IEEE - Aerospace and Electronic Systems - May 2022 - Tutorial XV - 87

Sorelli et al.
Table 1.
State of the Art of QI Experiments
Experiment
Frequency
Lopaeva
et al. [43]
Optical
Zhang
et al. [19]
Optical
Luong, Chang
et al. [15]-[17]
Microwave
Ns 1 @@ @
NB 1 @@ @
Optimal classical setup
Joint measurement
Quantum advantage
*
*
*
*
England
et al. [44]
Not specified
*
@ **
@
@ 20% (0.8 dB) **
While all three experiments mentioned above were
performed at optical frequencies, the recent development
of new sources of microwave entanglement enabled the
first quantum radar experiments in this frequency
domain [15]-[18]. All these works used JPA to produce
entangled photons in the GHz regime, and amplified the
signal and idler before sending it to the target region.
However, none of these experiments implement the QI
scheme discussed in the section " Gaussian QI. " In fact,
instead of performing joint measurements on the stored
idler and the signal coming back from the target region,
they heterodyne-detect the idler immediately after amplification,
and then compare it digitally with the heterodynedetected
return mode.
All these works present comparisons with some classical
radars, and show that their quantum devices outperform
them. However, all the considered classical and
quantum radars are not optimal, and therefore, these works
cannot be considered a proof of quantum advantage. In
fact, there are two important flaws in the abovementioned
described procedure that prevent all these experiment to
demonstrate a true quantum advantage.
The first of these flaws is that by heterodyne-measuring
the return and idler modes one introduces some additional
noise that deteriorates the correlations between the
two modes [23]. There are very efficient classical strategies
to counteract this noise which, however, cannot be
applied to the quantum case [5]. As a result, when the signal
and return mode are measured individually via heterodyne
detection, it is always possible to find a classical
radar that performs as well (sometimes even better) than
the quantum one. Furthermore, even an ideal heterodyne
measurement of the idler would only project the signal
beam onto a coherent state [23], [45], implementing de
facto a (suboptimal) CI strategy as defined in the section
" 'Gaussian QI. " The only way to exploit the benefits of
quantum correlations is to perform a joint measure on the
return and idler modes as in the cases of the OPA and FF-
SFG receivers [46]. The only microwave quantum
MAY 2022
@
Barzanjeh
et al. [18]
Optical Microwave
@
@
*
*
*
experiment that discusses this issue is the one by Barzanjeh
et al. [18], where the experimental data are used to
simulate a joint-measurement scenario and to prove that
in that case it is possible to show a quantum advantage.
The second criticality of these microwave experiments
is represented by the preamplification of signal
and idler before interrogating the target region. In fact,
quantum mechanics ensures that amplification with
gain G always come with some noise of variance G
1 that reduces the entanglement of the signal-idler
pair. In the experiment by Barzanjeh et al. [18] the
preamplification noise was actually large enough to
disentangle the signal and the idler before interrogating
the target region. In this case, as admitted by the
authors themselves, the strongest signature of the target
presence is given by the amplifier noise, and not by the
correlations between the signal and idler. Accordingly,
QI is not practically relevant in this scenario.
CONCLUSION
In this review, we explained to an audience not necessarily
familiar with quantum optics how quantum entanglement
can be used to improve target-detection performances in
presence of high losses and a strong thermal background.
In particular, we showed that in the low signal limit, QI
allows for a reduction of the error probability. At the same
time, the quantum advantage becomes less and less important
when the number ofphotons per mode used to interrogate
the target region is increased.
If we look closer at the physical processes that can
be used to produce entangled photons, in particular in
the microwave regime, the pulse powers available at
the source in the region of quantum advantage are
extremely low: 15 to 20 orders of magnitude below
what is typically used in radar applications. As a consequence,
in practical scenarios where the radar-to-target
round-trip transmissivity is incredibly low (up to
1020 for plane tracking applications) in order to have
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