Photonics-based quantum technologies advanced by improved filtering

Light-based quantum technologies hold immense promise for the future, enabling breakthroughs in fields like secure communications, quantum computing, and precision measurement.

A 2024 EQUS study demonstrates a significant leap for a classical photonics required by many of these new quantum photonic technologies.

Quantum photonics often depend on extremely weak signals, such as single photons—the smallest units of light.

However in order to create or manipulate those single-photon outputs, we often require much stronger control beams of light that end up ‘swamping’ the output.

The 2024 study delves into solving this pressing challenge: isolating those precious single output photons from the much stronger control beams required to produce or control them.

The advance hinges on a sophisticated optical filter called a fibre Bragg grating.

The fibre Bragg grating is a pattern of alternating high and low refractive index in the core of a hair-thin optical fibre. The result is destructive interference for light passing through the grating at specific wavelengths determined by the refractive index pattern spacing.

In the study, the researchers used an occasional change in the pattern to create constructive interference (high transmission) in the centre of the destructive interference region to create a filter.

This highly-effective filter device can block out ‘unwanted’ light from control beams while allowing the faint signal of single photons to pass through.

“It is a huge improvement over existing filter technologies used in transduction,” says EQUS PhD candidate Ben Field, who led the study.

The new filter is capable of attenuating control beams by over 10,000,000,000 times (100dB) – an astonishing reduction, equivalent to cutting the sound of a jet engine at take-off down to the level of a whisper.

Harnessing Australian capabilities for new applications in quantum technology

This breakthrough stems from pushing the limits of fibre Bragg grating fabrication. The researchers achieved unprecedented stability in the manufacturing process, enabling the creation of longer gratings with higher refractive index shifts. This refinement allowed for greater filtering performance than has been seen in similar devices.

The team also employed a wide array of measurement techniques to characterise the filters thoroughly. Although these techniques themselves aren’t new, the precision and level of filtering demanded novel approaches to ensure accuracy.

The project demonstrates the impressive capabilities of the newly established ANFF Advanced Fibre Bragg Grating Facility at the University of Sydney. “It has been fantastic to see the concept of combining expertise from classical photonics, astrophotonics and quantum optics go from idea to a demonstrated solution for a significant challenge that limits today’s quantum technologies,” says EQUS CI Dr John Bartholomew.

Just the beginning for FBG technologies in quantum tech

The impact of the work will be both immediate and far-reaching.

For quantum technologies, the filter’s significant improvement in the ability to isolate single photons could improve quantum transduction systems, where light must be converted between electronic signals and optical signals while maintaining quantum coherence.

On a broader scale, the filters demonstrate the potential for advancing classical photonic technologies to meet the stringent demands of quantum systems.

This work also paves the way for future research in designing even better filters, reducing losses, and improving overall performance.

One of the main challenges in designing optical filters is minimising insertion loss, which refers to the light lost as it passes through the filter. While the current design represents a significant leap forward, the team has identified pathways to refine the technology further. The team is exploring new designs to reduce insertion loss, which will be critical for integrating these filters into a broader range of real-world quantum devices.

While there is still more work to be done until these may see widespread adoption in the field there has already been a fair amount of interest. With the filters enabling an increase in the overall device efficiency of many transduction schemes.

A versatile filter platform for the EQUS Quantum Clock flagship

Today’s optical fibre technology is not sufficiently high-performance to meet the needs of future quantum technology.

EQUS’ Clock flagship has explored the limits of quantum hard drives as potential technologies for solid-state nuclear spin clocks. A key challenge for quantum hard drive and other photonic quantum technologies is to achieve low-loss, high dynamic range filtering.

While current optical fibre technology often falls short of these demanding standards, this study provides a promising path forward.

This story was first published in the 2024 EQUS annual report.