IEEE - Aerospace and Electronic Systems - April 2020 - 23

quest for better ways to identify genuine radar echoes amid
noise. This is not the only aspect of radars that can be
improved using quantum techniques, however. It is possible
to develop quantum-enhanced receivers, for instance, which
can detect signals at very low powers. Nevertheless, we will
focus on the signal-versus-noise problem in this article
because most work in the quantum radar has been in this
direction.
One type of the entanglement-based quantum radar,
called quantum two-mode squeezing radar (QTMS radar),
has been experimentally demonstrated to be a feasible
quantum range finder [6]. However, modern radars are not
just range finders. They are rich in possibilities and capabilities, such as a synthetic aperture radar (SAR), adaptive
array processing, and space-time adaptive processing
(STAP) [3], [7]. Can a quantum radar do all that? In the
case of QTMS radar, the answer is a definitive yes. This
shows that there exists a definite path to a quantum radar,
which can do everything modern radars can do. What is
needed now is the will to follow that path.
The following is a largely heuristic discussion and is
not meant to be exhaustive in terms of possibilities offered
by quantum physics. A rigorous discussion would require
material from quantum theory, which cannot be covered
in a single article. We hope, however, that it gives a flavor
for some of the possibilities of an entanglement-based
quantum radar.

HOW TO DISTINGUISH SIGNAL AND NOISE: MATCHED
FILTERING
The main function of a radar is to discriminate the signal
of interest from the interference background, which comprises thermal noise, clutter, and intentional interference
(jamming). Unlike other interference sources, thermal
white noise cannot be suppressed via signal processing.
Hence, distinguishing signal from noise is a fundamental
problem in radar design. In order to understand what parts
of a radar can be improved using quantum physics, it is
helpful to have a basic idea of how radars actually deal
APRIL 2020

with noise. (At this point, we will ignore other important
practical issues such as clutter and interference. These
will be discussed later). In order to keep the analysis simple, we will assume that the radar is attempting to detect a
single, stationary target with a high radar cross section
unless otherwise noted.
On a very basic level, the operation of many types of
radars is based on matched filtering, which can be summarized as follows.
1) Produce a signal and transmit it toward a target.
Retain a record of this signal.
2) Receive a signal, which may or may not contain an
echo of the transmitted signal.
3) Correlate the received signal with the record of the
transmitted signal.
4) If the correlation exceeds a preset threshold, declare
that a target is present.
The rationale behind this scheme is that a genuine
echo should be highly correlated with the originally transmitted signal. Therefore, an echo corrupted by noise may
still be more strongly correlated with the original signal
than pure noise. Note that the received signal does not
need to be highly correlated with the recorded signal. For
detection to succeed, it is only necessary that the detected
correlation be distinguishable from zero correlation. But
in order to achieve this distinguishability, the transmitted
signal should be as strongly correlated with the recording
as possible.
The quantum radar schemes that we consider in this
article offer potential improvements in the fidelity of the
record of the transmitted signal. In other words, quantum
radars could produce transmitted and recorded signals,
which are more highly correlated than any signals produced by a perfect "classical" source, irrespective of
future technical advances in such classical sources. The
reason this improves radar performance is that, the higher
the correlation at the transmit side, the more room we
have for noise to corrupt the received signal, yet still
maintain detection performance.

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IEEE - Aerospace and Electronic Systems - April 2020

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