Astronomy is undergoing a revolution. In fact, you might argue there are a number of them. Exoplanet research has progressed significantly in the last ten years, gravitational wave astronomy has developed as a new area, and the first photographs of supermassive black holes (SMBHs) have been taken.
Interferometry, a similar subject, has evolved tremendously as a result of very sensitive sensors and the ability to exchange and integrate data from observatories all over the world. The science of very-long baseline interferometry (VLBI) in particular is opening up completely new realms of potential.
According to a recent study by Australian and Singaporean researchers, a novel quantum approach might improve optical VLBI. STIRAP stands for Stimulated Raman Adiabatic Passage, and it allows quantum data to be transported without loss.
This approach, if imprinted into a quantum error correction code, might allow VLBI observations at wavelengths that were previously unavailable. This technology might enable for more thorough investigations of black holes, exoplanets, the Solar System, and the surfaces of distant stars once it is merged with next-generation instrumentation.
Zixin Huang, a postdoctoral research researcher at Macquarie University's Centre for Engineered Quantum Systems (EQuS) in Sydney, Australia, led the study. Gavin Brennan, a professor of theoretical physics at the National University of Singapore (NUSDepartment )'s of Electrical and Computer Engineering and the Centre of Quantum Technologies, and Yingkai Ouyang, a senior research fellow at NUS's Centre of Quantum Technologies, accompanied her.
To put it another way, the interferometry technology combines light from several telescopes to generate photographs of an object that would otherwise be impossible to resolve.
Very-long baseline interferometry is a radio astronomy technique that combines signals from astronomical radio sources (black holes, quasars, pulsars, star-forming nebulae, etc.) to provide precise pictures of their structure and activity.
VLBI has produced the most comprehensive photographs of the stars that circle Sagitarrius A* (Sgr A*), the SMBH in the heart of our galaxy, in recent years. It also enabled the Event Horizon Telescope (EHT) Collaboration to take the first picture of a black hole (M87*) and Sgr A*!
Classical interferometry, however, is still hampered by various physical restrictions, including information loss, noise, and the fact that the light collected is typically quantum in origin, as they pointed out in their work (where photons are entangled). VLBI might be utilized for considerably finer astronomical surveys if these restrictions are addressed.
"Current state-of-the-art large baseline imaging systems operate in the microwave band of the electromagnetic spectrum. To realize optical interferometry, you need all parts of the interferometer to be stable to within a fraction of a wavelength of light, so the light can interfere," Dr. Huang wrote in an email to Universe Today.
This is extremely difficult to do over long distances: sources of noise include the device itself, thermal expansion and contraction, vibration, and so on; besides, there are losses connected with the optical components.
"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."
The use of quantum communication techniques such as Stimulated Raman Adiabatic Passage, according to Dr. Huang and her colleagues, is the key to circumventing these constraints. STIRAP works by transferring optical data between two suitable quantum states utilizing two coherent light pulses.
According to Huang, when applied to VLBI, it would allow for efficient and selective population transfers between quantum states while avoiding noise and loss.
The mechanism they foresee would entail coherently coupling the starlight into "dark" atomic states that do not radiate, as they describe in their study ("Imaging stars with quantum error correction").
According to Huang, the next step is to combine the light with quantum error correction (QEC), a quantum computing approach that protects quantum information from mistakes caused by decoherence and other "quantum noise."
However, as Huang points out, this same approach might enable more precise and detailed interferometry:
"To mimic a large optical interferometer, the light must be collected and processed coherently, and we propose to use quantum error correction to mitigate errors due to loss and noise in this process."
"Quantum error correction is a rapidly developing area mainly focused on enabling scalable quantum computing in the presence of errors. In combination with pre-distributed entanglement, we can perform the operations that extract the information we need from starlight while suppressing noise."
To put their idea to the test, the researchers imagined a situation in which celestial light is collected by two facilities (Alice and Bob) separated by a large distance.
Each has pre-distributed entanglement and "quantum memories" into which the light is trapped, as well as its own set of quantum data (qubits) that it converts into some QEC code. A decoder imprints the acquired quantum states onto a common QEC code, which secures the data from future noisy operations.
The signal is recorded into the quantum memory in the "encoder" step using the STIRAP technology, which allows incoming light to be coherently linked into a non-radiative state of an atom.
The capacity to collect light from celestial sources that accounts for quantum states (and removes quantum noise and information loss) would revolutionize interferometry. Furthermore, these advancements would have far-reaching repercussions for other branches of astronomy that are now undergoing revolutions.
"By moving into optical frequencies, such a quantum imaging network will improve imaging resolution by three to five orders of magnitude," Huang added.
"It would be powerful enough to image small planets around nearby stars, details of solar systems, kinematics of stellar surfaces, accretion disks, and potentially details around the event horizons of black holes – none of which currently planned projects can resolve."
The James Webb Space Telescope (JWST) will analyze exoplanet atmospheres like never before, thanks to its powerful suite of infrared imaging equipment. The Extremely Large Telescope (ELT), Giant Magellan Telescope (GMT), and Thirty Meter Telescope (TMT) are examples of ground-based observatories (TMT).
These observatories will allow direct imaging investigations of exoplanets, revealing vital information about their surfaces and atmospheres, thanks to their enormous main mirrors, adaptive optics, coronagraphs, and spectrometers.
Observatories will be able to take photos of some of the most inaccessible and difficult-to-see objects in our Universe using new quantum methods and merging them with VLBI. The truths that may be revealed are certain to be revolutionary!
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