Ghost imaging is like a game of Battleship. Instead of seeing an object directly, scientists use entangled photons to remove the background and reveal its silhouette. This method can be used to study microscopic environments without much light, which is helpful for avoiding photodamage to biological samples.
So far, quantum ghost imaging has been limited to two dimensions, or to two planes at fixed z positions. In a new study, published in Optica, scientists at Lawrence Livermore National Laboratory (LLNL) developed a 3D quantum ghost imaging microscope - the first of its kind.
"This is a new way of 3D imaging that can do things with more sensitivity and gather more information without having to scan a sample," said LLNL scientist and author Audrey Eshun.
The method works based on the quantum phenomenon of entanglement. A laser illuminates a crystal that generates photon pairs that are entangled, or linked together in space and time. These pairs hit a mirror that separates them: one, called the "signal" photon, turns left toward the sample, while the other, the "idler" photon, continues straight to a camera-like detector.
The idler photons, which do not interact with the sample, form a uniform, featureless image on the detector.
Meanwhile, the signal photons move through a microscope objective that collects and focuses them onto a sample. In this case, the authors looked at metallic nanoparticle clusters.
The sample is tilted at a 45-degree angle relative to the incoming photons. When the photons hit it, they scatter in every direction.
Another microscope objective, positioned at a right angle to the incoming light, collects the scattered photons and directs them to a second detector. This camera captures a standard snapshot of the y-z plane of the nanoparticles.
Both detectors measure the exact arrival time of each photon. By matching the timestamps of photon pairs detected by both cameras, researchers can determine which idler photons correspond to signal photons that interacted with the sample. They remove every other photon from the featureless idler image, revealing a ghost image of the x-y plane of the sample.
"The standard image has y and z coordinates and a time for each pixel, and the ghost image has x and y coordinates and a time for each pixel," said Eshun. "By grouping all the photons that have the same timestamp, we can figure out the x, y and z position for each photon. These coordinates can then be plotted to form a 3D image."
In comparison to other techniques, 3D quantum ghost imaging doesn't require scanning the sample - it can happen all at once. It uses extremely low-light intensities, so it could be useful for imaging light-sensitive materials.
"This microscope is the first of its kind," said LLNL scientist and author Ted Laurence. "There was another 3D quantum ghost image, but in that case the resolution was about 3 centimeters. This is microns. We are getting three spatial dimensions of information at the micron scale."
Next, the team aims to use this method for high-speed tracking of the movement of cells in relation to each other.
This work was funded by the U.S. Department of Energy Biological and Environmental Research program.
