| Einstein called it "spooky."
He was referring to the quantum theory that two photons split from a single photon parent maintain a connection with each other-even if they fly off in different directions. Do something to one photon, and the other is affected, no matter if it's heading to the moon and the other is stuck in the lab.
Instead of being spooked by the phenomenon, Professors Bahaa Saleh, Alexander Sergienko, and Malvin Teich of the Electrical and Computer Engineering (ECE) Department have decided to put it to work. And task-masters they are. They see applications for entangled photons-those twins, by another name-in several areas, including cryptography, communication, imaging, and metrology. This wide range shows how engineers can make an esoteric discovery relevant to the real world. Take the case of imaging work Saleh, Sergienko, and Teich are doing in entangled photon microscopy, in collaboration with a biology professor at BU.
While techniques in microscopy have advanced remarkably in the past thirty years, it's still not possible to examine biological specimens such as tiny brain synapses without damaging them-it's that old problem of changing a thing simply by looking at it. The driving question in the field of biological imaging is simple: how can you get a light source weak enough that it won't damage living tissue but still strong enough to illuminate the specimens? Right now, the best method is two-photon microscopy, but even that is often too intense. Saleh, the chairman of ECE, Sergienko, and Teich realized they could apply their work on entangled photons to microscopy, and they're now working to move from conception to prototype.
Untangling Light
Entangled photons don't readily occur in nature. In fact, they were the stuff of theory until the invention of the laser. White light, of course, is made up of different colors-think of Isaac Newton and his experiments with prisms. The photons that make up the different colors have distinct energies proportional to their frequency (and inversely proportional to their wavelength). And it was long thought that the colors were constant, that "red remains red remains red in nature," as Saleh puts it.
But it turned out that colors can change when the intense light of a laser travels through certain types of crystal with nonlinear properties. "The energy of one single UV photon can be split into two photons, each with its own energies, but the energies add up to the energy of the mother photon," says Saleh. Thus the birth of entangled photons.
Entangled photons always maintain certain traits, such as simultaneity and correlation, which are being put to good use by the engineers. Take simultaneity, for instance. When the parent photons split, there are always two offspring, never more or less. That's helpful in dealing with the inherent randomness of photon emission. When light sources, even lasers, emit photons, "they are emitted at random, no matter what you do," says Saleh; it's always the number itself plus or minus its square root. "If you create 100 photons, nature is uncertain-that's part of the quantum physics-and it creates 100 photons plus or minus ten." If you have a very large number of photons, that discrepancy is unimportant, because the square root of a very large number is proportionally small. But if you're creating ten photons, that's ten plus or minus three, and that's a 30 percent difference, a wide latitude for error.
Lights, Camera...
In current systems of microscopy, high intensity beams of photons are used to illuminate biological specimens. That increases the likelihood that the light will interact with the cells and change their state. In neuroscience applications, for instance, scientists have been unable to see dendritic spines, the connection point where neurons receive nerve impulses, without damaging the specimens.
One solution is to use a less intense beam-fewer photons illuminating the object in question. But with fewer photons comes uncertainty: it's hard to use measurements with an error rate of plus or minus 30 percent. If you know the exact number of photons hitting the target you're imaging, "then you can precisely determine what that target does to your focus," says Saleh. Because of their property of simultaneity, entangled photons present an ideal solution. As the parent photons split into two, the twin offspring-call them A and B-can be recombined and focused on to a biological specimen.
Teich, who also holds appointments in the Department of Biomedical Engineering and the CAS departments of physics and cognitive and neural systems, says the idea of using entangled photons in microscopy stemmed from a convergence of factors. "By recognizing the needs for imaging in biology, by seeing people using pairs of photons, and through our work in entangled photons, we all of a sudden had a 'eureka' kind of thing, both of us more or less at the same time, realizing that we could use these entangled photon pairs and reduce the power. There are many subtleties that made that highly non-obvious."
Teich and Saleh patented their idea, and they and Sergienko have since teamed up with Biology Professor Kristen Harris of the College of Arts and Sciences to use it to examine the workings of dendritic spines, Harris's area of expertise. That work is being funded by a $960,000 grant from the David and Lucile Packard Foundation's Interdisciplinary Science Program. "With this collaboration, we're in a position to use this new way of imaging, which we hope will allow us to visualize the living cell in its native state," Harris says.
But before that begins, Saleh, Sergienko, and Teich are busy with preliminary physics experiments to understand how entangled photon pairs are absorbed in non-biological objects. Only after that's done will they move on the biological experiments. "Technically and theoretically, and on the basis of all the rules of the game, it should work, but we are still in the lab in development," Saleh says.
There's no prototype yet, but one is in the works. And that's just one of the many projects with entangled photons going on at the Quantum Imaging Laboratory that keep them busy. Among the other research topics listed on their Web site, one stands out: teleportation. Now that's spooky. 
For more information, see the Quantum Imaging Laboratory's Web page at www.bu.edu/qil. |
