Quantum key distribution technology could 
enable submarines to communicate securely both at depth and speed. 
Berenice Baker investigates how rapid underwater communication can be 
achieved at a level of secrecy protected by the very laws of physics 
themselves. 
        
        
Submarine communication challenges
Submarine communication is restricted by
 the depth at which vessels can exchange information and the speed at 
which they can do so through the medium of water.
Recently 
however, researchers have made impressive strides in solving this 
dilemma using a technique called Quantum Key Distribution (QKD).
QKD
 promises to guarantee secure communication through the principles of 
quantum mechanics, without sacrificing speed or forcing the submarine to
 rise nearer the surface.
For a submarine to retain all its 
tactical advantage, it must remain submerged in the mixed layer, which 
is around 60 to 100 metres deep, below which surface sonars cannot 
detect them. Submarine communications are currently carried out while 
submerged using ELF or VLF radio waves because only very low or 
extremely low frequencies can penetrate the water at those depths.
Using
 ELF and VLF presents a number of disadvantages, however. The 
transmission sites have to be very large, meaning the submarine must tow
 cumbersome antenna cables, plus it usually has to align on a specific 
orientation and reduce speed to obtain optimal reception.
The VLF 
and ELF frequencies only offer a very low bandwidth: VLF supports a few 
hundred bits a second while ELF sustains just a few bits each minute. 
This prevents the transmission of complex data such as video.
One 
potential solution is to carry out optical communications using a laser,
 a concept which has been around since the 1980s when experiments were 
carried out to demonstrate that it is possible to maintain an optical 
channel between a submarine and an airborne platform.
The Quantum 
Technologies group at defence technology specialist ITT Exelis is 
looking at taking this a step further through research into the 
feasibility of laser optical communication between a submarine and a 
satellite or an airborne platform, secured by using quantum information.
The
 work ITT Exelis carries out for the US Government includes research in a
 wide variety of quantum information topics, including the development 
of quantum algorithms, quantum sensors and novel solutions for quantum 
communication systems.
Perfectly secure keys
Dr. Marco 
Lanzagorta, the director of the Quantum Technologies group in the 
Information Systems department of ITT Exelis, explains that QKD is a 
protocol which uses quantum information to generate a pair of perfectly 
secure keys.
"Quantum information is different from classical 
information, because in classical information the unit is the bit and it
 can have the value of zero or one," said Lanzagorta. "The unit of 
quantum information is the qubit, which is a quantum state of a photon. 
It can be on zero, one or any superposition of zero and one. It's more 
of a concept of information than the classical one."
Quantum 
information has two important properties for securing communications. It
 cannot be copied which means it cannot be forged, and every time a 
quantum state is measured by an observer it gets collapsed, which means 
its properties are very difficult to detect.
Combined in QKD, 
these properties can be used to generate perfectly secure keys because 
the secrecy of the keys is guaranteed by the laws of physics.
Lanzagorta
 explains that in traditional cryptosystems - such as the public domain 
system RSA, Diffie-Hellman and ElGamal encryption methods - the security
 is based on the solution to a very hard mathematical problem.
However,
 there is no formal proof that this mathematical problem, for example 
prime factorisation in the case of RSA, could not be broken by an 
advanced algorithm. It has also been conjectured that hypothetical 
quantum computers could break these types of ciphers exponentially 
faster. Hence QKD would offer an unbeatably secure solution.
Optical communication
The
 technology for QKD already exists and is commercially available but it 
is currently carried out through an optical fibre, rather than photons 
travelling freely through air or water.
"Some
 experiments have been done on QKD using photons moving in free space," 
said Lanzagorta. "Most recently an experiment was done in the Canary 
Islands where they did first base QKD at a distance of 144km, showing it
 is feasible to have this free space quantum communication.
"Other
 work has been done on connecting a ground site with a satellite 
platform, but we're working not on a ground platform but on one that is 
submerged in the water."
In addition to the challenges of 
transmitting photons through water and free air, the researchers need to
 establish a laser link between the transmitter and a receiver on a 
satellite or airborne platform.
This is currently being tackled by a QinetiQ North America team which is developing a specialist tracking system.
Once
 the optical link between the submarine and the satellite is 
established, the ITT Exelis researchers' work takes over, investigating 
how to enable the QKD protocol to secure communications. This is done 
using a photosensor working in what is known as the Geiger mode, which 
effectively means it counts photons which arrive in a certain 
polarisation.
"For the transmission of quantum information, you 
need something that will polarise the photons, so the quantum state will
 be in a given basis, and to have a filter that detects this in the 
transmitter and receiver," said Lanzagorta.
"You cannot use 
regular lasers as you need specialist photon lasers, which is like a 
very diluted laser. These send one photon at a time and each photon has a
 well-determined quantum state."
Feasibility studies
The next 
stage for the programme will see the US Naval Research Lab carry out a 
series of experiments to establish how well a photon's quantum state is 
preserved as it travels through water to verify the accuracy of ITT 
Exelis' theoretical feasibility study.
If the experiments support 
the theoretical model and the research moves on to the next stage, an 
experimental prototype could be in place within five years. However, a 
number of factors are at play with such a radical new approach.
"It's not only a scientific technological question but also has to do with funding levels and politics," claimed Lanzagorta.
However,
 if the powers that be do see it through, the benefits could be 
substantial. The proposed system could potentially deliver perfectly 
secure transmission, the highest level of security available, at rates 
of up to 170kb a second, which is around 600 times more bandwidth than 
current VLF systems are capable of, easily coping with complex data such
 as video.
Additionally, there would be no loss of operational 
efficiency or stealth for the submarine itself, as in principle it would
 not have to slow down, remain at depths of less than 100m or change 
orientation to exchange data.
These factors would be addressed by 
the transmitting laser and receiving system part of the solution, which 
is being tackled by QinetiQ.
However, the entire success depends 
on how travelling through water affects the photon. "The biggest 
challenge is to see what is the best way to send the single photon 
pulses in such a way that the quantum state is protected even if it 
travels through water," said Lanzagorta. "We need to find a way to do 
some sort of encoding, like error correction encoding, that protects the
 quantum state of the photon so we can have a larger range of 
operations."
* Article 
publicat a Naval Technology. Les comunicacions amb submarins en immersió sempre han estat quelcom tant problemàtic com vital. Aquest interessantíssim article n'aporta algunes claus de futur