Towards the miniaturization of atomic sensors: a new readout method

Some technological devices that aim at high-precision sensing are based on optical pumping (that is, the process in which light is used to raise -or "pump"- electrons from a lower energy level in an atom or molecule to a higher one) and atomic vapor spin physics (that is, the physics associated with the evolution and the interactions of an intrinsic magnetic property of atoms, called spin). For instance, optically pumped magnetometers (OPMs), one of the most sensitive quantum devices to detect very weak magnetic fields, rely on these two principles.

There is an important element that influences the degree of sensitivity and that, consequently, can compromise the performance of such instruments: the readout of the electronic spins’ polarization (a property related to how atomic spins orient). Currently, there are state-of-the-art methods that enhance readout effectiveness, but their performance worsens as the dimensions of the sensors are reduced, holding back the miniaturization that most of the modern technologies pursue.

The issue has been recently addressed by ICFO researchers María Hernández, Yintao Ma (also member of the Xi’an Jiaotong University), Hana Medhat, Dr. Chiara Mazzinghi (also member of Instituto Nazionale di Ottica, Sesto Fiorentino) and Dr. Vito Giovanni Lucivero (also member of the Universitá degli Studi di Bari Aldo Moro), led by ICREA Professor at ICFO Morgan W. Mitchell. In a Physical Review Applied article, they have demonstrated a new nondestructive method to monitor the electron spin polarization of an atomic ensemble that, at the same time, allows for miniaturization.

A new spin-polarization monitoring technique

In the study, the authors utilized three different components (each of them well-established on its own) and brought them all together, something that no one had ever done before.

The first ingredient was the use of an atomic vapor and the exploitation of its optical properties. In particular, they used rubidium 87, a very regular choice when dealing with sensors as they are naturally sensitive to external changes in their environment.

The second one was the use of an optical resonator. These optical elements, also known as “optical cavities”, improve the spin readout by increasing the interaction length between the rubidium atoms and the infrared light, which is used to probe them. Moreover, the vapor was placed inside a so-called micro-electromechanical system (MEMS) vapor cell, since this kind of cell enables the miniaturization of sensing devices without this being detrimental to its quality.

And thirdly, the team performed the readout using the Pound-Drever-Hall (PDH) technique. The PDH method is widely used to measure changes in the frequency or phase of laser light with infinitesimal precision (for instance, in gravitational waves detection). In this case, by sending light into the medium and measuring the phase shift through PDH, the researchers were able to indirectly infer the atomic spin polarization (as the former depends on the latter). 

In summary, the team performed PDH readout of the atomic spin polarization of a rubidium 87 vapor housed in a MEMS cell within an optical resonator. The results suggest an enhanced readout (and thus better sensitivity) even in small cells, a two-fold improvement over previous attempts.

Benefits for future atomic sensors

The researchers claim that their method has potential to provide high efficiency readout for miniaturized atomic vapor sensing and metrology instruments. “This is an important result since it paves the way towards the miniaturization of atomic sensing devices that allow high spatial resolution in magnetic field detection. Moreover, in principle, the method can be quantum noise limited, which will enable quantum enhanced detection in miniaturized sensors”, shares María Hernández, leading author of the article.

Hernández has already started building an optical magnetometer based on this technique for micro-bio-magnetic applications. The miniaturization of atomic magnetometers could result in sensors with improved spatial resolution and that could also be suitable for spin squeezing, a milestone the team is looking forward to achieving.

Bibliographic reference:

1. Hernández, Y. Ma, H. Medhat, C. Mazzinghi, V. Giovanni, and M. W. Mitchell. Cavity-enhanced detection of spin polarization in a microfabricated atomic vapor cell. Phys. Rev. Applied 21, 064014 (2024).

Acknowledgements:

This work was supported by European Commission projects MACQSIMAL (820393), OPMMEG (101099379), and QUANTIFY (101135931); NextGenerationEU (PRTR-C17.I1); Spanish Ministry of Science (MCIN) project SAPONARIA (PID2021-123813NB-I00); “Severo Ochoa” Center of Excellence CEX2019-000910-S, Departament de Recerca i Universitats de la Generalitat de Catalunya Grant No. 2021 SGR 01453; Fundació Privada Cellex; and Fundació Mir-Puig. M.H.R. acknowledges support from Ayuda PRE2021-098880 financiada por MCIN/AEI/10.13039/ 501100011033 y por el FSE+. V.G.L. acknowledges financial support from European Union NextGenerationEU (PNRR MUR project PE0000023 – NQSTI) and from the Italian Ministry of University and Research (MUR) project “Budget MIUR - Dipartimenti di Eccellenza 2023 - 2027” (Law 232, 11 December 2016) - Quantum Sensing and Modelling for One-Health (QuaSiModO). H.M. acknowledges financial support from the European Union’s Horizon Europe research and innovation programme under the Marie SklodowskaCurie Grant Agreement No. 101081441. Y.M. acknowledges the support from China Scholarship Council (202206280171).The project is being carried out under the framework “Quàntica – Vall de la Mediterrània de les Ciències i les Tecnologies Quàntiques” Government Agreement GOV/51/2022, promoted by Secretariat of Digital Policies of the Government of Catalonia”.