Researchers Look to Cut Quantum Cryptography Costs

Researchers at the National Institute of Standards and Technology want cut the cost of quantum cryptography.

A team of researchers at the National Institute of Standards and Technology is touting a new method of cutting the costs associated with quantum key distribution.

In a soon-to-be-published paper, researchers at NIST outline a technique that simplifies the structure of a QKD system, slashing its costs by reducing the number of single photon detectors it needs.

In the paper, which will be published next month in IEEE Communications Letters, researchers present a DTBS (detection-time-bin-shift) scheme based on their previously developed conventional fiber-based QKD system.

DTBS uses time-division multiplexing of a single photon detector between two photon bases in a QKD system. The researchers' DTBS QKD system generated sifted keys at a rate of more than 1M bps with a quantum bit error rate of less than 2 percent over 1.1 kilometers of fiber.

Quantum cryptography uses the principles of quantum mechanics to enable two parties to produce a shared random bit string known only to them to encrypt and decrypt messages. QKD is considered by many to be highly secure because it enables the two communicating users to detect any attempts by an outside party to gain knowledge of the key. However, the price of implementation can be a barrier for many enterprises, said NIST researcher Xiao Tang.

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With quantum cryptography, "I would say the technology itself is there, [and it is] very clear how to do it," he said. "Still, when you want to have an experimental result transfer to the market, there are a lot of steps to do, and one of the most important things is just [to] reduce the cost, [and] then it would make it easier to make it a product."

In quantum cryptography, a recipient needs to measure a sequence of photons, or particles of light transmitted by the sender. In the most common polarization-based protocol, known as BB84, the recipient uses four single-photon detectors at a cost of roughly $5,000 to $20,000 each, according to NIST. One pair of detectors records photons with horizontal and vertical polarization, which could indicate 0 and 1, respectively. The other pair detects photons with diagonal, or plus or minus 45 degree polarization, in which the northeast and northwest directions alternatively denote 0 and 1.

In the new method, the researchers set up an optical component to make the diagonally polarized photons rotate another 45 degrees so they arrive later and in a separate time bin at the same detector than the horizontal/vertical polarized ones. Therefore, one pair of detectors can be used to record information from both kinds of polarized photons in succession, reducing the required number of detectors from four to two. In another protocol, called B92, the researchers were able to cut the necessary number of detectors from two to one.

Since writing the paper, the researchers have gone a step further so that the BB84 method now only requires one detector instead of four.

"In the beginning QKD is just point to point, only two parties talk to each other," Tang said. "Now we're trying to build a network. In our laboratory we actually demonstrated three users, which is a very simple network, but it's very complicated technology. ... The next step will be how to use this kind of setup [in] the existing fiber network environment."