VIEWPOINT Inducing Transparency with a Magnetic Field A magnetic eld applied to an atomic sample in an optical cavity generates optical transparency that could be used to enhance the frequency stability of lasers. by Ariel Sommer acroscopic objects vibrate because of their ther- mal energy, but we do not perceive these vibra- tions in daily life. However, thermal motion of M optical-cavity mirrors limits the frequency sta- bility of state-of-the-art lasers that are used in optical atomic clocks and in searches for new fundamental physical effects. James Thompson from the University of Colorado, Boul- der, and colleagues  now demonstrate a method to render laser light insensitive to mirror vibrations. They accomplish this by inserting an atomic vapor between the mirrors of an optical cavity and using an effect that is analogous to the phenomenon of electromagnetically induced transparency Figure 1: In Thompson and colleagues' experiment , a small (EIT). Unlike conventional EIT, the effect relies on a magnetic magnetic eld (not shown) applied to a gas of strontium atoms ﬁeld rather than a laser to modify the optical properties of (purple) placed between the mirrors (light blue) of an optical cavity an atomic medium, so the researchers term the phenomenon makes the cavity transparent to light (red) at a specic resonant frequency. At this frequency, optical energy traveling through the magnetically induced transparency (MIT). Like its electro- cavity resides mostly in two atomic excited states (inset), magnetic counterpart, MIT could also be employed to slow protecting the frequency of the resonance from being perturbed as light and store light pulses in an atomic ensemble. the cavity mirrors vibrate because of thermal noise. The coupling Electromagnetically induced transparency is a quantum of the light to the atoms at the magnetically induced transparency resonance relies on a small detuning of the light from the transition optical effect in which light at one frequency from a “control frequencies of the two excited states. The purple ellipses denote laser ” renders a medium transparent to light from a “probe clouds of atoms conned to pancake-shaped layers because of laser ” at a second frequency. The transparency, or strong the standing-wave structure of the cavity light. (APS/Alan drop in probe-light absorption, also leads to a strong change Stonebraker ) in the index of refraction of the medium, causing light pulses to travel slowly in the medium [2, 3]. In addition to trans- parency and slowing of light, EIT can enhance a material’s musical instrument’s pitch, an optical cavity often serves as a otherwise small nonlinear optical response, leading to appli- frequency reference to help stabilize the frequency of a laser. cations in frequency conversion and information processing By signiﬁcantly narrowing the cavity resonance, cavity EIT [4, 5]. would provide a more precise reference frequency for laser In 1998, Mikhail Lukin and colleagues  proposed a new stabilization. application of EIT based on enclosing an atomic medium be- The underlying reason for the reduced linewidth stems tween two mirrors to form an optical cavity. They predicted from the fact that in EIT, the medium temporarily absorbs that EIT would result in a new optical resonance in the cav- most of the probe light into a long-lived atomic excited state, ity, with a reduced linewidth compared to the empty-cavity which protects the light from being lost as it normally would linewidth. Just as a tuning fork aids in the adjustment of a through scattering by a shorter-lived excited state. This ef- fect only occurs for light in a narrow range of frequencies Department of Physics, MIT-Harvard Center for Ultracold Atoms, that is typically narrower than the width of the cavity reso- and Research Laboratory of Electronics, Massachusetts Institute of nance in an empty optical cavity. The transfer of light into a Technology, Cambridge, MA 02139, USA long-lived atomic excited state that occurs in EIT has also led physics.aps.org 2017 American Physical Society 26 June 2017 Physics 10, 70 to work using EIT to transfer light to highly excited atomic herited from EIT essentially remove the optical cavity from states, called Rydberg states, that induce extremely strong the problem. However, a beneﬁcial feature of the cavity re- optical nonlinearities [7–9]. mains, namely enhanced atom-light coupling, because light In its original form, EIT relies on the additional con- bounces back and forth between the cavity mirrors many trol laser to modify the optical properties of the medium. times. This enhanced coupling is particularly helpful when The transparency occurs when the difference in frequency accessing narrow optical transitions, which can be other- between the control laser and the probe laser matches a wise difﬁcult to observe (because of Doppler shifts from the speciﬁc value that depends on the atomic medium. There- motion of the atoms, for example). As Thompson and col- fore, conventional EIT can be used to stabilize the frequency leagues point out, one limitation of their scheme is that the difference of two lasers . However, a stable frequency signal from the narrow resonance becomes weaker the more difference is not sufﬁcient for applications requiring absolute one decouples it from the cavity, so one must choose an op- frequency stability, such as atomic clocks. The phenomenon timal operating point. It will be interesting to see future of MIT that Thompson and colleagues now introduce gives work building on this idea of inducing optical transparency access to the essential features of EIT, but it only requires a magnetically, for example, in photon storage and in the sta- static magnetic ﬁeld rather than an additional laser to mod- bilization of the frequency of narrow-linewidth lasers for ify the properties of the medium. MIT can thus be used high-precision measurements. to stabilize a single laser (the probe laser) to an absolute frequency deﬁned by the medium, rather than only stabi- This research is published in Physical Review Letters. lizing the laser relative to another laser. As in conventional cavity EIT, in MIT a narrow optical resonance appears in the cavity’s optical transmission spectrum. And similarly REFERENCES to EIT, this resonance arises because of a strongly coupled state of light and matter that can have a larger matter compo-  M. N. Winchester, M. A. Norcia, J. R. K. Cline, and J. K. nent than optical component. The larger matter component Thompson, ``Magnetically-Induced Optical Transparency on a makes the frequency of the resonance largely insensitive to Forbidden Transition in Strontium for Cavity-Enhanced Spec- troscopy,'' Phys. Rev. Lett. 118, 263601 (2017). thermal vibrations of the cavity mirrors.  K.-J. Boller, A. Imamo§lu, and S. E. Harris, ``Observation of To allow a magnetic ﬁeld to induce a narrow resonance, Electromagnetically Induced Transparency,'' Phys. Rev. Lett. Thompson and co-workers employ three energy levels of the 66, 2593 (1991). strontium-88 atom in a “V” conﬁguration: two long-lived ex-  S. E. Harris, ``Electromagnetically Induced Transparency,'' cited states and one ground state (Fig. 1). The excited states Phys. Today 50, No. 7, 36 (1997). have the same energy in the absence of a magnetic ﬁeld,  M. Fleischhauer, A. Imamo§lu, and J. P. Marangos, ``Elec- but in an applied magnetic ﬁeld, their energy is split by a tromagnetically Induced Transparency: Optics in Coherent small amount through the Zeeman effect. Because of the Media,'' Rev. Mod. Phys. 77, 633 (2005). near-degeneracy of the two excited states, a single laser can  D. E. Chang, V. Vuleti¢, and M. D. Lukin, ``Quantum Nonlinear make the atom transition to either excited state. At zero mag- OpticsPhoton by Photon,'' Nat. Photon. 8, 685 (2014). netic ﬁeld, the three-level system has a so-called dark state,  M. D. Lukin, M. Fleischhauer, M. O. Scully, and V. L. Velichan- sky, ``Intracavity Electromagnetically Induced Transparency,'' which consists of a coherent superposition of the two excited Opt. Lett. 23, 295 (1998). states and is unresponsive to the probe light. While this dark  V. Parigi, E. Bimbard, J. Stanojevic, A. J. Hilliard, F. Nogrette, state has the desired properties of possessing a precise res- R. Tualle-Brouri, A. Ourjoumtsev, and P. Grangier, ``Obser- onant frequency and being insensitive to mirror vibrations, vation and Measurement of Interaction-Induced Dispersive the fact that it doesn’t respond to probe light makes it im- Optical Nonlinearities in an Ensemble of Cold Rydberg Atoms,'' possible to access. However, the Zeeman splitting from an Phys. Rev. Lett. 109, 233602 (2012). applied magnetic ﬁeld modiﬁes the dark state slightly and  J. Ningyuan, A. Georgakopoulos, A. Ryou, N. Schine, A. allows it to respond to the probe light. The application of a Sommer, and J. Simon, ``Observation and Characterization of magnetic ﬁeld therefore causes a new narrow resonance to Cavity Rydberg Polaritons,'' Phys. Rev. A 93, 041802 (2016). appear in the cavity’s transmission spectrum. At sufﬁciently  J. Ningyuan, N. Schine, A. Georgakopoulos, A. Ryou, A. Som- small values of the applied ﬁeld, the desired characteristics mer, and J. Simon, ``A Strongly Interacting Polaritonic Quantum Dot,'' arXiv:1705.07475. of the dark state remain intact, providing a narrow feature  S. C. Bell, D. M. Heywood, J. D. White, J. D. Close, and R. E. that is largely decoupled from the thermal vibrations of the Scholten, ``Laser Frequency Offset Locking Using Electromag- cavity mirrors. netically Induced Transparency,'' Appl. Phys. Lett. 90, 171120 This narrow feature can serve as an absolute reference (2007). in the frequency domain. The key properties of narrow linewidth and insensitivity to thermal mirror vibrations in- 10.1103/Physics.10.70 physics.aps.org 2017 American Physical Society 26 June 2017 Physics 10, 70
Physics – American Physical Society (APS)
Published: Jun 26, 2017
It’s your single place to instantly
discover and read the research
that matters to you.
Enjoy affordable access to
over 12 million articles from more than
10,000 peer-reviewed journals.
All for just $49/month
Read as many articles as you need. Full articles with original layout, charts and figures. Read online, from anywhere.
Keep up with your field with Personalized Recommendations and Follow Journals to get automatic updates.
It’s easy to organize your research with our built-in tools.
Read from thousands of the leading scholarly journals from SpringerNature, Elsevier, Wiley-Blackwell, Oxford University Press and more.
All the latest content is available, no embargo periods.
“Hi guys, I cannot tell you how much I love this resource. Incredible. I really believe you've hit the nail on the head with this site in regards to solving the research-purchase issue.”Daniel C.
“Whoa! It’s like Spotify but for academic articles.”@Phil_Robichaud
“I must say, @deepdyve is a fabulous solution to the independent researcher's problem of #access to #information.”@deepthiw
“My last article couldn't be possible without the platform @deepdyve that makes journal papers cheaper.”@JoseServera