By C. Todd Lopez
ADELPHI, Md. (Oct. 27, 2016) -- At the U.S. Army Research Laboratory, scientists are looking at new ways to exploit the most fundamental or "quantum" component of light -- the photon -- to enhance communications, sensing and cryptography, and anything else they can think of.
"We don't really know what all the applications are. But our mandate, in part, is to find those applications," said Michael Brodsky, a physical scientist at Army Research Laboratory.
"If a sufficient number of parties share a sufficient number of entangled particles, is there any application?" Brodsky asked. "That's what we're looking for. Ten years from now, we'll have a better answer to that question."
Brodsky is setting up a new lab at the ARL, located about 12 miles north of the Pentagon in Adelphi, Maryland. He has boxes there that generate entangled photons -- the smallest measure of light. It's entangled photons that are of interest to Brodsky and the Army. A pair of entangled photons exhibits a unique property that Brodsky and his team hope to exploit.
A single photon, on its own, can be captured in a memory unit -- or "quantum storage" -- and subsequently measured. The measurements can be recorded as well. But when two entangled photons are captured and measured in the same way, they yield the same measurements every time, Brodsky said.
Those same two entangled protons could be split up, on different sides of the lab, on different sides of a research campus, or on different sides of the country, and still, because they are entangled, they behave the same way, and so they yield the same measurements no matter where they are.
The results of those measurements are unpredictable -- completely random -- and can be converted to a string of zeros and ones, Brodsky said, but "you get identical strings of zeros and ones at two remote locations. Which, for instance, could be used as a key for secure communications."
A critical part of cryptography and secure communications is the use of random numbers. On both sides of the communication, both parties will need the same string of random numbers to encrypt that communication. If both parties had one half of an entangled pair of photons, then they would both have an endless supply of random numbers at their disposal, and those random numbers would be the same. So a pair of entangled photons, distributed to two parties, could be used to encrypt communications between the two parties.
Distributing those entangled photons is a key issue for Brodsky and his team -- how to get the entangled photons to where they need to be, to multiple parties, so everybody can make use of the properties of the entangled photons. One way to do that is with fiber optics. A photon, which is light, can be sent by fiber optics.
"We've looked at how to take classical networking devices and integrate them in a fashion that would allow us to perform that function," said Robert J. Drost, also with ARL. "The idea is that a conventional networking design has requirements that are more restrictive than what we need to do. Because we have less restrictive requirements, we are able to optimize this design to minimize the loss that these photons experience through the network, and minimize the number of devices that are needed to be able to perform the function that we wanted."
It's not just two parties that can make use of the properties exhibited by entangled photons. It's multiple parties. And at ARL, they are looking at how to build networks of fiber optics with fiber-optic switching devices, to distribute those photons where they need to go, without degrading the photons themselves.
"The question is, how many switches you need to serve all possible entanglement requests from 6, 8 or 10 parties," Brodsky said. "We developed a theoretical framework, and subsequently performed some optimization routines, based on which we said these designs are optimal; the least number of switches you could possibly use to satisfy all possible entanglement requests."
Brodsky and his team will be working to develop networks where command posts or stations, called nodes, can get what they need, and where it's possible to distribute entangled photons in every combination to every node, but at the same time, also use the least amount of hardware so as to ensure the least amount of degradation.
Having entangled photons generated from a single, central node, and then distributed outward to other nodes, is one idea.
"At this central node, we're going to generate entangled photons, but we need to know how to get those entangled photons to the ... neighboring nodes that are necessary to allow one particular node and another to do some quantum communications," Drost said. "And then maybe an hour later, this same node wants to do some secure communications with a completely different node. We need a device that will allow us to reroute and send those entangled photons to those new pair of users that want to perform some sort of quantum protocol."
Another quirk of entangled photons is "entanglement swapping." Two photons can be entangled: A1 and A2, for instance. And two other photons are also entangled: B1 and B2. A1 and A2, because they are entangled, behave the same, as do B1 and B2.
Brian Kirby, a post-doctoral fellow at ARL, explained that by doing a particular "operation" on B2 and A1, those two photons will be destroyed. But as a result, A1 and B2 will become entangled, even if they've never been together.
"This idea of distributing entanglement in this way has been thought of before. And we're interested in how it would work for the warfighter, and how it would work in very noisy, realistic situations," Kirby said. "I did a lot of modeling on how this operation, this spreading of entanglement, would work if you are in a very noisy environment, places that have low signals and things like that. So for example, how it would be on the battlefield, where it's messy and loud and things don't work very well."
Finding ways to distribute entangled photons, and using those entangled photons for secure networking are just two challenges that ARL is working on now. But they are looking at other ways to use entangled photons as well, such as enhancing sensors and quantum computing, for instance.