For astronomers, one of the greatest challenges is capturing images of objects and phenomena that are difficult to see using optical (or visible light) telescopes. This problem has been largely addressed by interferometry, a technique where multiple telescopes gather light, which is then combined to create a more complete picture. Examples include the Event Horizon Telescope, which relies on observatories from around the world to capture the first images of the supermassive black hole (SMBH) at the center of the M87 galaxy, and of Sagittarius A* at the center of the Milky Way.
That being said, classic interferometry requires that optical links be maintained between observatories, which imposes limitations and can lead to drastically increased costs. In a recent study, a team of astrophysicists and theoretical physicists proposed how these limitations could be overcome by relying on quantum mechanics. Rather than relying on optical links, they propose how the principle of quantum entanglements could be used to share photons between observatories. This technique is part of a growing field of research that could lead to “quantum telescopes” someday.
The study was conducted by researchers from the Brookhaven National Laboratory (BNL) and Stony Brook University in New York, New York. Additional support was provided by Stephen Vintskevich, a theoretical physicist and independent researcher currently based in the United Arab Emirates. The paper that describes their findings recently appeared online and is being reviewed for publication in the scientific journal Optica.
In classical Michelson Interferometry, a beam of light is split so that one beam strikes a fixed mirror and the other strikes a movable mirror. An interference pattern is created when the reflected beams are brought back together. For the purposes of astronomy, the two beams are collected by two telescopes that are separated by some distance (called baseline interferometry). But despite its effectiveness, classic interferometry is subject to some limitations. Andrei Nomerotski, an astrophysicist with the BNL and a co-author on the paper, explained to Universe Today via email.
“Interferometry is a way to increase the effective aperture of telescopes and to improve the angular resolution or astrometric precision,” he said. “The main difficulty here is to maintain the stability of this optical path to very high precision, which should be much smaller than the photon wavelength, to preserve the photon’s phase. This limits the practical baselines to a few hundred meters.”
In recent years, scientists have investigated the possibility of using quantum principles to enable next-generation astronomy. The basic idea is that photons could be transferred between observatories without physical connections that are expensive to build and maintain. The key is to take advantage of quantum entanglement, a phenomenon where particles interact and share the same quantum state – despite being separated by considered distance. Quantum telescopes were initially proposed by researchers Daniel Gottesman, Thomas Jennewein, and Sarah Croke of the Perimeter Institute for Theoretical Physics and the Institute for Quantum Computing at the University of Waterloo.
The interferometer proposed by the BNL-led team borrows features from the Gottesman-Jennewein-Croke (GJC) proposal and the Narrabri Stellar Intensity Interferometer (NSII). Said Nomerotski:
“The proposal was to use a source of entangled photons and to employ correlations of photon counts in two stations and, therefore, to mostly remove the problem of photon phase stability. The intensity interferometers are used to measure the star diameters employing a technique based on the Hanbury Brown-Twiss (HBT) effect of photon bunching. In our scheme we use the same effect, only its phase-dependent part, to measure the opening angle between two stars, which now could be separated by a considerable angle. On the other hand, said Nomerotski, the second star also can be viewed as a source of coherent photons for the first star, hence the link to the Gottesman-Jennewein-Croke proposal.”
The team is currently developing a physical description, said Nomerotski, that would include both options. This could be generalized to multiple stations and quantum protocols to process quantum information in a “noisy” environment. To test their concept, the team built a bench-top version of the two-photon interferometer that used a narrow spectral line in two argon lamps (to simulate two stars). As they predicted based on previous theoretical research, the team noted HBT peaks and correlations of channels and measured its dependence on the photon phase.
The main advantage of this technique is improved angular resolution (the ability to discern details in objects) in telescopes. But as Nomerotski explained, the long-term benefits could be immeasurable:
“There could be multiple scientific opportunities that would benefit from substantial improvements in astrometric precision. Just to list a few: testing theories of gravity by direct imaging of black hole accretion discs, precision parallax, and the cosmic distance ladder, mapping microlensing events, exoplanets, peculiar motions, dark matter, and others.
“Of course, all this is quite long-term and will require proof-of-principle demonstrations and, importantly, improved sensitivity compared to what is achievable now. These improvements rely on the progress in the development of quantum networks and quantum repeaters like in the original GJC proposal. A lot of this development is driven by companies nowadays for completely different purposes, and good progress is being made so it could become a reality in the foreseeable future.”
This proposal for two-photon interferometry is one of many proposals for quantum telescopes in recent years. Other examples include a proposal by an MIT team to combine interferometry with quantum teleportation to drastically increase the resolution of observatories (without employing larger mirrors). There’s also the more recent idea of combining Stimulated Raman Adiabatic Passage (STIRAP) and pre-distributed entanglement to create a virtual Very-Long Baseline Interferometry (VLBI) telescope the size of planet Earth.
These quantum techniques could allow for observations in previously inaccessible wavelengths and more detailed studies of black holes, exoplanets, the Solar System, and the surfaces of distant stars. And as efforts to mature the technology behind quantum computing continue, the applications are sure to spin off into other fields of research (like quantum astronomy). As Nomerotski added:
“There is a variety of interesting conceptual ideas in this field, but most of them are theoretical and, therefore, quite futuristic. We believe that our work is one of the few that tackles the experimental difficulties of the approach, and we were making good progress there. A few of us will be organizing a one-day workshop, a companion meeting before the Quantum 2.0 conference in Denver in June to brainstorm these ideas.”
Further Reading: arXiv
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