IEEE Aerospace and Electronic Systems Magazine - November 2020 - 5

a range finding application. Such ideas should be explored if it can perform only a subset of the functions it does provide gain
in the functions. It is important that due consideration is given to the decades of work done by radar scientists/engineers. On
the other hand, it would be too limiting to reject all ideas that do not fully conform to what is currently thought of as a radar,
in much the same way that it would be too limiting to reject ideas that are not " fully quantum. " Many such theoretical radar
proposals may not be amenable to advanced radar signal processing (clutter suppression, array processing, etc.), but it may
still be useful as a less featured remote sensor (range finding).
Third, an important point is that it exploits practically a novel quantum aspect in the transmit and/or receive sides, and
that is not being exploited in current radars. This could be entangled photon pair generation, quantization of energy, quantized transitions (Rydberg), etc. Papers exploring such concepts are in the first issue.
Fourth, cost, size, weight, and power considerations are of course important in practice. However, these considerations
should not limit whether an idea is explored. It is likely that an initial prototype might have too many drawbacks to be relevant for practical applications. However, the concept should not be equated with the initial technological implementation.
After all, the initial implementation of the computer (Charles Babbage and Ada Lovelace's Analytical Engine, or later on the
vacuum tube-based ENIAC) was not very " practical. "
Still dwelling on the definition of a " QR, " one of the Associated Editors (A. Farina) remembers the interesting paper
[8] because it was previous to his paper [9] at the same symposium during the inaugural plenary session. Reading again
the paper [8], the following points seem interesting to quote in this foreword. " A radar is said to be a QR if at the detection level we have insufficient RF target photons density to invoke classical physics by means of the Gaussian central
limit theorem [8]. " Indeed, this definition of QR deserves to be reiterated. Continuing to quote the authors of [8]: " The
RF frequency electromagnetic waves are comprised of discrete photons. However, the energy of these photons is much
weaker than at visible frequencies; thus, the quantum nature of these signals is generally discounted, without deleterious
effects. At RF frequencies, the thermal noise is much larger than the energy of a single photon and so in general by the
time one sees a radar signal the law of large numbers has kicked in and quantum effects becomes invisible. "
Continuing with [8], the authors note: " As radar systems work with longer integration times, larger apertures, and
smaller noise figures we can no longer necessarily ignore the photon nature of RF energy. " The authors show that the
quantum effects arise in the area of phased arrays verses dishes in particular for radio astronomy application where
quantum effects might become substantial. An explanatory simulation of a " photon starved " phased array of 128 by
128 elements is compared to a dish of equal aperture. The authors show that the array loses m ¼ 128^2 photons compared to the dish. The authors' interpretation of this result is that the array pays a price for the ability to measure where
across the aperture the photon arrived. Knowledge always comes at a price in quantum theory and we see here that the
price is one photon per element. As a practical example, the authors reported the imaging of the 216 Kleopatra asteroid
by the Arecibo radio telescope in Puerto Rico (https://news.cornell.edu/stories/2000/05/radar-shows-giant-bone-shapedasteroid). They conclude that in general arrays would underperform dishes in low-noise environment. In short, these
are some lesson learned from [8].
The increased importance of quantum technologies is testified by the choice of IEEE Aerospace and Electronic Systems
Magazine to introduce a new category: Quantum Technologies for Aerospace. This will be managed by one of us (M. Frasca)
as an Associate Editor.
This special issue on QR contains five interesting papers that reflect both the radar engineer as well as the quantum physics viewpoints.
F. Daum raises some practical, system engineering aspects " Quantum Radar Cost and Practical Issues. " He notes the
many challenges in developing a practical system based on breadboard radar used in experimental investigations carried out
NOVEMBER 2020

IEEE A&E SYSTEMS MAGAZINE

https://news.cornell.edu/stories/2000/05/radar-shows-giant-bone-shaped-asteroid https://news.cornell.edu/stories/2000/05/radar-shows-giant-bone-shaped-asteroid

IEEE Aerospace and Electronic Systems Magazine - November 2020

Table of Contents for the Digital Edition of IEEE Aerospace and Electronic Systems Magazine - November 2020