IEEE Aerospace and Electronic Systems Magazine - November 2020 - 3

In This Issue -Technically
QUANTUM RADAR COST AND PRACTICAL ISSUES
We compute the minimum cost for an optimal quantum radar, and we compare it with the cost of actual real world classical
radars as a function of range. Our calculations show that the minimum cost quantum radar at X-band is many orders of magnitude more expensive than the corresponding classical radar, even assuming the most optimistic wideband-phased array radar
architecture. We also assume that the quantum radar is optimal; that is, the effective signal-to-noise ratio is 6 dB better
than for a classical radar with the same transmit power and bandwidth at low photon flux per mode. Finally, we discuss many
practical issues and potential solutions.

NOISY RECEIVERS FOR QUANTUM ILLUMINATION
Quantum illumination (QI) promises unprecedented performances in target detection, but there are various problems
surrounding its implementation. Where target ranging is a concern, signal and idler recombination forms a crucial barrier
to the protocol's success. This could potentially be mitigated if performing a measurement on the idler mode could still yield
a quantum advantage. In this article, we investigate the QI protocol for a generically correlated Gaussian source and study
the phase-conjugating (PC) receiver, deriving the associated SNR in terms of the signal and idler energies, and their crosscorrelations, which may be readily adapted to incorporate added noise due to Gaussian measurements. We confirm that a
heterodyne measurement performed on the idler mode leads to a performance, which asymptotically approaches that
of a coherent state with homodyne detection. However, if the signal mode is affected by heterodyne but the idler mode is
maintained clean, the performance asymptotically approaches that of the PC receiver without any added noise.

EVOLUTION OF QUANTUM RADAR CONCEPT TO NOISE RADAR CONCEPT
Preliminary results of the comparative analysis of quantum radar (QR) based upon quantum entanglement phenomenon and
noise radar (NR) based upon classical coherence and correlation processing of random signals are presented in this article.
Common and distinguishing features of NR and QR are briefly discussed. A novel concept of a stepped-frequency quantum
radar (SF-QR) for design of QR with range resolution capability is suggested. Field programmable gate array based digital
generation of entangled signals is proposed as well.

OPPORTUNITIES AND CHALLENGES OF QUANTUM RADAR
It has been recognized for over half a century that quantum mechanics defines the ultimate limits for sensing devices, and products
of that recognition are now on the horizon of practical implementation. One of the most promising of those products is quantum
radar, which offers a capability to significantly improve the tradeoff between energy and detection sensitivity compared to classical alternatives. We provide a back-of-the-envelope and implementation-agnostic analysis to glimpse the kinds of expected
improvement that a quantum radar could provide for applications of interest. Our analysis relies on relatively simplistic models
that are only intended to facilitate an intuitive grasp of quantum-versus-classical distinctions. Such analyses of course cannot lead
to definitive conclusions, but we believe they provide evidence that in some contexts quantum radar can be expected to offer
realizable practical advantages over classical alternatives. We conclude with a discussion of theoretical and practical obstacles
and a high-level summary of what can be said about the present status of quantum radar.

PROGRESS TOWARD AN ALL-MICROWAVE QUANTUM ILLUMINATION RADAR
Quantum illumination has been suggested as a way to improve the sensitivity of remote target detection by taking advantage
of quantum entanglement between a traveling electromagnetic beam of light interrogating the target, and a second beam kept
in the lab. In the microwave domain, superconducting quantum circuit utilizing superconducting Josephson tunnel junctions
can be used as quantum-limited amplifiers of faint microwave signals, as well as bright sources of quantum entangled signals.
These Josephson parametric amplifiers (JPAs) are poised to become fundamental building block in a future quantum illumination radar operating in the microwave domain. Demonstrating a practical advantage of quantum illumination with ambient
temperature signals is, however, challenging. Indeed, current quantum protocols are not practical as they require a complex
receiver with stringent phase-matching conditions, which cannot be met in the field. Notably, it is also not currently known
how one can transfer faint microwave signals out of cryogenic systems without destroying their fragile entanglement. In this
article, we discuss the recent progress and challenges regarding the development of an all-microwave quantum illumination
radar using the recently developed and implemented quantum-enhanced noise radar protocol. This quantum illumination
protocol is simpler to implement and procures a quantum enhancement over its classical analog. Finally, we discuss different
strategies that may resolve the issue of transmitting microwave entanglement under ambient conditions. We, thus, conclude
that successful transmission of entangled microwaves can be done in the near future, with possible demonstrations of
practical quantum advantage over short distances.

NOVEMBER 2020

IEEE A&E SYSTEMS MAGAZINE

IEEE Aerospace and Electronic Systems Magazine - November 2020

Table of Contents for the Digital Edition of IEEE Aerospace and Electronic Systems Magazine - November 2020