Qubits Keep Comms ‘Hush-Hush’

QKD uses diamonds to generate entangled protons.
Artist’s impression of the three-node quantum network. Credit: Matteo Pompili for QuTech

Quantum technologies could help revolutionise military communications, not least in the field of encryption.

Just a little quantum physics, there is no need to be afraid! Put simply, quantum physics is the study of atomic and subatomic particles. You would be forgiven for asking what this has to do with military communications? Potentially quite a lot. The emerging field of quantum communications has much to offer to the perennially vexing problem of how to keep military communications traffic secure.

Military communications carry voice, written information (data), still or moving images across high frequency (HF), and very/ultra-high frequency (V/UHF) radio channels. Traffic zooms across networks from one radio to another. Communications can be tactical, from a platoon commander to the company commander, for example. They can also be operational, from the brigade headquarters to the corps HQ. Beyond this, strategic communications link the corps HQ to the national command authority. There reside the military chiefs and politicians directing the war. While these examples are from the land domain, they are as applicable to the air and sea domains.

Traffic moving across these networks must be secure. Let us suppose a brigade is deployed to help a peacekeeping operation in a war-torn country. There is no way the brigade can tell whether hostile forces are eavesdropping on their radio traffic. In fact, this is the case for any user on a radio network, be they military or civilian. It is no more possible to tell if someone has bugged your cellphone than it is possible to tell if someone is listening to your conversation in a bar.

Steps can be taken to make radio traffic secure, but they cannot determine if someone is eavesdropping. One of the most ubiquitous means of securing radio traffic is encryption. Encryption follows a relatively simple principle which revolves around the use of a key. The key is like a password. It is typically a secret number shared with all the radios on a specific network. In theory, only radios possessing the key can decrypt the encrypted traffic and access its contents. The security risk is that if someone gets hold of the key who should not, they can load this into their radio and eavesdrop on the traffic. Referring to the encryption and decryption process relying on a key is apt. The key unlocks the encryption to reveal the radio traffic and likewise it encrypts traffic allowing it to be securely sent to its destination.

QKD Encryption

Encryption could be revolutionised by a process known as Quantum Key Distribution (QKD). Light particles, photons, are used as the medium for QKD. This is thanks to the ability to encode information in a property of photons, namely their polarization. A single photon can have a particular polarization while they travel. For example, they can have a vertical or horizontal polarization. This can be visualised as the photon oscillating up and down (vertical) or left and right (horizontal). Likewise, photons can spin clockwise or anti-clockwise on their axis. “QKD does not transmit secure messages, it creates a shared secret between users over unsecured communication links,” explained Dr. Daniel Twitchen, chief technologist at Element Six. Element Six develops and manufactures the diamonds necessary to enable quantum communications. “The shared secrets are then used to create secure messages that can subsequently be transmitted,” he said.

Let us suppose we have two people communicating. One is sending some traffic and the other is receiving it. The sender has a source of photons. The source is switched on and its beam of light contains a stream of photon particles. Each photon is either horizontally or vertically polarised according to whether that photon is denoting zero or one. Before diving into the nuts and bolts of QKD it is worth revisiting how binary works. Conventional computing encodes information into bits, hence why the power of your home Wi-Fi is measured in megabits-per-second. This shows how much information your Wi-Fi can handle each second. Each bit has a value of either zero or one, nothing else. These bits have a simple purpose. They denote the presence (one) or absence (zero) of an electrical signal. The bits tell a computer how to behave, and hence what task to perform. This is determined by which parts of that computer should be off or on to perform that task. The more complex the request, the longer the process takes. Consider how long takes your computer to download a complex video game compared to the time it takes to download a dinner reservation. In the field of encryption, a string of bits is used to define an encryption key and the longer this string is, the harder it is to break the encryption key.

AES-256 (Krypterix)
Confused? This crib sheet provides a pictorial representation of the US National Security Agency’s AES-256 encryption standard used for top secret information. (Krypterix)

Several QKD protocols were theoretically developed over the last decades, based on two quantum-mechanics phenomena, superposition and entanglement. The difference between a conventional bit and a quantum bit (or qubit) is that the qubit to be prepared into a ‘superposition’ of one and zero. Like the famous Schrodinger’s Cat, being dead and alive. The sender transmits a stream of photons with a random superposition of polarizations. The receiver will measure each individual photon and compare the results with the sender. The sender only needs to share over an unsecure channel which polarization directions are ‘correct’. They will not share the outcome of the measurement. Only qubits that measured in the ‘correct’ polarization, for example ‘vertical’ and ‘clockwise’ are maintained and can be used as a common encryption key.

Other options include using entanglement. Using a physical phenomenon, two photons are generated whose polarization is linked. The two resulting streams of photons are sent to two distant nodes, Radio A and Radio B. The polarization of each photon pair can be and chosen to be completely random. When Radio A measures the polarization of the incoming photon, it will know with 100 percent confidence the polarization of the corresponding entangled photon received by Radio B. By using several photon pairs the two nodes can now build a shared encryption key.

Element Six manufactures diamonds that can be used for QKD. Particular defects in diamond, such as nitrogen and silicon atoms with the carbon lattice act as sources of photon qubits. They can also entangle a photon pair as well as acting as repeaters that extend the range of quantum communications. Dr. Twitchen said that Element Six’s diamonds can perform these tasks at room temperature. This will help to make this quantum communications technology practical to use on the battlefield.

The beauty of QKD is that any attempt by a third party to intervene in this process instantly changes the polarization of the stream of photons. To verify that the link sharing the key is secure, Radio A and Radio B only need compare the statistics of their individual measurement without exchanging the results of the measurement, namely the encryption key. Should someone somehow gain access to the fibre optic link between Radio A and Radio B, the very act of trying to collate or decrypt the photon stream will corrupt the encryption key. This will destroy the correlation and thus they will immediately know the connection is not secure. Forget trying to eavesdrop on QKD encoded messages. “Even if two photons are on opposite sides of the galaxy, I must treat them the same. As soon as I attempt to observe one photon this will have a discernible effect on the other, and the whole thing collapses. The very nature of a third party trying to observe the message stops the whole thing working,” noted Dr. Twitchen. This means “you can be 100 percent certain that your message has not been intercepted by a third party.”.

A US Air Force Staff Sergeant installs an AES (Automatic Encryption Standard) chip into a Motorola XTS-5000 handheld tactical radio. (US DOD)

In Service

Quantum communications is not the stuff of science fiction. A July 2021 article in Scientific America said that the People’s Republic of China had shown that single photons could be transmitted across fibre optic links of up to 300 kilometres (187 miles). This was in excess of 100 times further than had been observed in previous experiments. “The technology is already there,” stated Dr. Twitchen. However, some hurdles must be jumped before the technology can proliferate. The strength of the signal sending the stream of photons down a fibre optic cable will invariably weaken the further it must travel. Dr. Twitchen said that quantum repeaters will be needed to ensure the photons can be moved efficiently around large networks. He believes there is some scope in the short term to begin using this technology for shorter distances. Deployed sensors on the battlefield could be connected using fibre optic cables carrying these communications. “This could happen in the next ten years,” he predicted.

One can see how QKD could be useful for fibre optic battlefield communications. Field telephones may seem antiquated but have good resistance to eavesdropping by the very nature of using a cable. QKD is clearly on the horizon and could improve the resilience of battlefield communications yet further.

by Dr. Thomas Withington