SUBMARINES must be able to talk securely with remote naval bases while remaining submerged. Could quantum communications allow them to pull off this technically challenging feat?
Submarines employ random "keys" known as one-time pads to encrypt messages. Each key can only be used once, making it impossible for eavesdroppers to crack the code.
One problem with this is that the key must be securely agreed before the submarine leaves base. There is a risk involved in having many keys on board, in case the sub is captured and they fall into hostile hands.
The other problem is that submarines receive messages using low-frequency radio waves that can penetrate water, but only a few characters per second can be transmitted at these frequencies. To receive high frequencies, which can boost the data rate, submarines have to surface and risk detection.
"You want the submarine to be undetectable for as long as possible - we're talking about several weeks," says Marco Lanzagorta, director of quantum technologies at US defence firm ITT.
He suggests that a technique called quantum key distribution (QKD) could solve these problems. It uses the quantum properties of photons, which are polarised in two different ways to encode 0s and 1s, to generate and exchange a key. Any attempt to intercept the photons disturbs these properties and raises the alarm.
To establish a secure link while remaining 100 metres underwater, submarines could transmit photons of laser light to satellites, for retransmission to base. With the key exchanged, the submarine could then communicate via laser pulses with guaranteed security.
Lanzagorta's simulations suggest it would be possible to transmit and receive data at 170 megabytes per second, enough for video communication. He will present his ideas next month at a cryptography workshop in Gaithersburg, Maryland, hosted by the US National Institute of Standards and Technology. Later he plans to conduct experiments at the US Naval Research Laboratory in Washington DC to investigate how well a photon's quantum state is preserved as it travels through water.
Rupert Ursin of the University of Vienna in Austria was part of a team that, in 2007, set a QKD record by sending photons 144 kilometres through air. That showed that quantum-encrypted signals can in principle be sent to and from satellites, though Ursin says such an experiment is still far off because much of the necessary equipment has never flown in space. It is "quite visionary" to contemplate quantum-encrypted signalling from Earth to a satellite, he says. "This submarine communication stuff is even more visionary."