Quantum radar

Quantum radar is a speculative remote-sensing technology based on quantum-mechanical effects, such as the uncertainty principle or quantum entanglement. Broadly speaking, a quantum radar can be seen as a device working in the microwave range, which exploits quantum features, from the point of view of the radiation source and/or the output detection, and is able to outperform a classical counterpart. One approach is based on the use of input quantum correlations (in particular, quantum entanglement) combined with a suitable interferometric quantum detection at the receiver (strongly related to the protocol of quantum illumination).

Paving the way for a technologically viable prototype of a quantum radar involves the resolution of a number of experimental challenges as discussed in some review articles,

Concept behind a microwave-range model

A microwave-range model of a quantum radar was proposed in 2015 by an international team

Although most of the original entanglement will be lost due to quantum decoherence as the microwaves travel to the target objects and back, enough quantum correlations will still remain between the reflected-signal and the idler beams. Using a suitable quantum detection scheme, the system can pick out just those photons that were originally sent by the radar, completely filtering out any other sources. If the system can be made to work in the field, it represents an enormous advance in detection capability.

One way to defeat conventional radar systems is to broadcast signals on the same frequencies used by the radar, making it impossible for the receiver to distinguish between their own broadcasts and the spoofing signal (or "jamming"). However, such systems cannot know, even in theory, what the original quantum state of the radar's internal signal was. Lacking such information, their broadcasts will not match the original signal and will be filtered out in the correlator. Environmental sources, like ground clutter and aurora, will similarly be filtered out.

History

One design was proposed in 2005 by defence contractor Lockheed Martin.

In 2015, an international team of researchers,

In 2019, a three-dimensional enhancement quantum radar protocol was proposed.

Review articles that delve more into the history and designs of quantum radar, in addition to the ones mentioned in the introduction above, are available on arXiv.

A quantum radar is challenging to be realized with current technology, even though a preliminary experimental prototype has been realized.

Challenges and limitations

There are a number of non-trivial challenges behind the experimental implementation of a truly-quantum radar prototype, even at short ranges. According to current quantum illumination designs, an important point is the management of the idler pulse that, ideally, should be jointly detected together with the signal pulse returning from the potential target. However, this would require the use of a quantum memory with a long coherence time, able to work at times comparable with the round-trip of the signal pulse. Other solutions may degrade too much the quantum correlations between signal and idler pulses to a point where the quantum advantage may disappear. This is a problem that also affects optical designs of quantum illumination. For instance, storing the idler pulse in a delay line by using a standard optical fiber would degrade the system and limit the maximum range of a quantum illumination radar to about 11 km.

Media speculation about applications

There is media speculation that a quantum radar could operate at long ranges detecting stealth aircraft, filter out deliberate jamming attempts, and operate in areas of high background noise, e.g., due to ground clutter. Related to the above, there is considerable media speculation of the use of quantum radar as a potential anti-stealth technology.