Via Photonics, Secure Communications

HANK HOGAN, CONTRIBUTING EDITOR, hank.hogan@photonics.com

Much of the world’s communications travel by light. So, data carried on photons moving through free space or over fiber must be made as secure as possible. Quantum key distribution helps do that. Also, security improvements via temporal cloaking and optomechanical crystals are on the horizon.

However, even when these and other new methods are deployed, there will still be gaps in the data armor. For that reason, a layered defense is best, such as the use of multiple levels of encryption, said Gregory Kanter, CEO of Evanston, Ill.-based NuCrypt LLC. The company was founded to commercialize a high-speed optical encryption device and now supplies quantum optics products.

Security involves trade-offs. Consider free space optics, with a transmitter and receiver communicating over a light beam. That configuration can enhance security.

“Typically, the eavesdropper has to be physically in the line of sight,” Kanter said.

Thus, if the beam has a small cross section, it can be difficult and expensive for an attacker to intercept. The downside, though, is that a tighter beam is harder to align, making it more difficult to use.

Something similar happens with optical fiber. It must be physically tapped, and that can be detected by, for instance, sending a signal down the line and looking to see if some unexpected result comes back. In practice, though, the system must be designed with margin, which makes spotting anything but a big tap difficult. What’s more, this assumes a network is completely clean of unwanted or unknown taps to begin with. That may not be a safe assumption in today’s world, according to Kanter.

Key exchange

Beyond physical security, the next layer of defense is to render the 1s and 0s of transmitted data into unintelligible gobbledygook. Encryption does that, with a key shared between sender and receiver forming the basis for the mathematical translation of information into nonsense and back again.

This can be done in an unbreakable way through a one-time pad, which uses a different random secret encrypting key for every character. But this requires a key longer than the message being sent that can never be reused and must always remain secret.

Such perfection may be impractical, but improvement upon current techniques is not. In quantum key distribution, entangled photons carry key information. The laws of quantum mechanics make it possible to detect when one has been intercepted and data potentially compromised, thereby improving the security of communications.

Exploiting the laws of quantum mechanics, systems can use photons to detect eavesdroppers and thereby improve the security of communications. Courtesy of ID Quantique/Copyright Massimo Brega/Kepach production.

That is one reason why quantum key distribution has attracted attention. Protocols on how to implement the idea appeared more than 30 years ago, with a commercial demonstration in 2004. Research around the world today aims to improve important performance parameters.

“A goal is to increase the range of quantum key distribution because it’s limited by optical attenuation,” said Grégoire Ribordy, CEO of ID Quantique SA of Geneva. The company makes products based on the discrete variable quantum key distribution approach.

In this technique, systems attenuate laser pulses to send single photons down a fiber, with single-photon receivers detecting them. The longer the distance, the fewer photons arrive and, therefore, the lower the bit rate. Eventually the transmission rate drops below what is useful for a given situation.

A distance of a few hundred kilometers currently is possible and this works for applications within a city. A reach of thousands of kilometers is needed for city-to-city use, Ribordy said. The distance barrier can be overcome via repeaters, but these must be physically secure. Another approach is to put the quantum key distribution system in low orbit, and then have it move over different parts of the globe. China launched a demonstration satellite based on a similar concept in August 2016.

The attenuated laser pulses can be overwhelmed by other light, either ambient in the case of free space optics or other traffic in the case of fiber. The latter can be lessened if the quantum and classical communication channels are placed on different wavelengths. Telecom equipment operators and suppliers are starting to investigate that, Ribordy said.

Squeezed light, homodyne detectors

Another way to solve this problem is being pursued by QuintessenceLabs Pty. Ltd. of Canberra, Australia. Instead of using weak signals and single-photon detectors, the company’s continuous variable quantum key distribution approach uses coherent or squeezed states of light and homodyne detectors, said Andrew Lance, senior research scientist.

Andrew Lance (front) and Raymond Chan at work in the QuintessenceLabs quantum key distribution facility. Courtesy of QuintessenceLabs Pty.

He said the technique “has gained increased interest recently because of the potential technology advantage it offers by leveraging commercial off-the-shelf homodyne detectors from the coherent optical communication industry. These detectors exhibit high-quantum efficiencies and gigahertz bandwidth.”

An advantage of this approach is that it is not hampered by high light levels. Consequently, a free space implementation will operate in bright-light conditions so it can be used 24 hours a day, Lance said.

The company’s technology is younger than the alternative. There has been steady progress in the continuous variable quantum key distribution theory and technology so that now the performance and security levels of the two techniques are similar, according to Lance.

A third, and much shorter-range, quantum key distribution example, was described in a 2017 Optics Express paper: “Handheld free space quantum key distribution with dynamic motion compensation.” A team demonstrated a system that could be used to make paying for purchases and other mobile transactions more secure by using quantum techniques to distribute encryption keys over distances of a half meter or so.

A handheld device for transmitting and receiving quantum cryptographic keys built from off-the-shelf components. The device could be miniaturized for use in a mobile device. Courtesy of Iris Choi, Oxford University.

The researchers used fast-switching resonant-cavity LEDs to transmit the quantum states via faint, nanosecond pulses. The short duration of the bursts of light brings two advantages, said co-author David Bitauld, principal researcher at the Nokia Corp. of Espoo, Finland.

“It is more convenient for the user, of course, but it is also helping to overcome the noise. Indeed, the shorter the light pulse, the less likely it is that a spurious photon would be detected in the meantime,” he said.

Bitauld added that a fundamental factor for the security of the transmission was the indistinguishability of the sources from one another. This arises from the inherent nature of the devices, the use of optical filters and other techniques. Bitauld said a key innovation was the beam steering, as this ensured that the sender and receiver stayed aligned despite hand movement.

Commercial implementation would require miniaturizing the prototype’s components. Implementing the transmitter on a chip rather than with free space elements should make the transmitter cheaper and smaller, Bitauld said. The steering system might be made smaller by leveraging the technologies developed for pico projectors, which use similar components, he added.

Temporal cloaking

There are also ways other than encryption to hide information. For instance, researchers are investigating temporal cloaking — it is the time analog of spatial invisibility, with part of a signal stream hidden from an eavesdropper.

Temporal cloaking can hide a data pattern, enhancing optical communication security. Courtesy of Joseph Lukens.

“Temporal cloaking offers security through steganography — communicating information when an eavesdropper thinks no message is being transmitted. By opening and closing time gaps in the eavesdropper’s probe field, temporal cloaks ensure that the eavesdropper still believes they could intercept the bit stream were it actually present, so as not to arouse suspicion,” said Joseph Lukens.

When Lukens, who is now a scientist with the Quantum Information Science group at Oak Ridge National Laboratory, was a graduate student at Purdue University, he was involved in temporal cloaking research. Using electro-optic modulators, dispersion-compensating fiber, fiber Bragg gratings and other elements of fiber optic technology, a Purdue team achieved cloaking at telecommunication data rates. In effect, a probe of the signal would have a gap created in it, with this subsequently closed. Any information in the gap would be invisible to an eavesdropper.

Temporal cloaking could be an additional layer of security, something done on top of encryption, Lukens said. Other possible uses involve packet switching or wavelength multiplexing, as the technique enables multiple signals to co-exist.

A final approach to security comes from a 2017 Nature Communications paper, “Nonlinear dynamics and chaos in an optomechanical beam.” Lead author Daniel Navarro-Urrios, a researcher at the Catalan Institute of Nanoscience and Nanotechnology in Barcelona, Spain, said that optical nonlinearities controlled by an optomechanical crystal and changing excitation laser parameters could potentially create secure data communications.

When excited by a laser, an optomechanical crystal creates a chaotic optical output, which could be used to secure information. Courtesy of Catalan Institute of Nanoscience and Nanotechnology.

The interaction of crystal and laser would create chaos in the signal, masking data. A second crystal-laser pair would remove the chaos and restore the information.

While the research is at an early stage and far from any commercial applications, the results so far are encouraging, Navarro said. “They are obtained using a low-cost technology with the potentiality of being mass-produced. At the same time, this technology will be competitive in terms of power efficiency, information-transfer speed, versatility and security.”

However, despite such innovations, no security approach is perfect. For instance, researchers have hacked quantum key distribution systems, said NuCrypt’s Kanter. He added that researchers investigating possible attack avenues have already been able to exploit flaws or unexpected behavior in detectors, in part because the actual implementation differs from the ideal. Consequently, this new technology faces challenges like those confronting previous optical communication security approaches.

As Kanter said, “Countermeasures to any known hack can always be put in place, but that still begs the question of how to make the QKD system as robust as possible to unknown hacking methods.”