US Navy device locates objects underwater
The US Navy has applied to patent a device that uses quantum photonics to identify any object hidden in the water, while keeping its own submarines hiddenPatentYogi

The US Navy has applied to patent a new device that is able to reveal almost anything hidden underwater, while preventing its submarines from giving away their exact locations to enemies.

At the moment, submarines and other underwater vessels that cruise in Arctic waters have to employ gyroscopes and accelerometers to measure the motion and rotation of the vessel in order to estimate current position, orientation and velocity of the submarine underwater, as GPS signals and radio frequencies are unable to penetrate water.

They could use an active sonar array detection system in order to spot ice canopies underwater to prevent accidental collisions, but the drawback of using this is that it reveals the position of the submarine to any enemies underwater. However, in areas of high latitude such as the North Pole, gyroscopes and compasses become unreliable due to proximity to the geomagnetic North Pole and the Earth's rotation axis, so the Navy has no choice but to use the active sonar array.

To solve this problem, a new patent application filed by the US government describes a quantum photonic imaging device that uses quantum entanglement to silently detect and identify targets. The device generates a pair of entangled photons, and one of the photons, known as the "signal photon", is transmitted into the unknown.

The signal photon is basically a probe, and when it comes into contact with an obstacle, the target reflects the radiation from the probe photon, which is picked up by a biologically-inspired quantum photo sensor, which contains the other photon in the pair, known as the "ancilla proton". If the correlation between the two photons is high, then the submarine knows that an object is present.

But unlike active sonar, the reflected radiation travels at levels that are below noise levels, so that the photons' activities are completely indistinguishable from other noise so it cannot be detected at all until it is correlated with the correct code sequence. In practice, the submarine would basically send out numerous entangled proton pairs, and they would essentially highlight the shape and dimensions of the man-made or natural obstacle.

The video below, created by PatentYogi, provides a handy visualisation that makes it easier to understand how the device would work: