IEEE Aerospace and Electronic Systems Magazine - November 2020 - 18

Quantum Radar Cost and Practical Issues
Third, the correct analysis in [16] shows that QR has a
6 dB better SNR than CR (with the same transmit power
and bandwidth for low photon flux per mode). However,
the analysis in [16] assumes that the target RCS is constant
over the QR dwell time, which is very rarely the case for
any real-world targets. Moreover, the analysis in [16]
assumes that the target is in one resolution cell in range,
Doppler, azimuth, elevation, and polarization, which is
also unrealistic for most practical applications. This
assumption is especially bad for ultrawideband radars
with long dwell times, which is the only case of practical
interest for QR. That is, in a long dwell time, a typical
real-world target would move through many ultrawideband range cells and Doppler cells. In CR, we can compensate for such effects using standard signal processing
methods, but there is no such theory for QR as of today.
For realistic targets with fluctuating RCS (and also with
changes to the polarization due to scattering), the theoretical advantage of QR over CR is much less than 6 dB in
SNR. See [20] for details on all of this. Even if the target
RCS was perfectly constant over the QR dwell time, the
troposphere and multipath and ducting can easily cause
the signal to fluctuate significantly in the real world. Likewise, multipath and Faraday rotation would change polarization even if the target scattering did not.
Fourth, the lack of any credible real-world experiments of QR has been a major source of skepticism among
radar engineers. This has been mitigated to some extend
by the recent experiments reported in [2] and [3]-[5].
However, both of these experiments are at an extremely
short range (about 1 m) inside a laboratory. There is a serious open question about whether QR can actually work at
ranges beyond 15 km (see [20]). Moreover, the target in
[2] had constant RCS, whereas there was actually no physical target in [3]-[5]. Furthermore, the target in [2] was in
one resolution cell for the entire QR dwell, which means
that it satisfied the mathematical assumptions in [16], but
it was not a realistic target for most practical applications.
Fifth, as with many new ideas, there has been much
hype. For example, it has been claimed that QR can
counter stealth and jamming much better than CR [17].
Such claims have been propagated in highly respected
archival peer reviewed technical journals like Popular
Mechanics as well as MIT Technology Review, and many
other magazines and newspapers. But these claims about
stealth and jamming are false; see [20] for details. Naturally, radar engineers would have their antennae up to
guard against other false claims of QR superiority over
CR. In fact, the word on the street among radar engineers
is that QR is hype with no substance.
Sixth, some papers claim (under certain conditions)
that quantum decision algorithms can maintain quantum
superiority over classical methods even though entanglement is destroyed by signal loss and noise [21]. Radar
engineers can easily buy the simple intuitive picture of
18

entanglement causing extra correlation that can be
exploited to improve on CR performance, but it is hard
to see how quantum superiority could be maintained after
the entanglement of photons is lost. This is extremely
counterintuitive and surprising. After being fed a steady
diet of hype and errors about QR, radar engineers are
unwilling to invest the time and energy to try to understand the mathematics and physics needed to grasp this
new quantum miracle. This is a pity because the theory
in [21] appears to be correct, and it is extremely interesting and maybe useful. Moreover, there are several new
concepts in quantum information theory that go beyond
entanglement. For example, both " quantum discord " and
" quantum contextuality " have the promise of clarifying
this confusing situation. Roughly speaking, these two
concepts attempt to measure quantum superiority even
after the loss of entanglement; see [33]-[36] for details.
The fact that there is still no useful quantum estimation
theory or quantum decision theory that quantifies quantum superiority over classical methods is yet another
source of skepticism.
Seventh, as shown in this article, QR are many orders of
magnitude more expensive than the corresponding CR for
any range and any target RCS. Even if all the previous six
items were fully resolved, the cost and SWAP of QR alone
would be sufficient to cause great skepticism among hard
boiled radar engineers. The calculations summarized in
Fig. 2 can be done on the back of an envelope using basic
well-known facts. No deep analysis or physics or software
or mathematics is required, but rather Fig. 2 was created
using simple arithmetic. Figure 2 shows that today (with the
maximum feasible LNA gain of 100 dB) the cost of transmit
power for a QR is ten orders of magnitude more than for a
CR. In the future, assuming the maximum theoretical signal
bandwidth (100%) and two orders of magnitude reduction in
the cost of cryogenic cooling with the maximum feasible
LNA gain (100 dB), the cost of transmit power for a QR is
five orders of magnitude more than for a CR.

ACKNOWLEDGMENTS
I would like to thank B. Balaji and A. Farina for inviting me
to write this article for a special issue on quantum radar
of the IEEE AES Systems Magazine. Thanks to S. Lloyd,
J. Shapiro, C. Wilson, N. Allen, J. Gray, B. Balaji, and
M. Lanzagorta for useful discussions about QR. Thanks
also to the ten anonymous IEEE reviewers of this article
for many good suggestions and questions. I am especially
grateful to reviewer #3, who did not like my simple explanation of how a QR works. I am also grateful to B. Balaji
for inviting me to give a plenary talk at the quantum radar
workshop in Waterloo last year, and asking me " to be as
provocative and blunt as possible, although I do not think I
need to explicitly state that, do I? "

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