  In the 1960s, computer engineers had to address the tyranny of numbers
problem in which improvements in computing and its applications required
integrating an increasing number of electronic components. From the first
computers powered by vacuum tubes to the billions of transistors fabricated on
a single microprocessor chip today, transformational advances in integration
have led to remarkable processing performance and new unforeseen applications
in computing. Today, quantum scientists and engineers are facing similar
integration challenges. Research labs packed with benchtop components, such as
tunable lasers, tables filled with optics, and racks of control hardware, are
needed to prepare, manipulate, and read out quantum states from a modest number
of qubits. Analogous to electronic circuit design and fabrication nearly five
decades ago, scaling quantum systems (i.e. to thousands or millions of
components and quantum elements) with the required functionality, high
performance, and stability will only be realized through novel design
architectures and fabrication techniques that enable the chip-scale integration
of electronic and quantum photonic integrated circuits (QPIC). In the next
decade, with sustained research, development, and investment in the quantum
photonic ecosystem (i.e. PIC-based platforms, devices and circuits, fabrication
and integration processes, packaging, and testing and benchmarking), we will
witness the transition from single- and few-function prototypes to the
large-scale integration of multi-functional and reconfigurable QPICs that will
define how information is processed, stored, transmitted, and utilized for
quantum computing, communications, metrology, and sensing. This roadmap
highlights the current progress in the field of integrated quantum photonics,
future challenges, and advances in science and technology needed to meet these
challenges.
