Underwater communication is vital for undersea exploitation and modern communication. Conventional ways which employ acoustical technique for underwater communication have their drawbacks including high path loss, narrow bandwidth, high bit error rate, among which unconditional security is more demanding due to commercial and secure interest.
Submarine communications have always been a challenge because radio waves can’t penetrate sea water. The obvious solution is to surface and raise an antenna above the sea level, then use ordinary radio transmissions. However, a submarine is most vulnerable when on the surface. Early submarines mostly travelled on the surface because of their limited underwater speed and endurance; they dived mainly to evade immediate threats. During the Cold War, however, nuclear-powered submarines were developed that could stay submerged for months. To communicate with submerged submarines several techniques are used.
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. These frequencies offer a very high path loss, narrow bandwidth, and high bit error rate. 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 which is too low to support high bandwidth data such as video.
These systems also impose severe operational limitations: these are extremely low bandwidth one-way systems that require towed antennas or buoys, and submarines need to steer specific courses and reduce their speed. 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 options such as submarines briefly surfacing or the use of towed antennae compromise the ability of the vessel to remain stealthy. 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.
The U.S. Navy operates 14 Ohio-class ballistic missile submarines, with at least five to six at sea at any given time. A typical deterrent patrol lasts an average of 70 days, during which time the submarines do their best to hide in the vastness of the world’s oceans and await orders to fire their missiles. An Ohio-class submarine at sea is America’s ace in the hole, ensuring that hundreds of nuclear warheads can survive a surprise nuclear attack on the U.S. The idea is that this strategy deters an enemy from launching an attack in the first place. All of this makes secure, reliable communications with submarines extremely important. A third party that could read messages between the “boomers” at sea and the Pentagon could determine their position and sink them.
“Secure communications with submarines are critical to maintain our nuclear deterrence capability and to enact the Network Centric Warfare doctrine of naval operations. As a consequence, the deployment of efficient and secure communication links with submarines is one of the greatest technological challenges presently confronted by the US Navy”, says Marco Lanzagorta, ITT Corporation.
“Indeed, due to their strategic and tactical importance, submarine communications require perfectly secure cryptographic protocols such Vernam (one-time) pads. Clearly, this solution not only presents the problem of efficient distribution of secret keys before the submarine departs the base, but also imposes a limit on the number of secret keys available onboard a submarine during prolonged seaborne missions,” he further says
Researchers are now exploring the possibility of provably secure communications with submerged submarines using quantum key distribution over an underwater optical channel. Quantum cryptography exploits the quantum properties of particles such as photons to help encrypt and decrypt messages in a theoretically unhackable way. Scientists worldwide are now endeavoring to develop satellite-based quantum communications networks for a global real-time quantum Internet. QKD promises to guarantee secure communication through the principles of quantum mechanics, without sacrificing speed or forcing the submarine to rise nearer the surface.
China is also reportedly looking into quantum communications to communicate with its submarines, and other undersea nuclear powers will likely follow suit.
Secure submarine communication using Quantum Key Cryptography (QKD)
Recent years, Quantum Key Distribution (QKD) based on photons has made great progress both in theoretical and experimental researches. The technology for QKD through optical fibers is already in use in various places and researchers have demonstrated quantum key distribution through free space over distances of up to 144 km. Quantum Key Distribution protect information by making use of the fundamental laws of quantum physics, make data transmission completely immune to hacker attacks in the future. Information in a quantum channel is carried by single photons that change irreversibly once an eavesdropper attempts to intercept them. Therefore, the legitimate users will instantly know about any kind of intervention.
In addition to beaming quantum communications signals across the air, through a vacuum, and within fiber optic cables, researchers have investigated establishing quantum communications links through water. Such work could lead to secure quantum communications between submarines and surface vessels, and with other subs, aircraft, or even satellites.
Long-distance quantum channels capable of transferring quantum states faithfully for unconditionally secure quantum communication have been so far confirmed to be feasible in both fiber and free-space air. Now the researchers are attempting to implement quantum communications through seawater, which covers more than 70% of the earth, can also be utilized, paving the way for global quantum communication.
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.”
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. 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.
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.
The Quantum Technologies group at defence technology specialist ITT Exelis is researching the feasibility of laser optical communication between a submarine and a satellite or an airborne platform, secured by using quantum information. An optical link between the submarine and the satellite is being established, after which a communication link shall be established between specialist photon lasers that sends individual photons at a time which shall be passed through filters that will polarize the photons, these polarized photons then be detected / counted by photo sensors working in Geiger mode.
Driven by the communication requirements of underwater sensor networks, submarines and all kinds of underwater vehicles, underwater laser optical communication has been developing rapidly in recent years. Blue green lasers between 400 to 500 nm wavelengths are being used for underwater communication. The new research shows that by using the current technology underwater wireless optical communication can transmit 350m in the clearest seawater with the bit rate of 10Mbps.
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.”
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.”
Chinese Scientists Use Lasers to Set Up First Underwater Quantum Communications Link
The Chinese scientists have demonstrated the feasibility of a seawater quantum channel, representing the first step towards underwater quantum communication, according to their study published in the journal Optics Express. Chinese researchers have experimentally demonstrated the distribution of polarization qubits and entangled photons over seawater channel. “While implemented in a short distance, the obtained high process fidelities indicate that the seawater associated with suspended particulate matter introduces very limited depolarization and disentanglement, which verify the feasibility of quantum communication and quantum cryptography in free-space seawater,” write the authors.
Ling Ji and others from State Key Laboratory of Advanced Optical Communication Systems and Networks, Tong University, Shanghai , China have experimentally demonstrated that polarization quantum states including general qubits and entangled states can well survive after travelling through seawater. The Chinese scientists bestowed photons from a laser with different polarizations (the direction their waves travel perpendicularly to the photon’s forward motion) by passing the light through a series of crystal, filters, and mirrors. Their experiment then splits the beam, keeps one of the two entangled photons on one side, and passes the other one through a ten-foot-long tube containing one of several seawater samples.
They performed experiments in a 3.3-meter-long tube filled with seawater samples collected in a range of 36 kilometers in Yellow sea. “Although quantum key distribution and quantum teleportation have been achieved via optical fiber installed underneath Geneva Lake and the River Danube respectively, experimental investigation in free-space seawater has never been done so far say authors. Analogous to the existence of transmission window around 800 nm in free-space air there is a “blue-green” optical window at the wavelength regime of 400-500 nm in free-space seawater , wherein photons experience less loss and therefore can penetrate deeper. “We have experimentally demonstrated the distribution of polarization qubits and entangled photons over seawater channel.
The high process fidelity indicate the seawater associated with suspended particulate matter introduces very limited depolarization, which verify the feasibility of quantum communication and quantum cryptography in free-space seawater.” Future explorations include field experiment in open sea, blue-green band quantum repeater and air-sea quantum communication interface.
Quantum repeater combining entanglement swapping and quantum memory in blue-green window, though technically more challenging, can efficiently extend the achievable distance. A scenario we have conceived is that, in a quantum communication network consisting of many non-stationary platforms, each platform communicate with neighbouring one in a moderate distance meanwhile can serve as quantum repeater node connecting two or more neighbouring platforms.
Another scenario we have thought of is that, a ship can connect one platform in free-space seawater and one in free-space air, where different wavelength windows of light can be exploited respectively, serving as a quantum repeater node as well as sea-air interface. As its elemental ingredients, short-wavelength quantum entanglement has been developed in semiconductor, and short-wavelength quantum memory may be achieved by virtue of frequency conversion in quantum regime.
The underwater free space QKD is faced with two propagation challenges: One is attenuation of seawater channel that reduces the number of received photons and influence the secret key generation rate of QKD. The second is that complex components and special optical properties of seawater scatter part of photons thereby changing the polarized state of photons which constitute the qubits, and increasing the error of information.
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.
“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 Dr. Marco Lanzagorta, the director of the Quantum Technologies group in the Information Systems department of ITT Exelis, “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.”
Peng Shi and pals at the Ocean University of China in Qingdao have calculated how far photons can travel through water while preserving the quantum information they carry. They conclude that it ought to be possible to send data at a rate of around 215 kilobits per second at a distance of 125 metres in clear ocean water. Their calculation was based on attenuation of 450 nanometre polarised light in sea water and as well as level of noise present and the level of error correction required.
Experts believe the proposed system could potentially deliver perfectly secure audio information and low bit rate encoded video information from an autonomous submarine. Of course, these results are the theoretical maximum, Peng and co suggest that placing 450 nm filters over the detectors or gating them so they only look for photons during the short time window over which they ought to arrive, thereby helping in screening the noise. 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.
New tests show that such links can be reliably established in turbulent waters at greater distances than previously reported
Underwater quantum links are possible across 30 meters (100 feet) of turbulent water, scientists have shown. Such findings could help to one day secure quantum communications for submarines. Prior work suggested that quantum links through water had a maximum length of 300 meters with 418-nanometer-wavelength light under clear conditions. Previous research also successfully established quantum communication across 55 meters of sheltered water, such as that which is found in coves and bays. However, until now, scientists had only reported quantum communications across 5.5 meters of turbulent water.
In the new study, researchers experimented with quantum communications in a “flume tank,” a water tank in which scientists can generate waves to mimic the ocean. They also tried two different strategies for quantum communications—one involving just the polarization of the light, and the other incorporating the polarization and the orbital angular momentum of the signals—to analyze how quantum communication protocols might differ in maximum distances and data transfer rates.
The researchers achieved quantum communication at up to 72 kilobits per second across up to 30 meters of turbulent water, the longest distance yet reported for such links. Although turbulence did result in significant wandering and distortion of light signals, those error rates didn’t prevent quantum links from successfully being established with either communication protocol. Unexpectedly, the researchers found they could keep quantum communication going even while the transmitter moved down the flume tank. “We had expected that this would not be possible without beam-tracking technology,” says Felix Hufnagel, a lead author and quantum physicist at the University of Ottawa in Canada.
After the scientists analyzed their data, they suggested the maximum distance for secure quantum communications might actually be 80 meters in turbulent water, although this would depend on factors such as the efficiency of the detectors used. Improving such factors might significantly boost the maximum communications distance, they say. In the future, the researchers aim to experiment with faster electronics and beam-tracking tools, “which will allow us to communicate between two- or multi-parties that are actually floating and moving around in the water,” Hufnagel says. They would also like to experiment with quantum links across air and choppy water, which will “present a whole new set of challenges, requiring dynamic beam correction on the sender and receiver sides,” he adds.
Ref:arxiv.org/abs/1402.4666: Feasibility of Underwater Free Space Quantum Key Distribution