If you want to ask questions about the universe, you can use photons as tiny probes, massless particles of light that enigmatically act like both matter and waves depending on the situation. But the universe has imposed limitations on the precision with which observations can be made. Just as we cannot use a ruler to measure the dimensions of objects smaller than the length between the ruler’s tick-marks, the precision of instruments that use photons to probe the universe is traditionally coarser than the wavelength of those photons.
Physicists at U of T have recently developed a technique to overcome this classical limitation by using “entangled photons.” Entanglement, described by Einstein as “spooky action at a distance,” allows two or more photons to be connected over distances of space yet behave like single photons of shorter wavelength. This cutting-edge research could one day prove useful in enhancing the resolution of interferometers, tools that already push the envelope of scientific measurements.
Interferometers are devices constructed to observe interference patterns, usually of light, with the aim of collecting data for which direct measurement is either impractical or impossible. The device consists of a set of mirrors that split and recombine a laser beam. This causes the photons of the laser to interfere with one another when they meet at the end. The brightness of the final beam, determined by the interference pattern, is affected by the distance between the mirrors.
Gravity wave detectors use a Michelson Interferometer, which measures microscopic changes in the distances between pairs of mirrors reflecting a laser beam. Even a minute change of several micrometers can be detected by examining this effect. But there is a limit to this precision because the interferometer can only track changes that are greater than the wavelength of the light.
To increase the sensitivity of the interferometer, U of T researchers took high-energy short-wavelength blue photons and split each of them into several lower-energy longer-wavelength red photons. They combined, or entangled, the red photons and used them in the interferometer. The result was interference with the resolution of short-wavelength blue photons while only using red photons.
Says graduate student and researcher Jeff Lundeen of the Physics Department at U of T, “There are a lot of measurements in physics and other areas of science, like medical science, that rely on interferometers. Any sort [of] measurement that relies on interferometers can be made more accurate by this [technique].”
The findings may lead to improved medical imaging systems by both increasing sensitivity and reducing patients’ exposure to high-energy radiation. Since entangled photons have a smaller wavelength, they can also inscribe smaller features and carve tinier circuitry, increasing the density limit of data on future CDs and DVDs. However, researchers warn that these applications are still decades from being realized.
Lundeen remarks, “It is pretty far from being applied to something commercial. Lasers took a long time to be commercial too, so this is not unusual.”
