There’s a revolution underway in astronomy. In fact, you might say there are several. In the past ten years, exoplanet studies have advanced considerably, gravitational wave astronomy has emerged as a new field, and the first images of supermassive black holes (SMBHs) have been captured. A related field, interferometry, has also advanced incredibly thanks to highly-sensitive instruments and the ability to share and combine data from observatories worldwide. In particular, the science of very-long baseline interferometry (VLBI) is opening entirely new realms of possibility.

According to a recent study by researchers from Australia and Singapore, a new quantum technique could enhance optical VLBI. It’s known as Stimulated Raman Adiabatic Passage (STIRAP), which allows quantum information to be transferred without losses. When imprinted into a quantum error correction code, this technique could allow for VLBI observations into previously inaccessible wavelengths. Once integrated with next-generation instruments, this technique could allow for more detailed studies of black holes, exoplanets, the Solar System, and the surfaces of distant stars.

The research was led by Zixin Huang, a postdoctoral research fellow with the Centre for Engineered Quantum Systems (EQuS) at Macquarie University in Sydney, Australia. She was joined by Gavin Brennan, a professor of theoretical physics with the Department of Electrical and Computer Engineering and the Centre of Quantum Technologies at the National University of Singapore (NUS), and Yingkai Ouyang, a senior research fellow with the Centre of Quantum Technologies at NUS.

To put it plainly, the interferometry technique consists of combining light from various telescopes to create images of an object that would otherwise be too difficult to resolve. Very Long Baseline Interferometry refers to a specific technique used in radio astronomy where signals from an astronomical radio source (black holes, quasars, pulsars, star-forming nebulae, etc.) are combined to create detailed images of their structure and activity. In recent years, VLBI has yielded the most detailed images of the stars that orbit Sagitarrius A* (Sgr A*), the SMBH at the center of our galaxy (see above).

It also allowed astronomers with the Event Horizon Telescope (EHT) Collaboration to capture the first image of a black hole (M87*) and Sgr A* itself! But as they indicated in their study, classical interferometry is still hindered by several physical limitations, including information loss, noise, and the fact that the light obtained is generally quantum in nature (where photons are entangled). By addressing these limitations, VLBI could be used for much finer astronomical surveys. Said Dr. Huang to Universe Today via email:

“Current state-of-the-art large baseline imaging systems operate in the microwave band of the electromagnetic spectrum. To realise optical interferometry, you need all parts of the intererometer to be stable to within a fraction of a wavelength of light, so the light can interfere. This is very hard to do over large distances: sources of noise can come from the instrument itself, thermal expansion and contraction, vibration and etc; and on top of that, there are losses associated with the optical elements.”

“The idea of this line of research is to allow us to move into the optical frequencies from microwaves; these techniques equally apply to infrared. We can already do large-baseline interferometry in the microwave. However, this task becomes very difficult in optical frequencies, because even the fastest electronics cannot directly measure the oscillations of the electric field at these frequencies.”


Aerial view of the Paranal Observatory showing the four 8.2-m Unit Telescopes (UTs) and various installations for the VLT Interferometer (VLTI). Credit: ESO

The key to overcoming these limitations, says Dr. Huang and her colleagues, is to employ quantum communication techniques like Stimulated Raman Adiabatic Passage. STIRAP consists of using two coherent light pulses to transfer optical information between two applicable quantum states. When applied to VLBI, said Huang, it will allow for efficient and selective population transfers between quantum
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