Nanophotonic Optical Isolator Controlled by the Internal State of Cold Atoms

The realization of nanophotonic optical isolators with high optical isolation even at ultralow light levels and low optical losses is an open problem. Here, we employ the link between the local polarization of strongly confined light and its direction of propagation to realize low-loss nonreciprocal transmission through a silica nanofiber at the single-photon level. The direction of the resulting optical isolator is controlled by the spin state of cold atoms. We perform our experiment in two qualitatively different regimes, i.e., with an ensemble of cold atoms where each atom is weakly coupled to the waveguide and with a single atom strongly coupled to the waveguide mode. In both cases, we observe simultaneously high isolation and high forward transmission. The isolator concept constitutes a nanoscale quantum optical analog of microwave ferrite resonance isolators, can be implemented with all kinds of optical waveguides and emitters, and might enable novel integrated optical devices for fiber-based classical and quantum networks.

Popular Summary

Optical isolators are crucial elements for optical signal processing: They allow light to pass in one direction, but they block light coming from the opposite direction. Most commonly, optical isolators make use of the Faraday effect; i.e., a magnetic field along the direction of light propagation defines the direction in which the light can pass. However, this working principle is difficult to implement in nanoscale, integrated optics. Here, we demonstrate a conceptually new type of optical isolator that is integrated while simultaneously featuring good isolation and low losses; furthermore, it can operate with single photons.

Our approach takes advantage of miniaturization by employing a special property of light fields that are guided through waveguides that are smaller than the optical wavelength: In this case, light becomes chiral; i.e., the local spin of the photons becomes locked to their propagation direction. Unidirectional light propagation can then be achieved by coupling the light to laser-cooled atoms prepared in suitable quantum states that absorb only photons with positive, but not negative, spin. In contrast to Faraday isolators, our optical diode does not fundamentally require a magnetic field. In addition, it can even operate with just a single atom as the control element. We demonstrate asymmetric transmission using laser-cooled atoms and light guided within a tapered optical fiber with a nanofiber waist. Since quantum applications require an infrastructure that is operational with single photons, our demonstration of an integrated isolator at the individual photon level constitutes an important step forward toward a global fiber-based quantum optical network. Our isolator concept is also compatible with various emitters and optical waveguide structures.

We anticipate that our findings will inspire the development of novel devices for future optical computer chips.

Figure 1

Chiral photons in evanescent fields coupled to spin-polarized atoms. (a) Polarization properties of the evanescent light field that surrounds an optical nanofiber (gray). A light field that propagates in the $(+z)$ direction and whose main polarization axis (double arrow) is along the $x$ axis is almost fully ${\sigma }^{+}$ polarized (green solid arrows) in the ($y=0$) plane. If it propagates in the $(-z)$ direction, it is almost fully ${\sigma }^{-}$ polarized (blue dashed arrows). The quantization axis is chosen along $y$, i.e., orthogonal to the propagation direction. An atom (light blue sphere) placed at a distance $r$ to the nanofiber surface couples to the evanescent field. (b) Relevant energy levels of the atom. The ground state $|g⟩$ is coupled to the excited states $|e_{-1}⟩$, $|e_0⟩$, and $|e_{+1}⟩$ via ${\sigma }^{-}$, $\pi$, and ${\sigma }^{+}$ transitions, respectively.

Figure 2

Schematics of the demonstrated nonreciprocal waveguides. (a) An optical nanofiber is realized as the waist of a tapered silica fiber. (b) Atoms are trapped in the vicinity of the nanofiber, interact with the evanescent field of the nanofiber-guided modes, and scatter light out of the nanofiber (wavy arrow) with a rate that depends on the direction of propagation. (c) A single atom is coupled to a whispering-gallery-mode (WGM) bottle microresonator. The atom-resonator coupling strength depends on the propagation direction of the field in the WGM resonator. The WGM resonator is coupled to the optical nanofiber via frustrated total internal reflection.

Figure 3

Nonreciprocal transmission of chiral photons that interact with an ensemble of spin-polarized atoms. Transmissions ${\mathcal{T}}_{-}$ (red) and ${\mathcal{T}}_{+}$ (blue) and isolation $\mathcal{I}$ (black dashed line) as a function of the chirality $\chi$, calculated at the position of the atoms. The lines are the result of a numerical calculation with $⟨N⟩\approx 27$. The error bars indicate the $1\sigma$ statistical error based on counting statistics. Inset: Cross section of the optical nanofiber (gray disk) including the trapped atoms (blue sphere) and the main polarization axis of the guided field (green double arrow). The main polarization axis of the guided field and the $x$ axis enclose the angle $\phi$.

Figure 4

Calculated nonreciprocal transmission of chiral photons that interact with a single spin-polarized resonator-enhanced atom. The transmission through the nanofiber in $(+z)$ (${\mathcal{T}}_{+}$, blue solid line) and $(-z)$ directions (${\mathcal{T}}_{-}$, red solid line) and the isolation ($\mathcal{I}$, black dashed line) are plotted as a function of the fiber-resonator coupling strength $\kappa$. A magnetic field of $B=4.5\mathrm{G}$ is assumed to be applied along the resonator axis.

Figure 5

Control of the nonreciprocal transmission using a single resonator-enhanced atom. (a) Measured transmission ${\mathcal{T}}_{-}$ as function of time. Because of optical pumping into the Zeeman ground state $|F=3,m_F=-3⟩$, the system changes its transmission properties and, after about $2\mu \mathrm{s}$, the directionality of the optical diode reverses. (b) Second-order correlation function $g^{(2)}$ of the light transmitted through the nanofiber in the ($+z$) direction. The thin blue (thick red) line corresponds to the measured data (theoretical prediction). For zero time delay one observes photon antibunching.