Quantum photonics is set to be a pivotal part of future quantum technology, if the right materials can be created, a new review paper has found.

Photonics provides one approach to developing quantum technology by using light. The authors believe that if a programmable photonic platform can be developed, it could enable exciting future technologies such as quantum neural networks and distributed quantum computing.

Many photonic components already exist and can be integrated onto a single silicon chip. However, while the pieces of the puzzle all exist, the challenge is to put the puzzle together in a way that can be manipulated efficiently – today’s technology is mostly locked down and cannot be changed, once manufactured.

The review was carried out by scientists from the ARC Centre of Excellence for Transformative Meta-Optical Systems (TMOS), which is led by Director Professor Dragomir Neshev.

“Programmable quantum photonic systems are an important future development for the Centre,” said Professor Neshev, from TMOS and the Department of Electronic Materials Engineering.

“We have made strong contributions, and it is important for us to merge these strengths.”

The review is published in Nature Photonics, with authors Professor Igor Aharonovich from the University of Technology Sydney, Professor Ken Crozier from the University of Melbourne and Professor Neshev.

The authors envision a new generation of quantum devices that are programmable, compared with existing widespread quantum devices that use a static quantum property, for example, lasers and transistors. This shift is known as the second quantum revolution.

A number of technological platforms are competing for parts of the future quantum market. However, the authors point out that photonics as a platform offers advantages because photons have low decoherence and can have information encoded into multidimensional quantum states. 

“Programmable quantum circuitry will play a pivotal role in transitioning quantum optics from proof-of-concept demonstrations to robust technological solutions for the second quantum revolution,” they said.

“Nonetheless, a missing link for the goal of programmable quantum photonics remains: multifunctionality. For many applications, one needs a system that can perform multiple functions, which requires multiple components to be reconfigured simultaneously.

“For example, would it be possible to alter the source wavelength as well as its polarisation and route the emitted photons to different detectors?”

The review covers key programmable elements, beginning with tunable quantum light sources. These need to create quantum-entangled photons, and also to control the structure of the light. This means controlling not only frequency but also amplitude, phase, and polarisation, enabling exotic states such as quantum vortices and vector beams, which would significantly increase the capabilities of photonic circuits.

While the most widespread current sources use nonlinear processes, such as spontaneous parametric down conversion, the authors believe single-photon sources have much potential.

In particular, van der Waals crystals can be tuned simply via how their constituent layers are assembled, or by twisting; microscopic electrically-powered mechanical systems have been developed that can tune both these and other on-chip components.

“It’s remarkable how much control is possible simply by changing how these ultra-thin materials are stacked on top of one another,” said Professor Crozier.

“However, van der Waals crystals do not yet compete with current leading sources, for example, quantum dots and colour centres, in terms of brightness, photon purity and coherence: more work is needed.”

The next key element is dynamic modulation of components, which is a “critical block” for programmable quantum photonics: traditional approaches to manipulating phase and frequency are bulky, lossy and slow to switch.

While a number of solutions are addressing these drawbacks, no single material platform can supply the essential criteria of efficient coupling of energy in and out, switching that is low energy, efficient and ultra-fast and scalability.

However, the authors believe 2D materials may, in the future, have the versatility to tick all these boxes.

“Essentially every component of a future quantum circuit can be engineered using 2D materials components,” Professor Aharonovitch said.

“Two-dimensional materials are powerful and have myriad functionalities. They enable bandgap engineering, optoelectronic and nonlinear functionalities and quantum light sources.

“These new materials with impressive tuning functionalities could provide a new toolkit of control.”