Resolving Starlight with Quantum Technology

Light is one of the most powerful tools we have for exploring the unknown. From a flashlight in a dark cave to starlight from distant galaxies, light illuminates the things and physical processes that surround us. In an article published yesterday in the American Physical Society’s Physical Review X, a team of scientists from the National University of Singapore describe how we can learn even more from light, using measurement techniques rooted in quantum mechanics. Their work could lead to dramatic improvements in the images we can resolve with microscopes and telescopes.

Imagine looking out into the dark sky and focusing on one pinprick of starlight. How do you know if you’re looking at a single star, two stars, or a billion stars? Zoom in with a powerful telescope and what looks like one star can transform into a star cluster, nebula, or even a galaxy. But what if the pinprick still looks like a single star? How can you be sure that it is one star and not, for example, a binary star system in which one star orbits a nearby star?

If two stars are close enough that their light overlaps on the path toward Earth, this can get really hard to determine. Computer programs do better than our eyes, but even image processing software is limited in its ability to resolve one light source from another. It all comes down to optics. Light diffracts as it travels through a telescope (or the lenses of your eyes) and this leads to blurring. If two objects are really close, diffraction limits our ability to resolve them. There are a few ways to get around this limit, but it becomes impossible as the distance between the stars approaches zero. At least it did until recently.

Last October, Mankei Tsang started thinking about the problem of resolving binary stars and other light sources. Tsang and his group study how quantum mechanical systems can help us measure things. Quantum metrology, as this field is called, explore ways to define units and make measurements based on the properties of photons and atoms. Quantum metrology is an emerging field that holds the promise of more precise, reliable, and sensitive measurements.

Tsang applied a quantum metrology approach to the problem of resolving two light sources. He sought help from postdocs Ranjith Nair and Xiao-Ming Lu and the work progressed quickly. They soon realized—after double and triple checking their calculations—that light coming from two stars (or other sources) contains more information about their separation distance than anyone realized. It turns out that the ability to resolve two sources isn’t limited by diffraction at all.

In the paper, the group outlined a way to measure the separation distance more accurately than ever before. The technique is based on cutting-edge quantum optics technology. Before the article was peer reviewed and published, Tsang posted a draft on arXiv, an online repository of physics papers (this is pretty standard practice). The preprint attracted a lot of interest—as of yesterday, when the final version of the paper was published, four groups based in three different countries had already experimentally demonstrated this technique.

Through this work we can learn more about our surroundings, both by looking outward and by looking inward. The technique Tsang’s team developed could also be applied to improving how well a microscope resolves fluorescent samples like biological molecules, drugs, or toxins. As both scales of this work moves forward, it reminds us that light is not only a tool for exploration, but also a rich source of information to explore.

—Kendra Redmond

For more on optics & astronomy, see our post on tilt-shift photography & miniature-faking: Galaxies Writ Small