IEEE - Aerospace and Electronic Systems - April 2020 - 53

Anderson et al.
EIT transient dynamics that reproduce the observed transient behavior in great detail. A detailed discussion and further characterization of the quantum physics and its
implementation in ultrafast RF detection method will be
the subject of future work.
Comparing Figure 2(b) and (c) it is noted that the EIT
line widths are quite different. This is due to the use of different laser-beam parameters and Rydberg states, leading
to different coupler and probe-beam Rabi frequencies in
the two cases. In Figure 2(b), the Rabi frequencies at the
laser-beam centers are Vp ¼ 2p Â 15 MHz for the probe
and Vc ¼ 2p Â 5 MHz for the coupler. These values are
small enough to largely avoid saturation broadening, leading to EIT lines that are less than about 10 MHz wide (in
coupler laser frequency). In Figure 2(c), the respective
Rabi frequencies are Vp ¼ 2p Â 37 MHz and Vc ¼
2p Â 10 MHz. In that case, the large probe Rabi frequency causes a larger amount of saturation broadening,
leading to EIT lines that are about 20 MHz wide.

ATOMIC RF PHASE DETECTORS
RF electric field sensing and measurement based on EIT
readout of field-sensitive Rydberg states of atoms in thermal
vapor cells has made rapid progress toward establishing
atomic RF E-field standards. Here, we describe an atomic
RF phase, amplitude, and polarization sensor that employs a
novel quantum-optical readout scheme from an RF fieldsensitive Rydberg vapor to achieve RF phase sensitivity [6].
The holography concept outlined in "Atomic Physics
and Field/Phase-Sensing Background" section can be
implemented from the optical into the RF domain. Measurements have been performed by combining RF signal
and reference waves in or close to Rydberg-EIT vapor
cells [31], [32], [33]. The magnitude of the coherent electric field sum of the object and reference RF or microwave
fields is measured using vapor-cell Rydbrg-EIT methods
within the atomic vapor cell or hybrid atom-cavity cell
structure, as described in "Atomic RF Electric Field
Sensing" section. According to principles of holography,
this allows measurement of amplitude and phase of the signal wave, with the reference wave providing the phase reference as well as amplification [31], [32], [33]. Toward
practical applications, a phase-sensitive recording of a
coherent electromagnetic field on a surface allows the
reconstruction of the field in all space. RF applications of
this reconstruction principle abound and include radars
based on interferometric schemes, such as SAR and
InSAR, and far-field characterization of antenna radiation
patterns based on near-field measurements of amplitude
and phase of the field emitted by the antenna under test. In
the last application listed, the measurement has to be performed on a surface, and a near-field to far-field transformation is applied to calculate the field in all space.
APRIL 2020

To achieve phase sensitivity in the holographic RF field
measurement, the reference wave can be interfered with the
waves emitted by or reflected from an object. The generation
of a clean RF reference wave presents a considerable problem. In optical holography, the reference wave typically is
an expanded, near-perfect plane-wave laser beam that interferes with the object scatter within a layer of photographic
emulsion (or an equivalent substance). The purity of the reference wave is important, i.e., it should be free of diffraction
rings caused by dust particles and other imperfections. Interference from specular reflections of the reference wave from
planar surfaces should also be avoided. In quantitative work,
it would also be important that the reference wave has a fixed
amplitude or, at least, a well-known, slowly varying amplitude function. In holographic measurements in the RF
domain, equivalent conditions are hard to meet. The preparation of a defect-free RF reference wave that has a smooth
amplitude behavior over a large surface presents a great
challenge. In some cases, it will be fundamentally impossible to prepare a stationary reference wave. This applies, for
instance, to SAR radar applications, where the detector is
mounted on a moving platform, like an airplane or a satellite,
or in cases where a millimeter-wave or microwave field
needs to be fully characterized over a large surface in space.
In another class of applications, the object waves are located
within close quarters where multiple reflecting surfaces cannot be covered with anechoic material ("urban radar"); there,
reflections from unknown surfaces spoil the reference wave.
The cited previous implementations of holographic RF
phase detection with atoms have required an antenna or similar for the generation of the reference RF wave, precluding the
approach from providing a stand-alone atomic detector solution for RF waves propagating in free space [31], [32], [33]. In
the following, we present the holographic scheme in which an
RF reference signal is provided via phase modulation of one
of the EIT laser beams [6]. Our presented approach removes
the need for RF reference waves, and therefore eliminates the
aforementioned shortcomings of RF reference waves.
For RF phase measurement using RF-modulated optical
beams, we consider a phase modulation imprinted on an
optical coupling laser beam via an electro-optic modulation
technique. Using a fiber-optic high-frequency modulator,
which is commercially available, the coupler beam is
phase-, frequency-, or amplitude-modulated with a signal at
frequency vRF that is near the frequency of the RF field to
be measured, and that is phase coherent with the RF field to
be measured. For the purpose of describing the basic concept, in the following, we consider a rubidium atom and a
case where the (optical) coupler field is phase-modulated at
a frequency that is identical with the RF signal frequency
vRF . Here, vRF also approximately equals half the separation between two neighboring Rydberg levels of rubidium,
nS1=2 and ðn þ 1ÞS1=2 , as shown in Figure 3(a). The carrier
frequency of the coupler laser beam is resonant with the forbidden transition 5P3=2 ! nP3=2 . Due to the quantum

IEEE A&E SYSTEMS MAGAZINE

IEEE - Aerospace and Electronic Systems - April 2020

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