IEEE Aerospace and Electronic Systems Magazine - November 2020 - 52

Opportunities and Challenges of Quantum Radar

Figure 15.

Figure 16.

Detection error probability bounds assuming a more complete
noise model Pqf (solid line) versus a pure solar-produced background noise Pqb (dashed line) as a function of target distance r
(in km).

Detection error probability bounds using the full noise model for
the entanglement-based Pqf (solid line) and nonentangled Plf
(dashed line) sensor with respect to the distance r (in km).

than 109 entangled photons is required to determine
the presence of a target 25 km away with detection
probability of about 0.8 in low visibility conditions.
If the target is in motion, which is the most realistic
scenario, the entire signal package needs to be generated, emitted, and processed in a short period of
time so that the process can be repeated multiple
times. This means that it will likely be necessary to
generate more than 109 entangled photons per millisecond if the target is in motion at a range of around
25 km (the exact amount will of course depend on
many factors such as the target speed, the target
cross section, and the quantum radar wavelength).
While the use of nonlinear crystals to generate
entangled optical photons is very well understood
from a theoretical and experimental point of view,
much less is known about how to do so in the microwave regime. Potentially viable proposals have been
made for the use of quantum dots, resonant QED
cavities, and down conversion from optical frequencies, but presently the generation of even a small
number of entangled microwave photons remains a
huge scientific and technological challenge [50]. In
addition, an ideal source of entanglement for sensing applications 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.

with a 40% fractional bandwidth and a 600-MHz
frequency bandwidth, a millisecond signal package
can only have about 4000 distinguishable optical
modes. This is substantially below the required 109
required for optimal quantum sensing at 25 km and
could lead to extremely large integration times [27].
Even though there have been some proposals to
address this problem, including the design of virtual
modes in a parallel architecture and using multipleinput multiple-output channels using beam-splitters,
considerable research is still required to overcome this
obstacle [28]-[30].

Distinguishable Optical Modes: A related challenge is
the number of distinguishable optical modes available
on each millisecond-long signal package [27], [29].
Because quantum radar exploits the photon-level correlations between entangled signal and idler photons,
it is necessary to have these photons in distinguishable
optical modes so they can be properly sorted out by
the system. In the case of X-band photons, for instance,
52

Quantum Memories: As described in Figure 1, the
idler photon needs to be stored in a robust and reliable quantum memory. This memory has to faithfully hold the quantum state of the idler photon for
a period of time determined by the time of travel of
the signal photon. In addition, the process of comparing the idler and received signal photons
requires the construction of nontrivial quantum
operators that act on both photon states. These
operations are accomplished using quantum gates
similar to those required for a quantum computer.
Fortunately, current research on fault-tolerant
quantum computation is making progress toward
near-term practical and dependable long decoherence-time quantum memories and fault-tolerant
gates [58]. In addition, some proposals such as
" quantum noise radar " may circumvent the need of
a quantum memory [40].
Single Photon Detectors: Equally fundamental for
the design of a fully functional quantum radar is the
development of improved single photon detectors
with very high detection efficiencies yet very small
dark currents and excess noise. As with the case of
quantum memories, such devices are receiving a

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