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.
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.
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.
Secure underwater communications for submarines
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.
“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
Submarine communications have always been a challenge because radio waves can’t penetrate sea water. Submarines since long have been using ELF or VLF radio waves for communications when submerged. However VLF and ELF frequencies offer a very high path loss, narrow bandwidth, and high bit error rate. 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 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.
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.
Underwater quantum key distribution
Researchers are now exploring the possibility of provably secure communications with submerged submarines using quantum key distribution over an underwater optical channel. QKD promises to guarantee secure communication through the principles of quantum mechanics, without sacrificing speed or forcing the submarine to rise nearer the surface.
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.
Physicists 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.
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 andas 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.