Waveguide-Coupled Rydberg Spectrum Analyzer from 0 to 20 GHz

We demonstrate an atomic rf receiver and spectrum analyzer based on thermal Rydberg atoms coupled to a planar microwave waveguide. We use an off-resonant rf heterodyne technique to achieve continuous operation for carrier frequencies ranging from dc to 20 GHz. The system achieves an intrinsic sensitivity of up to $-120(2)$, dc coupling, 4-MHz instantaneous bandwidth, and over 80 dB of linear dynamic range. By connecting through a low-noise preamplifier, we demonstrate high-performance spectrum analysis with peak sensitivity of better than $-145\mathrm{dBm/Hz}$. Attaching a standard rabbit-ears antenna, the spectrum analyzer detects weak ambient signals including FM radio, AM radio, WiFi, and bluetooth. We also demonstrate waveguide-readout of the thermal Rydberg ensemble by nondestructively probing waveguide-atom interactions. The system opens the door for small, room-temperature, ensemble-based Rydberg sensors that surpass the sensitivity, bandwidth, and precision limitations of standard rf sensors, receivers, and analyzers.

Figure 1

(a) Simplified experimental setup. Rydberg atoms are detected using a 780-nm and 480-nm beam, counterpropagating above a microwave circuit, with connections for input, output, and dc bias. Signals are detected in homodyne, using a balanced detector. (b) Microwave circuit. A coplanar waveguide transitions to a circuit region where the evanescent electric field area is matched to the Rydberg interrogation area. A third SMA cable, in the center, is used to add a dc bias to the circuit backplane, to help zero ambient dc fields. (c) Field simulations of the microwave circuit along a middle slice of the board. Atoms are interrogated over a 2-mm gap between the signal conductor and ground conductor. (d) Measured sensitivity versus frequency. Directly measured (blue), intrinsic PSN-limited (green), and directly measured using preamplification (red and orange) values are displayed. All error bars represent 2-dB total estimated standard deviation due to known statistical and systematic factors. The theoretically modeled intrinsic sensitivity, with no preamplification, is shown as a purple line. Note that lower and higher frequencies are shown on log and linear scales, respectively.

Figure 2

Sensor signals. (a) Detected signals versus probe detuning; optical amplitude quadrature in blue, phase quadrature in green, for (i) no microwaves, (ii) applied microwave LO at $f_{\mathrm{LO}}=10.2233$ GHz, but no signal field, and (iii) applied LO and signal field with offset $\delta =1$ kHz. (b) Enlarged view of (a)(iii) showing 1-kHz modulation.

Figure 3

Sensor performance. (a) Linear dynamic range for $f_{\mathrm{LO}}=2$ GHz, $\delta =700$ kHz. In a 1-s measurement, the sensor responds linearly over 80 dB. PSN-limited SNR is plotted. (b) Sensor response with $f_{\mathrm{LO}}=10.2233$ GHz, $\delta =700$ kHz. (c) Output signal versus instantaneous frequency $\delta$. The receiver has a 3-dB reduction at 4.0 MHz.

Figure 4

The rf signals observed inside the lab building using a rabbit-ears antenna. We tune the LO to $f_{\mathrm{LO}}=2$ MHz, 98.4 MHz, and 2.409 GHz to observe AM radio stations, FM radio stations, WLAN, and bluetooth signals. (a) AM radio stations sampled around 2 MHz. The doubled LO is directly observed at 4 MHz, since $f_{\mathrm{LO}}$ is within the instantaneous bandwidth. Other large peaks are spurious signals from lab equipment. (b) FM stations sampled instantaneously around 98.4 MHz. (c) WLAN and bluetooth signals detected over the course of several minutes around 2.409 GHz. The data are sampled in a max-hold configuration, showing packets sent over many WLAN subchannels (black ticks) and bluetooth bands (highlighted in green).

Figure 5

Microwave readout of Rydberg atoms. (a) Circuit diagram for microwave homodyne detection at 10.223 GHz. (b) Amplitude (blue) and phase (green) measurements of the microwaves transmitted through the chamber, versus probe detuning. The 3-mrad phase deflection, Lorentzian about Rydberg resonance, indicates the presence of Rydberg atoms. The lack of signal in the amplitude measurement indicates the Rydberg atoms do not absorb the field. (c) Enlarged view of the amplitude data in (b).

Figure 6

Homodyne readout sensitivity. The measured readout phase resolution, due to photon shot noise is shown in purple. Additional laser phase noise leads to poorer overall resolution, shown in blue.

Figure 7

In situ measured (blue) and comsol-predicted (orange) S21 of the coplanar waveguide.