IEEE Aerospace and Electronic Systems Magazine - November 2020 - 40

Opportunities and Challenges of Quantum Radar

Figure 1.
Conceptual description of an entanglement-based monostatic standoff quantum radar.

reflected signal photons will not be perfectly reflected
back toward the radar while some that should be perfectly reflected will be lost due to environmental attenuation, e.g., by being absorbed or scattered/deflected by
the intervening atmosphere. Rough back-of-the-envelope calculations suggest that the integration process
will require an excess of 109 entangled photons to have
a probability of detection of 0.8 for a static target (2 m
diameter flat circular target with 0.1 reflectivity oriented
in the specular direction) located 25 km away in low visibility atmospheric conditions (300 m visibility due to
fog or clouds) using a quantum radar operating in the Xband frequency (3.2 cm) and a 2 m diameter receiver/
transmitter. Each set of 109 photons is called a quantum
radar signal package.
An ideal source of entanglement would produce
entangled pairs on demand; that is, with near 100% certainty the source produces exactly one entangled pair at
the push of a button. However, such sources are in their
infancy and not commercially available. In optical frequencies, where most work on photon entanglement has
been carried out, one can generate entangled pairs via
spontaneous parametric downconversion (SPDC) using a
pulsed laser through a nonlinear crystal [23]. Pulsing the
laser allows one to timestamp the event; however, each
pulse has a very low probability of containing a photon
pair [24]. Even with this limitation, using a PPKTP crystal
and a 2 ps pulsed laser, it is possible to generate 105
entangled pairs per second for each milliwatt of pump
power [25], [26]. For a conservative model using off-theshelf technology, we can assume a pump power of
100 mW, and we will therefore have the capability to
40

generate 107 entangled pairs per second. Each of these
photons is temporally separated and their integration can
be carried out with standard equipment.
In the microwave regime the situation is far less
advanced, with the study of how to generate entangled
microwave photons only receiving significant attention in
the last few years. In addition, microwave frequencies
impose a limit on the number of distinguishable/independent optical modes available [27]. Indeed, the number of
distinguishable modes (M) available for a single illuminator-detector is M ¼ W Â T , where W is the bandwidth
and T the period of the signal. Assuming that a quantum
sensor operates in the X-band at 10 GHz, and works with
a 40% fractional bandwidth, then the bandwidth is approximately 4 GHz. If the signal pulses are emitted with a
period of 1 ms then M % 4 Â 103 , i.e., much less than the
M % 109 required in the previous example. In general, the
number of distinguishable modes available in the primary
radio-spectrum bands is impractically small. To overcome
the constraints imposed by small M, it has been proposed
a distributed architecture of quantum sensors for which
the effective size of M can be increased [28], [29]. A different approach to this problem is the use of multiple-input
multiple-output channels using beam-splitters [30].
In order to determine the expected performance of a
quantum radar, we use theoretical models that will be
described in more detail in subsequent sections [31].
According to these models, the advantage of quantum
radar (red) as compared to classical radar (blue) is shown
in Figure 2 in terms of the detection probability (Pd) versus the signal-to-noise ratio (SNR) in decibels. In the
quantum case, the theoretical SNR is computed using

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IEEE Aerospace and Electronic Systems Magazine - November 2020

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