The capacity to study the smallest objects in the universe depends on the resolution of the telescopes being used. This resolution is limited by the span of the primary aperture, which is the part of the instrument that collects light. Radio astronomers overcome this by combining signals from widely distributed dishes so they work like a single, much larger telescope, through an approach known as aperture synthesis. Two such examples are SKA-Low, now being built in Western Australia, and the global Event Horizon Telescope, which links observatories up to 10,000 kilometres apart.

Researchers at the International Centre for Radio Astronomy Research (ICRAR) at The University of Western Australia (UWA) are investigating whether quantum measurement techniques and long-distance AARNet fibre could allow the many advantages of aperture synthesis to be used further in optical astronomy.

Testing a 170 kilometre baseline optical telescope

Joshua Collier, a PhD researcher at the UWA node of ICRAR, is leading experiments that send signals emulating starlight over two 85 kilometre dark fibre links on the AARNet network. Together, the links simulate an optical interferometer spanning 170 kilometres.

Interferometry combines signals from multiple locations and uses the way they interact to extract more information. Although this method is well established in radio astronomy, it is much harder to apply to optical light because of the much higher frequencies being observed. While the carrier signal of radio waves can be recorded electronically, the carrier frequency of light is too high to capture directly, so the light needs to be interfered, or combined, before a signal can be recorded. 

Through AARNet, Australia’s national research and education network, Joshua and the team can access the long-distance dark fibre needed to test their approach at a scale difficult to reproduce in a laboratory. This dedicated, unlit fibre allows the researchers to transmit and control their own optical signals. Rather than simply carrying research data, AARNet’s network infrastructure connecting universities and research organisations has become part of the experiment’s system itself.

Over longer distances, optical signals are highly sensitive to changes in the fibre caused by temperature, vibration, and other environmental conditions. These changes can disrupt the relationship between the signals, preventing researchers from combining them accurately.

The team has shown that they could stabilise the two links and recover accurate imaging information after the emulated starlight had travelled over the 170 kilometre span.

“We are testing whether an idea that works in the laboratory can operate over distances relevant to astronomy,” Joshua said. “Long fibre links allow us to identify and solve the practical problems that will come up in building a telescope of this scale.”

Going beyond conventional telescope resolution

Such an instrument would represent a major increase over the scale achieved by existing optical interferometers, the largest of which spans about 330 metres. The team’s results suggest that extending the approach to around 300 kilometres could be feasible, potentially delivering resolution an order of magnitude greater than that of the Event Horizon Telescope.

Quantum imaging could improve that resolution further. Normally, the detail a telescope can distinguish is constrained by the size of the primary aperture, however quantum measurements can bypass this limit by extracting more imaging information from the available light.

Using single-photon detectors, UWA’s experiments make “quantum-optimal” measurements designed to recover as much useful imaging information as quantum physics allows. 

Joshua’s work forms part of a broader quantum imaging for astronomy project led by Dr David Gozzard, a Senior Research Fellow at ICRAR. 

“The result changes what we can realistically consider when designing a future telescope,” Dr Gozzard explains. “It suggests we could improve resolution not only by connecting telescopes over greater distances, but also by measuring the light they collect in a fundamentally different way.”

Lessons for quantum research

The work also demonstrates a capability required beyond astronomy to transmit, stabilise, and accurately recover weak, few-photon signals over long links of conventional telecommunications fibre. This can assist researchers developing certain kinds of quantum key distribution or link quantum sensors distributed across large distances, and shows how research network infrastructure can be used to test emerging technologies under conditions that cannot be otherwise recreated within a laboratory.

Photo: Dr David Gozzard (left) and Joshua Collier (right) with their quantum telescope experiment. Credit: ICRAR.