Photonic Integrated Circuits for Quantum Applications

Executive summary

Photonic Integrated Circuits (PICs) and opto-electronic integration are critical to enable and advance quantum systems. Optimized for quantum technologies, PICs would equip those technologies with new capabilities; improve performance; and reduce their size, weight, power and cost (SWaP-C), and thereby accelerate the market for emerging quantum products. PIC technologies comprise both active and passive circuits that may incorporate lasers, amplifiers, modulators, mixers, detectors, filters, multiplexers, as well as methods for hybrid or heterogeneous integration of disparate platforms.

PICs enable many applications—both classical and quantum— by generating, modulating, amplifying, and controlling light through microfabricated optical waveguides in a miniaturized form factor. They are widely used for optical communication, optical sensing (e.g., lidar range sensing), photonic neuromorphic computing, and high-speed optical interconnection/interfacing in high-performance computing or control systems. In addition, PIC components are in general more robust to vibration, shock, and radiation than bulk- and fiber-optic components. Many quantum systems also involve control of light, however at the early stages of development these systems are not generally integrated. As quantum technologies mature, it will be essential to develop PICs that are scalable and robust.

This report synthesizes the expert insights gained at the “Photonic Integrated Circuits for Quantum Applications” workshop held January 19-20, 2023, at the University of Colorado. Participants shared perspectives on the current PICs and quantum technology landscapes, gaps that need to be filled in order to enable PIC-based quantum systems, and recommendations on how to fill those gaps.

PICs research and development is inherently multidisciplinary: the hybrid, heterogenous, or monolithic integration of photonics intersects numerous areas, including silicon photonics, III-V photonics, and nonlinear photonics; likewise, the electronics components require expertise in application specific integrated circuits (ASICs), radio frequency integrated circuits, and integrated photonic technologies. Integration will be vital to the large-scale, low-cost manufacturability of chip-scale sub-system
components.

PIC technologies that have been matured for telecom or datacom applications can be leveraged, however, most quantum applications operate at wavelengths different from those used in telecom applications (1310 and 1550 nm). PICs technology at wavelengths in the UV, visible, and 600-1150 nm range relevant to quantum systems are much less mature.

A goal of this report is to inform public and private investors and the PIC/quantum communities about the state of technology and the research and development needs that must be addressed to support the emerging quantum industry—an industry that is critical to economic and national security. Several technical challenges must be overcome to allow commercially viable PIC technology for quantum applications, including the following.

• Quantum systems use light at a variety of wavelengths depending on the application and fundamental physics. Each wavelength requires specific materials, waveguides, optical components, etc. all of which must be miniaturized and made reliable and robust. This so-called “rainbow problem” could prove to be the industry’s greatest challenge. In addition, quantum systems often require higher powers and narrower linewidths.
• Interconnect fabrication is a challenge, including fiber-to-chip, chip-to-chip and chip-to-free space connections. Modular PIC architectures can contain several interconnects, and may require skilled workers performing complex alignment procedures, which can be prohibitively expensive on a large scale. In addition, new functionalities needed for quantum applications may require new or multiple material systems, and the technology to efficiently combine and integrate these multi-material platforms and components is still a work in progress.
• Reliability and ruggedization of PICs for quantum applications will need to be assessed and verified for the technology to scale and to move from the lab to fielded products. While some products, such as cloud-based quantum computers, may be in controlled environments, others will be subject to temperature, space radiation, and other extremes where PICs will need to perform for extended periods of time without maintenance or technical service.
• Commercial PICs for quantum applications require innovative packaging solutions that enable efficient and cost-effective integration of PICs into larger systems. Packaging requirements include efficient coupling between the on-chip photonic components and the external optical fibers, other PICs, or free-space optics. PICs packaging also involves direct current/radio frequency (DC/RF) electrical connectivity through wire bonds or solder bumps, as well as thermal management.

PIC technologies for quantum applications require sophisticated processes for design, fabrication, testing, and validation. Maturation of a technology from a concept that requires fundamental R&D to product can take 10 years or more. Investments in government, industry, and academic R&D is needed to be able to cross the “valley of death” from a promising laboratory prototype to volume production. Meanwhile, insufficient US-based fabrication capability with the necessary specialized skills is pressuring US quantum companies to turn overseas to meet their needs.

In addition to technical challenges, there are market challenges to the maturation of PICs for quantum technologies, with insufficient market demand to leverage the economies of scale of high-volume production. In many cases, the return on investment from quantum technologies enabled by PICs will be indirect: i.e., these systems will significantly grow and enhance fields as diverse as banking, medicine, communication and navigation, despite relatively small markets for the quantum technologies themselves. This indirect nature of return on investment only broadens the so called “valley of death” and is further reason for government investment to advance PICs technologies. At early stages, low-volume niche components can be essential to advancing high-impact quantum technologies, but ultimately larger markets will be needed to drive down cost.

The following are recommended actions to address the identified challenges and to accelerate the development and adoption of PICs technologies for the emerging quantum market. The actions may entail a combination of public and private investments.

• Invest in R&D for advanced PICs packaging, control, integration and ruggedization
• Create or enhance low-volume foundry capabilities, leveraging existing nascent commercial capabilities and government-supported resources such as the Microelectronics Commons hubs.
• Sustain long-term investment, especially by the government, for development and transition to manufacturing of PICs for quantum in order to capture both technological advantages and economic benefits.
• Stimulate the low-volume, high-cost markets for PICs and grow the foundational ecosystem, e.g, by identifying dual-use applications and developing modular PIC components. Government should accelerate development of such markets by being an early adopter of quantum technologies based on PICs for government missions.
• Encourage long-term partnerships between universities, national laboratories, and industry, each of which addresses parts of the innovation process from research to development to commercial deployment.
• Leverage PIC investments made for other applications, such as the telecom market.
• Boost the talent pipeline.