In 1949, the father of information theory Claude Shannon wrote a paper proving that it was possible to create a perfectly secure message, one where the code could never be cracked—even with all the time and computing power in the universe.

According to Shannon, this could be accomplished by using a randomly generated encryption key that is at least as long as the message itself and used only once. Which method of generating a random key doesn't matter—Shannon envisioned perfectly secure keys that used letters, words or even the amplitude of a video signal to encrypt the message. The important thing is that the key is totally random, meaning there is absolutely no relation to the original, unencrypted text. A message encrypted using a totally random, one time-use code would be perfectly secure because even assuming you had the computing power to calculate every single possible solution to the code, you'd end up with a ton of coherent messages which would make it impossible to tell which message was the actual solution.

In the years since Shannon's groundbreaking essay, cryptography has had a renaissance and now it is not uncommon to use methods of encryption that require computers to solve math problems that are so difficult the universe would end before today's computers would be able to solve them. Still, these forms of encryption aren't as secure as what Shannon was imagining because it's entirely possible that a more efficient algorithm capable of cracking them in much less time exists and mathematicians just haven't discovered it yet.

This perennial quest for a perfectly secure message spurred Seth Lloyd, a professor of quantum information at MIT, to put forth a theory of quantum enigma machines in 2013. This device, which derives its name from a Nazi-era cipher machine called Enigma, would use the quantum states of individual photons to encode and encrypt messages by altering properties of the photon wave, such as amplitude or wavelength. Unlike quantum key distribution, which uses principles of quantum mechanics to encrypt messages which are then sent over traditional communication channels like fiber optic cables or telephone lines, the quantum enigma machine would be capable of transmitting quantum states through a quantum channel between the sender and receiver. Moreover, the key used to encrypt this message is shorter than the message itself, an experimental method of encryption known as quantum data locking.

The idea for the device was intriguing, but due to technological limitations remained purely theoretical until last May, when a team of researchers managed to create a real quantum enigma machine in their lab for the first time.

The research team was led by Daniel Lum, a graduate student at the University of Rochester who hadn't even heard of a quantum enigma machine until a little over a year ago, when he stumbled upon Lloyd's arXiv paper describing the theoretical device. Intrigued, Lum brought up his discovery with the head of his lab, the physicist John Howell, at their next meeting and pitched it as a possible research project. Encouraged by his lab colleagues' interest in the idea, Lum reached out to Lloyd and a handful of researchers at the National Institute of Standards and Technology who assisted him in designing an experiment that would put Lloyd's quantum enigma theory to the test.

The quantum enigma machine ultimately created by Lum and his colleagues is elegantly simple in its design, but remarkably complex in its mechanics. At its most basic level the enigma machine consists of three core components: a device capable of generating single photons, two spatial light modulators, and an 8x8 array of nanowires.

To operate the device, a sender (we'll call her Alice) would launch a photon from the fiber optic cable through one of the spatial light modulators. This spatial light modulator manipulates the wave front of the photon by adding a tilt to the wave and changing its direction, kind of like what would happen if you shined a light at a mirror and then began to tilt the mirror. The modulation of this photon is how Alice is able to encode information onto the photon: the direction Alice sends the photon in after the modulation is aligned with one very specific spot on the 8x8 nanowire array which is controlled by the receiver (we'll call him Bob).

It might be helpful to visualize Bob's nanowire array as a keyboard, where each of the 64 nanowires represents one key. In this example, Alice is communicating with Bob by shooting photons at this nanowire keyboard to punch out a message.

If this was all the quantum enigma machine was doing, it would be easy for an eavesdropper (we'll call her Eve) to read messages being sent between Bob and Alice. All Eve would have to do is take a measurement of the system while the photon is moving through it to determine which key on the nanowire keyboard it is traveling toward. So in addition to manipulating the direction the photon is traveling in order to encode it with information, the spatial light modulator also scrambles the photon's wave front by applying a random pattern to the wave—if the wave front was originally smooth, this means that it is now a really rough. This is how Alice encrypts her message to Bob: making the wave front rough essentially un-focuses the photon, making the chances of it arriving at its intended spot on Bob's nanowire array incredibly small.

The random pattern applied to the photon's wave front is derived from a public code book that Bob, Alice and Eve all have access to. Using this code book, Bob and Alice were able to work out which random pattern would be applied to the photon before Alice sent it across the channel to Bob. Due to the immense number of random patterns that could possibly be applied to the photon, it is all but impossible for Eve to determine which pattern is needed to unscramble the photon's wave front without having access to the key agreed upon by Bob and Alice. When the photon reaches Bob, she uses her spatial light modulator to unscramble the photon's wave front, allowing the photon to reach the nanowire Alice had intended to send it to.

The advantages of this method of quantum data locking are manifold. In the first place, due to the current limitations of quantum memory, quantum states (such as that of the photon) cannot be stored indefinitely—they will eventually decohere and become absorbed in their environment. Normally if a classically encrypted message is intercepted, this message can be saved by the eavesdropper who can then keep trying different ways of cracking the code or wait until they have extra outside information that will help them crack the code.

But Eve gets one shot at unscrambling the photon sent between Bob and Alice—after she tries once, she will have corrupted the message due to a fundamental principle of quantum mechanics whereby measuring a quantum system actually results in changes to this system. Assuming she doesn't have access to the random patterns Bob and Alice agreed to apply to the photon beforehand, the odds of Eve just happening to guess the correct pattern on the first try are next to zero. Furthermore, since quantum states can't be stored indefinitely she has to take her one shot at cracking the code soon after intercepting the message before the photon's quantum state decoheres

In essence, what Lum and his colleagues have accomplished in their research is to improve upon Shannon's design for a perfectly secure message. This is because their key is actually far shorter than the message it is encrypting and also includes a new secret key to be used in reply. This ensures that a given key would never have to be used more than once.

In their experiment, Lum and his colleagues encrypted each photon with six bits of information.

Of these six bits, one bit was used to encode the message, 2.3 bits were used to encode the new secret key, and the remaining bits were used for forward error correction, which helps correct for information loss that occurs during transmission. Each time they sent a message, they would encode it in a stream of 63 photons carrying a total of 378 bits of information. This means that a key of 147 bits (2.3 bits per photon) yet was able to encrypt the entire packet of 378 bits of information (63 photons with six bits of info each).

Although Lum and his colleagues' enigma machine was only a proof of concept, it proved solid enough to pass review at Physical Review A, where the results of their research will be published in the coming months.

In the meantime, Lum and his colleagues are looking forward to continuing to develop their quantum data locking method. In addition to testing its robustness against different types of hacks, Lum said it was necessary to figure out how to cut back on the information loss that inevitably occurs during the transmission of quantum states.

"Our experiment was a proof of principle and we understand this particular implementation is never going to be useful for long range communication," said Lum. "A lot of people say it's not worthwhile pursuing because it's such a difficult problem, but I think it's very possible that we can get the losses low enough to reliably transmit secure data across a quantum channel. Quantum data locking is still in its infancy."

## No comments