IEEE Aerospace and Electronic Systems Magazine - November 2020 - 53

Lanzagorta and Uhlmann
significant level of research focus because of their
need in other quantum technologies such as secure
quantum key distribution [50].
Signal Processing: The signal photon is expected to
traverse an attenuating environment, so at near 0 dB
SNR most of the signal photons will be scattered or
absorbed by air molecules and will never reach the
detector. Furthermore, some of these photons may
be received out of synch with the idler photons
stored in the quantum memory. Consequently, there
is a strong need to develop robust signal processing
techniques able to extract as much useful target
information as possible from the few signal photons
that are received by the detector. This requires identification of the specific encodings of the signal photons that provide the best resilience to corruption
under the environmental conditions of relevance to
the application of interest [59]. In addition, we need
to develop robust techniques to optimize the integration time of the information from all the returning signal photons.
Detection and Ranging Protocols: To date, no efficient quantum ranging protocol has been developed.
While quantum illumination offers very interesting
advantages, it is only a detection protocol, i.e., is
only concerned with determining if a target is present or not at a specific point of space. Ranging protocols to actually estimate the distance to the target
have proved to be very challenging to design. The
likely reason is that most entanglement-based standoff quantum sensing devices appear to require single-photon control, where each signal photon and
its entangled idler counterpart need to be identified
to extract target information from the entanglement
correlations. It is clearly not feasible to illuminate
and probe a large region of space using these techniques. However, intriguing proposals such as
" progressive quantum scanning " (exploiting some
existing information about the target) and " quantum
noise radar " (exploiting the entanglement represented by the statistical properties of a group of photons) appear to be able to circumvent this problem
to some degree [40], [60]. Nonetheless, further
research in this area is required for the design of
quantum detection and ranging devices when no
other target information is available.
Quantum Hardware: In general, most detection and
ranging protocols will require sophisticated quantum hardware to perform operations and measurements on the photon states. For example, basic
quantum illumination requires a very specific type
of joint measurement [34]. Even though quantum
illumination has been demonstrated experimentally,
NOVEMBER 2020

this was done using a suboptimal receiver that only
achieved a 3 dB advantage (instead of the predicted
6 dB) [61]. Short of a general-purpose quantum
computer, even the most simple design of an optimal quantum illumination receiver requires sophisticated quantum operations on the pair of entangled
photons [62]. Therefore, it is necessary to develop
robust quantum hardware to implement any sophisticated quantum detection and ranging system.
Fortunately, quantum hardware is currently being
developed within the context of quantum
computation [63].
Target Interaction: Besides their interaction with the
attenuating environment, some of the signal photons
may also interact with the target before being reflected
back to the detector. In general, the target may have a
sophisticated geometry and may be moving in a nontrivial manner [64]. Also, the target may be composed
of layered materials with specific electromagnetic properties. Thus, we need to improve our understanding of
the details of these kinds of photon-target interactions
so they can be modeled with higher fidelity, e.g., for
better quantum radar cross section models that provide
more than simply " target in/outside range " classifications [36], [49], [65], [66]. For example, it is important
to determine how Doppler or other signatures in quantum radar can be used to determine the presence of
moving parts in the target (e.g., fan or helicopter
blades); to reduce the effects of clutter (e.g., due to hail
or radar countermeasures); or for target discrimination
(e.g., to determine if there is a single target or many
clustered targets). Even though radar cross section is an
important issue for classical radar systems, quantum
interactions and measurement protocols could lead to
novel effects not seen in the classical domain.
Quantum Attenuation Models: As quantum radar is a
standoff sensing technique, the signal photons are
expected to traverse a noisy and attenuating environment [31], [50]. That is, signal photons will be lost to
absorption and scattering produced by molecules in
the environment. Currently, the classical interaction
between signal photons and the environment is modeled with a macroscopic bulk material approximation [50]. However, the transition to quantum-based
sensing modalities offers an opportunity to develop
and exploit higher fidelity attenuation models using
the fully quantized Maxwell's equations in presence
of material media [67], [68]. Such models could create or significantly improve the quantum advantage
over classical in some scenarios and therefore represents a potentially high-impact research topic. Once
again, even though attenuation is an important issue
for classical radar systems, certain quantum

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IEEE Aerospace and Electronic Systems Magazine - November 2020

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