[Paper Review] Very-high- and ultrahigh- frequency electric field detection using high angular momentum Rydberg states

Roger C. Brown, Baran Kayim+4 more|arXiv (Cornell University)|May 25, 2022

Cold Atom Physics and Bose-Einstein Condensates4 citations

TL;DR

This paper demonstrates resonant detection of very-high- and ultrahigh-frequency (VHF/UHF) electric fields from 240 MHz to 900 MHz using high angular momentum Rydberg states (nF7/2 → nG9/2) in rubidium vapor via electromagnetically induced transparency (EIT). By employing three-photon infrared excitation, the authors achieve a noise floor of 13 μV/m/√Hz at n=50, enabling a new class of electrically small, atomic-based receivers with potential for sub-wavelength field sensing and high sensitivity beyond classical limits.

ABSTRACT

We demonstrate resonant detection of rf electric fields from 240 MHz to 900 MHz (very-high-frequency (VHF) to ultra-high-frequency (UHF)) using electromagnetically induced transparency to measure orbital angular momentum $L=3 ightarrow L'=4$ Rydberg transitions. These Rydberg states are accessible with three-photon infrared optical excitation. By resonantly detecting rf in the electrically small regime, these states enable a new class of atomic receivers. We find good agreement between measured spectra and predictions of quantum defect theory for principal quantum numbers $n=45$ to $70$. Using a super-hetrodyne detection setup, we measure the noise floor at $n=50$ to be $13\,\mathrm{μV/m/\sqrt{Hz}}$. Additionally, we utilize data and a numerical model incorporating a five-level master equation solution to estimate the fundamental sensitivity limits of our system.

Motivation & Objective

• To develop a new class of electrically small atomic receivers capable of resonant detection in the VHF and UHF bands (240–900 MHz), where classical antennas are band-limited by the Chu limit.
• To overcome the limitations of non-resonant detection and high principal quantum number (n > 130) requirements by utilizing high angular momentum Rydberg transitions (L=3→4) with lower energy splittings.
• To demonstrate practical, high-sensitivity RF field detection using electromagnetically induced transparency (EIT) with three-photon infrared excitation in a compact atomic vapor setup.
• To estimate fundamental sensitivity limits using a five-level master equation model and compare them to experimental noise performance.

Proposed method

• Employed three-photon infrared optical excitation to access high angular momentum Rydberg states (nF7/2 → nG9/2) in rubidium vapor, enabling resonant detection at VHF/UHF frequencies.
• Used electromagnetically induced transparency (EIT) to measure Autler-Townes splitting induced by rf electric fields, enabling high-sensitivity detection via probe transmission modulation.
• Implemented a super-heterodyne detection setup to down-convert the rf signal and measure the noise floor at n=50, achieving 13 μV/m/√Hz.
• Developed a five-level master equation model incorporating atomic parameters, laser intensities, and decoherence rates to simulate system response and estimate sensitivity limits.
• Calculated quantum projection noise and photon shot noise limits using the formula E_qpn = ℏ / (μ_R √(N T₂ t)) and ΔI_psn = √(2eηeΦ_p Δf), respectively, to benchmark experimental performance.
• Used transimpedance amplifier gain and receiver response to convert photodiode current noise into field sensitivity, yielding ΔE_psn = 1.6 μV/m/√Hz.

Experimental results

Research questions

• RQ1 Can high angular momentum Rydberg states (L=3→4) enable resonant detection of VHF and UHF electric fields at frequencies below 1 GHz?
• RQ2 What is the achievable sensitivity and noise floor of an EIT-based atomic receiver using three-photon infrared excitation at principal quantum numbers n=45 to 70?
• RQ3 How do experimental results compare to predictions from quantum defect theory and theoretical sensitivity limits (quantum projection and photon shot noise)?
• RQ4 To what extent can technical noise be reduced to approach the fundamental quantum limits in this system?

Key findings

• The measured noise floor at n=50 and f_rf=655 MHz is 13 μV/m/√Hz, with probe detection being the dominant noise source, well above the photon shot noise floor.
• Good agreement was observed between measured rf transition spectra and quantum defect theory predictions for principal quantum numbers n=45 to 70.
• The estimated quantum projection noise-limited sensitivity is 38 nV/m/√Hz, significantly lower than the experimental noise floor, indicating room for improvement.
• The photon shot noise-limited field sensitivity is estimated at 1.6 μV/m/√Hz, a factor of 40 lower than the current experimental noise floor, suggesting technical improvements could enhance sensitivity.
• The system achieves resonant detection of rf fields from 240 MHz to 900 MHz using n=75 down to n=40, avoiding the need for n>200 required by lower angular momentum transitions.
• The three-photon infrared excitation scheme enables Doppler-free and recoil-free excitation potential, supporting future improvements in spectral resolution and sensitivity.