IEEE Aerospace and Electronic Systems Magazine - November 2020 - 59

Credit: Image Licensed by Ingram Publishing

The use of superconducting circuits for radar application, however, poses a practical problem. These cryogenic
devices have output powers at the femtoWatt level and
must be kept at miliKelvin temperatures in a cryostat and
well isolated from room-temperature thermal noise.
Transmitting ultralow-power microwave signals out of
such a well-isolated and cold environment while preserving their initial entanglement is challenging. Successful
transmission of microwave entanglement to the atmosphere would, however, have a tremendous scientific and
technological impact well beyond quantum radar, paving
the way to numerous new quantum-based sensing, communication, and computing applications.
In this article, we present the basic concept, requirements, and possible attainable gains of a quantum noise
radar using superconducting quantum circuits. After providing a short overview of the classical noise radar system,
we then review the quantum signals basics in a matter that
will hopefully be familiar to nonspecialists and provide
quantum description of the classical noise radar. We then
present the new QI protocol we recently suggested and
successfully realized in the microwave domain [8], [19].
We describe its physical implementation using superconducting microwave circuits and derive the predicted quantum enhancement observed experimentally. While its
performance may not be optimal, we show how this novel
QI protocol is much more practical than previous protocols, as it eliminates the need for joint measurements, photodetectors, and delay lines. Finally, we approach the
entanglement-transmission problem by exploring how
quantum-limited amplification and multiplexing may
solve the issue.

TARGET DETECTION WITH CLASSICAL NOISE RADAR
A typical (or classical) noise radar system uses random
waveforms for target detection and ranging [11], and a
simple schematic of such system is illustrated in
Figure 2(a). Here, a single zero-mean, Gaussian noise
voltage signal is generated and sent to a power divider to
create two (ideally) correlated copies, here referred as the
NOVEMBER 2020

signal and the idler. While the signal is transmitted
through free space, heterodyne measurements of the inphase (Ib ) and quadrature (Qb ) voltages of the idler are
recorded and retained. The transmitted signal is reflected
off a distant target, and a small fraction h ( 1 is
recorded by the receiver using heterodyne measurements, giving the voltage quadratures Ia and Qa . The
fraction h encapsulates both the target reflectivity and
the atmospheric losses.
The presence or absence of the target is determined
not by the intensity of the returned signal, but rather by
the amplitude of the signal-idler correlations at the
receiver [11], [19]. These correlations are computed
through postprocessing of the measurement records
where the retained idler acts as a matched-filter on the
received signal. This is done by constructing a covariance matrix R corresponding to the expectation value
R ¼ E½xxT where x ¼ ½Ia ; Qa ; Ib ; Qb T is the vector of
the instantaneous measurements of the different quadrature voltages.
For a; b being correlated zero-mean Gaussian
signals with relative phase f, the measured covariance
matrix from heterodyne measurements can be simplified
to [11], [19],

Figure 1.
Typical QI protocol. A quantum source generates entangled signal (blue) and idler (pink) photon pairs. Idler photons are kept in
a delay line, whereas signal photons are sent to a distant target
and reflected photons are collected back by the antenna. If the
time delay is right, the quantum receiver then performs a joint
measurement by recombining the entangled pair and measuring
the mixed signal with a single photon detector.

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