We describe simple quantum lidar and show it provides the best, most rapid identification of high reflectivity nearby targets. For lower reflectivities or more distant targets quantum lidar is impractical so we describe a protocol that mimics the relevant feature of quantum lidar via random intensity modulation of a classical beam. This provides a degree of covertness together with many of the other useful properties of a quantum lidar without the complication of producing quantum states and crucially, without the limit on mean photon number. Hence it is useful for identifying much lower reflectivity targets than is practical with quantum states.
Optical quantum technologies such as quantum sensing, quantum cryptography and quantum computation all utilize properties of non-classical light, such as precise photon-number and entangled photon-pair states, to surpass technologies based on the classical light. A common route for obtaining heralded single photons is spontaneous four-wave mixing in optical fibers, allowing for a well-defined spatial mode, for high efficiency integration into optical fiber networks. These fibers are typically pumped using large, commercial, pulsed lasers requiring high-power (~10 W) pump lasers and are limited to ~MHz repetition rate. Here we propose a cost- efficient, compact and mobile alternative. Photon pairs at 660 nm and 960 nm will be created using four-wave mixing in commercial birefringent optical fiber, pumped using transform limited picosecond pulses with GHz repetition rates derived from a 785 nm CW laser diode using cavity-enhanced optical frequency comb generation. The pulses are predicted to have average power of 275 mW, a peak power of >40 W, and predicted photon yield of >2000 pairs detected per second. This design will be later utilized to implement a quantum illumination scheme based on a coincidence count between idler and signal photons -- instead of joint measurement between signal and idler. This will allow for quantum advantage over classic LIDAR without the requirement for maintaining an interferometric stability in free space.
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