will allow the implementation of quantum logic gates - a key enabler of quantum computing. The programmability of these quantum gates is achieved through a user-friendly software platform that unlocks the full potential of programmable photonics.

A photo of QuiX Quantum's upcoming 52-mode quantum photonic processor. The system contains 2,652 tunable elements that enable reconfiguration and programming of the processor.

QuiX Quantum (Netherlands) is the global market leader in photonic quantum computing hardware, thanks to its proven base technology based on silicon nitride waveguides. They develop and market the most powerful quantum photonic processors on the market: these processors have become the de facto standard for photonic quantum computing throughout Europe. quiX uses the patented TriPleX silicon nitride platform to provide a scalable approach to quantum computing with the advantage of room temperature operation; it has the potential to be used in applications such as personalized medicine, self-driving cars and energy storage. The company's next step is to expand their quantum photonic processor technology to implement a photonic quantum computing architecture, as well as cloud access to one of their complete systems, where it will be possible to run specific quantum computing tasks.

SiPhotonIC (Denmark) aims to bring fast and accurate quantum photonic integrated circuit (QPIC) prototyping to commercial customers and research labs. The company designs and manufactures custom QPICs based on 220 and 250 nm silicon on insulator (SOI) technology. the platform is developed for custom low-loss photonic components for quantum photonic applications. Depending on customer requirements, it relies on deep UV and electron beam lithography processes with lead times of less than one to three months, which is attractive for fast R&D iteration cycles requiring small numbers of SOI samples. For custom QPIC projects, other materials and processes can be considered. The company also offers additional on-demand back-end services through third parties.

SiPhotonIC has a proven track record of fabricating QPICs for internationally recognized research teams and companies, and has published articles in top journals. Indicatively, the generation or sampling of quantum states of light, quantum teleportation, quantum entanglement, QKD, error-proof quantum bits, and other quantum communication and quantum photonic computing concepts have been demonstrated with QPICs using New Light Information's SOI platform.

One of the most complex systems developed by SiPhotonIC is a single programmable QPIC designed for multidimensional quantum entanglement. to achieve this, they fabricated a programmable photonic chip that integrates more than 550 photonic elements onto a single chip, including 16 identical photon pairs of sources. The multidimensional system demonstrates previously unexplored quantum applications such as quantum randomness scaling and self-testing of multidimensional states.

SiPhotonIC's expansion goals include the release of a proprietary platform based on state-of-the-art silicon nitride (SiN) and lithium niobate thin film (TFLN) technologies, photonic prototyping for quantum and classical applications, and product development. The company's target applications include quantum photonic computing, quantum communications, and quantum sensing.

05Highly flexible, low-loss photonic multi-chip integration

In a full-featured optical quantum computer, the above promising platforms need to be integrated to generate, process and detect photons.Vanguard Automation's (Germany) photonic wire bonding and 3D printed microlens technologies combine the complementary nature of these different optical integration platforms into advanced photonic multi-chip modules, enabling the achievement of the full-featured optical quantum computer's the compactness, high performance and tremendous design flexibility required to meet the challenging requirements. The technology relies on highly accurate direct-write 3D laser lithography to print free-form single-mode waveguides between optical chips, thus providing a path to fully automated photonic packaging up to mass production without the need for active alignment.

3D nano-printing can also be used to fabricate faceted beam-shaping elements on optical chips and fibers, resulting in low-loss coupling with high alignment tolerances and for wafer-level probing of optical devices. Photonic harnesses and 3D-printed microlenses have demonstrated their capabilities in quantum applications.

Electron microscope image of the photonic line bond (shown in blue) forming a low-loss connection between a single-mode fiber and a silicon waveguide.

Finally, although much of this work is in the developmental stage, it is clear from the progress made so far that we are entering a paradigm shift in quantum computing and quantum communication and are on the cusp of a second quantum revolution.

Reference links:

https://www.laserfocusworld.com/optics/article/14282714/integrated-photonics-for-quantum-applications