A team of scientists has developed a way of overcoming the diffraction limit of telescopes, which has the potential to significantly improve the angular resolution of even moderately size telescopes. The research, published this month in Optics Letters, could benefit many astronomical applications.
The angular resolution of a telescope is the smallest angle between two objects that still can be resolved as separate things; in a telescope with high angular resolution, those objects can be very close together and yet still appear distinct.
The usable angular resolution of ground-based telescopes can be increased using adaptive optics systems, which compensate in real-time for the blurring effects of earth’s atmosphere and restore imagery to diffraction-limited resolution.
However, as telescopes increase in size, the correction becomes increasingly more complex, explained adaptive optics expert Aglaé N Kellerer, from the University of Cambridge. ‘In 1989, the first astronomical prototype had 19 correction elements and a 150Hz sampling rate. Current systems have several thousand correction elements and sampling rates above 1,000Hz – and this is not the end of the line.’
The researchers from the University of Cambridge and the Technion–Israel Institute of Technology have now proposed a way of improving the angular resolution of a telescope beyond the diffraction limit, by using a combination of photon amplification and the statistical properties of stimulated photons versus spontaneous photons.
Figure 1: The team used a combination of photon amplification and the statistical properties of stimulated photons versus spontaneous photons. Credit: University of Cambridge / Technion–Israel Institute of Technology
Consider a photon emitted by an astronomical object. Before the photon is actually detected by a given telescope, all that is known of its location is that it exists at some point on an immense spherical wave centred on the astronomical object and extending all the way to the telescope. Once the telescope’s detector records the photon, however, the photon’s pathway is narrowed to within an area constrained by the telescope’s aperture. The Heisenberg uncertainty principle indicates that, because the path of the photon is now better known, the corresponding uncertainty in its momentum must increase. This limits the resolution of the telescope.
However, this limit applies only to independent photons – with sets of coherent or entangled photons, the limit can be smaller, explained Kellerer: ‘We propose to use photon amplification – stimulated emission – to overcome the diffraction limit in astronomy.’
Specifically, the researchers propose that excited atoms could be placed between the telescope aperture and its photon detector. When an astronomical photon enters the telescope, it will stimulate the emission of identical photons. ‘These photons arrive simultaneously on the detector and spread over the diffraction pattern,’ commented Kellere. ‘If the incoming photon stimulates the emission of 100 photons, the precision on the determination of the photon’s incoming direction is improved by a factor of 10.’
The stimulated emission would be accompanied by spontaneous emission that contributes noise. For that reason, scientists previously had discarded the idea of using photon amplification to improve astronomical imaging. Kellerer and Ribak, however, suggest using only stimulated photon bursts that are above a particular size. Astronomical photons that generate small photon bursts have a larger noise component and are discarded, reducing the overall noise. ‘This might allow us to overcome the diffraction limit,’ Kellerer said.
One potential downside of the proposed technique is a loss of sensitivity in the images produced. ‘It is a price to pay,’ Kellerer remarked, ‘but it is reassuring: if we found a means to overcome the diffraction limit at no cost, we would be in contradiction with the Heisenberg uncertainty principle, and we would thus certainly be wrong.’ Kellerer added that the loss of sensitivity can partly be overcome by increased exposure times.
Achieving extremely high angular resolution would be beneficial for many astronomical applications. For example, recent research carried out by Kellerer’s team led to the discovery of Earth-like planets around an ultracool dwarf star, located 39 light years away. ‘Even though these planets are close by astronomical standards… it will be extremely difficult to build telescopes that are sufficiently large, or interferometers that have a sufficiently long baseline, to image their surfaces,’ she explained. ‘This will require a technological breakthrough.’