[Paper Review] Enhanced quantum sensing with amplification and deamplification

Min Jiang, Yushu Qin+4 more|arXiv (Cornell University)|Sep 1, 2023

Quantum and electron transport phenomenaPhysics and Astronomy3 citations

TL;DR

This paper demonstrates enhanced quantum sensing using Fano resonance in coupled alkali-metal and noble-gas spin systems, enabling signal amplification and noise deamplification without requiring quantum entanglement. The method achieves magnetic field sensitivity 54 dB below photon-shot noise, surpassing squeezed-light techniques and enabling sub-femtotesla-level detection in higher frequency ranges, with applications in dark matter searches and archaeometry.

ABSTRACT

Quantum sensing is a fundamental building block of modern technology that employs quantum resources and creates new opportunities for precision measurements. However, previous methods usually have a common assumption that detection noise levels should be below the intrinsic sensitivity provided by quantum resources. Here we report the first demonstration of Fano resonance between coupled alkali-metal and noble gases through rapid spin-exchange collisions. The Fano resonance gives rise to two intriguing phenomena: spin amplification and deamplification, which serve as crucial resources for enhanced sensing. Further we develop a novel scheme of quantum sensing enhanced by amplification and deamplification, with relaxed requirements on the detection noise. The coupled systems of alkali-metal and noble gases act as amplifiers or de-amplifiers, enabling to extract small signals above the detection noise before final detection. We demonstrate magnetic-field measurement about 54 decibels below the photon-shot noise, which outperforms the state-of-the-art squeezed-light technology and realizes femtotesla-level sensitivity. Our work opens new avenues to applications in searches for ultralight dark matter with sensitivity well beyond the supernova-observation constraints.

Motivation & Objective

• To overcome the stringent requirement of low detection noise in quantum sensing by introducing a mechanism that enhances sensitivity despite high noise levels.
• To explore and demonstrate the existence of Fano resonance in coupled alkali-metal and noble-gas spin systems via rapid spin-exchange collisions.
• To leverage both spin amplification and deamplification as dual resources for enhanced metrology, relaxing constraints on detection noise.
• To extend the operational bandwidth of self-compensating quantum sensors beyond the near-zero frequency range, enabling high-frequency applications.
• To develop a generic sensing framework applicable to various systems exhibiting Fano resonance, including metamaterials and plasmonic-atomic systems.

Proposed method

• Utilizes a vapor cell containing 20 torr 129Xe, 250 torr N2, and isotopically enriched 87Rb to enable rapid spin-exchange collisions between alkali-metal and noble-gas atoms.
• Employs circularly polarized pump laser at the D1 line and linearly polarized probe laser detuned by 110 GHz from the D2 line to polarize and probe 87Rb spins.
• Models the coupled spin system as two harmonic oscillators with distinct linewidths: broad (Γa ≈ 30 kHz) for 87Rb and narrow (Γb ≈ 7 mHz) for 129Xe, leading to Fano resonance.
• Exploits interference between the continuum (87Rb) and discrete (129Xe) responses to generate asymmetric Fano profiles, enabling resonant amplification and deamplification.
• Applies a non-Hermitian Hamiltonian model with an exceptional point (EP) at δ=0 and J/β=1, yielding sublinear frequency splitting Δω̃a,b = 2β^{1/2}|δ|^{1/2} and amplification factor (β/δ)^{1/2} ≫ 1.
• Demonstrates signal amplification of 54 dB below photon-shot noise and background noise suppression of ~24 dB through deamplification, enabling high-sensitivity detection.

Figure 1: Principle of enhanced sensing using Fano resonance. a , Experimental setup. The key element is a 0.5 cm 3 vapor cell containing 20 torr $\rm{}^{129}Xe$ , 250 torr $\rm N_{2}$ , and a droplet of isotopically enriched $\rm{}^{87}Rb$ . $\rm{}^{129}Xe$ spins are polarized and coupled with $\rm

Experimental results

Research questions

• RQ1 Can Fano resonance in coupled alkali-metal and noble-gas spin systems enable both signal amplification and noise deamplification for enhanced quantum sensing?
• RQ2 How does the Fano resonance mechanism relax the requirement for low-noise detection in quantum metrology?
• RQ3 Can the self-compensating mechanism based on Fano resonance extend the operational bandwidth of comagnetometers beyond the near-zero frequency range?
• RQ4 What is the sensitivity limit of this enhanced sensing scheme in terms of magnetic field detection, particularly in the presence of background noise?
• RQ5 Can this approach be generalized to other systems exhibiting Fano resonance, such as metamaterials or plasmonic-atomic systems, for broader sensing applications?

Key findings

• The first experimental observation of Fano resonance in a coupled alkali-metal–noble-gas spin system via rapid spin-exchange collisions was achieved in a 0.5 cm³ vapor cell with 20 torr 129Xe and 250 torr N2.
• Signal amplification of 54 dB below the photon-shot noise level was demonstrated, enabling magnetic field sensitivity well below the standard quantum limit.
• Background magnetic noise was suppressed by approximately 24 dB through deamplification, significantly improving signal-to-noise ratio.
• The method achieves sub-femtotesla-level sensitivity (0.75 fT/Hz^{1/2} at ~0.1 Hz), outperforming state-of-the-art squeezed-light-based sensors.
• The Fano resonance-based framework explains and extends previous self-compensated comagnetometry to higher frequencies, including above 100 Hz, for the first time.
• The technique enables enhanced sensitivity to hypothetical particles such as ultralight axions and dark photons, surpassing supernova-observation constraints by two orders of magnitude.

Figure 2: Demonstration of Fano resonance, amplification, and deamplification. a , Fano response profile as a function of the frequency of $y$ -directed measured field. $B_{z}$ is set to about -8.59 mG as an example. The data are well fit with theoretical Fano profile with Fano parameter $q$ , which