Simultaneous Detection, Demodulation, and Angle-of-Arrival Determination of Communication Signals Using a Dual Ladder Rydberg Receiver

Abstract

In a typical Rydberg mixer, modulated communication signals are detected using a radio frequency (RF) heterodyne technique. The mixer outputs an intermediate frequency (IF), which must be filtered and mixed down to baseband. In this work, we apply an RF-homodyne technique to demonstrate simultaneous detection and a direct, baseband readout of the in-phase (I) and quadrature (Q) components of standard communication signals using a dual ladder Rydberg receiver. We further show that the inherent polarization sensitivity of this receiver can be used to determine the signal's angle of arrival. We also compare the dual ladder system with a typical Rydberg mixer. The RF-heterodyne-based system's maximum detectable symbol rate is constrained by a signal amplitude which decays with the heterodyne field's detuning from the Rydberg-Rydberg atomic transition used to detect the signal, but the dual ladder design is not subject to this limitation. However, the dual ladder system is more sensitive to low-frequency noise. As a result, its performance is degraded relative to its conventional counterpart when subjected to pink noise. We show that once pink noise effects have been accounted for, both systems behave comparably.

Figures (10)

• Figure 1: Schematic of the optical setup and the orientation of the RF horn-antennas relative to the vapor cell. The black arrows out of the fiber couplers indicate the direction of laser propagation. Red laser beam paths indicate $780nm$ light, while blue paths indicate $480nm$ light. The angle $\theta$ describes the relative angle between LO 1 and the signal horn, i.e., the AoA. The z-direction is taken to be opposite the direction of the probe's propagation at the vapor-cell.
• Figure 2: Above shows the energy level diagram for $85$-Rubidium used to construct the DLRR. To achieve independent sensing, the second EIT system interacts with the same states as the first, but accesses them through a different Doppler class of atoms. The atom velocity which shifts the beams to resonance is provided on the left, where the positive z-direction is taken to be opposite the direction of the probe's propagation.
• Figure 3: Above displays the EIT spectra acquired by measuring the transmission profile of each probe beam while scanning the pump beams of the DLRR. Since all four beams are spatially overlapped, each spectrum shows two pairs of EIT features separated by the detuning between the pump beams. The smaller peak seen in each pair corresponds to the $54D_{3/2}$ state, while the larger peak corresponds to the $54D_{5/2}$ state. The difference between these peaks was used to calibrate the frequency axis. The features marked with a $*$ correspond to the frequency-overlapped features, the larger of which was used for this work. The unmarked features result from beam cross-talk and do not impact the experiment.
• Figure 4: Above shows three APSK and three 16QAM I/Q diagrams as a function of the received signal's measured AoA; each constellation is compressed or expanded along each axis according to equation \ref{['IQ-WarpEqn']}. The individual points in each sub-plot are measured data, while each black circle with a distinct color inside represents a nominal constellation point. (a) APSK with AoA $=30^{\circ}$. (b) APSK with AoA $=45^{\circ}$. (c) APSK with AoA $=60^{\circ}$. (d) 16QAM with AoA $=30^{\circ}$. (e) 16QAM with AoA $=45^{\circ}$. (f) 16QAM with AoA $=60^{\circ}$.
• Figure 5: (a) displays the AoA found by averaging all five measurement runs. Vertical error bars represent one standard deviation, while the horizontal error bars represent absolute limits on the signal horn position. (b) displays the phase-averaged AoA measurement results of each measurement run. The color coding used here to represent different modulation schemes is maintained throughout this work.